WO2015077235A1 - Concentrated solar power systems and methods utilizing cold thermal energy storage - Google Patents

Concentrated solar power systems and methods utilizing cold thermal energy storage

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Publication number
WO2015077235A1
WO2015077235A1 PCT/US2014/066166 US2014066166W WO2015077235A1 WO 2015077235 A1 WO2015077235 A1 WO 2015077235A1 US 2014066166 W US2014066166 W US 2014066166W WO 2015077235 A1 WO2015077235 A1 WO 2015077235A1
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WO
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Prior art keywords
thermal energy
cold thermal
working fluid
power
power cycle
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PCT/US2014/066166
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French (fr)
Inventor
Luke ERICKSON
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Abengoa Solar Llc
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Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING WEIGHT AND MISCELLANEOUS MOTORS; PRODUCING MECHANICAL POWER; OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03GSPRING, WEIGHT, INERTIA OR LIKE MOTORS; MECHANICAL-POWER PRODUCING DEVICES OR MECHANISMS, NOT OTHERWISE PROVIDED FOR OR USING ENERGY SOURCES NOT OTHERWISE PROVIDED FOR
    • F03G6/00Devices for producing mechanical power from solar energy
    • F03G6/06Devices for producing mechanical power from solar energy with means for concentrating solar rays
    • F03G6/065Devices for producing mechanical power from solar energy with means for concentrating solar rays having a Rankine cycle
    • F03G6/067Devices for producing mechanical power from solar energy with means for concentrating solar rays having a Rankine cycle using an intermediate fluid for heat transfer
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24HEATING; RANGES; VENTILATING
    • F24SSOLAR HEAT COLLECTORS; SOLAR HEAT SYSTEMS
    • F24S90/00Solar heat systems not otherwise provided for
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25BREFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT-PUMP SYSTEMS
    • F25B27/00Machines, plant, or systems, using particular sources of energy
    • F25B27/002Machines, plant, or systems, using particular sources of energy using solar energy
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02ATECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A30/00Adapting or protecting infrastructure or their operation
    • Y02A30/20Adapting or protecting infrastructure or their operation in buildings, dwellings or related infrastructures
    • Y02A30/27Relating to heating, ventilation or air conditioning [HVAC] technologies
    • Y02A30/272Relating to heating, ventilation or air conditioning [HVAC] technologies using solar thermal energy
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B10/00Integration of renewable energy sources in buildings
    • Y02B10/20Solar thermal
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B10/00Integration of renewable energy sources in buildings
    • Y02B10/20Solar thermal
    • Y02B10/24Air conditioning or refrigeration systems
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00Energy generation through renewable energy sources
    • Y02E10/40Solar thermal energy
    • Y02E10/46Conversion of thermal power into mechanical power, e.g. Rankine, Stirling solar thermal engines

Abstract

Embodiments include concentrated solar power systems and methods including a solar receiver configured to heat a power cycle working fluid with concentrated solar flux. The systems and methods also utilize a powered cold thermal energy reservoir. The cold thermal energy reservoir houses a cold thermal energy medium which may be cooled, utilizing refrigeration, heat pumps or another cooling technique, to a temperature which is below the contemporaneous ambient air temperature. The systems and methods further include apparatus to the convert thermal energy of the working fluid to mechanical energy according to a power cycle. Waste heat from the power cycle working fluid is rejected into the cold thermal energy reservoir thereby increasing overall system efficiency. Electrical energy from any source may be stored as thermal energy in a cold thermal energy storage system associated with the cold thermal energy medium.

Description

CONCENTRATED SOLAR POWER SYSTEMS AND METHODS UTILIZING

COLD THERMAL ENERGY STORAGE

TECHNICAL FIELD

[0001] The embodiments disclosed herein relate to concentrated solar power (CSP) electricity generation. In particular, the disclosed embodiments relate to CSP systems and methods utilizing waste heat rejection at less than ambient temperature and/or cold thermal energy storage.

BACKGROUND

[0002] Concentrating Solar Power (CSP) systems utilize solar energy to heat a working fluid which drives a thermal power cycle for the generation of electricity. CSP technologies include parabolic trough, linear Fresnel, central receiver or "power tower," and dish/engine systems. Considerable interest in CSP has been driven by renewable energy portfolio standards applicable to energy providers in the southwestern United States and renewable energy feed-in tariffs in Spain. CSP systems are typically deployed as large, centralized power plants to take advantage of economies of scale.

