WO2013102537A2 - Electro-thermal energy storage system with improved evaporative ice storage arrangement and method for storing electro-thermal energy - Google Patents

Electro-thermal energy storage system with improved evaporative ice storage arrangement and method for storing electro-thermal energy Download PDF

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Publication number
WO2013102537A2
WO2013102537A2 PCT/EP2012/075206 EP2012075206W WO2013102537A2 WO 2013102537 A2 WO2013102537 A2 WO 2013102537A2 EP 2012075206 W EP2012075206 W EP 2012075206W WO 2013102537 A2 WO2013102537 A2 WO 2013102537A2
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cold
storage
storage medium
thermal
electro
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PCT/EP2012/075206
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French (fr)
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WO2013102537A3 (en
Inventor
Andreas Z'graggen
Jaroslav Hemrle
Lilian Kaufmann
Mehmet Mercangoez
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Abb Research Ltd
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Publication of WO2013102537A3 publication Critical patent/WO2013102537A3/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
    • F01K3/00—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein
    • F01K3/006—Accumulators and steam compressors
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
    • F01K25/00—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for
    • F01K25/08—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for using special vapours
    • F01K25/10—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for using special vapours the vapours being cold, e.g. ammonia, carbon dioxide, ether
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
    • F01K3/00—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein
    • F01K3/12—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein having two or more accumulators
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D20/00—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00
    • F28D20/0034—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using liquid heat storage material
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F28—HEAT EXCHANGE IN GENERAL
    • F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
    • F28D20/00—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00
    • F28D20/02—Heat storage plants or apparatus in general; Regenerative heat-exchange apparatus not covered by groups F28D17/00 or F28D19/00 using latent heat
    • F28D20/028—Control arrangements therefor
    • Y—GENERAL 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
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/14—Thermal energy storage

Definitions

  • the present invention relates generally to electro-thermal energy storage, previously also referred to as thermoelectric energy storage. It relates in particular to a system and method for storing thermal energy on the cold side of a electro-thermal energy storage system using an improved evaporative ice storage arrangement.
  • ice making methods such as scraped ice production, supercooled ice production and evaporative ice making.
  • a compressor is used to increase the pressure of the resulting water vapor, leading to a super-triple point condensation process.
  • the water vapor is at saturation conditions and therefore the increased saturation pressure also increases the saturation temperature. If the saturation temperature becomes high enough, the vapor can be condensed at above 0°C as liquid water and can easily be removed from the system.
  • Such a super-triple point condensation process may be useful for applications requiring continuous ice production.
  • An alternative ice production method is based on the desublimation of vapor (ice formation) at a "condenser" surface.
  • the main disadvantage of such simple evaporative ice production method is that the "condenser” that removes the heat from the system needs to be at temperature below the triple point conditions in an evaporation chamber. This results to so called sub-triple point ice production method, the main disadvantage of which is that solid ice accumulates on the "condenser" by desublimation. If this cannot be prevented, the ice has to be periodically removed or the performance of the condenser deteriorates gradually, as e.g. ice thickness of just about 1 to 2 mm may double the thermal resistance. Additionally, the heat transfer is not constant due to the changing ice thickness and therefore control and system measures have to be adopted to ensure constant conditions on the C0 2 side. In summary, such ice accumulation on heat exchange surfaces results in a deterioration of the heat transfer.
  • the deposited ice is removed periodically by quick heating of the "condenser" surface.
  • the heating of the whole heat exchanger results however in efficiency loss of the system.
  • ETES electro-thermal energy storage
  • An example of an industrial application in which a cold storage feature may be used is a transcritical electro-thermal energy storage (ETES) system, as described by the applicant in an earlier patent application EP 2157317.
  • ETES electro-thermal energy storage
  • Such a ETES system converts excess electricity to heat in a charging cycle, stores the heat, and converts the heat back to electricity in a discharging cycle, when required.
  • Such an energy storage system is robust, compact, site independent and is suited to the storage of electrical energy in large amounts.
  • Thermal energy can be stored in the form of sensible heat via a change in temperature or in the form of latent heat via a change of phase or a combination of both.
  • the round-trip efficiency of an electrical energy storage system can be defined as the percentage of electrical energy that can be discharged from the storage in comparison to the electrical energy used to charge the storage, provided that the state of the energy storage system after discharging returns to its initial condition before charging of the storage.
  • the efficiency of electro-thermal energy storage is limited for various reasons rooted in the second law of thermodynamics. Firstly, the conversion of heat to mechanical work in a heat engine is limited to the Carnot efficiency. Secondly, the coefficient of performance of any heat pump declines with increased difference between input and output temperature levels. Thirdly, any heat flow from a working fluid to a thermal storage and vice versa requires a temperature difference in order to happen. This fact inevitably degrades the temperature level and thus the capability of the heat to do work.
  • thermodynamic irreversibility factor is the transfer of heat over large temperature differences.
  • a relatively large maximum temperature difference between the thermal storage medium and the working fluid reduces the roundtrip efficiency.
  • relatively large heat exchangers could be constructed or phase change materials can be used for thermal storage.
  • the temperature difference between the thermal storage medium and the working fluid may be reduced using intermediate thermal storage tanks as described in the applicant's earlier patent application EP 2275649.
  • an efficient latent heat storage system such as an ice production and storage system which may be used, inter alia, in combination with the cold side of a ETES system to minimize the maximum temperature difference.
  • the ice production and storage system preferably has relatively small temperature approaches, low parasitic losses and convenient scalability. Such factors would also enable the ETES system to have a higher round-trip efficiency.
  • This objective is achieved by a electro-thermal energy storage system having an evaporative ice storage arrangement according to claim 1 and a method according to claim 13. Preferred embodiments are evident from the dependent claims.
  • a electro-thermal energy storage system comprises a hot thermal storage arrangement comprising one or more hot storage heat exchangers; a cold thermal storage arrangement comprising one or more cold storage heat exchangers; a thermodynamic cycle unit comprising at least one working fluid circuit for circulating one or more working fluids through at least one hot storage heat exchanger as well as through at least one cold storage heat exchanger, each working fluid circuit further comprising at least one compressor or pump for compressing the at least one working fluid, at least one expander, preferably a turbine or an expansion valve, for expanding the at least one working fluid, the thermodynamic cycle unit configured to transfer thermal energy from the cold storage unit to the hot storage unit in a charging mode, convert thermal energy from the hot storage unit into mechanical and subsequently electric energy in a discharging mode, wherein the cold storage arrangement comprises a cold thermal storage medium; a vacuum evaporation chamber adapted to evaporate liquid phase cold thermal storage medium to obtain solid phase cold thermal storage medium and gaseous phase cold thermal storage medium; a first fluid connection for supplying gaseous phase cold thermal
  • a method in accordance with the present invention for storing energy in a electro-thermal energy storage system comprising a hot thermal storage arrangement, a cold thermal storage arrangement, a thermodynamic cycle unit comprising at least one working fluid circuit for circulating one or more working fluids through the hot thermal storage arrangement as well as through the cold thermal storage arrangement, each working fluid circuit further comprising at least one compressor or pump for compressing the working fluid, at least one expander, preferably a turbine or an expansion valve, for expanding the working fluid, comprises the steps of (a) transferring, by means of the thermodynamic cycle unit, thermal energy from the cold storage arrangement to the hot storage arrangement in a charging mode, (b) converting, by means of the thermodynamic cycle unit, thermal energy from the hot storage arrangement into mechanical and subsequently electric energy in a discharging mode, wherein in the charging mode (c) a liquid cold thermal storage medium is evaporated in a vacuum evaporation chamber to obtain solid phase cold thermal storage medium and gaseous phase cold thermal storage medium, (d) gaseous
  • a electro-thermal energy storage system which has a charging cycle for providing thermal energy to a hot thermal storage arrangement and an evaporative ice storage arrangement, and a discharging cycle for generating electricity by retrieving the thermal energy from the hot thermal storage arrangement and the evaporative ice storage arrangement.
  • the electro-thermal energy storage system comprises a working fluid circuit adapted to circulate a working fluid through the hot thermal storage arrangement and the evaporative ice storage arrangement.
