Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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THERMAL STORAGE F~RNACE SYSTEM
This invention was made under contract with or supported by the Electric
Power Research Institute, Inc.
ackground of the Invention
Field of the Invention:
The invention relates, in general, to a thermal storage furnace system for
convection heating the interior volume of a structure or housing and in
particular to a thermal storage furnace system having a mixing means for
mixing high temperature fluid from a heat source with low temperature fluid
from a low temperakure fluid source, such as the return from the interior
volume of the structure.
Descriptlon of the prlor art:
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The electric utility industry experiences wlde variations in their daily
load cycle. During peak demand periods they generally use premium fuel to
generate power. During minimum demand periods coal and nuclear generating
plants are under utilized. As a result, utilities can offer electric power at
reduced rates to their customers during the minimum demand time intervals in
their daily load cycles.
A significant use of electric power during peak load periods is the direct
conversion of electric energy to thermal energy. Apparatus is available which
converts off-peak electric power to thermal energy, and stores it for use
during peak demand time periods. These devices have generally been too costly
to achieve general utilization, due in large part to the economic burden
imposed by the large heat storage medium. AccordLngly it would be desirable
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to have a thermal storage furnace system having a low cost thermal storage
medium capable of being cycled to high storage temperatures and a means for
delivering the high temperature heat stored therein to the interior volume of
a structure at a lower temperature.
Summary of the Invention:
~ riefly, the present invention is a novel thermal storage furnace system
for convectively heating the interior volume of a structure or housing, by
means of circulating a heated fluid, including a thermal storage means or
chamber, a thermal storage means bypass conduit adapted for fluid
communication with the interior volume of the structure to be heated and in
fluid communication with the thermal storage chamber for routing the return
unheated fluid of the structure or a portion thereof past or through the
thermal storage chamber, and a mixing means in fluid communication with the
thermal storage chamber and the thermal storage chamber bypass conduit for
mixing the unheated fluid from the thermal storage chamber bypass conduit and
the heated fluid ~rom the thermal storage chamber to provide for storage of
heat in the thermal storage chamber at a first predetermined higher
temperature than desired for heating the interior of the structure while
providing a second predetermined temperature fluid for space hea~ing the
interior of the structure by mixing the heated and unheated fluids. The
thermal storage furnace system also includes fluid heating means in fluid
communication with the thermal storage chamber and the mixing means and a
fluid moving means in f~uid communication with the mixing means for moving
fluid within the thermal storage furnace system. The mixing means of the
thermal storage furnace system further includes a fluid directing means for
directing heated fluid through the thermal storage chamber in a first
predetermined direction during the charging phase when heat is being
transferred into the thermal storage chamber and in the opposite direction
during the discharging phase when heat is being transferred out of the thermal
storage chamber theraby achieving optimum counter flow effects within the
thermal storage chamber. The mixing means of the thermal storage furnace
system also includes a directed heated fluid discharge means in fluid
communication with the thermal storage chamber and the thermal storage chamber
bypass conduit for directing heated fluid in a predetermined direction into
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the unheated fluid flowlng in the opposite direction exiting the
thermal storage chamber bypass conduit to augment mixing of the
hea~ed and unheated fluids.
