Note: Descriptions are shown in the official language in which they were submitted.
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EXTRACTION FROM LARGE THERMAL STORAGE SYSTEMS USING PHASE
CHANGE MATERIALS AND LATENT HEAT EXCHANGERS
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from United States Patent Application
No.
61/581,306 filed on 29 DEC 2011, United States Patent Application No.
61/673,860 filed
on 20 JUL 2012, and United States Patent Application No. 61/673,861 filed on
20 JUL
2012.
FIELD OF THE INVENTION
[0002] The present invention relates generally to energy storage. More
particularly, the present invention relates to thermal heat extraction from,
and charging of,
a large thermal storage tank (LIST) containing thousands of megawatt hours of
thermal
energy, using the phase change of heat collection fluid (HCF) and the phase
change of
molten phase change material (PCM) for thermal storage use in generating
electricity,
steam, or for other industrial processes as implemented in the field of solar
energy
collection, thermal storage and extraction.
BACKGROUND OF THE INVENTION
[0003] Energy
systems in terms of geothermal and solar heating are well known
in the thermal energy arts. In terms of geothermal heating systems, wells in
the Earth
have been used to facilitate transfer of heat from the ground to a usable
energy system.
Likewise, solar heating systems utilize the sun's heat to facilitate transfer
of heat from the
sun's rays to a usable energy system.
[0004] Two recent
examples of geothermal systems are illustrated by US Patent
Application Publication No. 2009/0320475, published 31 DEC 2009 to Parrella
and US
Patent Application Publication No, 2010/0276115, published 04 NOV 2010 to
Parrella.
Each of these systems relate to either wells drilled specifically to produce
heat or wells
that have been drilled for oil and gas exploration that are either depleted,
or have never
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produced oil or gas, usually remain abandoned and/or unused and may eventually
be
filled.
[0005] Numerous
other known geothermal heat/electrical methods and systems
for using the geothermal heat/energy from deep within a well (in order to
produce a
heated fluid (liquid or gas) and generate electricity therefrom) exist. More
specifically,
geothermal heat pump (GHP) systems and enhanced geothermal systems (EGS) are
well
known systems in the prior art for recovering energy from the Earth. In GHP
systems,
geothermal heat from the Earth is used to heat a fluid, such as water, which
is then used
for heating and cooling. The fluid, usually water, is actually heated to a
point where it is
converted into steam in a process called flash steam conversion, which is then
used to
generate electricity. These systems use existing or man-made water reservoirs
to carry
the heat from deep wells to the surface.
[0006] Geothermal
energy is present everywhere beneath the Earth's surface. In
general, the temperature of the Earth increases with increasing depth, from
400
Fahrenheit (F) to 1800 F at the base of the Earth's crust to an estimated
temperature of
6300 F to 8100 F at the center of the Earth. In a conventional geothermal
system, such
as for example and enhanced geothermal system (EGS), water is pumped into a
well
using a pump and piping system. The water then travels over hot rock to a
production
well and the hot, dirty water is transferred to the surface to generate
electricity. In some
situations, a phase change is involved such that the water may actually be
heated to the
point where it is converted into steam. The steam then travels to the surface
up and out
of the well. When it reaches the surface, the steam is used to power a thermal
engine
(electric turbine and generator) which converts the thermal energy from steam
into
electricity whereby the steam cools and is returned to the liquid phase as
water for reuse
deep in the piping system. This type of conventional geothermal system can be
highly
inefficient in very deep wells because of the need for large quantities of
water are very
limited. Furthermore, these water-based systems often fail due to a lack of
permeability
of hot rock within the Earth, as water injected into the well never reaches
the production
well that retrieves the water.
[0007] In either
geothermal or solar heating systems, energy concentrators assist
in maximizing collection of thermal energy. In accomplishing concentration of
thermal
energy, there is often more heat produced than is immediately usable thereby
creating a
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need for storage of heat. In the instance of solar heating systems, the need
for storage of
heat is readily apparent in that sunshine is not a constant. Still further,
thermal energy
concentration and storage may also be found in any industrial process whereby
heat is an
intended or waste byproduct. In all of these thermal systems, water as a heat
transfer
medium cannot offer maximum transfer and storage characteristics. Accordingly,
alternatives exist in terms of phase change material (PCM). Such PCM are
materials that
use phase changes (e.g., solidify, liquefy, evaporate, or condense) to absorb
or release
large amounts of latent heat at relatively constant temperature. Suitable
materials include
paraffin, salt hydrates, or water-based solutions. Regardless of the type of
materials, such
PCM leverage the natural property of latent heat to help maintain a
temperature for
extended periods of time.
[0008] Several
solutions exist for PCM based thermal systems including
"PureTemp PCM" from ENTROPY SOLUTIONS INC. of Plymouth, Minnesota.
"PureTemp PCM" uses small encapsulated PCMs to store heat at constant
temperature
and deal with the low thermal conductivity of the PCMs. However, while
encapsulation
works for small volume PCMs it cannot store large volumes of heat, nor can it
do so
efficiently at a temperature above 300 F.
[0009] Another
approach based on the use of molten salt that does not
incorporate the use of PCMs is the thermal storage systems included in the
"Solar Power
Towers" from SolarReserve of Santa Monica, California that provide power
plants
configured to capture and focus the sun's thermal energy with heliostats. A
tower resides
in the center of a heliostat field. The heliostats focus concentrated sunlight
on a receiver
which sits on top of the tower. Within the receiver, the concentrated sunlight
heats molten
salt to over 1000 F. The heated molten salt then flows into a thermal storage
tank where
it is stored and eventually pumped to a steam generator where the steam drives
a
standard turbine to generate electricity. The molten salt storage loop enables
the plant to
generate electricity regardless of sunshine. Although
such molten salt storage
technology operates at medium temperatures to extract sensible heat, this
solution
requires two tanks, extensive piping and expensive external heat exchangers
and does
not take advantage of the concentrated energy storage a PCM solution offers.
It also has
an upper limit in size where it will become economically unfeasible.
