EP2547976A2

EP2547976A2 - Energy storage vessel, systems, and methods

Info

Publication number: EP2547976A2
Application number: EP11712371A
Authority: EP
Inventors: John P. Bell; Joseph M. Bell; Michael S. Bower; Gerard C. Walter
Original assignee: Bell Independent Power Corp
Current assignee: Bell Independent Power Corp
Priority date: 2010-03-16
Filing date: 2011-03-16
Publication date: 2013-01-23
Also published as: WO2011116040A3; US20110226780A1; WO2011116040A2

Abstract

A vessel for a thermal energy storage system comprising a bottom wall joined to a surrounding side wall, and an inner liner disposed within the bottom wall and side wall and comprising an inner liner bottom and an inner liner side wall. One aspect of the inner liner bottom and side wall is that they are configured to repeatedly expand and contract during the thermal cycling of the storage system. A thermal energy storage system comprising the containment vessel and an array of heat exchangers is also disclosed. The heat exchangers are disposed in the vessel, and arranged so as to enclose a volume within the vessel. Each of the heat exchangers is suspended by a suspension assembly. The assembly may be comprised of a central support hanger, a spring loaded upper hanger, and a lower hanger.

Description

ENERGY STORAGE VESSEL SYSTEMS, AND METHODS

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from U.S. provisional patent Application No. 61/314,313, filed March 16, 2010, the disclosure of which is incorporated herein by reference. This application is also related to the following copending commonly owned United States Patent applications: Application No. 12/033,604 of Geiken et al., filed February 19, 2008; Application No. 12/172,673 of Flynn et al., filed July 14, 2008; and Application No. 12/842,203 of Bell et al., filed July 23, 2010. The disclosures of these United States patent applications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The inventions disclosed herein relate generally to energy storage and, more particularly, to thermal energy storage systems and methods thereof. BACKGROUND ART

Worldwide, there are ever-growing demands for electricity due to increasing populations, technology advancements requiring the use of electricity, and the proliferation of such technology to more and more countries around the world. At the same time, there is an increasing push to harness reusable sources of energy to help meet these increasing electricity demands and offset and/or replace traditional carbon- based generators which continue to deplete natural resources around the world.

Many solutions have been developed to collect and take advantage of reusable sources of energy, such as solar cells, solar mirror arrays, and wind turbines. Solar cells produce direct current energy from sunlight using semiconductor technology. Solar mirror arrays focus sunlight on a receiver pipe containing a heat transfer fluid which absorbs the sun's radiant heat energy. This heated transfer fluid is then pumped to a turbine which heats water to produce steam, thereby driving the turbine and generating electricity. Wind turbines use one or more airfoils to transfer wind energy into rotational energy which spins a rotor coupled to an electric generator, thereby producing electricity when the wind is blowing. All three solutions produce electricity when their associated reusable power sources (sun or wind) are available, and many communities have benefited from these clean and reusable forms of power. Unfortunately, when the sun or wind is not available, such solutions are not producing any power. In the case of solar solutions, non-reusable energy solutions are often employed during nighttime hours. Similar issues arise for wind turbines during calm weather. Therefore, some form of energy storage is needed to store excess energy from the reusable power sources during power generation times to support energy demands when the renewable power source is unavailable or unable to meet peak demands for energy.

Solar mirror arrays generate and transfer heat as an inherent part of their operation. Solar cells and wind turbines which typically generate electricity can also selectively be used to drive heaters to generate heat and/or transfer heat from windings to a heat transfer fluid. Several solutions have been developed to store heat from these renewable energy sources for use in non-energy-generating times. In particular, one or more large vessels of a liquid may be used to temporarily contain the sensible heat contained by oil heated in a solar field, and delivered to the liquid in the vessels by heat exchangers disposed therein. Hot oil is delivered to heat exchangers in a vessel containing relatively cold liquid, and the liquid in contact with the heat exchangers is heated up. The liquid may initially be in solid form, with some portion of the energy transfer being in the latent heat of fusion in the solid to liquid transition.

During nighttime, or on overcast days when the solar field is down, the flow through the heat exchangers in the thermal energy storage vessel is directed to a steam generator which drives a turbine, which in turn is connected to an electrical generator. Heat is transferred from the heated liquid back into the oil in the heat exchangers, which is pumped to the steam turbine, to drive the turbine and generate electrical power.

One suitable energy storage medium is a molten inorganic salt, with a liquid phase temperature range from about 400 to 750 degrees Fahrenheit (°F). This temperature range is effective for providing high pressure steam to drive large scale steam turbines for electrical power generation. Additionally, inorganic salts typically have a substantially higher specific gravity than water or organic heat transfer oils, in the range of about two to about four times that of water. On a volumetric basis, a molten inorganic salt can contain more sensible heat than an equal volume of a typical organic heat transfer oil, and is thus preferred in this regard.

Thermal energy storage systems which use molten salt are known, such as the "Solar Two" system which was built near Barstow, California, and was operational from about 1994 to 1999. This system used molten salt comprised of a combination of about 60% sodium nitrate and about 40% potassium nitrate as an energy storage medium, which was circulated from a cold storage tank, through the solar field where it was heated, to a hot storage tank. A summary of a similar system is described and shown in FIG. 2 of the aforementioned copending United States patent application 12/033,604 of Geiken et al.

Another known thermal energy storage system is the single-tank thermocline energy storage system, which is also described in the aforementioned patent application 12/033,604 of Geiken et al., and shown in FIG. 3 thereof. The thermocline tank of this system contains a hot molten salt in the top portion thereof and a relatively cool molten salt in the bottom portion thereof. When the solar field 52 is active, a hot heat transfer fluid is pumped from the solar field to a heat exchanger. The relatively cool molten salt is pumped from the bottom of the thermocline tank out to the heat exchanger where it is heated by proximity to the hot heat transfer fluid from the solar field. The heated molten salt is then returned to the top of the thermocline tank. When the solar field is not active, the flow to and from the thermocline tank is reversed. Heated molten salt is pumped out of the top of the thermocline tank to the heat exchanger, where it transfers its heat to the heat transfer fluid. The heat transfer fluid is pumped to a steam turbine system for generating electricity. The molten salt which gave up some of its heat in the heat exchanger is then returned to the bottom of the thermocline tank. While this system takes advantage of a vertical temperature gradient within the thermocline tank to enable simplification to a single tank, challenges with respect to the pumping, valving, and delivery of molten salt through a complex network of piping to the solar field, and to the power generator remain present.

It is therefore preferable to have a thermal energy storage system in which the molten salt is simply contained within a vessel as the thermal storage medium, and not pumped throughout the overall system. Instead, the thermal energy is transferred from the solar field to the vessel of molten salt by the circulation of a heat transfer fluid, such as an organic heat transfer oil, from the solar field through one or more heat exchangers immersed in the molten salt in the vessel. In like manner, during nighttime or overcast days, the thermal energy is transferred from the vessel of molten salt to the power generator by the circulation of the heat transfer fluid therebetween. The handling of heat transfer fluid by standard pumps, valves and piping is much simpler than the equivalent handling of a molten inorganic salt, which is corrosive, and which is abrasive if some portion of solid phase salt (crystals) is present, and which can solidify in the pumps, valves, and piping at temperatures lower than 400°F, thus requiring a disruptive thawing process step to restart the system.

By using a heat transfer fluid to perform the energy transfer from the solar field to the thermal storage vessel, and from the vessel to the power generator, the problems to be addressed are simplified to those relating to containment of the molten salt, and heat transfer to and from the molten salt. This simplification notwithstanding, the problems of containing the molten salt, and transferring thermal energy to and from it are significant, particularly at the scale needed by a public utility for an economically viable thermal energy storage process. The main problems are summarized briefly as follows:

1 . The containment vessel is of economic necessity quite large, and the molten salt contained therein is quite dense, on the order of two to four times the density (or specific gravity) of water. Consequently, the structural loads on the vessel bottom and side wall are very high.

2. The molten salt within the vessel may reach temperatures of up to about 750°F.

Therefore, the containment vessel bottom and side walls must retain the required structural strength to contain the molten salt at temperatures much higher than the ambient environment.

3. The containment vessel undergoes repeated thermal expansion and contraction, and must be able to accommodate such extreme thermal cycling. During system fabrication, the vessel is constructed at substantially ambient temperature, and is then cycled up to as high as 750°F during startup and operation. The vessel is then cycled between its minimum operating temperature (about 400°F to about 500°F) to its maximum operating temperature of about 750°F on a daily basis when in operation. The vessel may also be occasionally cycled back down to ambient temperature of between about 0°F and about 90°F during process shutdowns. Given the requisite size of the containment vessel (on the order of 25 feet in diameter by 30 feet high in one embodiment), the dimensional changes of the vessel (and any insulation contained therein or applied thereto) over the full temperature range are significant, and must be accommodated. Accordingly, the repeated cycles of thermal expansion and contraction must occur without any structural failures of the vessel, which could result in leaks of the molten salt therefrom and/or collapse of the vessel. 4. The heat exchangers contained within the vessel must also be able to withstand the above-described thermal cycling, and must remain securely fixed within the vessel during such cycling.