[0003] Much recent work has been directed toward improving the overall efficiency, cost and grid-compatibility of CSP generation systems, in order to compete more effectively with non-renewable energy generation sources.

[0004] Certain known subsystems such as thermal energy storage (TES) can increase the grid-compatibility of a CSP system by extending energy production into periods without sufficient sunshine. Nonetheless, the peak efficiency of power generation utilizing a thermal power cycle is constrained by certain immutable laws of physics. As noted above, most CSP systems are based upon a working fluid heated to an operational temperature by concentrated solar flux. The thermal energy within the working fluid is then converted to mechanical energy in one or more thermal power cycles to drive an electrical energy generator. Various thermal power cycles or combinations thereof are commonly used to generate power. For example, CSP systems can utilize a Brayton cycle, a Rankine cycle or a combination or series of cycles to generate power. Other CSP systems utilize a Stirling engine to generate power. [0005] The theoretical highest efficiency of any thermal power cycle is limited by the maximum hot and cold working fluid temperatures present in the cycle according to the Carnot efficiency limit. The actual highest efficiency of any CSP system will be significantly less than the theoretical highest efficiency, but actual efficiency is still constrained by the maximum hot and cold working fluid temperatures of the cycle. In most CSP systems, the maximum hot temperature of the working fluid is limited by the maximum temperature allowable in the receiver or the maximum temperature possible before the integrity of the working fluid or an intervening heat transfer fluid is compromised. Typical CSP systems operate with a minimum cold temperature equal to or greater than the ambient air temperature of the system environment.

[0006] The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.

SUMMARY OF THE EMBODIMENTS

[0007] One embodiment disclosed herein is a concentrated solar power system including a solar receiver configured to directly or indirectly heat a power cycle working fluid with concentrated solar flux. In typical embodiments, the solar receiver is associated with one or more reflective heliostats, parabolic trough reflectors, dish reflectors, lenses or similar apparatus configured to concentrate sunlight on the receiver. The power cycle working fluid may be heated directly at the receiver or through heat exchange with an intervening heat transfer fluid. The system also features a powered cold thermal energy reservoir. The cold thermal energy reservoir houses a cold thermal energy medium which may be cooled, utilizing refrigeration, heat pumps or another cooling technique, to a temperature which is below the contemporaneous ambient air temperature. The system also includes apparatus to convert thermal energy of the working fluid to mechanical energy according to a power cycle. The apparatus required to exploit a power cycle may be comprised of multiple elements including but not limited to heat exchangers, compressors, turbines and associated machinery. Waste heat from the power cycle working fluid is rejected into the cold thermal energy reservoir which is maintained at less than ambient temperature, thereby increasing overall system efficiency compared to a similar cycle rejecting heat to ambient temperature.

[0008] The electricity or other power required to cool the cold thermal energy storage medium may be obtained from several types of sources or a combination of sources. For example, electricity to power a refrigeration cycle to charge the cold thermal energy reservoir may be obtained locally, from an array of one or more photovoltaic panels, a wind driven generation system or a water driven generation system for example. Alternatively, the electricity required to power a refrigeration cycle to charge the cold thermal energy reservoir may be obtained from the electric grid.

[0009] The cold thermal energy storage medium may be cooled below ambient temperature according to any desired method including but not limited to vapor compression, absorption chiller, or hybrid refrigeration cycles. Alternatively, cooling may be accomplished with one or more heat pumps. In embodiments utilizing a heat pump to cool the cold thermal energy storage medium, thermal energy from a material heated at the solar receiver with concentrated solar flux may be used as the high temperature thermal input to the heat pump.

[0010] Certain embodiments of the disclosed concentrated solar power systems may utilize a phase change material as one or more of the working fluid, cold thermal energy storage medium or a heat storage material to enhance system efficiency. In some embodiments the working fluid may be carbon dioxide, krypton, argon, nitrogen and air. The system may include all apparatus necessary to implement a transcritical power cycle, a supercritical power cycle, a Brayton power cycle, a Rankine power cycle or some type of hybrid or blended power cycle.

[0011] The system may include a cold thermal energy storage material in a cold thermal energy storage system associated with the cold thermal energy reservoir. The cold thermal energy storage material may be cooled through contact with the cold thermal energy medium thereby providing for the thermal storage of electrical energy.

