HEAT CONVERSION INTO MECHANICAL ENERGY INSTALLATION FOR COOLING OPTIMIZED BY A SYSTEM FOR RECOVERING AND STORING A PORTION OF THE THERMAL ENERGY FROM THE WORKING FLUID

Abstract

Installation for converting heat into mechanical energy comprising: - a heat engine capable of subjecting a working fluid to a thermodynamic cycle, - a reservoir of sensible heat thermal storage material, as a means of storing part of the heat evacuated by the cooling circuit of the working fluid, the reservoir being capable of re-emitting this part of the heat into the cooling circuit of the working fluid of the heat engine during the hours when the outside temperature is coldest in order to evacuate the stored heat.

Description

technical field

[0001] The present invention relates to a system for recovering and storing part of the thermal energy (heat) available on the cooling circuit of the working fluid of a thermal machine for a heat-to-mechanical energy conversion installation with improved efficiency under high heat, according to claim 1, and a method of operating such an installation.

[0002] Although the invention is particularly suited to a concentrated solar power plant, due to its potential location, as described below, it can equally be implemented on any installation for converting heat into mechanical energy and in particular subsequently into electricity using an expansion turbine and therefore, an associated cooler. State of the art

[0003] Concentrating solar thermal technology involves concentrating solar radiation using mirrors to heat a heat transfer fluid that serves as the heat source in a thermodynamic cycle. This concentration allows for varying temperatures to be reached, resulting in different levels of thermodynamic conversion efficiency.

[0004] The technologies developed are distinguished by their method of concentrating solar rays, transporting (and possibly storing) heat (heat transfer fluids) and thermodynamic conversion (steam turbines, gas turbines, Stirling engines).

[0005] There are typically four main families of concentrated solar power plants (also called concentrated solar thermal power plants or solar thermal power plants, from the English acronym CSP for "Concentrating Solar Power Plant"): systems with cylindrical-parabolic collectors with linear focus, those with linear Fresnel concentrators, tower systems with central receiver and finally, parabolic systems with moving focus.

[0006] A CSP plant is a power plant that concentrates the sun's rays using mirrors to heat a heat transfer fluid. This fluid serves as a heat source for an expansion turbine, which drives a generator to produce electricity.

[0007] We now describe an example of a parabolic trough CSP power plant already installed, notably in Spain at Andasol with a capacity of 50MWe.

[0008] In this power plant, thermal oil is heated in an area consisting of multiple rows of parabolic trough mirrors grouped together in what is commonly called a "solar field." The heat from this oil is either stored in a storage fluid, which is typically a mixture of nitrate salts in the case of these parabolic trough plants, or sent to heat exchangers to directly produce steam. This steam is then expanded in a turbine to generate electricity.

[0009] At the outlet of the expansion turbine, the steam is condensed by an air cooler using outside air or water as a cold source. The condensed steam then returns to the heat exchangers to be evaporated again before being sent back to the expansion turbine.

[0010] In such a configuration, there can therefore be two distinct fluid circuits, namely that of the working fluid integrating the expansion turbine and connected via heat exchangers to the main thermal oil circuit and that of the condenser cooling of the circuit integrating the expansion turbine.

[0011] The efficiency of the expansion turbine is directly related to the steam inlet temperature and the temperature of the lowest point of the cycle, i.e. the temperature at the condenser.

[0012] The expansion turbine, as a thermodynamic conversion device, is limited by the theoretical Carnot efficiency according to the equation n = 1 - Tcold / Thot, where T is in Kelvin. Thus, the theoretical efficiency of an expansion turbine depends on the temperature of both the hot and cold sources.

[0013] Therefore, for a given hot temperature which depends on the solar receivers in the case of a CSP plant, or on a combustion chamber in the case of a gas or coal plant, the higher the cold temperature (temperature at the condenser), the lower the efficiency of the expansion turbine.

[0014] In the case of a concentrated solar power (CSP) plant, installation is preferentially carried out in a region with high levels of sunshine, which can therefore experience high daytime temperatures. A simple solution for the cold source is to draw water from a river or groundwater, but this is a scarce resource in desert or arid regions. Groundwater aquifers in Africa, for example, are of Quaternary age. Moreover, even when such a cold source exists, it is increasingly difficult and expensive to exploit due to environmental regulations. In addition, treating the extracted water is costly and requires a regular supply of chemicals.

[0015] Publication [1] discusses this issue at length. In particular, it investigates four potential sites for CSP plants in Africa, taking into account current and projected average temperatures for the coming years. This investigation reveals that the predicted global warming will particularly affect regions already considered hot, including the sites being considered for CSP plants.

