FR3014182A1 - ADVANCED SYSTEM FOR ENERGY STORAGE BY COMPRESSED AIR - Google Patents

Abstract

The invention relates to a system for storing and recovering energy by compressed air, comprising means for storing compressed air and means for storing heat. The system comprises a plurality of stepped compression means, a plurality of stepped relaxation means for energy recovery, and a plurality of air heat storage elements at the outlet of each compression stage, the elements comprising several levels. temperature storage temperature, the temperature levels being between the outlet temperature of the air of the first compressor and the storage temperature of the compressed air.

Description

The field of the present invention relates to the storage of energy by compressed air (CAES for "Compressed Air Energy Storage"). In particular, it is a method for optimizing energy recovery.

In this system, the energy, which can come from electricity, which one wishes to use at another time, can be stored in the form of compressed air. The electricity produced in excess thus supplies one or more compressors whose purpose is to compress a given quantity of air. This air compression involves an increase in temperature.

Several variants currently exist for this system. These include: - CAES (Compressed Air Energy Storage) in which the air is stored at room temperature and the heat due to compression is removed. - ACAES (Adiabatic Compressed Air Energy Storage) in which air is stored at the temperature due to compression. - AACAES (Advanced Adiabatic Compressed Air Energy Storage) in which air is stored at room temperature and the heat due to compression is also stored in a TES (Thermal Energy Storage).

In the CAES processes, only the mechanical energy of the air is used, that is to say that all the heat produced during the compression is rejected. Air is typically stored at 8 MPa (80 bar) and at room temperature. This implies that, if we wish to recover the energy by a relaxation, the decompression of the air will again follow an isentropic curve, but this time starting from the initial conditions of 8 MPa and 300 K. The air is therefore cooling down. at unrealistic temperatures (83 K = -191 ° C). It is therefore necessary to heat it up, which is done using a gas burner, or other fuel.

The present invention particularly aims to avoid spending heat by combustion during the recovery of energy. Thus, it is the AACAES system that is favored and the main objective is to propose improvements to the heat storage used in the AACAES approach.

More generally, the main idea is to store heat so as to minimize energy losses during storage under pressure of the air, and to maximize the temperature level achievable during the return of heat to the air. air during its decompression after passing through turbines.

Thus, the present invention relates to a system for storing and recovering energy by compressed air, comprising compressed air storage means and heat storage means. The system comprises a plurality of stepped compression means, a plurality of stepped expansion means for the recovery of energy, and a plurality of air heat storage elements at the outlet of each compression stage, said elements comprising several levels. heat storage temperature, said temperature levels being between the air outlet temperature of the first compressor and the storage temperature of the compressed air.

The heat storage elements located downstream of a compression stage may be at the pressure of the air at the outlet of said stage. The heat storage elements may include phase change materials (PCMs). The staged compression means may comprise at least four compressors, the heat storage means may comprise at least three temperature levels, and the expansion means may comprise at least four turbines.

Storage can be done substantially at 20 ° C and 80 bar. The present invention will be better understood and its advantages will appear more clearly on reading the description which follows, illustrated by the appended figures, of which: FIG. 1 shows an example of a cooling and heating temperature profile for three storage temperature levels in three heat stores. FIG. 2 shows the rise in temperature as a function of the compression ratio for different yields; FIGS. 3a and 3b illustrate an example of storage and removal of air according to the invention with 3 storage temperature levels of 4a and 4b illustrate another example of storage and retrieval of air according to the invention with two levels of temperature of storage of heat. It should be noted that heat can be stored in latent or sensitive form. If we want to store heat at a temperature Tsi and our source has a temperature T1, the quantity of stored heat Q is: Qm.Cp (Ti-Ts1) where m and Cp are respectively the mass and the thermal capacity of source. For air, about mf = 1 kJ / ° C. In the case of sensible heat, when our source is warmed up with the stored heat, the temperature level of the heat storage decreases. The maximum attainable temperature Te, "is: m 'Cp' T si- mCpT 'id T = mCp + m' Cp 'with Tcold, source temperature, m' and Cp 'are the mass and thermal capacity of the storage, respectively It is clear that Te, "is always clearly inferior to Tsi.