[0012] Systems such as described herein may be operated more efficiently than systems lacking a powered cold thermal energy reservoir because the maximum efficiency of a thermal power generation cycle is dictated by the maximum hot and cold temperatures in the cycle according to the Carnot efficiency limit. Thermal-to-electric conversion efficiency can be increased more by decreasing the cold temperature than by increasing the hot temperature the same amount.

[0013] In a concentrated solar power system, the hot temperature is limited by the maximum temperature allowable in the receiver or by the maximum temperature which may be sustained by a heat transfer fluid or the working fluid. Current concentrated solar power plant designs typically utilize power cycles which rely upon the rejection of waste heat at ambient air temperature, typically through an air-cooled condenser or similar apparatus. In the embodiments disclosed herein, the concentrated solar power plant is coupled with a cold thermal energy reservoir thus reducing the cold temperature of the power cycle working fluid and increasing the maximum possible Carnot efficiency.

[0014] Alternative embodiments include methods of generating electricity from concentrated solar power using a system featuring a cold thermal storage reservoir as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. 1 is a schematic diagram of a prior art CSP system featuring waste heat rejection to the ambient environment.

[0016] Fig. 2 is a graph representation of typical power cycle efficiency at various hot and cold operating temperatures.

[0017] Fig. 3 is a schematic diagram of a CSP system as disclosed herein featuring waste heat rejection at a temperature below the ambient temperature and including cold thermal energy storage.

[0018] Fig. 4 is a flow chart representation of a method as disclosed herein.

DETAILED DESCRIPTION

[0019] Unless otherwise indicated, all numbers expressing quantities of ingredients, dimensions, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about".

[0020] In this application and the claims, the use of the singular includes the plural unless specifically stated otherwise. In addition, use of "or" means "and/or" unless stated otherwise. Moreover, the use of the term "including", as well as other forms, such as "includes" and "included", is not limiting. Also, terms such as "element" or "component" encompass both elements and components comprising one unit and elements and components that comprise more than one unit unless specifically stated otherwise. [0021] All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any combination, with the exception of combinations of mutually exclusive features and/or steps.

[0022] CSP systems utilize concentrated sunlight to directly or indirectly heat a working fluid which is used to drive one or more power generation cycles. The power cycles occur within machinery such as turbines and compressors or heat engines which in turn drive electric generators. Some CSP systems utilize an initial heat transfer fluid (HTF) circuit where HTF is directly heated to operational temperatures by solar energy and a separate power cycle working fluid is thermally charged by heat exchange with the HTF. For example, FIG. 1 illustrates a highly simplified prior art CSP system 100 featuring a receiver 102 situated at or near the top of a tower 104. The receiver 102 receives concentrated reflected sunlight from a field of multiple heliostats 106 positioned to optimally reflect solar flux onto surfaces of the receiver 102. Within the receiver 102 a primary HTF circuit 108 carries HTF through the receiver 102 where the HTF is heated to an operational temperature. Thermal energy from the HTF may be stored at any point in the HTF circuit in thermal energy storage devices 110 or 112 to extend the operational timeframe of the system.

[0023] In the simplified diagram of Fig. 1, the heated HTF is conveyed to a heat exchanger 114 from the receiver 102 or from TES 110 where thermal interchange with the HTF causes the heating of pressurized working fluid flowing in a working fluid circuit 116.

[0024] The thermal energy of the working fluid is utilized to drive a thermal power cycle

118. In the particular embodiment of Fig. 1, the thermal power cycle 118 is represented as a highly simplified Brayton cycle featuring a turbine 120 and compressor 122 connected by an axle 124. Expansion of the heated working fluid within the turbine 120 converts thermal energy to mechanical energy, thereby driving the compressor 122 and outputting work (represented by rotational arrow 126) which can be utilized to drive an electric generator. Other types of thermal power cycle or heat engines may also be driven with thermal energy obtained initially from concentrated solar flux.

[0025] The efficiency of any thermal power cycle is physically limited by the maximum hot and minimum cold temperatures of the working fluid according to the Carnot efficiency theorem. Thus, the maximum theoretical efficiency of a thermal power cycle (η) may be expressed as follows, where Tc is the absolute temperature of the cold working fluid and TH is the absolute temperature of the hot working fluid:

(Equation 1)

Figure imgf000007_0001

[0026] A useful estimate of the practical limit of the efficiency of an actual power cycle is 75% of the idealized Carnot efficiency. In addition, as is illustrated in Fig. 2, which graphically represents 75% of the Carnot efficiency limit at various hot and cold working fluid temperature combinations, the thermal-to-mechanical conversion efficiency of a thermal power system can be increased more by decreasing the cold temperature than by increasing the hot temperature the same absolute amount.

