DESCRIPTION

This invention relates to an innovative plant and a method for the storage of thermal energy, in particular obtained starting with surplus electric energy, with the aim of subsequently being able to generate electric energy. The system according to this invention may be sized in such a way that it is able to store for subsequent use quantities of energy ranging from several tens of kilowatt hours (kWh) to tens, hundreds or even several thousand megawatt hours (MWh).

In the recent past, electric energy was produced above all in large plants by burning fossil fuels such as coal, fuel oil and natural gas, or in thermonuclear plants. Then, energy crises, incidents at nuclear plants, the ever increasing attention to environmental issues, but above all the urgent need for an ever greater amount of energy, prompted international bodies to promote the development of "new" renewable energy sources such as wind and photovoltaic.

At least in some countries, policies for incentivising and deregulating energy markets allowed these new renewable sources and other forms of distributed energy generation (such as plants for recovering energy from industrial processing) to develop and increase their share of the energy mix of the various countries. Obviously, these energy sources have many advantages compared with traditional sources. But, at the same time, they bring new challenges linked to their integration in the national and international electric system.

In particular, these alternatives to traditional sources (whether renewable or not) are based on direct use of energy sources that have peaks which are difficult to predict (for example, due to solar radiation or wind in the case of renewable sources) which do not usually correspond to peaks in energy needs.

In order for these energy sources to really be alternatives to traditional sources (coal, fuel oil, natural gas and nuclear power), it is therefore crucial to develop energy storage plants. In fact, thanks to plants of this type, the electric energy produced by such alternative sources which is in excess of what is required by the electric system can be stored so that, when production levels are below demand, the stored energy can be re-converted into electric energy and fed into the network to meet demand.

Therefore, storage technologies are an integral and essential part of an electric system based on renewable sources and on distributed generation. Moreover, storage systems are useful for reducing the load variations of traditional plants which, following deregulation of the electric market and the strong spread of renewable source systems, found themselves operating in a market where renewable energy sources have priority dispatch.

Consequently, traditional plants are forced to follow the load and not operate at stable levels as they did in the past. However, load following involves not just daily start-ups and shut-downs, but above all hourly variations in output. Rapid and prolonged ramping up or down of the output and frequent start-ups and shut-downs of traditional plants cause high levels of stress on the components of output systems that in the long-term result in big reductions in the working life of the devices.

As is known, in order to be stored, electric energy may be converted into various forms. However, it is important to consider that every storage technology has its own particular features and problems, which make it suitable for some applications rather than others. In particular, parameters such as energy and power density, response speed, cost and economy of scale, working life, check and control systems, management efficiency and simplicity are factors that lead to the choice of the best technology to be used for a particular application.

Specifically, electric energy can be stored in the form of potential energy of water (Pumped Hydro Energy Storage or Pumped Hydro Storage) or as compressed air (Compressed Air Energy Storage), as electrochemical energy in batteries, as chemical energy in fuel cells, as kinetic energy in flywheels, as a magnetic in inductors, as an electric field in capacitors and as thermal energy in suitable containers.

Water-based storage or Pumped Hydro Energy Storage (PHES) or Pumped Hydro Storage (PHS) is the oldest, best known and most widespread storage system. The operating principle is based on management of the potential energy of water: the water is pumped from a downstream basin to an upstream basin during periods of low demand for electricity or when the cost of that energy is low. In contrast, during periods of maximum demand for electricity or when the energy selling price is high, the water flows from the upper basin to the lower basin, driving one or more hydraulic turbines. Since these machines are fitted to an electric generator, the potential energy of the water is converted into mechanical energy and then into in electric energy. In this case, the storage capacity is proportionate to the volume of the upstream basin and to the difference in height between the two basins. This technology may be applied both for seasonal storage and for daily storage. The start-up time for these systems is several seconds, whilst the working life of a PHS plant is between 30 and 50 years. Efficiency is around 65-75% with installation costs of between 500-1500 euros/kW or 10-20 euro cents/kWh. It should be noticed that the larger the upstream basin is, the easier it is to plan plant output. But, in spite of this, it is always necessary to take into account water availability, which depends on the place where the plant is installed. As already indicated, this technology has been extensively developed and there are many plants operating worldwide, despite their installation being greatly influenced by the presence of a suitable installation site morphology.

Storage using compressed air, or Compressed Air Energy Storage (CAES), is based on gas turbine technology. The energy is stored in the form of compressed air in underground caverns. As for PHS, during periods of low demand for electricity when electricity prices are low, excess output is used to drive compressors which suck in air from the surrounding environment, compress it and inject it into underground caverns. When required, that is to say, during periods of network high demand for electricity, the compressed air is taken from the storage cavern, heated, mixed with natural gas. The mixture obtained in that way is then burned and therefore expanded in one or more turbine bodies, generating electric energy. Therefore, pressure energy is converted into rotational kinetic energy. The exhaust gases exiting the turbine are used for heating the air arriving from the cavern before it is mixed with the natural gas.

A typical pressure value for air injection into the cavern is 75 bar, whilst typical power values for a CAES vary from 50 MW to 300 MW. Start-up times for this system are around 9 minutes for an emergency start-up and 12 minutes for a normal start-up. Obviously, CAES requires a geologically stable cavern and therefore installation of such a system is only possible in the presence of suitable underground cavities. It should be noticed that the system may even be built using man-made tanks. Obviously, this factor increases plant construction costs. The estimated working life of a system of this type is around 40 years, with around 71 % efficiency.

There are not many plants using this type of storage technology operating around the world, even though, currently, CAES is being widely studied in the United States given the presence of geologically stable caverns.

Electrochemical storage of surplus energy in batteries is another method for storing large quantities of electric energy. Batteries are the most widely used storage technology available on the market. Energy is stored in the form of electrochemical energy, in a set of cells, connected in series or in parallel, in such a way as to obtain the desired voltage. Each cell consists of two conductive electrodes and an electrolytic solution, both inserted in a special container and connected to an external source or load. Several studies are currently in progress on different types of batteries which seem very promising both for network storage and for electric vehicles. In any case, these technologies are currently hardly used for network storage, given the high costs that limit their spread.

Thermal storage is another of the most important and widespread energy storage methods, even though it is still not really used for storing electric energy. In particular, commercial applications cover both heating and cooling systems that use waste heat or solar energy, and high temperature systems for generating heat for industrial processes. In general, the heat may be stored in the form of sensible heat (sensible heat storage system), latent heat (latent heat energy storage system) or by means of the heat deriving from the splitting or breaking of chemical bonds (thermochemical energy storage).