Latent heat is the heat of change of state of a body. For pure bodies, the change of state is done at constant pressure and temperature. Thus, when using phase change materials (PCM), storage and reheating are done at a constant temperature. Thus, the maximum reheating temperature is substantially the storage temperature.

It therefore appears that the major advantage of storage in the form of latent heat is the greatest amount of heat that can be stored per unit volume (or mass). Indeed, in the case of sensible heat (for a hypothesis of a temperature cycling of 10 ° C), we arrive at about 20 MJ / m3, while in latent heat, we can reach 200 MJ / m3. In the description which follows, the process will be illustrated by its preferred implementation with the use of latent heat. The use of sensible heat storage is possible but brings less benefit.

Generally, when it is desired to compress air, several compression stages are used with a low compression ratio at each stage and cooling between each stage. Indeed, during its compression, the air heats up to a hot temperature. In order to reduce the storage volume, the air is cooled to a Tfroed temperature. Thus, the available heat is: Cp (hot T - cold T) If the heat is stored at a single temperature level Tsi, it is desired that this temperature is attainable during reheating. That is, the stored heat is equal to the heat of reheating. We thus have in an ideal case: Cp (hot -Ts1) = Cp (cold Ts1-T) = hot + cold Ts1 2 and the stored energy is: CP (hot TS 1) = ch Tc warm hot HUD T + T Thus, only half of the available heat is stored. This leads to the fact that the reheating temperature is well below the hot temperature. In order to overcome this limitation, it is proposed to store at several temperature levels between two stages of compression. It is assumed that one stores at N temperature levels Ti, S TS2, ..., TSN with hot> TS1> TS2> ...> TSN> Tfroed. The stored heat must be equal to the heat of reheating for each temperature level: Cp (Hot -Ts1) = Cp (Tsi-Ts2) Cp (Tsi-Ts2) Cp (Ts2-Ts3) Cp (TS (Ni) -TSN ) Cp (TsN -T cold) The resolution of this system (with the assumption of constant Cp) makes it possible to obtain the temperature for stage n: (N + 1- n) hot + nTfroid N +1 where 20 25 T Sn = The temperature that can be reached during reheating is Ts1. It is noted that this temperature tends to Tchaud when N tends to infinity.

Figure 1 shows an example of AACAES for three temperature levels. In this example, the air exits at a temperature of 140 ° C from one stage of the compressor and it is desired to cool it to 20 ° C before the next stage of compression. The horizontal lines A, B, C represent the temperatures of the phase change material (here three), respectively 115 ° C, 82 ° C, 50 ° C.

The lines according to the arrows 1, 2, 3 which go down from left to right show the evolution of the air temperature when it is cooled after compression, in energy storage mode. The parallel lines with the arrows 4, 5, 6 illustrate the temperature of the air in heating mode, when the air is released in energy recovery mode, from a storage temperature at 15 ° C. Compression stages The temperature increase through the compressor depends on the compression ratio and the efficiency of the compressor. Figure 2 presents this phenomenon by assuming a polytropic compression.

The compression ratio is substantially equal to the expansion ratio. Since the storage temperature must be equal to or greater than the turbine inlet temperature, the number of compression stages must be equal to or less than the number of expansion stages.