In accordance with a broad aspect of the invention there
is provided a thermal ætorage furnace system for heating the
interio~ volume of a structure or housing by means of circulating
a heated fluid, comprising:
(a) a heating means for heating a fluid;
(b) a thermal storage means disposed in fluid communication
with said heating means for transferring heat from said heated
fluid and storing said transferred heat, and for transferring said
stored heat to an unheated fluid;
~ c) a thermal storage means bypass conduit disposed in fluid
communication with a source of unheated fluid for bypassing
unheated fluid past said thermal storage means; and
(d) a mixing means in fluid communication with said thermal
storage means and said thermal storage means bypass conduit for
mixing said heated fluid fxom said thermal storage means with said
unheated fluid ~rom said thermal storage means bypass conduit to
provide for storage of heat in said thermal storage means at a
first predetermined higher temperature than desired for heating
the interior of said structure while providing a second
prede~ermined temperature fluid for heating the interior of said
structure;
(e) a fluid directing means disposed in said thermal storage
means bypass conduit for directing heated fluid through said
thermal storage means in a first predetermined direction during
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the charging phase when heat is being transferred lnto said
thermal storage means and in the opposite director during the
discharging phase when heat is being transferred out of said
thermal storage means to provide optimum counterflow effects
within said thermal storage means;
(f) said mixing means includes a directed heated fluid
discharge means disposed in fluid communication with said thermal
storage means and said thermal storage means bypass conduit for
directing said heated fluid stored at said first predetermined
high temperature within said thermal storage means in a third
predetermined direction into the fluid flowing in a fourth
di.rection within said thermal storage means bypass conduit to
augment mixing of said heated and unheated fluid, said directed
heated fluid dis~harge means being located to discharge said
heated fluid immediately upstream of a turn within said mixing
means to further augment fluid mixing;
said thermal storage means having first and second ends, said
first end being ln fluid communication with said mixing means and
said heating means and said second end being in fluid
communication with the thermal storage means bypass conduit to
provide for routing said heated fluid from said heating means
through said first end, then said second end of said thermal
storage means and into said thermal storage means bypass conduit
to provide for mixing said heated fluid exiting said second end of
said thermal storage means with said unheated fluid within said
thermal storage means bypass conduit to provide for heating said
thermal storage means to said first predetermined higher
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temperature, while providing a second predetermined temperature
fluid for heating the interior of the structure by combining said
heated fluid exiting said second end o~ said thermal storaye means
with said unheated ~luid ~rom said thermal storage means bypass
conduit and deliveriny said combined heated and unheated fluids to
said mixing means for mixing said heated and unheated fluids.
In accordance with another broad aspect o~ the invention
there is provided a thermal storage furnace system for heating the
interior volume of a structure or housing by means of circulating
a heated fluid, comprising,
(a) a heating means for heating a ~luid;
(b) a thermal storage means having a top portion, a middle
portion and a bottom portion disposed in ~luid communication with
said heating means for transferrlng heat from said heated fluid
and storing said transferred heat, and ~or transferring said
storecl heat to an unheated fluid;
(c) a charge fan ~or mo~ing unheated ~luld through said
heating means and into said thermal storage means; and
(d) a charge level control subclrcuit including 1st and 2nd
outdoor thermal switches disposed in thermal communication with
the external temperature of the structure to be heated for
electrically open circuiting in response to ls~ and 2nd
predetermined outdoor temperatures, respectively, and lst~ 2nd and
3rd storage vessel thermal switches for electrically open
circuiting in response to 1st, 2nd and 3rd predetermined storage
vessel temperatures, respectively, said 1st and 2nd storage vessel
thermal switches being disposed in thermal communication with the
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middle and bottom of the ~hermal storage means respectively and
electrically in series with said 1st and 2nd outdoor thermal
switches respectively, said 3rd storage vessel thermal switch
being disposed in thermal communication with the top of the
thermal storage means and electrically in parallel with each of
the series combinations of said 1st outdoor and storage vessel
thermal switches and said 2nd outdoor and storage vessel thermal
switches, respectively, said charge ~an and said heating means
being disposed electrically in series with and controlled by said
charge level control subcircuit to provide for 1st, 2nd and 3rd
predetermined levels of charge in said thermal storage means in
response to whether the exterior temperature of the structure to
be heated is higher than, in between, or lower than said 1st and
2nd predetermined outdoor temperatures respectively.
Brief De~cription of ~he Drawinqs~
The invention may be understood and further advantages
and uses thereof more readily apparent, when considered in view of
the following detailed description of exemplary embodiments, with
the accompanying drawings, in which:
Figure 1 is a cross-sectional view taken through a
thermal storage furnace system constructed according to the
teachings of the present invention;
Figure 2 iæ an isometric view with parts broken away of
a preferred embodiment of thermal storage furnace system
constructed according to the teachings of the invention;
Figure 3 is a fxont cross~sectional view of the
preferred embodiment of the thermal storage furnace sy~tem of
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Figure 2;
Figure ~ is a side cross-sectional view of the thermal
storage furnace system preferred embodiment of Figure 2;
Figure 5 is a top view with a portion broken away of the
thermal storage furnace system preferred embodiment of Figure 2;
and
Figure 6 is a schematic diagram of a control circuit for
use with the thermal storage furnace system preferred embodlment
of Figure 2.