[0016] Under these conditions, the cooling needs of these CSP plants are likely to be crucial.

[0017] We can refer to publication [2] in which a presentation of the different cooling technologies for power generation plants is carried out.

[0018] Four technologies are predominantly used, and we will now briefly describe how they work.

[0019] Open wet cooling systems draw water from a river or sea to cool the steam in the condenser. The water is then discharged downstream, at a higher temperature, directly back into the river or sea. As a guideline, the required water volume is approximately 160 liters / kWh (kilowatt-hour of electricity), with no water evaporating.

[0020] Closed-loop wet cooling systems significantly reduce external water withdrawals because the water circulating within the circuit is cooled by cooling towers. The water consumed is only what is needed to maintain a constant level in the closed loop, as some of the water evaporates through the cooling towers. For reference, the approximate volume of water required is 6 liters / kWh, of which 2 liters / kWh represents evaporation.

[0021] Air-cooled systems include an additional circuit, which is intermediate between the expansion turbine condenser and the heat exchanger. Water consumption is reduced by 95% compared to open wet-cooling systems, with the remaining water used for blowdown and leaks. For example, a parabolic trough CSP power plant currently operating in the California desert with a wet-cooling system consumes approximately 3 liters / kWh (kilowatt-hours of electricity). With an air-cooled system, consumption is reduced to 3 to 3.4 liters / kWh, of which 0.75 liters / kWh are used for mirror cleaning.

[0022] However, air cooling systems have many disadvantages. Indeed, the investment cost is much higher than for wet cooling systems, the noise generated is significant, efficiency is greatly affected in hot weather, electricity consumption is high, and finally, the footprint is also large.

[0023] Hybrid cooling systems combine, in various forms, air cooling with water spray. The use of water spray improves the efficiency of the air-cooled condenser in warm outdoor conditions. Water consumption is lower than that of wet cooling systems.

[0024] In a waterless or very low-water cooling system, either a so-called dry air condenser is used, in which the steam from the expansion turbine is condensed, or a so-called dry air cooler is used which cools a transfer fluid, often glycol water to avoid possible nighttime freezing, which ensures the condensation of the steam from the turbine inside the condenser.

[0025] The architecture is the same for air-cooled condensers and air-cooled condensers, with supply and return piping for steam or transfer fluid and fans that circulate ambient air. The fans are positioned a few meters above the ground to allow sufficient airflow to prevent pressure loss and avoid drawing in dust.

[0026] One option for cooling a CSP plant is to use a cooling system that directly utilizes ambient air as a cold source via a dry air-cooled condenser. In such a system, ambient air is used to dissipate the residual heat from the expansion turbine, which represents approximately 65% ​​to 85% of the heat produced by the solar receivers, depending on the conversion efficiency. This efficiency ranges from around 15% for expansion turbines in small Rankine cycle heat engines to around 35% for large steam turbines. A dry air-cooled condenser cooling system is implemented, for example, by Novatec in CSP plants with linear Fresnel concentrators.

[0027] As already mentioned, the main advantage of such a system is water conservation. The major drawbacks are the energy cost, which is dedicated to powering the fans, and the loss of efficiency of the unit when the outside air temperature exceeds 30 to 35°C.

[0028] The switch from a wet cooling system to an air cooling system represents a loss equivalent to 3% of the annual electrical energy produced in Spain, and to a value of 5 to 10% in North Africa, according to the publication [1].

[0029] Another source of information estimates the additional cost of electricity production for an air cooling system at 10%, and the loss of energy produced at 7%: publication [3].

[0030] A hybrid cooling system makes it possible to limit the loss of energy produced to 1% for a water consumption of around 50% of that of a wet system.

[0031] The penalty induced by a cooling system with a dry air condenser, i.e. the loss of energy produced due to its operation, is the greatest during the day when it is very hot outside, which unfortunately corresponds to periods of strong sunshine, therefore of strong electricity production, and therefore of strong cooling needs at the condenser of the expansion turbine.

[0032] One possible solution for dissipating the full thermal power at the condenser of the heat-to-electricity converter, in cases of very high outside temperatures, is to oversize the air-cooled condensers. This means installing units calibrated to dissipate this full thermal power in the most extreme conditions, corresponding to the highest possible temperatures. The major drawbacks of this solution are the very high investment cost and the year-round electricity consumption of the fans, while periods of extreme heat are limited.

[0033] Other cooling systems for the condensers of solar power plant electrical conversion machines are currently being considered, although they remain largely at the study and / or prototype stage. One example is systems based on the use of a desiccant heat transfer fluid between the condenser and the ambient air: see publication [4]. This type of system offers superior performance compared to conventional air-cooling systems and provides a natural heat storage function based on the principle of a desiccant fluid and variations in outside air temperature.

