System for reversibly storing electrical energy as thermal energy

Abstract

A system for reversibly storing electrical energy as thermal energy. The system can include a reversible subcritical vapor-liquid cycle energy storage system with a single hot storage fluid tank and cold storage fluid tank that are inter connected by an inter storage tank flow path. The inter storage tank flow path includes an inter storage heat exchanger in the vapor-liquid cycle that enables sensible heat transfer between the working fluid and storage fluid as the storage fluid passes between the hot storage fluid tank and the cold storage fluid tank. This supplements latent heat transfer between the working fluid and the hot storage fluid tank and the cold storage fluid tank.

Claims

1. A system for reversibly storing electrical energy as thermal energy comprising: a reversible subcritical vapour-liquid cycle having a working fluid flow path cycling through a hot storage fluid heat exchanger configured and arranged to exchange thermal energy between a working fluid, as the working fluid changes phase, and a hot storage fluid, a vapour pressure changing apparatus configured to change pressure of the working fluid in a vapour phase and arranged between and fluidly adjacent the hot storage fluid heat exchanger and a cold storage fluid heat exchanger, the cold storage fluid heat exchanger arranged fluidly adjacent the vapour pressure changing apparatus, configured and arranged to exchange thermal energy between the working fluid, as the working fluid changes phase, and a cold storage fluid, a liquid pressure changing apparatus arranged fluidly adjacent the cold storage fluid heat exchanger opposite the vapour pressure changing apparatus, configured to change pressure of the working fluid in a liquid phase, and an inter storage heat exchanger arranged fluidly between the liquid pressure changing apparatus and the hot storage fluid heat exchanger, configured and arranged to exchange sensible heat of the working fluid with a storage fluid; a hot storage fluid flow path cycle passing through the hot storage fluid heat exchanger and a hot storage fluid tank storing the hot storage fluid; a cold storage fluid flow path cycle passing through the cold storage fluid heat exchanger and a cold storage fluid tank storing the cold storage fluid; and an inter storage tank flow path cycle fluidly connecting the hot storage fluid tank to the cold storage fluid tank via the inter storage heat exchanger.

2. A method for generating electrical energy from thermal storage comprising the steps of: evaporating a working fluid of a vapour-liquid cycle in a hot storage fluid heat exchanger against a hot storage fluid circulating through a cycle of the hot storage fluid heat exchanger and a hot storage fluid tank; expanding the evaporated working fluid in a turbine of the vapour-liquid cycle to drive a generator to generate electricity; condensing the expanded working fluid in a cold storage fluid heat exchanger against a cold storage fluid circulating through a cycle of the cold storage fluid heat exchanger and a cold storage fluid tank; pressuring the condensed working fluid in a pump; and heating the pressured working fluid in an inter storage heat exchanger against a storage fluid passing from the hot storage fluid tank to the cold storage fluid tank.

3. A method for storing electrical energy as thermal energy comprising the steps of: isenthalpically throttling a working fluid of a vapour-liquid cycle using a throttle valve; evaporating the throttled working fluid in a cold storage fluid heat exchanger against a cold storage fluid circulating through the cold storage fluid heat exchanger and a cold storage fluid tank; compressing the evaporated working fluid in a compressor driven by a motor thereby inputting electrical energy in the vapour-liquid cycle; condensing the compressed working fluid in a hot storage fluid heat exchanger against a hot storage fluid circulating through the hot storage fluid heat exchanger and a hot storage fluid tank; and cooling the condensed working fluid in an inter storage heat exchanger against a storage fluid passing from the cold storage fluid tank to the hot storage fluid tank.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic of an energy storage system according to an exemplary embodiment;

(3) FIG. 2 is a temperature-entropy diagram of a charging cycle of the storage system of FIG. 1; and

(4) FIG. 3 is a temperature-entropy diagram of a dis-charge cycle of the storage system of FIG. 1.

DETAILED DESCRIPTION

(5) Exemplary embodiments of the present disclosure are now described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiment disclosed herein.

(6) FIG. 1 shows an exemplary embodiment of a system for storing electrical energy as thermal energy using a reversible subcritical vapour-compression cycle. The reversible subcritical vapour-compression cycle includes a working fluid flow path 47 that makes up a working fluid circuit 30, a hot storage fluid flow path 7, a cold storage fluid flow path 17 and an inter storage tank flow path 27 that connecting storage tanks 5,15 in the hot storage fluid flow path 7 to the cold storage fluid flow path 17 respectively.

(7) In an exemplary embodiment shown in FIG. 1 the working fluid flow path 47 comprises as closed loop. A hot storage fluid heat exchanger 9 in the working fluid flow path is configured and arranged to transfer thermal energy of the working fluid as it changes phase to a hot storage fluid. A vapour phase pressure changing device 32, such as a compressor/turbine configured either as a single unit or as two separate units either provides electrical energy into the working fluid flow path 47 by means of a motor 34 when the vapour phase pressure changing device 32 operates as a compressor or release electrical energy from the working fluid flow path 47 by means of a generator when the vapour phase pressure changing device 32 operates as a turbine. A further cold storage fluid heat exchanger 19 utilised a further change in phase of the working fluid to exchange thermal energy with a cold storage fluid wherein the cold storage fluid heat exchanger 19 changes phase of the working fluid in the opposite direction to that of the hot storage fluid heat exchanger 9. A fluid liquid phase pressure changing device, comprising either a pump 36 or throttle valve 38 for isenthalpic throttling, depending on the direction of working fluid flow in the reversible cycle, changes the pressure of the of the working fluid while it is in the liquid phase. A further inter storage heat exchanger 29, within the working fluid flow path 47, is used as a sensible heat exchanger.