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[0010] The paper "ADVANCED HIGH TEMPERATURE LATENT HEAT
STORAGE SYSTEM - DESIGN AND TEST RESULTS; authored by D. Laing, T. Bauer,
W.-D. Steinmann, D. Lehmann; June 14-17, 2009; from the Institute of Technical
Thermodynamics, German Aerospace Center (DLR), states: "Different options have
been
investigated to overcome the limitation resulting from the low thermal
conductivity of the
storage material. The sandwich concept has been identified as the most
promising option
to realize cost-effective latent heat storage systems. Here, fins enhance the
heat transfer
within the storage material. The heat transfer area is increased by mounting
the fins
vertically to the axis of the tubes. The characteristic height of these fins
exceeds the
dimensions which are commercially available as finned tubes. Decisive for the
successful
implementation of this approach is the selection of the fin material." This
approach is a
batch approach and is not continuous as once the PCM has frozen on the fin
surface the
PCM insulates the fin and has to be melted by a heat source. This approach is
also
inefficient in terms of heat flux extracted.
[0011] In the DLR paper, a fin heat exchanger is described therein as the
state of
the art and, although not implemented in the field, can be used in reverse
mode to extract
heat from the HCF and transfer it to a molten salt which is stored in a large
storage tank.
This heat exchanger however does not incorporate phase change on both the
input and
output side of the heat exchanger. Its input uses sensible heat derived from
the solar HTF
(typically high pressure steam or hot oil) and its output uses phase change to
heat the
molten salt which is then pumped to a large storage tank. That prior art
approach
decreases the efficiency of the heat transfer process as phase change is only
used on
one side of the heat exchanger. Furthermore, common technologies used in the
field of
solar energy collection, conversion and storage are shell and tube heat
exchangers which
collect thermal energy from high pressure steam or high temperature oil and
transfer
sensible heat to a molten salt, no phase change is employed. Improvement over
such
heat exchangers as implemented in the field of solar energy collection,
conversion and
storage is therefore needed.
[0012] It is, therefore, desirable to provide a thermal energy
storage solution
that overcomes the undesirable effects of the prior art.
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SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to obviate or mitigate at
least one
disadvantage of previous thermal energy storage solutions.
[0014] The present invention provides storage and recovery of large amounts
of
thermal energy for many days using phase change, for use in generating
electricity,
steam or for other industrial processes. Moreover, the present invention
provides the
ability to continuously extract medium temperature heat (1000 to 600 Celsius
(C)) from a
molten phase change storage material with low thermal conductivity while
overcoming
PCM tendencies to solidify and thereby insulate the surface which extracts the
heat.
[0015] The present invention does not utilize encapsulation used in small
volume
PCMs, but rather uses an alternative approach to eliminate the problems
associated with
low thermal conductivity storage material freezing on heat transfer surfaces.
[0016] The present invention uses molten PCM, which freezes over a
temperature range that is bounded by its liquidus and solidus states, to
extract large
quantities of medium temperature heat efficiently while maintaining a
temperature within
the freezing range of the PCM storage medium.
[0017] The present invention provides that the temperature of the thermal
storage
medium remains at an almost constant temperature (bounded by its liquidus and
solidus
states) when extracting latent heat energy and only requiring a single tank
and does not
suffer the wide temperature swings that occur when extracting sensible heat
energy from
traditional two tank heat storage mediums which store sensible heat.
[0018] The present invention provides for a reduction of the storage
footprint and
capital costs typically by 75% by utilizing a single tank, eliminating need
for hot/cold
cycling which requires two tanks and more storage material which can be as
much as
four times the storage volume.
[0019] The present invention significantly extends the time period over
which
large amounts of heat energy can be withdrawn from storage as a result of the
reduced
storage size required.
[0020] in a first aspect, there is provided a method of thermal energy
storage and
extraction for large systems using phase change materials with low thermal
conductivity,
the method including: heating a phase change material; transferring the phase
change
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material to a storage tank having a heat exchange drum and concentric scraper
mechanism; and maintaining fairly constant heat transfer at a surface of the
heat
exchange drum via the scraper mechanism.
[0021] In a further aspect, there is provided an apparatus for thermal
energy
storage and extraction within large systems using phase change materials with
low
thermal conductivity, the apparatus including: a heat transfer loop for
heating a phase
change material; a heat storage loop for transferring the phase change
material to a
storage tank having a heat exchange drum and concentric scraper mechanism; and
a
working loop for maintaining constant heat transfer to working fluid via a
surface of the
heat exchange drum by way of the scraper mechanism.
[0022] In a further aspect, there is provided a system for thermal energy
storage
and extraction using phase change materials with low thermal conductivity, the
system
including: a heat source; a heat transfer loop for heating a phase change
material with
heat from the heat source; a heat storage loop for transferring the phase
change material
to a storage tank having a heat exchange drum and concentric scraper
mechanism; and
a working loop for maintaining constant heat transfer to working fluid of a
power plant via
a surface of the heat exchange drum by way of the scraper mechanism.
[0023] In a further aspect, there is provided an method of thermal energy
extraction and storage, the method including: placing a molten phase change
material in
a thermal storage tank; at least partly submerging a first side of a heat
transfer surface
within the molten phase change material; moving heat transfer fluid across a
second side
of the heat transfer surface such that heat from the molten phase change
material
transfers from the molten phase change material to the heat transfer fluid;
facilitating
constant heat transfer from the molten phase change material to the heat
transfer fluid by
using a scraper mechanism for removal of solidified phase change material from
the first
side of the heat transfer surface.
[0024] In a further aspect, there is provided an apparatus for thermal
energy
extraction and storage, the apparatus including: a thermal storage tank for
retaining a
phase change material in a heated state; a heat exchanger at least partly
submerged
within the phase change material, the heat exchanger including a first heat
transfer
surface and a second heat transfer surface, the phase change material in
contact with the
first heat transfer surface; a heat transfer fluid in contact with the second
heat transfer
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surface and arranged such that heat is transferred from the phase change
material to the
heat transfer fluid; and a scraper mechanism for removal of the phase change
material
from the first heat transfer surface formed via solidification of the phase
change material
upon the first heat transfer surface.
[0025] In still a further aspect, there is provided an apparatus for dual
stage
thermal energy extraction and storage, the apparatus including: a large
thermal storage
tank (LTST) for retaining a phase change material in a heated state; a
decoupled thermal
storage extractor (DTSE) for receiving the phase change material from the
LTST; a latent
heat to latent heat extractor (LHTLHE) for receiving heat collection fluid
(HCF) in a
vaporized state and for receiving the phase change material from the LTST and
selectively from the DTSE, the LHTLHE having heat exchanger coils through
which the
phase change material flows and exits to the LTST, the heat exchanger coils
configured
for exposure to the HCF to enable heat transfer between the phase change
material and
the HCF.
