METHOD FOR STORING ENERGY IN A STEAM ACCUMULATOR

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of steam accumulators. It finds a particularly advantageous application in the storage of energy in a steam accumulator by direct condensation, in particular when the stored gas comprises incondensable gases, such as, for example, in the case of geothermal energy or thermodynamic power plants for producing electricity. STATE OF THE ART

[0002] Water vapor is one of the heat transfer fluids frequently used in industry. It has the advantage of being able to transport a large quantity of heat per unit of mass: because it transports most of it in the form of latent heat, through its liquid-vapor phase transition. This heat transfer fluid can be used, for example, to produce electricity in thermodynamic power plants.

[0003] Steam storage in industry aims to store steam energy when it is not needed and release it when a process requires it. Thermal storage thus allows for the phase shifting of steam production and its use. Thermal storage can also stabilize the supply of steam to a process by smoothing out fluctuations related to production. Document EP2 444 594 A1 describes a system and method for accumulating steam in tanks for solar use.

[0004] In the renewable electricity production sectors, steam storage makes it possible to store thermal energy during periods when electricity demand is low in order to release it during peak electricity consumption in order to limit the use of fossil fuel backup resources.

[0005] There are several ways to store steam, including direct condensation, for example by storing it in pressurized liquid water tanks called steam accumulators.

[0006] In several industrial applications such as thermodynamic power plants producing electricity from geothermal energy, steam may contain non-condensable gases such as carbon dioxide (CO2). The use of a steam accumulator in this type of application is therefore traditionally avoided.

[0007] Indeed, since the accumulator tank is closed, its storage capacity decreases significantly, because the incondensable gases transported by the injected water vapor accumulate in the tank's top layer during the charging phase. The partial pressure of incondensable gases increases rapidly during the charging phase to the detriment of the partial pressure of vapor. The latter is then lower than the total pressure at the end of charging, which is a given of the system. The presence of incondensable gas therefore results in a reduction in the tank's storage capacity.

[0008] The aim of the present invention is to propose a solution which makes it possible to limit the impact of incondensable gases on the storage capacity of a steam accumulator.

[0009] Other objects, features, and advantages of the present invention will become apparent from the following description and accompanying drawings. It is understood that other advantages may be incorporated. SUMMARY

[0010] To achieve this objective, the invention provides a method for storing thermal energy in a steam accumulator comprising a tank according to claim 1, advantageously closed, preferably pressure-tight, containing a liquid and a gas canopy arranged above the liquid, a device for injecting gas entering the tank and a device for discharging gas leaving the tank, the gas comprising steam and at least one incondensable gas.

[0011] The method according to the invention thus makes it possible to limit the partial pressure of incondensable gas in the tank and more specifically in the gas headspace located above the liquid contained in the tank. However, the partial pressure of gas in the tank is conventionally the parameter limiting the energy capacity of the accumulator, because the partial pressure of gas limits the partial pressure of vapor for a fixed total gas pressure. The storage capacity of a steam accumulator is therefore optimized by the present method. The present invention makes it possible to apply the storage method to thermal storage systems of the existing steam accumulator type. The invention makes it possible to increase the storage capacity without increasing the size of the accumulator or the maximum operating pressure of the storage.

[0012] By reducing the fraction of non-condensable gases in the tank's gas headspace, storage capacity increases. The same amount of energy can therefore be stored in a smaller volume accumulator, which reduces the cost of the installation, or for the same volume of accumulator, a greater amount of energy will be stored, which allows for a more cost-effective installation.