Descri tion of -the Pr~ferred Embodi_ents~
Thermal storage system 10 includes heating means 12 for
heating a fluid suitable for convective or conductive heating,
such as for lnstance ambient alr, ~hermal storage means 14 for
transferring heat from the heated fluid and storing the
transferred heat therewithin, thermal storage means bypass conduit
16 for bypassing thermal storage means 14 wi~h unheated fluid such
as for example from the cold fluid return of the interior volume
of the structure or housing to be heated, mixinq means 18 for
mixing heated fluid from thermal storage means 14 or heating means
12 with unheated fluid from thermal storage
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means bypass conduit 16, fluid moving means 20 for moving fluid within thermal
furnace storage system 10 and fluid directing means 22 for directing heated
fluid through thermal storage means 14 in a first predetermined direction
during the charging phase and in the opposite direction during the discharginy
phase. Thermal storage means 14 is in fluid communication with heating means
12. Thermal storage means bypass conduit 16 is adapted for fluid
communication with the interior volume of a structure or housing to be
heated. ~iixir~g means 18 is in fluid communication with heating means 12,
thermal storage means 14 and tharmal storage means bypass conduit 16. Fluid
moving means 20 is in fluid communication with mixing means 18 and fluid
directing means 22 is disposed in thermal storage bypass conduit 16.
Heating means 12 is for heating a fluid suitable for convection or
conduction circula tion through the interior volume of a structure or housing
and includes charging branch 32 which consists of lint filter 34 standpipe 36
and thermal isolation damper 38, all in fluid communication with electric
resistance heater 40 having electric resistance elements 42. Thermal storage
means 14 for transferring heat from the heated fluid exiting heating means 12
and storing the heat transferred there:Erom, includes multiple layers of trap
rock 52. Trap rocl; 52, relative to other rock has high density, high specific
heat, does not contain Eree quartz, is chemically inert, and is particularly
strong and durable when cycled to temperatures as high as 1800F. In
addition, trap rock is currently produced in large quantities, thereby having
a low material cost, and is widely distributed thereby having a low shipping
cost. The multiple layers oE trap rock 52 comprise storage chamber or bed 54
of thermal storage means 14. Thermal storage means bypass conduit 16 includes
balancing damper 62, structure or housing interior volume cold fluid return
64, storage unit bypass line 66 and mixer damper 68. Mixing means 18 is for
mixing the heated fluid from thermal storage means 14 or heating means 12 with
the unheated fluid from the thermal storage means bypass conduit 16 to provide
for storage of heat in the thermal storage means 14 at a first predetermined
higher temperature than desired for heating the interior of the structure
while providing a second predetermined temperature fluid for heating the
interior of the structure by mixing the unheated fluid from the thermal
storage means bypass conduit 16 with the heated fluid from the thermal stor2se
means 14, and includes air mixer 72 and directed heated fluid discharge means
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76 disposed in fluid communication with heating means 12, thermal storage
means 14 and thermal storage means bypass conduit 16 for directing the heated
fluid from heating means 12 and the heated fluid stored at the first
predetermined high temperature within thermal storage means 14 in a
predetermined direction opposite to the unheated fluid flowing within thermal
storage means bypass conduit 16 to augment mixing of the heated and unheated
fluids. Directed heated fluid discharge means 76 being further disposed
immediately upstream of turn 78 of mixing means 18 to further augment heated
and unheated mixing. If desired, thermal storage furnace system 10 may be
converted to a totally closed system simply by placing charging branch 32 in
fluid communication with cold fluid return 64 by means of, for exa;nple, a
charging branch ~o cold air return conduit which is shown in Figure 1 in
phantom at 82.
The operation of thermal storage furnace system 10 consists basically of a
charging phase and a discharging phase. The operation of thermal storage
furnace system 10 will be described with reference to a warm air convection
furnace system because the present invention was developed for such a
system, The invention, however, is broadly applicable to thermal storage
furnace systems employing any fluid both gaseous and liquid which is suitable
for conveying heat within the system.
