U.S. patent application number 13/220217 was filed with the patent office on 2011-12-22 for fuel cell based power generation systems and methods of operating the same.
This patent application is currently assigned to NuCellSys GmbH. Invention is credited to Uwe LIMBECK.
Application Number | 20110311891 13/220217 |
Document ID | / |
Family ID | 38895731 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110311891 |
Kind Code |
A1 |
LIMBECK; Uwe |
December 22, 2011 |
Fuel Cell Based Power Generation Systems and Methods of Operating
the Same
Abstract
A power generation system has a fuel cell stack and at least one
condensation point in the system at which water present after
shutdown of the power generation system can condense or collect.
Drying after shutdown is improved by maintaining a temperature
gradient between the condensation point and at least one other
component in the power generation system after shutdown. In one
embodiment, the temperature gradient is maintained by housing the
fuel cell stack in a thermally insulated container and arranging
the condensation point outside of the insulating container. In
another embodiment, drying after shutdown is accomplished with an
adsorption unit having a water-adsorbing material arranged in a
desired location within the power generation system.
Inventors: |
LIMBECK; Uwe; (Kirchheim,
DE) |
Assignee: |
NuCellSys GmbH
Kirchheim/Teck-Nabern
DE
|
Family ID: |
38895731 |
Appl. No.: |
13/220217 |
Filed: |
August 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11587140 |
May 23, 2007 |
8043755 |
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PCT/US05/13773 |
Apr 22, 2005 |
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13220217 |
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60631705 |
Nov 30, 2004 |
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Current U.S.
Class: |
429/414 |
Current CPC
Class: |
H01M 8/04007 20130101;
H01M 8/04253 20130101; H01M 8/04268 20130101; H01M 8/04164
20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/414 |
International
Class: |
H01M 8/06 20060101
H01M008/06 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 2004 |
DE |
10 2004 020 029.7 |
Claims
1. A fuel cell based power generation system comprising: a
water-collecting device; and a water adsorbing material coupled to
the water-collecting device; wherein the water adsorbing material
is located within the power generation system to attract water
present within the system after an operational shutdown of the
system.
2. The power generation system of claim 1, further comprising: a
fuel cell stack having at least one fuel cell; and a compressor to
supply an oxidant stream to the fuel cell stack; wherein the
water-collecting device is disposed between the fuel cell stack and
the compressor.
3. The power generation system of claim 1, wherein the water
adsorbing material is one of a silica gel, a zeolite and an ion
exchange resin.
4. The power generation system of claim 1, further comprising: a
heating element operable to regenerate the adsorbing material.
5. The power generation system of claim 1, further comprising: a
first component operable to condense water present in the power
generation system, the first component having a first temperature
after an operational shutdown of the power generation system, at
least one other component having a second temperature that is
higher than the first temperature of the first component after the
operational shutdown of the power generation system; and a
subsystem to maintain a temperature gradient between the first
component and the at least one other component for a substantial
amount of time after the operational shutdown of the power
generation system.
Description
[0001] This application is a divisional of U.S. application Ser.
No. 11/587,140, filed Oct. 20, 2006, and claims the priority of
German patent document 10 2004 020 029.7, filed Apr. 23, 2004 and
U.S. Ser. No. 60,631,705, filed Nov. 30, 2004 (PCT International
Application No. PCT/US2005/013773, filed Apr. 22, 2005), the
disclosures of which are expressly incorporated by reference
herein.
BACKGROUND OF THE INVENTION
[0002] The present systems and methods relate to fuel cell based
power generation systems for the generation of electrical energy,
and particularly, to improving the freeze-start capability of such
systems.
[0003] Fuel cells are electrochemical energy converters, which
directly convert chemical energy to electrical energy. For this
purpose, the fuel cell is supplied with a fuel and with oxidant
(such as air) as reactants. The reactants are spatially separated
by an electrolyte, through which ion exchange takes place. Example
fuels include hydrogen, or methane. There are several known types
of fuel cells, including polymer electrolyte membrane fuel cells
(PEMFC), phosphoric acid fuel cells (PAFC), and solid oxide fuel
cells (SOFC). This list is not complete and the present systems and
methods are not limited to a specific type of fuel cell, nor to a
specific fuel, nor to a specific application. For example, the
application of the present systems and methods invention in a motor
vehicle is only one possible embodiment.