[0027] As noted above, all CSP systems which drive thermal power cycles are subject to the Carnot efficiency limitation. The maximum hot temperature of a CSP system is limited by the maximum temperature allowable in the receiver or the maximum temperature which may be sustained by the HTF or working fluid within the receiver. Certain improvements in overall CSP system efficiency may therefore be made by selecting receiver materials and HTF substances which permit increased hot temperatures. As noted above however, a potentially more effective method of improving CSP system efficiency is to lower the minimum cold temperature of the working fluid within the power cycle. Thus, the apparatus and methods disclosed herein improve efficiency and reduce the cost of operating a CSP system by rejecting heat from the power cycle at a lower than ambient temperature, thereby lowering Tc. Certain embodiments disclosed herein also feature cold energy storage which may further enhance efficiency and the ability of a CSP system to mesh effectively with the electric grid.

[0028] A thermal power cycle requires the working fluid to undergo distinct transitions between working fluid energy and enthalpy levels. Accordingly, a working fluid is required, according to well-known engineering principles, to be pressurized, heated, expanded and cooled at specific times and within specific apparatus during a thermal energy cycle. Thus, a thermal power cycle requires waste heat from the working fluid to be removed from the system.

[0029] Conventional CSP plant designs reject heat from the system at the ambient air temperature through an apparatus such as an air-cooled condenser, for example the condenser flowing to the ambient atmosphere. On the contrary, in the embodiments disclosed herein, waste heat is rejected into a reservoir or system which is maintained at lower than ambient temperature. Thus, as described in detail below, the apparatus and methods disclosed herein improve efficiency and thereby reduce the cost of operating a CSP system by rejecting waste heat from a power cycle at a lower Tc then the ambient temperature.

[0030] A simple schematic drawing of a CSP system featuring both waste heat rejection at a temperature which is lower than ambient temperature and cold thermal energy storage is shown in Fig. 3. The solar concentrating apparatus 300 of Fig. 3 is substantially identical to that illustrated in Fig. 1 and includes a solar receiver 302 positioned upon a tower 304 to receive concentrated sunlight reflected from a field of heliostats 306. Within the receiver 302 a primary HTF circuit 308 carries HTF through the receiver 302 where the HTF is heated to an operational temperature. Thermal energy from the HTF may be stored at any point in the HTF circuit, for example in thermal energy storage devices 310 or 312. Stored thermal energy may be used to extend the operational timeframe of the system.

[0031] In the simplified diagram of Fig. 3, the heated HTF is conveyed to a heat exchanger 314 from the receiver 302 or from TES 310 where thermal interchange with the HTF causes the heating of a working fluid flowing in a working fluid circuit 316.

[0032] The thermal energy of the working fluid is converted to mechanical energy according to a thermal power cycle 318. In the particular embodiment of Fig. 3, the thermal power cycle 318 is represented as a highly simplified Brayton cycle implemented with mechanical elements such as a turbine 320 and compressor 322 in mechanical communication with each other through at least an axle 324. Expansion of the working fluid within the turbine 320 converts thermal energy to mechanical energy, thereby driving the compressor 322 and outputting work (represented by rotational arrow 326) which can be utilized to drive an electric generator. Other types of thermal power cycle or heat engines may also be driven with thermal energy obtained initially from concentrated solar flux.

[0033] Alternative system embodiments can include other types of solar concentrator apparatus including but not limited to parabolic trough collection apparatus, linear Fresnel collectors, central and dish/engine systems. Alternative system embodiments may also be implemented with different machinery suited to implement a different power cycle. [0034] The system 300 of Fig. 3 uniquely features waste heat rejection at a temperature less than ambient temperature. For example, waste heat in the Fig. 3 embodiment is removed from the power cycle 318 to a cold thermal energy reservoir 327 which is maintained at less than ambient temperature. In particular, during power generation operations, heated and pressurized working fluid flowing in the working fluid cycle 316 expands to drive the turbine 320.

Downstream from the turbine, lower pressure and slightly cooled working fluid exchanges heat in a recuperator 328 with pressurized working fluid flowing toward the heat exchanger 314 for reheating. The heat exchange processes occurring in the recuperator 328 additionally cools the working fluid. At this point in a conventional process, additional heat within the working fluid would be rejected to the atmosphere at the ambient temperature.