[0026] Other aspects and features of the present invention will become
apparent
to those ordinarily skilled in the art upon review of the following
description of specific
embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Embodiments of the present invention will now be described, by way
of
example only, with reference to the attached Figures, wherein:
[0028] FIGURE 1 is an illustration of a first preferred embodiment of the
present
invention as utilized with a solar linear Fresnel reflector using heat pipes.
[0029] FIGURE 2 is an illustration of another embodiment of the present
invention
as utilized with a solar trough or linear Fresnel reflector system.
[0030] FIGURE 3 is an illustration of the storage tank with heat exchanger
in
accordance with the present invention.
[0031] FIGURE 4 is an illustration of the inner and outer drum of the heat
exchanger in accordance with the present invention.
[0032] FIGURE 5 is an exploded view of the heat exchanger as shown in
FIGURE 4 with the inner and outer drum separated for purposes of illustration.
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[0033] FIGURES 6
and 7 illustrate an alternative embodiment similar to that
shown in FIGURE 3, but having a scraping mechanism formed by bars or wires.
[0034] FIGURE 8
illustrates a further alternative embodiment having a scraping
mechanism formed by an auger arranged internal to a tubular heat exchanger.
[0035] FIGURE 9
illustrates yet another alternative embodiment having multiple
scraping mechanisms formed by rotating blades arranged external to plate-like
heat
exchangers.
[0036] FIGURES 10,
11, and 12 illustrates yet still another embodiment having a
decoupled thermal storage extractor with a large storage tank and related
latent heat to
latent heat extractor.
DETAILED DESCRIPTION
[0037] Generally,
the present invention is a thermal storage extractor (TSE) that
provides heat extraction and storage of medium temperature thermal energy
within large
systems using phase change from phase change materials with low thermal
conductivity.
[0038] The TSE is
generally described herein as a thermal storage tank
environment containing a thermally non-conductive molten PCM which resides on
the
outside of a heat transfer surface and a heat transfer fluid (HTF) flowing on
the inside of
the heat transfer surface at a lower temperature than the PCM. This invention
mechanically removes the PCM solids formed as a result of the cooling of the
heat
transfer surface due to the transfer of heat from the PCM to the HTF through
the heat
transfer surface walls. Removal of PCM solids occurs by using various scraping
methods
and configurations which cause the solids not only to be removed but to fall
to the bottom
of a tank where they are either re-heated, collected, or removed from the
tank. After
removal of the solids from the heat transfer surface, the surface is thereby
refreshed
allowing for the further transfer of heat from the molten PCM through the heat
transfer
surface walls to HTF until more solids form on the heat transfer surface to be
removed by
scraping.
[0039] The present
invention uses a phase change material (two phase material ¨
liquid, solid) by continuously removing thermal resistance that comes from the
phase
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change material allowing the system to operate at a much higher rate of
efficiency. The
inventive heat exchanger is advantageously provided inside the storage tank
thereby
reducing heat losses, capital costs and space requirements compared to
existing thermal
storage systems. This enables need for only one storage tank such that three
major
components have been collapsed into a single unit. Moreover, the present
invention
facilitates the temperature of the storage medium remaining at a relatively
constant
temperature when extracting thermal energy and does not suffer the wide
temperature
swings that occur when extracting thermal energy from sensible traditional
heat storage
mediums.
[0040] The present
invention is able to use a wide variety of low cost high latent
heat PCMs to continuously extract latent heat at a fraction of the cost of
existing large
thermal storage systems.
[0041] The present
invention is able to continuously extract latent heat from a
PCM at high heat flux rates. This is accomplished by removing thermal
resistance that
comes from the PCM solidifying upon and insulating the heat transfer surface,
allowing
the system to operate at a much higher rate of efficiency. The temperature of
the
storage medium operates between the liquidus and solidus temperatures when
extracting
thermal energy and does not suffer the wide temperature swings that occur when
extracting thermal energy from sensible traditional heat storage mediums.
[0042] This
invention is a device to efficiently extract and optionally store large
amounts of medium temperature heat energy from a thermal storage material
using latent
heat of fusion phase change. The inventive TSE includes a storage tank
containing
molten PCM with a submerged or partially submerged heat exchanger with
horizontal or
vertical orientation. The heat
exchanger can take many forms and various
implementations are shown and described herein as they perform the same
function that
is, remove solids from the majority of their heat exchange surface using
scrapers. Four
possible mechanical scraper embodiments of the heat exchangers are described
in more
detail hereinbelow. However, it should be readily apparent that removal of the
PCM
solids from the heat transfer surface may also be achieved by using ultrasound
in
conjunction with an insert surfactant. This approach is also considered within
the scope
of the present invention.
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[0043] With reference to FIGURE 1, the present invention 100 is a device
to
efficiently extract and optionally store large amounts of medium temperature
heat energy
from a thermal storage material using latent heat of fusion phase change.
Here, heat flux
Qin is provided from heat concentrated from a collection system. The
collection system
may be of any known type such as, but not limited to, solar arrays, solar
linear Fresnel
reflector arrangements, solar troughs, solar towers or any suitable solar
collection
system. Likewise, the collection system may also be any heat generation source
aside
from solar including, but not limited to, heat derived with or without
concentration
mechanisms from industrial machinery or any heat emitting device or machine.
In this
regard, the heat flux Qin may be provided in any available medium which is
either an open
or closed loop which provides the heated medium to a heat pipe 107 and, after
heat
transfer to the heat pipe 107, returns the cooled medium to the heat
generation source.
[0044] The heat pipe 107 may be of any suitable configuration. One such
suitable configuration is shown by way of a heat pipe disclosed in US Patent
No.
7,115,227 issued on 03 OCT 2006 to Mucciardi et al.
In terms of the Mucciardi et al. example, such a heat pipe would include an
assembly under vacuum or pressure and having a liquid working substance
charged
therein, including generally an evaporator adapted to evaporate the working
substance
and a condenser. The heat exchanging condenser is in fluid flow communication
with the
evaporator. The condenser is adapted to condense evaporated working substance
received from the evaporator and has a reservoir, located at a higher
elevation than the
evaporator, for collecting liquid working fluid therein. A discrete,
impermeable liquid return
passage permits the flow, by gravity, of the liquid working substance from the
reservoir to
the evaporator. The liquid return passage extends through the evaporator and
terminates
near the closed leading end thereof, and is fitted with a vent line that
diverts ascending
vapor to the top of the condenser. A flow modifier is positioned within the
evaporator,
causing swirling working fluid flow in the evaporator, whereby the flow
modifier ensures
that un-vaporized liquid entrained with evaporated working substance is
propelled against
inner surfaces of the evaporator by centrifugal force to ensure liquid
coverage of the inner
surfaces, thereby delaying onset of film boiling. The vapor/liquid loop (i.e.,
VAPOR and
LIQUID RETURN as shown) between the heat pipe 107 and condenser/heat exchanger
106 is a closed loop providing transfer of heat flux Qin to a phase change
material (PCM)
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as shown in FIGURE 1. This effectively segregates heat collection 101 from
heat transfer
to storage 102.