5. Many environments where a thermal energy storage system may be located are seismically active. Accordingly, the containment vessel and heat exchangers must be able to withstand seismic events such as earthquakes without significant damage, and without any leakage of the molten salt.

6. Heat transfer rates are reduced when solid salt forms on the heat exchanger.

7. For greatest energy efficiency, it is desirable that the containment vessel of the thermal energy storage system has as much insulation as possible, thereby minimizing heat loss while still accommodating the expansion and contraction of the vessel during thermal cycling.

It is further noted that at minimum, for any thermal energy storage system that comprises a molten inorganic salt as a thermal storage medium, the above problems 1 - 4 must be simultaneously addressed or otherwise rendered inconsequential by the features and capabilities of the system. A system that further addresses problems 5, 6, and/or 7 will be further advantageous.

DISCLOSURE OF THE INVENTION

In accordance with the present disclosure, the problem of containing a large mass of molten salt, which undergoes repeated extreme thermal cycling in a thermal energy storage system is solved by providing a vessel comprising a bottom wall joined to a surrounding side wall, and an inner liner disposed within the bottom wall and side wall and comprising an inner liner bottom and an inner liner side wall. One aspect of the inner liner bottom and side wall is that they are configured to repeatedly expand and contract during the thermal cycling of the storage system in a manner that avoids stress concentrations within the liner, which could otherwise cause fractures and leaks in the liner. The liner bottom and side wall are both comprised of means for expanding radially outwardly when heated, and contracting radially inwardly when cooled, without producing stress concentrations.

The inner liner bottom may be comprised of a central plate surrounded by an array of radially arranged sector-shaped plates. Each of the sector-shaped plates may be joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector-shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint. The inner liner side wall is joined to the inner liner bottom and may be comprised of a plurality of panels. Each of the panels may be joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent panel by a lateral flexible joint. The vessel may be cylindrical, with the bottom wall of the vessel correspondingly being circular.

The inner liner contained within the vessel is constructed so as to be able to expand and contract under the loading thereof with the molten salt, and in particular, due to the thermal expansion and contraction caused by contact with the molten salt at temperatures up to about 750°F. During thermal cycling, the radial and inner flexible joints of the liner bottom and the lateral flexible joints of the liner side wall flex and accommodate the thermal expansion and contraction of the sector shaped plates and arcuate panels, so as to prevent localized stress concentrations in the liner. The central plate may be circular, or the central plate may be a polygon. The number of sides of the polygon may be equal to the number of sector-shaped plates. The number of radially arranged sector-shaped plates may be between three and twelve, or more, depending upon the size of the vessel. A flexible joint which joins a sector shaped plate at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent plate may be comprised of an arcuate shaped member formed and joined to the adjacent plates.

Depending upon the particular molten salt, the inner liner may be made of a material selected from, but not limited to, plain carbon steels; steels alloyed with copper, manganese, molybdenum, nickel, silicon, tungsten, titanium, vanadium and chromium, individually or in any combination thereof such as chromium-molybdenum, or nickel-chromium-molybdenum; and stainless steels. In one embodiment, the inner liner is made of 316 stainless steel.

The vessel may further include an outer liner comprising an outer liner bottom and an outer liner side wall joined to the outer perimeter of the outer liner bottom. The outer liner contains the inner liner within it, and is separated from the inner liner by thermal insulation.

The vessel may further include thermal insulation disposed between the bottom wall of the vessel and the inner liner bottom. The bottom thermal insulation may be comprised of a plurality of support members of a first insulating material interspersed within a second insulating material. The support members of the first insulating material provide structural support to the liner bottom, thereby enabling the second insulating material to have a higher R-value without needing to provide significant structural support. Hence the combination of the first and second insulating materials provided in this manner solves the problem of having insulation that has the required structural strength and the required high R-value insulating capability. In embodiments in which an outer liner is provided, the thermal insulation is disposed between the bottom wall of the vessel and the outer liner bottom.

The bottom wall of the vessel may be made of concrete, and preferably, steel- reinforced concrete. The bottom wall of the vessel may be formed upon a mud slab that is formed in the supporting ground. The side wall of the vessel may be comprised of a network of structural members. A plurality of insulating support members may be disposed between the network of structural members and the inner liner side wall. In embodiments in which an outer liner is provided, the insulating support members may be disposed between the network of structural members and the outer liner side wall. The vessel may further include thermal insulation covering the plurality of insulating support members and the network of structural members.

In accordance with the invention, a thermal energy storage system may include the instant vessel comprising a bottom wall joined to a surrounding side wall, and an inner liner disposed within the bottom wall and side wall as described above; a roof structure disposed on the surrounding side wall; and a heat exchanger assembly comprising a heat exchanger comprised of an upper region, a central region, and a lower region, a central support hanger connected to the central region of the heat exchanger and suspended from the roof structure, a spring loaded upper hanger connected to the upper region of the heat exchanger and suspended from the roof structure, and a lower hanger suspended from the central support hanger and connected to the lower region of the heat exchanger. The heat exchanger assembly may include an array of heat exchangers disposed in the vessel and arranged so as to enclose a volume within the vessel. Each of the heat exchangers may be comprised of an upper manifold connected to a lower manifold by a plurality of heat exchanger tubes. The heat exchangers of the array may be connected in series. The system may be further comprised of a mixer comprising a shaft and at least one impeller disposed within the volume enclosed by the array of heat exchangers.

The inner liner of the vessel and the heat exchanger suspension system of the thermal energy storage system may be separately useful in other applications involving high temperature material processing. Accordingly, there is provided an expandable liner for a vessel comprising a liner bottom comprising a central plate surrounded by an array of radially arranged sector-shaped plates, each of the sector-shaped plates joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector-shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint; and a liner side wall joined to the liner bottom and comprising a plurality of panels, each of the panels joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent panel by a lateral flexible joint.

There is also provided a heat exchanger assembly comprising a heat exchanger comprising an upper region, a central region, and a lower region, and a suspension system suspendable from a structure and connected to the heat exchanger. The suspension system may be comprised of a central support hanger proximate to the central region of the heat exchanger and suspendable from the structure, a spring loaded upper hanger connected to an upper region of the heat exchanger and suspendable from the structure, and a lower hanger suspended from the central support hanger and connected to a lower region of the heat exchanger. The heat exchanger may be comprised of an upper manifold connected to a lower manifold by a plurality of heat exchanger tubes.

In accordance with the invention, methods for using thermal energy are also provided. The methods may include storing the thermal energy received from a source and delivering the thermal energy to a power generating station. In one embodiment, a method comprises providing a thermal energy storage system as recited above, connecting at least one heat exchanger of the system to a thermal energy source, and delivering heated heat transfer fluid from the thermal energy source to the heat exchangers, thereby heating a thermal energy storage substance contained in the vessel. The method may further comprise connecting the heat exchanger to a power generating station and delivering heated heat transfer fluid from the heat exchanger to the power generating station.

In another embodiment, a method comprises providing a thermal energy storage system comprised of a vessel containing a heat exchanger, connecting the heat exchanger to a first thermal energy source, charging the vessel with a solid thermal energy storage substance, and delivering heated heat transfer fluid from the first thermal energy source to the heat exchanger, thereby heating and melting the thermal energy storage substance contained in the vessel. The charging of the vessel with a solid thermal energy storage substance and delivering heated heat transfer fluid from the first thermal energy source to the heat exchanger may be performed simultaneously. The first thermal energy source may be a solar array. The thermal energy storage substance may be an inorganic salt. The method may be further comprised of mixing the thermal energy storage substance. The method may be further comprised of connecting the heat exchanger to a second thermal energy source and delivering heated heat transfer fluid from the second thermal energy source to the heat exchanger. The second thermal energy source may be a solar array. BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be provided with reference to the following drawings, in which like numerals refer to like elements, and in which:

FIG. 1 is a schematic illustration of a thermal storage system and method in accordance with the present disclosure;

FIG. 2 is a perspective view of a containment vessel and thermal oil delivery unit of the thermal storage system, with the vessel shown enclosed in thermal insulation;

FIG. 3 is a perspective view of the vessel of FIG. 2 shown with the thermal insulation removed;

FIG. 4 is a perspective cutaway view of the containment vessel of FIG. 3, showing the heat exchanger array contained therein;

FIG. 5 is a side elevation cross-sectional view of a thermal storage system, including a containment vessel, heat exchangers, and a mixer, taken through a vertical plane passing through the center of the vessel;

FIG. 6 is a plan view of the containment vessel of FIG 2, with the roof support structure, top cover, and top insulation removed;

FIG. 7 is a detailed cross sectional single-plane elevation view of a central plate of the liner bottom, taken along the line 7 - 7 of FIG. 6;

FIG. 8 is a detailed cross sectional elevation view of a lateral flexible joint between arcuate panels of the inner liner side wall, taken at the location indicated in FIG. 6; FIG. 9 is a detailed cross-sectional plan view of the vessel liners and insulation at a structural column of the vessel, taken at the location indicated in FIG. 6, along line 9 - 9 of FIG. 1 1 ;

FIG. 10 is a plan view of the bottom of the containment vessel with the liner bottom removed, showing the locations of a plurality of insulating support members;

FIG. 1 1 is a side elevation cross-sectional view of a containment vessel, including vessel liners, insulation, and a vessel bottom foundation;

