U.S. patent application number 12/878896 was filed with the patent office on 2011-05-26 for liquid metal thermal storage system.
Invention is credited to Arlon J. Hunt.
Application Number | 20110120669 12/878896 |
Document ID | / |
Family ID | 43733098 |
Filed Date | 2011-05-26 |
United States Patent
Application |
20110120669 |
Kind Code |
A1 |
Hunt; Arlon J. |
May 26, 2011 |
LIQUID METAL THERMAL STORAGE SYSTEM
Abstract
Embodiments of this invention relate generally to high
temperature thermal energy storage, and more specifically, to the
use of the latent heat of fusion of melting and solidifying metals
to receive from and provide heat to a gaseous medium. Embodiments
of this invention are also known as the Liquid Metal Thermal
Storage system or LIMETS. Also described are methods of containing
the storage material, heat transfer means, and choices of metals
and alloys for thermal storage materials.
Inventors: |
Hunt; Arlon J.; (El Cerrito,
CA) |
Family ID: |
43733098 |
Appl. No.: |
12/878896 |
Filed: |
September 9, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61276269 |
Sep 10, 2009 |
|
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|
Current U.S.
Class: |
165/67 ;
165/104.19 |
Current CPC
Class: |
F24S 60/30 20180501;
F24S 60/00 20180501; F28F 21/02 20130101; Y02P 90/50 20151101; Y02E
60/14 20130101; F28D 20/02 20130101; F24S 60/10 20180501; F28F
21/04 20130101; Y02E 10/40 20130101; F24S 20/20 20180501; Y02E
10/46 20130101; F28D 20/021 20130101 |
Class at
Publication: |
165/67 ;
165/104.19 |
International
Class: |
F28F 9/00 20060101
F28F009/00; F28D 15/00 20060101 F28D015/00 |
Claims
1. A system for storing and retrieving thermal energy from gas
heated by a high temperature source comprising: a chamber
containing heat exchanger elements wherein the heated gas is passed
through the chamber containing the heat exchanger elements which
heat exchanger elements are in thermal communication with a
non-alkali metal or metal alloy that melts at a specified
temperature between 600.degree. C. and 1400.degree. C. to stores
thermal energy; and the same or a different chamber containing the
same or different heat exchanger elements wherein a gas to be
heated is passed through the same or the different chamber
containing the same or different heat exchanger elements that are
in thermal communication with the same metal or metal alloy that at
least partially solidifies, giving up the thermal energy
stored.
2. The system of claim 1 including the metal alloy composition for
a latent heat thermal storage system that melts at a specified
temperature by varying the fraction of each component.
3. The system in claim 1 wherein the heat exchanger elements are
tubes which tubes are composed of a high temperature ceramic
material.
4. The system in claim 1 wherein the heat exchanger elements are
tubes and the tubes are composed of a high temperature metal alloy
with a substantially higher operating temperature than the melting
temperatures of the metal or metal alloy.
5. The system in claim 1 wherein the heat exchanger elements are
tubes and the tubes are composed of graphite.
6. The system in claim 1 wherein the heat exchanger tubes are tubes
and the tubes are composed of graphite that is clad in a metal or
ceramic to prevent oxidation when using oxidizing gasses as the
heat transfer medium.
7. The system in claim 1 wherein the heat exchanger elements are
tubes and the tubes are solid metal or solid graphite with a
cladding.
8. The system in claim 1 wherein the heat exchanging elements are
tubes and the tubes are hollow and contain a element or compound
with a boiling temperature above the metal or metal alloy to carry
the heat from the solar source into the metal or metal alloy.
9. The system in claim 1 wherein the heat exchanging elements are
tubes and the tubes are hollow and contain a element or compound
with a boiling temperature below the metal or metal alloy to carry
the heat from the metal or metal alloy to the gas stream to be
heated.
10. The system in claim 1 wherein the heat exchanging elements are
tubes and the tubes are arranged in such a fashion so as to
maximize the heat transfer between the tubes and heat transfer
gas.
11. The system in claim 1 wherein the heat exchanging elements are
tubes and the tubes are one of hollow or solid rods have radial or
axial fins to improve the heat transfer.
12. The system in claim 1 wherein the heat exchanging elements are
tubes and the cross section of the tubes is designed so as to
maximize the heat transfer between the tubes and the heat transfer
gas.