At present, electric energy storage in the form of heat is only known in the emerging technology called Pumped Thermal Electricity Storage (PTES), described in patent application EP 2 220 343 A2. This technology, recently developed and based on the heat pump principle during the energy storage step and a heat engine during the energy output step, constitutes a valid alternative to PHS, to CAES and to batteries. The system uses storage in the form of sensible heat and consists of two tanks, one hot and one cold, two heat exchangers, a compressor, a turbine and an engine/generator. During periods of low demand for electricity the system is used for storing electric energy in the form of thermal energy using the heat pump principle. Surplus electric energy in the network is used to drive a compressor. That machine increases the pressure of the fluid but, due to irreversibilities, a temperature increase is also generated. The hot fluid is then sent to a tank (called the hot tank) where it transfers heat to the material which constitutes the tank. When it exits that tank, the cold gas is expanded in such a way that it returns to a low pressure (and temperature) and is injected into another tank, called the cold tank. In general, the hot tank operates at a temperature of between 500 and 1000 °C and at a pressure of between 4 and 10 bar. In contrast, the cold tank is at ambient pressure and temperatures between -70 and -150 °C. In order to guarantee pressure and temperature levels upstream of the compressor and the turbine, two heat exchangers are used. Their task is to keep the temperature of the fluid at the machine infeed constant and equal to the value defined during the system design process.

When demand for electricity is high, the thermal energy contained in the tanks is re-converted to electric energy by reversing the cycle.

The machines used (compressor and expander) may be either reciprocating or turbomachines.

There are currently no known commercial applications based on PTES. There is only a prototype application of it.

A comparison of the above-mentioned storage technologies immediately reveals that PHS and CAES are technologies that, in order to be advantageous, require the installation site to have a special morphology. In the case of water storage, an upstream basin and a downstream basin are needed which are positioned at different heights. In the case of CAES, an underground cavern with stable geological structure is required. As for electrochemical systems, these are still being developed, but their spread is above all limited by costs and short life. In contrast, PTES is a good alternative to the above systems, even if the main problems relate to the temperature level to be used in the two tanks and the energy to dissipate/supply in the two heat exchangers necessary for cycle stability. In addition to the PTES technology described above, it should be noticed that other solutions relating to electric energy storage systems using the storage of thermal energy are covered by many patent documents. Some of them are briefly described below.

Patent application EP 0 003 980 A1 describes a thermal energy storage system using a gas, preferably ambient air, which comprises an alternately working engine/generator, a compressor and a turbine. By means of pipes, the compressor and the turbine are connected to each other and also to a heat accumulator and to a regenerator. That regenerator is connected upstream of the compressor during the charging process and downstream of the turbine during the discharging process. Switching is guaranteed by suitable switching units.

Patent application WO 2007/093277 A1 describes a method for storing and recovering energy, in particular solar and wind energy, that is converted into electric energy and is used to charge and discharge a heat accumulator with the aid of a circulatory process, which operates as a thermal pump. Patent application WO 2007/096656 A1 describes a system for storing energy and using it to generate electric energy or drive a propeller, and comprises the use of a cryogenic fluid.

Patent application WO 2009/044139 A2 describes an apparatus for storing electric energy comprising two tanks (one "hot" and one "cold"), a compressor, a turbine (both reciprocating machines) and an engine/generator. The cycle operates like a heat pump during the charging process and like a heat engine during the discharging cycle. The gas used may be air, whilst the tanks may be filled with particles of solid materials or packed fibres. Heat exchangers are also inserted to counteract cycle irreversibilities.

Patent applications EP 2 570 759 A1 and EP 2 147 193 A2 also propose a system for storing electric energy in the form of thermal energy, constituted of two storage tanks, a compressor and a turbine.

Other solutions are described in patent documents EP 2 905 432 A2, EP 2 157 317 A2, CN 102400872 A, CN 102852742 A, CN 202300882 U, CN 202419944 U, CN 204064069 U, DE 10 2011 017311 A1 , US 8,739,533 and WO 2014/003491 A1 .

Further studies on PTES, thermal storage systems and systems that use a compressor and expander are described in various articles in scientific literature, including first the following:

[01 ] Desrues, T., Ruer, J., Marty, P., Fourmigue, J.. A thermal energy storage process for large scale electric applications. Applied Thermal

Engineering 2010;30(5):425-432. (Appendix 17).

[02] White, A., Parks, G., Markides, C.N.. Thermodynamic analysis of pumped thermal electricity storage. Applied Thermal Engineering

2013;53(2):291 -298. (Appendix 18).

[03] McTigue, J.D., White, A.J., Markides, C.N.. Parametric studies and optimisation of pumped thermal electricity storage. Applied Energy 2015;137:800-811 . (Appendix 19).

[04] Thess, A.. Thermodynamic efficiency of pumped heat electricity storage. Physical review letters, 2013, 111 (11 ), 110602. (Appendix

20).

Desrues et al. (2010) (Ref. [01 ]) proposed a process for storing electric energy in the form of thermal energy based on the high temperature heat pump cycle followed by a Thermal Engine. The system is called Pumped Thermal Electricity Storage (PTES) or Pumped Heat Electricity Storage (PHES). The first distinctive feature of this system, according to the authors, is that at the time (2010) energy storage in the form of thermal energy had not been developed for storing electric energy. The system proposed consists of a HP (high pressure) tank and an LP (low pressure) tank, four turbomachines (one compressor and one turbine - first pair of machines - are used during the loading process, whilst another compressor and another turbine - second pair of machines - are used during the delivery process) and two heat exchangers. The cycle working fluid is Argon. The cycle is a closed cycle. The two tanks are filled with refractory material and used alternatively for storing or delivering energy. These tanks have a square cross-section and have a volume of 21622 m ³ .

During the loading process the "Hot Turbomachine" is a compressor, whilst the "Cold Turbomachine" is a turbine. In contrast, in the delivery process, the "Hot Turbomachine" is a turbine, whilst the "Cold Turbomachine" is a compressor.

Desrues et al. assume that at the start of the process the low pressure LP Tank is at the temperature of 500 °C whilst the high pressure HP Tank is at 25 °C.

During the loading process the gas leaves the low pressure LP Tank at a temperature of 500 °C and a pressure of around 1 bar and enters the compressor ("Hot Turbomachine") where its pressure and temperature are increased. In this case, the compression ratio is selected in such a way that at the compressor outlet the gas temperature is equal to 1000 °C. At this point, the gas is injected into the high pressure HP Tank, where it transfers its heat to the refractory material (which is at a temperature of 25 °C) constituting the above-mentioned tank. The refractory material therefore starts to heat up whilst the gas cools down. That gas leaves the high pressure HP Tank at a temperature of 25 °C to enter the turbine ("Cold Turbomachine"). In this turbomachine the gas is expanded. The expansion ratio is set in such a way as to obtain at the machine outlet a gas temperature equal to -70 °C. Therefore, the gas enters the low pressure LP Tank that initially was at 500 °C and begins cooling the refractory material of that tank. Since it is a closed cycle, the gas is then sent to the compressor and the process continues. Obviously, as time passes and the loading process progresses, the temperature of the high pressure tank will rise (compared with the 25 °C design condition), whilst that of the low pressure tank will fall (compared with the 500 °C design condition). This means that in order to keep the temperature constant at the compressor inlet and turbine inlet, heat exchangers have to be inserted, which supply energy ("Hot Heat Exchanger" located upstream of the compressor) on one side and dissipate it ("Cold Heat Exchanger" located upstream of the turbine) on the other. The two heat exchangers located upstream of the turbomachines therefore have the fundamental task of keeping the temperature constant at the inlet to those machines. In particular, the heat exchanger located upstream of the compressor ("Hot Heat Exchanger") is designed to heat the gas to the temperature of 500 °C in such a way as to guarantee, at the end of compression, a temperature of 1000 °C. This is necessary because as time passes the low pressure LP Tank gradually cools (initially it is at 500 °C, whilst at the end of the loading process it is at -70 °C), therefore, the gas exiting that tank will no longer be at 500 °C as at the start, but its temperature will gradually drop until it is -70 °C, that is to say, the end of loading process or maximum loading condition. In contrast, the exchanger located at the turbine inlet has the task of dissipating heat, since the temperature of the gas exiting the high pressure HP Tank gradually increases (initially the tank is at 25 °C, whilst at the end of loading it would be 1000 °C). Therefore, in order to keep that temperature equal to 25 °C and to guarantee -70 °C at the expander outlet, the exchanger must dissipate the surplus heat. It should also be remembered that the cycle maximum pressure is 4.6 bar (at the compressor outlet and/or the high pressure tank inlet), whilst at the turbine outlet the pressure is approximately 1 bar.