[0035] In the embodiments disclosed herein however, downstream from the recuperator

328, but prior to compression in the compressor 322, the working fluid is further cooled, to a temperature below ambient temperature by heat exchange with a cold thermal energy medium in a heat exchanger 330. The cold thermal energy medium is another, separate, HTF flowing in a cold thermal energy medium circuit 332. The cold thermal energy medium is maintained at a suitably low temperature by a refrigeration cycle 334 and/or a heat pump as described below. In addition to receiving waste heat from the working fluid of the power cycle 318, the cold thermal energy medium may be used to facilitate the storage of electrical energy in the form of cold thermal energy. For example, electrically cooled cold thermal energy medium may be stored directly, or used to exchange heat with a cold thermal energy storage material in a cold thermal energy storage system 336. Collectively, the cold thermal energy medium, cold thermal energy medium circuit 332, refrigeration cycle 334, heat exchanger 330 and cold thermal energy storage system 326 are referred to herein as the cold thermal energy reservoir 327. Various elements of the cold thermal energy reservoir 327 are described in more detail below.

[0036] The efficiency of many different types of power cycle or combinations of power cycles may be enhanced according to the general principles noted above. Specific material and machinery configurations will be influenced by engineering variables include the maximum temperature achievable by the CSP system (and which may be sustained by the associated primary HTF), the cold thermal energy reservoir temperature, the relative energy contributions of the CSP and cold storage system, the cost and complexity of various subsystems, and the feasibility of coupling heat rejected from the power cycle to a heat pump for charging the cold reservoir.

[0037] A system 300 can be designed to use a relatively small contribution from the cold thermal energy reservoir 327 and thus operate at cold temperatures of, for example, between 20°C and -80°C. Such a system would be well suited for use to accept waste heat from a transcritical or Rankine carbon dioxide power cycle. In certain embodiments, the power cycle could be beneficially operated in two modes. A first high efficiency mode could feature the rejection of heat to the cold thermal energy reservoir 327 with a second operational mode rejecting heat to ambient air, for example when the cold thermal energy storage system 336 is nearly discharged. Supercritical or transcritical Brayton or Rankine power cycles utilizing krypton as the working fluid may be implemented with lower cold thermal energy reservoir temperatures, for example approximately less than -70°C. Even lower cold thermal energy reservoir temperatures, for example less than -130°C would be useful for power cycles using argon as the working fluid. Power cycles utilizing nitrogen, oxygen, or air would achieve high efficiencies with even lower waste heat rejection temperatures of, for example, less than -180°C.

[0038] As noted above, the described systems may include a cold thermal energy storage system 336 housing a cold thermal energy storage material. The cold thermal energy storage system 336 may contain any or several of many useful storage materials. Representative cold thermal energy storage materials include but are not limited to:

• Ice at near 0°C,

• Freeze point suppressed ice using either organic solutions such as ethelyne glycol or salts as freezing point suppressants at temperatures between 0°C and -80°C.

• Organic or inorganic solid-to-liquid phase change materials

• Liquid-to-gas phase change materials such as air, nitrogen, oxygen, krypton, argon, or others.

[0039] The cold thermal energy reservoir 327 may be charged by any of several refrigeration or cooling methods including but not limited to vapor compression cycles, absorption chiller cycles, or hybrid cycles. Refrigeration cycles require a power input, typically an electrical power input. In certain embodiments power to the refrigeration cycle 334 may be obtained locally, for example from a photovoltaic array 338, wind driven turbine or a water driven turbine 340. Alternatively, power to drive the refrigeration cycle 334 may be obtained from the electric grid 342. In other embodiments, the cold thermal energy reservoir 327 may be charged for the operation of a heat pump utilizing a high temperature thermal input heated by the CSP system 300.

[0040] The working fluids used within the refrigeration cycle 334 can be selected to produce many different cold reservoir temperatures. For example, ammonia or R-134a could be used as the refrigeration cycle working fluid to achieve cold storage temperatures typical of domestic and commercial refrigeration temperatures. Carbon dioxide could be used as refrigeration cycle working fluid at slightly lower temperatures. Air or nitrogen could be utilized as a refrigeration cycle working fluid to achieve cryogenic temperatures. As noted above, thermally-driven heat pumps could also be used to implement some or all of the refrigeration cycle 334.