Referring again now to Figure 1, the charging phase occurs during the
utility's low load or off-peak time intervals and is initiated either by an
internal clock or by a telemetric signal initiated by the host utility. This
system has been designed so that the charging phase may be either continuous
or intermittent i.e., the utility may elect to provide charge energy over, for
example, two distinct 4 hour periods. The fan or fluid moving means is turned
on which draws ambient air into the system and discharges it to the furnace
plenum. The chamber or bed 54 of thermal storage means 14 has been designed
to minimize pressure loss while assuring good flow distribution. The buoyancy
vector opposes the flow momentum vector which it stabilizes for the heating
mode (with upward flow, the buoyancy vector would be destabilizing). Upward
flow of the discharge air is also stabilizing. This is important for low
pressure loss beds where the buoyancy force is a significant (relatively)
fluid body force. The fan or fluid moving means runs continuously during the
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charging cycle and draws air into the system from branches 32 and 64. The air
dxawn into the charging branch 32 passes through lint filter 34, up standpip-
36, past thermal isolation damper 38, and into electric resistance heater
40. Electric resistance elements 4~, heat the air. The power to the heatin~
elements is controlled to maintain a constant air temperature at variable air
flow rates. The other option is to modulate flow which is a technically more
difficult and costly alternative. This feature assures high utilization of-
the media storage capacity with simple, low cost flow control components.
The air is heated as it passes through the resistance heater 40 and flows
downward into and through chamber bed 54 of thermal storage means 14. ~s the
air passes through the chamber bed 54 its thermal energy is transferred to the
trap rock, storage media 52, and is itself, cooled. When the charging air is
leaving the thermal energy storage means 14 rises, the house fan is turned
on. The unheated air in thermal storage means bypass conduit 16 (from cold
fluid return air stream 64 mixes with the air leaving the bed). An acceptabla
flow split between the two air streams is passively maintained by an
adjustable flow orifice flow 62. The combined air flows through thermal
storage means bypass conduit 16, through mixer damper 68, mixing means 18, and
into fan 20. ~ thermostat within the living space determines whether thermal
energy for space heating is required.
If a space heating demand occurs during the charging time interval, mixer
damper 68 opens to pass part oE the hot air leaving the electric resistance
heater so that the desired air temperature and flow for space heating is
achieved at the exit of mixing means 18. In this mode mixing means 18 blends
the hot air stream and the colder air stream from the thermal storage means
bypass conduit 16 to provide a uniform temperature air stream for space
heating at the desired temperature level. The charging phase is terminated by
an internal clock, by a telemetric signal, or if the mixed air temperature in
the bypass line exceeds a predetermined temperature limit.
During the discharging phase the charging air branch 32 is isolated by a
flow restriction,such as for example by closure of a thermal isolation damper
38. The fan is started upon a demand from the house thermostat. All of the
air is cold return air. The air stream then splits into two streams. One
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stream flows through tne bypass line to the air mixing plenum 68. The other
stream enters thermal storage means 14 and flows upward through the bed 54 (in
the opposite direction to that of the charging flow in order to achieve
optimum counterflow effects). This air is convectively heated by the transfer
of thermal energy stored in the rock to the air. The heated air leaves
thermal storage means 14 and passes through the hot leg of air mixer damper
68. Mixer damper 68 regulates the flow split between the two air streams in
order to achieve the desired temperature in the combined air stream downstream
o~ the air mixing plenum a mixing means 18.
The flow paths, damper arrangement, and operation during the charging
phase were selected to best utilize the element storage means 14 capacity. As
the charging phase progresses the temperature of the air leaving the bottom of
the storage chamber 54 at opening 80 slowly rises. Ultimately the temperature
will exceed the allowable temperature for the house air distribution system
and the charging phase must be stopped. By using fan capacity in order to
draw in a second air stream of unheated air at cold fluid return 64, time of
cessation of the charging phase is significantly delayed. Therefore, more
energy can be stored and cost is reduced.