In conclusion, the electric energy from the network is converted into thermal energy by using a compressor, that is to say, exploiting the inefficiency of compression. Then it is stored in two tanks: one high pressure and one low pressure. The control strategy developed by Desrues et al. involves stopping the loading process when the difference in temperature between the outlet of one of the two tanks and its nominal value exceeds a predetermined tolerance.

Before analysing the delivery process, several points must be considered relating to the system proposed by Desrues et al., in order to identify some problems with the system proposed.

First, Desrues et al. do not explain how the low pressure LP Tank is initially brought to the temperature of 500 °C and do not consider the energy used in order to bring the low pressure tank to that energy level. Moreover, during normal operation, it is necessary to use a heat exchanger (identified as the "Hot Heat Exchanger") in order to keep the temperature at the compressor inlet constant and equal to 500 °C. This is not mentioned by Desrues et al. during the quantification of the loading process efficiency: the closer the temperature of the low pressure tank moves towards -70 °C, the higher the energy to be supplied with that heat exchanger will be. It is also known that the hotter the gas is, the greater the power absorbed by compressor will be in order to compress that fluid. Consequently, the process proposed therein is inefficient in terms of energy.

At the turbine inlet there is another exchanger (identified as the "Cold Heat Exchanger") whose task is to dissipate surplus heat, since the temperature at the machine inlet must be kept at 25 °C. The temperature of the gas exiting the high pressure HP Tank will gradually increase, therefore the power to be dissipated with that heat exchanger will increase with an increase in the system loading state. Obviously, this energy dissipation is a source of loss. It should also be remembered that Desrues et al. do not explain either how they administer the heat or how they dissipate it. That is to say, they do not indicate the fluids used in the exchangers or the powers supplied/dissipated during the system loading process.

Therefore, in this system it cannot be claimed that the energy stored in the tanks is supplied only by compression because part of it is administered by the heat exchanger located upstream of the compressor. Desrues et al. also consider the system completely ideal, that is to say, they overlook pressure drops and do not assess the effects of incorrect distributions of the fluid in the tank in the radial direction; assumptions that are implausible given the considerable dimensions of the tanks (21622 m ³ ).

The system control strategy, developed by Desrues et al., in reality allows the tanks to be only partly loaded, since the full load condition, which requires the entire high pressure HP Tank to be at the uniform temperature of 1000 °C and the cold LP Tank at -70 °C, is highly inefficient. In contrast, in the delivery process the gas at the temperature of 1000 °C is injected into the "Hot Turbomachine" (which in this case is a turbine/expander) and, consequently, expanded. The Argon exits the turbine at 500 °C and is sent to the low pressure LP Tank. There, it transfers heat to the refractory material of that tank, which starts to heat up (initially the tank is at -70 °C). At this point, the gas is sucked into the compressor ("Cold Turbomachine") at -70 °C and compressed. At the outlet of that turbomachine the temperature must be 25 °C. Therefore, the gas enters the high pressure HP Tank and starts to cool it, completing the cycle. It should be noticed that, as time passes, the temperature of the low pressure LP Tank tanks increases, therefore, at the compressor outlet the temperature of the fluid will be greater than 25 °C. In order to keep that temperature constant Desrues et al. provided a heat exchanger which dissipates the surplus heat, guaranteeing at the inlet of the hot HP Tank a constant temperature of 25 °C, that is to say, the design condition. During delivery, the "Hot Heat Exchanger" is not needed, but a variable turbine expansion ratio is used in order to maintain a constant temperature for gas infeed into the LP Tank.

Again, the control strategy involves interrupting the delivery process when the difference in temperature between the outlet of one of the two tanks and the nominal temperature of the tank exceeds the predetermined tolerance. The management strategy adopted allows a reduction in the power dissipated in the "Cold Heat Exchanger", but also causes a reduction in the discharge depth. This means limiting use of the plant; a necessary condition to avoid compromising the subsequent loading process.

As in the preceding case, White et al. (2013) (Ref. [02]) also propose a system for storing electric energy in the form of thermal energy. The system proposed by White et al. is also able to store large amounts of energy and can compete with the "Lange Scale Energy Storage" that includes "Pumped Hydro Energy Storage" and "Compressed Air Energy Storage". In their work, White et al. propose a system constituted of the following components: a compressor, an expander, two heat exchangers and two storage tanks: one hot and one cold. The two tanks are positioned vertically and filled with an appropriate material for thermal storage: for example a bed of pebbles or a matrix of ceramic. The vertical positioning prevents thermal front instability. The electric energy, as in the case of Desrues et al. (Ref. [01 ]), is converted to thermal energy, then stored in the form of sensible heat. The compressor and the expander may be turbomachines or reciprocating machines, although, in the analysis by White et al., reciprocating machines are used. In this case too, the use of two heat exchangers is necessary, to reject heat from the cycle to the environment and combat the effects of system irreversibilities.

The cycle working fluid is once again Argon. As in the article by Desrues et al. (Ref. [01 ]), during charge, the system operates as a high temperature heat pump, using electric energy to extract heat from the cold tank and deliver heat to the hot tank.

In contrast, during discharge the process is reversed. That is to say, the devices operate as a heat engine: heat is returned from the hot tank to the cold tank and the electric energy is produced and fed to the network. Note that if reciprocating machines are used, the flow direction can be reversed without problems and one compressor and one expander can be used. In the case presented by Desrues et al. (Ref. [01 ]) in contrast the compressor and the turbine were turbomachines, therefore, it was necessary to use one pair of machines for loading and another pair of machines for delivery. Obviously, the effect on the system of using two compressors and two expanders considerably increases plant costs.

White et al., (Ref. [02]) unlike Desrues et al. (Ref. [01 ]), set the initial temperature of both tanks equal to ambient temperature, that is to say, 25 °C (Desrues et al. set the initial temperature of the high pressure tank at 25 °C and that of the low pressure tank at 500 °C), whilst the operating temperatures of the hot and cold tanks are respectively set at 505 °C and - 150 °C. The cycle maximum pressure is 10.5 bar, whilst the minimum pressure is equal to 1 .05 bar (in Desrues et al. the pressure values were approximately 4.6 and 1 bar).