  • the evaporative ice storage arrangement comprises a heat exchanger coupled to an cold slurry storage tank via a cold thermal storage medium circuit, wherein in the charging cycle system, the heat exchanger is adapted to function as a desublimation evaporator, and an evaporation chamber is coupled between said desublimation evaporator and the cold slurry storage tank in the cold thermal storage medium circuit.
  • the heat exchanger is adapted to function as a condenser, and the cold slurry storage tank is connected to said condenser via the cold thermal storage medium circuit.
  • the evaporative ice storage arrangement of the present invention is also descriptively referred to herein as sub-triple-point evaporation arrangement.
  • a further heat exchanger positioned in the cold thermal storage medium circuit at the outlet of said condenser and an inlet of the cold slurry storage tank in the discharging cycle system.
  • the heat exchanger and the cold slurry storage tank are directly coupled together in both the charging cycle system and the discharging cycle system.
  • the cold thermal storage medium is a mixture of a liquid first component, preferably water, and at least a second, preferably liquid component.
  • the vacuum evaporation chamber comprises a spraying device adapted to spray the cold thermal storage medium into the chamber where triple point conditions, preferably of a predominant component in the mixture are maintained.
  • a disadvantage associated with known ice production through desublimation of vapor at a condenser surface is that ice accumulation on heat exchange surfaces results in a deterioration of the heat transfer coefficient.
  • the present invention overcomes this problem by selecting a second component of the cold thermal storage medium which lowers a freezing point of the liquid first component.
  • the second component of the cold thermal storage medium is chosen such that it is more volatile than the first component, i.e. at given temperature the concentration of the second component in vapor above the liquid phase of cold thermal storage medium is higher than in the cold thermal storage medium itself.
  • a saturated vapor pressure above the cold thermal storage medium decreases as a fraction of the second component in the cold thermal storage medium increases.
  • methanol, ethanol, glycol, or a mixture thereof is preferably used as second component for the cold thermal storage medium, i.e. as volatile additive miscible with water.
  • a fraction of the second component is preferably chosen between 1 % and 12%, preferably between 3% and 10%, most preferably at least approximately 4% of the cold thermal storage medium, e.g. as a mass or volume fraction.
  • the working fluid is preferably carbon dioxide (C0 2 ), or may comprise ammonia (NH 3 ) and/or an organic fluid (such as methane, propane or butane) and/or a refrigerant fluid (such as R 134a (1 ,1 , 1 ,2-Tetrafluoroethane), R245 fa (1 , 1 ,1 ,3,3-Pentafluoropropane)).
  • NH 3 ammonia
  • an organic fluid such as methane, propane or butane
  • a refrigerant fluid such as R 134a (1 ,1 , 1 ,2-Tetrafluoroethane
  • R245 fa (1 , 1 ,1 ,3,3-Pentafluoropropane
  • the evaporative ice storage technology of the present invention can be used in various commercial applications for energy efficiency improvements and load leveling, especially for processes with periodical switching between ice production and ice melting.
  • Examples include air conditioning and district cooling as well as gas turbine air inlet cooling, in which ice could be generated during nighttime and consumed during daytime. It is also envisaged that other applications without such periodical switching could also benefit from the present invention; for example, in food processing applications or in mine cooling.
  • Figure 1 shows a simplified schematic diagram of a charging cycle of a electro-thermal energy storage system of the present invention having an evaporative ice storage arrangement
  • Figure 2a shows a simplified schematic diagram of a discharging cycle of a electrothermal energy storage system of the present invention having an evaporative ice storage arrangement
  • Figure 2b shows a simplified schematic diagram of an alternative embodiment of a discharging cycle of a electro-thermal energy storage system of the present invention having an evaporative ice storage arrangement
  • Figure 3 shows an ideal entropy-temperature diagram of the cycles in a transcritical ETES system
  • Figure 4 shows an entropy-temperature diagram of the cycles in a transcritical ETES system using real mechanical components (turbo machines) and the evaporative ice storage arrangement of the present invention with very small approach temperatures between I and II.
  • Figure 5 shows a phase T-x diagram including evaporation and melting lines of water mixture with ethanol at different mass rations/concentrations.
  • the same reference numerals are used to denote similar elements illustrated throughout the figures.
  • FIGs 1 and 2 schematically depict a charging cycle system and a discharging cycle system, respectively, of a sub-triple-point evaporation arrangement in accordance with an embodiment of the present invention, when connected to a ETES system.
  • the charging cycle system 10 shown in Figure 1 comprises a working fluid circuit of a work recovering expander 12, an evaporator 14, a compressor 16 and a heat exchanger 18.
  • a working fluid circulates through these components as indicated by the solid line with arrows in Figure 1 .
  • the system further comprises a cold-fluid storage tank 20 and a hot- fluid storage tank 22 containing a fluid thermal storage medium, coupled together via the heat exchanger 18.
  • the thermal storage medium flows between these components as indicated by the dashed line with arrows in Figure 1.
  • the system includes a sub-triple- point evaporation arrangement 24 comprising an ice slurry storage tank 26 and a vacuum evaporator chamber 28 and the evaporator 14.
  • Cold thermal storage medium in form of a water-ethanol mixture having a mass ratio of at least approximately 96:4, in various states, circulates through the components of the sub-triple-point evaporation arrangement 24.
  • the circulation of the cold thermal storage medium is indicated by the dotted line with arrows in Figure 1 , where the direct connection between the evaporator 14 and the ice slurry storage tank 26 is an optional feature.
  • Three properties of ethanol and water mixture - as an exemplary cold thermal storage medium having a liquid first component and a second component - contribute to the present inevention.
  • ethanol has lower freezing point than water and may be used as freezing depressant for water, i.e. the freezing point of water-ethanol mixture drops with increasing ethanol concentration in the mixture.
  • ethanol is more volatile than water, i.e. at given temperature the concentration of ethanol in vapor above the liquid phase of water-ethanol mixture is higher than in the liquid itself.
  • the saturated vapor pressure above the mixture at freezing conditions is decreasing with increasing concentration of ethanol in the liquid. Two mechanisms compete here: On one hand, increasing the concentration of more volatile ethanol tends to increase the saturation pressure, but on the other hand, the freezing is depressed to lower temperatures as the concentration of ethanol increases which reduces the pressure. The second effect dominates.
  • the charging cycle system 10 performs a transcritical cycle and the working fluid flows around the ETES system in the following manner.
  • the working fluid in the evaporator 14 absorbs heat from the sub-triple-point evaporation arrangement 24 and evaporates.
  • the vaporized working fluid is circulated to the compressor 16 and surplus electrical energy is utilized to compress and heat the working fluid to a supercritical state. (In such a supercritical state, the fluid is above the critical temperature and critical pressure.)
  • the working fluid is fed through the heat exchanger 18 where the working fluid discards heat energy into the thermal storage medium.
  • a transcritical cycle is defined as a thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. There is no distinction between a gas phase and a vapor phase beyond the supercritical pressure and therefore there is no evaporation or boiling (in the regular meaning) in the transcritical cycle.
  • the compressed working fluid exits the heat exchanger 18 and enters the work recovering expander 12. Here the working fluid is expanded to the lower pressure of the evaporator 14. The working fluid flows from the expander 12 back into the evaporator 14.
  • the thermal storage medium is pumped from the cold-fluid storage tank 20 through the heat exchanger 18 to the hot-fluid storage tank 22.
  • the heat energy discarded from the working fluid into the thermal storage medium is stored in the form of sensible heat.
  • the sub-triple-point evaporation arrangement 24 functions in the following manner during the charging cycle. Cold thermal storage medium in liquid form is sprayed into the vacuum evaporation chamber 28 to significantly enhance the evaporation surface.
  • Triple point conditions are at least approximately maintained in the chamber and so part of the cold thermal storage medium droplets evaporate to form vaporized cold thermal storage medium and consequently another part of the cold thermal storage medium droplets freeze to form solid cold thermal storage medium in the evaporation chamber 28 as solid/liquid slurry (a mass ratio of evaporated to solidified cold thermal storage medium is approximately 1 to 7).
  • the solidified cold thermal storage medium produced in the vacuum evaporation chamber 28 is pumped to slurry storage tank 26.