[0045] As further shown in FIGURE 1, heat transferred from the heat flux
Qin to
the PCM provides a closed PCM loop 102a, 102b. The PCM loop 102a, 102b
involves
heated PCM being routed into a storage tank 105 and subsequently routed out as
a PCM
slurry to again be heated in the condenser/heat exchanger 106 discussed above.
One
aspect of the present invention is that the storage tank 105 contains hot
phase change
liquid material with integrated heat exchanger drums 104. The heat exchanger
drums
104 are shown in more detail in FIGURES 3 through 5 and are submerged in the
hot
phase change liquid and the entire surface of the storage tank is covered by
the drums
except for gaps to allow space for scrapers or other material removal devices.
The
purpose of the scrapers is to remove surface build-up of solidified and
solidifying PCM on
the heat exchanger drum outer surface. In doing so, continuous heat transfer
from the
PCM to the working fluid (i.e., taõt) is enabled from the PCM during phase
change. The
working fluid is typically a loop 103a, 103b to a power plant 103 for
electricity or steam
generation for industrial purposes.
[0046] In terms of FIGURE 2, as similar system 200 is shown with identical
heat
collection 201 and heat transfer to storage 202 operation and functional
elements
provided except that a simple heat exchanger 206 with input of hot oil or
pressurized
steam from heat pipes 207 again via a closed loop (i.e., HOT FLUID and FLUID
RETURN) is utilized to heat the phase change material via PCM loop 202a, 202b.
The
PCM storage liquid or slurry to be heated at 202b is provided to the heat
exchanger 206
and returned to the at 202a to the storage tank 105 whereby the heat exchanger
104
extracts via working fluid at 103a the heat flux CLut for use in a power plant
103 before
return of the working fluid at 103b.
[0047] With further regard to FIGURE 3, a storage arrangement 300 is shown
where each drum has an intake 311 and exhaust 303 pipe at its center, which
extends
outside the storage tank 301 on opposite sides. As mentioned, the working
fluid to be
used for electricity generation or other application, is pumped at a
temperature lower than
that of the hot phase change liquid, flowing from 310 to 304 as shown through
the inside
of the drum entering through the intake and exiting from the exhaust side of
the drum for
use in its electricity or steam generation application. The working fluid
extracts heat from
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the hot phase change liquid through the walls of the drum. This hot phase
change liquid
is a molten PCM held at a working level 302 in the tank 301. The working fluid
in the heat
exchange drums is chosen based on its subsequent use. For some applications,
it may
be a liquid that is heated in the drum. An example could be oil. In other
applications, it
may be pressurized liquid (e.g., water) which is boiled in the drums to
produce a high
pressure vapor (e.g., steam). The discharge is then fed to, for example, a
Rankine cycle
plant to generate electricity. If the discharge is a high pressure vapor, it
can be fed
directly to a turbine. If it is a hot liquid (e.g., oil), it can be used to
create a high pressure
vapor for the turbine from a variety of liquid feeds such as organics, water,
etc.
[0048] The PCM solution includes a binary or multi-component system. While
a
single component system may also be used, it is the preferred embodiment to
use a PCM
solution that freezes over a temperature range and as such is characterized by
varying
degrees of solid fractions in the solidifying layer which makes its removal
easier. The
multi component combinations of compounds may consist of, but are combinations
of
compounds of, but not restricted to, a material selected from the group
consisting of
Potassium Nitrate, Potassium Nitrite, Potassium Hydroxide, Potassium
Carbonate,
Potassium Chloride, Sodium Hydroxide, Salt ceramics (NaCo3-BaCo3/Mg0) Sodium
Nitrate, Sodium Nitrite, Sodium Hydroxide, Sodium Carbonate, Sodium Chloride,
Zinc
Chloride, Lithium Nitrate, Lithium Nitrite, Lithium Chloride, Magnesium
Chloride, Nitrate
salts, Nitrite salts, Carbonate salts, Calcium Nitrate, Calcium Nitrite,
Pentaerythritol. The
PCM material may also be formed by Aluminum Silicon alloy.
[0049] Still further, it should be understood that the phase change
material in the
tank is chosen such that it comprises a solution which has a freezing range
(i.e., liquidus
and solidus temperature) at temperatures required by the application
(electricity
generation or other). The solution may be binary or multi-component. It should
therefore
be readily apparent that many different PCM mixtures may be used without
straying from
the intended scope of the present invention. Suitable salt combinations for
phase change
materials can be a mixture based on the desired operating temperature and also
selected
in terms of the ratio of the individual compounds/components to achieve ideal
maximum
and minimum operating temperature points of the slurry. Moreover, any
combination of
compounds may be selected such that their molar composition (i.e., mixture)
will operate
near, but not at, the eutectic temperature point on the corresponding binary
or tertiary
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phase diagram. This ensures that above the desired maximum operating
temperature
the mixture will be a liquid and below the minimum desired operating
temperature the
mixture will be a solid, whereas in between the maximum and minimum operating
temperatures it will be a slurry formed of a mixture of solid and liquid.
[0050] With further reference to FIGURE 3, PCM flow is shown from input 308
of
molten PCM to output 306 of molten PCM with some PCM solids. The transfer of
heat
energy from the hot phase change liquid surrounding the outside wall of the
drum, to the
working fluid inside the drum, causes the hot phase change liquid with low
thermal
conductivity to start solidifying on the outer surface of the drum wall,
thereby insulating
the drum from absorbing more heat. As shown, hot molten PCM rises in the tank
while
PCM solids fall to the bottom 306. Because the phase change storage material
has a
freezing range, the solidification on the outer surface of the drum should be
characterized
by a 'mushy' region of slurry (i.e. mixture of liquid and solid) which is
easily removed. One
aspect of the present invention is to remove the solidified and mushy phase
change
material that insulates the drum, from the outer wall of the drum, at a
constant rate by
using a scraper blade 307 rotating via motor 309 around the outer wall of the
drum,
thereby allowing the heat transfer to the working fluid inside the drum to
continue at a
relatively fast rate. The outer scraper mechanism 312 with blade 307 is
concentric with
the inner heat exchanger drum.