FIG. 12A is a detailed cross-sectional elevation view of the junction of the vessel bottom and side wall, taken at the location indicated in FIG. 1 1 ;

FIG. 12B is a detailed cross-sectional elevation view of insulation of the side wall of the vessel at the junction of a structural column and ring channel, taken at the location indicated in FIG. 1 1 ;

FIG. 12C is a detailed cross-sectional elevation view of insulation of the side wall of the vessel at a ring channel only;

FIG. 12D is a detailed cross-sectional elevation view of the junction of the vessel top and side wall, taken at the location indicated in FIG. 1 1 ;

FIG. 13 is a perspective view of a heat exchanger array of the thermal energy storage system;

FIG. 14A is a top view of the thermal storage system of FIG. 5 shown with the top cover and roof structure removed, and including the heat exchanger array of FIG. 13, and a mixer;

FIG. 14B is a top cross sectional view of the thermal storage system of FIG. 14A, taken approximately halfway down the heat exchanger of FIG. 14A;

FIG. 15 is a detailed cross-sectional side elevation view of the heat exchanger array of the thermal energy storage system of FIG. 5, including a suspension system for supporting the array;

FIG. 16A is a side elevation view of a suspension system for a heat exchanger;

FIG. 16B is a detailed side elevation view of a spring loaded suspension member of the suspension system of FIG. 16A; and

FIG. 17 is a flow chart depicting a method of storing and releasing thermal energy using the applicants' thermal energy storage system.

The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

BEST MODE FOR CARRYING OUT THE INVENTION

For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements.

FIG. 1 is a schematic illustration of a thermal storage system and method in accordance with the present disclosure. The thermal energy storage system 100 is comprised of a containment vessel 101 , which contains a heat exchanger or an array 400 of heat exchangers immersed in a molten salt. The system 100 is further comprised of a liquid transfer unit 490 (that may be of a modular design), which circulates a heat transfer fluid through the heat exchangers during operation of the system 100. The liquid transfer unit 490 is comprised of liquid piping, various switching and control valves, a pump 492, and a pressure relief tank (not shown). The liquid transfer unit 490 may further include a heater loop 494 comprising an oil heater 496 and an expansion tank (not shown) for heating and circulating heat transfer fluid through the heat exchanger array 400 when the system 100 is not online.

In daytime operation of the system 100, the control valves of the liquid transfer unit 490 are positioned such that heat transfer oil is recirculated by the pump 492 from the solar field 10, through the heat exchanger array 400 in the vessel 101 , and back out to the solar field 10. In the solar field 10, the heat transfer fluid passes through piping in the solar collectors, which collect and concentrate the solar radiation by suitable means such as mirrors and/or lenses onto the piping. The heat transfer fluid passing therethrough is heated to a high temperature of as much as 800°F. This hot heat transfer fluid is then pumped through the heat exchanger array 400 in the containment vessel 101 . Heat from the hot heat transfer fluid is transferred into the molten salt contained in the vessel 101 , and the heat transfer oil is cooled. The relatively cold heat transfer oil is recirculated back to the solar field 10 for reheating. During a startup condition, the solid salt in the vessel is heated and melted into a liquid (molten) state, and is then further heated up beyond its melting point. The molten salt thus contains energy in the form of the latent heat of fusion of the salt, as well as the sensible heat of the heated liquid. It is also noted that during the delivery of hot oil from the solar field 10 to the system 100, some portion of the hot oil may be routed directly to a power generating station 20 by pumps, piping, and valving (not shown) for driving the power generating station 20.

On overcast days and during nighttime, this sensible and latent heat is available to be transferred back into the heat transfer oil, and delivered to an electrical power generating station or "power block" 20. During this portion of the operation, the control valves of the liquid transfer unit 490 are positioned such that heat transfer oil is recirculated by the pump 492 from the power block 20, through the heat exchanger array 400 in the vessel 101 , and back out to the power block 20. In the power block 20, the heat transfer fluid passes through a steam generator, which produces high pressure steam that is used to drive a turbine, which drives an electrical generator. The heat transfer fluid passing therethrough is cooled substantially as its thermal energy is transferred to produce steam. This relatively cold heat transfer fluid is then pumped through the heat exchanger array 400 in the containment vessel 101 , where it is reheated and delivered back to the power block 20. This cycle may continue, with the thermal energy contained in the molten salt that was transferred from the solar field being used to generate electrical power during nighttime or overcast days, for as long as the molten salt contains enough thermal energy to heat the heat transfer fluid to a temperature suitable for producing steam in the steam turbines of the power block 20.

The salt used for thermal energy storage may be a salt mixture, or a eutectic salt mixture. Suitable salts include, without limitation, lithium nitrate, potassium nitrate, sodium nitrate, sodium nitrite, calcium nitrate, lithium carbonate, potassium carbonate, sodium carbonate, rubidium carbonate, magnesium carbonate, lithium hydroxide, lithium fluoride, beryllium fluoride, potassium fluoride, sodium fluoride, calcium sulfate, barium sulfate, lithium sulfate, lithium chloride, potassium chloride, sodium chloride, iron chloride, tin chloride, and zinc chloride, and mixtures and solutions thereof.

A thermal energy storage system containing a molten salt will be particularly useful in the commercial generation of electrical power if it can contain a minimum of 3,400 MMBTU (million British Thermal Units) for transfer to the power block 20 during nighttime or overcast days. Such a system would have the capacity to maintain generating operation of a 50 megawatt power block for about 6 hours.

This need for large thermal capacity in a thermal energy storage system results in the previously recited problems of vessel size, related structural loads, high operating temperatures, and repeated thermal cycling. These problems are solved in the thermal energy storage system, containment vessel, heat exchanger array, and related suspension system.

The containment vessel is comprised of a bottom wall joined to a surrounding containment side wall, and an expandable liner disposed within the bottom wall and side wall and comprising a liner bottom and a liner side wall. The expandable liner bottom may be comprised of a central plate surrounded by an array of radially arranged sector-shaped plates. Each of the sector-shaped plates may be joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector- shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint. The liner side wall is joined to the outer perimeter of the liner bottom and may be comprised of a plurality of arcuate panels. Each of the panels may be joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent arcuate panel by a lateral flexible joint. An array of heat exchangers is contained within the vessel. The heat exchangers are suspended from a roof structure by a suspension system configured to accommodate the thermal expansion of the heat exchangers resulting from the large temperature fluctuations that occur during the thermal process. The vessel may further include a second, outer liner comprising an outer liner bottom and an outer liner side wall joined to the outer perimeter of the outer liner bottom. In such an embodiment, the outer liner contains the inner liner within it, and is separated from the inner liner by thermal insulation.

In the following disclosure, the containment vessel, liners, and thermal insulation of the thermal energy storage system 100 will be described first, followed by descriptions of the heat exchanger array and mixer. The respective relationships between these components and sub-assemblies to form the overall thermal storage system 100, and methods for making and assembling the system 100, vessel, liners, and heat exchanger array will also be provided.

Turning first to FIG. 2, the thermal energy system 100 is shown in perspective view, with the containment vessel covered in insulation. The system 100 is comprised of the containment vessel 101 and a liquid transfer unit 490. Referring also to FIGS. 3 - 5, only the vessel 101 is shown, without the liquid transfer unit 490. (Additionally, in FIGS. 3 - 4, the thermal insulation on the vessel side wall and roof is not shown.) In the embodiment depicted in FIGS. 3 - 5, the containment vessel 101 is cylindrical, with the bottom wall of the vessel correspondingly being circular, although other vessel shapes may be employed. The vessel 101 is comprised of a bottom wall 1 10 joined to a surrounding containment side wall 120. The bottom wall 1 10 may be made of concrete, and preferably, steel-reinforced concrete comprised of steel rebar (not shown) embedded in the concrete. The bottom wall 1 10 of the vessel 101 may be formed at a constant thickness as shown in FIG. 5, or it may be formed so as to decrease in thickness from the central region thereof to the perimeter region thereof as shown in FIG. 3 of the aforementioned U.S. Provisional Application No. 61/228,351 .

The bottom wall 1 10 may be formed upon a mud slab 1 12, which in turn is formed in the ground. The mud slab 1 12 may be comprised of an engineered fill. For simplicity of illustration, the outer edge 1 16 of the mud slab 1 12 is shown as being terminated at the same location as the outer edge 1 14 of the bottom wall 1 10 in FIGS. 2 - 4; however, it is to be understood that the mud slab 1 12 may extend beyond the edge 1 14 of the bottom wall 1 10. Additionally, the ground 2 (soil, rocks, etc.) would be backfilled against the edges of the mud slab 1 12 and bottom wall 1 10 after construction thereof as shown in FIG. 1 1 , but such ground 2 is also not shown in FIGS. 2 - 4.