[0013] Another aspect relates to a steam accumulator comprising a tank according to claim 14, advantageously closed, preferably pressure-tight, containing a liquid and a gas canopy, a device for injecting gas entering the tank, a device for discharging gas leaving the tank, a management module comprising a computer and a control unit comprising control valves for the gas injection device and the device for discharging gas leaving the tank. BRIEF DESCRIPTION OF THE FIGURES

[0014] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which: THE Figures 1A to 1C represent a diagram of a steam accumulator according to the invention. The Figure 1A represents the accumulator according to a first possibility. The Figure 1B represents the accumulator according to a second possibility. The Figure 1C represents the accumulator according to a third possibility. The figure 2 represents the evolution of the pressure in a steam accumulator without incondensable gas according to example 1. The figure 3 represents the evolution of energy in a steam accumulator without incondensable gas according to example 1. The figure 4represents the evolution of the pressure (partial vapor, partial gas and total) in a steam accumulator with a mass fraction of 2.8% of non-condensable gases according to example 2. The Figure 5 represents the evolution of energy in a steam accumulator with a mass fraction of 2.8% of incondensable gases according to example 2. The figure 6 represents the evolution of the pressure (partial steam, partial gas and total) in a steam accumulator with a mass fraction of 2.8% of non-condensable gases according to example 3 with a lower injected gas flow rate than in example 2. The figure 7 represents the evolution of energy in a steam accumulator with a mass fraction of 2.8% of incondensable gases according to example 3 with a lower injected gas flow rate than in example 2. The figure 8 represents the evolution of the storage capacity of a steam accumulator as a function of the mass fraction of CO2 in the gas. The figure 9 represents the evolution of the pressure in a steam accumulator with a mass fraction of 2.8% of incondensable gases in the steam according to an example of the invention illustrated in example 4. The figure 10 represents the evolution of energy in a steam accumulator with a mass fraction of 2.8% of incondensable gases in the steam according to an example of the invention illustrated in example 4.

[0015] The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. DETAILED DESCRIPTION

[0016] Before commencing a detailed review of embodiments of the invention, optional features are set out below which may optionally be used in combination or alternatively: According to one example, the gas discharge at a second gas flow rate is intermittent during the charging phase.

[0017] According to one example, the gas discharge at a second gas flow rate is continuous during the charging phase. This arrangement ensures great ease of management of the charging phase.

[0018] In one example, the second gas flow rate is lower than the first gas flow rate. In this way, charging is achieved while limiting the partial pressure of non-condensable gas in the tank.

[0019] According to one example, the gas discharge at a second gas flow rate is carried out from the gas ceiling.

[0020] According to one example, the injection of gas at a first gas flow rate is continuous during the charging phase.

[0021] According to one example, the injection of gas at a first gas flow rate comprises a main gas flow rate and a complementary gas flow rate. Preferably, the first gas flow rate consists of a main gas flow rate and a complementary gas flow rate.

[0022] According to one example, the gas injection at a complementary gas flow rate is intermittent during the charging phase. According to this arrangement, the first gas flow rate is continuous during the charging phase, but the first gas flow rate is variable, increasing and decreasing depending on the intermittency of the complementary flow rate.

[0023] According to one example, the injection of gas at a complementary gas flow rate is continuous during the charging phase.

[0024] In one example, gas injection at a complementary gas flow rate is carried out into the liquid contained in the tank. This arrangement makes it possible to optimize the thermodynamic balance between the liquid and the gas canopy.

[0025] According to one example, the injection of gas at a complementary gas flow rate is carried out in a gas canopy arranged above the liquid contained in the tank. In this arrangement, the gas canopy is swept by the complementary flow rate. Control of the charging phase is facilitated according to this embodiment.

[0026] According to one example, the additional gas flow rate is lower than the main gas flow rate. The additional gas flow rate corresponds to at least partially compensating for the second gas discharge flow rate so as to, in particular, comply with an identical charging time setpoint, regardless of the fraction of non-condensable gas accompanying the vapor in the gas.

[0027] In one example, the second gas flow rate is greater than or equal to the additional gas flow rate.

[0028] In one example, the liquid contained in the tank is water and the gas vapor is water vapor comprising at least one non-condensable gas.

[0029] It is specified that in the context of the present invention, the term flow rate is understood as a mass flow rate in kg / s.

[0030] The amount of non-condensable gas in the gas comprising the vapor is given as a mass fraction or percentage relative to the total mass quantity of gas.