  • the vaporized cstm vapor produced in the vacuum evaporation chamber 28 flows into the evaporator 14, which acts as condenser for the vaporized cold thermal storage medium.
  • a small fraction of the vaporized cold thermal storage medium may deposit (or "desublimate") on the evaporator surfaces as solid cold thermal storage medium.
  • conditions maintained in the vacuum evaporation chamber 28 preferably correspond at least approximately to triple point conditions of water, which in the present embodiment serves as predominant component of the cold thermal storage medium.
  • a temperature and/or pressure is maintained at values between 10% below and/or 5% above of the corresponding triple point values, preferably at values between 3% below and/or 1 % above, and most preferably at values between 1 % below and or 0.2% above.
  • pressure is preferably maintained between 6.0mbar and 6.4mbar, most preferably at at least approximately 6.2mbar, while temperature is preferably maintained between -1.0°C and -2.0°C, most preferably at at least approximately -1.5°C.
  • the evaporator comprises a housing containing cooling conduits.
  • the vaporized cold thermal storage medium in the evaporator 14 is condenses on the surface of the cooling conduits, so that the evaporator acts as a condenser.
  • the dimensions of the evaporator 14 are adapted to allow optimal removal of the heat from the sub-triple-point evaporation arrangement by evaporation of the working fluid passing through or flowing around the cooling conduits.
  • the discharging cycle system 30 shown in Figure 2a comprises a working fluid circuit of a pump 32, a heat exchanger 18, a turbine 34, and a condenser 36.
  • a working fluid circulates through these components as indicated by the solid line with arrows in Figure 2a.
  • the system further comprises a cold-fluid storage tank 20 and a hot-fluid storage tank 22 containing a fluid thermal storage medium coupled together via the heat exchanger 18.
  • the thermal storage medium flows between these components as indicated by the dashed line with arrows in Figure 2a.
  • the system also includes a sub-triple-point evaporation arrangement 24 comprising slurry storage tank 26, a slurry heat exchanger 40 and the condenser 36.
  • Cold thermal storage medium in various states, circulates through the components of the sub-triple-point evaporation arrangement 24 and through the condenser 36.
  • the circulation of cold thermal storage medium is indicated by the dotted line with arrows in Figure 2a.
  • the discharging cycle system also performs a transcritical cycle and the working fluid flows around the ETES system in the following manner.
  • Heat energy is transferred from the thermal storage medium to the working fluid causing the working fluid to go through transcritical heating.
  • the working fluid then exits the heat exchanger 18 in a supercritical state and enters the turbine 34 where the working fluid is expanded thereby causing the turbine 34 to generate electrical energy.
  • the working fluid passes through the condenser 36 and subsequently the slurry heat exchanger 40, where the working fluid is condensed by exchanging heat energy with the sub-triple-point evaporation arrangement 24.
  • the condensed working fluid exits the sub-triple-point evaporation arrangement 24 via an outlet and is pumped again beyond its critical pressure into the heat exchanger 18 via the pump 32.
  • the sub-triple-point evaporation arrangement 24 functions in the following manner. Initially, the working fluid in gas phase enters the condenser 36 and increases the temperature in the condenser 36 as it condenses. This partially melts any solid cold thermal storage medium or ice that may have built up on the cooling conduits during the charging cycle, causing it to drop off the conduits, from where it may be transferred to slurry storage tank 26.
  • FIG. 2b illustrates an alternative discharging cycle system which similarly comprises a working fluid circuit of a pump 32, a heat exchanger 18, a turbine 34 and a condenser 36.
  • a separate slurry heat exchanger is not required, as the function of the slurry heat exchanger is performed by directing a liquid cold thermal storage medium circuit through the condenser.
  • the solid-line quadrangle shown in the entropy-temperature diagram of Figure 3 represents both the ideal charging and ideal discharging cycles of a standard transcritical ETES system in which on the cold side latent heat is exchanged with the surroundings at ambient temperature.
  • the charging cycle as illustrated, follows a counterclockwise direction and the transcritical discharging cycle follows a clockwise direction.
  • the working fluid is assumed to be carbon dioxide for this exemplary embodiment.
  • latent heat exchange at constant temperature with increasing entropy between point I and point II can clearly be seen and also isentropic compression and expansion with increasing temperature between point II and point III can be seen for charging and discharging, respectively.
  • the entropy of the working fluid falls from 1.70 KJ/kg-K to 1.20 KJ/kg-K during the transcritical cooling by sensible heat exchange between point III, at 120°C, and point IV, at 42°C, in the charging cycle.
  • the transition between point IV and point I occurs with a change in temperature and isentropic compression and expansion for discharging and charging, respectively.
  • Figure 4 shows an entropy-temperature diagram of the charging and discharging cycles in a transcritical ETES system using the sub-triple-point evaporation arrangement 24 of the present invention.
  • the charging cycle is indicated by a dotted line and follows a counter- clockwise direction and the discharging cycle is indicated by a solid line and follows a clockwise direction.
  • the working fluid is assumed to be carbon dioxide for this exemplary embodiment.
  • Latent heat exchange at constant temperature with increasing entropy between point I and point II can clearly be seen.
  • heat is transferred from the cold storage fluid at 0°C to the working fluid at -°2 C.
  • discharging heat is transferred from the working fluid at 1 °C to the cold storage fluid at 0°C.
  • Point I and point II in Figure 4 can be equated with the working fluid passing through the evaporator 14 in Figure 1 and the condenser 36 in Figures 2a and 2b.
  • Point IV in Figure 4 can be equated with the working fluid passing through section A of the working fluid circuit in Figures 1 , 2a and 2b.
  • the heat transfer on the cold side heat exchanger is isothermal.
  • the sub-triple-point evaporation arrangement can be run at small approach temperatures due to latent heat exchange with solid cold thermal storage medium on the cold side of the ETES and thus the sub-triple-point evaporation arrangement of the present invention has greater efficiency than a system exchanging sensible heat on the cold side, e.g. with a cold river.
  • Fig. 5 illustrates the solid cold thermal storage medium generation as described above by means of a phase T-x diagram showing evaporation (bubble) lines 102 and condensation (dew) lines 101 of water mixture with ethanol at different concentrations/mass rations, with pure water being on the right of the scale.
  • the phase change lines are plotted for a number of pressures, in particular for 6.2mbar which is close to the equilibrium pressure above freezing mixture with about 4% ethanol.
  • the diagram additionally includes a freezing line 103 showing the dependence of a freezing temperature of the mixture at varying ethanol concentration. While the evaporation and condensation lines are at constant pressure, the pressure drops at higher ethanol concentrations along the freezing line.
  • the freezing point i.e.
  • the triple point, of a mixture with 4 % ethanol at the pressure of 6.2 mbar will be at temperature of about -1.5 °C. If the mixture evaporates at these conditions, the latent heat of evaporation will be consumed by the formation of ice as in the case when pure water is used. However, the ethanol concentration in the vapor phase increases to about 27%, as seen on the evaporation line at temperature -1.5 °C. At ideal conditions, i.e. without any pressure drop, this vapor mixture will enter the evaporator 14. As seen from the T-x diagram, if a surface of evaporator 14 has temperature of e.g.
  • condensate cold thermal storage medium forming on the evaporator surface will have higher concentration of ethanol of about 12 %, which is higher than the concentration in the evaporation chamber 28. Because the freezing point is suppressed at this concentration to about -6 °C the condensate cold thermal storage medium at such conditions would be in liquid form.
  • the condensate cold thermal storage medium may be returned to the evaporation chamber so the ethanol forms essentially a closed loop with the water as the carrier.
  • Pure water is preferably supplied to the evaporation chamber 28 continuously, while essentially pure ice (subcooled to -1.5 °C) is formed in the evaporation chamber 28 and can be transferred to the slurry storage tank 26.
  • the process described here assumes that there are no non-condensing gases in the system. With condensing gases, in particular air, the system may operate even at high pressures, but circulation ventilators would be needed to achieve reasonable heat transfer rates.