Referring now to Flgure 2, there is shown an isometric view with parts
broken away of a preferred embodiment of the thermal storage furnace system
constructed according to the teachings of the invention. Thermal storage
furnace system 100 utilizes a two-fan arrangement, charge fan 102 for charging
the thermal storage means and house fan 104 for moving the heated fluid
throughout the thermal storage Purnace system and structure or housing to be
lleated. Thermal storage system 100 again has two primary modes or phases,
charging and discharging, each day of the heating season. The operation of
thermal storage furnace system 10 will be described relative to a residential
central air heating configuration as before. Warm air is generated within the
storage furnace on demand by the time residence thermostat in either of these
phases of the daily cycle. Circulation of cold return air from the residence
to the furnace and, distribution of warmed air from the furnace to the
residence is accomplished by the activation of the house fan 104 in a manner
similar to conventional central air furnaces.
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The charging phase occurs during the utility's low load or off-peak time
intervals and is initiated either by an internal clock or by a telemetric
signal by the host utility. This heating system has designed so that the
charging phase may be continuous or intermittent. Furthermore, numerous
charging and discharging modes interspersed throughout the day can be
accommodated by the system.
Referring now to Figures 3, 4 and 5, at the beginning of a charge phase
ambient air is drawn into the furnace through an inlet filter 106 by the
charging fan 102. The air leaves the fan and flows through an optional, wide
open, resistance heater shutoff damper 108, through an interconnecting pipe
110 and through a distribution manifold 112 to an electric resistance heater
114. The resistance heater accepts off-peak electric power, converts it to
thermal energy, and delivers the thermal energy to the flowing charge air
stream. The hot air leaves the electric resistance heater and enters the
upper plenum 116 of the thermal storage vessel 118.
The storage vessel encloses the thermal energy storage bed 120 which
consists of low cost storage media 122 such as crushed basalt rock or natural
basalt pebbles. the hot air flows downward through the bed, heats the storage
media, and is, itself cooled. The rock bed ls such a good receiver of heat
that the air temperature leaving the storacJe vessel is at or near ambient
temperature except near the end of the charge phase of the most severe heating
days. In order to maximize the amount of stored energy for these peak heating
days of the heating season, the air temperature at the bottom of the storage
is allowed to rise above the normal warm air delivery temperature to the
residence. At this condition the house fan is turned on so as to mix cold
return air with the air leaving the storage bed. Approximately 20~ mDre
thermal energy can be stored within the bed without increase in heating system
size and cost, and without adverse effect on residents with regard to
temperature or sensation of drafts. It may be noted that the downward flow of
hot air through the bed with respect to gravity at low total bed pressure loss
permits the buoyancy force to stabilize and balance the air flow (and,
therefore, the temperature) distribution within the storage bed. The air is
discharged from the bottom plenum of the storage vessel 124 into the air
mixing plenum 126. This exiting air stream is allowed to dissipate through
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the house ducting when its temperature is low. At higher discharge
temperature the house fan i9 on and the tempered air mixture is forced through
the warm distribution ducts (at an acceptable temperature level).
If the residence thermostat initiates a demand for heat at any time during
the charging phase, the house fan is turned on and the hot air temperature
control damper 128 opens to allow part of the hot charge air flow leaving the
resistance heater to bypass the bed through the hot air duct 130. This hot
air damper modulates in order to provide the desired air temperature to the
residence through the warm air ducting. The furnace has been designed ~o
provide a continous supply of warm du-ring the charging phase while fully
charging the storage bed.
During the normal discharge phase, the charge circuit flow elements
tcharge fan, isolation damper, and resistance heater) are deactivated. On
demand for residence heat, the house fan is turned on and the hot air damper
opens. A fixed adjustable air flow orifice 132 is provided to cause cold
return air from the residence to split into two parallel streams. The larger
of two streams passes through the orifice 132 into a mixing plenum. The
smaller of the two air streams enter the stora~e vessel through the lower
plenum from whence it flows upward through the bed. The air is heated by its
passage through the storage media, and the storage media is, itself, cooled.
The heated air leaves there storage vessel via the upper plenum and flows
downward through the hot air duct to join and mix with the other cold air
stream. The hot air damper modulates the air flow rate through the bed in
order to provide the desired temperature of the warm air to be distributed
through the residence heating system.