[0051] It is a preferred embodiment of this invention that the frozen layer
on the
outside of a drum be easily removed by having it exist as a mushy material
which is
amenable to removal by a scraping or other suitable mechanical device. It is
also a
preferred embodiment that the drum remains stationary with the scraper
rotating around
it, so as to simplify the construction and operation of the drum units. In one
possible
implementation, the scraper is rotated by an external motor and chain attached
to its pipe
along its center axis and which extends outside the storage tank but around
the intake
and exhaust pipe of the stationary drum. However in other embodiments within
the scope
of the present invention, the drum may rotate around a fixed scraping device.
[0052] Although a particular embodiment of scraper is shown and described,
it
should be readily apparent that more than one scraper device configuration is
possible
and the scraping device may be a straight blade the length of the drum
rotating around
the drum. Moreover, any potential methods for renewing the heat transfer
surface may
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be provided without straying from the intended scope of the present invention.
Such
variations may include mechanical separation methods such as, but not limited
to: 1) flat
plates with wiper arms (windshield wiper like devices); 2) turning around drum
with
stationary scrapers; 3) netting, grating or tightened wire that rotates
closely around the
surface of the drum or is stationary with a moving drum; or 4) using rollers
to crush the
frozen material off of the drum. As well, such variations may include
mechanical,
chemical or other separation methods such as, but not limited to: 1)
ultrasonic bursts that
remove the solid from the heat transfer surface; 2) using jets of cold slurry
to separate the
frozen material off of a drum or plate; 3) using jets of inert liquid to
separate the frozen
material off of a drum or plate; or 4) treating the heat transfer surface with
a non stick
coating.
[0053] In terms of a spiral scraper blade described in more detail herein,
such
configuration may be one preferred method of scraping the drum. The spiral
scraper
blade appears like a cork screw around the outside of the drum with a sharp
blade on its
inner side which scrapes the surface of the drum. The spiral scraper blade may
have a
height above the drum sufficient to displace and move hot phase change liquid
away from
the surface of the drum as it rotates. The spiral scraper blade's inner side
may touch the
surface of the drum provided that this contact does not affect its rotational
ability, or it
may be a distance of up to 1/8" above the surface of the drum.
[0054] As the scraper operates, the scraped phase change material is
ideally in
the mushy state (e.g., a `slurry'), but is denser than the hot phase change
liquid, and falls
to the bottom of the storage tank. There, the phase change material is
reheated and
liquefied by incoming hot phase change liquid heated from a heating source
such as a
concentrated solar collector as illustrated in embodiment #1 (FIGURE 1). At
the bottom
of the tank and on the opposite side from the arriving hot phase change liquid
from the
heating source, phase change liquid (and/ or slurry) is pumped out of the
storage tank to
its heating source such as a concentrated solar collector where its
temperature is raised
to a temperature higher than the storage tank's before returning to the bottom
of the tank
to convert solid phase change material to liquid. The cycle of exhausting
phase change
liquid heating and returning it, is contained in a closed loop.
[00551 The construction of the storage tank is stainless or carbon steel or
other
suitable material capable of storing medium temperature molten PCM. To enable
the
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efficient transfer of heat from the hot phase change material surrounding the
main drum
(the outer drum) to the working fluid inside, a drum of lesser diameter (the
inner drum) is
positioned inside the outer drum so that the layer of working fluid flowing
between them
that is to be heated is thin, thereby promoting faster heat transfer. The
difference in
diameter between the outer and inner drums will vary depending on, the planned
use of
the working fluid and is a function of the required heat extraction rate. As
can be seen by
way of FIGURES 4 and 5, the inner drum contains 408 circular flow guides 401
positioned in the gap between the inner 404 and outer 405 drums. The flow
guides 401
force the working fluid to swirl around the inner drum 404 from point of entry
406 to point
of exit 403 via respective channels 407 and 402, pushing the heavier and
colder working
fluid to the inner surface of the outer drum. Further alternative embodiments
of the
mechanical removal of solidified PCM from the heat exchange surface are shown
and
described later herein with regard to FIGURES 6, 7, 8, and 9.
[0056] With specific reference to FIGURES 6 and 7, a respective side view
600
and end view 700 of an alternative embodiment of the scraper mechanism are
shown
which is similar to that shown in FIGURES 4 and 5. In contrast to FIGURES 4
and 5, the
embodiment of FIGURES 6 and 7 include scrapers 610 in the form of either
aircraft wire
or metal bars. Such aircraft wires may include braided strands of stainless
steel or any
similarly durable material. The use of scrapers in the form of wires which are
inherently
flexible allows for positive contact of the wire scraper with the outer
surface of the drum
which forms the heat exchange surface. Similar to the arrangement shown in
FIGURES
4 and 5, the heat transfer fluid (HTF) passes in flow 613 between the inner
606 and outer
607 drums. Here, the inner 606 and outer 607 drums along with the rotating
scrapers
610 (either wire or bar type) are submerged in the molten PCM 601 within a
tank 603 that
holds all aforementioned elements.
[0057] As such molten PCM flows from an inlet 608 to an outlet 605, the HTF
flows from an inlet (at 611) to an outlet (at 602) through the space 701
between the inner
606 and outer 607 drum. In effect, the inner and outer drum form a tubular
cylinder with
which cylinder's walls is the aforementioned space. As the HTF flows through
this space
701, heat from the molten PCM in which the drums are submerged thereby
transmits into
the HTF for subsequent external use in electricity or steam generation. As
more heat is
transferred, the molten PCM will cool near and collect upon the outer drum
surface.
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However, each pass of the rotating scrapers 610 (either wire or bar type) will
remove
such collection of cooled PCM from the drum surface so as to refresh and renew
the
ability of heat transfer from the outer drum surface through to the HTF.
Accordingly, hot
molten PCM will enter at 608 while molten PCM with PCM solids will exit at
605.
[0058] As shown in
the embodiment of FIGURES 6 and 7, rotation of the rotating
scrapers 610 (either wire or bar type) may be accomplished via an external
motor (not
shown) with a driveshaft 612 having a worm gear drive 609. The worm gear would
then
drive a geared disc to effect rotational movement upon the scrapers which are
mounted
on rotational bearings 604. Such mechanics of worm gears and bearing structure
are
well within the skill of those in the mechanical arts and are not further
described herein.