[0034] Furthermore, other cooling systems have already been proposed.

[0035] Thus, US patent application 2009 / 0158736 proposes a system where the ground is used as a cold source, with a heat exchanger embedded in the ground. The drawbacks of the disclosed system are numerous and can be listed as follows: Significant lengths of piping are required to ensure heat exchange with the ground, which represents a financial and energy cost and necessitates substantial maintenance. Heat transfer is inefficient, and thermal regeneration of the ground—that is, the return of the ground to its initial thermal configuration before heat exchange—is not guaranteed due to the ground's low thermal conductivity. In this case, the effectiveness of the ground as a cold source tends to decrease with repeated use.

[0036] From US patent 4054246, we also know of systems that store the sun's heat during the summer or the cold of the ground in a bed of rocks arranged under a dwelling.

[0037] Instead of creating cooling systems for the condensers of thermal power conversion machines, other solutions involve removing some of the thermal energy from the expansion turbine, i.e. upstream of the condenser.

[0038] Publication [5] thus envisions a system that combines an absorption chiller for cooling, cold fluid storage, and then the release of the stored cold to the air inlet of an air-cooled condenser via a heat exchanger. The proposed cooling system has several major drawbacks, which can be summarized as follows: The system is particularly complex as it combines several components and machines; electrical energy consumption is particularly high as the absorption machine and the added circuits all need to be powered; setting up such a system requires a large storage volume.

[0039] Publication [6] proposes a phase change material (PCM) storage system to delay the release of some of the thermal power from the expansion turbine. The proposed storage system is expensive to manufacture and complicated to implement, particularly due to the addition of an extra heat exchanger.

[0040] US patent 7340899 B1 describes a solar thermal power plant with a heat engine cooling circuit comprising two tanks in series: a hot water storage tank and a cold water storage tank. In this patent, the hot water storage tank is sized to temporarily store the cooling fluid after it has cooled the working fluid. When the cooling fluid has reached a temperature low enough to circulate again through the heat engine's condenser, it can then be transferred to the cold water tank. The disclosed cooling system thus involves storing all the cooling fluid first in the hot tank and then in the cold tank, which implies a very large size for the storage tanks that could be prohibitive in terms of cost and / or installation capacity.Furthermore, even with a very large capacity, it is not certain that the disclosed tanks can dissipate excess thermal power (heat) produced by the machine. Moreover, US patent 7340899 B1 is silent regarding the solution provided to this problem of heat dissipation. US patent 7340899 B1 describes a heat-to-mechanical energy conversion system according to the preamble of claim 1.

[0041] Therefore, there is a need to improve the efficiency of the condenser cooling system in a heat-to-power plant under very high outdoor temperatures. In other words, there is a need to maintain the nominal performance of a condenser in a heat-to-power plant under very high outdoor temperatures.

[0042] More generally, there is a need to improve the efficiency of a heat-to-mechanical energy conversion installation and in particular afterwards into electricity, especially of a concentrated solar power (CSP) plant incorporating such an installation, in the event of very high outside temperatures.

[0043] The general purpose of the invention is to meet at least part of this need(s). Description of the invention

[0044] To achieve this, the invention first of all relates, according to one aspect, to a heat-to-energy conversion installation according to claim 1.

[0045] In other words, the invention relates to a heat-to-mechanical energy conversion installation comprising: a heat engine capable of subjecting a working fluid to a thermodynamic cycle, a cooling device comprising: a cooling circuit capable of allowing the circulation of a cooling fluid, a means of cooling by forced ventilation or pulsed air (air heater) allowing the cooling of the cooling fluid heated by the working fluid of the heat engine, a tank of thermal storage material by sensible heat, the tank being designed to store part of the heat removed by the cooling circuit of the working fluid and the tank being designed to re-emit this part of the heat into the cooling circuit of the working fluid of the heat engine during the hours when the outside temperature is coldest in order to remove the stored heat.

[0046] According to an advantageous embodiment, the invention relates to a heat-to-electricity conversion installation comprising: a heat engine capable of subjecting a working fluid to a thermodynamic cycle known as the Rankine cycle, the heat engine comprising a steam expansion turbine, a reservoir of sensible heat thermal storage material, as a means of storing part of the heat evacuated by the turbine cooling circuit, the reservoir being capable of re-emitting this part of the heat into the cooling circuit of the working fluid of the heat engine during the hours when the outside temperature is coldest in order to evacuate the stored heat.