In this case, White et al. limit themselves to theoretical analysis of the effects of the various inefficiencies of the processes and conclude that the round-trip efficiency and storage density increase with a rise in the compression ratio. This also means high temperatures and therefore high plant costs. White et al. do not refer to system control strategies, but supply a purely theoretical analysis.

A comparison between the work proposed by Desrues et al. (Ref. [01 ]) and that published by White et al. (Ref. [02]) allows the following to be observed: the initial temperatures set in the two tanks are identical for the hot tank, but completely different for the cold tank: Desrues et al. assume it is 500 °C, whilst White et al. set the temperature at ambient temperature (25 °C). The choice made by White et al. is more plausible and allows system energy costs to be minimised. Also in White et al., the use of reciprocating machines allows a reduction in system complexity, since one compressor and one expander are used (two devices). In Desrues et al., since the compressor and the turbine are turbomachines, two pairs have to be used (four devices), a solution that increases installation costs and layout complexity.

Even the choice of storage tanks is different: Desrues et al. assume the use of square cross-section tanks filled with refractory material having special channels for gas transit, whilst White et al. consider cylindrical tanks containing a bed of pebbles or a matrix of ceramic. However, both agree on the positioning of the tanks (vertical) and on the use of Argon as the working fluid. Since the fluid is a gas other than air, in both cases, a closed circuit is necessary.

McTigue et al. (2015) (Ref. [03]) propose a system identical to that developed by White et al. (Ref. [02]) (White is also one of the authors of this article Ref. [03]) but, in this case, the analysis aims to assess and optimise the parameters that influence the system proposed. McTigue et al. refer to the study of a 2 MW PTES system able to store 16 MWh operating with Argon as the working fluid and cycle maximum pressure and temperature respectively equal to 10.5 bar and 500 °C. The plant layout is identical to that of White et al. (Ref. [02]). However, in this case the volumes of the storage tanks are different and respectively equal to 71 m ³ (hot tank) and 117 m ³ (cold tank). Notice that Desrues et al. assumed a tank volume of 21622 m ³ . The tanks are again cylindrical and positioned vertically, but they contain spheres made of Fe ₃ O ₄ with a void fraction of 0.35. As in the article by White et al. (Ref. [02]), reciprocating machine are used (one compressor and one expander), and two heat exchangers are needed in order to counter cycle irreversibilities. Even the temperature and the pressure of the cold tank are kept the same as those proposed in the article by White et al. (Ref. [02]), that is to say, -150 °C and 1 .05 bar whilst, once again, it is assumed that the initial temperature of the bed contained in the tanks is 25 °C.

In conclusion, the study reveals which parameters influence the main features of the system.

Thess (2013) (Ref. [04]) in contrast simply presents a theoretical study on the operation and potential of "Pumped Heat Electricity Storage" (PHES). With reference to the articles described above (Ref. [01 -04]) it may be concluded that the systems proposed are able to store electric energy in the form of thermal energy during the charging/loading process and to "return it" during the discharging/delivery step. Obviously, output is not uniform, since there are cycle irreversibilities linked to energy dissipation during the charging process.

Unlike Desrues et al. (Ref. [01 ]) in the publications by White et al. (Ref. [02]) and McTigue et al. (Ref. [03]) smaller tank volumes are used, which makes the proposed plant more feasible from a technical viewpoint. Naturally, the presence of a high temperature tank (1000 or 500 °C) and a low temperature tank (-70 or -150 °C) implies the use of special materials and efficient insulation in order to reduce dispersions: factors which increase plant installation costs.

Continuing the review of the scientific literature, Singh, H., Saini, R., Saini, J.. A review on packed bed solar energy storage systems. Renewable and Sustainable Energy Reviews 2010;14(3):1059-1069. (Ref. [05]), proposed a review of all methods for storage of thermal energy. They focused on the storage of the thermal energy "captured" by solar collectors.

Bolland, O., M. Forde, and B. Hande. "Air bottoming cycle: use of gas turbine waste heat for power generation." Journal of engineering for gas turbines and power 11 8.2 (1996): 359-368, (Ref. [06]) present a system for the recovery of waste thermal energy, for example from the exhaust gases of a turbine powered with syngas, using a compressor, a heat exchanger and a turbine. The system is called the "Air Bottoming Cycle" (ABC) because it uses air as the working fluid. The ABC uses the same components as a normal gas turbine, but the combustion chamber is replaced with a heat exchanger.

In use, a traditional gas turbine operating in an open cycle or an industrial process release into the atmosphere exhaust gases with a high energy content due to their temperature which is, on average, high (300-500 °C). Using the Air Bottoming Cycle developed by Bolland et al., it is possible to recover that heat and produce further electric energy in a simple way and above all with limited costs, weights and volumes. A predetermined flow rate of air is sucked in at ambient conditions, compressed and sent to a heat exchanger where it is heated by the hot gases released by the gas turbine or by the industrial process. The high pressure and high temperature hot air then enters a turbine where it expands, producing electric power. The compressor and the turbine of the Air Bottoming Cycle are usually fitted on the shaft of the electric generator. This system guarantees the recovery of waste heat with a simple method and using a fluid that is non-toxic, nonflammable and above all plentiful and free of charge.

Mahlia, T. M. I., Saktisahdan, T. J., Jannifar, A., Hasan, M. H., & Matseelar, H. S. C. (2014). "A review of available methods and development on energy storage; technology update". Renewable and Sustainable Energy Reviews, 33, 532-545 (Ref. [07]) and ZHANG, Huili, et al. "Thermal energy storage: Recent developments and practical aspects". Progress in Energy and Combustion Science, 2016, 53: 1 -40 (Ref. [08]) provide further reviews of the types of thermal storage available.

As has already been partly discussed, all of the prior art technical solutions are not without disadvantages.

With reference to the works proposed by Desrues et al., White et al., McTigue et al. and Thess (Ref. [01 -04]) at least the following problems are identified:

- necessary presence of two tanks with related installation costs;

- conversion of electric energy into thermal energy by the compression process. In use, the increase in the temperature of the fluid is required of the compression process. Consequently, it is not possible to heat a variable flow rate of fluid or to heat the fluid to a temperature greater than or less than that established as the system design condition;

- the need to always use, for moving the fluid, one pair (one compressor and one expander) or two pairs of fluid machines for the charging step and one pair (one compressor and one turbine) for the discharging step;

- the need to use Argon as the working fluid in a closed cycle, with related installation and management costs (for example, in the event of circuit leaks, the fluid will be released into the atmosphere and, therefore, when the system is restarted, the cycle working fluid will have to be topped up);

- the need to work at relatively high operating pressures, with consequent costs for the tanks which have to be built in such a way that they can withstand the pressures;

- in the plant configurations proposed in the literature, during the charging process, a "hot" heat exchanger, for keeping the compressor inlet temperature constant, and a "cold" heat exchanger for keeping the expander inlet temperature constant are needed (see, for example, Desrues et al. Ref. [01 ]);

- in the plant configurations proposed in the literature, during the discharging process, a "cold" heat exchanger is necessary for rejecting heat to the environmental and keeping the hot tank inlet temperature constant; - cycle maximum temperatures are between 500 °C and 1000 °C and cycle minimum temperatures are between -70 °C and -150 °C, with consequent limitations in terms of the materials usable;

- since the fluid is heated mainly by compression, high compression ratios are required: approximately 4.6 in the case of Desrues et al. (Ref. [01 ]) and 10.5 in the case of McTigue et al. (Ref. [03]);

- satisfactory operation of the plants usually requires large storage systems with consequent installation costs and making it impossible to use the technology for medium/small applications.