[0059] During flow
of molten PCM, flow of HTF, and related removal of solidified
PCM by the scrapers, it should also be understood that PCM solids will fall by
normal
gravity to the bottom of the tank. As can be seen in FIGURE 7, the drums are
arranged
in the warmer top half of the tank as heated molten PCM rises. In the course
of molten
PCM flow from the inlet to the outlet, such solids will be also moved out of
the tank.
Subsequent re-heating of the PCM from a mixed slurry state to a fully molten
state will
then be allowed to occur via solar heating in a manner as previously
discussed. The fully
molten PCM will then be returned to the tank for continuous heating of the HTF
and
therefore continuous electrical or steam generation by way of the HTF. It
should be
readily apparent that any heat driven elements such as, but not limited to,
steam turbines,
Stirling engines, heating fins, or the like may use the heat transferred to
the exiting HTF
without straying from the intended scope of the present invention.
[0060] Accordingly,
the first two embodiments of the present invention use drums
as the heat exchangers with their outer surfaces acting as their heat exchange
surfaces,
in one case scraped by an auger and in the other scraped by airplane wires or
metal
bars.
[0061] Within the
third embodiment 800 of the present invention, there is shown
by FIGURE 8 a further heat exchanger within a tank 802 of molten PCM 812
itself flowing
from an inlet 810 to an outlet 805. Here, the HTF flows from an inlet 803 to
an outlet 808
through the space 801 between the inner 806 and outer 807 drum where the inner
and
outer drum form a hollow, tubular cylinder with which cylinder's walls is the
aforementioned space 801. As the HTF flows through this space 801, heat from
the
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molten PCM in which the drums are submerged thereby transmits via the inner
drum
surface into the HTF for subsequent external use in electricity or steam
generation. As
more heat is transferred, the molten PCM will cool near and collect upon the
inner drum
surface.
[0062] However, each pass of a rotating auger type scraper 804 will remove
such
collection of cooled PCM from the drum surface so as to refresh and renew the
ability of
heat transfer from the outer drum surface through to the HTF. As before, any
solidified
PCM will fall to the bottom of the tank where the flow of molten PCM will mix
therewith
and serve to remove the molten/solid slurry of PCM to the outlet for re-
heating. In this
embodiment, a chain or belt 811 driven by an external motor (not shown) may be
used to
rotate the auger 804 which itself is rotatably mounted on an axle or pipe 809
to rotate
within the stationary heat exchanger.
[0063] Yet another embodiment 900 of the present invention is shown in
FIGURE
9. Here, the tubular cylinder structure formed previously by drums of the
prior
embodiments is replaced with flattened and hollowed disc-like plates 908. Each
plate
908 is circular in shape with a central aperture though which a rotating axle
or pipe 901
extends and to which scrapers 902, 906 are attached. In this manner, the HTF
flows (via
inlet 910 and outlet 905) through the hollowed interior of each disc-like
plate 908. The
outer surfaces of these plates act as the heat exchange surfaces. Similar to
before, any
build-up of solidified PCM will be removed by the scrapers 902, 906. In this
specific
embodiment, the scrapers 902, 906 are configured as multiple blades that look
somewhat
like the propellers of the engine of a turboprop airplane. These blades are
placed in such
a manner so as to scrape both the top (via 902) and bottom (via 906) surfaces
of each
plate. As the blades sweep over the plate surfaces, any solidified PCM will be
moved off
the plate surfaces and thereby drop to the bottom of the tank 903 where the
flow of
molten PCM will move the mixed slurry to the outlet 907 for subsequent re-
heating and
return to the molten PCM inlet 909 as in previous embodiments to maintain a
molten
PCM level 904. It should be readily apparent that this configuration using
plates
advantageously increased the surface area available for heat transfer.
[0064] In each of the above-described embodiments of the present invention,
the
heat exchangers are submerged in molten PCM contained in a thermal storage
tank. The
heat exchangers all have an intake and exhaust pipe for passage of the HTF
through
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them. These pipes extend past the walls of the thermal storage tank containing
the
molten PCM and heat exchanger enabling the HTF to be injected and recovered
from
outside the thermal storage tank. The HTF to be used for electricity
generation or other
application is pumped at a temperature lower than that of the molten PCM,
through the
heat exchanger entering through the intake pipe and exiting from the exhaust
pipe. The
HTF extracts heat from the molten PCM through the walls of the heat exchanger.
The
type of HTF used for this task is chosen based on its subsequent use. For some
applications, it may be a liquid that is heated in the heat exchanger. An
example could be
an oil. In other applications, it may be pressurized liquid (e.g., water)
which is boiled in
the heat exchangers to produce a high pressure vapor (e.g., steam). The
exhaust HTF is
then fed to, for example, a Rankine cycle plant to generate electricity. If
the discharge is
a high pressure vapor, it can be fed directly to a turbine. If it is a hot
liquid (e.g., oil), it
can be used to create a high pressure vapor for the turbine from a variety of
liquid feeds
such as organics, water, etc.
[0065] The PCM in
the thermal storage tank is chosen such that it includes a
molten solution typically with high latent energy, which has a freezing range
(i.e., liquidus
and solidus temperature) at temperatures required by the application
(electricity
generation or other). The PCM solution comprises a binary or multi-component
system.
While a single component system may also be used, it is the preferred
embodiment to
use a PCM solution that freezes over a temperature range and as such is
characterized
by varying degrees of solid fractions in the solidifying layer which makes its
removal
easier. The transfer of heat energy from the molten PCM surrounding the
outside wall of
the heat exchanger, to the HTF inside the heat exchanger, causes the molten
PCM with
low thermal conductivity to start solidifying on the outer surface of the heat
exchanger
wall, thereby insulating the heat exchanger from absorbing more heat. Because
the
phase change storage material has a freezing range, the solidification on the
outer
surface of the heat exchanger should be characterized by a 'mushy' slurry
region (i.e., a
mixture of liquid and solid) which is easily removed.
[0066] One of the
goals of this invention is to remove the solidified and mushy
PCM that insulates the heat exchanger, from the outer wall of the heat
exchanger, at a
constant rate by using a scraper blade rotating around or on the outer or
inner wall
(depending on the heat exchanger configuration as previously described above)
of the
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heat exchanger, thereby allowing the heat transfer from the PCM to the HTF
through the
heat exchanger walls to continue at a relatively fast rate. It is a preferred
embodiment of
this invention that the solidified layer on the outside of a heat exchanger be
easily
removed by having it exist as a mushy material which is amenable to removal by
a
scraping device. It is also a preferred embodiment that the heat exchanger
surface
remain stationary with the scraper blade rotating around or on the outer or
inner wall
(depending on the heat exchanger configuration), so as to simplify the
construction and
operation of the heat exchanger units.