[0004] Water is present in fuel cells. It may be produced in the
fuel cell as a product of the reaction of hydrogen and oxygen, or
it may be supplied to the fuel cell for cooling or humidification
purposes. For example, the membrane serving as electrolyte in PEM
fuel cells must be humidified to allow an efficient cation
exchange. Thus, typically either or both of the fuel and oxidant
streams may be humidified with water in a humidifier upstream of
the fuel cell. Water may also be conducted through special cooling
channels in the fuel cell to cool it.
[0005] After an operational shutdown of a fuel cell, the
temperature of the fluids contained therein and of the components
of the power generation system gradually drop to ambient
temperature. During this period, water vapor that is still present
in the fluid channels of the power generation system condenses and
precipitates as liquid water. If the ambient temperature drops
below 0.degree. C., any water present in the power generation
system may freeze. Such water may be located in the fuel cells, but
may also be present in other areas of the power generation system,
such as circulation devices (e.g., pumps, compressors, fans,
blowers) for the reactants, valves, or in the flow channels that
conduct the reactant streams or the cooling water through the power
generation system. Often, the flow channels have areas in which
water can accumulate, such as in corners or at the end of dead ends
where sensors are located.
[0006] Problems may occur upon resumption of power generation if
condensed water drops or ice are still present in the system. The
presence of ice or condensed water may obstruct the flow of
reactants, and the presence of ice in particular may affect the
proper functioning of system components, such as valves, sensors,
or circulation devices. In some situations, this may result in
damage to the components.
[0007] To prevent accumulation of water drops and ice and to
improve the freeze-starting capability of fuel cell systems, a
conventional approach is to purge (i.e., blow dry gas through) the
flow channels of the system immediately after operation ceases.
However, this method has disadvantages. The use of purging requires
considerable amounts of time and energy. Moreover, as the quantity
of water present in the system is unknown, it is difficult to
estimate whether the amount of purge gas and the duration of the
purging will be adequate for sufficient drying. Furthermore, it is
difficult for the purge gas to reach water that has been deposited
at poorly accessible spots of the flow channel system, such as at
the ends of flow channels and in corners. Moreover, the membranes
of PEM fuel cells can normally not be dried completely. There will
always be small remaining reservoirs, from which water is able to
diffuse to other locations and, in particular, to critical
positions in the power generation system.
[0008] Japanese patent document JP 2003-142136 proposes the
provision of a condenser for drying the internal fluid channels of
a fuel cell stack, where the condenser is cooled during a power
generating operation. For vehicular fuel cell stacks, Japanese
patent document JP 2003-142136 proposes disposing the condenser
just behind the radiator grill, so that it is cooled by the air
draft. The object of this arrangement is to permit water vapor that
is still present in the fluid channels of the fuel cell stack after
operation ceases to travel through an open path to the condenser,
and to precipitate there. Thus, the condenser forms a predetermined
condensation point.
[0009] An alternate approach to minimize or avoid the problems
associated with freezing water in a fuel cell stack is to house the
fuel cell stack in a thermally insulated container. This approach
is described in published U.S. Patent Application No. 2003/0162063.
One drawback of using the thermally insulated container housing
only the fuel cell stack, however, is that the other components of
the power generation system may retain water in inaccessible
locations. The system described in US 2003/0162063 keeps residual
water in the fuel cell stack from freezing by maintaining the
temperature in the insulating container sufficiently above freezing
with a heating system arranged in the insulating container.
However, water droplets may remain in the reactant channels of the
power generation system after an operational shutdown.
BRIEF SUMMARY OF THE INVENTION
[0010] The present invention provides a power generation system
having a fuel cell stack composed of one or more fuel cells, as
well as at least one predetermined condensation point, on which
water that is present in the power generation system after an
operational shutdown condenses. Methods and apparatus are provided,
which are intended and designed to inhibit an equalization
(reduction) of the temperature gradient between the predetermined
condensation point and at least other parts of the power generation
system after operation of the system ceases. Thus, after shutdown
the temperature gradient may be maintained by force, or at least
its weakening may be attenuated.