The containment side wall 120 may be comprised of a network of structural members. The structural members may be structural steel members, such as steel I- beams and steel channels. In the embodiment depicted in FIGS. 3 and 4, the network of steel members is comprised of a plurality of vertical I-beams 121 A - 121 H, and a plurality of arcuate channels 131 A - 131 E, 132A - 132E, 133A - 133H (see also FIG. 6), and additional arcuate channels on the back side of the vessel 101 not shown. The arcuate channels are disposed between pairs of vertical I-beams and joined to the I- beams by suitable means such as welding. The arcuate channels thus form a set of radially constraining rings 131 - 133 that serve to support and prevent radial expansion of the vessel liner 200 contained therein. For added rigidity, the vertical I-beams 121 A - 121 H may be anchored or embedded in the concrete slab bottom wall 1 10. Alternatively, referring to FIG. 12A, a thick structural ring 1 18, such as a steel ring may be embedded in the concrete or secured to the concrete vessel bottom 1 1 by anchor bolts 124 and 126 and nuts 125 and 127, with the vertical I-beams 121 A - 121 H joined thereto by suitable means such as welding. Additionally, a network 140 of roof structural members is joined to the upper ends of the vertical I-beams 121 A - 121 H. The roof network 140 serves to join the I-beams 121 A - 121 H together, as well as to provide a structure for suspension of the heat exchanger array 400, as will be described subsequently herein. The roof structure 140 may further include a bottom sheet covering 142 which provides a protective barrier to prevent personnel from falling into the containment vessel during inspection and service, and also provides a structural support, upon which thermal insulation 330 (FIG. 1 1 ) can be disposed, to further conserve the heat energy contained within the vessel 101 . The roof structure 140 may be further provided with a salt fill port (not shown), which extends downwardly through the bottom sheet 142. The salt fill port may be provided with a removable cap (not shown). In that manner, the vessel 101 can be filled with granular solid salt through the fill port at the time of startup of the thermal storage system 100. One aspect of the roof structure 140 is that it may be of modular construction.

In another embodiment depicted in FIGS. 5, 14A, and 14B, a containment vessel 102 of a smaller size is provided in which the network of steel members is comprised of vertical I-beams 122A - 122D, and a plurality of upper arcuate channels 134A - 134D, as well as additional lower arcuate channels not shown, beneath channels 134A - 134D. A network 141 of roof structural members may be joined to the upper ends of the vertical I-beams 121 A - 121 H for added rigidity and for support of a heat exchanger array 401 .

In order to contain the liquid molten salt within the containment vessel 101 , a liner 200 is provided within the bottom wall 1 10 and containment side wall 120. The liner 200 is comprised of a liner bottom 201 and a liner side wall 230, both of which may be configured to repeatedly expand and contract during the thermal cycling of the storage system 100. Referring again to FIGS. 3 - 4, and also to FIG. 6, the liner bottom 201 may be comprised of a central plate 204, surrounded by an array of radially arranged sector-shaped plates. The central plate 204 may be circular, or the central plate may be a polygon. The number of sides of the polygon may be equal to the number of sector-shaped plates, such that the linear inner perimeter portions of the sector-shaped plates are contiguous with the sides of the polygon.

The number of radially arranged sector-shaped plates may be between three and twelve, or more, depending upon the size of the vessel. In the embodiment depicted in FIG. 6, the liner bottom 201 is comprised of eight sector-shaped plates 202A - 202H, and a circular central plate 204. In the alternative embodiment depicted in FIGS. 14A and 14B, the liner 250 of the vessel 102 is comprised of a liner bottom 251 comprised of four sector-shaped plates 252A - 252D, and a circular central plate 254. Referring to FIGS. 4 and 6, the sector-shaped plates 202A - 202H may be joined at radial portions of their perimeters to radial portions of the perimeters of adjacent sector-shaped plates by respective radial flexible joints 206A - 206H. The sector shaped plates 202A - 202H may also be joined at the inner portions of their perimeters (e.g., 208A and 208D) to the central plate 204 by an inner flexible joint 210. Referring to FIG. 7, the inner flexible joint 210 may be formed as an angular riser which transitions upwardly from the plane of a sector shaped plate (e.g. plate 202C or plate 202G) to the plane of the central plate 204. In like manner, in the alternative embodiment depicted in FIGS. 14A and 14B, the four sector-shaped plates 252A - 252H are joined at radial portions of their perimeters to radial portions of the perimeters of adjacent sector-shaped plates by respective radial flexible joints 256A - 256D. The sector shaped plates 252A - 252D are also joined at the inner portions of their perimeters to the central plate 254 by an inner flexible joint 258.

Referring again to FIGS. 4 and 6, the liner side wall 230 is joined to the outer perimeter of the inner liner bottom 201 . The liner side wall 230 may be comprised of a plurality of arcuate panels 232A - 232H. The panels 232A - 232H may be joined to form the side wall 230, wherein lateral portions of their perimeters are joined to the lateral portions of the perimeters of adjacent arcuate panels by respective lateral flexible joints 234A - 234H. In like manner, in the alternative embodiment depicted in FIGS. 14A and 14B, the four arcuate panels 282A - 282D of the liner side wall 280 are joined at lateral portions of their perimeters to lateral portions of the perimeters of adjacent arcuate panels by respective lateral flexible joints 284A - 284D.

The inner flexible joint 210, radial flexible joints 206A - 206H, and lateral flexible joints 234A - 234H are means for the liner to expand radially outwardly when heated, and contract radially inwardly when cooled, without producing stress concentrations. The radially arranged sector-shaped plates of the liner bottom 201 and the panels of the liner side wall 230 may be provided in substantially greater numbers than shown, with corresponding flexible joints between them, to the point where the liner side wall 230 and liner bottom 201 are provided essentially as corrugated structures.

Depending upon the particular molten salt, the liner 200 may be made of a material selected from, but not limited to, plain carbon steels; steels alloyed with copper, manganese, molybdenum, nickel, silicon, tungsten, titanium, vanadium and chromium, individually or in any combination thereof such as chromium-molybdenum, or nickel-chromium-molybdenum; and stainless steels. In one embodiment, the liner is made of 316 stainless steel.

The containment vessel may further include a second, outer liner comprising an outer liner bottom and an outer liner side wall joined to the outer perimeter of the outer liner bottom. The outer liner contains the first, inner liner within it, and is separated from the inner liner by thermal insulation. The outer liner serves to provide secondary containment of the molten salt in the event that the inner liner develops a leak and allows molten salt to pass through it.

FIG. 1 1 is a side elevation cross-sectional view of a containment vessel, including vessel liners and insulation. FIGS. 12A - 12D are detailed cross sectional views at various locations of the vessel wall and bottom, as indicated in FIG. 1 1 . For the sake of simplicity of illustration, the network of roof structural members is not shown in FIGS. 1 1 and 12D. The second, outer liner 260 is comprised of a liner bottom 261 and a liner side wall 270. Solid, substantially incompressible solid thermal insulation material 301 is disposed between the inner liner bottom 201 and the outer liner bottom 261 . In like manner, solid insulation material 31 1 is disposed between the inner liner wall 230 and the outer liner side wall 270. The solid insulation material 301 and 31 1 serves to keep the inner liner 200 and the outer liner 260 separated by a substantially constant distance while also limiting the rate of heat transfer therebetween.

The outer liner 260 may be provided with flexible joints in the bottom wall 261 and side wall 270 in a manner similar to that described for the inner liner 200. Referring to FIG. 8, a first outer arcuate panel 272B is joined along its lateral perimeter to the lateral perimeter of a second arcuate panel 272C by a flexible joint 274B. The flexible joint 274B of the outer side wall 270 follows substantially the same contour as the flexible joint 234B of the inner side wall, and is thus separated from the inner flexible joint 234B by a distance that is approximately the thickness of the solid thermal insulation 31 1 . In the event that the solid thermal insulation is not easily formed or flexible, a different insulating material 312, such as a ceramic blanket insulation may be disposed between the inner flexible joint 234B and the outer flexible joint 274B.

Although not shown, corresponding flexible joints may be provided in the outer liner bottom 261 , which follow the respective contours of the flexible joints 206 of the inner liner bottom 201 . In such an embodiment, the outer liner bottom 261 is comprised of a plurality of sector-shaped plates having substantially the same shape as the sector shaped plates 202A - 202H of the inner liner bottom 201 . Additionally, referring to FIG. 7, the outer liner bottom 261 may be comprised of a central plate 264, and a flexible joint 266, which may be formed as an angular riser which transitions upwardly from the plane of an outer sector shaped plate of the outer liner bottom 261 to the plane of the outer central plate 264. In that manner, separation is maintained between the inner central plate 204 and the outer central plate 264. Solid thermal insulation 301 may be disposed between the plates 204 and 264, with a flexible batt- type or ceramic blanket insulation 302 disposed between the flexible joints 210 and 266. The volume 209 directly beneath the plates 204 and 264 may also be filled with batt or ceramic blanket insulation.

Referring to FIGS. 5, 14A, and 14B, and in the alternative embodiment depicted therein, the containment vessel 102 may be further comprised of an outer liner 290 comprising a liner bottom 291 and a liner side wall 295. Referring also to FIGS. 14A and 14B, the outer liner 295 may be provided with lateral flexible joints that follow the contour of the lateral flexible joints 284A - 284D of the inner liner 280. In like manner, the radial flexible joints (not shown) of the bottom 291 of the outer liner 290 follow the contour of the radial flexible joints 256A - 256D of the inner liner 280. Solid, substantially incompressible solid thermal insulation material 302 is disposed between the inner liner bottom 251 and the outer liner bottom 291 . In like manner, solid insulation material 313 is disposed between the inner liner wall 280 and the outer liner side wall 295.