[0067] The scraper
is rotated by an external motor and chain ("drive system")
which rotates a drive shaft that either directly rotates the scrapers or does
so through use
of a gear and chain system as shown in the illustrations of all embodiments.
One method
of rotating the scraper is to attach the drive system to a drive shaft along
its center axis
and which extends outside the storage tank but around the intake and exhaust
pipe of the
stationary heat exchanger. However in other embodiments the heat exchanger may
rotate around a fixed scrapping device. As well, more than one scraper device
configuration is possible and the scraping device may be a straight blade the
length of the
heat exchanger rotating around the heat exchanger. A spiral scraper blade may
be the
most practical method of scraping the heat exchanger as shown in the
illustration of the
first and second embodiments. As noted above with regard to the first
embodiment, a
spiral scraper blade looks like a cork screw around the outside of the heat
exchanger with
a sharp blade on its inner side which scrapes the surface of the heat
exchanger. The
spiral scraper blade may have a height above the heat exchanger sufficient to
displace
and move molten PCM away from the surface of the heat exchanger as it rotates.
The
spiral scraper blade's inner side may touch the surface of the heat exchanger
provided
that this contact does not affect its rotational ability.
[0068] In
operation, the scraped PCM is ideally in the mushy slurry state and is
denser than the molten PCM so that it falls to the bottom of the thermal
storage tank.
There, it may be reheated and liquefied by merely incoming molten PCM heated
from a
heating source such as a concentrated solar collector as illustrated in FIGURE
1, but may
also be removed from the thermal storage tank for reheating and liquefied
elsewhere. In
the instance where the denser scraped PCM falls to the bottom of the tank and
is
reheated elsewhere, it falls on the opposite side from the arriving molten
PCM. The
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mixed slurry of PCM is then pumped out of the storage tank to its heating
source such as
a concentrated solar collector where its temperature is raised to a
temperature higher
than the storage tank's before returning to the bottom of the tank. The cycle
of
exhausting PCM slurry, heating and returning it, is contained in a closed
loop.
[0069] As
mentioned, the construction of the storage device is stainless or carbon
steel or other suitable material capable of storing medium temperature molten
PCM. In
the instance of Chlorides and other corrosive PCMs, suitable materials will
have to be
selected for the thermal storage tank and heat exchanger that enable it to
operate at
medium temperatures for many years.
[0070] In
operation, the present invention effectuates the efficient and continuous
storage and discharge of heat at high fluxes at relatively constant
temperatures. The
typical industrial uses include:
= Use for the continuous generation of electricity from solar thermal heat
concentration systems such as solar trough, solar tower, Linear Fresnel
Reflectors
as well as from other heat concentration or collection systems.
= Use for reducing the levelized cost of electricity for solar thermal heat
concentration systems such as solar trough, solar tower, Linear Fresnel
Reflectors
to near or at grid parity rates. This is achieved by significantly increasing
the
capacity of the solar thermal plant to produce electricity around the clock.
= Use of steam for water and plant heating.
= Use in industrial manufacturing processes.
= Use in generating steam for utility use.
= Use in storage and reuse of industrial waste heat for various
applications.
= Use in storage for other thermal power generating technologies (such as
coal or natural gas) enabling their base load power stations to provide peak
load
power.
[0071] In yet a further embodiment of the present invention, there is shown
in
FIGURES 10, 11, and 12 an improvement upon extraction from large thermal
storage
systems using phase change materials and latent heat exchangers utilizing the
above-
referenced and described thermal storage extractors (TSEs).
Specifically, the
improvement includes thermal heat extraction from, and charging of, a large
thermal
storage tank (LTST) containing thousands of megawatt hours of thermal energy,
using
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both the phase change of heat collection fluid (HCF) and the phase change of
molten
phase change material (PCM) for thermal storage use in generating electricity,
steam, or
for other industrial processes as implemented in the field of solar energy
collection,
thermal storage and extraction. This dual use of phase change is termed herein
as
forming a latent heat to latent heat exchanger (LHTLHE) (shown and described
later with
specific regard to FIGURE 11). In terms of the LHTLHE as implemented in the
field of
solar energy collection, conversion and storage, the following paragraphs of
description
shall refer to the process of adding thermal energy at a specific phase change
temperature to a mix of molten and solid PCM contained in a LTST as "charging"
the
LTST and the process of extracting thermal energy from an LTST containing
molten PCM
at a specific temperature is herein referenced to be "extraction."
[0072] As an overview, FIGURE 12 shows an embodiment 1200 of the present
invention in a generalized schematic format with a heat collection segment
1201 along
with transfer and storage elements formed by LTST 1013, DTSE 1001, and via
heat
exchanger 1205 a subsequent useful output Qout for use by power plant 1207.
The
LHTLHE shown as 1100 in FIGURE 12 is likewise shown in more detail in FIGURE
11.
The LTST shown as 1013 and DTSE shown as 1001 in FIGURE 12 are likewise shown
in
more detail in FIGURE 10. It should be readily apparent that variations in
mechanical
implementation of each element are possible without straying from the intended
scope of
the present invention.
[0073] With specific regard to FIGURE 10, an embodiment 1000 is shown.
Here,
an LTST 1013 is shown which includes a decoupled thermal storage extractor
(DTSE)
unit 1001. The DTSE 1001 is substantially identical. to the TSE described
above except
that the DTSE 1001 is constructed so that the volume of the tank containing
the heat
exchanger is not much larger than the volume occupied by the heat exchanger.
Thus,
the heat exchanger 1205 in the DTSE 1001 is large enough to allow the heat
exchanger
to be 100% surrounded by (i.e., submerged within) molten PCM, but not large
enough for
the tank to act as a molten PCM thermal storage device. The heat exchanger,
and thus
the DTSE as a whole, is sized in accordance with the heat extraction
requirement for a
particular application. In other words, more heat requirements in terms of the
HTF output
at 1003 for a given application (e.g., power plant 1207 such as turbine) would
require a
larger heat exchanger and reduced heat requirements a smaller heat exchanger.