[0047] According to an advantageous embodiment, the heat engine is an organic Rankine cycle (ORC) engine. Like publication [6], which considers heat storage to delay the release of some of the thermal energy from the expansion turbine, the invention also proposes storing some of the thermal energy in case of high temperatures. However, unlike that publication, the sensible heat storage according to the invention is simple to implement and easy to carry out, and furthermore, it is performed on the cooling circuit of the working fluid and not on the working fluid itself.

[0048] The storage means according to the invention makes it possible to store excess heat discharged by the turbine during the few hours per day when the outside temperature is too high in order to ensure sufficient efficiency of the air-cooled heat exchanger (in particular an air-cooled condenser or an air-cooled condenser) which ensures the cooling of the working fluid, and to release this heat when the heat exchanger is less stressed because the production of steam and therefore electricity is lower and at the same time the outside temperature allows for easier heat discharge.

[0049] The heat exchanger of the thermal machine can be sized as usual, or even undersized compared to existing thermal machines. This reduces its operating cost and increases the overall efficiency of the electricity conversion system, and therefore of a CSP solar power plant.

[0050] The invention, implemented on an intermediate water cooling circuit, can allow the use, as an air condenser / air cooler for cooling the condenser of the thermal machine, of both a dry air condenser / air cooler and a wet air condenser / air cooler, even though implementation on a dry air condenser / air cooler would compensate for all or part of the current decrease in plant efficiency linked to the overconsumption of electricity by the fans of dry air condensers / air coolers and to outdoor temperature peaks.

[0051] In other words, the invention makes it possible to increase the conversion efficiency of a heat-to-electricity conversion installation during periods of high or even very high heat, at a reasonable cost.

[0052] The invention can be advantageously implemented in regions with very high daytime temperatures, generally arid climate zones, and low nighttime temperatures.

[0053] More specifically, potential areas for the installation of CSP solar power plants are those, particularly in arid semi-desert climates with mostly clear skies, where the temperature difference between day and night is significant in all seasons.

[0054] For example, in the Ouarzazate region, where CSP solar power plants have been built, the average temperature difference between day and night is 15 to 20°C, and this difference remains even on the hottest days of the year.

[0055] According to the invention, this temperature difference between day and night can be used to store some of the heat evacuated by the turbine during the hottest hours of the day and to release it during the coldest hours of the night to the circuit of the thermal machine, which allows the air condenser / air cooler to operate in its nominal regime at all times and therefore does not affect its efficiency.

[0056] Advantageously, the storage material reservoir is constructed as a liquid storage system known as a "thermocline." This can be advantageously achieved using a pit in the ground, the pit being connected to the cooling circuit of the heat engine's working fluid by at least one fluid circuit. It should be noted here that a thermocline liquid storage system is a single reservoir containing a liquid with a hot zone, a cold zone, and a small intermediate zone called the "thermocline," where thermal stratification occurs within the reservoir.

[0057] The pit to be constructed to form the heat storage tank according to the invention can be created during the construction of a new installation. It can also be created in an existing installation.

[0058] Liquid storage using a thermocline can also be advantageously implemented in the form of a fluidized bed stabilized by a rock bed. In this case, heat storage according to the invention is therefore carried out both in the heat transfer fluid and in a rock bed. For the application of the invention to a solar power plant, storage in a rock bed is advantageous because the rocks can be present in situ at the plant site and in abundance. Other sensible heat storage materials can be suitable alone or in combination with thermocline liquid storage. Generally, inexpensive and readily available materials can be considered, such as scrap ceramics, bricks, or used ball bearings.

[0059] According to an advantageous feature of the invention, the heat transfer fluid in the cooling circuit is simply water. According to the prior art, glycol water is generally used as the heat transfer fluid in cooling circuits because the glycol prevents nighttime freezing. The heat stored according to the invention allows the use of simple water instead of this glycol water, since the stored heat also prevents nighttime freezing.

[0060] According to an advantageous variant, the pit is in the shape of a truncated pyramid with its base positioned deepest in the ground. This truncated pyramid shape advantageously allows for a reduced pit depth for the same volume.

[0061] The invention also relates to a method of operating the installation described above, according to which the following steps are carried out: i / at night, heat exchange between the thermal storage tank and the cooling circuit of the working fluid of the thermal machine, ii / during the day, if the power dissipated by the working fluid is less than a predetermined threshold value, fluidic insulation of the tank, iii / during the day, if the power dissipated by the working fluid is greater than a predetermined threshold value, heat exchange between the cooling circuit of the working fluid of the thermal machine and the thermal storage tank in order to store part of the heat dissipated by the working fluid in the tank.