13. The system in claim 1 that uses air, carbon dioxide, argon,
helium, or nitrogen as the heat transfer gas.
14. The system in claim 7 wherein the source including a solar
receiver to heat the gas to provide heat to melt the metal or metal
alloy.
15. The system in claim 1 including a gas turbine that uses the
stored heat to operate a gas turbine to provide mechanical
power.
16. The system in claim 14 wherein the source including a windowed
high temperature solar receiver that uses small particles to absorb
concentrated sunlight and heat gases including at least one of air,
carbon dioxide, helium or nitrogen in which they are entrained.
17. The system in claim 1 including a metal alloy that is made from
two or more elements whose melting temperature is determined by the
choice of the fraction of the two or more elements.
18. The system in claim 1 wherein the metal alloys including
aluminum and silicon with a melting temperature from 600.degree. C.
to 1400.degree. C.
19. A method for storing and retrieving thermal energy from and to
a gas heated by a high temperature source including the steps of:
passing the heated gas through a chamber containing heat exchanger
elements that are in thermal communication with a non-alkali metal
or metal alloy that melts at a specified temperature between
600.degree. C. and 1400.degree. C. to store thermal energy in the
form of the latent heat of fusion of the metal or metal alloy; and
passing the gas to be heated through the same or a different
chamber containing the same or different heat exchanger elements
that are in thermal communication with the same metal or metal
alloy that at least partially solidifies, giving up the thermal
energy stored in the form of the latent heat of fusion.
20. The method of claim 19 including choosing the metal alloy
composition for a latent heat thermal storage system so that the
metal alloy melts at a specified temperature by varying the
fraction of each component of the metal alloy.
21. A system for storing and retrieving thermal energy from a gas
heated by a high temperature source comprising: a chamber
containing heat exchanger elements; and a non-alkali metal or metal
alloy contained in the heat exchanger elements adapted for storing
heat from the heated gases.
22. The system of claim 1 wherein the metal or metal alloy melts at
a specified temperature of between about 600.degree. C. and
1400.degree. C.
23. A system for storing and retrieving thermal energy from a gas
heated by a high temperature source comprising: a first channel
having first heat exchanger elements; a second channel having
second heat exchange elements; a chamber containing a non-alkali
metal or metal alloy that is adapted for storing heat from the
heated gas; and the first and the second heat exchanger elements in
part extending into the chamber.
24. The system of claim 1 wherein the metal or metal alloy melts at
a specified temperature of between about 600.degree. C. and
1400.degree. C.
25. A method for storing and retrieving thermal energy from gas
heated by a high temperature source including the steps of: passing
the heated gas through a chamber containing heat exchanger elements
that are in thermal communication with a non-alkali metal or metal
alloy that melts to store thermal energy; and passing the gas to be
heated through the same or a different chamber containing the same
or different heat exchanger elements that are in thermal
communication with the same metal or metal allow that at least
partially solidifies, giving up the thermal energy stored.
Description
CLAIM OF PRIORITY
[0001] This application claims benefit of U.S. Provisional
Application No. 61/276,269, filed Sep. 10, 2009, which is
incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The use of solar thermal power for generating electricity on
a utility scale is having a resurgence of interest in light of
global warming issues caused largely by the release of carbon
dioxide, methane, and absorbing particulates in the atmosphere and
to the increasing of fossil fuels. The use of concentrated solar
energy to heat a working fluid to a high temperature to operate
Rankin, Brayton, and Sterling cycle engines to provide mechanical
power to operate a generator for utility scale electric power
production offers an attractive alternative to the use of fossil
fuels. However, utilities are generally requesting that power
production facilities provide dispachable power on the order of 75%
of the year. Since the sun is only above the horizon 50% of the
time on a yearly basis, there is a need to provide some form of
storage to operate the plant when there is no sunlight
available.