[0041] One advantage of the system and method embodiments disclosed herein over the state of the art is a "boosting" benefit achieved by coupling two systems with independent power cycles. Another benefit is the ability of the cold thermal energy reservoir 327 to store electrical energy from the grid 342, a local source for example photovoltaics 338 or another source in a cold thermal energy medium. The synergistic effect of reduced temperature waste heat rejection and cold thermal energy storage is illustrated in the following example.

[0042] Typically, a concentrated solar power plant in the southwest United States may operate during the day rejecting heat from the power cycle to an ambient temperature of 35°C. If the CSP plant is a molten salt tower, it may supply thermal energy to the power cycle at 550°C. The Carnot efficiency of a cycle operating between these two temperatures is (1-308K/823K) = 62.6%. Practically, a state of the art steam Rankine power cycle may achieve 70% of the ideal efficiency, (0.7*(1-308K/823K)) = 43.8% in this case. Thus, (100%-43.8%) = 56.2% of the thermal energy transferred to the power cycle working fluid must be rejected to the environment. If instead the plant was operated by rejecting the waste heat to a cold thermal energy reservoir at 0°C, the practical efficiency limit would be (1-273K/823K) = 46.8% requiring (100%-46.8%) = 53.2% of the heat to be rejected to cold reservoir.

[0043] During the night, when a typical ambient air temperature could be 15°C, the cold reservoir could be charged by operating a heat pump with a Carnot coefficient of performance of (273K/(285K-273K)) = 22.8 or, practically, 70% of the Carnot efficiency, 15.9. [0044] When operated cyclically, the round trip efficiency of stored electricity is the additional electricity generated by the power cycle (46.8%-43.8% = 3%) divided by the electricity input to the heat pump for the cooling system which is the thermal demand divided by COP (53.2%/15.9 = 3.3%) which is 3%/3.3% = 89.1%. Thus, the described systems can store electricity from night time to day time as or more efficiently than state of the art systems.

[0045] Alternative embodiments disclosed herein include methods of operating a concentrated solar power system to generate electricity. For example, as shown in Fig. 4, one representative method includes the steps of heating a power cycle working fluid with

concentrated solar flux (step 402). Heat from the heated power cycle working fluid is converted to mechanical energy (step 404) which in turn is utilized to power electricity generation (step 406). In certain embodiments the power cycle working fluid is heated directly with concentrated solar flux. In other embodiments, the power cycle working fluid is heated indirectly through heat exchange with a heat transfer fluid.

[0046] During the conversion of thermal energy to mechanical energy, the working fluid must reject waste heat from the system. In the disclosed embodiments, waste heat is rejected from the heated power cycle working fluid to a cold thermal energy medium (step 408). The cold thermal energy medium is typically flowing within one or more circuits associated with a cold thermal energy reservoir. The cold thermal energy medium is therefore cooled to a temperature below ambient temperature before, during or after accepting waste heat rejected from the power cycle working fluid (step 410). The method may also include the storage of electrical energy in a cold thermal energy storage material (step 412). Thus, the cold thermal energy medium may be chilled (step 410) through heat exchange with the cold thermal energy storage material or through direct refrigeration in a refrigeration cycle.

[0047] Various embodiments of the disclosure could also include permutations of the various elements recited in the claims as if each dependent claim was a multiple dependent claim incorporating the limitations of each of the preceding dependent claims as well as the independent claims. Such permutations are expressly within the scope of this disclosure.

[0048] While the embodiments disclosed herein have been particularly shown and described with reference to a number of alternatives, it would be understood by those skilled in the art that changes in the form and details may be made to the various configurations disclosed herein without departing from the spirit and scope of the disclosure. The various embodiments disclosed herein are not intended to act as limitations on the scope of the claims. All references cited herein are incorporated in their entirety by reference.