  • Figure 5 also illustrates the real effect of vapor pressure drop, also referred to as throttling, between evaporation chamber 28 and the evaporator 14. For this phase change lines are shown for several pressure values. As is seen, the freezing line 103 crosses melting lines 102 at lower pressures as ethanol concentration increases. The fact that the freezing temperature drops faster with increasing ethanol concentration than the melting temperature at constant pressure is important, because as a result the conditions on colder evaporator 14 are further from the freezing line which allows the condensation to liquid to take place.
  • the relative size of the evaporator component 14 in the charging cycle system (and equivalently the condenser component 36 in the discharging cycle) is an important factor in the efficient functioning of the sub-triple-point evaporation arrangement.
  • the surface area of the cooling conduits in the evaporator 14 are preferably sized so that it balances well the power duty requirement, allowing enough surface area to limit a thickness of any solid cold thermal storage medium that may built up.
  • the heat exchanger 18, cold-fluid storage 20, hot-fluid storage 22 and thermal storage medium of the ETES system are preferably common to both cycles.
  • the evaporator 14 and the condenser 36 in the sub- triple-point evaporation arrangement 24 may be the same apparatus operated in opposing directions for the charging and discharging cycles respectively.
  • the turbine 34 and the compressor 16 roles as well as the pump 32 / expander 12 roles may be carried out by the same machinery, capable of achieving both tasks.
  • the charging and discharging cycles may be performed consecutively, not simultaneously.
  • a pipe may be connected directly between the evaporator 14 and the slurry storage tank 26 during a charging cycle. Parts of solid cstm that may build up on the evaporator 14 which are spontaneously released or actively removed may be moved to the slurry storage tank 26 via said pipe.
  • the temperature difference between the thermal storage medium and the working fluid may be reduced using intermediate storage tanks as described in the applicant's earlier patent application EP09163084 and thereby increasing the system efficiency.
  • the heat exchanger is a counterflow heat exchanger, and the working fluid of the cycle is preferably carbon dioxide.
  • the compressor 16 is an electrically powered compressor.
  • the counterflow heat exchanger may have a minimal approach temperature, ATmin, of 3 K (ie. the minimal temperature difference between the two fluids exchanging heat is 3 K).
  • ATmin the minimal temperature difference between the two fluids exchanging heat is 3 K.
  • the approach temperature should be as small as possible.
  • the evaporator 14 is a vertically oriented desublimation evaporator of the "shell and tube” type, having working fluid circulating inside of the "tubes” (referred to as conduits herein), and the vaporized cold thermal storage medium on the "shell” side.
  • the surface of the cooling conduits can be increased by utilizing cross sections other than circular, or by the addition of fins.
  • the fins improve the deposition heat transfer by the increasing surface.
  • the preferred working fluid for circulating in the ETES system is carbon dioxide; mainly due to the higher efficiencies in heat transfer processes and the amiable properties of carbon dioxide as a natural working fluid i.e. non-flammable, no ozone depletion potential, no health hazards etc.
  • the thermal storage medium in the hot side of the ETES system is generally water (if necessary, in a pressurized container), other materials, such as oil or molten salt, may also be used.
  • the present invention has the following advantages; minimal system irreversibilities through gentle heat transfer due to relatively small temperature approaches, low parasitic losses as no large fans or compressors are required, and good scalability as the size of the heat exchanger on the cold side of the cycle may be decoupled from the size of the cold storage.
  • sub-triple point evaporative ice storage technology of the present invention can be used in various commercial applications where energy efficiency improvements and load leveling are sought. Examples include air conditioning and district cooling, gas turbine air inlet cooling, food processing, or mine cooling. This list of examples is not exhaustive.

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Abstract

A system and method for electro-thermal energy storage is described. The system has a charging cycle (10) for providing thermal energy to a hot thermal storage arrangement (18, 20, 22) and an evaporative ice storage arrangement (24) and a discharging cycle (30) for generating electricity by retrieving the thermal energy. The evaporative ice storage arrangement (24) comprises a heat exchanger (14, 36), an ice slurry storage tank (26), a vacuum evaporation chamber (28) and a slurry heat exchanger (40). The evaporative ice storage arrangement (24) of the present invention functions as a dedicated cold storage for the electro-thermal energy storage system. The cold storage is realized by producing an ice-water mixture during charging of the storage, and using the stored ice-water mixture to condense the working fluid during the discharge cycle (30). Use of this subtriple-point evaporation arrangement functions to increase the round-trip efficiency of the ETES system through minimising the maximum temperature difference between the working fluid and the thermal storage medium during operating cycles.

Description

ELECTRO-THERMAL ENERGY STORAGE SYSTEM WITH IMPROVED
EVAPORATIVE ICE STORAGE ARRANGEMENT AND METHOD FOR STORING
ELECTRO-THERMAL ENERGY
FIELD OF THE INVENTION
The present invention relates generally to electro-thermal energy storage, previously also referred to as thermoelectric energy storage. It relates in particular to a system and method for storing thermal energy on the cold side of a electro-thermal energy storage system using an improved evaporative ice storage arrangement.
BACKGROUND OF THE INVENTION
Storage systems for cold or ice are valuable in many industries and applications. Various ice making methods are known, such as scraped ice production, supercooled ice production and evaporative ice making. In evaporative ice making systems, a compressor is used to increase the pressure of the resulting water vapor, leading to a super-triple point condensation process. The water vapor is at saturation conditions and therefore the increased saturation pressure also increases the saturation temperature. If the saturation temperature becomes high enough, the vapor can be condensed at above 0°C as liquid water and can easily be removed from the system. Such a super-triple point condensation process may be useful for applications requiring continuous ice production. However, known super-triple point evaporation systems are not suitable for certain industrial applications because the size of the compressor limits the capacity of the evaporative ice making system to approximately 3 to 5 MW thermal duty. Such thermal duty is too small for many commercially-viable, industrial applications.
An alternative ice production method is based on the desublimation of vapor (ice formation) at a "condenser" surface. The main disadvantage of such simple evaporative ice production method is that the "condenser" that removes the heat from the system needs to be at temperature below the triple point conditions in an evaporation chamber. This results to so called sub-triple point ice production method, the main disadvantage of which is that solid ice accumulates on the "condenser" by desublimation. If this cannot be prevented, the ice has to be periodically removed or the performance of the condenser deteriorates gradually, as e.g. ice thickness of just about 1 to 2 mm may double the thermal resistance. Additionally, the heat transfer is not constant due to the changing ice thickness and therefore control and system measures have to be adopted to ensure constant conditions on the C02 side. In summary, such ice accumulation on heat exchange surfaces results in a deterioration of the heat transfer.
In another method, where ice is produced on plates or on tubes, the deposited ice is removed periodically by quick heating of the "condenser" surface. The heating of the whole heat exchanger results however in efficiency loss of the system.
An example of an industrial application in which a cold storage feature may be used, is a transcritical electro-thermal energy storage (ETES) system, as described by the applicant in an earlier patent application EP 2157317. Such a ETES system converts excess electricity to heat in a charging cycle, stores the heat, and converts the heat back to electricity in a discharging cycle, when required. Such an energy storage system is robust, compact, site independent and is suited to the storage of electrical energy in large amounts. Thermal energy can be stored in the form of sensible heat via a change in temperature or in the form of latent heat via a change of phase or a combination of both.
The round-trip efficiency of an electrical energy storage system can be defined as the percentage of electrical energy that can be discharged from the storage in comparison to the electrical energy used to charge the storage, provided that the state of the energy storage system after discharging returns to its initial condition before charging of the storage. The efficiency of electro-thermal energy storage is limited for various reasons rooted in the second law of thermodynamics. Firstly, the conversion of heat to mechanical work in a heat engine is limited to the Carnot efficiency. Secondly, the coefficient of performance of any heat pump declines with increased difference between input and output temperature levels. Thirdly, any heat flow from a working fluid to a thermal storage and vice versa requires a temperature difference in order to happen. This fact inevitably degrades the temperature level and thus the capability of the heat to do work.
It is established that a thermodynamic irreversibility factor is the transfer of heat over large temperature differences. A relatively large maximum temperature difference between the thermal storage medium and the working fluid (whether charging or discharging) reduces the roundtrip efficiency. In order to minimize this maximum temperature difference, relatively large heat exchangers could be constructed or phase change materials can be used for thermal storage. Problematically, these solutions result in a high capital cost and therefore are not generally practical.