[0003] Thermal energy storage may be accomplished by storing the
energy in the form of heat, either as sensible heat or latent heat
(or a combination thereof). Current solar collectors utilize
heliostats, parabolic troughs or liner Fresnel reflectors to
concentrate sunlight on solar receivers. These receivers are heated
by the concentrated sunlight and utilize steam, oil, liquid salt,
liquid alkali metal or gas as the heat collection and transfer
fluid. This fluid may be used as the heat storage medium itself or
the heat may be transferred to another medium to provide the
storage. Thermal storage may be provided by the sensible heat in
tanks of oil, oil and rock, liquid salts, or liquid alkali metals
as discussed by Geyer in Winter et. al. The fluids heated by the
concentrated sunlight are used to generate steam, heat a working
fluid for energy conversion or they may be stored at high
temperatures (or in combination). After giving up their heat for
energy conversion the cooled fluids are stored separately from the
hot fluids. This may be accomplished in separate hot and cold tanks
or by using a thermocline configuration. In the thermocline system,
the colder (denser) fluid forms the bottom layer with the hotter
(less dense) fluid forms the upper layer. In any of these
configurations, when the sun is not providing heat, the stored hot
liquid may be pumped through the heat exchanger to heat the working
fluid for power production and then to the cold side of the storage
to complete the cycle.
[0004] Alternatively, the latent heat of fusion may be used to
store thermal energy. Liquid salts or alkali metals that undergo a
phase change to store or release heat at their melting temperature
have been used in thermal storage systems. An advantage of this
form of heat storage is that the heat is released at a nearly
constant temperature, providing the optimum operating conditions
for the energy conversion cycle. Another advantage to the use of
latent heat energy storage occurs because the amount of storage
material can be significantly decreased. To clarify, the amount of
energy stored in specific heat is determined by the product of the
specific heat and the temperature change. For example; the specific
heat of water is 1 cal/gm, if the temperature is lowered by
1.degree. C., one gram of the water releases 1 calorie of heat.
Alternatively, the latent heat of fusion of water is about 80
cal/gm so that the energy released in freezing or solidifying one
gram of ice is 80 calories of heat at a nearly constant
temperature. Thus, the amount of water needed to store the same
amount of heat that is provided by freezing one gram of ice is 80
times greater than that to change the temperature of the water by
1.degree. C.
[0005] In latent heat storage systems using high temperature salts,
there are restrictions in the heat flow into and out of the storage
material due to the low conductivity of salt. This is further
aggravated by the fact that as the heat storage is discharging, the
salt freezes around the pipes carrying the heat exchange fluid
which stops convective heat transfer. This reduces the rate at
which the heat that can be extracted from the storage system. Thus
the combination of the low conductivity of the salt and the
curtailment of convection due to the immobility of the salt
presents obstacles to the utilization of this type of latent heat
storage system.
[0006] Another consideration in the operation of a solar power
system is the operating temperatures. To make concentrated solar
thermal power systems as cost effective as possible, it is
desirable to maximize the efficiency of the power conversion cycle.
This reduces the cost of the most expensive component of the plant;
the concentrating collectors. Since the maximum efficiency of a
heat engine is determined by the temperature difference between the
hot and cold reservoirs between which it operates, operating at the
highest temperature possible is most desirable. Oils break down at
temperatures above about 400.degree. C. Most thermal storage
systems using salt operate below about 570.degree. C. No effective
means have been found to store steam at the pressures and
temperatures required to run efficient Rankin cycle engines.
Brayton or gas cycle engines use gas as a working fluid and again
it is impracticable to store gas at very high temperatures and
pressures. The Brayton cycle provides the highest efficiencies for
power tower concentrating systems because the operating
temperatures are only limited by the turbine inlet temperatures
(well over 1000.degree. C.). Storing high temperature gas is not a
realistic energy storage option.
INCORPORATION BY REFERENCE
[0007] The following references are incorporated herein in their
entireties:
[0008] U.S. Pat. No. 4,512,388, issued Apr. 23, 1985, by Terry D.
Claar et al., entitled "High-Temperature Direct-Contact Thermal
Energy Storage Using Phase-Change Media"; Simensen "Comments on the
Solubility of Carbon in Molten Aluminum" Metallurgical Transactions
A Vol. 20A January 1989, p. 191;
[0009] Winter, Sizmann, @ Vant-Hull, Solar Power Plants, Chapter 6,
Springer, Verlag 1991; and
[0010] Guthy and Makhlouf "The aluminum-silicon eutectic reaction:
mechanisms and crystallography" Journal of Light Metals Vol. 1, No.
4, November 2001, pp. 199-218.