[0011] There are various measures that may be implemented to
inhibit the temperature equalization. In one embodiment, the
apparatus used for this purpose may comprise at least a thermally
insulating jacket, for example in the form of an insulating
container that houses at least the fuel cell stack, where the
predetermined condensation point is located outside of this
insulating container. Other components of the power generation
system, such as the reactant supply circulation devices, may be
located in the same or in another insulating container. Similarly,
it may be advantageous to dispose some or all of the sensors and
valves of the power generation system in the insulating container.
In one embodiment, only the predetermined condensation point is
located outside of the insulating container, together with any
adjoining parts of a pipe system in which the reactants and/or the
cooling water are transported.
[0012] The insulating container of the present invention serves to
maintain a temperature gradient between the components installed in
the insulating container and a condenser arranged on the
outside.
[0013] Not all of the components of the power generation system
that are to be insulated with respect to the predetermined
condensation point need be housed within the same insulating
container. In alternative embodiments, two (or even more) separate
insulating containers may be present to house various components of
the power generation system. In addition, in some embodiments,
there may be a thermal bridge of a material that is a good thermal
conductor (e.g., copper) provided between the different insulating
containers. In still other embodiments, there may be a thermal
bridge between an insulating container and one or more individual
components located outside of the insulating container.
[0014] Inhibition of temperature equalization may comprise--instead
of or in addition to the insulating container--a cooling fan
cooling the predetermined condensation point. In alternative
embodiments, a heater may be provided for the insulating
container.
[0015] In other embodiments of the present systems and methods, the
power generation system includes a fuel cell stack having one or
more fuel cells, and a passive water collecting component, which is
intended and designed to attract water that is still present in the
power generation system after an operational shutdown of the
system. The water-collecting component contains a water-adsorbing
material.
[0016] An adsorbing material can also aid in achieving the goal of
collecting any water still present (in vapor or droplet form) in
the power generation system after shutdown at a predetermined
location in order to prevent the water from remaining at undesired
points within the power generation system. In contrast to for
example a pump, adsorbing materials act passively (i.e., no
additional energy is required to attract the water), which has a
positive effect on the overall energy balance of the power
generation system. Suitable candidates for the adsorbing materials
include, in principle, all materials that have water-adsorbing
properties, such as ion exchange resins, silica gels, or zeolites.
The quantity of adsorbing material may be determined based
on--among other factors--the quantity of water employed during
operation of the power generation system and the expected residual
quantity of water in the system after shutdown.
[0017] In one embodiment, the water-collecting component that
contains the adsorbing material is disposed between the fuel cell
stack and a compressor, connected upstream of the stack, that
supplies a reactant stream to the fuel cell stack. The reactant
stream is heated by the compression, and as a rule moisture will be
extracted from the reactant stream, i.e., the reactant stream will
become drier. By conducting the warm and dry reactant stream, which
is available at the compressor outlet, across the adsorbing
material, the latter can be desorbed (regenerated) very efficiently
upon resumption of operation. Alternatively, a separate heating
element may be provided to regenerate the adsorbing material.
[0018] Those of ordinary skill in the art will appreciate that the
two aspects described above may be combined. For example, the
predetermined condensation point and the adsorbing material may be
at locations within the power generation system that are far apart,
or alternatively the water-collecting component that contains the
adsorbing material to be located in immediate proximity to a cold
spot that forms the predetermined condensation point. In still
another alternative embodiment, the water-collecting component may
be located at the coldest point in the system.
[0019] The present invention also includes a method of improving
the freeze-starting capability of a power generation system
including a fuel cell stack of one or more fuel cells, whereby
moisture is extracted from the power generation system after an
operational shutdown by means of at least one desired condensation
point. According to the invention, equalization of a temperature
gradient between the desired condensation point(s) and other (i.e.,
to-be-dried) components of the power generation system after the
operational shutdown of the power generation system is inhibited.
The temperature equalization may be inhibited by means of a
thermally insulating container, in which at least the fuel cell
stack is disposed, with at least one desired condensation point
arranged outside of the insulating container.
[0020] In an alternative embodiment of the present methods, the
freeze-starting capability of a power generation system may be
improved by extracting moisture from the power generation system
after shutdown by means of a passive water-collecting component,
wherein a water-adsorbing material is employed for moisture
removal.