On the hot side of a ETES cycle, the temperature difference between the thermal storage medium and the working fluid may be reduced using intermediate thermal storage tanks as described in the applicant's earlier patent application EP 2275649.
Thus, there is a need to provide an efficient latent heat storage system such as an ice production and storage system which may be used, inter alia, in combination with the cold side of a ETES system to minimize the maximum temperature difference. The ice production and storage system preferably has relatively small temperature approaches, low parasitic losses and convenient scalability. Such factors would also enable the ETES system to have a higher round-trip efficiency.
DESCRIPTION OF THE INVENTION
It is an objective of the invention to provide a electro-thermal energy storage system for converting electrical energy into thermal energy to be stored and converted back to electrical energy with an improved round-trip efficiency. This objective is achieved by a electro-thermal energy storage system having an evaporative ice storage arrangement according to claim 1 and a method according to claim 13. Preferred embodiments are evident from the dependent claims.
A electro-thermal energy storage system according to the invention comprises a hot thermal storage arrangement comprising one or more hot storage heat exchangers; a cold thermal storage arrangement comprising one or more cold storage heat exchangers; a thermodynamic cycle unit comprising at least one working fluid circuit for circulating one or more working fluids through at least one hot storage heat exchanger as well as through at least one cold storage heat exchanger, each working fluid circuit further comprising at least one compressor or pump for compressing the at least one working fluid, at least one expander, preferably a turbine or an expansion valve, for expanding the at least one working fluid, the thermodynamic cycle unit configured to transfer thermal energy from the cold storage unit to the hot storage unit in a charging mode, convert thermal energy from the hot storage unit into mechanical and subsequently electric energy in a discharging mode, wherein the cold storage arrangement comprises a cold thermal storage medium; a vacuum evaporation chamber adapted to evaporate liquid phase cold thermal storage medium to obtain solid phase cold thermal storage medium and gaseous phase cold thermal storage medium; a first fluid connection for supplying gaseous phase cold thermal storage medium from said vacuum evaporation chamber to at least one cold storage heat exchanger for evaporating working fluid through condensation of vaporized cold thermal storage medium during the charging cycle; a second fluid connection for connecting said vacuum evaporation chamber to a cold slurry storage tank for storing a mixture of solid phase cold thermal storage medium and liquid phase cold thermal storage medium during the charging cycle; and a third fluid connection for connecting said cold slurry storage tank to at least one cold storage heat exchanger for cooling or condensing working fluid during the discharge cycle.
A method in accordance with the present invention for storing energy in a electro-thermal energy storage system, said electro-thermal energy storage system comprising a hot thermal storage arrangement, a cold thermal storage arrangement, a thermodynamic cycle unit comprising at least one working fluid circuit for circulating one or more working fluids through the hot thermal storage arrangement as well as through the cold thermal storage arrangement, each working fluid circuit further comprising at least one compressor or pump for compressing the working fluid, at least one expander, preferably a turbine or an expansion valve, for expanding the working fluid, comprises the steps of (a) transferring, by means of the thermodynamic cycle unit, thermal energy from the cold storage arrangement to the hot storage arrangement in a charging mode, (b) converting, by means of the thermodynamic cycle unit, thermal energy from the hot storage arrangement into mechanical and subsequently electric energy in a discharging mode, wherein in the charging mode (c) a liquid cold thermal storage medium is evaporated in a vacuum evaporation chamber to obtain solid phase cold thermal storage medium and gaseous phase cold thermal storage medium, (d) gaseous phase cold thermal storage medium from said vacuum evaporation chamber is condensed in the cold thermal storage unit in order to evaporate at least one working fluid, (e) a mixture of solid phase cold thermal storage medium and liquid phase cold thermal storage medium is stored in a cold slurry tank; and in the discharging mode, (f) solid phase cold thermal storage medium from the cold slurry tank is used to cool or condense at least one working fluid.
According to a further aspect of the invention, a electro-thermal energy storage system is provided which has a charging cycle for providing thermal energy to a hot thermal storage arrangement and an evaporative ice storage arrangement, and a discharging cycle for generating electricity by retrieving the thermal energy from the hot thermal storage arrangement and the evaporative ice storage arrangement. The electro-thermal energy storage system comprises a working fluid circuit adapted to circulate a working fluid through the hot thermal storage arrangement and the evaporative ice storage arrangement. The evaporative ice storage arrangement comprises a heat exchanger coupled to an cold slurry storage tank via a cold thermal storage medium circuit, wherein in the charging cycle system, the heat exchanger is adapted to function as a desublimation evaporator, and an evaporation chamber is coupled between said desublimation evaporator and the cold slurry storage tank in the cold thermal storage medium circuit. In the discharging cycle system, the heat exchanger is adapted to function as a condenser, and the cold slurry storage tank is connected to said condenser via the cold thermal storage medium circuit. (The evaporative ice storage arrangement of the present invention is also descriptively referred to herein as sub-triple-point evaporation arrangement.)
Thus, it is understood that where the sub-triple-point evaporation arrangement operates in connection with a standard ETES system, then ice is produced in the sub-triple-point evaporation arrangement during the ETES charging cycle (via working fluid evaporation), while ice would be consumed during the ETES discharging cycle (via working fluid condensation).
Utilisation of the sub-triple-point evaporation arrangement (also referred to herein as the evaporative ice storage arrangement) of the present invention is highly advantageous for the transcritical ETES system because relatively small approach temperatures are achievable.
In a preferred embodiment, a further heat exchanger positioned in the cold thermal storage medium circuit at the outlet of said condenser and an inlet of the cold slurry storage tank in the discharging cycle system. In an alternative embodiment, the heat exchanger and the cold slurry storage tank are directly coupled together in both the charging cycle system and the discharging cycle system.
In the present invention, the cold thermal storage medium is a mixture of a liquid first component, preferably water, and at least a second, preferably liquid component. The vacuum evaporation chamber comprises a spraying device adapted to spray the cold thermal storage medium into the chamber where triple point conditions, preferably of a predominant component in the mixture are maintained.
A disadvantage associated with known ice production through desublimation of vapor at a condenser surface is that ice accumulation on heat exchange surfaces results in a deterioration of the heat transfer coefficient. However, the present invention overcomes this problem by selecting a second component of the cold thermal storage medium which lowers a freezing point of the liquid first component. Preferably, the second component of the cold thermal storage medium is chosen such that it is more volatile than the first component, i.e. at given temperature the concentration of the second component in vapor above the liquid phase of cold thermal storage medium is higher than in the cold thermal storage medium itself. Preferably, in addition, a saturated vapor pressure above the cold thermal storage medium decreases as a fraction of the second component in the cold thermal storage medium increases. Thus, in particular when water is used as a liquid first component of the cold thermal storage medium, methanol, ethanol, glycol, or a mixture thereof is preferably used as second component for the cold thermal storage medium, i.e. as volatile additive miscible with water. A fraction of the second component is preferably chosen between 1 % and 12%, preferably between 3% and 10%, most preferably at least approximately 4% of the cold thermal storage medium, e.g. as a mass or volume fraction.
Advantageously, exchanging latent heat with ice allows for relatively compact cold storage and relatively small mass flows when compared with sensible-heat water storages. Thus, the footprint of the ETES system is reduced when connected to the evaporative ice storage of the present invention. The working fluid is preferably carbon dioxide (C02), or may comprise ammonia (NH3) and/or an organic fluid (such as methane, propane or butane) and/or a refrigerant fluid (such as R 134a (1 ,1 , 1 ,2-Tetrafluoroethane), R245 fa (1 , 1 ,1 ,3,3-Pentafluoropropane)). Advantageously, the evaporative ice storage technology of the present invention can be used in various commercial applications for energy efficiency improvements and load leveling, especially for processes with periodical switching between ice production and ice melting. Examples include air conditioning and district cooling as well as gas turbine air inlet cooling, in which ice could be generated during nighttime and consumed during daytime. It is also envisaged that other applications without such periodical switching could also benefit from the present invention; for example, in food processing applications or in mine cooling.
Further advantage is gained in that the instant invention does not require a compressor to function and therefore the high costs and space requirements associated with compressors are avoided.