SUMMARY OF THE INVENTION
[0011] Embodiments of the invention relate to the use of melting
and solidifying or freezing metals and metal alloys to store and
release the high latent heat of fusion of certain metals and alloys
to store large amounts of heat energy at very high temperatures
suitable for operating a gas turbine or other purposes. In
particular, the alloy may consist of two or more metals with
melting and eutectic temperatures in the range that is compatible
with the energy conversion device to be used.
[0012] In the first embodiment considered here, the metal or alloy
is contained in an array of tubes located in an insulated channel
through which the high temperature gas is circulated. The system is
charged by passing gas, from the solar receiver or other heat
source, past the tubes in order to heat and melt the metal/alloy
contained within the tubes. The system is discharged by passing the
air to be heated through the same channel until the metal or alloy
has changed phase (liquid to solid) and the temperature has dropped
to the optimum operating temperature for the system.
[0013] In another embodiment, the metal or alloy is contained in an
insulated container equipped with heat transfer elements or tubes
that thermally communicate with the heat source. In this case, the
system is charged by transferring heat from a high temperature gas
circulating in a channel or passageway through a wall into the
chamber containing the solid/liquid metal or alloy until it melts.
The system is discharged by passing heat out of the chamber with
the same or different heat transfer elements or tubes that
communicate with the channel carrying the gas to be heated.
[0014] In any of the embodiments above, there is a wide choice of
alloys to be used. In another embodiment two elements are combined
to form an alloy with a melting temperature determined by the
fraction of each metal present, which is in turn chosen by the
desired operating temperature. In a particular embodiment, the
alloy composed of aluminum and silicon is chosen. By varying the
ratio of these elements the operating point may be chosen from
about 600.degree. C. to 1411.degree. C. This very wide temperature
range provides for the operation of a variety of turbine inlet
temperatures including the upper range of Rankine steam cycles.
[0015] The tubes containing the metal or metal alloy in the first
embodiment may be made from ceramic, metal, or clad graphite. The
graphite must be clad in metal or ceramic in the case of air or
other oxidizing gas (e.g., carbon dioxide) in the heat exchanger as
otherwise the graphite would be subject to oxidation at the
operating temperatures considered here.
[0016] In the embodiment using the heat transfer elements or tubes
that transfer the heat to and from the metal enclosed in a separate
insulated chamber, the tubes may be composed of solid metal of
suitably high melting temperature e.g. copper, steel, nickel, or
high temperature alloys of these or other metals. The elements may
also be composed of graphite in direct contact with the molten
metal if there is minimum chemical reaction with the heat storage
metal or metal alloy, but with appropriate cladding in the sections
that they may be exposed to an oxidizing atmosphere.
[0017] In another embodiment these heat transfer elements or tubes
may also be closed hollow tubes composed of a high temperature
metal ceramic or graphite containing a relatively small amount of
an element or compound with a boiling temperature that is above
that of the melting point of the metal or metal alloy storage
material. In this case the element or compound is boiled within the
lower end of the tube by the gas passing through the channel below
the storage tank with the upper end imbedded in the metal or metal
alloy storage material. This heat pipe arrangement is very
effective as a heat exchanger. In this case the thermal storage is
discharged by similar tubes, but that contain an element or
compound with a lower boiling temperature than the melting point of
the metal or metal alloy storage material. The lower end of the
heat pipe is in the metal or alloy storage material while the upper
end passes through the upper side of the storage chamber and into a
separate gas carrying channel. In this case the storage is
discharged by passing a gas through the upper channel.
[0018] There are several advantages to this form of latent heat
storage. First, most of the heat is released at a constant
temperature which allows a gas turbine to operate at its design
point. This is a consideration as off-design operation of gas
turbines can significantly lower their conversion efficiency.
Charging the thermal storage is accomplished by the gas at any
reasonable temperature above the melting point of the metal.
Because all suitable metals and alloys contract on melting there is
no reason for metals to break their containing tubes or storage
containers. The high thermal conductivity of the metal in both
liquid and solid form provides excellent heat transfer within the
metal. This avoids problems encountered in using liquid salts or
alkali metals wherein low conductivity regions of the solid and
solidifying material slow the release of heat.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The foregoing aspects and others will be readily appreciated
by the skilled artisan from the following description of
illustrative embodiments when read in conjunction with the
accompanying drawings.
[0020] FIG. 1a is a schematic illustration of the top view of an
embodiment of the a heat exchanger of an embodiment of the
invention.