[0031] A steam accumulator comprises a tank 1 containing a liquid 2 and a gas canopy 3. The tank is advantageously closed, more preferably sealed so as to maintain the fluids: the liquid 2 and the gas canopy 3, inside the tank 1. The tank 1 is advantageously configured to allow pressurization of the liquid 2 and the gas canopy 3. The tank 1 is advantageously gas- and liquid-tight.

[0032] The tank 1 comprises a gas injection device 5 and a gas evacuation device.

[0033] The gas injection device 5 is intended to inject a gas into the tank 1. Preferably, the gas is steam, for example water vapor comprising at least one incondensable gas. The gas injection is at least partially carried out in the liquid 2. According to one possibility, the injection device comprises an injector 10, for example in the form of an injection line dipping into the liquid 2 of the tank 1, preferably in the lower part. The injection line comprises, for example, a plurality of outlets preferably distributed in the tank 1. The gas injection by the gas injection device 5 is carried out at a first gas flow rate.

[0034] The gas evacuation device is intended to evacuate the gas from the tank 1. The gas evacuation is carried out from the gas head 3. Preferably, the gas is steam, for example water vapor comprising at least one undesirable gas. According to one possibility, the evacuation device comprises an evacuation pipe 4 opening at one end into the tank 1 and extending outside the tank 1. The gas evacuation by the gas evacuation device is carried out at a second gas flow rate.

[0035] According to one embodiment, the tank 1 comprises a device 6 for filling and emptying the liquid 2. The device 6 for filling and emptying the liquid 2 is used to fill or empty the tank 1 with liquid 2. Preferably, this filling and emptying device 6 is arranged in the lower part of the tank 1 to facilitate its emptying. According to one possibility, the filling and emptying device 6 comprises a valve 9 ensuring the opening and closing of the filling and emptying device 6 making it possible to control the filling and emptying of the tank 1.

[0036] The steam accumulator comprises, according to one embodiment, a control unit comprising controlled valves for controlling the injection and evacuation of gas into and out of the tank 1. The control unit advantageously comprises a discharge valve 7 arranged on the evacuation device, preferably on the evacuation pipe 4. The discharge valve 7 makes it possible to initiate or stop the evacuation of gas from the tank 1. Preferably, the discharge valve 7 is configured to control the second flow rate, i.e. the gas evacuation flow rate.

[0037] The control unit advantageously comprises an injection valve 8 arranged on the injection device 5, preferably on the injection line. The injection valve 8 makes it possible to initiate or stop the injection of gas into the tank 1. Preferably, the injection valve 8 is configured to control the first flow rate, i.e. the gas injection flow rate.

[0038] The accumulator comprises, according to one possibility, at least one measuring device, in particular for operating parameters of the accumulator. At least one measuring device is a pressure sensor of the total pressure in the tank 1.

[0039] The steam accumulator comprises, according to one embodiment, a management module. The management module is intended to optimize the storage capacity of the accumulator as a function of the mass fraction of non-condensable gas in the vapor of the stored gas.

[0040] The management module preferably comprises traditional computing means, in particular a processor, a memory, and a computer program stored in the memory and comprising a series of instructions configured to process input data, in particular received from measuring devices or data from the accumulator.

[0041] The invention advantageously relates to thermal storage in the form of steam by direct condensation. The steam is accumulated in the tank 1 containing a liquid 2 under pressure.

[0042] According to a preferred embodiment, the liquid 2 contained in the tank 1 is water in the liquid state while the gas comprises vapor such as water vapor and at least one incondensable gas.

[0043] A steam accumulator according to the invention operates to implement a thermal energy storage method. The thermal energy storage method comprises a charging phase during which thermal energy is stored in the accumulator. The method advantageously comprises a discharging phase during which thermal energy is released from the accumulator.

[0044] In the present description, gas is understood to mean a mixture of gases comprising vapor such as water vapor and at least one non-condensable gas.