In this context, the technical purpose which forms the basis of this invention is to provide a plant and a method for the storage of energy and the subsequent production of electric energy which overcome the above- mentioned disadvantages.

In particular, the technical purpose of this invention is to provide a plant and a method for the storage of energy and the subsequent production of electric energy that is based on thermal energy storage.

It is also the technical purpose of this invention to provide a plant and a method for the storage of energy and the subsequent production of electric energy that allows the achievement of performance comparable to that of prior art energy storage systems, and that is easy to install and manage. The technical purpose specified and the aims indicated are substantially achieved by a plant and method for the storage of energy and the subsequent production of electric energy as described in the appended claims.

Further features and the advantages of this invention are more apparent in the detailed description, with reference to the accompanying drawings which illustrate several preferred, non-limiting embodiments of a plant and method for the storage of energy and the subsequent production of electric energy, in which:

- Figure 1 is a diagram of a part of a plant made in accordance with this invention, relating to thermal energy generation and storage;

- Figure 2 is a diagram of a different part of a plant made in accordance with this invention, relating to the generation of electric energy from stored thermal energy;

- Figure 3 is a complete diagram of a first version of a plant made in accordance with this invention;

- Figure 4 is a complete diagram of a second version of a plant made in accordance with this invention;

- Figure 5 is a complete diagram of a third version of a plant made in accordance with this invention;

- Figure 6 is a schematic vertical section view of a thermal storage tank in accordance with this invention; and

- Figures 7 and 8 are two possible variants of heating means that can be associated with the tank of Figure 6.

The following is a description first of the method according to this invention and then of the plant. The latter operates based on the method.

The method for the storage of energy and the subsequent production of electric energy according to this invention comprises on one hand a sequence of thermal energy storage steps which is implemented until a predetermined amount of thermal energy has been stored, and on the other hand a sequence of generating steps which allow part of the stored thermal energy to be converted to electric energy.

Those sequences of steps are performed one after another and are advantageously continuously repeated. In fact, in the preferred embodiments of this invention, the cycle for repetition of the steps is advantageously equal to twenty-four hours, although different repetition frequencies are possible.

The sequence of storage steps is based first on the operating step of preparing one or more thermal storage bodies inside at least one chamber covered with a thermally insulating material; advantageously the chamber is made inside one or more tanks, and the thermal storage is achieved with the aid of beds of pebbles, rocks or the like or packed fibres (which constitute the thermal storage bodies) or any material able to store heat. In particular, in the preferred embodiment of this invention, the thermal storage bodies (which remain in the solid state throughout the range of operating temperatures) are constituted of a material that has a specific heat equal to at least 400 J/kg K and/or a volumetric heat capacity equal to at least 10 ⁶ J/m ³ K (preferably both), such as aluminium, aluminium oxide (alumina), aluminium sulphate, copper, earth, brick, magnesium-based brick, concrete, iron, cast iron, potassium sulphate, sodium carbonate, calcium chloride, potassium chloride and various stones (granite, marble, sandstone, limestone). At least for some applications that require relatively rapid storing and generating times, the preferred materials are those with the highest conductivity and thermal diffusivity. Furthermore, in order to allow at least the subsequent use of the stored thermal energy, the thermal storage bodies must be positioned in such a way as to allow the flow of working fluid to pass between them. In particular, it is particularly advantageous for them to be positioned with a predetermined degree of void or forming channels. Once the chamber containing the one or more thermal storage bodies has been prepared (or the chambers if more than one is required, to be used in parallel or one after another), the sequence of storage steps in general comprises heating the one or more thermal storage bodies until they reach a temperature that is less than or equal to a predetermined maximum temperature. For that purpose, the primary energy source to be used is either electric energy or thermal energy. In particular, this invention is advantageously applied where the thermal energy used as a primary energy source is waste thermal energy from industrial processes (for example, thermal energy resulting from industrial processes) or thermal energy from renewable sources (such as that which can be generated in solar collectors), and where the electric energy used as the primary energy source is surplus electric energy produced compared with local requirements and/or distribution network demand (it may typically be electric energy produced by renewable sources that cannot be regulated, such as photovoltaic or wind, or from the energy recovery systems of industrial plants) or electric energy produced at low cost (that is to say, at a cost that, even considering the output and the management costs of the method according to this invention, is less than the cost of the electric energy that can subsequently be produced at a different time or during different hours). If electric energy is used, advantageously, the step of heating the one or more thermal storage bodies in turn comprises the sub-steps of generating a flow of a storage fluid (first working fluid), converting the electric energy into heat, transferring at least part of the heat generated in this way to the storage fluid and striking the one or more thermal storage bodies with the heated storage fluid. Advantageously, the electric energy can be converted to heat by means of the Joule effect, using, for example, resistors positioned directly inside the flow of storage fluid. In the preferred embodiments the storage fluid is a gaseous fluid at ambient temperature. In particular, the storage fluid may advantageously be ambient air. Moreover, if necessary, the working fluid may be suitably filtered in advance. Moreover, the flow of storage fluid may advantageously be generated at low pressure. In fact, it is enough that the system used to generate the flow of storage fluid be able to overcome the pressure drops that the flow of storage fluid suffers until the moment when it is released to the environment. Regarding the flow rate, this will have to be selected each time at the design stage, so as to guarantee the flow a contact time with the one or more thermal storage bodies long enough to allow a good heat transfer.

In the preferred embodiments in which the above-mentioned flow of storage fluid is used, it is advantageously the case that the sequence of storage steps also comprises the operating steps of recovering the storage fluid that has struck the one or more thermal storage bodies (that is to say, the storage fluid that exits the chamber that contains the one or more thermal storage bodies) and of pre-heating the flow of storage fluid upstream of the tank using the residual heat of the storage fluid that has struck the one or more thermal storage bodies (Figure 1 ). That pre-heating step therefore affects the flow of storage fluid before it is further heated using the heat generated with the electric energy.

However, preferably, the step of pre-heating the storage fluid is only performed when the difference between the temperature of the recovered storage fluid that has struck the one or more thermal storage bodies, and the temperature of the storage fluid to be pre-heated, is equal to at least a predetermined minimum value.

In a different embodiment that also comprises the step of heating the one or more thermal storage bodies carried out using electric energy, in contrast the electric energy is converted to heat directly inside the chamber, and at least part of the heat generated in this way is transferred to the one or more thermal storage bodies by radiation and/or convection and/or conduction. In this case too, the heat may advantageously be generated by the Joule effect.