[0062] The invention also relates to a concentrated solar power (CSP) plant, comprising an installation described above. More generally, the invention relates to any electricity generation installation using heat conversion that operates in hot climates and whose efficiency depends on the outside temperature.

[0063] The invention also relates to the use of an installation described above in a region where the difference in outside temperature between night and day is at least equal to 10°C. Detailed description

[0064] Other advantages and features of the invention will become clearer upon reading the detailed description of the invention, given by way of illustration and not limitation, with reference to the following figures, among which: Figure 1 is a general schematic view of a CSP solar power plant already operated by the applicant; Figure 2 is a simplified schematic view of the CSP power plant according to Figure 1; Figure 3 is a simplified schematic view of a CSP power plant with an example of a turbine heat removal system according to the invention during a phase of storing this heat in a reservoir; Figure 4 is a simplified schematic view of a CSP power plant with an example of a turbine heat removal system according to the invention during a phase of releasing this heat; Figure 5 is a schematic cross-sectional view of an advantageous example of a storage reservoir for part of the thermal power removed by the heat engine turbine, according to the invention.Figure 6 is a graph showing the characteristic curves of the power delivered by the CSP plant according to the state of the art, illustrated in Figures 1 and 2, these curves having been taken during a hot summer day; Figure 7 shows the characteristic curves of the power delivered by the CSP plant according to Figure 6 and shows a first operating mode of the system according to the invention; Figure 8 shows the characteristic curves of the power delivered by the CSP plant according to Figure 6 and shows a second operating mode of the system according to the invention.

[0065] In the description that follows, the terms "inlet", "outlet", "upstream", "downstream", are used by reference to the direction of fluid flow within an installation according to the invention.

[0066] The terms "above", "below", "lower", "upper" are to be considered with reference to the ground placement of a heat storage tank according to the invention.

[0067] For clarity, the same elements in an installation 3 according to the state of the art and an installation 3 according to the invention are designated by the same numerical references.

[0068] The thermodynamic solar power plant (CSP) 1, which is schematically represented in Figure 1, is a prototype power plant and is already operated by the applicant.

[0069] We will not describe in detail here part 2 of the power plant 1 which carries out the actual heat production. In essence, this part 2 comprises two groups of pluralities of parabolic trough mirrors forming what are commonly referred to as solar fields 20, 21. The heat transfer fluid, which as shown in Figure 1 is oil, circulates in a circuit 22 which supplies its energy to a heat conversion unit 3 which converts the heat from the oil into electricity and is more specifically relevant to the invention.

[0070] More specifically, the electrical conversion heat machine 3 is a machine operating according to a thermodynamic cycle called the Organic Rankine Cycle (ORC).

[0071] This machine 3 includes first of all a main circuit 3a with an expansion turbine 30 enabling the production of electricity from steam, an evaporator 31 to recover heat energy from the circuit 22 and vaporize the working fluid, which is a refrigerant in the example illustrated in Figure 1, to send it downstream to the turbine 30 via a pump 32 which circulates the working fluid in the circuit 3a.

[0072] A condenser 33 is provided downstream of the turbine 30: its function is to evacuate, by condensation, the energy not transformed by the turbine 30.

[0073] Condensation is made possible in the condenser 33 by a dedicated cooling circuit 3b through which a coolant, also called a transfer fluid, circulates. In the example shown in Figure 1, this coolant is glycol water. The glycol water is then cooled by a dry air cooler 34, which incorporates a fan supplying it with outside air. Circulation of the coolant in circuit 3b is ensured by a pump 35.

[0074] Instead of having an additional circuit 3b with a dedicated transfer fluid as shown in figures 1 and 2, we can also have a single cooling circuit 3a, the condenser 33 and the air cooler 34 then constituting a single component, which is a dry air condenser, in which the steam from the expansion turbine is directly condensed.

[0075] By "air condenser", we mean here and within the framework of the invention, a heat exchanger in which a fluid in vapor form circulates and is condensed with ambient air blown by a fan.

[0076] By "air cooler" we mean a heat exchanger in which a liquid or gaseous fluid is cooled by an outside airflow blown by a fan.

[0077] After consulting the typical annual temperature profiles at the site of this CSP 1 power plant and the usual cooling requirements of the ORC 3 thermal machine, the air-cooled condenser 34 was sized during construction for a given power output at an outside temperature of approximately 25°C. Outside air temperatures of 30°C and 35°C were considered infrequent and rare, respectively, and were therefore not initially used as sizing parameters.