The containment vessel may further include thermal insulation disposed between the bottom wall of the vessel and the inner liner bottom. In embodiments in which an outer liner is provided, the thermal insulation is disposed between the bottom wall of the vessel and the outer liner bottom. The bottom thermal insulation may be comprised of a plurality of support members of a first insulating material interspersed within a second insulating material. This is best understood with reference to FIGS. 4, 10, 1 1 , and 12A. A plurality of insulating support members 320 are distributed upon the bottom wall 1 10 of the vessel 101 with a substantially even spacing. A second, higher R-value insulation 322 is disposed in the volume between the outer liner bottom 261 and the vessel bottom 1 10. The insulating support members 320 of the first insulating material provide structural support to the liner bottom 261 , thereby enabling the second insulating material 322 to be selected from a higher R-value material, without the need for the second material to provide significant structural support. The side wall of the liner (or liners, if an inner and outer liner are provided) may be supported by the network of structural members of the containment vessel so as to prevent expansion and contraction of the liner(s) during thermal cycling. However, it is preferable that the liner(s) not be in direct contact with the structural members; because the structural members are made of steel or another high thermal conductivity material, any such contact would cause a substantial heat loss through the vessel side wall 120. To address this problem, a plurality of insulating support members may be disposed between the network of structural members and the inner liner side wall. In embodiments in which an outer liner is provided, the insulating support members may be disposed between the network of structural members and the outer liner side wall. The vessel may further include a second thermal insulation material covering the plurality of insulating support members, the network of structural members, and the portions of the exterior of the liner wall that are not covered by the insulating support members. In a manner similar to the thermal insulation of the bottom of the vessel liner, the insulating support members of the first insulating material provide structural support to the liner side wall, thereby enabling the second insulating material to be selected from a higher R-value material, without the need for the second material to provide significant structural support.

This is best understood with reference to FIGS. 1 1 - 12D. Referring first to FIG. 1 1 , the network of structural members may comprise a plurality of vertical I-beams surrounding the vessel liner(s), including I-beams 121 C and 121 G. (See also FIG. 4.) FIGS. 12A, 12B, and 12D depict detailed cross-sectional elevation views of the vertical I-beam 121 C, the inner and outer liner side walls 230 and 270, and insulating support members disposed between vertical I-beam 121 C and the outer liner side wall 270, for the bottom, middle, and top regions of the vessel 101 , respectively. For the sake of simplicity of illustration, the second non-supporting wall thermal insulation is not shown covering I-beam 121 C in FIGS 12A, 12B, and 12D.

Referring now to FIGS. 12A and 12B, a first solid insulation member 304A is disposed between the lower portion of vertical I-beam 121 G and the outer liner wall 270. The solid insulation member 304A extends from the lower arcuate channel 131 B to the middle arcuate channel 132B. Referring to FIGS. 12B and 12D, a second solid insulation member 304B is disposed between the upper portion of vertical I-beam 121 G and the outer liner wall 270. The solid insulation member 304B extends from the middle arcuate channel 132B to the upper arcuate channel 133B. Referring also to FIG. 9, which is taken along line 9 - 9 of FIG. 1 1 , but oriented as indicated in FIG. 6, the solid insulating members such as insulation member 304B may be formed to have the same width as the corresponding I-beam 121 G. In like manner, first and second solid insulation members may be disposed between the outer liner wall 270, and the other vertical I-beams 121 A - 121 G and 121 H.

Each of the lower and middle arcuate channel sections (e.g. sections 131A and 132A of FIG. 4) and the upper arcuate channel sections 133A - 133H are provide with solid insulation members disposed between them and the outer liner wall 270. By way of illustration, referring to FIG. 12A, a solid arcuate insulation member 306A is disposed between arcuate channel section 131 B and the outer liner side wall 270; referring to FIG. 12B, a solid arcuate insulation member 306B is disposed between arcuate channel section 132B and the outer liner side wall 270; and referring to FIG. 12C, a solid arcuate insulation member 306C is disposed between arcuate channel section 133B and the outer liner side wall 270. It can be seen in FIGS. 12A, 12C, and 12D, and in FIG. 12B in particular, the height of the solid arcuate insulation member 306B may be made to be the same as the height of the side of the corresponding arcuate channel section 132B.

Referring again to FIGS. 1 1 and 12A - 12D, a second thermal insulation material 324 is provided, which covers the plurality of insulating support members, the network of structural members, and the portions of the exterior of the liner wall that are not covered by the insulating support members. As can be seen particularly in FIG. 12C, in regions of the outer liner side wall 270 that are not adjacent to any of the I- beams or arcuate channel sections, such as regions 276, the second thermal insulation material 324 is disposed directly onto such surfaces. In that manner, with the second thermal insulation material 324 being of a higher R-value than that of the solid insulation members in contact with the network of structural members, heat loss from the containment vessel 101 is minimized.

A wire mesh 326, or other supporting material may be attached to the network of structural members to provide support to the second thermal insulation material 324 during fabrication of the vessel 101 and operation of the thermal storage system 100. The second thermal insulation material 324 may be covered with a layer of waterproof and weather-resistant protective material 328 to keep the insulation material 324 dry, thereby maintaining its high R-value. The liners within the vessel, and the inner liner in particular are constructed so as to be able to expand and contract under the loading thereof with the molten salt, and in particular, due to the thermal expansion and contraction caused by contact with the molten salt at temperatures up to about 750°F. During thermal cycling, the radial and inner flexible joints of the liner bottom and the lateral flexible joints of the liner side wall flex and accommodate the thermal expansion and contraction of the sector shaped plates and arcuate panels. Having described the construction of the vessel walls, liners, and insulation, the manner in which the vessel liners expand and contract during thermal cycling will now be explained.

The network of structural members, and the solid substantially incompressible thermal insulation members disposed between the structural members and the liner side wall serve to dimensionally constrain the liner side wall and prevent substantial radial thermal expansion of the vessel liner(s) during the heating portion of thermal cycling. Accordingly, each of the sector-shaped plates 202A - 202H of the inner liner bottom 201 is prevented from significant expansion radially outwardly along its outer arcuate perimeter portion. Thus during heating, each sector-shaped plate is free to expand along the radial portions of its perimeter, and radially inwardly at its inner perimeter portion. Along the radial portions of the perimeters of the sector-shaped plates, a radial flexible joint accommodates the circumferential or angular thermal expansion by flexing. At the inner perimeter portions of the sector-shaped plates, the flexible joint 210 which borders the central circular plate 204 accommodates the radially inward thermal expansion by flexing as well.

By way of illustration, referring to FIGS. 6 and 8, the circumferential thermal expansion of sector-shaped plates 202B and 202C is indicated by arrows 212. The radial flexible joint 206B flexes to accommodate this expansion, with its bend radius increasing as a result. The radially inward thermal expansion of sector-shaped plate is indicated by arrow 214. Referring also to FIG. 7, the flexible joint 210 bordering the central circular plate 204 accommodates this radially inward thermal expansion by flexing radially inwardly, as indicated by arrows 216.

In a similar manner, the thermal expansion of the arcuate panels 232A - 232H is accommodated by the lateral expansion joints provided between them. Again by way of illustration, referring to FIGS. 6 and 8, the lateral (circumferential) thermal expansion of arcuate panels 232B and 232C is indicated by arrows 236. The lateral flexible joint 234B flexes to accommodate this expansion as indicated by arrows 238 and 239, with its bend radius increasing as a result.

Referring also to FIG. 12D, the arcuate panels 232A - 232H of the liner wall 230 are terminated at the upper ends 231 thereof below the supporting plate 142 of a vessel cover 240. In that manner, the side wall 230 of the vessel is free to expand and contract vertically during thermal cycling as indicated by bidirectional arrow 233.

It will be apparent that the thermal expansion of the outer liner 260, constructed similarly with sector-shaped plates in the bottom 261 thereof, and arcuate panels in the side wall 270 thereof, is accommodated in a similar manner by its respective radial and lateral flexible joints.

Referring back to FIG. 1 , the thermal energy storage system may include an array of heat exchangers. The array of heat exchangers are disposed in the vessel and arranged so as to enclose a volume within the vessel. Each of the heat exchangers may be as shown for heat exchanger 402 of FIG. 4, comprising an upper manifold 403 connected to a lower manifold 404 by a plurality of heat exchanger tubes 405. The tubes may also include fins (not shown) to increase the rate of heat transfer between the heat exchanger and the surrounding environment. The heat exchangers may be constructed of a variety of materials, for example, but not limited to plain carbon steels; steels alloyed with copper, manganese, molybdenum, nickel, silicon, tungsten, titanium, vanadium and chromium, individually or in any combination thereof such as chromium-molybdenum, or nickel-chromium-molybdenum; and stainless steels.