[0045] Non-condensable gas is a gas that cannot be condensed under the operating conditions of the steam accumulator. Examples of non-condensable gases are carbon dioxide (CO2) or H2S, N2, CH4, Ar, O2, H2, He, C2H6, C3H8. In several industrial applications such as thermodynamic power plants for generating electricity on geothermal sites, steam may contain non-condensable gases.

[0046] The principle of a charging phase and a discharging phase of a steam accumulator is described below.

[0047] The charging phase allows thermal energy to be stored in the form of steam.

[0048] At the start of the charging phase, tank 1 contains a quantity of liquid 2 at saturation and a saturated gas layer 3 at the minimum operating pressure. The discharge valve 7 is closed.

[0049] A gas at a higher pressure than the pressure in the tank 1 is injected, preferably from the bottom of the tank 1, through the injection device 5. Preferably, the loading valve 8 is open. The vapor bubbles condense on contact with the colder liquid 2 and release energy, which increases the temperature of the liquid 2. If the liquid 2 and the gas head 3 are in thermodynamic equilibrium, this increase in temperature results in an increase in the pressure in the tank 1 by evaporation of the liquid 2 at its free surface. If the thickness of liquid 2 is insufficient to ensure the condensation of all the vapor in contact with the liquid 2, the increase in pressure in the tank 1 is linked to the accumulation of vapor in the gas head 3. This vapor, at an overpressure relative to the saturation pressure, condenses at the free surface of the liquid 2 to tend towards thermodynamic equilibrium.The free surface of the liquid is understood as the surface of the liquid 2 at the interface with the gas ceiling 3.

[0050] Liquid level 2 also increases, resulting from the condensation of the injected steam.

[0051] During this charging phase, the pressure in the tank 1 increases, possibly continuously. According to one embodiment, the storage stops when the pressure reaches a target maximum pressure. According to one possibility, the target pressure is the pressure of the injection device 5, more precisely the maximum pressure of the injected gas. The liquid level and the pressure are maximum at the end of the charging phase.

[0052] The discharge phase is described below. The discharge phase is intended to provide thermal energy in the form of steam to a final source. For example, the final source may be a steam turbine.

[0053] During the discharge phase, the injection device 5 is closed, preferably the charging valve 8 is closed. The discharge valve 7 is open.

[0054] When the discharge device is opened, the pressure in the tank 1 decreases, causing vaporization of the liquid 2. In fact, the liquid 2 is at a temperature very slightly higher than the saturation temperature, leading to rapid vaporization of the liquid 2. This vaporization requires energy, latent heat of vaporization, which is provided by the liquid 2. The temperature of the liquid 2 decreases accordingly. The level of liquid 2 decreases during this discharge phase, because a portion of the liquid 2 is evaporated. The accumulator produces saturated vapor.

[0055] This discharge phase stops when tank 1 reaches a target minimum pressure. Preferably, the target minimum pressure is a function of the minimum pressure accepted by the final source.

[0056] It may happen that there is a thermodynamic imbalance with a pressure higher than the equilibrium pressure at the charge and a lower pressure at the discharge. The hypothesis of thermodynamic equilibrium is therefore favorable. The imbalance is unfavorable by interrupting the charge and discharge earlier.

[0057] According to the invention, the gas injected into the accumulator comprises at least one non-condensable gas. The presence of the latter in the steam has a significant impact on the storage capacity of a steam accumulator.

[0058] The tank is closed, its storage capacity decreases sharply, because the incondensable gases transported by the injected steam accumulate in the gas 3 headspace during the charging phase. The partial pressure of incondensable gases increases during the charging phase to the detriment of the partial pressure of steam. The latter is then lower than the maximum target pressure also called total end-of-charge pressure which is a predefined value. However, the temperature of the liquid follows the pressure of the steam. The final temperature of liquid 2 in tank 1 will therefore be lower than it would have been in the absence of incondensable gas. Since the stored energy is proportional in the first order to the temperature rise, less energy will be stored in tank 1 in the presence of incondensable gases. This reduction in energy due to a lower partial pressure of the steam is illustrated in Figures 2 and 3 corresponding to example 1.

