Note: Descriptions are shown in the official language in which they were submitted.
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RESIDENTIAL SOLAR THERMAL POWER PLANT
STATEMENT OF FEDERALLY SPONSORED DEVELOPMENT
[00021 The United States Government has rights in this invention
pursuant to Contract No. W-7405-ENG-48 between the United States
Department of Energy and the University of California for the
operation of Lawrence Livermore National Laboratory.
III. FIELD OF THE INVENTION
100031 This invention relates to solar-thermal energy systems. In
particular, the invention relates to a highly efficient residential solar
thermal energy collection, storage, and utilization system having a
parabolic trough-type solar concentrator rotatably mountable on a
preferably fixed structure, such as a residential rooftop, and a tubular
heat collector coaxially positioned to receive concentrated sunlight
from the concentrator, with the concentrator and collector shaped and
oriented to maximize solar collection efficiency and thermal energy
delivery to a heat-powered engine for optimizing mechanical and
electrical power generation.
IV. BACKGROUND OF THE INVENTION
100041 Despite over a century of attempts to make solar power
commercially viable, solar energy currently makes up an insignificant
proportion of per capita energy supply. This has been due primarily to
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performance and cost inefficiencies of existing solar energy collectors,
concentrators, and interfaces to heat storage media which have
prevented widespread adoption and use for commercial and
residential applications. For example, the SEGS, Solar Electric
Generating System, plants in Southern California represent the state of
the art today in deployed CSP, Concentrating Solar Power. Based on
the experience with the existing SEGS plants, the cost of electricity
from newly constructed plants using currently available technology is
approximately 10C/kWh. This cost is much greater than the cost to
generate electricity by burning coal, which is approximately 3C/kWh.
[0005] Various solar energy collectors and concentrators, and
interfaces to heat storage media and heat engines are known for use in
solar thermal electric energy systems, such as the SEGS plants. A few
examples include: U.S. Pat. No. 4,586,334 to Nilsson, and U.S. Pat. No.
6,487,859 to Mehos. The Nilsson patent discloses "... a solar energy
power generation system which includes means for collecting and
concentrating solar energy; heat storage means; Stirling engine means
for producing power", and "... the means for collecting and
concentrating solar energy is a reflective dish; and the heat transfer
means includes first and second heat pipes; the heat storage means is
preferably a phase change medium ..." The Mehos patent discloses:
sodium heat pipe receivers for dish/Stirling systems", and cites
references demonstrating: "...sodium vapor temperatures up to 790
C." Additionally, U.S. Pat. No. 4,125,122 discloses a heat pipe receiving
energy from a solar concentrator, U.S. Pat. No. 6700054B2 describes
connecting to a Stirling engine, among other things, and U.S. Pat. No.
4088120 describes a parabolic trough with a heat pipe at the focus
connected to a heat storage medium. U.S. Pat. No. 787,145 describes an
elliptical dish mirror that is oriented to track the sun, with a boiler to
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produce steam at the focus of the mirror. U.S. Pat. No. 3,982,526
describes a device for turning a solar collector about a polar axis, and
U.S. Pat. No. 6,886,339B2 describes a parabolic trough solar
concentrator with a sun tracking system. U.S. Pat. No. 4,205,657
describes a parabolic trough solar concentrator with a steam generation
system. U.S. Pat. No. 4,108,154 describes a parabolic trough solar
collector with a windshield.
[0006] One particular limitation of currently available solar
collectors/concentrators, however, is their relatively low thermal
gathering efficiency, which is the ratio of the thermal heat delivered by
the heat collecting element relative to the solar heat incident on the
concentrating mirror surface area. Based on recent field
measurements, the best available collector's, (such as the UVAC heat
collector from Solel or the PTR 70 heat collector from Schott, using an
oil based heat transfer fluid heated to 400 C), achieve a maximum
value of only 50% thermal gathering efficiency at a solar incidence of
800 W/m2. At either higher or lower solar irradiance levels, the
thermal efficiency is even lower. This efficiency is low primarily
because the solar concentration factor for these collectors is relatively
low. For example, in the current generation of SEGS plants, the
diameter of the absorbing surface in the heat-collecting element is 7 cm,
while the width of the parabolic trough aperture is 5.77 m, and the
ratio of the concentrator aperture area to collector absorber area, the
solar concentration factor, is only 26. Another limitation associated
with the relatively low concentration factors of parabolic trough
collectors is that the axial length of the collector relative to the
concentrator aperture width is quite large. In the DISS case, for
example, the length to width ratio is 46.
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10007] Another efficiency loss factor that is characteristic of the current
state of the art parabolic trough collectors is associated with their
horizontal deployment. Averaging over the range of solar incidence
angles both through the day and through the year, leads to an average
geometrical foreshortening factor of 87%.
[0008] Since the efficiency of conversion from solar irradiance to
power is known to have a great impact on the cost of electricity, it
would be advantageous to provide a highly efficient solar thermal
power system for the economical utilization of solar thermal energy in
the context of a residential/commercial unit which overcomes the
limitations of current solar energy technology for reducing energy
costs. And in particular an apparatus and method capable of
increasing the solar concentration factor for parabolic trough collectors
to beyond about 160 and improving the average geometrical
foreshortening factor to greater than about 90%, would be particularly
beneficial to substantially raise the thermal gathering efficiency of such
solar thermal power plants.
V. SUMMARY OF THE INVENTION
100091 One aspect of the present invention includes a solar thermal
power plant comprising: a parabolic trough mirror having a
longitudinal focal axis for concentrating sunlight therealong; means for
rotating said mirror about a longitudinal rotation axis to follow the
sun; and a heat collector comprising an elongated heating tube
surrounding a flow channel, said flow channel having an oblong cross-
sectional shape characterized by major and minor axes with a largest
diameter of the channel along the major axis and a smallest diameter of
the channel along the minor axis and with the major axis aligned with
a longitudinal plane of symmetry of the parabolic trough mirror, said
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heating tube coaxially positioned along the focal axis of said mirror to
receive
concentrated sunlight therefrom so that a working fluid in said heating tube
is
heated thereby and provided for use through an outlet end of said heating
tube.
[00101 Another aspect of the present invention includes a solar thermal power
plant
comprising: a parabolic trough mirror having a longitudinal focal axis for
concentrating sunlight therealong; means for rotating said mirror about the
rotation
axis to follow the sun; and a tubular heat collector comprising an optically
transparent
thick-walled heating tube having an inner wall surface forming a flow channel
and a
convex curvilinear outer wall surface for magnifying the dimensions of the
flow
channel, said inner wall surface coated with a sunlight absorbing material,
and said
heating tube coaxially positioned along the focal axis to receive concentrated
sunlight
from said mirror so that a working fluid in the flow channel is heated thereby
and
provided for use through an outlet end of the heating tube.
100111 And another aspect of the present invention includes a solar thermal
power
plant comprising: a parabolic trough mirror having a longitudinal focal axis
for
concentrating sunlight therealong; means for mounting said mirror so that the
focal
axis is parallel with the earth's rotational axis and said mirror is rotatable
about a
longitudinal rotation axis thereof; means for rotating said mirror about the
rotation axis
to follow the sun; and an elongated tubular heat collector forming a flow
channel and
coaxially positioned along the focal axis to receive concentrated sunlight
from said
mirror so that a working fluid in the flow channel is heated thereby and
provided for
use through an outlet end of said heat collector.
[0012] And another aspect of the present invention includes a solar thermal
power
plant comprising: a parabolic trough mirror having a longitudinal focal axis
for
concentrating sunlight therealong; means for mounting said mirror so that the
focal
axis is parallel with the earth's rotational axis and said mirror is rotatable
about a
longitudinal rotation axis thereof; means for rotating said mirror about a
longitudinal
rotation axis to follow the sun; and a tubular heat collector comprising an
optically
transparent thick-walled heating tube having an inner wall surface forming a
flow
channel and a convex curvilinear outer wall surface for magnifying the
dimensions of
the flow channel, said flow channel having an oblong cross-sectional shape
characterized by major and minor axes with a largest diameter of the channel
along the
major axis and a smallest diameter of the channel along the minor axis and
with the
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major axis aligned with a longitudinal plane of symmetry of the parabolic
trough
mirror, said inner wall surface coated with a sunlight absorbing material, and
said
heating tube coaxially positioned along the focal axis to receive concentrated
sunlight
from said mirror so that a working fluid in the flow channel is heated thereby
and
provided for use through an outlet end of the heating tube.
10012A1 Accordingly, this writing has disclosed at least the following
concepts.
Concepts 1. A solar thermal power plant comprising:
a parabolic trough mirror having a longitudinal focal axis for
concentrating sunlight therealong;
means for rotating said mirror about a longitudinal rotation
axis to follow the sun; and
a heat collector comprising an elongated heating tube
surrounding a flow channel, said flow channel having an oblong
cross-sectional shape characterized by major and minor axes with a
largest diameter of the channel along the major axis and a smallest
diameter of the channel along the minor axis and with the major axis
aligned with a longitudinal plane of symmetry of the parabolic trough
mirror, said heating tube coaxially positioned along the focal axis of
said mirror to receive concentrated sunlight therefrom so that a
working fluid in said heating tube is heated thereby and provided for
use through an outlet end of said heating tube.
Concepts 2. The solar thermal power plant of concepts 1, wherein the focal
ratio of
said mirror is about f/0.25.
Concepts 3. The solar thermal power plant of concepts 1, wherein said
mirror
rotating means includes a timer for rotating the parabolic trough mirror to
track the sun
based on a predetermined rotation schedule.
Concepts 4. The solar thermal power plant of concepts 1, wherein the ratio
of the
largest diameter to the smallest diameter is about 2:1.
Concepts 5. The solar thermal power plant of concepts 1, wherein the oblong
cross-
sectional shape of the flow channel is formed by two parabolic surfaces joined
along the
major axis to form two opposing vertices.
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Concepts 6. The solar thermal power plant of concepts 5, wherein each of
the
opposing vertices forms an angle of about 90 degrees.
Concepts 7. The solar thermal power plant of concepts 1, wherein the oblong
cross-
sectional shape of the flow channel has four sides with two opposing vertices
along the
major axis and two opposing vertices along the minor axis.
Concepts 8. The solar thermal power plant of concepts 1, wherein said
heating tube
is a sunlight-absorbing thin-walled tube.
Concepts 9. The solar thermal power plant of concepts 8, wherein said heat
collector further comprises an evacuated optically transparent thin-walled
tube
telescopically surrounding and radially spaced from the heating tube.
Concepts 10. The solar thermal power plant of concepts 1, wherein said heating
tube
comprises an optically transparent thick-walled tube having an inner wall
surface
forming the flow channel and a convex curvilinear outer wall surface for
magnifying
the dimensions of the flow channel, said inner wall surface coated with a
sunlight
absorbing material.
Concepts 11. The solar thermal power plant of concepts 10, wherein the ratio
of the
outer wall surface diameter of the optically transparent thick envelope to the
largest
diameter of the flow channel along the major axis is at least 3:1.
Concepts 12. The solar thermal power plant of concepts 10, wherein said heat
collector further comprises an evacuated optically transparent thin tube
telescopically
surrounding and radially spaced from the optically transparent thick-walled
heating
tube.
Concepts 13. The solar thermal power plant of concepts 1, wherein said heating
tube
is longer than said mirror and positioned to extend beyond each end of said
mirror.
Concepts 14. The solar thermal power plant of concepts 13, wherein said heat
collector is positioned to extend beyond each end of said mirror by up to an
amount
substantially equal to the focal length of said mirror times tan (23.5
degrees).
Concepts 15. The solar thermal power plant of concepts 1, wherein the ratio of
a
largest outer diameter of the heating tube to the width of said mirror is
about 0.45%.
Concepts 16. The solar thermal power plant of concepts 1, further comprising
means
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for mounting said mirror so that the focal axis is parallel with the earth's
rotational axis
and said mirror is rotatable about a longitudinal rotation axis thereof.
Concepts 17. The solar thermal power plant of concepts 16, wherein the mirror
mounting means is capable of mounting said mirror so that the focal axis
thereof is
also the rotation axis.