[0078] However, as the inventors were able to see, the cooling requirement for the 3 ORC thermal machine turned out to be greater than initially planned, and the occurrences of outside air temperatures above 30°C were more significant than anticipated.

[0079] Consequently, it was found that the installed air-cooled condenser 34 was undersized for the hottest hours in summer, when the ORC heat engine 3 is operating at full capacity. This has the unavoidable consequence of forcing the operating team to defocus certain mirror lines of the solar arrays 20 and 21, that is, to change their orientation so as not to concentrate the sun's rays on these parts of solar arrays 20 and 21 during the hottest periods. This therefore leads to a reduction in the power generation efficiency of the plant 1, since the full potential of solar arrays 20 and 21 is not available at these times. The actual power output of the ORC heat engine 3 is thus reduced compared to the maximum theoretical power output, the latter representing the maximum possible power output given the single available solar resource.

[0080] To address this problem of loss of efficiency of the solar power plant 1 CSP during very hot hours, which is induced by the loss of efficiency of the air cooler 34 directly supplied by outside air, the inventors thought of creating a system to store part of the heat evacuated by the turbine 30 during the hottest hours.

[0081] As illustrated in Figures 3 and 4, this system 4 includes a storage tank consisting of a thermocline liquid storage tank in the form of a stabilized fluidized bed in a rock bed 40 to store said part of the turbine heat 30 at the hottest hours, in order to release it at the coldest hours via circuits 3c, 3d to the working fluid circuit 3a of the heat engine 3.

[0082] Thus, circuit 3c, 3d is an additional cooling circuit, derived from the main cooling circuit 3b.

[0083] In this system 4, heat storage according to the invention is advantageously achieved both in the heat transfer fluid and in the rock bed. With such a storage system 4, the heat transfer fluid can simply be water, whereas in state-of-the-art cooling systems, glycol must be added to water to prevent nighttime freezing. In other words, the heat stored according to the invention can eliminate the need for glycol, as is the case with state-of-the-art systems.

[0084] More specifically, as illustrated in Figure 3, when the thermal power removed by the heat engine is below a predetermined threshold value, at least valve 42 is in its closed position.

[0085] When the heat output from turbine 30 exceeds a predetermined threshold value, corresponding to the hottest hours of a summer day, the valve system 42 is actuated to allow a portion of the heat flow to circulate through circuit 3c and thus into the storage tank 40 designed for this purpose. The remaining heat output from turbine 30 is discharged through the air cooler 34 as usual. During storage, valve 43 is open and valve 44 is closed. In its open position, valve 43 allows fluid connection between the outlet of storage tank 40 and the inlet of air cooler 34 via circuit 3c. In this way, the portion of the heat output exceeding the predetermined threshold value is stored in tank 40, while the cooling fluid from the auxiliary cooling circuit 3c flows into the main cooling circuit 3b.

[0086] As illustrated in Figure 4, when it is necessary to release the heat stored in reservoir 40, valve 42 is opened, valve 43 is closed, and valve 44 is opened. In its open position, valve 44 connects the outlet of storage reservoir 40 to the outlet of the air cooler 34 via circuit 3d. The fluid circulating in circuit 3b can then flow through circuit 3d back to reservoir 40, which releases its stored heat upstream of the turbine 30. Thus, the heat stored within reservoir 40 can be released back into the main cooling circuit 3b.

[0087] In certain configurations for the heat storage and release phases in / from reservoir 40, the pump 35 initially dedicated to the circulation of the cooling fluid in circuit 3b is sufficient on its own to circulate the fluid through circuits 3c, 3d and reservoir 40. If necessary, one or more other pump(s) can be considered to achieve this circulation.

[0088] As seen in figure 3, the direction of circulation of the cooling fluid allows the single tank 40 to be thermally charged by bringing the fluid into the hot part of the tank and pushing it towards its cold part.

[0089] Conversely, as seen in figure 4, the direction of circulation of the cooling fluid allows the single reservoir 40 to be thermally discharged by amending the cooling fluid in the cold part and pushing it towards its hot part.

[0090] A configuration according to figures 3 and 4 allows us to take advantage of the circuits already installed for the destocking of heat at night but for the purpose of increasing electricity production, like that of figure 1.

[0091] A configuration according to figure 4 allows the circuit 3c to be completely isolated, thus dedicated solely to the evacuation of the heat from the turbine 30.

[0092] As shown in Figure 5, the rock bed is preferably made in a pit 40. In this Figure 5, the preferred shape of the pit 40 is also shown: the pit 40 is in the shape of a truncated pyramid with the base arranged deepest in the ground.