In contrast, a further alternative embodiment uses one or more thermal storage bodies made of a material that is heatable using electromagnetic induction (preferably but not necessarily ferromagnetic), and the one or more thermal storage bodies are heated by converting electric energy into heat directly inside the one or more thermal storage bodies, using electromagnetic induction heating.

As already indicated, the second preferred primary energy source is thermal energy. In this case, the step of heating the one or more thermal storage bodies may advantageously comprise use of gaseous substances (for example, of natural origin or industrial process waste) which themselves possess the thermal energy, and striking the one or more thermal storage bodies directly with these gaseous substances in the chamber.

Alternatively, fluid substances may be used which are either industrial process waste or generated from renewable sources, themselves possessing the thermal energy, and at least part of that thermal energy is extracted by heat exchangers or dissipaters, then is transferred to the one or more thermal storage bodies. Similarly to what was described relative to the use of electric energy, even if these fluid substances are used, the step of heating the one or more thermal storage bodies may in turn comprise the sub-steps of generating a flow of a storage fluid, of transferring at least part of the thermal energy of the fluid substances to the storage fluid and of striking the one or more thermal storage bodies with the heated storage fluid. Furthermore, even in the case of use of a flow of storage fluid heated using other fluid substances, there may be the same pre-heating steps as described above relative to the flow of storage fluid heated with electric energy.

Alternatively to the use of the storage fluid, the step of heating the one or more thermal storage bodies by means of the thermal energy of the above- mentioned fluid substances, may comprise transferring at least part of that thermal energy to the one or more thermal storage bodies by radiation and/or convection and/or conduction using suitable exchangers and/or dissipaters located inside the chamber.

In general, it is advantageously the case that the sequence of storage steps continues until the temperature of what has struck the one or more thermal storage bodies (that is to say, what exits the chamber in which those thermal storage bodies are contained) is equal to a predetermined maximum temperature, or to a temperature that is less than it. That predetermined maximum temperature may be selected by the designer based on practical and technological aspects. In fact, the higher that temperature is (for example, 1000 °C), the higher the technological level of the materials used to make the plant and its components must be; consequently, the higher the costs of the plant will be. According to the Applicant, a good technical - economic compromise is guaranteed by a predetermined maximum temperature of approximately 500-600 °C.

The sequence of generating steps advantageously comprises one after another the operating steps of:

sucking in a generating fluid (second working fluid, which may or may not coincide with the first working fluid - again preferably being a gaseous fluid, and advantageously air at ambient temperature; in this case too, if necessary, the working fluid may be filtered in advance);

increasing the pressure of the generating fluid sucked in, (advantageously, if using air, generating compressed air);

striking the one or more thermal storage bodies with the pressurised generating fluid, the pressurised generating fluid taking heat away from the one or more thermal storage bodies and itself heating up, whilst the one or more thermal storage bodies cool down;

converting at least part of the pressure energy and thermal energy of the pressurised and heated generating fluid into kinetic energy, advantageously using a fluid machine, such as a turbine or an expander (even if in general it is possible to use either rotary machines or reciprocating machines); and using the kinetic energy generated in this way to drive an electric generator (which in the traditional way may advantageously be fitted on the shaft of the turbine). If the generating fluid coming out of the step of converting at least part of its pressure energy and its thermal energy into kinetic energy, still has a significant amount of thermal energy, in the method the sequence of generating steps may also comprise the operating step of pre-heating the pressurised generating fluid before making it pass through the one or more thermal storage bodies, using the residual heat of the generating fluid that has been subjected to the step of converting at least part of the pressure energy and the thermal energy into kinetic energy. Advantageously, it may be considered, for example, that the generating fluid coming out still has a significant amount of thermal energy when the difference between the temperature of the generating fluid that has been subjected to the step of converting at least part of the pressure energy and the thermal energy into kinetic energy, and the temperature of the pressurised generating fluid to be pre-heated, is equal to at least a predetermined minimum value.

Although the method described above may be using in many different ways, below are descriptions of the preferred types of plants according to this invention. It should be noticed that, respectively, what is described above with reference to the method, and what is described below with reference to the plant, shall also be considered valid respectively for the plant and for the method.

In its most general form, the plant for the storage of energy and the subsequent production of electric energy according to this invention comprises at least one tank 1 which comprises an inner chamber 2 and, in fluid communication with it, an inlet 3 and an outlet 4. Therefore, the inlet 3 is in fluid communication with the outlet 4 through the inner chamber 2. The inner chamber 2 is also covered with a thermally insulating material.

Therefore, this invention only requires one tank 1 for storing thermal energy. That tank 1 may be filled or covered or built with any material able to store thermal energy (the one or more thermal storage bodies 5 referred to below). Obviously, the use of more tanks in series or in parallel allows system storage capacity and flexibility to be improved. However, it is important to emphasise that the tank 1 , according to this invention, may be a single unit or divided into multiple parts/layers, may be positioned vertically and/or horizontally, may be one or more than one. If the tank 1 is a single unit, the flow of fluid must travel along its entire length, whilst if the tank 1 is divided into multiple parts/layers, the flow of fluid may even travel along only one or more parts. The latter solution allows only parts/modules of the tank 1 to be charged or discharged, guaranteeing system flexibility. One or more thermal storage bodies 5 are contained in the tank 1 inner chamber 2. Advantageously, they are a plurality of thermal storage bodies 5 of the type described above and they fill the tank 1 leaving a void fraction of between 0.2 and 0.6, preferably between 0.3 and 0.5.

Extending between the inlet 3 and the outlet 4 of the tank 1 , and through the inner chamber 2, there is a movement path that brushes the thermal storage bodies 5 and that can be followed by a working fluid (in Figure 6 the movement path is constituted of the sequence of empty spaces in communication with each other that are formed between the various thermal storage bodies 5). In the simplest embodiments, the tank 1 has an elongate shape and is positioned vertically with the inlet 3 at the top and the outlet 4 at the bottom. In contrast, the movement path extends along a roughly vertical line. However, in other embodiments the shape of the tank 1 may be different and inside the chamber 2 there may be panels or the like that cause the movement path to extend in a more complex way.

As already indicated, depending on the applications, there may be either a single tank 1 which forms a single inner chamber 2, or the tank 1 may comprise a plurality of inner chambers that are fluidly independent and selectively connectable to the inlet 3 and to the outlet 4, or there may be a plurality of tanks of the type described above, which are positioned fluidly in parallel and are alternatively connectable to the rest of the plant (individually or in groups). Hereinafter reference will be made to the simplest case of a single tank 1 with a single chamber 2, but it shall be understood that all assessments are valid, even for cases with more tanks and/or more chambers for each tank 1 .

The plant also comprises heating means for heating the thermal storage bodies 5 inside the chamber 2, heating means which may have different structures depending on the embodiments, as described in more detail below.

Furthermore, the plant comprises a discharging circuit that is intended to be used for generating electric energy from the thermal energy stored in the thermal storage bodies 5.