[0059] The decrease in stored energy is all the more marked as the fraction of non-condensable gas is high, as illustrated in figure 8 . For example, according to Examples 1 to 3, a 33% decrease in stored energy with a mass fraction of non-condensable gas of 2.8%, and a 64% decrease in stored energy with a mass fraction of non-condensable gas of 10% ( figure 8 ). These values ​​are obtained by assuming thermodynamic equilibrium between the liquid and the gas in tank 1. In the event of thermodynamic imbalance, the effect of the presence of non-condensable gases will be accentuated.

[0060] The present invention proposes that during the charging phase, the presence of incondensable gas is compensated by management of the pressure of the gas ceiling 3 to reduce the partial pressure of incondensable gas in favor of the partial pressure of vapor.

[0061] For this purpose, the charging phase includes, in addition to the injection of a gas at a first flow rate into the tank 1 of the accumulator, an evacuation of gas at a second flow rate from the gas ceiling 3.

[0062] The charging phase therefore comprises an injection of gas at a first gas flow rate. The injected gas comprises a fraction of non-condensable gas. Preferably, the injected gas is a mixture of steam, preferably water vapor, and at least one non-condensable gas. The gas is injected by a gas injection device 5. The gas is preferably injected continuously throughout the charging phase at a first gas flow rate which may be stable or variable.

[0063] During the charging phase, the method also comprises an evacuation, which can also be called gas extraction advantageously from the gas ceiling 3. According to a first possibility, the evacuation of gas during the charging phase is carried out by the gas evacuation pipe 4 also used for the evacuation of gas during the discharge phase, as illustrated in Figures 1A and 1B . According to a second possibility, the gas evacuation during the charging phase is carried out by an evacuation 14, preferably equipped with a discharge valve 13. The gas evacuation during the discharge phase is carried out by the different evacuation pipe 4 as illustrated in Figure 1CThe evacuation device comprises a gas evacuation 14 separate from the evacuation pipe 4 used for the discharge phase. This second possibility makes it possible in particular to adapt the evacuation to the evacuated flow rate. In the charging phase, the evacuated flow rate is lower than that evacuated in the discharge phase so that the diameters of the evacuation pipe 4 and of the evacuation 14 are thus adapted. The evacuated gas corresponds to the gas in the gas ceiling 3. According to the invention, the evacuated gas comprises a fraction of incondensable gas. Preferably, the evacuated gas is a mixture of vapor, preferably water vapor, and at least one incondensable gas.

[0064] The gas is discharged continuously or intermittently at a second gas flow rate which can be stable or variable during the charging phase.

[0065] The second gas flow rate is advantageously lower than the first gas flow rate so as to allow the accumulator to be charged, i.e. the pressure in the tank 1 and therefore the stored energy to be increased. For example, the second gas flow rate represents from 2% to 50% of the first gas flow rate.

[0066] The gas is discharged from the gas ceiling 3. The second gas flow is advantageously controlled by the control unit and in particular an evacuation valve 7.

[0067] According to one possibility, the gas is evacuated throughout the charging phase at a stable or variable flow rate, the gas evacuation is continuous. According to another possibility, the gas is evacuated intermittently during the charging phase, i.e. at least one degassing, preferably successive degassings, take place. The second gas flow rate is then either identical during all degassings or different.

[0068] According to one embodiment, the injection of gas at a first flow rate comprises a main flow rate and a complementary flow rate of gas.

[0069] The first gas flow rate is the sum of the main gas flow rate and the supplementary gas flow rate.

[0070] The injection of gas at the main flow rate is advantageously continuous during the charging phase and carried out in the liquid 2.

[0071] The gas injection at the complementary flow rate may be continuous during the charging phase or intermittent. The gas injection at the complementary flow rate is intended to at least partially compensate for the gas evacuation. The complementary flow rate increases the injected gas flow rate compared to a prior art accumulator providing a charging phase without gas evacuation. The complementary flow rate may be stable or variable during charging.