[0093] The construction of the rock bed 4 in an underground pit 40 is not mandatory, and any suitable type of reservoir 40 can be provided in any installation 3 which can store part of the heat removed by the expansion turbine 30 during the hottest hours, in order to release it at night by exchange with the circuit 3a of the thermal machine 3. Preferably, a single liquid storage reservoir by thermocline is chosen.

[0094] Figure 6 illustrates a typical situation that occurred during a summer day on prototype solar power plant 1, as illustrated in Figure 1.

[0095] Figure 6 shows the following quantities: “Text”: outside air temperature, in °C; “P_CS”: maximum power in kW recoverable from the solar field 20, 21 at a given time. This quantity is independent of the operation of the ORC machine 3 and the air cooler 34; “Pelec ORC”: electrical power delivered by the ORC machine 3 based on P_CS and an unlimited air cooler 34. This value therefore corresponds to the maximum electrical power that can be generated at a given time; “Pcond ORC”: power that must be dissipated at the condenser 33 of the ORC machine 3 when it is supplied with P_CS at its evaporator 31; “Paéro max”: maximum power of the air cooler 34 depending on the outside temperature conditions.

[0096] It is specified here that the powers indicated "P_CS" and "Pcond ORC" have been dimensionless with the value of the maximum theoretical power recoverable on the solar field 20, 21. The power "Pélec ORC" has on the other hand been dimensionless with the value of the maximum theoretical electrical power produced by the machine 3 ORC.

[0097] As can be seen in Figure 6, during a summer day, the outside temperature exceeds 35°C at midday, and the power supplied to machine 3 approaches its nominal value. Under these conditions, the air cooler 34, which was initially installed and is supplied directly by outside air, is clearly insufficient to meet the cooling requirements of machine 3, because Paéromax <PcondORC.

[0098] In other words, during period D shown in Figure 6, the power that can be dissipated by condenser 33 is less than the power actually required, due to excessively high temperatures. As already mentioned, the only current solution to prevent the shutdown of machine 3 due to insufficient cooling of condenser 33 is to defocus the mirrors of the solar fields 20 and 21 to reduce the heat input (Pcs) to machine 3. Consequently, the actual electricity production then falls to 59% of the potential electricity production.

[0099] The thermal power that an air-cooled condenser 34 can dissipate is expressed according to the following law: Pth = K * S * ΔT With Pth: heat exchangeable power in W; K: average surface heat transfer coefficient (in W / m² / K); S: heat exchange surface area in m²; ΔT: average temperature difference between hot and cold fluid (in K)

[0100] Thus, according to the invention, when the excess heat evacuated by the turbine 30 can no longer be evacuated by the air cooler 34, it is stored in the thermal storage material 40 by sensible heat, that is to say during the hottest hours of the day, and this stored power is evacuated during the coldest hours, at the end of the day or during the night.

[0101] In general, the operating method of the system according to the invention comprises the following steps: i / at night, heat exchange between the thermal storage tank 40 and the working fluid circuit 3a of the machine 3 via the circuit 3b, ii / during the day, if the power dissipated by the working fluid is less than a predetermined threshold value, fluidic insulation of the tank 40, iii / during the day, if the power dissipated by the working fluid is greater than a predetermined threshold value, heat exchange between the working fluid circuit 3a of the machine 3 and the thermal storage tank 40 via the circuit 3b, in order to store part of the heat dissipated by the working fluid in the tank 40.

[0102] Two modes of operation can be considered.

[0103] If the thermal power supplied by turbine 30 exceeds a predetermined limit, the excess power is sent to the thermal storage tank 40. When the power supplied by turbine 30 falls below this predetermined limit, the heat from tank 40 can be removed by the air cooler 34. These different periods of excess power storage from turbine 30 and release from tank 40 are shown as hatched areas in Figure 7.

[0104] According to a second method, the electrical power of the air cooler 34 is fixed. The maximum power that can be evacuated by the air cooler varies throughout the day: it is maximum when outside temperatures are low, and minimum during periods of heat and strong sunshine.

[0105] As before, when the thermal power supplied by turbine 30 exceeds a predetermined limit, the excess power is sent to the thermal storage tank 40. When the power supplied by turbine 30 falls below this predetermined limit, the heat from tank 40 can be removed by the air cooler 34. The hatched areas in Figure 8 also show the storage and release of excess thermal power from turbine 30.

[0106] The first mode (predetermined limited thermal power) allows for a reduction in the electrical consumption of air coolers, especially outside of periods of electricity production.