An exemplary heat exchanger array and heat exchanger suspension system will now be described with reference to FIGS. 5 and 13 - 16B. Referring first to FIGS. 5 and 13 - 14B, a heat exchanger array 401 is suspended in the containment vessel 102. The heat exchangers are arranged so as to enclose a volume within them and within the vessel. In the embodiment depicted in FIGS. 13 - 14B, the heat exchanger array 401 is comprised of four heat exchangers 412, 414, 416, and 418 arranged in a square pattern, and enclosing a rectangular volume. Each of the heat exchangers 412 - 418 is comprised of an upper manifold connected to a lower manifold by a plurality of heat exchanger tubes 41 1 , the tubes 41 1 also optionally including fins 409. In an alternative embodiment, a single heat exchanger may be provided comprising upper and lower manifolds and connecting tubes therebetween, formed so as to enclose a volume within the heat exchanger. The heat exchangers of the array may be connected in parallel or in series. In the embodiment depicted in FIGS. 13 - 14B, the heat exchangers 412 - 418 are connected in series, with the lower manifolds 415 and 417 of heat exchangers 414 and 416 connected to each other, and the lower manifolds 413 and 419 of heat exchangers 412 and 418 connected to each other. In that manner, the flow of heat transfer fluid within successive heat exchangers around the array is in vertically opposed directions, as indicated by the flow arrows provided in FIG. 13. Other heat exchanger connection arrangements may be suitable.

Referring to FIG. 5, the thermal energy storage system may be further comprised of a roof structure 141 disposed on the top of the containment side wall 123, wherein each of the heat exchangers 412 - 418 is suspended from the roof structure 141 .

FIG. 15 is a detailed view of the heat exchanger array 401 contained within the vessel 102 of FIG. 5, depicting suspension systems 420 for suspending the heat exchangers 412 - 418 within the vessel 102. FIG. 16A is a detailed view of the suspension system suspending one of the heat exchangers 416. (For the sake of simplicity of illustration, only the upper manifold 408, a central vertical tube 407 and fins 409, and the lower manifold 417 of heat exchanger 416 are shown. Additional vertical tubes 41 1 parallel to the central tube 407 are not shown.)

The suspension system 420 is comprised of a central support hanger 422 suspended from the roof structure 141 proximate to the central region 441 of the heat exchanger 416, a spring loaded upper hanger 424 suspended from the roof structure 141 and connected to an upper region 443 of the heat exchanger 416, and a lower hanger 426 suspended from the central support hanger 422 and connected to a lower region 445 of the heat exchanger 416. The central support hanger 422 may be comprised of a horizontal support member 428 that is suspended from the roof structure 141 by threaded rods 429 and nuts 430. The lower hanger 426 may be comprised of a threaded rod 427 secured to the horizontal support member 428 by nuts 421 .

Referring also to FIG. 16B, the spring loaded upper hanger 424 may be comprised of a container 431 holding a spring 432 disposed on the container bottom 433. A suspension member 434 such as a threaded rod extends upwardly through the container bottom, through the spring 432, and through a retainer plate 435 disposed on the top of the spring 432. A nut 436 on the threaded end of the suspension member 434 secures the retainer plate 435 to the top of the spring 432. The suspension member 434 extends downwardly and is joined to the upper region 443 of the heat exchanger 416. The container 431 is provided with a top lug 437, which may be engaged with an eyebolt 438 that is joined to the roof structure 141 .

In the embodiment depicted in FIGS. 15 and 16A, in which the heat exchanger 416 is comprised of an upper manifold 408 connected to a lower manifold 417 by vertical tubes 407, the spring loaded upper hanger 424 may be connected to an upper pipe hanger 425 which supports the upper manifold 408, and the lower hanger 426 may be connected to a lower pipe hanger 423 which supports the lower manifold 417. The heat exchanger 416 may further include a tube guide plate 447, and the horizontal support member 428 may be connected to the guide plate 447.

When the heat exchanger array 401 is installed in the containment vessel 102, each of the heat exchangers 412 - 418 is provided with a support system 420. During installation, when the heat exchanger 416 is first suspended from the central and lower support hangers 422 and 426, all of the weight of the heat exchanger is supported by those hangers. But when the spring loaded upper hanger 424 is added to the support system 420, and the nut 436 is turned down on the support rod 434, then the spring 432 begins to compress, and an increasing amount of the weight of the heat exchanger 416 becomes carried by spring loaded upper hanger 424. The nut 436 may be turned down sufficiently such that approximately half, or slightly more than half of the weight of the heat exchanger 416 is borne by the spring loaded upper hanger 424, with the remainder of the weight being borne by the central and lower support hangers 422 and 426. The container 431 of the spring loaded upper hanger 424 may be provided with indicia 439 of a compression scale of the spring 432, and the retainer plate 435 may be provided with a pointer 440 proximate to the indicia, so that adjustment of the spring 432 can be made quantitatively. In that manner, when the weights of the heat exchangers 412 - 418, spring constant of the springs 432, and dimensions of the heat exchangers 412 - 418 and suspension system 420 are known, the adjustments of spring compression can be made according to the indicia, without the need for individual tension measurements of the support hangers 422, 424, and 426.

In operation of the thermal energy storage system 100, when the molten salt is thermally cycled, the heat exchanger suspension systems 420 provide for expansion of the heat exchangers in both vertical directions, and also prevent stress of the heat exchangers due to any difference in thermal expansion coefficients of the heat exchanger tubes and the hanger rods 427, 429, and 434. Some of the thermal expansion of the heat exchangers 412 - 418 will be in downward from the horizontal support member 428, and some of the expansion will be in upward from the horizontal support member 428 by the action of the spring loaded upper hanger 424.

The system 100 or 102 may be further comprised of a mixer comprising a shaft and at least one impeller disposed within the volume enclosed by the array of heat exchangers. In the embodiment depicted in FIGS. 5, 14A, and 14B, the mixer 450 is comprised of a drive motor 452 mounted on the vessel roof support 141 , a shaft 454, and upper and lower impellers 456 and 458. Referring also to FIG. 15, the impellers 456 and 458 may be configured to cause flow of molten salt downward through the internal volume of the heat exchanger array 401 , and upward along the exterior of the array 401 as indicated by arrows 459. The motor 452 may be made rotationally reversible so that the direction of flow through the heat exchanger array 401 may be reversed.

Details of one exemplary design of the applicants' thermal energy storage system and containment vessel will now be provided. These details are to be considered as exemplary only, and not limiting. Many other designs of the system and vessel are possible, and are within the scope of the present invention. In this embodiment, the side wall 120 of the vessel may be about 23 feet in diameter and about 22 feet high. The bottom wall 1 10 of the vessel may be of steel rebar reinforced concrete about 24 inches thick. The mud slab 1 12 beneath the bottom wall 1 10 may also be about 24 inches thick. The concrete used in the bottom wall 1 10 may be 4000 psi (pounds per square inch) concrete. The vertical I-beams 121 of the vessel wall 120 may be 8-inch X 8-inch steel I-beams, and the ring channels 131 , 132, and 133 may be formed from 8-inch wide steel channel.

The plates of the inner liner bottom 210 and the arcuate panels of the side wall 230 of the liner 200 are preferably made of metal, which provides sufficient structural strength at high temperatures, and which is joinable at the job site by suitable means such as welding. The particular metal must also be resistant to corrosion by the molten salt. Suitable metals include stainless steel, titanium, and other metals and metal alloys previously cited herein with regard to the liner. In one embodiment, the inner liner 200 may be made of 304 stainless steel having a thickness of about ¼ inch. The outer liner, which serves as secondary containment in the event of a leak through the inner liner, is not directly exposed to the molten salt. In one embodiment, the outer liner is made of carbon steel.

The flexible joints of the liner bottom 201 and liner side wall 230 may be made by suitable forming means such as a metal brake, or by stamping with a die. The flexible joints may be made of the same material as the sector shaped plates 202A - 202H and the arcuate panels 232A - 232H.

The Applicants have determined that modularity of the instant thermal storage system is preferred. In construction of the vessel and overall thermal energy storage system, a system that is of a modular design is more cost effective. In a modular system, a major portion of the system components can be fabricated in a shop environment, which is better equipped and controlled, and then shipped to the job site. Final construction and assembly can then be done on the job site.

Accordingly, one aspect of the liner 200 of the containment vessel 101 is that it may be designed in a modular manner, i.e. such that the various pieces of the liner bottom 210 and liner side wall 230 may be cut and formed in a workshop or factory, and then transported to the job site to be assembled. The panels of the side wall 230 may be formed from upper and lower arcuate panels that are pre-formed in a shop and welded together at the job site. In one embodiment, the lower panels may be thicker than the upper panels to contain the higher fluid pressures near the bottom of the containment vessel 101 . The respective sector plates 202A - 202H, center plate 204, radial flexible joints 206A - 206H, and center plate perimeter joints 210 may also all be pre-cut and formed in a shop.

Other configurations of the sectoral portions of the liner bottom 201 are also possible. In one embodiment, each sectoral section 202A - 202H may be provided with a flexible joint formed along one radial edge, with the other radial edge being a simple straight edge. In that manner, when the sectoral plates are joined together, a flexible joint is provided between each of them. Additionally, the center plate 204 could be formed with its perimeter flexible joint 210 integrally formed therewith. Many other configurations are possible, with the operative requirement being that the respective adjacent plates are separated by flexible joints to accommodate the expansion and contraction of the liner 200 during thermal cycling.