Concepts 18. The solar thermal power plant of concepts 16, wherein said mirror
mounting means includes means for substantially aligning the focal axis with
the
North Star for use in northern hemisphere locations.
Concepts 19. The solar thermal power plant of concepts 16, wherein said mirror
mounting means includes means for angling the focal axis of said mirror from
horizontal by an angle equal to the latitude of the mounting location.
Concepts 20. The solar thermal power plant of concepts 16, wherein for non-
zero
latitude mounting locations the mirror mounting means is capable of mounting
said
mirror so that the outlet end of the heating tube is elevated higher than the
opposite
end.
Concepts 21. The solar thermal power plant of concepts 1, further comprising a
thermal storage reservoir operably connected to the outlet end of the heating
tube to
storably receive thermal energy transferred to the reservoir by the heated
working
fluid.
Concepts 22. The solar thermal power plant of concepts 21, wherein said
thermal
storage reservoir is also operably connected to an inlet end of the heating
tube to return
the working fluid thereto in a closed-loop thermal storage cycle.
Concepts 23. The solar thermal power plant of concepts 21, wherein said
thermal
storage reservoir is fluidically connected to the outlet end of the heating
tube to
storably receive the heated working fluid therefrom.
Concepts 24. The solar thermal power plant of concepts 23, wherein said
thermal
storage reservoir is also fluidically connected to an inlet end of the heating
tube to
return the working fluid thereto in a closed-loop thermal storage cycle.
Concepts 25. The solar thermal power plant of concepts 21, wherein said
thermal
storage reservoir contains water and rock for use as the thermal energy
storage
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medium.
Concepts 26. The solar thermal power plant of concepts 21, wherein said
thermal
storage reservoir has a heat exchange surface connectable to a water supply
line for
heating water supplied thereby.
Concepts 27. The solar thermal power plant of concepts 1, further comprising a
heat-
powered engine operably connected to receive thermal energy from said heat
collector
for producing power.
Concepts 28. The solar thermal power plant of concepts 27, further comprising
a
thermal storage reservoir operatively connected to both the outlet end of the
heating
tube for storing thermal energy received therefrom, and said heat-powered
engine for
supplying thermal energy thereto.
Concepts 29. The solar thermal power plant of concepts 28, wherein said heat-
powered engine is a steam engine fluidically connected to said thermal storage
reservoir to receive steam stored therein.
Concepts 30. The solar thermal power plant of concepts 29, wherein the steam
engine is fluidically connected to said thermal storage reservoir to also
return water
back into said reservoir in a closed-loop power cycle.
Concepts 31. The solar thermal power plant of concepts 30, wherein said
thermal
storage reservoir is also operably connected to an inlet end of the heating
tube to return
the working fluid thereto in a closed-loop thermal storage cycle which
operates
independently of the closed-loop power cycle.
Concepts 32. The solar thermal power plant of concepts 31, wherein water is
also the
working fluid for the closed-loop thermal storage cycle.
Concepts 33. The solar thermal power plant of concepts 32, wherein said
thermal
storage reservoir is fluidically connected to the outlet end of said heat
collector to
storably receive steam therefrom and to the input end of said heat collector
to return
water thereto, so that the same working fluid is used for both the closed-loop
power
cycle and the closed-loop thermal storage cycle.
Concepts 34. The solar thermal power plant of concepts 27, further comprising
an
electric generator operably connected to said heat-powered engine.
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Concepts 35. The solar thermal power plant of concepts 1, wherein the outlet
end of
the heating tube is fluidically connectable to a water storage tank, and the
heating tube
has an inlet end fluidically connectable to a water source.
Concepts 36. The solar thermal power plant of concepts 35, further comprising
a
water pump for pumping water from the water source into the inlet end and
heated
water from the outlet end into the water storage tank.
Concepts 37. A solar thermal power plant comprising:
a parabolic trough mirror having a longitudinal focal
axis for concentrating sunlight therealong;
means for rotating said mirror about the rotation axis to
follow the sun; and
a tubular heat collector comprising an optically transparent
thick-walled heating tube having an inner wall surface forming a
flow channel and a convex curvilinear outer wall surface for
magnifying the dimensions of the flow channel, said inner wall
surface coated with a sunlight absorbing material, and said heating
tube coaxially positioned along the focal axis to receive
concentrated sunlight from said mirror so that a working fluid in the
flow channel is heated thereby and provided for use through an
outlet end of the heating tube.
Concepts 38. The solar thermal power plant of concepts 37, wherein the ratio
of
the outer wall surface diameter to the inner wall surface diameter of the
heating tube
is at least 3:1.
Concepts 39. The solar thermal power plant of concepts 37, wherein the flow
channel has an oblong cross-sectional shape characterized by major and minor
axes
with a largest diameter of the channel along the major axis and a smallest
diameter
of the channel along the minor axis and with the major axis aligned with a
longitudinal plane of symmetry of the parabolic trough mirror.
Concepts 40. The solar thermal power plant of concepts 39, wherein the oblong
cross-sectional shape of the heat pipe is formed by two parabolic segments
joined
along the major axis to form two opposing vertices.
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Concepts 41. The solar thermal power plant of concepts 40, wherein each of the
opposing vertices forms an angle of about 90 degrees.
Concepts 42 The solar thermal power plant of concepts 39, wherein the ratio of
the
largest diameter of the channel to the smallest diameter of the channel is
about 2:1.
Concepts 43. The solar thermal power plant of concepts 37, wherein said heat
collector further comprises an evacuated optically transparent thin tube
telescopically
surrounding and radially spaced from the heating tube.
Concepts 44. The solar thermal power plant of concepts 37, wherein the ratio
of a
largest outer diameter of the heating tube to the width of said mirror is
about 0.45%.
Concepts 45. A solar thermal power plant comprising:
a parabolic trough mirror having a longitudinal focal axis for
concentrating sunlight therealong;
means for mounting said mirror so that the focal axis is
parallel with the earth's rotational axis and said mirror is rotatable
about a longitudinal rotation axis thereof;
means for rotating said mirror about the rotation axis to follow
the sun; and
an elongated tubular heat collector forming a flow channel and
coaxially positioned along the focal axis to receive concentrated
sunlight from said mirror so that a working fluid in the flow channel is
heated thereby and provided for use through an outlet end of said heat
collector.
Concepts 46. The solar thermal power plant of concepts 45, wherein the mirror
mounting means is capable of mounting said mirror so that the focal axis
thereof is also
the rotation axis.
Concepts 47. The solar thermal power plant of concepts 45, wherein said mirror
mounting means includes means for substantially aligning the focal axis with
the North
Star for use in northern hemisphere locations.
Concepts 48. The solar thermal power plant of concepts 45, wherein said mirror
mounting means includes means for angling the focal axis of said mirror from
horizontal by an angle equal to the latitude of the mounting location.
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Concepts 49. The solar thermal power plant of concepts 45, wherein for non-
zero
latitude mounting locations the mirror mounting means is capable of mounting
said
mirror so that the outlet end of the heat collector is elevated higher than
the opposite
end.
Concepts 50. The solar thermal power plant of concepts 45, wherein the ratio
of a
largest outer diameter of the heating tube to the width of said mirror is
about 0.45%.
Concepts 51. A solar thermal power plant comprising:
a parabolic trough mirror having a longitudinal focal axis for
concentrating sunlight therealong;
means for mounting said mirror so that the focal axis is
parallel with the earth's rotational axis and said mirror is rotatable
about a longitudinal rotation axis thereof;
means for rotating said mirror about a longitudinal rotation
axis to follow the sun; and
a tubular heat collector comprising an optically transparent
thick-walled heating tube having an inner wall surface forming a flow
channel and a convex curvilinear outer wall surface for magnifying the
dimensions of the flow channel, said flow channel having an oblong
cross-sectional shape characterized by major and minor axes with a
largest diameter of the channel along the major axis and a smallest
diameter of the channel along the minor axis and with the major axis
aligned with a longitudinal plane of symmetry of the parabolic trough
mirror, said inner wall surface coated with a sunlight absorbing
material, and said heating tube coaxially positioned along the focal axis
to receive concentrated sunlight from said mirror so that a working fluid
in the flow channel is heated thereby and provided for use through an
outlet end of the heating tube.
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10012A1 Accordingly, in one aspect the present invention resides in a solar
thermal
power plant comprising: a parabolic trough mirror having a longitudinal focal
axis for
concentrating sunlight therealong; means for mounting said mirror so that the
focal axis
is parallel with the earth's rotational axis and said mirror is rotatable
about a longitudinal
rotation axis thereof; means for rotating said mirror about the longitudinal
rotation axis to
follow the sun; and a heat collector comprising an elongated heating tube
surrounding a
flow channel, said flow channel having an oblong cross-sectional shape
characterized by
major and minor axes with a largest diameter of the channel along the major
axis and a
smallest diameter of the channel along the minor axis and with the major axis
aligned
with a longitudinal plane of symmetry of the parabolic trough mirror, said
heating tube
coaxially positioned along the focal axis of said mirror to receive
concentrated sunlight
therefrom so that a working fluid in said heating tube is heated thereby and
provided for
use through an outlet end of said heating tube; wherein said heating tube
comprises an
optically transparent heating tube having an inner wall surface forming the
flow channel
and a convex curvilinear outer wall surface for magnifying the dimensions of
the flow
channel; and wherein said inner wall surface is coated with a sunlight
absorbing material.
[0012131 In a
further aspect, the present invention resides in solar thermal power plant
comprising: a parabolic trough mirror having a longitudinal focal axis for
concentrating
sunlight therealong; means for mounting said mirror so that the focal axis is
parallel with
the earth's rotational axis and said mirror is rotatable about a longitudinal
rotation axis
thereof; means for rotating said mirror about the rotation axis to follow the
sun; and an
elongated tubular heat collector forming a flow channel and coaxially
positioned along
the focal axis to receive concentrated sunlight from said mirror so that a
working fluid in
the flow channel is heated thereby and provided for use through an outlet end
of said heat
collector; wherein said flow channel has an oblong cross-sectional shape
characterized by
major and minor axes with a largest diameter of the channel along the major
axis and a
smallest diameter of the channel along the minor axis and with the major axis
aligned
with a longitudinal plane of symmetry of the parabolic trough mirror; wherein
said heat
collector comprises an optically transparent heating collector having an inner
wall surface
forming the flow channel and a convex curvilinear outer wall surface for
magnifying the
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dimensions of the flow channel; and wherein said inner wall surface is coated
with a
sunlight absorbing material.
100131
Generally, the residential solar thermal power plant of the present invention
is
largely based on the solar thermal power plant used in the solar thermal
aircraft described
herein. As such, the residential solar thermal power plant of the present
invention has
several main components, including a solar concentrating mirror capable of
focusing/concentrating sunlight and rotating about a rotation axis, a heat
collector/heating
tube positioned to absorb the concentrated sunlight, a thermal energy storage
reservoir
connected to an outlet end of the heat collector, and a heat-powered engine
operably
connected to the thermal energy storage reservoir, all of which are similar in
construction
and operation to those previously described for the solar thermal aircraft.
The residential
solar thermal aircraft. The residential solar thermal power plant,
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however, includes additional efficiency-improving features which are
enabled in part by being mountable on a preferably fixed structure,
such as the roof of a building, and which together operate to improve
the overall efficiency of the power plant.
[0014] For example, in one particular embodiment the heating tube of
the heat collector has an oblong cross-sectional profile which increases
the solar concentration factor, i.e. the ratio of the aperture area of the
concentrator mirror to the sunlight absorbing area of the heating tube.
In another embodiment, an optically transparent thick-walled heating
tube is used so that the outer surface of the heating tube operates to
magnify the dimensions of the flow channel formed by an inner
surface, to increase the solar concentration factor further still.
Furthermore, in still another embodiment, the concentrator mirror and
the heat collector are capable of being mounted so that the focal axis of
the mirror and the heat collector are aligned parallel with the earth's
rotational axis. This minimizes the foreshortening effect of solar
incidence for different times of the year to improve solar concentration.