Utilization of this two-fan arrangement and the adjustable air flow
orifice simplifies, and minimizes the cost of the air flow control components
and controller logic. Only one modulating damper 12~ is required for furnace
function, while achieving the desiredl high efficiency counterflow effects
within the storage bed. The optional resistance heater isolation damper can
be replaced by a flow restriction which serves to limit flow through that path
during the discharge phase. A moderate amount of flow through does not
diminish thermal performance of the furnace and does favorably mitigate
thermal exposure of the hot air damper.
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Referring now to Figure 6, a schematic diagram of the furnace control
system constructed according to the teachings of the invention is shown.
Attention should be given to the novel charge level control subcircuit 140.
With this simple circuit of thermal switches, incremental levels of charge can
be achieved in response to outdoor conditions throughout the course of the
heating season. As shown, the circuit is comprised of two switches 142 and
144 respectively, which may be located on the exterior and -three switches 146,
148 and 150 respectively, which may be positioned on the storage vessel or
thermal storage means along its axis. If the outdoor temperature is moderate
(for example above 15 F), a higher setpoint switch 142 triggers. In this
state the charging air circuit is disabled as soon as the top-most temperature
switch 146 on the storags vessel triggers at a predetermined temperature
setpoint, thus charging stops at a reduced amount of stored energy
(approximately 45~). The bottom of the storage bed remains at near residence
interior temperature an no appreciable heat is conveyed to the residence as a
result of the charge air flow leaving the storage vessel. If the outdoor
temperature is intermediate (for example, above -5 F), the second, lower
setpoint switch 144, on the exterior of the house triggers. In this state,
the charging air clrcuit is disabled as soon as the middle position, axially,
swltch 148 on the storage vessel triggers at a predetermined temperature
setpoint. Thus charging stops at approximately 80~ of the amount of the
maximum design heat level. q~he bottom of the storage bed and the charge
exiting the storage vessel never rises above the normal warm air delivery
temperature for central heating systems. A finite, but small amount of
thermal energy is conveyed to the residence which is commensurate with the
rate of thermal energy consumption of the residence at these outdoor
temperature conditions. Finally, independent of outdoor temperature, and the
third temperature switch 150 on the storage vessel (located at the bottom of
the storage vessel) will disable the charging air circuit when it triggers at
its temperature setpoint, and thereby affecting the full charge state at
maximum design storage.
This control scheme optimizes furnace function with the use of low cost
and high reliability components. It does not require the use of conventional
temperature sensors, signal conditioning, and logic devices, as is, by
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comparison, very cost effective and rugged. The specific temperature
setpoints of the five temperatures switched can be selected to meet the needs
of arbitrary climatic zones.
The storage vessel 118 is of cylindrical, thin shell construction
permitting economical use of material while providing adequate mechanical
strength to withstand cylical mechanical forces. These forces result from the
effec:ts of differential expansion between the storage vessel material and the
storage media. The vessel is externally insulated 152 so that low cost
thermal insulation may be used. The optimum insulation is vermiculite
granules because of cost, performance, and allowable service temperature
range. Because these granules may compact and settle over time, a high
performance insulation gives adequate secondary thermal hazard protection even
at excessive compa~tion of the granular insulation.
In conclusion, what has been disclosed is a thermal storage furnace system
having a rnixing means in fluid communication with both a thermal storage means
and a ther,nal storage means bypass conduit for mixing both heated fluid from
the thermal storage means and unheated fluid from the thermal storage means
bypass conduit to provide for storage of heat in the thermal storage means at
a first predetermined higher temperature than desired for heating the interior
of the structure, while providing a second predetermined temperature fluid for
space heating the interior of the structure by mixing the heated fluid from
the thermal storage meahs with the unheated fluid from the thermal storage
means bypass conduit. The means of mixing the heated and unheated air
according to the teachings of the invention are such that only a single low
cost air moving means is required and that the thermal storage means may have
heat transferred to it for an extended period of time and rise to a storage
temperature much higher than that desired for heating the interior of a
desired structure or housing. These advantages result in a significant cost
reduction for the thermal storage furnace system. Another major cost
reduction, results from the use of a rock bed as the storage medium for the
thermal storage means, Trap rock, a material with low material costs and wide
distribution have been identified, by test as having uniquely suitable
properties for the heat storage medium.
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