The thermal insulation of the containment vessel 101 will now be described. Referring first to FIGS. 1 1 and 12A, the substantially incompressible solid thermal insulation material 301 that is disposed between the bottoms 201 and 261 of the inner and outer liners 200 and 260 may be made from one inch thick sheets of MARINITE® 1 insulation manufactured by BNZ Materials, Inc. of Littleton CO. This insulation is primarily comprised of calcium silicate. In like manner, the thermal insulation material 31 1 that is disposed between the side walls 230 and 270 of the inner and outer liners 200 and 260 may be made of two-inch thick MARINITE® 1 insulation. The sheets may be scored or routed vertically to facilitate their being formed to conform to the arcuate panels of the liner side walls.

The insulating support members or piers 320 beneath the liner bottom walls 201 and 261 may also be made of MARINITE® I insulation. In one embodiment, the piers 320 are 12 inches in diameter, 16 inches high, and are distributed as shown in FIG. 10 over the 23-ft. diameter vessel bottom. The second insulation 322 that is disposed in the volume between the outer liner bottom 261 and the vessel bottom 1 10 may be Foamfrax® insulation manufactured by the Unifrax Corporation of Niagara Falls NY. Foamfrax® is a monolithic three-component insulation system comprised of bulk ceramic or soluble fibers, an inorganic binder, and an organic foaming binder which may be applied by spray gun. In the side wall 120 of the vessel 101 , the various members 304A - 304C and 306A - 306C of solid thermal insulation material may also be made of MARINITE® I insulation having a thickness of about two inches. The second thermal insulation material 324 of the side wall 120, and the roof insulation 330 may also be made of Foamfrax® insulation, applied to a thickness of between about 12 and 16 inches.

The heat exchangers 402 of the system 100 may be made of "2-1/4 Cr-1 Mo alloy," (also known as ASME SA-213-T22 tubing), a steel alloy containing molybdenum and chromium, which is particularly suitable for high temperature pressure vessels in corrosive environment. The upper and lower manifolds of the heat exchangers may be

8-inch diameter pipes, with 31 rows of five tubes connecting them. The tubes may be

¾-inch diameter and have twelve fins each.

A method of making the applicants' thermal energy storage system 100 may be substantially as described in the aforementioned United States Provisional Patent Application No. 61/228,351 of Bell et al., except that the preparation of the system site such that the system my be partially submerged in the ground would likely not be performed.

FIG. 17 is a flow chart depicting a method of storing and releasing energy from the applicants' thermal energy storage system. It is to be understood that although the steps of the method 500 of FIG. 17 are illustrated in a serial order, the method is not limited to being done in the order depicted in FIG. 17. Certain steps of the method 500 may be performed in parallel, and/or in orders other than depicted in FIG. 17.

One aspect of the instant system 100 and method 500 is that the latent heat contained in the system 100 will be extracted in an optimal manner. In transferring thermal energy out of the system 100, when most of the sensible heat has been removed from the molten salt and the salt is near its freezing point, the salt will begin to freeze on the outside of the heat exchanger tubes 41 1 . The molten salt will be agitated by the mixer 450 to bring hotter molten salt in contact with the cooler solidified salt on or near the heat exchanger tubes 41 1 , thus allowing maximum heat removal from the salt bath. The heat transfer oil within the heat exchangers will have a lower temperature rise across the salt bath, but can still be utilized overnight to heat oil in the solar field 10 or to preheat components in the power generating station 20.

From calculations, it is estimated that approximately 15% of the latent heat in the salt bath can be recovered for producing power, and the oil temperature from thermal storage will be below 520°F. The low grade heat that is not hot enough to produce steam in the power generating station 20 can be used to preheat the oil in the solar field 10 and to preheat feed water in the power generating station 20. This will enable a faster daily startup of the power generating station 20, which will bring the turbine cycle of the station 20 to full load sooner and increase the daily energy production of the station 20.

A method for storing thermal energy using the applicants' thermal energy storage system will now be described. Referring in particular to FIGS. 1 and 20, as a first step 510 of the method 500, a thermal energy storage system comprising a vessel, a heat exchanger or heat exchanger array, and a heat transfer fluid delivery system is provided. The thermal energy storage system may be the system 100 as described herein. The system 100 may be fabricated according to a method substantially as described, or similar to that described and shown in FIG. 12 of the aforementioned United States Provisional Patent Application No. 61/228,351 of Bell et al. The system is connected 520 to a thermal energy source such as a solar field 10 and a power generating station 20. The fluid delivery system including the piping to and from the solar field 10 and the power generating station 20, and the heat exchanger array 400 is filled 530 with heat transfer fluid. The heat transfer fluid may be circulated through the heat exchanger array 400 at normal operating pressure or higher pressure to confirm that the entire system is leak-free.

To facilitate the startup process of the thermal energy storage system 100, the Applicants have determined that it is desirable that the initial melting of the salt be performed in the containment vessel, rather than melting the salt at a remote location and pumping it to the containment vessel, as is widely practiced in the art currently. Accordingly, in step 540, the vessel 101 is charged with solid phase salt. The solid salt is typically in a coarse granular state, and is delivered through a charging port (not shown) in the top of the vessel 101 . The salt may be delivered manually, or the system may be provide with a temporary or permanent hopper and/or auger, or other conveying means (not shown) to deliver the granular salt into the vessel 101 . In one embodiment, as the incoming granular salt contacts the lower regions of the heat exchangers 412 - 418, the melting process can be started. Valves 495 and 497 of heater loop 494 are positioned to circulate heat transfer fluid through heater 496, and through the heat exchanger array 400, and back to the heater loop 494. The heat transfer oil is heated to above the melting point of the salt by the heater 496, and then circulated through the heat exchanger array 400, such that the initial granular salt contacting the heat exchangers 412 - 418 is melted early in the charging process. As the liquid salt level rises, it submerges the lower impeller 458, such that the mixer functions to circulate a slurry of liquid salt and solid granules as the melting process proceeds. With mixing and therefore enhanced heat transfer occurring during almost the entire salt melting process, the initial salt melting process is accelerated. This melting process is also beneficial in that if the vessel 101 were fully filled with granular salt and then melted, a second charge of salt would need to be added. This is because the granular salt has a large void volume, and once that first charge of granular salt were melted, the vessel would not be full to capacity, thus requiring a second charge of salt.

Once the salt is completely melted, or nearly so, such that the mixer 450 can circulate substantially the entire contents of the vessel 101 , the contents may be maintained 550 in the pre-heated state by using the heater loop 494 if necessary until thermal energy is available 555 from the solar field 10 or other thermal source. The various valves of the fluid delivery system 490 are switched, circulating hot heat transfer fluid from the solar field 10 through the heat exchanger array 450 and back, thereby delivering 560 thermal energy to the vessel from the solar field 10. Any remaining solid salt in the vessel is fully melted, and the molten salt is heated further, typically to a temperature of as much as 800°F. When the molten salt is at its maximum operating temperature, the vessel 101 is at its maximum thermal capacity 565. This maximum thermal energy is available to deliver to the power generating station 20.

This is typically needed from evening to morning, or during cloudy periods of the day. The various valves of the fluid delivery system 490 are again switched, circulating hot heat transfer fluid through the heat exchanger array 450 in the vessel 101 to the power generating station 20 and back, thereby delivering 570 thermal energy from the vessel 101 to the power generating station 20. The heat transfer rate from the molten salt to the heat transfer fluid in the heat exchangers 412 - 418 is enhanced by use of the mixer 450. This may continue until the molten salt in the vessel 101 is cooled to a point where it begins to solidify on the upstream portion of the inlet heat exchanger 414, at which point the available thermal energy in the vessel 101 is substantially depleted. It is preferable that the system 100 not be operated such that the molten salt completely freezes and encloses the heater exchangers 412 - 418. This is because the enhanced heat transfer provided by the fluid flow caused by the mixer would then no longer be available. After depletion of the thermal energy in the vessel 101 , the salt is maintained 550 in a heated state with minimal heat loss by virtue of the substantial amount of thermal insulation surrounding the entire vessel. In the event that it is necessary to completely shut down the process, or to interrupt the process for a prolonged period of time, it is permissible to allow some or all of the entire contents of the vessel to solidify. The solid salt in the vessel can be re-melted by resuming the circulation of hot heat transfer fluid through the heat exchangers 412 - 418.

It is also noted that it is not necessary to have the vessel fully charged to maximum thermal capacity 565 before delivering 570 thermal energy to the power generating station 20, as indicated by dotted line arrow 568. If conditions warrant, a partial thermal charge may be delivered 570 to the power generating station 20. Additionally, even if the temperature of the molten salt in the vessel 101 is relatively low, and only a small amount of thermal energy is available in the vessel 101 , that energy may still be beneficially used. As described previously, the low grade heat that is not hot enough to produce steam in the power generating station 20 can be used to preheat the oil in the solar field 10 and/or to preheat feed water in the power generating station 20. This will enable a faster daily startup of the power generating station 20, which will bring the turbine cycle of the station 20 to full load sooner and increase the daily energy production of the station 20.

It is, therefore, apparent that there has been provided, in accordance with the present invention, a vessel for containing a thermal energy storage liquid, a thermal energy storage system comprising such vessel, and methods for making and using the vessel and thermal energy storage system. Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.