[0021] FIG. 1b is a schematic illustration of the side view of an
embodiment of a heat exchanger of an embodiment of the
invention.
[0022] FIG. 1c is a schematic illustration one of the tubes
containing the metal or metal alloy of an embodiment of the
invention.
[0023] FIG. 1d is a schematic of an embodiment of the invention
using a vertical flow configuration wherein the gas is moving
parallel to the alignment of the storage tubes.
[0024] FIGS. 1e and 1f depict alternative embodiments of the tubes
of the invention.
[0025] FIG. 2 is a schematic of another embodiment of the invention
showing the charging plenum at the bottom and the discharging
plenum above the metal or metal alloy storage container.
[0026] FIG. 3a is a schematic illustration how an embodiment of the
invention is implemented with a gas turbine generator in solar only
mode.
[0027] FIG. 3b is a schematic illustration how an embodiment of the
invention is implemented with a gas turbine generator during
thermal discharging.
[0028] FIG. 3c is a schematic illustration how an embodiment of the
invention is implemented with a gas turbine generator during hybrid
operation wherein power for the turbine is supplied from storage
and the solar receiver.
[0029] FIG. 4 is an equilibrium diagram for the Al--Si system
showing metastable extensions of liquidus and solidus line.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] The embodiments of the invention are illustrated in the
context of a Brayton cycle solar thermal electric power plant. The
skilled artisan will readily appreciate, however that the materials
and methods disclosed herein will have application in a number of
other contexts where high temperature thermal storage is
desirable.
[0031] One embodiment of the invention, also know as a Liquid Metal
Thermal Storage system (LIMETS) consists of substantially four
items ; the metal or metal alloy thermal storage material, the
tubes or a compartment containing the metal or metal alloy, the
insulated cavity enclosing the tubes, and the heat transfer medium
(gas). FIG. 1a is a top view of a schematic drawing of the system
showing the insulated cavity 100, the ceramic or clad graphite
tubes 101 containing the metal or metal alloy and the insulated
container 102. FIG. 1b is a side view of the same components. FIG.
1c is a cross sectional view of the tube and metal showing one tube
101 and metal 103 with the open top. FIG. 1d depicts a perspective
view of this embodiment. By way of example only, tubes 101 can
include fins or other appendages or structures that increase the
surface area of the tubes and the rate of heat transfer to and/or
from the tubes (FIG. 1e). The tubes also have cross-sections that
increase the rate of heat transfer (FIG. 1f). Such cross-sections
increase the surface area of the cross-section by including, for
example, a star-shaped cross-section. The tubes and enclosure are
to be arranged so as to maximize the heat transfer considering the
temperature and nature of the gas transfer medium. The Reynolds
number is determined by the properties of the gas and the
characteristic dimensions of the tubes and the design should be
optimized for these factors to maximize the heat transfer to and
from the tubes.
[0032] FIG. 2 illustrates another embodiment utilizing the tubes. A
vertical orientation of the tubes is useful so as to utilize the
down corner from the solar receiver located at the top of tower and
to provide an alternative design to optimize the heat transfer to
the tubes. Ducting arrangements can allow flow of the gas in either
up or down past the vertical tubes. In both of these arrangements
the system is charged by passing hot gas from the solar receiver
over the tubes until melting takes place. Since most metals and
metal alloys expand when melting the lower density melt will rise
to the top, leaving the bottom to melt last. This has a consequence
of encouraging good mixing to ensure that the metal or metal alloy
is nearly isothermal. In this embodiment the metal or metal alloy
104 is contained in a separate insulated container 105 that
thermally communicates to the heated air via either high
conductivity metal or metal clad graphite rods, or preferably by
using hollow heat pipes or tubes 106 and 107. In this embodiment
there are two channels, one channel 108 for the hot (charging) gas
below the metal or metal alloy container and one channel 109 above.
The hot gas passes through the lower channel 108 and the heat pipes
or tubes or rods 106 and carry the heat to the metal or metal alloy
to melt the storage material 104. To discharge the storage, a
similar set of heat pipes or tubes or rods 107 carries the heat to
the upper channel when cooler gas is pumped through the upper
channel. The heat transfer may be substantially improved by using
heat pipes in which an element or compound with a suitable boiling
point is encapsulated within the tubes. As an example only, such
element or compound can include potassium that may be used from
about 500.degree. C. to 1000.degree. C., sodium from 500.degree. C.
to 1000.degree. C., and lithium from 900.degree. C. to 1700.degree.