[0021] Other objects, advantages and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] In the drawings, identical reference numbers identify
similar elements or acts. The sizes and relative positions of
elements in the drawings are not necessarily drawn to scale. For
example, the shapes of various elements and angles are not drawn to
scale, and some of these elements are arbitrarily enlarged and
positioned to improve legibility. Further, the particular shapes of
the elements as drawn are not intended to convey any information
regarding the actual shape of the particular elements, and have
been solely selected for ease of recognition in the drawings.
[0023] FIG. 1 is a schematic view of a power generation system
housed in an insulating container according to one illustrated
embodiment of the invention;
[0024] FIG. 2 is a schematic view of a power generation system with
only a fuel cell stack housed in an insulating container according
to another embodiment;
[0025] FIG. 3 is a schematic view of a power generation system with
a fuel cell stack housed in a first insulating container and a fan,
sensor, and valve housed in a second insulating container according
to another embodiment;
[0026] FIG. 4 is a schematic view of the power generation system of
FIG. 3 with a thermal bridge connected between the first insulating
container and the second insulating container; and
[0027] FIG. 5 is a schematic view of the power generation system of
FIG. 2 with a thermal bridge connecting the insulating container to
a recirculation fan.
DETAILED DESCRIPTION OF THE INVENTION
[0028] In the following description, certain specific details are
set forth in order to provide a thorough understanding of various
embodiments of the invention. However, one skilled in the art will
understand that the invention may be practiced without these
details. In other instances, well known structures associated with
fuel cells and power generation systems have not been shown or
described in detail, to avoid unnecessarily obscuring descriptions
of the embodiments of the invention.
[0029] Unless the context requires otherwise, throughout the
specification and claims which follow, the word "comprise" and
variations thereof, such as, "comprises" and "comprising", are to
be construed in an open, inclusive sense, that is as "including,
but not limited to."
[0030] The headings provided herein are for convenience only and do
not interpret the scope or meaning of the claimed invention.
[0031] It has been observed that drying of the fuel cell stack and
other components of the power generation system, such as
circulation devices, valves, and sensors, may be insufficient if it
relies solely on the temperature gradient established during
operation between the condensation point and the other components
in the system. Uncontrolled temperature equalization often takes
place so rapidly after shutdown that insufficient quantities of the
residual water are captured in various regions of the system.
[0032] FIG. 1 shows a power generation system 10 having a fuel cell
stack 12. The fuel cell stack 12 generally comprises a multiplicity
of individual fuel cells combined into the stack 12; however for
the purpose of clarity, only a single fuel cell 14 is illustrated
in FIG. 1. The fuel cell 14 has an anode 16 and a cathode 18,
separated by an electrolyte 20. Although not shown in detail in
FIG. 1, a fuel stream is supplied to anode 16, while an oxidant
stream, such as air, is supplied to cathode 18. In one embodiment,
the fuel cell 14 is a polymer electrolyte membrane fuel cell, for
use in a motor vehicle. In addition, the fuel cell 14 includes a
cooling area 22, through which a coolant, such as water, flows for
cooling purposes.
[0033] During operation of the power generation system 10, an
amount of residual fuel (i.e., not consumed in fuel cell 14) is
recirculated in an anode recirculation loop 24 and an amount of
fresh fuel is mixed with the residual fuel on an anode-inlet side,
which is not described in any detail but known in the art. The
anode recirculation loop 24 includes a recirculation device, such
as a recirculation fan 26, that conducts the residual fuel to the
inlet of anode 16. The power generation system 10 may include
sensors 28, 30, which can serve to measure various characteristics
of the residual fuel, such as pressure, temperature, concentration,
and/or relative humidity.
[0034] A compressor 32 delivers an air stream to an inlet side of
the cathode 18. One or more sensors 34 may measure various
characteristics of the air stream, such as pressure, temperature,
concentration, and/or relative humidity.
[0035] Reactant gases supplied to the anode 16 and the cathode 18
are humidified with water with humidifiers to maintain the moisture
of the electrolyte 20 during operation of the power generation
system 10. The humidified water added to the reactant gases is used
for cooling the recirculation fan 26 and/or compressor 32.