Since increased efficiency, with negligible impact on system capital
cost, directly increases the power generation rate to lower the cost of
the electric power, these efficiency improving features of the residential
solar thermal power plant of the present invention independently as
well as in combination provide energy/power generation at reduced
cost.
[0015] Table 1 lists several efficiency factors which are well known
(based on the experience with commercially running power plants,
such as the SEGS plants in Southern California) to contribute to the
overall efficiency of parabolic trough systems. Additionally, Table 1
shows how these efficiency factors are improved by the present
invention.
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Table 1
Component Parabolic Troughs Present Remarks
from S&L 2004 Invention
Heat Collector 0.862 0.944 Oblong shape and size of heating tube
Efficiency greatly lowers radiation losses
Incidence Angle .873 .959 Polar orientation reduces fore-
shortening
Optical Efficiency 0.704 0.774 Polar orientation essentially
eliminates end
losses. Single unit has no "row to row"
shadowing
Piping Thermal 0.965 1 Direct connection to thermal storage
Losses virtually eliminates piping loss
Thermal to Power 0.934 1 Loss not significant for storage in
very close
Plant Efficiency contact with heat engine
Parasitics 0.883 0.998 Only moving part (outside engine) is
the
trough itself
0.422 0.699 Product of above Six Factors
The numerical values in Table 1 for conventional parabolic troughs are
taken from the Sargent Lundy report for 2004 parabolic trough
technology. The net efficiency advantage of the present invention, i.e.
the product of all the individual efficiency factors, is shown in the last
row in the table.
100161 Since there is little in the current configuration that incurs
additional cost relative to those well known in the SEGS plants, it is
possible to estimate the cost of electricity by scaling the conventional
SEGS cost by the inverse of the relative efficiency factor from Table 1.
Assuming no significant increase in capital costs, the Levelized
Electricity Cost (LEC) is estimated to be cut from 10C/kWh to 6C/kWh.
In the residential application, the economic value of the heating
derived from the cooling water feed to the steam engine can be
estimated based on the quantity of avoided heating fuel. This
economic value is approximately 2C per kWh of heating energy. The
heating energy derived from cooling the engine is approximately
double the power produced by the engine. Reducing the LEC cost by
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the economic benefit derived from water and space heating leads to a
cost for the electric power that is less than 4C/kWh. Since this cost is
much less than the retail price of electric power, approximately
10C/kWh for a typical customer in Northern California, this shows that
residential solar thermal power based on the configuration of the
present invention is indeed economically competitive.
VI. BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The accompanying drawings, which are incorporated into and
form a part of the disclosure, are as follows:
[0018] Figure 1 is a perspective view of an exemplary embodiment of
the solar thermal aircraft of the present invention.
[0019] Figure 2 is a side cross-sectional view of the solar thermal
aircraft taken along the line 2-2 of Figure 1.
[0020] Figure 3 is a cross-sectional view of the solar thermal aircraft
fuselage taken along the line 3-3 of Figure 2.
[0021] Figure 3a is an enlarged cross-sectional view of the heat
collection element and back-reflector enclosed in circle 3a of Figure 3.
[0022] Figure 4 is an enlarged cross-sectional view of the heat
collection element enclosed in the circle 4 of Figure 3a.
[0023] Figure 5 is a perspective view of the heat storage vessel coupled
to a heat engine.
[0024] Figure 6 is a cross-sectional view of the heat storage vessel
taken along the line 6-6 of Figure 5.
[0025] Figure 7 is a cross-sectional view of the heat storage vessel and
the heat engine taken along the line 7-7 of Figure 5.
[0026] Figure 8 is an enlarged cross-sectional view of the crankshaft
pumping structure enclosed in the circle 8 of Figure 7.
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[0027] Figure 9 is an enlarged cross-sectional view of the lithium
hydride containment shell structure.
[0028] Figure 10 is an enlarged cross-sectional view of the multi-layer
insulation structure.
[0029] Figure 11 is a heliostat circuit diagram for sun-tracking mode.
[0030] Figure 12 is a heliostat circuit diagram for sun-searching mode.
[0031] Figure 13 is a heliostat mode switching circuit diagram.
[0032] Figure 14 is a perspective view of a twin engine/twin collector
solar thermal aircraft.
[0033] Figure 15 is a perspective view of single engine/twin pusher
propeller solar thermal aircraft.
[0034] Figure 16 is a cross-sectional view of a Stirling engine.
[0035] Figure 17 is a graph of hydrogen vapor pressure in equilibrium
with LiH-Li mixture.
[0036] Figure 18 is a side cross-sectional view of a ducted fan
embodiment of the solar thermal powered aircraft.
[0037] Figure 19 is a cross sectional view through an alternative heat
pipe embodiment comprising a 6 channel structure.
[0038] Figure 20 is a cross-sectional view of an alternative heat storage
vessel and heat engine including a hermetically sealed reservoir of
working fluid.
[0039] Figure 21 is a perspective view of an exemplary embodiment of
the residential solar thermal power plant of the present invention,
mounted at a northern hemisphere location.
[0040] Figure 22 is an axial cross-sectional view of an exemplary
embodiment of the concentrator mirror and heat collector of the
present invention shown protected by a windshield.
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[0041] Figure 23 is a cross-sectional view of the embodiment shown in
Figure 21 taken along the line 23-23 showing representative sunrays at
the summer solstice.
[0042] Figure 24 is a cross-sectional view similar to Figure 23 of the
embodiment shown in Figure 21 and showing representative sunrays
at the winter solstice.
[0043] Figure 25 is an enlarged cross-sectional view of the exemplary
heat collector enclosed in circle 25 in Figure 22.
[0044] Figure 26 is an enlarged cross-sectional view of a second
exemplary embodiment of the heat collector of the present invention
having a thin-walled heating tube with oblong cross-sectional profile
surrounded by an evacuated optically transparent tubular envelope.
[0045] Figure 27 is an enlarged cross-sectional view of a third
exemplary embodiment of the heat collector of the present invention
which is an optically transparent thick-walled heating tube.
[0046] Figure 28 is an enlarged cross-sectional view of a fourth
exemplary embodiment of the heat collector of the present invention
having an optically transparent thick-walled heating tube similar to
Figure 27 surrounded by an evacuated optically transparent tubular
envelope.
[0047] Figure 29 is a schematic diagram illustrating an exemplary
steam generation embodiment of the present invention.
[0048] Figure 30 is a perspective geometric view of the parabolic
trough mirror of the present invention.
[0049] Figure 31 is an enlarged cross-sectional view of a fifth
exemplary embodiment of the heat collector of the present invention
having four sides and four opposing vertices.
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V. DETAILED DESCRIPTION
A. Solar Thermal Aircraft
[0050] Reference numerals used in the following description for the
solar thermal aircraft are listed in Table 2.
Table 2
MU I (Multi-layer insulation) layers of highly
100 Solar thermal powered aircraft 132
reflective material
102 Wing 133 LiH containment shell structure
Hydrogen& other dissociation products of
103 Fuselage 134
LiH
104 Rudder 135 Spacers between MLI layers
105 Elevator 136 Lithium hydride and lithium
106 Aileron 137 Lithium impervious alloy
107 Transmission 139 Gold layer
108 Cooling air inlet channel 140 Heat engine
109 Propeller 141 Cooling fins
110 Concentrator mirror 142 Hot side heat exchanger
111 Ruddervator 143 Regenerator heat exchanger
112 Transparent fuselage skin 144 Cold side heat exchanger
113 Back-reflector 145 Crankshaft pump
114 Solar concentrator support 146 Filter
115 Solar concentrator drive motor 147 Crank mechanism
116 Heliostat 148 Crankshaft
117 Heliostat Photovoltaic A 149 Crankcase pressure relief valve
118 Heliostat Photovoltaic B 150 Ducted fan
119 Heliostat Photovoltaic C 151 Expansion space
120 Heat collector 152 Displacer piston
121 Antireflection coating 153 Compression space
122 Heat collector envelope 154 Power piston
123 Evacuated space 155 Crankcase space
124 Heat collector coating 156 Displacer piston gap
125 Stainless steel shell 157 Power piston gap
126 Vapor phase sodium 158 Bend region of heat pipe
127 Liquid phase sodium 160 Working fluid pressure vessel
128 Sodium condenser 161 Working fluid reservoir
129 Heat pipe 162 Gas tight journal bearing
130 Thermal battery 163 Hydrogen permeable cap
131 Highly reflective vacuum shell 164 Operational amplifier
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[0051] Turning now to the drawings, Figures 1 and 2 show an
exemplary embodiment of the aircraft of the present invention,
generally indicated at reference character 100. The aircraft 100 is
shown having a conventional fixed- wing airplane body configuration
comprising a fuselage 103, and wings 102 and horizontal and vertical
stabilizing fins extending from the fuselage. As used herein and in the
claims, the term "aircraft body" generally includes the fuselage, the
wings, and the horizontal and vertical stabilizing fins, among other
structural components connected to and extending from the fuselage.
Additionally, attitude control is provided by rudder 104, elevators 105
(or a ruddervator 111 shown in Figure 15) and ailerons 106. And a
propulsion device, such as a propeller 109 in Figure 1, is coupled to an
engine, such as heat engine 140 to propel the aircraft, and thereby
produce lift and sustain free flight of the aircraft. Exemplary
alternative embodiments of the aircraft body are shown in Figures 14,
15 and 17 discussed in greater detail below.
[0052] Figures 1 and 2 also show the solar thermal power plant of the
aircraft 100 generally positioned in the interior of the aircraft body,
namely the fuselage 103. The solar thermal power plant includes a
heat engine 140, heat storage means i.e. a thermal battery 130
including a heat storage container and medium, a solar tracking
concentrator 110, and a heat collection/transport conduit, device, or
other means 120. The heat engine 140 is shown mounted in the
fuselage 103 at a forward end, with the thermal battery 130 (and in
particular the heat storage medium) in thermal contact with a hot side
of the heat engine. Due to its internal location, a cooling air inlet
channel 108 may be provided to direct ambient air backwash from the
propeller 109 to a cold side of the heat engine for cooling. An
alternative exemplary embodiment shown in Figure 18 comprises a
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rear mount of a heat engine 140, with ambient air sucked past cooling
fins 141 by a rearward mounted ducted fan 150. The solar tracking
concentrator 110 is movably mounted for actuation in an optically
transparent section 112 of the aircraft body, shown in Figure 2 as a
section of the fuselage 103. The optically transparent section 112 has a
fuselage skin which is made of an optically transparent, ultraviolet
resistant, lightweight material, such as TEDLAR from DuPont, that
allows most of the incident solar energy to be transmitted therethrough
and to the solar concentrator 110.
Solar Tracking Concentrator
[0053] Figures 2 and 3 show the solar concentrator, i.e. the
concentrator mirror 110, in the preferred form of a parabolic trough-
shaped reflector, which is movably mounted to a support structure 114
connected to the fuselage. In particular, the concentrator mirror is
mounted so as to freely rotate about a rotational axis, which is
preferably a focal axis of the parabolic trough reflector. Furthermore,
the rotational axis may also be located to be coaxial with the central
axis of the fuselage. In any case, the concentrator mirror may be made
of a lightweight, thin plastic film, for example, stretched over a
skeleton array of formers and coated with a thin layer of highly
reflective metal, such as gold or silver. And the solar concentrator
support structure 114 is preferably a space frame that allows most of
the incident solar flux to be transmitted to the concentrator mirror 110.
The entire solar concentrator assembly is balanced, so that no torque is
required to hold a particular orientation.
[00541 Rotational control of the solar concentrator is provided by a
solar tracking device or means including a device or means for
determining whether the solar concentrator is optimally aligned with
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the sun, and a device or means for actuating, e.g. rotating, the solar
concentrator mirror into optimal alignment with the sun based on the
optimal alignment determination. As used herein and in the claims,
"optimal alignment" is that alignment and angle producing the highest
concentration of solar flux, i.e. a position "directly facing" the sun.