If technically possible but not explicitly mentioned, also combinations of embodiments of the invention described in the above and in the following may be embodiments of the method and the system. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments, which are illustrated in the attached drawings, in which: Figure 1 shows a simplified schematic diagram of a charging cycle of a electro-thermal energy storage system of the present invention having an evaporative ice storage arrangement; Figure 2a shows a simplified schematic diagram of a discharging cycle of a electrothermal energy storage system of the present invention having an evaporative ice storage arrangement; Figure 2b shows a simplified schematic diagram of an alternative embodiment of a discharging cycle of a electro-thermal energy storage system of the present invention having an evaporative ice storage arrangement;
Figure 3 shows an ideal entropy-temperature diagram of the cycles in a transcritical ETES system;
Figure 4 shows an entropy-temperature diagram of the cycles in a transcritical ETES system using real mechanical components (turbo machines) and the evaporative ice storage arrangement of the present invention with very small approach temperatures between I and II.
Figure 5 shows a phase T-x diagram including evaporation and melting lines of water mixture with ethanol at different mass rations/concentrations. For consistency, the same reference numerals are used to denote similar elements illustrated throughout the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Figures 1 and 2 schematically depict a charging cycle system and a discharging cycle system, respectively, of a sub-triple-point evaporation arrangement in accordance with an embodiment of the present invention, when connected to a ETES system. The charging cycle system 10 shown in Figure 1 comprises a working fluid circuit of a work recovering expander 12, an evaporator 14, a compressor 16 and a heat exchanger 18. A working fluid circulates through these components as indicated by the solid line with arrows in Figure 1 . The system further comprises a cold-fluid storage tank 20 and a hot- fluid storage tank 22 containing a fluid thermal storage medium, coupled together via the heat exchanger 18. The thermal storage medium flows between these components as indicated by the dashed line with arrows in Figure 1. The system includes a sub-triple- point evaporation arrangement 24 comprising an ice slurry storage tank 26 and a vacuum evaporator chamber 28 and the evaporator 14. Cold thermal storage medium in form of a water-ethanol mixture having a mass ratio of at least approximately 96:4, in various states, circulates through the components of the sub-triple-point evaporation arrangement 24. The circulation of the cold thermal storage medium is indicated by the dotted line with arrows in Figure 1 , where the direct connection between the evaporator 14 and the ice slurry storage tank 26 is an optional feature. Three properties of ethanol and water mixture - as an exemplary cold thermal storage medium having a liquid first component and a second component - contribute to the present inevention. First, ethanol has lower freezing point than water and may be used as freezing depressant for water, i.e. the freezing point of water-ethanol mixture drops with increasing ethanol concentration in the mixture. Second, ethanol is more volatile than water, i.e. at given temperature the concentration of ethanol in vapor above the liquid phase of water-ethanol mixture is higher than in the liquid itself. Third, the saturated vapor pressure above the mixture at freezing conditions is decreasing with increasing concentration of ethanol in the liquid. Two mechanisms compete here: On one hand, increasing the concentration of more volatile ethanol tends to increase the saturation pressure, but on the other hand, the freezing is depressed to lower temperatures as the concentration of ethanol increases which reduces the pressure. The second effect dominates.
In operation, the charging cycle system 10 performs a transcritical cycle and the working fluid flows around the ETES system in the following manner. The working fluid in the evaporator 14 absorbs heat from the sub-triple-point evaporation arrangement 24 and evaporates. The vaporized working fluid is circulated to the compressor 16 and surplus electrical energy is utilized to compress and heat the working fluid to a supercritical state. (In such a supercritical state, the fluid is above the critical temperature and critical pressure.) The working fluid is fed through the heat exchanger 18 where the working fluid discards heat energy into the thermal storage medium.
It is noted that in the heat exchanger 18 the working fluid pressure will be above the critical pressure, however the working fluid temperature may go below the critical temperature. Therefore, whilst the working fluid enters the heat exchanger 18 in a supercritical state, it may leave the heat exchanger 18 in a subcritical state. A transcritical cycle is defined as a thermodynamic cycle where the working fluid goes through both subcritical and supercritical states. There is no distinction between a gas phase and a vapor phase beyond the supercritical pressure and therefore there is no evaporation or boiling (in the regular meaning) in the transcritical cycle.
The compressed working fluid exits the heat exchanger 18 and enters the work recovering expander 12. Here the working fluid is expanded to the lower pressure of the evaporator 14. The working fluid flows from the expander 12 back into the evaporator 14.
The thermal storage medium is pumped from the cold-fluid storage tank 20 through the heat exchanger 18 to the hot-fluid storage tank 22. The heat energy discarded from the working fluid into the thermal storage medium is stored in the form of sensible heat. In operation, the sub-triple-point evaporation arrangement 24 functions in the following manner during the charging cycle. Cold thermal storage medium in liquid form is sprayed into the vacuum evaporation chamber 28 to significantly enhance the evaporation surface. Triple point conditions are at least approximately maintained in the chamber and so part of the cold thermal storage medium droplets evaporate to form vaporized cold thermal storage medium and consequently another part of the cold thermal storage medium droplets freeze to form solid cold thermal storage medium in the evaporation chamber 28 as solid/liquid slurry (a mass ratio of evaporated to solidified cold thermal storage medium is approximately 1 to 7). The solidified cold thermal storage medium produced in the vacuum evaporation chamber 28 is pumped to slurry storage tank 26. The vaporized cstm vapor produced in the vacuum evaporation chamber 28 flows into the evaporator 14, which acts as condenser for the vaporized cold thermal storage medium. Depending on the operating conditions, a small fraction of the vaporized cold thermal storage medium may deposit (or "desublimate") on the evaporator surfaces as solid cold thermal storage medium.
As mentioned, conditions maintained in the vacuum evaporation chamber 28 preferably correspond at least approximately to triple point conditions of water, which in the present embodiment serves as predominant component of the cold thermal storage medium. In particular, a temperature and/or pressure is maintained at values between 10% below and/or 5% above of the corresponding triple point values, preferably at values between 3% below and/or 1 % above, and most preferably at values between 1 % below and or 0.2% above. E.g., for the cold thermal storage medium chosen as described above, pressure is preferably maintained between 6.0mbar and 6.4mbar, most preferably at at least approximately 6.2mbar, while temperature is preferably maintained between -1.0°C and -2.0°C, most preferably at at least approximately -1.5°C.
The evaporator comprises a housing containing cooling conduits. As described above, the vaporized cold thermal storage medium in the evaporator 14 is condenses on the surface of the cooling conduits, so that the evaporator acts as a condenser. The dimensions of the evaporator 14 are adapted to allow optimal removal of the heat from the sub-triple-point evaporation arrangement by evaporation of the working fluid passing through or flowing around the cooling conduits.
The discharging cycle system 30 shown in Figure 2a comprises a working fluid circuit of a pump 32, a heat exchanger 18, a turbine 34, and a condenser 36. A working fluid circulates through these components as indicated by the solid line with arrows in Figure 2a. The system further comprises a cold-fluid storage tank 20 and a hot-fluid storage tank 22 containing a fluid thermal storage medium coupled together via the heat exchanger 18. The thermal storage medium flows between these components as indicated by the dashed line with arrows in Figure 2a. The system also includes a sub-triple-point evaporation arrangement 24 comprising slurry storage tank 26, a slurry heat exchanger 40 and the condenser 36. Cold thermal storage medium, in various states, circulates through the components of the sub-triple-point evaporation arrangement 24 and through the condenser 36. The circulation of cold thermal storage medium is indicated by the dotted line with arrows in Figure 2a.
In operation, the discharging cycle system also performs a transcritical cycle and the working fluid flows around the ETES system in the following manner. Heat energy is transferred from the thermal storage medium to the working fluid causing the working fluid to go through transcritical heating. The working fluid then exits the heat exchanger 18 in a supercritical state and enters the turbine 34 where the working fluid is expanded thereby causing the turbine 34 to generate electrical energy. Next, the working fluid passes through the condenser 36 and subsequently the slurry heat exchanger 40, where the working fluid is condensed by exchanging heat energy with the sub-triple-point evaporation arrangement 24. The condensed working fluid exits the sub-triple-point evaporation arrangement 24 via an outlet and is pumped again beyond its critical pressure into the heat exchanger 18 via the pump 32.