[0036] After an operational shutdown of the power generation system
10, water in either vapor or droplet form generally remains in the
reaction stream flow channels and the cooling water channels. The
water vapor may condense when the temperature of the various
components of power generation system 10 decreases over time. It is
even possible for ice to form in power generation system 10 if the
temperature falls below the freezing point of water. When resuming
operation of the power generation system 10, any ice crystals that
may have formed or water droplets that may remain within the fuel
cell 14, or in other components of the power generation system 10,
such as the recirculation fan 26 or the compressor 32, may lead to
problems. Ice and/or water droplets can hinder the flow of the
reactants and the coolant, and in particular, ice may adversely
affect some system components with moving parts, such as the
recirculation fan 26, the compressor 32 and/or the valves within
the power generation system 10. (Although no valves are shown in
FIG. 1, it is understood that power generation systems of the type
described herein are generally equipped with a number of valves to
control the fluid flows.)
[0037] After operation of power generation system 10 ceases, for
example when a vehicle equipped with power generation system 10 is
parked, the various system components are dried as completely as
possible by collecting residual water at one or more desired
collecting points. The collecting points include a condenser 36 and
an adsorption unit 38, which contains a water adsorbing material 40
according to the illustrated embodiment. The condenser 36 is
integrated into anode recirculation loop 24 where it is connected
downstream of the recirculation fan 26. The adsorption unit 38 is
integrated into the air supply/conducting system. Specifically, the
adsorption unit 38 is disposed between the compressor 32 and the
inlet of the cathode 18 so that warm compressed air from the outlet
of the compressor 32 passes over the adsorbing material 40.
[0038] The condenser 36 acts as a condensation point because it is
the coldest point in anode recirculation loop 24. Because of the
temperature gradient, any residual water that is still present in
the anode recirculation loop 24 after shutdown of the power
generation system 10 is attracted to the condenser 36 where the
water condenses. The temperature gradient between the condenser 36
and the fuel cell stack 12, as well as the other components of the
anode recirculation loop 24, such as the recirculation fan 26, may
be established during operation of the power generation system 10.
For example, if the power generation system 10 is installed in a
vehicle, the air draft may be used to cool the condenser 36. In
other embodiments, it is possible to link the condenser 36 with a
cooling fan 42, which selectively cools the condenser 36.
[0039] In order to maintain the temperature gradient between the
condenser 36 and the other components of the anode recirculation
loop 24, which includes the fuel cell stack 12, for a sufficiently
long time after the operational shutdown of the power generation
system 10, an insulating container 44 may be provided. In one
embodiment, the insulating container 44 houses at least the fuel
cell stack 12, the recirculation fan 26, and sensors 28, 30. The
insulating container 44 has a thermally insulating effect and
inhibits an equalization of the temperature of the components
housed therein with respect to an ambient temperature external to
the container 44. The condenser 36, on the other hand, is arranged
outside of insulating container 44, and accordingly cools toward
ambient temperature at a faster rate than components located within
the container 44.
[0040] The configuration of condenser 36 may take various forms. In
one embodiment, the condenser 36 is a pot-shaped container, in
which the attracted water is collected. In another embodiment, the
condenser 36 is formed from a curved piece of pipe. In yet another
embodiment, the condenser 36 is a drain valve (not shown),
which--when opened--discharges the collected water, as
schematically indicated in FIG. 1 by arrow 46. The specific
configuration of condenser 36 will depend on an expected quantity
of residual water in the power generation system 10, especially in
the anode recirculation loop 24, and also depend on the energy
generated during condensation. For this reason, the condenser 36 is
generally configured with a sufficiently large surface area to
quickly radiate away the heat generated during condensation.
[0041] In order to further maintain the temperature gradient
between the components inside the insulating container 44 and the
condenser 36, the cooling fan 42 may continue to run after
operation of the power generation system 10 has ceased according to
at least one embodiment. In particular, it may be possible to cool
the condenser 36 to a temperature below the ambient temperature by
means of the cooling fan 42.
[0042] Additionally or alternatively, the adsorption unit 38
provides another option to extract residual water from the power
generation system 10 and to selectively collect the water at a
predetermined point. While the embodiments described herein are not
limited to a specific type of adsorbing material, when choosing the
type and quantity of adsorbing material 40, the overall expected
quantity of residual water in power generation system 10 should be
considered.