The actuation device or means may comprise, for example, a drive
motor 115 (Figure 2) mounted on the rotational axis of the solar
concentrator assembly. And the device or means utilized for
determining optimal alignment may be a heliostat 116 adapted to
determine the alignment of the sun with respect to the focal axis of the
concentrator mirror 110 and operably connected to the drive motor 115
to control the rotational actuation of the solar concentrator. In
particular, the heliostat is adapted to detect a shadow of a heat
collection and transport element (heat pipe) along the focal axis for use
in the optimal alignment determination. The heliostat 116 is shown in
Figure 2 mounted on the concentrator mirror, and in particular, along a
symmetric plane of the reflective parabolic trough. The heliostat 116
includes sensing elements which are preferably solar cells (e.g. 117-
119) and which are preferably symmetrically arranged about the
symmetric plane of the concentrator mirror 110. In a preferred
embodiment, the solar cells include a center cell 118, and two outer
cells 117, and 119 on opposite sides of the center cell.
[0055] A preferred method of heliostat operation uses the one center
and two outer solar cells in a closed loop feedback stabilization system
involving two modes of operation: a sun-searching mode, and a sun-
tracking mode, shown in Figures 11-13. First, in the sun-tracking
mode, the sun is already aligned with the symmetric plane of the
reflective parabolic trough, and deviations from alignment are
detected. When the solar concentrator is properly, i.e. optimally,
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aligned to the sun, both outer cells 117, 119 of the heliostat 116 are
equally illuminated, while the central cell 118 is in the shadow of the
back-reflector 113 of the heat collector 120 (or the shadow of the heat
collector itself if a back-reflector is not used). As the alignment
deviates slightly from the optimal, one of the outer solar cells 117, 119
in the heliostat 116 gets a greater solar exposure, while the opposing
cell exposure decreases. These sensors feed into a control mechanism
(not shown) known in the art, operably connected to the actuating
mechanism, e.g. motor 115, for adjustably rotating the solar
concentrator 110 on the support structure 114 to maintain optimal
alignment of the concentrator mirror to the projected direction to the
sun. An example of such a system is shown in Figure 11. In this
figure, the voltage sent to the DC electric motor 115 is the difference of
the voltages across the photodiodes 117 and 119, and is proportional to
the deviation from the aligned position, and has a nearly linear
restoring torque for a certain range of deviations.
[0056] In the sun-searching mode, photo-diodes associated with the
two outer cells 117 and 119 are connected electrically as shown in
Figure 12. As long as some solar illumination is present, the DC motor
115 produces a driving torque on the solar concentrator structure.
Under the condition that no shadow falls on any of the photo-diodes,
and they are all equally illuminated, the average voltage of the end
photo-diodes (which are driving the motor) is less than the voltage
across the central diode. In this case, the output of operational
amplifier 164 is low, and the polarity switch is in sun-searching mode.
The transition from sun searching mode to sun tracking mode occurs
as the shadow of the axial heat collector back-reflector falls onto center
photo-diode 118. As the central photo-diode becomes sufficiently
shaded, its voltage drops below the average voltage of the outer two
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photo-diodes 117 and 119. A circuit that exploits this drop in the
central diode voltage to change the relative polarity of the diode 117
and 119 connections to the motor 115, is displayed in Figure 13. As the
central diode 118 becomes sufficiently shaded, its voltage drops, and
the output of the operational amplifier goes high, thus triggering the
sun-tracking mode. Friction of the mechanical structure serves to
damp oscillations about the properly aligned orientation.
[0057] It is appreciated that sun-searching mode is required at sunrise
once per day, and also each time the aircraft heading becomes very
close to the projected direction to the sun, and the heliostat is not
sufficiently illuminated to maintain sun-tracking. Additionally, the
solar cell sensors are adapted to provide power to directly drive the
axial rotation actuator, i.e. DC motor 115, and no external power
source is required. In this manner, the mass and complexity required
for the heliostat system are greatly reduced.
Heat Collection and Transport Element (Heat Pipe)
[0058] Once the parabolic trough reflector 110 is aligned to the sun,
solar radiance is focused onto the center of a heat collector 120 shown
best in Figures 2 and 3 as being located along the focal axis of the
parabolic trough reflector 110 (shown also as the central axis of the
fuselage 103). As shown in Figures 3a and 4, the heat collector 120
includes a central heat pipe 129 and a heat collector envelope 122,
which is a transparent vacuum vessel that allows focused sunlight to
transmit to the central heat pipe 129. In a preferred embodiment, the
envelope material is fused silica, by virtue of its high transparency,
high strength, and tolerance to high temperature. The transparent heat
collector envelope 122 is constructed to support a sufficiently high
vacuum in the evacuated space 123 to prevent significant conductive or
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convective heat loss from the central heat pipe 129. The heat collector
envelope 122 may have an antireflection coating 121 that decreases the
transmission loss of sunlight to the central heat pipe, and minimizes
radiative heating of the envelope by the hot central heat pipe. As
shown in Figure 4, both an inner surface and an outer surface of the
heat collector envelope 122 are coated with the antireflection coating
121.
[0059] As shown in Figure 4, the heat pipe 129 preferably has a
triangular micro-heat pipe structure 129 with a single triangular
channel, which configuration is especially suited for small aircraft
applications. For larger aircraft applications, however, heat pipes
having a network of multiple capillary channels in parallel are
preferred. An example of the multiple capillary channel configuration
is shown in Figure 19, illustrating a close packed assembly of six
parallel channels each having a triangular cross-section. The heat pipe
129 contains a heat transfer working fluid that operates to collect solar
energy and transport heat to the heat storage medium and/or heat
engine (see Figures 5 and 6). The heat transfer working fluid is
preferably sodium, in both liquid phase 127, shown as a meniscus
along the three corners of the triangular heat pipe structure, and vapor
phase 126. Alternatively lithium may be utilized as the heat transfer
working fluid. In any case, the radius of curvature of the heat pipe
working fluid meniscus varies across the length of the heat acceptance
region of the heat collector and produces a pressure drop that drives
vapor from the hot end of the heat pipe, located along the focal axis of
the solar concentrator, to a sodium condenser 128 located inside the
thermal battery 130. A corresponding return flow of liquid sodium
drains from the condenser into the hot section. This drain is primarily
driven by capillary forces, but is also supplemented by gravity in a
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bend region 158 of the heat pipe illustrated in Figure 5 and discussed in
greater detail below.
[0060] The shell 125 of the heat pipe shown in Figure 4 is preferably
constructed of high strength, high temperature material, such as
stainless steel, with an outer coating 124 that absorbs sunlight very
efficiently, while at the same time having relatively low thermal
emissivity. According to the reference: "Reducing the Cost of Energy
from Parabolic Trough Solar Power Plants: Preprint", by H. Price and
D. Kearney, available from the National Technical Information Service,
report number NREL/CP-550-33208, published in January 2003, an
envelope solar transmittance of 96%, a coating solar absorptance of
94.1%, and a coating thermal emittance of 9.1% have been shown to be
practical for solar energy collection systems. Assuming these values for
the optical properties of the collection element, the efficiency for
operation of the heat pipe at 1150 K, near the boiling point of sodium
would be approximately 85% for an equilateral triangle cross section
heat pipe 129 having a base width equal to 0.35% of the aperture of the
concentrator mirror 110.
[0061] With the addition of a highly reflective, semi-circular back-
reflector 113, shown in Figure 3a, this efficiency increases to
approximately 90%. The back-reflector is positioned adjacent the heat
collector 120 at a side opposite the parabolic trough and preferably
rotatably mounted to the solar concentrator support structure 114
together with the solar concentrator. In the preferred embodiment, the
back-reflector 113 has a semi-circular cross-section that is concentric to
the heat pipe, and thus much of the thermal radiation from the heat pipe
emitted in the direction away from the concentrator mirror is not lost,
but is instead reflected back and refocused onto the heat pipe. Heat
pipes having diameters significantly greater than 0.35% of the
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concentrator aperture absorb somewhat more power, but have greater
radiating surface area and are thus less efficient. Heat pipes having
diameters significantly less than 0.35% of the concentrator aperture are
significantly smaller than the projected image of the sun on their
surface, and thus have low collection efficiency. The efficiency of 90%
with the back-reflector 113 represents the fraction of the solar energy
incident on the concentrator mirror that is realized as heat to the hot
side of the heat engine and is available for thermal storage. The solar
collection coating 124 extends only over the portion of the heat pipe
that is illuminated by the solar concentrator. For the interval between
the end of the solar absorption region and the thermal battery, the heat
pipe outer surface is high reflectively material, such as gold. This
reduces the thermal emission from the heat pipe in regions where it is
not designed to be collecting solar energy.
[0062] The fabrication methods for the heat collector 120 are well
known to those skilled in the art of electronic vacuum tube fabrication.
Indeed, the overall structure is similar to a long cylindrical "light
bulb", consisting of a transparent envelope with a central high
temperature "filament", i.e. the heat pipe 129. As is well known in the
art, such vacuum vessels can maintain a vacuum of sufficient quality to
maintain thermal insulation between the filament and the glass
envelope for years. A getter, such as titanium, (not shown) may be
deposited on the inside of the heat collector envelope in the section
between the solar concentrator region and the thermal battery in order
to help maintain the requisite vacuum quality, and yet not degrade the
heat collection efficiency.
Thermal Diode Action of Heat Pipe
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[0063] Since the heat transport mechanism in the heat pipe 129 is
predominantly driven by capillary action when sunlit, a gentle bend in
the heat collector 120 may be employed for the convenience of
coupling the heat collector 120 to the thermal battery 130. Moreover, a
bend such as downward sloping bend 158 in Figure 5 between the
thermal battery 130 and the solar concentrator 110, also provides a
"thermal diode" action for the heat pipe. The downward slope in the
bend 158 away from the thermal battery 130 serves as a "drain" for the
heat pipe working fluid during dark periods. Since the heliostat device
acts autonomously to maintain the solar concentrator pointed at the
sun whenever sunlight is available; during sunlit hours the sodium, for
example, in the heat pipe remains active as a heat transfer medium.
During periods of extended darkness, sodium in remote regions of the
heat pipe from the thermal battery 130 will liquefy and then solidify.
Liquid sodium will drain out of the thermal battery 130 by gravity
down the bend 158 in the heat pipe 120. Eventually, almost all of the
sodium will be frozen in regions of the heat pipe below the bend region
158. The remaining thermal connection out of the thermal battery is
the thin stainless steel heat pipe shell, and the thin glass envelope,
neither of which have significant thermal conductivity. In this fashion,
the heat pipe acts as a thermal diode to prevent significant loss of heat
from the thermal battery during periods of extended darkness, as at
night, or during extended periods of heavy cloud cover, while having
very high heat transport efficiency during sunlit periods.
Thermal Battery Container
[0064] As previously mentioned and further shown in Figures 6 and 7,
the thermal battery 130 includes (1) a heat storage container
comprising layers 131, 132, 133 and (2) a heat storage medium, i.e.
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thermal battery core 136 contained in the heat storage container. With
respect to the heat storage container, i.e. thermal battery container, it
includes several layers of thin, highly reflective materia1132, separated
by spacers 135, and a highly reflective outer vacuum vesse1131,
surround a containment shell structure 133. As shown in Figure 9, the
containment shell structure 133 is further comprised of a primary
containment she11137 and a gold layer 139, described in detail below.
The layers of highly reflective material act as radiation shields, and
provide thermal insulation of the hot thermal battery core 136. The
spacers 135 separating the multiple layers of reflective material in the
preferred embodiment are simply pointed dimples in the reflective
material, having very little mass, and providing very little thermal
contact between layers. The vesse1131 is evacuated to prevent
conductive or convective degradation of the thermal insulation. A
certain quantity of getter material, such as titanium, (not shown) may
be deposited on the interior of the vacuum vesse1131 in order to
maintain sufficiently high vacuum quality that the thermal insulation
quality of the multi-layer insulation is preserved. As is well known to
the person of ordinary skill in the art, for such a multi-layer insulation
structure, designed to have negligible conductive and convective
thermal loss, for a reflective material having an emissivity of 0.03 (as is
typical of goal coatings) in a total of 15 layers, and an inner
temperature of 1200 K, the effective thermal emissivity is 0.001, and the
radiative cooling power loss rate is approximately only 120 W/m2.