Temperature Levels If the compression ratio and compressor efficiency are known, it is possible to determine the outlet temperature of the compressor. Once this value is known, it is necessary to determine at which temperature the heat will be stored in the case of latent heat. Ideally you have to be able to heat the air as much as possible. Two examples based on a four-stage compression train are described. Pinch in thermal transfer systems is neglected here. The compressor outlet temperature is 140 ° C. It is proposed to store the heat at 2 or 3 temperature levels. - Case of two storage levels: 10 - stored at 100 ° C and 60 ° C. The heat was thus recovered between 140 and 100 ° C and between 100 and 60 ° C. The heat between 60 ° C and 20 ° C (hypothesis on the compressor inlet temperature) is lost. During the discharge, if the air is stored at 20 ° C, the first temperature level is used to heat the air to 60 ° C and the second allows the progression to 100 ° C. Thus, at the turbine inlet, the air 15 has been heated up to 100 ° C. - Case of three levels of storage: - one stores at 110 ° C, 80 ° C and 50 ° C. The heat was thus recovered between 140 and 110 ° C, between 110 and 80 ° C and between 80 and 50 ° C. The heat between 50 ° C and 20 ° C (hypothesis on the compressor inlet temperature) is lost. During the discharge, if the air is stored at 20 ° C, the first temperature level can heat the air to 50 ° C and the second allows to continue the progression to 80 ° C and finally the last allows to reach 110 ° C. Thus, at the turbine inlet, the air will have been heated up to 110 ° C.

The first solution does not make it possible to reach 110 ° C at the turbine inlet. Now in this example, the turbine outlet temperature will be lower (12 ° C) than the storage temperature (20 ° C). Thus during the next stage of relaxation, the available heat will not achieve the same temperature (98 ° C against 100 ° C). Figures 3b and 4b show the evolution of the temperature in both cases. Note that the inlet temperature of the turbines decreases in the case where the temperature is stored at two temperature levels. Overall implementation In this example, we consider the case of four compression stages with 3 temperature levels and four stages of energy recovery turbines. The implementation can be represented as in Figures 3a and 3b. FIG. 3a thus illustrates how, from left (low pressure) to right (high pressure), the air passes successively through four compressors C1, C2, C3, C4 which increase the pressure, but also the temperature, and after each stage of compression in three phase change material (PCM) exchangers. Figure 3a illustrates a heat recovery "matrix" consisting of 12 heat storage elements at four pressures (3, 9, 27 and 80 bar) and three temperatures (50, 80 and 110 ° C). The 12 heat storage elements can be independent for each element of the matrix or can be common for the same temperature level. We will choose to use the most cost-effective technology for each element of the matrix.

Figure 3b illustrates the retrieval by four turbines, from storage at 20 ° C and 80 bar. The air passages in the heat storage elements allow to heat the air at 110 ° C after each of the relaxation, and to obtain a final relaxation at 20 ° C.

Embodiments for the elements of the storage array are conventional exchangers, in particular with phase change material (PCM). Hereinafter some examples for the storage of one or more elements: each element may consist of an exchanger in which circulates a heat transfer fluid stored thereafter by temperature level; each element may consist of a material with immobile phase change integrated in the exchanger. In this case, the heat of the hot air passing through the exchanger is stored on site. The volume of the exchanger will then be larger but no external storage space is needed. In this case, the storage is independent by temperature and pressure torque. Figures 4a and 4b illustrate the case of storage at two temperature levels: 60 ° C and 100 ° C. There is a greater loss of heat than in the case of three temperature levels.

Claims (5)

CLAIMS1) System for storing and recovering energy by compressed air, comprising means for storing compressed air and heat storage means, characterized in that it comprises a plurality of staged compression means, a plurality of staged detent means for the recovery of energy, and a plurality of air heat storage elements at the output of each compression stage, said elements comprising several levels of temperature of heat storage, said temperature levels being understood between the air outlet temperature of the first compressor and the storage temperature of the compressed air. 2) System according to claim 1, wherein the heat storage elements located downstream of a compression stage are at the pressure of the air output of said stage. 3) System according to one of the preceding claims, wherein said heat storage elements comprise phase change materials (PCM). 4) System according to one of the preceding claims, wherein the staged compression means comprise at least four compressors, the heat storage means comprise at least three temperature levels, and the expansion means comprise at least four turbines. 5) System according to one of the preceding claims, wherein the storage is substantially at 20 ° C and 80 bar.