Claims

CLAIMS What is claimed is:
1. A concentrated solar power system comprising:
a solar receiver configured to heat a power cycle working fluid with concentrated solar flux;
a powered cold thermal energy reservoir providing for the cooling of a cold thermal energy medium to a temperature which is below a contemporaneous ambient air temperature; and
a heat exchanger in thermal communication with the power cycle working fluid and the cold thermal energy medium, wherein the system utilizes thermal energy from the power cycle working fluid to produce mechanical energy and thereby to generate electrical power and wherein waste heat from the power cycle working fluid is rejected into the cold thermal energy medium.
2. The concentrated solar power system of claim 1 wherein the power cycle working fluid is heated by heat transfer with a heat transfer fluid flowing through the solar receiver.
3. The concentrated solar power system of claim 1 further comprising a hot thermal energy storage reservoir comprising a heat storage material in thermal communication with at least one of the heat transfer fluid and the power cycle working fluid.
4. The concentrated solar power system of claim 1 further comprising a locally generated electricity source to power a refrigeration cycle to cool the cold thermal energy medium to a temperature which is below a contemporaneous ambient air temperature, wherein the locally generated electricity source comprises at least one of a photovoltaic panel, a wind driven generation system or a water driven generation system.
5. The concentrated solar power system of claim 1 further comprising an electrical connection to the electric grid providing electricity to power a refrigeration cycle to cool the cold thermal energy medium to a temperature which is below a contemporaneous ambient air temperature.
6. The concentrated solar power system of claim 1 further comprising a heat pump in thermal communication with the solar receiver and the cold thermal energy medium, wherein the heat pump cools the cold thermal energy medium using thermal energy from a material heated at the solar receiver as a high temperature thermal input.
7. The concentrated solar power system of claim 3 wherein the heat storage material is a phase change material which will undergo a phase change when the heat storage material is heated to an operational temperature.
8. The concentrated solar power system of claim 1 wherein the cold thermal energy medium is a phase change material which will undergo a phase change when the cold thermal energy medium is cooled to an operational temperature.
9. The concentrated solar power system of claim 3 wherein the power cycle working fluid is a phase change material which will undergo a phase change when the power cycle working fluid is heated to an operational temperature.
10. The concentrated solar power system of claim 1 wherein the power cycle working fluid is utilized to implement at least one of a transcritical power cycle, a supercritical power cycle a Brayton power cycle or a Rankine power cycle.
11. The concentrated solar power system of claim 1 wherein the system utilizes one of carbon dioxide, krypton, argon, nitrogen and air as the power cycle working fluid.
12. The concentrated solar power system of claim 1 further comprising a cold thermal energy storage material in a cold thermal energy storage system which cold thermal energy storage material is in thermal communication with the cold thermal energy medium.
13. A method of operating a concentrated solar power system comprising:
heating a power cycle working fluid with concentrated solar flux;
converting heat from the heated power cycle working fluid to mechanical energy to power electricity generation; and
rejecting waste heat from the heated power cycle working fluid to a cold thermal energy medium in an electrically powered cold thermal energy reservoir, wherein the cold thermal energy medium is cooled to a temperature which is below a contemporaneous ambient air temperature.
14. The method of claim 13 further comprising storing thermal energy in a heat storage material within a hot thermal energy storage reservoir in thermal communication with the power cycle working fluid.
15. The method of claim 13 further comprising cooling the cold thermal energy medium to a temperature which is below the contemporaneous ambient air temperature with a refrigeration cycle powered by a locally generated electricity source, wherein the locally generated electricity source comprises at least one of a photovoltaic panel, a wind driven generation system or a water driven generation system.
16. The method of claim 13 further comprising cooling the cold thermal energy medium to a temperature which is below the contemporaneous ambient air temperature with a refrigeration cycle powered by electricity obtained through a connection to the electric grid.
17. The method of claim 13 further comprising cooling the cold thermal energy medium with a heat pump in thermal communication the cold thermal energy medium, wherein the heat pump uses a high temperature thermal input heated with concentrated solar flux.
18. The method claim 14 wherein the heat storage material is a phase change material which will undergo a phase change when the heat storage material is heated to an operational temperature.
19. The method of claim 13 wherein the cold thermal energy medium is a phase change material which will undergo a phase change when the cold thermal energy medium is cooled to an operational temperature.
20. The method of claim 13 wherein the power cycle working fluid is a phase change material which will undergo a phase change when the power cycle working fluid is heated to an operational temperature.
21. The method of claim 13 wherein the power cycle working fluid is utilized to implement at least one of a transcritical power cycle, a supercritical power cycle a Brayton power cycle or a Rankine power cycle.
22. The method of claim 13 wherein the system utilizes one of carbon dioxide, krypton, argon, nitrogen and air as the power cycle working fluid.
23. The method of claim 13 further comprising storing cold thermal energy in a cold thermal energy storage material of a cold thermal energy storage system, which cold thermal energy storage material is in thermal communication with the cold thermal energy medium.
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