C.
[0033] Because heat pipes carry heat most efficiently in an upward
direction, in this embodiment there are two sets 106 and 107 . The
element or compound within the lower pipes or tubes 106 is
preferably chosen to have a operating point above the melting
temperature of the metal or metal alloy storage material. The
element or compound within the upper pipes or tubes 107 is
preferably chosen to have a operating point below the meeting
temperatures of the metal or metal alloy storage material.
[0034] The hot gas passing through the lower channel heats the
lower end of the tubes and the element or compound in the tubes
vaporizes and moves upward and condense at the cooler end in the
storage material. When the storage material has melted and heat is
needed to run a turbine, the gas to be heated is pumped through the
upper channel. The upper heat pipes contain an element or compound
that has an operating temperature below that of the melting
temperature of the storage material. Therefore, when cooler air is
pumped through the upper channel, the element or compound in the
upper heat pipes condenses on the upper end transferring the heat
to the gas to operate the turbine. The heat transfer is controlled
by the flow of gases, moving upwards when heat is needed. There is
an added advantage to this heat pipe system because the upper and
lower channel may be at different pressures and the storage
material need not be in a pressure container. Thus the system can
take heat from air at ambient pressure, store the heat and
discharge the heat at a convenient pressure for gas turbine
operation.
[0035] The choice of the metal or alloy rod or tube is determined
by, for example, 1) the melting temperature, 2) latent heat of
fusion, 3) heat conductivity, 4) its viscosity and thermal
convection characteristics, 5) expansion and contraction upon phase
change, 5) chemical reactivity with containment and heat transfer
elements and 6) effects of contaminants. For any given application,
the melting temperature may be determined by the choice of metal,
or be more finely tuned by the selection of alloy. Other
considerations include crystallite size, effects of contaminates
and alloy separation during the solidifying or freezing and
re-melting. Another consideration is the price of the metal or
metal alloy in current metal markets and what its future price will
be at the decommissioning of the plant as this is likely to
represent a significant investment.
[0036] Pure non-alkali metals that may be used for thermal storage
include aluminum (m.p. 660.degree. C., I.h. 95 cal/gm), copper
(m.p. 1084.degree. C., I.h. 49 cal/gm), iron (m.p. 1536.degree. C.,
I.h. 65 cal/gm), and magnesium (m.p. 650.degree. C., I.h. 88
cal/gm) (m.p. =melting point, I.h.=latent heat). The other pure
metals have impractically high or low melting temperatures, are
rare, expensive, radioactive, or toxic. However, alloys of the
above mentioned and other metals form a very large class of
possible alternatives for thermal storage materials. One reason for
this is that two metals with differing melting temperatures often
form a eutectic mixture when melted together that has a lower
melting point than either metal by itself. Sometimes these effects
can significant lower the melting point in a range of materials
that could be useful for new metal alloy storage materials.
[0037] Another embodiment of the invention includes the specific
choice of aluminum and silicon as a thermal storage material.
Silicon is a common component of aluminum alloys; particularly at
the composition of AlSi12 (approximately 88% aluminum and 12%
silicon with a small amount of impurities such as iron). This is a
particularly advantageous combination of materials, because of the
physical properties resulting therein. While aluminum has a melting
point of about 660.degree. C., and silicon has a melting point of
1411.degree. C., the melting point at the eutectic mixture of
AlSi12 is about 600.degree. C. Thus, it can be seen that by varying
the composition, the melting point of the resulting alloy ranges
from 600.degree. C. at the eutectic point to 1411.degree. C. for a
pure Si composition. This is illustrated in FIG. 4 which depicts a
graph of melting temperatures vs. compositions. This is a very wide
and convenient range for high temperature latent heat storage
materials.