The discharging circuit comprises first at least one compressor 6 comprising an intake opening 7 and a delivery opening 8. The intake opening 7 is in fluid connection with a source of the generating fluid (directly or via a suction duct), advantageously with the outside environment for sucking in air (when the generating fluid is ambient air). In contrast, the delivery opening 8 is fluidly connected or connectable to the inlet 3 of the tank 1 , for in use feeding the generating fluid pressurised to the tank 1 . In the preferred embodiments, in particular, extending between the delivery opening 8 and the inlet 3 of the tank 1 there is at least one first duct 9. If the delivery opening 8 is selectively connectable to the inlet 3 of the tank 1 , the first duct 9 will be equipped with first valves that allow fluid communication to be selectively enabled or inhibited.

In the discharging circuit there is at least one driving machine 10, comprising an infeed section 11 fluidly connected or connectable to the outlet 4 of the tank 1 by means of a second duct 12, for in use receiving from the tank 1 the pressurised generating fluid, generated by the compressor 6, after it has travelled along the movement path and been heated up. Mechanically connected to the driving machine 10 is an electric generator 13 which is driven by the driving machine. In the preferred embodiment in which the generating fluid is gaseous (in particular, air), the driving machine 10 is a gas-type driving machine 10, advantageously a turbine or an expander. An outfeed section 43 of the driving machine 10 is connected to the source of the generating fluid (to the outside environment if air is used) either directly or by means of third duct 14 for returning the generating fluid to it. If the infeed section 11 is selectively connectable to the outlet 4 of the tank 1 , the second duct 12 will be equipped with second valves that allow fluid communication to be selectively enabled or inhibited. Both the compressor 6 and the driving machine 10 may in general be either turbomachines or reciprocating machines.

Returning to the heating means, as already indicated, these may have different forms.

In the preferred embodiment illustrated in the accompanying figures, the heating means comprise a charging circuit in which there is first at least one ventilator 15 connected or connectable to the inlet 3 of the tank 1 by means of a feed duct 16. That ventilator 15 is designed in use to feed the storage fluid (advantageously, ambient air) to the tank 1 during the step of thermal energy storage. For that purpose, the ventilator 15 is fluidly connected with a source of the storage fluid (advantageously with the outside environment in the preferred embodiment - directly or via a suction duct - for sucking in ambient air, if necessary suitably filtered and pre-treated). If the ventilator 15 is selectively connectable to the inlet 3 of the tank 1 , the feed duct 16 will be equipped with third valves that allow fluid communication to be selectively enabled or inhibited. Depending on requirements, the ventilator 15 may be constituted of the compressor 6 part of the discharging circuit or, preferably, is constituted of an independent element (such as a low output pressure fan, since it simply has to move the storage fluid with enough force to compensate for the pressure drops of the charging circuit). In general, the ventilator 15 may be constituted either of a turbomachine or of a reciprocating machine.

At least one heating element 17 is mounted along the feed duct 16 for in use heating the storage fluid that is fed into the feed duct 16. Advantageously, the heating element 17 is constituted either of an electrically powered device (comprising, for example, one or more resistors), or of a fluid radiator. The latter in turn may be fed either with a hot fluid already available, or with a hot fluid obtained by means of electric heating.

In the preferred embodiment, in the charging circuit there is also at least one charging heat exchanger 18 comprising a first primary circuit 19 (in which in use the hotter fluid flows) and a first secondary circuit 20 (in which in use the colder fluid flows) which are coupled in such a way as to allow heat exchange. The first secondary circuit 20 is part of the feed duct 16, whilst the first primary circuit 19 is part of a fourth duct 21 fluidly connected or connectable to the outlet 4 of the tank 1 , for in use receiving the storage fluid exiting the tank 1 and returning it to the related source (to the environment if air is used). If the first primary circuit 19 is selectively connectable to the outlet 4 of the tank 1 , the fourth duct 21 will be equipped with fourth valves that allow fluid communication to be selectively enabled or inhibited.

Preferably, in order to allow use of the charging heat exchanger 18 only when this is energy efficient, advantageously present in the charging circuit there is a first delivery bypass circuit 22 and/or a first return bypass circuit 23 (preferably both for minimising pressure drops if the charging heat exchanger 18 is not used).

The first delivery bypass circuit 22 is positioned parallel to the first secondary circuit 20, and is connected to the feed duct 16 by at least one first delivery flow diverter 24 (advantageously a three-way valve) which in use allows the storage fluid arriving from the ventilator 15 to be selectively directed to the first secondary circuit 20 or to the first delivery bypass circuit 22; in the embodiments illustrated in the accompanying figures, there are two first delivery flow diverters 24 mounted at the two ends of the first delivery bypass circuit 22.

The first return bypass circuit 23 is positioned parallel to the first primary circuit 19, and is connected to the fourth duct 21 by at least one first return flow diverter 25 (advantageously a three-way valve) which in use allows the storage fluid arriving from the tank 1 to be selectively directed to the first primary circuit 19 or to the first return bypass circuit 23; in the embodiments illustrated in the accompanying figures, there are two first return flow diverters 25 mounted at the two ends of the first return bypass circuit 23. In a first alternative embodiment, in place of the charging circuit described, the heating means may comprise at least one heat dissipater 26 mounted in the tank 1 , preferably at least partly surrounded by the thermal storage bodies 5. For example, the heat dissipater 26 may be positioned between the walls of the tank 1 and the thermally insulating material 27, on the inner walls of the chamber 2, or immersed between the thermal storage bodies 5 (Figure 8). However, advantageously, a plurality of heat dissipaters 26 are present.

Advantageously, like the heating element 17, the heat dissipater 26 is also constituted either of an electrically powered device (comprising, for example, one or more resistors), or of a fluid radiator. The latter in turn may be fed either with a hot fluid already available, or with a hot fluid obtained by means of electric heating.

According to a further embodiment illustrated in Figure 5, the heating means may comprise at least one feeder of a hot gaseous fluid 28 (such as industrial fumes) selectively connected or connectable to the inlet 3 of the tank 1 for in use feeding the hot gaseous fluid 28 to the tank 1 and making it advance along the movement path. In this case, as illustrated in Figure 5, the plant may comprise a feed duct 16 and a fourth duct 21 , and if necessary a charging heat exchanger 18, similar to those described above (except for the absence of the ventilator 15 and the heating element 17). In a further embodiment, the thermal storage bodies 5 may be constituted of a material heatable by electromagnetic induction and the heating means may comprise at least one inductor 29 mounted around the inner chamber 2 (Figure 7), or in the inner chamber 2 (solution not illustrated), and electromagnetically coupled to the thermal storage bodies 5 for heating them by induction.

In the preferred embodiments, it is also possible that the discharging circuit also comprises at least one discharging heat exchanger 30, which in turn comprises a second primary circuit 31 and a second secondary circuit 32 that are coupled in such a way as to allow heat exchange.

The second primary circuit 31 is part of the third duct 14, whilst the second secondary circuit 32 is part of the first duct 9. The second primary circuit 31 is therefore fluidly connected or connectable downstream of the driving machine 10 for in use receiving the generating fluid exiting it, whilst the second secondary circuit 32 is fluidly connected or connectable between the compressor 6 and the inlet 3 of the tank 1 . In this way, in use, the pressurised generating fluid generated by the compressor 6 circulates in the second primary circuit 31 . If they are selectively connectable, again in this case there will be valves present that allow fluid communication to be selectively enabled or inhibited.