Thermal Battery Core
[00651 With respect to the heat storage medium, i.e. thermal battery
core 136 contained by the thermal battery container, the utility of LiH
as a thermal energy storage medium was previously discussed in the
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Background, and is due to the very high thermal energy per unit mass
characteristic of LiH. However, in order to address the problem of
lithium-hydride containment for high temperatures, e.g. 700 C and
above, a small admixture of lithium is utilized in order to prevent a
hydrogen explosion. Thus the heat storage medium, i.e. the thermal
battery core 136, consists of a mixture of lithium hydride and lithium
metal, in equilibrium with various dissociation products 134, including
hydrogen gas and liquid phase lithium and lithium hydride. The most
significant contribution to the total vapor pressure is the partial pressure
of hydrogen. The equilibrium hydrogen pressure is a function of both
the temperature and the fraction of Li in a LiH-Li mixture, as is
displayed in Figure 17. Theoretically, pure LiH has an infinite hydrogen
vapor pressure just above the melting point of LiH. It is therefore
necessary either to provide a certain small quantity of Li along with the
LiH in the thermal battery core, or to allow some hydrogen to permeate
out of the container prior to final sealing.
[0066] The fabrication of the LiH and Li mixture may be achieved by
starting with an initially pure quantity of LiH in the thermal battery
fabrication process, and after initial hermetic sealing of the LiH in its
primary containment shell 137, consisting of a LiH-Li impervious alloy,
test the quality of the seal by heating the LiH to just below the melting
point. Some possible alloys that are relatively inert to Li are Mo-Z, Mo-
Re, and Nb-Zr, as described in "High Temperature Liquid Metal Heat
Pipes", by A. Bricard, T. Claret, P. Lecocq and T. Alleau, in the
Proceedings of the 7th International Heat Pipe Conference, (1993). In
addition, very low carbon steel is also inert to Li and LiH. According to
the reference: "Compatibility of potential containment materials with
molten lithium hydride at 800 C", by SJ. Pawel, published in the
Journal of Nuclear Materials
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vol. 207, pp. 136-152, in 1993, "Stabilized (Nb and Ti) low carbon
(<0.06%) steels are observed to be essentially inert in LiH at 800 C
with stable carbides and no grain growth." The initial "seal test" heating
step causes a significant pressure of hydrogen to build up in the LiH
container. If the seal is bad, a relatively high hydrogen pressure will be
observed. In contrast, if the container is well sealed, a much lower
hydrogen pressure will still be seen outside the container, due only to
hydrogen permeation. After a small quantity of hydrogen has been
allowed to permeate out of the container, the LiH may be slowly raised
(in order to avoid an excessive pressure spike) above the melting point,
and sufficient hydrogen removed by permeation to bring the Li metal
fraction remaining in the core 136 up to a desirable value.
[0067] As an example, by getting to a 2% Li metal mixture, the
hydrogen pressure at a working temperature of 1100 K will be just over
one atmosphere, as can be read from the plot in Figure 17. Once the
desired LiH-Li mix has been reached, heating may be ended, and the
LiH container allowed to cool. In order to prevent further significant
hydrogen permeation, the inner Lill containment shell is coated with a
gold layer 139. The outermost layer of gold 139 provides a permeation
barrier to the evolution of hydrogen. A gold layer of approximately
0.001" is estimated to yield a hydrogen containment lifetime of over a
year. Gold has the additional advantage of having low thermal
emissivity (approximately 3%), and thus provides for low thermal
radiative cooling loss through the muti-layer thermal insulation.
[0068] Inner cavities inside the thermal battery 130 provide good
thermal contact to both the sodium condenser 128 at the end of the heat
pipe 129, as illustrated in Figure 6. The external surface of the sodium
condenser 128 is primarily cooled by hydrogen "boiling" as the LiH
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dissociates. Hydrogen bubbles rise to the vapor space, with some
hydrogen-lithium recombination occurring in the liquid phase 136, and
some recombination occurring in the vapor phase 134, until
equilibrium is reached. The sodium condenser is sufficiently large to
assure that the heat flux through the sodium condenser 128 into the
thermal battery is below the critical heat flux marking the onset of so-
called "transition" boiling, and thus maintains a high heat transfer
efficiency.
Heat Engine
[0069] Figure 16 illustrates a Stirling engine of the beta form, well
known to practitioners in the art of heat engines, which serves as a
preferred embodiment of the heat engine 140. Generally, a crank
mechanism 147 converts the reciprocating motion of the Stirling engine
to rotary motion of a propeller by a crankshaft 148, as is well known to
those skilled in the art. The Stirling engine has a hot side and a cold
side, represented by a hot side heat exchanger 142 and a cold side heat
exchanger 144, respectively. The Stirling engine mechanism forces a
working fluid, such as for example air or helium hermetically sealed
therein, to cyclically pass from the expansion space 151 through the hot
side heat exchanger 142, the regenerator 143, the cold side heat
exchanger 144, the compression space 153, and back. The working
fluid goes through a pressure cycle that is phased to deliver net power
over the course of a cycle, through the power piston 154 to the
crankshaft 148. The phase of the variation of the compression space
volume 153 relative to the expansion space volume 151 is
approximately 900. The gap 156 around the displacer piston is
sufficiently large that only an insignificant pressure drop is developed
between the expansion space 151 and the compression space 153. In
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contrast, the gap 157 around the power piston is sufficiently small that
almost no working fluid between the compression space 153 and the
crankcase space 155. Still, over many cycles, sufficient working fluid
does flow through the power piston gap 157 that equilibrium is
reached between the average pressure in the compression space 153
and the average pressure in the crankcase space 155.
[00701 As illustrated in Figure 7, the thermal battery 130 generally and
the heat storage medium in particular, e.g. the LiH/Li mixture, is in
thermal contact with the hot side of the heat engine 140 for supplying
heat thereto from the stored heat transported by the heat collection and
transporting conduit, i.e. heat pipe 120. The hot side heat exchanger
142 is primarily heated by conduction from the hot liquid phase 136
through the thin container wall 133. Waste heat is removed from the
cold side heat exchanger 144 of the heat engine 140 by forced
convective cooling provided by ambient air flowing in through the
inlet channel 108 past a set of cooling fins 141. Since the air
temperature at high altitude is very low, approximately 220 K between
km and 40 km, the cold side of the heat engine can be held relatively
cool, and the resulting Carnot heat engine efficiency may exceed 70%.
Achieving such efficiency is aided by the design of the air cooling
channel 108 shown in Figure 2. The cool air forced past the cooling fins
141 may be driven by the airflow past the aircraft, a forward propeller
109 or a rearward ducted fan 150. As displayed in Figure 7, the full
length of the hot side heat exchanger 142 lies within the thermal
battery core, while the full span of the regenerator 143 extends across
the gap between the thermal battery core and the outer vacuum vessel
wall, and the cold side heat exchanger 144 lies within the range of the
cooling fins 141. This arrangement maximizes the thermal contact to
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both the hot and cold thermal reservoirs, and produces a nearly linear
temperature gradient across the regenerator.
Stirling Engine Power Modulation
100711 The power produced by the Stirling engine tends to increase
with the mean pressure in the expansion space 151 inside the engine.
Thus, venting the engine crankcase, through the crankcase pressure
relief valve 149 shown in Figure 7, to the ambient air, for example,
serves to decrease the output power. Correspondingly, increasing the
crankcase pressure serves to increase the output power.
100721 Pressurization of the crankcase above the ambient atmospheric
pressure is preferably achieved by the action of a crankshaft pump 145
that produces a pumping action as the crankshaft rotates, to self-
pressurize the crankcase. The crankshaft pump 145 comprises at least
one helical groove on either the crankshaft surface or a journal
surrounding the crankshaft. It is appreciated that one or more helical
grooves may be utilized in the same direction for greater pumping
performance. And a filter 146 prevents particulate contamination in
the working fluid from clogging the passageways in the crankshaft
pump 145.
100731 In the preferred embodiment, the crankcase pressurizes to a
value determined by the pressure drop across the crankshaft pump
and the outside atmospheric pressure, for the case that the working
fluid is simply ambient air. This pressure drop is in turn determined
by the design of the grooves, both in terms of the number of grooves,
and the groove shape. The steady state speed of the crankshaft pump
is designed to produce a given mean operating pressure inside the
crankcase of the engine. A pressure drop of one atmosphere across the
crankcase pump, for example, produces an operating pressure that is
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relatively insensitive to the operating altitude of the aircraft. At an
altitude corresponding to 10% of atmospheric pressure, the engine
operating pressure would be approximately 50% that corresponding to
sea level.
[0074] An alternative embodiment is shown in Figure 20 using helium
as the working fluid in the Stirling engine, includes a closed and sealed
reservoir 160 (the working fluid pressure vessel) serving to contain
helium that is vented from the crankcase pressure relief valve 149, and
return the released helium to the crankshaft pump 145 in a closed cycle
through a filter 146. The pressure of the helium in the sealed chamber
is much less than the engine operating pressure, and thus the outer
crankshaft journal bearing 162 may readily act as a gas tight seal to
prevent significant loss of helium to the ambient air. In another
embodiment, the working fluid may be hydrogen, and in addition, a
hydrogen permeable cap 163 (even high temperature steel will be
adequate to this end under many circumstances) may be used on the
hot end of the Stirling engine. In this case, the slow loss of hydrogen
from the thermal battery core 136 may be balanced by a slow gain from
the Stirling engine hydrogen working fluid through the end cap 163,
thereby extending the hydrogen containment lifetime of the thermal
battery to an arbitrary degree.
Alternative Configurations
100751 And Figures 14, 15, and 18 show alternative arrangements of
the solar thermal power plants for aircraft of various configurations.
Figure 14 illustrates the aircraft 100 having two solar power plants, one
on each wing 102 of the aircraft. In particular, Figure 14 shows
multiple wing-mounted solar energy collection and storage systems
directly coupled to a corresponding wing-mounted heat engine. Thus
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each solar power plant of each wing is self-sustainable and
independently operable. Figure 15 shows a fuselage-mounted solar
energy collection and storage system with a multiplicity of wing
mounted propellers driven by a transmission system 107. It is
appreciated that the propellers may be arranged to push the aircraft, as
specifically shown in Figure 15, or alternatively to pull the aircraft (not
shown). And Figure 18 shows a fuselage-mounted solar energy
collection and storage system with a stern mounted ducted fan
propulsion system 150. As shown, the heat engine 140 and cooling fins
141 in particular are cooled via an air inlet 108 that also serves to
supply airflow to the ducted fan propulsion system.
B. Residential Solar Thermal Power Plant
100761 The solar thermal power plant which was previously discussed
for solar powered aircraft can also be incorporated for use in
residential and commercial ground-based applications, hereinafter
referenced collectively as "residential solar-thermal power plants."
When used in such fixed, stationary implementations additional
benefits may be realized such as for example cost efficiencies which can
make such residential solar thermal power plants economically
attractive for domestic consumption. While the following description
focuses primarily on fixed structure applications, it is appreciated
however that the residential solar thermal power plant of the present
invention may also be mounted on other structures which are not
necessarily fixed or ground based, such as for example on boats, trains,
or other mobile but earth-bound platforms, to realize similar benefits of
efficient solar-thermal energy generation.
[0077] Reference numerals used in the following description for the
residential solar thermal power plant are listed in Table 3.