[0072] Advantageously, the injection of gas at the additional flow rate is carried out by the injection device 5.

[0073] According to a first possibility illustrated in the Figure 1A the injection of gas at the complementary flow rate is carried out in the liquid 2. According to this possibility, the injection of gas at the complementary gas flow rate is preferably carried out by the same injection device 5 as the injection of gas at the main flow rate, that is to say for example the injection line illustrated in Figure 1A. It may be necessary to oversize the injection device 5. In this case, the valve 8 ensures the control of the injection and advantageously also of the first flow rate of injected gas. According to an alternative, the injection device 5 comprises an injector 10 for the injection of gas at the main flow rate and a complementary injector 11 for the injection of gas at the complementary flow rate. According to this alternative of this first possibility, the complementary injector 11 is arranged in the tank 1 immersed in the liquid 2 preferably in the lower part. In this case, the complementary injection 11 comprises a complementary valve 12 controlling the injection of complementary gas and advantageously the flow rate of complementary gas.

[0074] According to a second possibility illustrated in the Figure 1B, the injection of gas at the complementary flow rate is carried out in the gas head 3. According to this possibility, the injection of gas at the complementary gas flow rate is preferably carried out by the injection device 5 comprising an injector 10 for the injection of gas at the main flow rate and a complementary injector 11 for the injection of gas at the complementary flow rate. According to this second possibility, the complementary injector 11 is arranged in the tank in the gas head 3. In this case, the complementary injection 11 comprises a complementary valve 12 controlling the injection of complementary gas and advantageously the complementary gas flow rate.

[0075] According to a first embodiment, the gas injection is continuous at a first main flow rate which can be stable or variable during the charging phase. The gas injection is continuous at the first complementary flow rate which can be stable or variable during the charging phase. The gas evacuation is continuous at the second flow rate which can be stable or variable.

[0076] According to a second embodiment, the gas injection is continuous at the first main gas flow rate which may be stable or variable during the charging phase. The gas injection is intermittent at the first complementary flow rate which may be stable or variable during the charging phase. The gas evacuation is continuous at the second flow rate which may be stable or variable during the charging phase.

[0077] According to a third embodiment, the gas injection is continuous at the first main gas flow rate which may be stable or variable during the charging phase. The gas injection is intermittent at the first complementary flow rate which may be stable or variable during the charging phase. The gas evacuation is continuous at the second flow rate which may be stable or variable during the charging phase. In this third embodiment, the gas injection at the complementary flow rate and the gas evacuation at the second flow rate may be carried out alternately or simultaneously, without being dependent on each other.

[0078] The method according to the invention is intended to control the first gas flow rate and the second gas flow rate as a function of the mass fraction of non-condensable gas to achieve optimal energy storage.

[0079] For this purpose, the management module controls the first gas flow rate and the second gas flow rate as a function of the non-condensable gas fraction, preferably also the maximum target pressure and the minimum target pressure of the accumulator, preferably also the pressure measured by the pressure measuring device, preferably also the desired charging time.

[0080] According to an advantageous embodiment, the gas extracted by the evacuation device during the charging phase can be recovered by being, for example, re-injected into a step of a gas utilization process.

[0081] The present invention finds a particular application in the field of deep geothermal energy in which the aim is to recover the energy from brine (highly mineralized water), taken from underground, by supplying heat networks and producing electricity for example.

[0082] Steam storage in this sector allows thermal energy to be stored during periods of low demand in order to release it during peak consumption.

[0083] Generally, the quantity of incondensable gases in the brine is not negligible, with for example a mass fraction of incondensable gas between 2% and 20% depending on the geothermal site.

[0084] The following table shows the evolution of the first flow rate corresponding to gas injection and that of the second flow rate corresponding to gas extraction during charging as a function of the CO2 content, a non-condensable gas, to store 3.71 MWh in 4 hours. If the mass fraction of CO2 increases, the first flow rate and the second flow rate increase in order to store the same energy (Table 1). Mass fraction of CO2 inlet [%] Mass flow rate (steam+CO2) at inlet - load [kg / s] Mass flow rate (steam+CO2) at outlet (extract) - load [kg / s] % exit 2,8 0,46 0,0253 5,5 5 0,53 0,0901 17 10 0,7 0,266 38

[0085] Example 1: Evolution of pressure and energy in a steam accumulator without incondensable gases.