(8) As shown in FIG. 1, an exemplary embodiment further includes a hot storage fluid tank 5 for storing a hot storage fluid a hot storage fluid flow path 7 that connects the hot storage fluid tank 5 to the hot storage fluid heat exchanger 9.

(9) As shown in FIG. 1, an exemplary embodiment further includes a cold storage fluid tank 15 for storing a cold storage fluid and a cold storage fluid flow path 17 that connects the cold storage fluid tank 15 to the cold storage fluid heat exchanger 19.

(10) As shown in FIG. 1 an exemplary embodiment further includes an inter storage tank flow path 27 that fluidly connects the hot storage fluid tank 5 to the cold storage fluid tank 15 via an inter storage heat exchanger 29.

(11) The reversible cycle shown in FIG. 1 is operable in both a charging phase and a discharging phase.

(12) In the charging phase the reversible cycle operates as a heat pump in which electrical energy is converted into thermal energy stored in the hot storage fluid tank 5. The thermal cycle of this phase is shown in FIG. 2. After evaporation 48 against the cold storage fluid in the cold storage fluid heat exchanger 19 the working fluid is compressed 40 by the compressor 32 using electrical energy input from a motor 34. In this arrangement the cold storage fluid heat exchanger 19 is an evaporator. The thermal energy of this high pressure vapour is then stored by de-superheating and condensing 42 the vapour against the hot storage fluid in the hot storage fluid heat exchanger 9. In this arrangement the hot storage fluid heat exchanger 9 is a condenser. Sensible heat 44 of the liquid phase working fluid is then removed in an inter storage heat exchanger 29 against storage fluid transferred from the cold storage fluid tank 15 to the hot storage fluid tank 5 via the inter storage tank flow path 27. During the condensation step, to minimize entropy generation, it is preferable to maintain a small temperature difference across the hot storage fluid heat exchanger 9 by ensuring that the flow rate of storage fluid in the hot storage fluid flow path 7 is kept high. After cooling down, the working fluid is isenthalpically throttled 46 using a throttle valve 38 before being evaporated 48 in the cold storage fluid heat exchanger the working fluid absorbs heat from low temperature storage. Again, to be achieved minimum entropy generation it is preferable to maintain a small temperature difference across the cold storage fluid heat exchanger 19 by ensuring that the flow rate of storage fluid in the cold storage fluid flow path 17 is kept high.

(13) In a configuration with one hot storage fluid tank 5 and one cold storage fluid tank 15 the need for high volumes storage fluid to achieve high flow rates through the Hot storage fluid heat exchanger 9 and the cold storage fluid heat exchanger 19 is respectively are avoided. Nonetheless, during the charging cycle, the temperature of the hot storage fluid tank 5 will rise while the temperature of the cold storage fluid tank 15 will fall.

(14) The use of an inter storage fluid heat exchanger 29 has the advantage to storing sensible heat as well as latent heat. An the exemplary embodiment shown in FIG. 1 takes storage fluid from the cold storage fluid tank 15 to the hot storage fluid tank the portion of the sensible heat lays exactly between the high and low temperature condensation/evaporation parts of the working fluid cycle enabling optimum utilisation of sensible heat storage.

(15) During a dis-charging the cycle, as shown in FIG. 3 the charging process is reversed. A pump 36 increases the pressure of liquid phase working fluid. At first, the working fluid absorbs sensible heat 45 against storage fluid passing through the inter storage flow path 27 from the hot storage fluid tank 5 to the cold storage fluid tank 15 as it pass through the inter storage fluid heat exchanger 29. The working fluid is then evaporated 48 in the hot storage fluid heat exchanger 9 against hot storage fluid. In this arrangement the hot storage fluid heat exchanger 9 is an evaporator. As in the charging cycle, the hot storage fluid is circulated in the hot storage fluid flow path at a sufficient flow rate to lead to only a small temperature decrease across the hot storage fluid heat exchanger 9 before being fed back to the hot storage fluid tank 5. The now gaseous working fluid is expanded 41 in a turbine 32 and used to drive a generator 34. The cycle is closed by condensing the working fluid against cold storage fluid in a cold storage fluid heat exchanger 19. In this arrangement the cold storage fluid heat exchanger 19 is operated as a condenser. As the discharging process continues the hot storage fluid tank 5 cools down while the cold storage fluid tank 15 heats up. In addition, storage fluid passed from the hot storage fluid tank 5 to the cold storage fluid tank 15.

(16) As a consequence of the irreversibility's of the cycle not all energy that was charged with the compressor can be discharged by the turbine. The remaining energy can be found in the tanks, as a higher temperature of either the hot tank or the cold tank. This heat has to be removed from the cycle, wherein the latter is only an option if the cold tank temperature is above any available heat rejection opportunity. The heat from the high temperature tank can be used for the purpose of room heating or warm water preparation.

(17) As suitable storage fluid for described exemplary embodiments is water, nonetheless, other storage fluids matching required thermodynamic requires could be used.

(18) Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, it will be appreciated by those skilled in the art that the present disclosure can be embodied in other specific forms. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.

REFERENCE NUMBERS

(19) 5 Hot storage fluid tank 7 Hot storage fluid flow path 9 Hot storage fluid heat exchanger 15 Cold storage fluid tank 17 Cold storage fluid flow path 19 Cold storage fluid heat exchanger 27 Inter storage flow path 29 Inter storage heat exchanger 30 Working fluid flow circuit 32 Compressor/turbine 34 Motor/Generator 36 Pump 38 Expander valve 40 Compression 41 Expansion 42 Condensation, de-superheating 44 Sensible heat storage 45 Sensible heating 46 Sensible heat storage Isenthalpic throttle 47 Working fluid flow path 48 Evaporation