During the discharging cycle, the sub-triple-point evaporation arrangement 24 functions in the following manner. Initially, the working fluid in gas phase enters the condenser 36 and increases the temperature in the condenser 36 as it condenses. This partially melts any solid cold thermal storage medium or ice that may have built up on the cooling conduits during the charging cycle, causing it to drop off the conduits, from where it may be transferred to slurry storage tank 26. Both the solid/liquid cold thermal storage medium slurry and the working fluid after exiting the condenser 36 may be subsequently circulated through the slurry heat exchanger 40 where the temperature of the working fluid is reduced thereby condensing the remaining working fluid vapor and consequently solid cold thermal storage medium in the slurry is subsequently melted. Figure 2b illustrates an alternative discharging cycle system which similarly comprises a working fluid circuit of a pump 32, a heat exchanger 18, a turbine 34 and a condenser 36. However, in this embodiment a separate slurry heat exchanger is not required, as the function of the slurry heat exchanger is performed by directing a liquid cold thermal storage medium circuit through the condenser. The further features of this alternative discharging cycle system are identical to those described in respect of Figure 2a.
The solid-line quadrangle shown in the entropy-temperature diagram of Figure 3 represents both the ideal charging and ideal discharging cycles of a standard transcritical ETES system in which on the cold side latent heat is exchanged with the surroundings at ambient temperature. Specifically, the charging cycle, as illustrated, follows a counterclockwise direction and the transcritical discharging cycle follows a clockwise direction. The working fluid is assumed to be carbon dioxide for this exemplary embodiment. In this diagram, latent heat exchange at constant temperature with increasing entropy between point I and point II can clearly be seen and also isentropic compression and expansion with increasing temperature between point II and point III can be seen for charging and discharging, respectively. In the exemplary embodiment shown in Figure 3, the entropy of the working fluid falls from 1.70 KJ/kg-K to 1.20 KJ/kg-K during the transcritical cooling by sensible heat exchange between point III, at 120°C, and point IV, at 42°C, in the charging cycle. The transition between point IV and point I occurs with a change in temperature and isentropic compression and expansion for discharging and charging, respectively. Figure 4 shows an entropy-temperature diagram of the charging and discharging cycles in a transcritical ETES system using the sub-triple-point evaporation arrangement 24 of the present invention. The charging cycle is indicated by a dotted line and follows a counter- clockwise direction and the discharging cycle is indicated by a solid line and follows a clockwise direction. The working fluid is assumed to be carbon dioxide for this exemplary embodiment. Latent heat exchange at constant temperature with increasing entropy between point I and point II can clearly be seen. During the charging cycle heat is transferred from the cold storage fluid at 0°C to the working fluid at -°2 C. Inversely, during discharging heat is transferred from the working fluid at 1 °C to the cold storage fluid at 0°C. Point I and point II in Figure 4 can be equated with the working fluid passing through the evaporator 14 in Figure 1 and the condenser 36 in Figures 2a and 2b.
Polytropic compression and expansion with increasing temperature between point II and point III can be seen for charging and discharging, respectively. As a consequence of thermodynamic irreversibilities, such as non zero approach temperatures in the heat exchangers and non isentropic compression and expansion in the turbomachines, the charging cycle and discharging cycle follow differing entropy-temperature paths. In the exemplary embodiment shown in Figure 4, the entropy of the working fluid falls from -0.8 KJ/kg-K to -1.6 KJ/kg-K during the transcritical cooling by sensible heat exchange between point III, at 120°C, and point IV, at 12°C, in the charging cycle. The transition between point IV and point I occurs with a change in temperature and polytropic compression and expansion for discharging and charging, respectively. Point IV in Figure 4 can be equated with the working fluid passing through section A of the working fluid circuit in Figures 1 , 2a and 2b. Thus, it will be clear to the skilled person that the heat transfer on the cold side heat exchanger is isothermal. Importantly, the sub-triple-point evaporation arrangement can be run at small approach temperatures due to latent heat exchange with solid cold thermal storage medium on the cold side of the ETES and thus the sub-triple-point evaporation arrangement of the present invention has greater efficiency than a system exchanging sensible heat on the cold side, e.g. with a cold river.
Fig. 5 illustrates the solid cold thermal storage medium generation as described above by means of a phase T-x diagram showing evaporation (bubble) lines 102 and condensation (dew) lines 101 of water mixture with ethanol at different concentrations/mass rations, with pure water being on the right of the scale. The phase change lines are plotted for a number of pressures, in particular for 6.2mbar which is close to the equilibrium pressure above freezing mixture with about 4% ethanol. The diagram additionally includes a freezing line 103 showing the dependence of a freezing temperature of the mixture at varying ethanol concentration. While the evaporation and condensation lines are at constant pressure, the pressure drops at higher ethanol concentrations along the freezing line. The freezing point, i.e. the triple point, of a mixture with 4 % ethanol at the pressure of 6.2 mbar will be at temperature of about -1.5 °C. If the mixture evaporates at these conditions, the latent heat of evaporation will be consumed by the formation of ice as in the case when pure water is used. However, the ethanol concentration in the vapor phase increases to about 27%, as seen on the evaporation line at temperature -1.5 °C. At ideal conditions, i.e. without any pressure drop, this vapor mixture will enter the evaporator 14. As seen from the T-x diagram, if a surface of evaporator 14 has temperature of e.g. about -4 °C, condensate cold thermal storage medium forming on the evaporator surface will have higher concentration of ethanol of about 12 %, which is higher than the concentration in the evaporation chamber 28. Because the freezing point is suppressed at this concentration to about -6 °C the condensate cold thermal storage medium at such conditions would be in liquid form. The condensate cold thermal storage medium may be returned to the evaporation chamber so the ethanol forms essentially a closed loop with the water as the carrier.
Pure water is preferably supplied to the evaporation chamber 28 continuously, while essentially pure ice (subcooled to -1.5 °C) is formed in the evaporation chamber 28 and can be transferred to the slurry storage tank 26. The process described here assumes that there are no non-condensing gases in the system. With condensing gases, in particular air, the system may operate even at high pressures, but circulation ventilators would be needed to achieve reasonable heat transfer rates. Figure 5 also illustrates the real effect of vapor pressure drop, also referred to as throttling, between evaporation chamber 28 and the evaporator 14. For this phase change lines are shown for several pressure values. As is seen, the freezing line 103 crosses melting lines 102 at lower pressures as ethanol concentration increases. The fact that the freezing temperature drops faster with increasing ethanol concentration than the melting temperature at constant pressure is important, because as a result the conditions on colder evaporator 14 are further from the freezing line which allows the condensation to liquid to take place.
However, it is seen from Figure 7, that if too high a pressure drop takes place between the evaporation chamber 28 and the evaporator 14, the conditions at the evaporator 14 may shift to or below the freezing line, i.e. the condensation would take place below freezing line 103.
The relative size of the evaporator component 14 in the charging cycle system (and equivalently the condenser component 36 in the discharging cycle) is an important factor in the efficient functioning of the sub-triple-point evaporation arrangement. Specifically, the surface area of the cooling conduits in the evaporator 14 are preferably sized so that it balances well the power duty requirement, allowing enough surface area to limit a thickness of any solid cold thermal storage medium that may built up.
Whilst the charging cycle system 10 of Figure 1 and the discharging cycle systems 30 of Figures 2a and 2b have been illustrated separately, the heat exchanger 18, cold-fluid storage 20, hot-fluid storage 22 and thermal storage medium of the ETES system are preferably common to both cycles. The evaporator 14 and the condenser 36 in the sub- triple-point evaporation arrangement 24 may be the same apparatus operated in opposing directions for the charging and discharging cycles respectively. Similarly the turbine 34 and the compressor 16 roles as well as the pump 32 / expander 12 roles may be carried out by the same machinery, capable of achieving both tasks. The charging and discharging cycles may be performed consecutively, not simultaneously.
In an alternative embodiment of the sub-triple-point evaporation arrangement 24 of the present invention, a pipe may be connected directly between the evaporator 14 and the slurry storage tank 26 during a charging cycle. Parts of solid cstm that may build up on the evaporator 14 which are spontaneously released or actively removed may be moved to the slurry storage tank 26 via said pipe.