[0086] For example, for a 345 m 3< steam accumulator, 5MWh of energy can be stored in 4 hours with a charge rate of 0.58 kg / s, between 4.5 bars absolute and 7.5 bars absolute, i.e. between 50.5 and 94 psi relative ( Figures 2 and 3 ), under conditions of thermodynamic equilibrium between the two liquid and vapor phases.

[0087] The energy can be approximately evaluated in this case by the relation: E ≈ m L CpL T sat P max − T sat P min

[0088] With (unit of the International System) E: energy m L: mass of liquid in the accumulator CpL average heat capacity of the liquid T sat< (P min ): saturation temperature of the liquid at minimum target pressure T sat< (P max ): saturation temperature of the liquid at maximum target pressure

[0089] Example 2-- : Evolution of pressure and energy in a steam accumulator with incondensable gases.

[0090] The same experiment as Example 1 is carried out with an injected gas comprising steam and a mass fraction of 2.8% of non-condensable gases. The injected gas flow rate is kept identical to Example 1.

[0091] The stored energy is greatly reduced ( Figure 5 ) compared to example 1. Here, it is reduced to 3.34MWh, i.e. -33% less than in example 1.

[0092] Charging stops when the maximum pressure (7.5 bars absolute) is reached, i.e. after 2 hours 40 minutes instead of 4 hours ( Figure 4 ). This is due to the increasing presence of incondensable gases in the gas headspace 3 of tank 1, the partial vapor pressure is then only 6.3 bars absolute at the end of the charge.

[0093] The energy can be evaluated approximately, being calculated in this case by the relation: E ≈ m L Cp T sat P steam , max − T sat P min P steam , max = P max − P CO 2 , max With (International System unit) E: energy mL: mass of liquid in the accumulator Cp: average heat capacity of the liquid Tsat(Psteam, max)): saturation temperature of the liquid at maximum target vapor pressure Tsat (Pmin): saturation temperature of the liquid at minimum target pressure Pmax: maximum target pressure PCO 2,max: maximum non-condensable pressure

[0094] Example 3 : Evolution of pressure and energy in a steam accumulator with incondensable gases.

[0095] The same experiment as example 2 is carried out with an injected gas comprising steam and a mass fraction of 2.8% of non-condensable gases. The injected gas flow rate is adapted to maintain a charging duration of 4 hours. The incoming steam flow rate is reduced from 0.58 kg / s to 0.39 kg / s (i.e. less than 33% than in examples 1 and 2) while maintaining the same conditions as in example 1, the charging then stops after 4 hours, and the same amount of energy is stored as in example 2 (3.34 MWh) ( Figures 6 And 7 ).

[0096] Example 4 : Evolution of pressure and energy in a steam accumulator with incondensable gases according to the method of the invention.

[0097] The same experiment as example 3 is carried out. A steam accumulator with a volume of 345 m 3< and a gas comprising steam with a mass fraction of 2.8% of non-condensable gases in the steam are used. The charging lasts 4 hours and the discharging lasts 4 hours. The mass flow rate of injected gas is 0.46 kg / s and the extraction rate is 0.0253 kg / s or 5.5% of the input flow rate. An energy of 3.71 MWh is stored in 4 hours of charging considering a thermodynamic equilibrium between the minimum and maximum target pressures of the previous examples, i.e. 4.5 bar absolute and 7.5 bar absolute, i.e. between 50.5 and 94 psi relative.

[0098] The storage capacity is therefore increased from 3.34MWh to 3.71MWh, i.e. an 11% increase compared to example 2 or 3.