HTF
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return from power plant 1207 completes the HTF loop at 1002. The other details
regarding the DTSE will not be further described again as they correlate to
the four
embodiments of the TSE fully described hereinabove. Because the DTSE does not
technically serve to store heat due to these sizing constraints, it
effectively provides a
TSE that is decoupled from the storage aspect. Hence, the name decoupled TSE
or
simply DTSE.
[0074] It should
therefore be readily apparent that certain advantages exist using
the DTSE as shown.
[0075] With
particular to decoupling of the TSE and LTST and the extraction of
thermal energy, by decoupling the TSE from its LTST, the TSE's size,
placement, and
operation becomes independent of the LTST's size, operation, and location. As
well,
decoupling the TSE from its LTST enables multiple TSEs to be used to extract
heat from
its LTST. Moreover, decoupling the TSE from its LTST enables heat to be
extracted from
the LTST at the same time the LTST is being charged. Still further, decoupling
the TSE
from its LTST enables distributed storage configurations of smaller LTSTs.
Even more,
decoupling the TSE from its LTST simplifies access and maintenance of the TSE,
and
there is no concern of heat energy distribution in the LTST as long as the TSE
has
access to molten PCM.
[0076] With
particular regard the LHTLHE as implemented in the field of solar
energy collection, the present inventive LHTLHE provides the most efficient
method of
transferring thermal energy collected by a solar thermal energy collection
system to
thermal storage where it can be extracted. This is accomplished by the
transfer between
two materials in three states (i.e., solid, liquid, and vapor) at each
material's respective
phase change temperature.
[0077] This
decoupled TSE in terms of the presently discussed LTST/LHTLHE
embodiment mechanically removes the PCM solids formed as a result of the
cooling of
the heat transfer surface due to the transfer of heat from the PCM to the HTF
through the
heat transfer surface walls, by using various scraping methods and
configurations which
cause the solids not only to be removed but to fall to the bottom of a tank.
Unlike the
earlier embodiments, here the solids (i.e., mixed slurry of molten PCM and
solids) are
pumped from the tank either directly to the LHTLHE for reheating, or
optionally, to the
LTST for heating at some later time. As can be seen in FIGURE 10 and also by
way of
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FIGURE 12, placement of the DTSE 1001 is close to the top of the LTST 1013
where it
accepts molten PCM from the top of the tank and exhausts molten and solid PCM
(at
1004) back to the LTST (at 1006) or directly to the LHTLHE for immediate
reheating (at
1005) via an actuated valve 1210. Moreover, a chimney pipe 1011 is provided
within the
PCM bath (denoted by liquid line 1012) which allows hot molten PCM to flow
freely from
the input source 1102 to the top of the LTST tank. The decision of whether to
exhaust
molten and solid PCM back to the LTST (at 1006) or directly to the LHTLHE for
reheating
(at 1005) is a function of the specific system design for the particular solar
collection and
electricity or steam generation system and whether the LHTLHE is operational.
[0078] With further regard to FIGURES 10 through 12, the charging process
of a
LTST will now be described. Here, the LTST 1013 can be seen with the DTSE 1001
outputting to the LHTLHE 1100 when charging the LHTLHE 1100 and to the LTST
1013
when not charging the LHTLHE 1100.
[0079] Initially, a heat collection fluid, HCF, (i.e., working fluid) is
initially heated
Qin by a heat source (not shown) such as a solar thermal heat collection
system or any
number of sources. Having been mentioned earlier, such heat sources are not
shown or
described further here. The HCF is heated to a temperature where it vaporizes
in the
LHTLHE 1100, and such temperature is set and controlled by the pressure in the
HCF
pipe 107 by well-known thermodynamic principals via a gas trap valve 1107. The
LHTLHE chamber 1101 operates at the same pressure as the HCF pipe 107. The HCF
vapor is released into the top (at 1108) of the LHTLHE chamber 1101. There,
the HCF
vapor surrounds PCM heat exchanger coils 1106. These coils 1106 are pipes that
effectively cool the HCF vapor as the pipes containing the mix of molten and
solid PCM is
at a lower temperature than the HCF vapor. The input 1109 illustrated in
FIGURE 11 is
formed by the molten/solid PCM mix of loops 1005 and 1008.
[0080] Upon giving up its latent heat, the HCF vapor incurs a phase change
and
becomes a liquid. This liquid then falls to the bottom of the LHTLHE chamber
1101 and
is pumped through the HCF liquid return 1104 back to its heat source to be
vaporized.
Fluid level is maintained at the working fluid liquid line 1103 as shown by
means of
adding (or removing) HCF via the HCF fluid charging port 1105.
[0081] The mix of molten and solid PCM input (at 1109) to the LHTLHE 1100
originates from the DTSE 1001 which has extracted latent energy (via the
liquid to solid
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phase change) from the molten PCM (received at 1203) of the LTST 1013. The
internal
functioning of the DTSE 1001, having been described earlier, will not be
further discussed
here. The DTSE 1001 thereby exhausts a mix of molten and solid PCM (at 1004),
which
is pumped (during this charging scenario) through a pipe into the LHTLHE
chamber. It
should be readily apparent that the mix of molten and solid PCM is at a lower
temperature
than both the HCF vapor and the molten PCM in the LTST. As the mix of molten
and
solid PCM (from loops 1005 and/or 1008 ¨ or collectively shown at 1109) passes
within
the PCM heat exchanger coils 1106 and through the vapor in the LHTLHE chamber
1101,
heat transfers to the PCM mix so as to acquire the latent heat from the HCF
vapor. This
latent heat transfer melts the PCM solid in the molten solid mix. Thus, the
temperature of
the PCM in the coils 1106 rises to one which is slightly higher than the
temperature of the
PCM in the LTST 1013. This molten PCM is then pumped from the LHTLHE chamber
back to the LTST (at 1102) where it is stored until its latent heat is
extracted by the DTSE
and circulated for charging again.
[0082] It should be
understood that during periods of time when the heat source
(not shown) is insufficient to heat the HCF vapor, there may be a requirement
to add heat
to the LTST 1013 to ensure sufficiently molten PCM. In such scenario,
auxiliary heater
tubes 1009 are provided for heating via propane or any other suitable
auxiliary fuel.
Sensing and monitoring of the temperatures in the LTST 1013 may be
accomplished by
selectively placed heat probes or via a heat tracer wire 1007 as shown.
[0083] The above-
described embodiments of the present invention are intended
to be examples only. Alterations, modifications and variations may be effected
to the
particular embodiments by those of skill in the art without departing from the
scope of the
invention, which is defined solely by the claims appended hereto.
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