[0107] According to publication [1], the air-cooled condenser is typically sized in such a way that the hottest 1% of hours of the year are not taken into account. In other words, the air-cooled condenser is undersized for these hottest 1% of hours, and the plant will inevitably operate at a reduced capacity during these times. In the case study considered here, this roughly corresponds to a sizing for an outside temperature of 35°C. Such sizing is advantageous because it minimizes the investment costs of the air-cooled condensers / air-cooled condensers in the CSP plant, as well as their annual operating costs.

[0108] The major drawback is the obligation to operate the air-cooled condensers / air-coolers in degraded mode during periods of very strong sunshine, which penalizes the electrical production efficiency of the CSP plant and therefore its profitability.

[0109] The invention just described makes it possible to maintain or even minimize the usual sizing of air condensers / air coolers without affecting the production efficiency of a CSP plant.

[0110] Other variations and improvements can be envisaged without departing from the scope of the invention.

[0111] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0112] The expression "containing one" should be understood as synonymous with "containing at least one", unless otherwise specified. References cited

[0113] [1]: Kevin Damerau and al. « Costs of reducing water use of concentrating solar power to sustainable levels: Scénarios for North Africa », Energy Policy, 2011[2]: «Comparison of alternate cooling technologies for california power plants, Economies, environmental and other tradeoffs», California Energy Commission, Février 2002[3]: « Realising the potential of concentrating solar power in Australia », IT power, Mai 2012[4]: C. Martin and al. « Novel Dry Cooling Technology for Power Plants », SunShot Concentrating Solar Program Review 2013[5]: V.Gadhamshetty and al. « Improving Air-Cooled Condenser Performance in Combined Cycle Power Plants », Journal of Energy Engineering ASCE, Août 2006[6]: Lorenzo Pistocchini and al. « Feasability Study of αn Innovative Dry-Cooling System with Phase-Change Material Storage for CSP Multi-MW Size Power Plant », Journal of Solar Energy Engineering, Août 2011

Claims

1. An installation (1) for converting heat into mechanical energy, characterized in that it comprises: - a thermal machine (3) capable of subjecting a working fluid to a thermodynamic cycle, - a reservoir (4, 40) of sensible heat thermal storage material, as means for storing a part of the heat released by the working fluid cooling circuit, the reservoir being capable of returning this part of the heat into the working fluid cooling circuit (3a) of the thermal machine in hours when the outside temperature is coldest in order to release the stored heat.

2. The installation (1) as claimed in claim 1, the installation being an installation for converting heat into electricity comprising: - a thermal machine (3) capable of subjecting a working fluid to a so-called Rankine thermodynamic cycle, the thermal machine comprising a working fluid vapor expansion turbine (30), - a reservoir (4, 40) of sensible heat thermal storage material, as means for storing a part of the heat released by the cooling circuit of the turbine, the reservoir being capable of returning this part of the heat into the working fluid cooling circuit (3a) of the thermal machine in hours when the outside temperature is coldest in order to release the stored heat.

3. The installation (1) as claimed in claim 2, the thermal machine being an Organic Rankine cycle (ORC) machine.

4. The installation (1) as claimed in one of claims 1 to 3, the reservoir being a so-called "thermocline" liquid storage, that is to say a single reservoir in which a liquid is present with a hot zone, a cold zone and an intermediate zone of small volume in which a thermal laying is established within the reservoir.

5. The installation (1) as claimed in claim 4, the thermocline liquid storage being stabilized by a bed of rocks.

6. The installation as claimed in any one of the preceding claims, in which the heat transfer fluid of the cooling circuit is water.

7. The installation (1) as claimed in one of the preceding claims, the reservoir being produced in the form of a pit in the ground, the pit being linked to the circuit of the working fluid of the thermal machine by at least one fluid circuit (3c, 3d).

8. The installation (1) as claimed in claim 7, the pit being in the form of truncated pyramid with the base arranged deepest in the ground.

9. A thermodynamic solar power plant (CSP), comprising an installation as claimed in any one of claims 2 to 8.

10. A method for operating the installation as claimed in one of claims 1 to 8, according to which the following steps are carried out: i / at night, exchange of heat between the thermal storage reservoir (4, 40) and the working fluid cooling circuit (3a) of the thermal machine, ii / in the day, in case of power release by the working fluid below a predetermined threshold value, fluidic isolation of the reservoir, iii / in the day, in case of power released by the working fluid above a predetermined threshold value, exchange of heat between the working fluid cooling circuit (3a) of the thermal machine and the thermal storage reservoir in order to store a part of the heat released by the working fluid in the reservoir (4, 40).

11. The use of an installation as claimed in any one of claims 1 to 8 in a region where the outside temperature deviation between night and day is at least equal to 10°C.