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Table 3
210 Parabolic trough concentrator mirror 231Collector loop water valve
212 Transparent windshield 232 Top of liquid level
213 Mirror plane of symmetry 233 Spent steam line
214 Concentrator mirror support 234Top of boiling region
215 Concentrator mirror mount/rotator 235 Collector loop water pump
220 Heat collector 236 Engine loop water pump
222 Borosilicate thin-walled envelope 237 Automatic check valve
223 Immersion lens thick-walled tube 238 Engine loop steam valve
224 Evacuated space 239 Engine loop water valve
226 Textured steel heating tube 240Heat-powered engine (e.g. steam
engine)
227 Black coating 241Pressure vessel
228 Heat transfer fluid space; flow channel 242 Rock pebbles
229 Representative sunray 244 Condensed water tank
229A Representative sunray A 245 Upper pebbles
229B Representative sunray B 248 Crankshaft
229C Representative sunray C 249Generator
229D Representative sunray D 250 Cold water supply line
229E Representative sunray E 251 Warm water return line
229F Representative sunray F 260 Residential hot water supply
229G Lowest sunray absorption point 261Radiator
229H Highest sunray absorption point 262Cold water supply
230 Thermal energy storage reservoir 270 North star
[0078] Figure 21 in perspective view shows an exemplary embodiment
of the residential solar thermal power plant of the present invention
having several main components, including a solar concentrating
mirror 210 capable of rotating about a rotation axis and focusing
sunlight along a focal axis, a heat collector 220 (similar to heat collector
120) positioned along the focal axis of the mirror to absorb the
focused/concentrated sunlight, a thermal energy storage reservoir 230
connected to an output end of the heat collector, and a heat-powered
engine 240 operably connected to the thermal energy storage reservoir,
all of which are similar in construction and operation to those
previously described for the solar thermal aircraft. In particular, the
preferred shape of solar concentrating mirror 210 for use in the
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residential solar thermal power plant is also that of an elongated
parabolic trough, as illustrated in Figure 30, which has a length L in the
longitudinal direction of its focal axis and a parabolic curve cross-
section with a reflective inner surface that focuses sunlight on the focal
axis. The concentrator mirror has a width W, and a longitudinal plane
of symmetry 213 that passes through both the focal axis of the
parabolic curve halfway along the width W, and the center of the
parabolic curve at the base of the trough, as shown in Figure 30.
[00791 And Figure 22 shows an axial cross-sectional view of the
concentrating mirror 210 and heat collector of the residential solar
thermal power plant having heating tube 226 (representing heat
collector 220 as its primary component) coaxially positioned along the
focal axis of the mirror so that sunlight focused by the mirror is
incident on the heating tube 226 to heat a working fluid (not shown)
inside the tube. In order to rotate the concentrating mirror about its
rotation axis (e.g. focal axis), an actuator device, motor, or other means
215 for rotating the mirror similar to that described for the solar
thermal aircraft is preferably used, with the exception that the actuator
device is preferably a clockwork drive which operates to turn the
mirror based on a predetermine rotation schedule, such as 24 hours per
cycle, so as to follow the sun during the day and maintain focused
sunlight concentrated onto heating tube 226.
[00801 Unlike the solar thermal aircraft, however, these main
components of the residential solar thermal power plant are preferably
mounted on a fixed structure that is sufficiently exposed to the sun,
such as for example a residential rooftop shown in Figure 21. Also
unlike the solar thermal aircraft, the reject heat from the heat-powered
engine is preferably further exploited for its heating value rather than
simply dumped to the environment. As such, the thermal energy
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collected by the residential solar thermal power plant may be used in
various ways for domestic or commercial consumption, such as for use
directly to offset domestic heating requirements, for conversion into
mechanical energy for pumping water via the heat engine, or for
further conversion into electrical energy with an electric generator. For
example, Figure 21 illustrates the residential solar thermal power plant
for use in a combined water heating and power application, where
useful hot water is derived by connecting a cold water utility line to the
heat engine to provide engine cooling. In particular, domestic cold
water supply line 250 is shown connected to heat-powered engine
system 240 of the power plant and then to hot water storage tank 260
via warm water return line 251. Figure 21 also shows the residential
solar thermal power plant connected by crankshaft 248 to an electric
generator 249 for generating electricity.
[0081] As illustrated in Figure 21, typical residential power
consumption needs are such that the concentrating mirror, which is the
single largest component of the current system, need occupy only a
few square meters per person (which is a small fraction of a typical
rooftop area), especially in relatively sunny regions such as for
example the Southwestern United States. In contrast to the SEGS
plants discussed in the Background section, and most other currently
deployed centralized power plants using parabolic trough solar
collectors, there is no "row to row" shadowing produced by the
concentrating mirror of the residential solar thermal power plant
because it is isolated from other mirrors which may be mounted on the
rooftops of other buildings or structures. In a centralized power plant,
the cost of land becomes a factor, and there is a tradeoff between the
acreage required and the degree of self-shadowing. In contrast, in the
residential case presented here, with more than enough roof-top area
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available for the concentrator mirror, there is no need to incur the self-
shadowing penalty. Thus the roof-top area per kW of capacity devoted
to the solar collector is less than a third the corresponding land area
per kW needed in large centralized parabolic trough solar thermal
power plants.
100821 It is appreciated that when mounted as such for residential
applications, the residential solar thermal power plant is often directly
exposed to the elements, e.g. wind, rain, snow, dirt, etc. To protect
them from environmental effects, a windshield assembly is preferably
provided to surround mirror 210 and tube 226. Figure 22 shows a
preferred embodiment of the windshield assembly having a
transparent window 212 and mirror support structure 214. The
windshield prevents wind from unduly cooling the surface of tube 226
which can lower the system heat transport efficiency. Additionally the
protection provided by the windshield allows the structure of collector
mirror 210 to be made of lightweight material. Furthermore in an
exemplary embodiment of the residential application, a portion of the
home space heating requirement in winter can be supplied by
circulating air from the home through the interior of the windshield
volume where it is heated by the heat collector tube.
Polar Alignment of Focal Axis of Collector Mirror
100831 In the exemplary embodiment of the residential solar thermal
power plant shown in Figure 21, the focal axis of mirror 210 is
preferably parallel to the Earth's rotation axis, and is thus substantially
aligned with the North Star 270 for northern hemisphere locations.
The heat collector 220 is also preferably coaxially positioned along the
focal axis of the concentrator mirror so that it too is aligned parallel
with the earth's rotational axis, and substantially aligned with the
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North Star for northern hemisphere locations. First, in order to
properly adjust the orientation of the mirror and the heat collector, a
suitable mounting structure known in the art, shown generically as 215
in Figure 21, is provided to enable one end of the mirror and heat
collector (i.e. the outlet end) to be elevated higher than the other end of
the mirror and heat collector (i.e. the inlet end). For example each end
may be mounted via adjustable mounting brackets. Furthermore, the
mounting structure preferably mounts the mirror and heat collector so
as to rotate about the focal axis, i.e. the focal axis is the rotational axis
of the mirror.
[0084] To achieve proper alignment with the earth's rotational angle,
various methods may be utilized. For northern hemisphere locations,
one example utilizes a small telescope provided with and held parallel
to the mirror/collector assembly to locate the North Star on a clear
night, as shown in Figures 21, 23, and 24. In this manner, during
installation or after possible house settling, slight adjustments of the
alignment of the collector may be performed so that the North Star is
no more than a few minutes of arc off center. Substantial alignment of
the focal axis of the mirror to point to the North Star is most
expeditious in the Northern hemisphere by virtue of the easy visibility
of the North Star, but the corresponding South Celestial Pole alignment
is also possible in the Southern hemisphere as well by observation of
fainter reference stars.
[0085] An alternative method of achieving correct parallel alignment
with the earth's rotational axis uses the latitude coordinate of the
mounting location and a compass to determine the direction of due
north, as shown in Figures 23 and 24. In this case, the mounting
structure would angle the focal axis above a horizontal plane by an
angle equal to the local angle of latitude, and inclined towards one of
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the Poles (for non-zero latitudes). Angular gradations may be
provided on the mounting structure to enable this manner of angular
adjustment. For northern hemisphere locations the focal axis is
inclined towards the North Celestial Pole, and for southern hemisphere
locations the focal axis is inclined towards the South Celestial Pole.
[0086] With the focal axis of the parabolic trough substantially parallel
with the earth's rotational axis, the angle between the normal to the
collector axis and the direction to the sun will not vary by more than
23.5 over the course of a year, i.e. rays from the sun are never more
than 23.5 from normal incidence to the aperture plane of the
concentrating mirror. These extreme deviations occur on the summer
solstice, the longest day, and on the winter solstice, the shortest day.
The paths for a pair of extreme rays from the sun on the summer
solstice are illustrated in Figure 23, and the paths for a pair of extreme
rays from the sun on the winter solstice are illustrated in Figure 24.
The lowest axial position, throughout the course of a year, struck by
concentrated sunlight is represented by point 229G in Figure 23, and is
reached at noon on the summer solstice. Similarly, the highest axial
position, reached at noon on the winter solstice, is point 229H in Figure
24. The active length of collector assembly 220 that is ever exposed to
concentrated sunlight over the course of the year extends only from
point 229G to point 229H. The maximum degree of foreshortening in
the polar aligned case is only attained on the solstices and is only 91.7%
in the extreme.
[0087] The limited length of exposed collector tube and the small
degree of foreshortening in the polar aligned case is in contrast to that
for the horizontal deployment typical of commercial parabolic trough
collectors. The annual average foreshortening factor associated with
this incident angle effect is listed in the first row of Table 1, shown in
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the Summary section. Similarly, the end losses associated with
conventional horizontal collectors for solar angles for which the
focused sunlight converges at positions along the axis beyond the
extent of the collector tube are listed. In the polar orientation of the
present invention this loss is avoided by having a heating tube that is
slightly longer than the trough itself, as illustrated in Figures 23 and 24.
The heating tube is shown positioned to extend beyond both ends of
the mirror by up to an amount substantially equal to the focal length of
the mirror times tan(23.5 degrees), in order to capture all of the
concentrated sunlight, including during the solstices. This incurs very
little extra cost, but improves the collection efficiency. This efficiency
factor is listed in the second row in Table 3.
[0088] By having the axis of the solar collector inclined at an angle
substantially equal to the local latitude, and parallel to the earth's
rotation axis, several benefits are obtained over the case with a
horizontal collector. As previously mentioned, since the angle of the
sun's rays to the axis of the solar collector does not deviate by more
than 23.5 from normal incidence over the course of the year, the
projected mirror area available for solar collection changes by only +/-
4% over the course of the year. This is in contrast to horizontally
deployed parabolic troughs, typical of current commercial solar
thermal energy power plants such as SEGS, for which the mean
incidence angle cosine is significantly less. Accounting for the
variation of this angle of incidence throughout the year, the
conventional horizontally deployed parabolic troughs have a
geometrical efficiency factor of 87.3%, while for the case that the angle
of the trough is aligned with the North Star, this geometrical efficiency
factor increases to 95.9%. The increase in overall solar collection
efficiency with respect to horizontal troughs from this deployment
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angle alone is thus approximately 9%. Another advantage of inclined
orientation: by having the thermal energy storage located at the upper
end of the solar collector, the liquid phase of the two-phase working
fluid in the heat collector may be very effectively returned from the
condenser to the boiler primarily by gravitational action. Such heat
collectors are called thermo-siphons, and are well known in the art and
are commercially available. Another significant advantage of having
the collector axis aligned with the North Star is that rotation of the
parabolic reflector may be driven by relatively simple and inexpensive
clockwork, with only occasional need for adjustment to either run a bit
faster or run a bit slower. The control mechanism needed for such
gradual adjustments can be very simple and inexpensive.
Concentrating Mirror Shape
[00891 As previously mentioned the preferred shape of the
concentrator mirror 210 is that of a parabolic trough which is straight
in the longitudinal direction and which has a parabolic curve cross-
section in the perpendicular plane defining the trough width.
Furthermore, the focal length, f, for the parabolic curve is preferably
equal to 25% of the full width W of the trough. In other words, the
focal ratio, designated by f/# in optics nomenclature, is preferably
about f/0.25. At this ratio, the relative size of the absorber (e.g. the
outer surface of tube 226 in Figures 22 and 25) required to fully capture
all reflected sunrays, assuming a perfect parabolic figure for mirror
210, is minimal and the corresponding solar concentration factor is
maximal compared to any other f/# focal ratio. In particular, for this
shape and f/0.25, solar rays incident at the extreme edge of the trough
are reflected by approximately 900, as shown in Figure 22 for
representative incoming sunray 229. Since the angular diameter of the
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sun, as seen from Earth, is approximately 1/2 degree, the rays reflected
from any given point on mirror 210 diverge by this angle as they
approach the focal axis. The divergence of such ray bundles from three
separate, representative points is shown in Figure 22 with a great deal
of exaggeration in their angular spread. In particular, rays 229A and
229B correspond to sunlight that has reflected from the left hand
extreme of mirror 210, i.e. from incoming sunray 229. Similarly, rays
229C and 229D correspond to light reflected at an intermediate
position on mirror 210, while rays 229E and 229F correspond to light
reflected from near the middle of mirror 210. With the f/0.25 as the
focal ratio, the spread near the focus of parabolic mirror 210 between
rays 229A and 229B is twice as great as the spread between rays 229E
and 229F. This can be seen in the illustration in Figure 25 showing a
close up in the vicinity of the focal axis as shown in Figure 22, but
without exaggeration in the angular spread of the various sunrays.