Claims

We claim:

1 . A vessel comprising a bottom wall joined to a surrounding side wall, and an inner liner disposed within the bottom wall and side wall and comprising:

a) an inner liner bottom comprising a central plate surrounded by an array of radially arranged sector-shaped plates, each of the sector-shaped plates joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector-shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint; and b) an inner liner side wall joined to the inner liner bottom and comprising a plurality of panels, each of the panels joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent panel by a lateral flexible joint.

2. The vessel of claim 1 wherein the vessel is cylindrical, and the vessel bottom wall is circular.

3. The vessel of claim 1 , wherein the central plate is circular.

4. The vessel of claim 1 , wherein the central plate is a polygon.

5. The vessel of claim 1 , wherein the number of radially arranged sector-shaped plates is between three and twelve inclusive.

6. The vessel of claim 5, wherein the number of radially arranged sector-shaped plates is eight.

7. The vessel of claim 1 , wherein the radial flexible joint between each pair of adjacent sector-shaped plates is an arcuate member.

8. The vessel of claim 1 , wherein the lateral flexible joint between each pair of adjacent panels of the side wall is an arcuate member.

9. The vessel of claim 1 , wherein each of the lateral flexible joints are integrally formed along one of the radial portions of the perimeters of each of the sector-shaped plates.

10. The vessel of claim 1 , further comprising an outer liner comprising an outer liner bottom having an outer perimeter and an outer liner side wall joined to the outer perimeter of the outer liner bottom, the outer liner containing the inner liner therein and separated from the inner liner by thermal insulation.

1 1 . The vessel of claim 1 , further comprising bottom thermal insulation disposed between the bottom wall of the vessel and the inner liner bottom, the bottom thermal insulation comprising a plurality of support members of a first insulating material interspersed within a second insulating material.

12. The vessel of claim 1 , wherein the surrounding side wall is comprised of a network of structural members, and the vessel is further comprised of a plurality of insulating support members disposed between the network of structural members and the inner liner side wall.

13. The vessel of claim 12, further comprising thermal insulation covering the plurality of insulating support members and the network of structural members.

14. A vessel comprising a bottom wall joined to a surrounding side wall, and a liner disposed within the bottom wall and side wall, the liner comprising:

a) a liner bottom comprised of means for expanding radially outwardly when heated and contracting radially inwardly when cooled; and

b) a liner side wall joined to the inner liner bottom and comprised of means for expanding radially outwardly when heated and contracting radially inwardly when cooled.

15. A vessel comprising:

a) a bottom wall joined to a surrounding side wall;

b) a liner disposed within the bottom wall and surrounding side wall and comprising a liner bottom; and

c) bottom thermal insulation disposed between the bottom wall of the vessel and the liner bottom, the bottom thermal insulation comprising a plurality of support members of a first insulating material interspersed within a second insulating material.

16. The vessel of claim 15, wherein the vessel side wall is comprised of a network of structural members, and the vessel is further comprised of a plurality of insulating support members disposed between the network of structural members and the liner side wall.

17. An expandable liner for a vessel, the expandable liner comprising:

a) a liner bottom comprising a central plate surrounded by an array of radially arranged sector-shaped plates, each of the sector-shaped plates joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector-shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint; and

b) a liner side wall joined to the liner bottom and comprising a plurality of panels, each of the panels joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent panel by a lateral flexible joint.

18. The liner of claim 17 wherein the liner side wall is cylindrical, and the liner bottom is circular.

19. The liner of claim 17, wherein the central plate is circular.

20. The liner of claim 17, wherein the central plate is a polygon.

21 . The liner of claim 17, wherein the number of radially arranged sector-shaped plates is between three and twelve inclusive.

22. The liner of claim 17, wherein the radial flexible joint between each pair of adjacent sector-shaped plates is an arcuate member.

23. The liner of claim 17, wherein the lateral flexible joint between each pair of adjacent panels of the side wall is an arcuate member.

24. The liner of claim 17, wherein each of the lateral flexible joints are integrally formed along one of the radial portions of the perimeters of each of the sector-shaped plates.

25. A heat exchanger assembly comprising:

a) a heat exchanger comprising an upper region, a central region, and a lower region;

b) a central support hanger connected to the central region of the heat exchanger and suspendable from a structure;

c) a spring loaded upper hanger connected to the upper region of the heat exchanger and suspendable from the structure; and

d) a lower hanger suspended from the central support hanger and connected to the lower region of the heat exchanger.

26. A thermal energy storage system comprised of:

a) a vessel comprising a bottom wall joined to a surrounding side wall, an inner liner disposed within the bottom wall and surrounding side wall and comprising a liner bottom, and bottom thermal insulation disposed between the bottom wall of the vessel and the inner liner bottom, the bottom thermal insulation comprising a plurality of support members of a first insulating material interspersed within a second insulating material;

b) a roof structure disposed on the surrounding side wall; and

c) a heat exchanger assembly comprising a heat exchanger comprised of an upper region, a central region, and a lower region, a central support hanger connected to the central region of the heat exchanger and suspended from the roof structure, a spring loaded upper hanger connected to the upper region of the heat exchanger and suspended from the roof structure, and a lower hanger suspended from the central support hanger and connected to the lower region of the heat exchanger.

27. The system of claim 26, wherein the heat exchanger assembly is further comprised of an array of heat exchangers disposed in the vessel and arranged so as to enclose a volume within the vessel.

28. The system of claim 27, wherein each of the heat exchangers of the array is suspended from the roof structure by a suspension assembly comprised of a central support hanger suspended from the roof structure and connected to a central region of the heat exchanger, a spring loaded upper hanger suspended from the roof structure and connected to an upper region of the heat exchanger, and a lower hanger suspended from the central support hanger and connected to a lower region of the heat exchanger.

29. The system of claim 27, further comprising a mixer comprised of a shaft and at least one impeller disposed within the volume enclosed by the array of heat exchangers.

30. The system of claim 26, wherein the inner liner bottom is comprised of a central plate surrounded by an array of radially arranged sector-shaped plates, each of the sector-shaped plates joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector-shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint; and the inner liner is further comprised of an inner liner side wall joined to the inner liner bottom and comprising a plurality of arcuate panels, each of the panels joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent arcuate panel by a lateral flexible joint.

31 . The system of claim 30, wherein the vessel is cylindrical, and the vessel bottom wall is circular.

32. The system of claim 30, wherein the central plate is circular.

33. The system of claim 30, wherein the central plate is a polygon.

34. The system of claim 30, wherein the number of radially arranged sector-shaped plates is between three and twelve inclusive.

35. The system of claim 30, wherein the radial flexible joint between each pair of adjacent sector-shaped plates is an arcuate member.

36. The system of claim 30, wherein the lateral flexible joint between each pair of adjacent panels of the side wall is an arcuate member.

37. The system of claim 30, wherein each of the lateral flexible joints are integrally formed along one of the radial portions of the perimeters of each of the sector-shaped plates.

38. The system of claim 30, further comprising an outer liner comprising an outer liner bottom having an outer perimeter and an outer liner side wall joined to the outer perimeter of the outer liner bottom, the outer liner containing the inner liner therein and separated from the inner liner by thermal insulation.

39. The system of claim 30, wherein the surrounding side wall is comprised of a network of structural members, and the vessel is further comprised of a plurality of insulating support members disposed between the network of structural members and the inner liner side wall.

40. The system of claim 39, further comprising thermal insulation covering the plurality of insulating support members and the network of structural members.

41 . A method for using thermal energy comprising:

a) providing a thermal energy storage system comprised of a vessel containing at least one heat exchanger and comprising a bottom wall joined to a surrounding side wall, and an inner liner disposed within the bottom wall and surrounding side wall and comprising:

i) an inner liner bottom comprising a central plate surrounded by an array of radially arranged sector-shaped plates, each of the sector-shaped plates joined at a radial portion of its perimeter to a radial portion of the perimeter of an adjacent sector-shaped plate by a radial flexible joint and joined at an inner portion of its perimeter to the central plate by an inner flexible joint; and

ii) an inner liner side wall joined to the inner liner bottom and comprising a plurality of panels, each of the panels joined at a lateral portion of its perimeter to a lateral portion of the perimeter of an adjacent panel by a lateral flexible joint; and

b) delivering heated heat transfer fluid from a thermal energy source to the heat exchanger, thereby heating a thermal energy storage substance contained in the vessel and causing thermal expansion of the inner liner to be accommodated by flexing of the radial, inner, and lateral flexible joints.

42. The method of claim 41 , further comprising delivering heated heat transfer fluid from the at least one heat exchanger in the vessel to a power generating station.

43. A method for using thermal energy comprising:

a) providing a thermal energy storage system comprised of a vessel containing a heat exchanger;

b) charging the vessel with a solid thermal energy storage substance; and c) delivering heated heat transfer fluid from a first thermal energy source to the heat exchanger, thereby heating and melting the thermal energy storage substance contained in the vessel.

44. The method of claim 43, wherein charging the vessel with the solid thermal energy storage substance and delivering heated heat transfer fluid from the first thermal energy source to the heat exchanger are performed simultaneously.

45. The method of claim 43, further comprising connecting the heat exchanger to a power generating station and delivering heated heat transfer fluid from the heat exchanger to the power generating station.