[0043] After residual water has been adsorbed in the cooling phase
of the power generation system 10, regeneration of the adsorbing
material 40 may be necessary during normal operation. In one
embodiment, the heated and compressed air available at the outlet
of the compressor 32 allows a highly effective desorption of the
adsorbed water. In this embodiment, the adsorption unit 38 is
arranged in spatial proximity to the compressor 32 to more
effectively utilize the higher temperature compressed air.
[0044] It should be noted that in an alternative embodiment, the
anode recirculation loop 24 contains a predetermined condensation
point. In addition, other areas of the power generation system 10
may be similarly equipped. For example, alternatively or
additionally, the oxidant supply system of the power generation
system 10 may contain a condenser. Likewise, the coolant loop may
contain a condenser. Similarly, the power generation system 10 may
include more than one adsorption point. In the embodiment shown in
FIG. 1, the adsorption point is located in the air supply system,
however the adsorption point can alternatively or additionally be
located in the anode recirculation loop 24 and/or the coolant
loop.
[0045] In still other embodiments, the adsorption unit may be
disposed in the anode recirculation loop 24 near the condenser 36.
For example, the adsorption unit may be disposed in the immediate
proximity of the condenser 36. Likewise, the oxidant system of the
power generation system 10 may contain a condenser in addition to
the adsorption unit 38.
[0046] In FIGS. 2 through 5, similar components or components with
similar functions are identified by the same reference labels as in
FIG. 1, with a lower-case letter appended. Unless specified
differently in the following discussion, the
functionality/composition of the components is generally as
described above.
[0047] In the embodiment shown in FIG. 2, only the fuel cell stack
12a is housed within the insulating container 44a, while the
recirculation fan 26a of the anode recirculation loop 24a is
arranged outside of the insulating container 44a. Two condensers
36a are disposed in the anode recirculation loop 24a, with one of
the condensers positioned upstream of the recirculation fan 26a and
the other condenser positioned downstream of the recirculation fan
26a.
[0048] A valve 48a serves to control the quantity of humidifying
water being supplied to the power generation system 10a, while a
valve 50a allows an amount of fuel to be discharged, or purged,
from the power generation system 10a.
[0049] In the embodiment shown in FIG. 2, the valves 48a, 50a and
the sensors 28a, 30a are disposed outside of the insulating
container 44a. In the embodiment shown in FIG. 3, a second
insulating container 52b is provided, which houses the
recirculation fan 26b, the sensors 28b, 30b and the valves 48b,
50b. Only the two condensers 36b are located outside of the
insulating containers 44b, 52b. It is understood other components
of the power generation system 10b, which are not shown in FIG. 3,
such as components related to the air supply system or the cooling
loop of fuel cell stack 12b, may also be housed in one of the two
insulating containers 44b, 52b.
[0050] The embodiment depicted in FIG. 4 includes a thermal bridge
54c, which extends between the two insulating containers 44c, 52c.
The thermal bridge is formed of a material that is a good thermal
conductor (e.g., copper) and provides for temperature equalization
between the two insulating containers 44c, 52c; specifically for
temperature equalization between the interiors of the containers
44c, 52c. The thermal bridge 54c allows the heat generated during
operation of fuel cell stack 12c to be used to heat the components
in insulating container 52c. This may be advantageous if the
components housed in insulating container 52c generate no heat or
only a small amount of heat during operation of the power
generation system 10c.
[0051] FIG. 5 shows one insulating container 44d that houses the
fuel cell stack 12d. The recirculation fan 26d is connected to the
insulating container 44d with a thermal bridge 54d. The thermal
bridge 54d allows selective heat transfer to individual components
located outside of the insulating container 44d, for example the
heat transfer from the insulating container 44d to the
recirculation fan 26 as illustrated in the present embodiment. The
thermal bridge 54d can also be connected to other "critical"
components to transfer heat and thus remove residual water when
operation of the power generation system 10d ceases. As those of
ordinary skill in the art will appreciate, in other embodiments
additional thermal bridges may connect insulating container 44d to
other components, such as other circulation devices (fans, pumps,
compressors, blowers, etc.) of power generation system 10d, or to
valves.
[0052] From the foregoing it will be appreciated that, although
specific embodiments of the invention have been described herein
for purposes of illustration, various modifications may be made
without deviating from the spirit and scope of the invention.
Accordingly, the invention is not limited except as by the appended
claims.
* * * * *