[00901 The relative efficiency for f/# values differing slightly from the
optimal f/0.25 varies as follows. For f/# values between f/0.2 and
f/0.3, the relative concentration factor decreases by 2% from the
maximum possible at f/0.25, while for f/# values between f/0.16 and
f/0.4 the maximum achievable concentration factor decreases by 10%.
Collector Tube
[00911 The primary component of the heat collector 220 shown in
Figure 21 is the heating tube 226 shown as a cross-section in Figure 22
coaxially positioned along the focal axis of the parabolic trough
concentrating mirror 210. In Figure 22, the heating tube 226 is shown
centered between opposing edges of the parabolic profile of mirror 210
at the focus of the preferably f/0.25 mirror. In general, the heating
tube is positioned at the focus (i.e. focal axis) of the mirror, whatever its
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focal length. The heat collector 220 and the heating tube 226 are similar
to the heat collector 120 and heat pipe 129, respectively, previously
discussed with respect to the solar thermal aircraft. Various
embodiments of the heat collector cross-sectional shape are shown in
Figures 25-28, and 31 that enable highly efficient operation. The
heating tube may be an optically transparent thin-walled tube, such as
shown in Figures 25 and 31, or in the alternative, the heating tube may
be an optically transparent thick-walled tube 223 functioning as an
immersion lens (Figure 27) to magnify an inner surface forming a flow
channel. As shown in Figure 26, the heat collector 220 may optionally
also include additional components, such as a tubular glass envelope
222A providing vacuum insulation around heating tube 226. And as
shown in Figure 28, the thick-walled tube may also additionally have
an optically transparent thin-walled evacuated tube/envelope 222C
providing vacuum insulation around collector tube 226. In any case,
the improved collection efficiency enables the heating tube 226 to be
much shorter, relative to the width of collector mirror 210 than in the
conventional art. For example, in the prior art DISS, Direct Solar
Steam, arrangement, the length to width ratio is approximately 46.
Such an unfavorable aspect ratio would require a great deal of
"folding" to fit onto a typical residential rooftop, and this incurs a
significant degree of extra piping, as well as extra inefficiency. In the
present case, the length to width ratio can be as low as one or two
without undue efficiency loss.
[0092] Preferably, heating tube 226 comprises a hollow type-316
stainless steel tube with a sputter-etched surface. Such surfaces on
type-316 stainless steel are known to be resistant to deterioration, and
are feasible for use in air at temperatures up to 400 C. The preparation
and characteristics of such surfaces are known in the art and described
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in, for example, "Sputter Etched Metal Solar Selective Absorbing
Surfaces for High Temperature Thermal Collectors", by G.L. Harding
and M. R. Lake, published in Solar Energy Materials, vol. 5 (1981), pp.
445-464. Solar absorptances for sputter-etched stainless steel are
observed to be 93%, with a thermal emittance of only 22%. It is further
known that type-316 stainless steel is suitable for use with Sodium,
Potassium or high pressure steam as heat transfer fluids.
Collector Tube Shape
[0093] Figure 25 shows an enlarged view of the circle 25 of Figure 22
and of an exemplary embodiment of tube 226 surrounding a flow
channel having cross-sectional profile that is oblong in shape having a
major axis corresponding to the largest diameter of the channel and a
minor axis corresponding to the smallest diameter of the channel, and
roughly resembling a lemon shape. In the exemplary embodiment of
Figure 25, the oblong profile is preferably produced by two facing
parabolic surfaces joined to form two opposing vertices, with the angle
formed at each of the opposing vertices preferably 90 . In another
exemplary case shown in Figure 31, the oblong cross-sectional profile
is preferably produced by an oblong diamond-like shape having four
sides with two opposing vertices along the major axis and two opposing
vertices along the minor axis. In either case, the oblong profile
preferably has a major to minor axis length ratio of 2 to 1, but with
either straight outer sides, as shown in Figure 31, or curved sides, as
shown in Figures 25 through 28.
[0094] In any case, the major or long axis of this profile is preferably
located within the longitudinal symmetry plane 213 (shown in Figure
25 and in Figure 30) of concentrator mirror 210, and must thus rotate
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along with the mirror to follow the sun. In the interior of tube 226 is a
channel 228 for the passage and transport of a heat transfer fluid, i.e.
working fluid. The length-to-width ratio for the oblong cross-section
of tube 226 (where the length is measured along the major axis, and the
width is measured along the minor axis) is preferably two to one. As
illustrated in Figures 24 and 25, such a profile allows the interception
of all focused sunlight from mirror 210 with a substantially reduced
(compared to a circle) surface area for tube 226, assuming that mirror
210 has a perfect parabolic figure. In fact the surface area
corresponding to such an oblong tube fashioned of two facing
parabolic segments is only 73% that of a circular tube having the same
diameter as the major axis of the oblong tube. Also, the hydraulic
diameter (i.e. four times the central channel flow area divided by the
perimeter of the central channel) is only 58% that of the circular case,
neglecting the wall thickness. This decreased hydraulic diameter is
helpful for heat transfer purposes.
[00951 It is also important to note the angle of incidence of the
concentrated sun rays as they meet the surface of tube 226. Rays 229A
and 229B encounter the surface of tube 226 at an incidence angle of 45 .
In contrast, for a circular collector tube having the same diameter as the
major axis of the oblong shape, the incidence angle for such rays would
be 900. On the other hand, rays 229E and 229F encounter the surface of
tube 226 at an incidence angle of 90 , while for the circular tube case,
the incidence angle would be 45 . Since the marginal rays can
encounter the surface of a minimally sized tube 226 at relatively high
angles of incidence, it is important for the absorptance of the surface to
remain high, even for such grazing angles. According to the reference
by Harding and Lake mentioned in the previous section, the relative
solar absorptance for sputter etched type 316-stainless steel is above
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90% at an incidence angle of 600, and is about 80% at an incidence
angle of 80 . Because the solar absorptance remains high at very high
incidence angles, it is feasible for the major axis of collector tube 226 to
be no larger than approximately 0.45% of the width W shown in Figure
30. It is notable that at the closest approach of the earth to the sun, the
sun's angular diameter, viewed from earth, is such that the major axis
of the collector tube would need to be precisely 0.474% to cover the
image with a perfect f/0.25 parabolic concentrating mirror, while at the
farthest distance from the sun, the collector tube major axis would need
to be 0.458%.
[0096] It is appreciated that with proper suppression of convective
losses, collector assembly heat losses tend to be dominated by thermal
radiation from the hot central tube. In turn, the power loss associated
with thermal radiation is directly proportional to the area of the
radiating surface. By decreasing the area of the radiating surface as
described with the oblong profile, the efficiency of the collector is
improved by the factor listed in the first row in Table 1 in the Summary
section. The magnitude of the thermal power loss does increase with
higher temperature. The numerical value in Table 1 is calculated
assuming a temperature of 400 C, as is currently used in the SEGS
plants.
[0097] Since the radiating area of the present tube is so much reduced
compared to the conventional art, it is feasible to attain higher heat
transfer fluid temperatures than for the conventional parabolic trough
solar collectors. This can enable more efficient heat engines to be
employed. On the other hand, if the conventional heat transfer fluids
are used, such as those in the SEGS plants, and the temperature is
limited to 400 C, the efficiency will improve substantially by virtue of
the decreased thermal radiation losses.
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100981 It is appreciated, however, that while a circular shaped tube is
not quite as efficient as the oblong cross-sectional tube, it is not
necessary to rotate a circular tube with the collector mirror, and a
circular tube can thus be completely stationary with respect to the
ground, and this can offer a compensating simplicity of operation.
Vacuum Envelope
100991 Although windshield 212 substantially reduces wind generated
convective cooling of tube 226, an optional transparent glass envelope
may be provided to further protect and provide thermal insulation to
tube 226. Figure 26 shows an exemplary embodiment of a collector
assembly 220A having a tube-shaped, circular profile, transparent glass
envelope 222A that is preferably radially spaced from and arranged
coaxial to tube 226, with a vacuum 224 maintained within transparent
glass envelope 222A to eliminate convective cooling of tube 226. In this
embodiment, heat collector assembly 220A is considered the
combination of tube 226, glass envelope 222A, and vacuum insulation
224 therebetween. Such vacuum tube construction is well known in
the art for parabolic trough solar collectors. With thin walled glass
envelopes, there is essentially no degradation of the benefit of the
lemon shaped collector itself. There is, however, an approximately 5%
loss of sunlight intensity, assuming the benefit of an anti-reflection
surface coating (not shown), associated with transmission through
glass envelope 222A. The glass vacuum envelope may be employed
especially in applications where natural convection is expected to
produce a greater loss of power than 5%, such as for example with very
high temperature operation as is necessary for the aircraft
embodiment. For residential applications the glass vacuum envelope
may be used, for example, where the collector tube is not used directly
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for heat recovery, such as previously described where a portion of
residential heating is provided by passing air through the windshield
interior. It is appreciated that in portions of the system for which
concentrated sunlight illumination is not present, such as the section
between the collector mirror and the thermal storage shown in Figure
21, while it may be advantageous to have a vacuum containing
envelope surrounding heating tube 226, it is not necessary that it be
transparent.
Immersion Lens Heat Collector
1001001 Figure 27 shows an alternative exemplary heat collector
embodiment 220B having an optically transparent thick-walled heating
tube 223 having a convex curvilinear outer surface and an inner surface
forming a flow channel, with a sunlight absorbing material (e.g. black
coating 227) coating the inner surface. As such, the outer surface
functions as an immersion lens for magnifying the dimensions of the
inner surface and the flow channel. The thickness of the tube wall
preferably has a ratio of an outer surface diameter to the largest inner
surface diameter (e.g. length of the major axis of the oblong cross-
sectional tube 226) preferably being at least three to one. The result of
having such a thick-walled optically transparent heating tube is that, as
viewed from the outside, the central oblong shaped flow channel
appears to be magnified. The degree of magnification depends on the
index of refraction of the glass. For inexpensive borosilicate glass, e.g.
"Pyrex", the magnification factor is 140% to 150%. The significance of
this magnification factor is that the size of the flow channel needed to
absorb all of the sunlight focused onto the axis of parabolic trough
concentrator mirror 210 can be reduced to about 2/3 the size of an
unmagnified tube.
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[00101] An example of the effect of this lens action on the converging
sunlight is illustrated in Figure 27, drawn to the same scale as Figure
26, for rays 229A and 229B. As these incoming rays encounter the
surface of the thick glass, they bend by refraction, and the solar flux
becomes more highly concentrated as it is absorbed at surface 227.
Such immersion lens action is well known, as in the context of oil
immersion microscopy, for example. Since the collector tube appears
optically to be larger, it is possible to achieve a higher concentration of
the incident sunlight than is ordinarily thought to be feasible with
parabolic trough solar collectors.
[00102] Additionally, with such a reduced cross-section of the collector
tube, the axial length of tube 226 relative to the width of collector 210
may be reduced by more than a factor of 25 relative to conventional
parabolic trough geometry, such as that studied in the prior DISS,
Direct Steam Generation, experiments, and still maintain equivalent
heat transfer. This allows the collector to be much more compact than
for conventional parabolic trough collectors, and facilitates the
packaging of such systems on typical residential rooftops.
[00103] Figure 28 shows another exemplary embodiment which
modifies the immersion lens 220B of Figure 27 by providing a radially-
spaced thin-walled glass vacuum envelope 222C to surround the thick
glass envelope with a vacuum region 224 between them to provide
even greater thermal insulation.