[0099] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the claims. LIST OF REFERENCES

[0100] 1. Tank 2. Liquid 3. Gas ceiling 4. Evacuation device 5. Injection device 6. Liquid filling and draining device 7. Discharge valve 8. Loading valve 9. Liquid filling and draining valve 10. Injector 11. Additional injector 12. Additional loading valve 13. Discharge valve 14. Evacuation

Claims

1. Method for storing thermal energy in a steam accumulator comprising a tank (1) containing a liquid (2) and an expansion space (3) arranged above the liquid (2), a device (5) for injecting gas entering into the tank (1) and a gas discharge device exiting the tank (1), the gas comprising steam and at least one incondensable gas, the method comprising: - an energy charging phase comprising a gas injection at a first gas flow rate at least partially in the liquid contained in the tank (1) of the accumulator by the gas injection device (5) and the steam from the gas is condensed on contact with the colder liquid (2) and releases energy, which increases the temperature of the liquid (2), and - a discharge phase comprising the opening of the discharge device and the drop of pressure in the tank (1) causing a vaporisation of the liquid (2), characterised in that the method further comprises, during the energy charging phase, a gas discharge at a second gas flow rate outside of the tank (1) of the accumulator by the gas discharge device (4), the first gas flow rate and the second gas flow rate being controlled according to the incondensable gas mass fraction in the injected gas to reach an optimal energy storage, the second gas flow rate being less than the first gas flow rate.

2. Method according to the preceding claim, wherein the gas discharge at a second gas flow rate is intermittent during the charging phase.

3. Method according to claim 1, wherein the gas discharge at a second gas flow rate is continuous during the charging phase.

4. Method according to any one of the preceding claims, wherein the gas discharge at a second gas flow rate is performed from the expansion space (3).

5. Method according to any one of the preceding claims, wherein the gas injection at a first gas flow rate is continuous during the charging phase.

6. Method according to any one of the preceding claims, wherein the gas injection at a first gas flow rate comprises a main gas flow rate and a complementary gas flow rate.

7. Method according to the preceding claim, wherein the gas injection at a complementary gas flow rate is intermittent during the charging phase.

8. Method according to claim 6, wherein the gas injection at a complementary gas flow rate is continuous during the charging phase.

9. Method according to any one of claims 6 to 8, wherein the gas injection at a complementary gas flow rate is performed in the liquid (2) contained in the tank (1).

10. Method according to any one of claims 6 to 8, wherein the gas injection at a complementary gas flow rate is performed in an expansion space (3) arranged above the liquid (2) contained in the tank (1).

11. Method according to any one of claims 6 to 8, wherein the complementary gas flow rate is less than the main gas flow rate.

12. Method according to any one of claims 6 to 8, wherein the second gas flow rate is greater than or equal to the complementary gas flow rate.

13. Method according to any one of the preceding claims, wherein the liquid (2) contained in the tank (1) is water and the steam from the gas is water vapour comprising at least one incondensable gas.

14. Steam accumulator being adapted to implement the method according to claim 1, comprising a tank (1) containing a liquid (2) and an expansion space (3), a device (5) for injecting gas entering into the tank (1), a device (4) for discharging gas exiting from the tank (1), a management module comprising a computer and a control unit comprising valves for controlling the gas injection device (5) and the device (4) for discharging gas exiting from the tank (1), characterised in that the gas comprising steam and at least one incondensable gas, the management module is configured to implement the following actions during the energy charging phase: a) a gas injection at a first gas flow rate at least partially in the liquid contained in the tank (1) of the accumulator by the gas injection device (5), the steam from the gas is condensed on contact with the colder liquid (2) and releases energy, which increases the temperature of the liquid (2), b) a gas discharge at a second gas flow rate outside of the tank (1) of the accumulator by the gas discharge device (4), the first gas flow rate and the second gas flow rate being controlled according to the incondensable gas mass fraction in the injected gas to reach an optimal energy storage, the second gas flow rate being less than the first gas flow rate, and the management module is configured to implement the following action during the energy discharge phase: c) the opening of the discharge device and the drop of pressure in the tank (1) causing a vaporisation of the liquid (2).