[0038] There is another beneficial advantage of this combination of
materials. While the latent heat of aluminum is relatively quite
high at 95 cal/gm compared to other metals, the latent heat of
fusion of silicon is amongst the highest known at 430 cal/gm. For
example, it can be seen from the figure that at approximately a
50-50 atomic percentages, the melting temperature of the mixture is
about 1000.degree. C. If a linear interpolation between the latent
heats of fusion of aluminum and silicon is used, the latent heat of
the resulting mixture is about 263 cal/gm. This may be compared to
value for sodium which has been used for a latent heat storage
medium at 27 cal/gm. (about 1/10th that of the mixture--requiring
10 times the storage mass). Other potential storage materials
include zinc with a latent heat of fusion of 27 cal/gm, copper at
49 cal/gm or lead of 5.5 cal/gm. Thus, it can be seen that there is
a very substantial reduction in required material in using the AlSi
combination.
[0039] Another advantage of the combination of silicon and aluminum
is the relatively low cost of these materials in the industrial
grades sufficient for this purpose compared to other metals with
suitable melting temperatures.
[0040] Yet another consideration is the selection of the
containment tubes. The size and shape of the tubes should be chosen
to maximize the heat transfer with the gas and optimize the melting
rates and patterns of the enclosed metal. In some circumstances
radial or axial fins can be added to improve heat transfer to the
tubes. High temperature ceramic materials are suitable because of
the high melting temperatures of the metals involved (600-1200'
C.). However, certain high temperature alloy tubes may be
considered for containment in the lower part of that temperature
range. Another choice of materials is graphite. Graphite has high
thermal conductivity and low reactivity with aluminum as discussed
by Simensen and is widely used in aluminum refining for electrodes
and containment materials. However, graphite may not be used in the
presence of oxidization gases such as air or carbon dioxide because
it will oxidize to carbon dioxide and fail as a containment or heat
transfer means. The graphite may be clad with metals or ceramics to
prevent its oxidation. The choice of the tube material should be
guided by the desired operating temperatures and potential
metal--containment tube interactions. The tubes may be closed or
open depending on the choice of gas and metals. If air is the heat
transfer medium the tubes should be closed to eliminate possible
oxidation or other reactions between the metal and the components
of the air. If helium, nitrogen or carbon dioxide is used the tubes
may be open at the top if there are no interactions between the
metal and gasses. For other gasses the potential interactions must
be taken into consideration.
[0041] To illustrate the operation of a liquid metal thermal
storage system embodiment of the invention in conjunction with a
heat source and turbine, an embodiment of the overall system is
illustrated in FIGS. 3a, 3b, and 3c. FIG. 3a illustrates the
components of the system without the heat storage system 111 being
connected or in the "pure solar" mode Air enters the turbo
compressor 112 and is compressed before arriving at the heat source
113. This may be a high temperature solar receiver heating a gas by
direct or indirect of absorption of sunlight or a non-solar high
temperature heat source. Further, the heat source 113 can be a
windowed high temperature solar receiver that uses small particles
to absorb concentrated sunlight 116 and heats the gas in which they
are entrained. An example of such a receiver is discussed in "Solar
Test Results of an Advanced Direct Absorption High Temperature Gas
Receiver (SPHER)," by
[0042] A. J. Hunt and C. T. Brown, Proc. of the 1983 Solar World
Congress, International Solar Energy Society, Perth, Australia,
Aug. 15-19, 1983, LBL-16947, and "Heat transfer in a directly
irradiated solar receiver/reactor for solid-gas reactions" by
Klein, H. H., Karni, J., Ben-Zvi, R. and Bertocchi, R. Solar Energy
81 (2007) 1227-1239. which are incorporated herein by reference.
After being heated to a high temperature the gas is routed into the
expansion turbine 114 that provides power to run the compressor and
turn the generator 115 before being exhausted or recycled. FIG. 3b
illustrates the arrangement for charging the storage wherein all
the gas is routed through the storage system before passing through
the expansion turbine. FIG. 3c illustrates operation of the system
in "hybrid" mode in which the gas is selectively routed both
through the storage and through the turbine, in parallel, adjusted
with the controlling valves 117 and 118. Valve 117 can divert
gasses directly to the solar receiver or heat source 113 (for the
operation of the embodiment of FIG. 3a) or directly to the heat
storage system 111 for the operation of the embodiment of FIG. 3b).
Valve 118 can divert gasses to the heat storage system 111 or to
the expansion turbine 114. Various positions of the valves 117 and
118 can allow the expansion turbine 114 to run directly on energy
provided by the receiver or heat source 113, or alternatively on
energy provided by the heat storage system 111, or both.
* * * * *