Residential Thermal Energy Storage
[00104] The power plant of the present invention preferably also
includes a thermal storage reservoir, such as 230 in Figure 21
operatively connected to the outlet end of the heat collector.
Preferably, the thermal storage reservoir and the heat collector are
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fluidically connected so that the heat transfer is achieved by using the
same working fluid for both the heat collector and the thermal storage
unit. The preferred medium for thermal energy storage in the
residential embodiment is a combination of water and rock, as it is
much less hazardous and much less expensive than the LiH-Li material
needed for the aircraft embodiment. Furthermore, water is also
suitable as the heat transfer medium used in heat collector tube 226,
replacing the more expensive and more hazardous sodium preferred in
the aircraft embodiment. In a third role, water is also suitable as the
working fluid for the heat engine, which thus becomes a familiar steam
engine 240, and provides a less expensive, and more readily
replaceable medium than the hydrogen or helium preferred in the
aircraft embodiment. Finally, in a fourth role, water is also suitable as
a consumable. The use of a single substance, water, for all four of the
roles: heat transfer at the heat collector, thermal energy storage, engine
working fluid, and hot water supply virtually eliminates the heat
exchange inefficiencies associated with transfer of heat from the heat
transport fluid to the thermal energy storage reservoir, from the
thermal energy storage reservoir to the working fluid of the heat
engine, and from the thermal energy storage reservoir or the heat
engine to the consumable hot water supply. As such heat exchange
processes inevitably incur temperature drops, their elimination can
translate either into more efficient operation, or lower maximum
temperature requirements for a given level of efficiency. Use of water
as a thermal medium is sufficiently benign that, with appropriately
clean, oil-free pumps, valves and engine components, the hot water
may be used directly for washing dishes, cleaning clothes, or even
cooking. Financially, the cost of water and rock as the thermal energy
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storage medium is so low that it is essentially only the cost of the
containment that matters.
Thermal Energy Conversion and Power Generation
1001051 Although single gas phase heat engines, such as the Stirling
engine embodiment discussed above in the context of the aircraft
embodiment, can take advantage of the very low ambient air
temperature at high altitude and can thereby achieve very high
thermal efficiency, in the context of the ground-based environment,
without such low temperature capability, the familiar steam engine is
preferred. This is especially so, considering the advantages of water as
the thermal energy medium.
1001061 The use of steam to generate power is very well known and
very well developed technology, and there is such a myriad of
approaches that the optimal configuration will depend strongly on the
nature of the desired energy product. At one extreme, it may be that
all that is required is a supply of high pressure, high temperature
steam for some particular process of interest, and there may be
relatively little requirement for power. At another extreme, it may be
that it is essentially only electric power that is required, and the reject
heat is just a nuisance. In the next section, among these myriad cases,
the specific case appropriate for the average power and energy needs
of a residential consumer is considered. In this example, in line with
the needs of a typical residential energy consumer, comparable
quantities of heating energy and electrical energy are needed over the
course of a year, but with more heating required during the winter,
and more electric power required during the summer. It is therefore
important to have flexibility in the conversion of concentrated solar
energy into heat or electric power.
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[00107] As shown in Figure 21, the thermal storage reservoir is
preferably in contact with the heat-powered engine. As such, there is
also not an extensive piping component between the thermal energy
storage reservoir and the heat engine, as there is in the SEGS plants, for
example. Instead, the thermal energy storage reservoir is in very close
thermal contact with the heat engine, and this loss is virtually
eliminated. Since essentially all of the heat transfer occurs through
extremely effective autonomous, phase-change boosted effects,
involving both boiling in the collector and condensation in the thermal
storage reservoir, there is very little parasitic power loss associated
with actively pumping heat transfer fluids around through extensive
piping interconnections and heat exchangers.
[00108] One of the benefits of thermal energy storage in the residential
case is that momentary interruptions in the solar illumination do not
cause corresponding upsets in the heat supply to the engine. While the
primary role of the thermal energy storage in the solar aircraft
application is to enable overnight flight, in the residential application it
is not always necessary to store an entire day's worth of heat. In some
cases it may be economically advantageous to have only a relatively
short storage duration capability. Another benefit of thermal energy
storage in the residential case is that the normal noon-time peak in the
solar illumination may be distributed over a number of hours in the
afternoon, thus allowing a lower maximum electric generation capacity
design, and thereby a less expensive heat engine and electric generator.
Furthermore, by storing thermal energy, the typical noontime peak in
solar energy supply may be better matched to the typical mid-
afternoon peak in electric energy demand. At another extreme, for
energy self-sufficiency, the thermal energy storage capacity may be
made great enough for weeks to months of storage, so that the
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dependence of solar power on the vagaries of the weather may be
virtually eliminated.
Example System Operation: Direct Steam Generation
[00109] Figure 29 shows a schematic diagram of an exemplary steam
power plant embodiment of the residential solar thermal power plant
of the present invention. As shown, heat collector tube 226 is inclined
from a lower end to an upper end, with the upper end connected to the
top of thermal energy storage reservoir 230 through an automatic
pressure regulating check valve 237, and the lower end of heating tube
226 connected to the bottom of thermal energy reservoir 230 via water
pump 235 and water valve 231 to form a fluidic circuit characterized as
the collector loop. Arrows indicate the normal flow direction of water
through this circuit. Similarly, a second independent fluidic circuit,
characterized as the engine loop, connects in series the top of thermal
energy reservoir 230, steam valve 238, steam engine 240, condensing
radiator 261, condensed water tank 244, water pump 236, water valve
239, and returns back to the bottom of the thermal energy storage
reservoir 230.
100110] Collector loop water valve 231 controls the flow of water from
the thermal energy storage into the bottom of heating tube 226, while
water pump 235 controls the water pressure in the collector loop and
automatic check valve 237 prevents excessive pressure from building
up in the collector loop. Similarly, steam valve 238 controls the flow of
superheated vapor to steam engine 240, while engine loop water pump
236 determines the pressure within thermal storage reservoir 230.
[00111] The transfer of heat to thermal storage reservoir 230 from the
solar collector and the transfer of heat from the thermal storage
reservoir to the steam engine 240 take place in two independent
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process flows. The collector flow operates in proportion to the solar
heating supply, while the engine flow operates in proportion to the
power demand. Regarding the collector flow, during periods when
adequate sunlight is available, so that sufficient steam pressure is
produced in collector tube 226 by the absorption of concentrated
sunlight to force open automatic valve 237, heat from the concentrated
sunlight is transferred to the water in tube 226, and then transferred to
the top of thermal storage reservoir 230. Conversely, at night, or
during periods of obscured sun, valves 237 and 231 are closed. It is
appreciated that throughout day and night, concentrator mirror 210 is
continuously rotated on its axis so that whenever direct sunlight is
available, the alignment of the collector is such that heating of the
water in tube 226 will occur. And regarding the engine loop, during
periods of demand for power, both valves 238 and 239 are opened and
high pressure steam from the top of thermal energy storage reservoir
230 is admitted to steam engine 240, and after expansion, is condensed
in radiator 261 and drains as liquid water into water tank 244. In
winter, when temperatures are low enough to require space heating,
the flow of cooling air past radiator 261 may provide a supplemental
supply of warm air for space heating purposes. In contrast, during
summer, when temperatures are high enough that further space
heating is undesirable, radiator 261 simply rejects heat to the outdoors.
[00112] The heating process, in more detail is this: cold pressurized
water is forced into the lower end of tube 226 by collector loop
circulating pump 235 and heated along the axis of the collector. The
upward tilt in the axis of tube 226 enables very high heating rates of
the steam compared to horizontal tubes as is known in the art. Under
normal operating conditions, as the water is heated by the concentrated
sunlight, it reaches boiling temperature at a point indicated by level
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232. Between the onset of boiling at level 232 and the onset of
superheating at level 234, the steam transitions from very wet to very
dry at substantially constant temperature. Above level 234, the steam
is superheated, and its temperature increases to the design maximum.
Once raised in temperature to the design point, the superheated steam
flows to thermal storage reservoir 230, and/or to steam engine 240.
[00113] In a "cold start" case, corresponding to the lowest quantity of
heat in storage, pressure vessel 241 is almost entirely filled with near
room temperature water, with a relatively small vapor space at the top,
and water tank 244 is almost empty. In this state, the top of the liquid
level 232 is near the top of pressure vessel 241. Very shortly after
concentrated sunlight is focused onto tube 226, superheated steam is
forced into the top of pressure vessel 241, through automatic valve 237.
At the same time, cold water is pumped by pump 235 from the bottom
of pressure vessel 241 through valve 231. As this steam is blown
against rock pebbles 245 at the top of thermal energy storage reservoir
230, the pebbles begin to heat up. A portion of the incoming steam
initially condenses on pebbles 245 and drips down to the water level
232 and begins to heat the water in reservoir 230. Because of the
relatively low conductivity of gaseous steam, there is relatively little
drop in the gaseous steam temperature, and valve 238 may be opened
shortly after sunlight becomes available to provide superheated vapor
to steam engine 240. As superheated steam continues to flow into the
top of reservoir 230, while liquid water continues to be pumped out of
the bottom, the liquid water temperature continues to increase until it
reaches the boiling point. Also as superheated steam continues to flow
past upper pebbles 245, their temperature also soon exceeds the boiling
point of the pressurized water in vessel 241. As the water in the
pressure vessel boils, as steam is provided to engine 240, and as water
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is pumped out the bottom of reservoir 230, liquid level 232 drops, and
a larger fraction of the rock pebbles 242 are exposed above water level
232, and they too begin to increase in temperature above the
pressurized water boiling point. This process may be allowed to
proceed until water level 232 has dropped to the lowest permitted safe
level; at which point thermal energy storage reservoir 230 has reached
its maximum capacity, and essentially all the pebbles 242 above the
saturated water level 234 are at the superheated steam temperature,
and most of the water in the system is contained in liquid water tank
244. At this point, further removal of liquid water by pump 235 from
reservoir 230 must be made up by water pump 236 pumping
condensed water from tank 244 through valve 239 back into the bottom
of reservoir 230.
[00114] After sunset, or during extended periods without available .
concentrated sunlight, valves 237 and 231 are closed and the collector
loop is no longer operative. In this case, as superheated steam is
provided to steam engine 240 through valve 238, makeup water is
pumped into the bottom of reservoir 230 by pump 236 through valve
239. As water level 232 rises in reservoir 230, so does the saturated
vapor level 234, and heat is transferred from the newly immersed hot
rock pebbles 234 to the surrounding water and more steam is
generated. This process may continue until the saturated vapor level
234 in reservoir 230 reaches the level of the steam valve 283. At this
point, it is typically undesirable to continue to operate the steam
engine on the saturated water, but extraction of heat from thermal
energy storage reservoir 230 by the heating of water from cold water
supply 262 and delivery to residential hot water supply 260 is still
desirable, especially in winter for space heating purposes. In the limit
that practically all of the heat stored in reservoir 230 is extracted
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overnight, then the diurnal cycle is complete, and a "cold start"
condition is again obtained. It is convenient with this system that the
natural time of need for heat is at night, which corresponds to the
period of relatively lower mean water temperature in reservoir 230,
while the natural time of need for power is during the day,
corresponding to the period of relatively higher steam temperature and
more efficient electric power generation.
[00115] The approximate division of the incoming solar energy may be
estimated, based on typical steam engine thermal efficiencies, to be 1/4
to 1/3 to power and most of the balance to heating. With such a
system, well over 90% of the incident solar energy may be exploited for
the combination of heating and power. The division between heat and
power with such a system is thus quite well matched to the typical heat
vs. power consumption for a typical residential consumer in the South
Western United States, and especially so in winter.
[00116] After sundown, on cold winter nights when there is a
possibility of water in collector tube 226 freezing, it is advantageous to
allow dry steam from thermal storage reservoir 230 to flow backwards
through the collector tube and flush any liquid water out of tube 226.
[00117] While particular operational sequences, materials,
temperatures, parameters, and particular embodiments have been
described and or illustrated, such are not intended to be limiting.
Modifications and changes may become apparent to those skilled in
the art, and it is intended that the invention be limited only by the
scope of the appended claims.