It is noted that on the hot side of a ETES, the temperature difference between the thermal storage medium and the working fluid may be reduced using intermediate storage tanks as described in the applicant's earlier patent application EP09163084 and thereby increasing the system efficiency.
In the present embodiment, the heat exchanger is a counterflow heat exchanger, and the working fluid of the cycle is preferably carbon dioxide. In a preferred embodiment, the compressor 16 is an electrically powered compressor.
In a preferred embodiment of the present invention, the counterflow heat exchanger may have a minimal approach temperature, ATmin, of 3 K (ie. the minimal temperature difference between the two fluids exchanging heat is 3 K). The approach temperature should be as small as possible.
In the present embodiment, the evaporator 14 is a vertically oriented desublimation evaporator of the "shell and tube" type, having working fluid circulating inside of the "tubes" (referred to as conduits herein), and the vaporized cold thermal storage medium on the "shell" side. Advantageously, the surface of the cooling conduits can be increased by utilizing cross sections other than circular, or by the addition of fins. The fins improve the deposition heat transfer by the increasing surface. The preferred working fluid for circulating in the ETES system is carbon dioxide; mainly due to the higher efficiencies in heat transfer processes and the amiable properties of carbon dioxide as a natural working fluid i.e. non-flammable, no ozone depletion potential, no health hazards etc. Whilst the thermal storage medium in the hot side of the ETES system is generally water (if necessary, in a pressurized container), other materials, such as oil or molten salt, may also be used.
Thus, it is clear to the skilled person that the present invention has the following advantages; minimal system irreversibilities through gentle heat transfer due to relatively small temperature approaches, low parasitic losses as no large fans or compressors are required, and good scalability as the size of the heat exchanger on the cold side of the cycle may be decoupled from the size of the cold storage.
Importantly, the skilled person will be aware that the sub-triple point evaporative ice storage technology of the present invention can be used in various commercial applications where energy efficiency improvements and load leveling are sought. Examples include air conditioning and district cooling, gas turbine air inlet cooling, food processing, or mine cooling. This list of examples is not exhaustive.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Claims

1) An electro-thermal energy storage system comprising
a) a hot thermal storage arrangement comprising one or more hot storage heat
exchangers (18),
b) a cold thermal storage arrangement comprising one or more cold storage heat exchangers (14, 36, 40),
c) a thermodynamic cycle unit comprising
i) at least one working fluid circuit for circulating one or more working fluids through at least one hot storage heat exchanger (18) as well as through at least one cold storage heat exchanger (14, 36, 40), each working fluid circuit further comprising
ii) at least one compressor or pump (16, 32) for compressing the at least one working fluid,
iii) at least one expander (12, 34), preferably a turbine or an expansion valve, for expanding the at least one working fluid,
iv) the thermodynamic cycle unit configured to
(1) transfer thermal energy from the cold storage unit to the hot storage unit in a charging mode,
(2) convert thermal energy from the hot storage unit into mechanical and subsequently electric energy in a discharging mode,
characterized in that
d) the cold storage arrangement comprises
i) a cold thermal storage medium,
ii) a vacuum evaporation chamber (28) adapted to evaporate liquid phase cold thermal storage medium to obtain solid phase cold thermal storage medium and vapor phase cold thermal storage medium,
iii) a first fluid connection for supplying gaseous phase cold thermal storage medium from said vacuum evaporation chamber (28) to at least one cold storage heat exchanger for evaporating working fluid through condensation of vaporized cold thermal storage medium during the charging cycle, iv) a second fluid connection for connecting said vacuum evaporation chamber (28) to a cold slurry storage tank (26) for storing a mixture of solid phase cold thermal storage medium and liquid phase cold thermal storage medium during the charging cycle, V) a third fluid connection for connecting said cold slurry storage tank to at least one cold storage heat exchanger for cooling or condensing working fluid during the discharge cycle.
2) The electro-thermal energy storage system of claim 1 , wherein the cold thermal
storage medium comprises a liquid first component, preferably water, and a second, preferably liquid component, which lowers a freezing point of the liquid first component.
3) The electro-thermal energy storage system according to claim 2, wherein the second component of the cold thermal storage medium is more volatile than the first component. 4) The electro-thermal energy storage system according to claim 2 or 3, wherein a
saturated vapor pressure above the cold thermal storage medium decreases as a fraction of the second component in the cold thermal storage medium increases.
5) The electro-thermal energy storage system according to one of the preceding claims, wherein the first fluid connection and the third fluid connection connect to the same cold storage heat exchanger (14, 36).
6) The electro-thermal energy storage system according to one of the preceding claims, wherein the cold storage heat exchanger (14, 36) is adapted to function as a condensation evaporator (14) in the charging mode, and as condenser (36) in the discharging mode.
7) The electro-thermal energy storage system according to one of the preceding claims, wherein the cold storage heat exchanger (14, 36) and the cold slurry storage tank (26) are directly coupled together in both the charging mode and the discharging mode.
8) The electro-thermal energy storage system according to one of the preceding claims, wherein the first fluid connection connects to a first cold storage heat exchanger (14, 36) and the third fluid connection connects to a second cold storage heat exchanger
(40). 9) The electro-thermal energy storage system according to the preceding claim, wherein the second cold storage heat exchanger (40) is positioned after an outlet of the first cold storage heat exchanger (14, 36) in the discharging mode.
10) The electro-thermal energy storage system according to the preceding claim, wherein the second cold storage heat exchanger (40) is positioned before an inlet of the cold slurry storage tank (26) in the discharging mode.
1 1) The electro-thermal energy storage system according to claim 8, 9 or 10,
characterized in that the cold storage arrangement comprises a fourth fluid connection to allow for a transfer of solid cold thermal storage medium from the first cold storage heat exchanger (14, 36) to the cold slurry storage tank (26) in a discharging mode.
12) The electro-thermal energy storage system according to one of the preceding claims, wherein the evaporation chamber (28) comprises a spraying device adapted to spray the cold thermal storage medium into the evaporation chamber (28) where triple point conditions of the cold thermal storage medium are at least approximately maintained.
13) A method for storing energy in a electro-thermal energy storage system, said electrothermal energy storage system comprising
a) a hot thermal storage arrangement,
b) a cold thermal storage arrangement,
c) a thermodynamic cycle unit comprising
i) at least one working fluid circuit for circulating one or more working fluids through the hot thermal storage arrangement as well as through the cold thermal storage arrangement, each working fluid circuit further comprising ii) at least one compressor or pump (16, 32) for compressing the working fluid, iii) at least one expander (12, 34), preferably a turbine or an expansion valve, for expanding the working fluid,
d) the method comprising the steps of
i) transferring, by means of the thermodynamic cycle unit, thermal energy from the cold storage arrangement to the hot storage arrangement in a charging mode, ii) converting, by means of the thermodynamic cycle unit, thermal energy from the hot storage arrangement into mechanical and subsequently electric energy in a discharging mode,
characterized in that
e) in the charging mode
i) a cold thermal storage medium is evaporated in a vacuum evaporation chamber (28) to obtain solid phase cold thermal storage medium and gaseous phase cold thermal storage medium,
ii) gaseous phase cold thermal storage medium from said vacuum
evaporation chamber (28) is condensed in the cold thermal storage unit in order to evaporate at least one working fluid,
iii) a mixture of solid phase cold thermal storage medium and liquid phase cold thermal storage medium is stored in a cold slurry tank (26); f) in the discharging mode
i) solid phase cold thermal storage medium from the cold slurry tank (26) is used to cool or condense at least one working fluid.
14) The method according to claim 13, wherein the cold thermal storage medium
comprises a liquid first component, preferably water, and a second, preferably liquid component, which lowers a freezing point of the liquid first component.
15) The method according to claim 13 or 14, wherein triple point conditions of
predominant component of the cold thermal storage medium are at least
approximately maintained in the vacuum evaporation chamber (28).
PCT/EP2012/075206 2012-01-03 2012-12-12 Electro-thermal energy storage system with improved evaporative ice storage arrangement and method for storing electro-thermal energy WO2013102537A2 (en)

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