Preferably, in order to allow use of the discharging heat exchanger 30 only when this is energy efficient, advantageously present in the discharging circuit there is a second delivery bypass circuit 33 and/or a second return bypass circuit 34 (preferably both for minimising pressure drops if the discharging heat exchanger 30 is not used).

The second delivery bypass circuit 33 is positioned parallel to the second secondary circuit 32, and is connected to the first duct 9 by at least one second delivery flow diverter 35 (advantageously a three-way valve) for in use selectively directing the generating fluid arriving from the compressor 6 to the second secondary circuit 32 or to the second delivery bypass circuit 33; in the embodiments illustrated in the accompanying figures, there are two second delivery flow diverters 35 mounted at the two ends of the second delivery bypass circuit 33.

The second return bypass circuit 34 is positioned parallel to the second primary circuit 31 , and is connected to the third duct 14 by at least one second return flow diverter 36 (advantageously a three-way valve) for in use selectively directing the generating fluid arriving from the driving machine 10 to the second primary circuit 31 or to the second return bypass circuit 34; in the embodiments illustrated in the accompanying figures, there are two second return flow diverters 36 mounted at the two ends of the second return bypass circuit 34.

As shown in Figure 4, the charging heat exchanger 18 and the discharging heat exchanger 30 may also coincide.

Whilst Figures 1 and 2 show a preferred embodiment respectively of only the charging circuit and only the discharging circuit, Figures 3, 4 and 5 show three possible complete layouts of a plant according to this invention, in which the charging and discharging circuits are more or less integrated with each other.

In detail, Figure 3 shows a plant equipped both with the charging circuit and with the discharging circuit, in which the feed duct 16 and the first duct 9 coincide in the final stretch 37 upstream of the inlet 3 of the tank 1 . Upstream of that final stretch 37 a three-way valve 38 selectively puts in fluid communication with the tank 1 either the ventilator 15 or the compressor 6. In contrast, downstream of the tank 1 , the second duct 12 and the fourth duct 21 share a first stretch 39, downstream of which a further three-way valve 40 selectively puts in fluid communication the outlet 4 of the tank 1 and either the driving machine 10 or the outside environment.

Figure 5 shows a similar layout to that of Figure 3, except for the fact that the feed duct 16 is connected to the external feeder of a hot gaseous fluid 28 and is without the ventilator 15 and the heating element 17. Finally, Figure 4 shows a layout of a plant in which the charging circuit and the discharging circuit are broadly integrated and share the same heat exchanger, which acts alternatively as a charging heat exchanger 1 8 and as a discharging heat exchanger 30. The fourth duct 21 therefore shares its first part 41 with the second duct 12 and its final part 42 with the third duct 14. All switching is always achieved by means of three-way valves. Finally, an additional bypass circuit 44 is provided, starting from the first duct 9 and bypassing the part of the feed duct 16 that contains the heating element 17. In conclusion, obviously other layouts are also possible, which use other intercepting and diverting systems.

Operation of the plant disclosed is, in general, easily inferred from the structural description above and corresponds to a possible implementation of the method described above. However, the following is a brief description of operation of a plant comprising the charging circuit.

As regards operation during thermal storage, the storage fluid, for example ambient air, (at the thermodynamic conditions of point A) is sucked in by the ventilator 15 which increases the pressure of the air to a value such that it overcomes the pressure drops of the circuit in which it will operate (bringing it to the thermodynamic conditions of point B). As already indicated, in order to reduce system energy consumption, it is, in fact, appropriate for the storage fluid to be expelled from the system (at point F) at a pressure level close to that of the environment from which it was drawn (the outside environment if a storage fluid is used). The pressurised storage fluid at the conditions of point B can then flow through the first delivery bypass circuit 22 along the path delineated by points B-B'-C, or through the charging heat exchanger 18 following the direct path B-C. In principle, it will be appropriate to make the fluid flow through the bypass when the temperature of the storage fluid at point E, that is to say, at the tank 1 outfeed, is close to the temperature of the fluid at point B, whilst it is appropriate to recover the energy of the storage fluid exiting the tank 1 when the temperature of the storage fluid at point E is higher.

The storage fluid at the thermodynamic conditions of point C is then heated to the system design temperature T by the heating element 17.

At this point, the storage fluid is at the thermodynamic conditions of point D is introduced into the tank 1 where it will transfer its heat to the one or more thermal storage bodies 5. The storage fluid, after having transferred part of its thermal energy, leaves the tanks 1 at the thermodynamic conditions of point E, and is made to flow either through the first return bypass circuit 23 (along the path defined by points E-E'-F) or through the charging heat exchanger 18 (along the direct path E-F), depending on the control logic implemented and whether or not it is appropriate to recover part of its energy.

Advantageously, the system charging process stops when the one or more thermal storage bodies 5 are at a temperature close to the system design temperature T. However, obviously, the system may even be partly charged, even if that condition does not guarantee system full storage capacity, that it to say, reduces the time for which the system is able to supply a predetermined energy.

In contrast, as regards the generating step, considering that it is started when the one or more thermal storage bodies 5 (or at least most of them) are at the system design temperature, the process is performed as follows. The generating fluid, for example ambient air, is drawn at the thermodynamic conditions of point M and is pressurised by the compressor 6. The pressurised generating fluid at the thermodynamic conditions of point N will be sent either through the bypass (following the path N-N'-O) or through the discharging heat exchanger 30 (HX3) (direct path N-O) to the tank 1 . The generating fluid flowing through the tank 1 gradually acquires the thermal energy contained in it and gradually cools the tank. At the tank 1 outfeed (point P), the generating fluid will be at high pressure and temperature and its energy will be partly transformed into mechanical energy in the driving machine 10 (turbine in the accompanying figures). Since the driving machine 10 is fitted on the same shaft as the compressor 6 and the electric generator 13, part of the power developed by the driving machine 10 will be used to keep the compressor 6 running, whilst the remaining part will be converted into electric energy by the electric generator 13. It will be possible to feed that electric energy generated into the network or consume it directly.

The generating fluid at the outfeed of the driving machine 10 (point Q) will be able to flow either through the bypass (path Q-Q'-R) or in the discharging heat exchanger 30 (direct path Q-R) depending on its energy level defined by the control strategy adopted. From point R the generating fluid will be returned to the related source (to the environment if air is used).

As already indicated, this invention is particularly advantageous for application where the stored thermal energy is obtained starting with electric energy. In other words, with this invention it is possible to store electric energy in the form of thermal energy and, when required, re-convert the stored thermal energy into electric energy.

Therefore, advantageously, the storing period usually coincides with the hours of the day when demand for electric energy is lower than the electric energy produced. In contrast, during hours when demand for electric energy is high, the system uses up the stored thermal energy, converting it into electric energy.

Furthermore, this invention is equally advantageously applied for recovering the waste heat of production processes, storing it and, when required, generating electric energy for meeting for example peaks in demand. Moreover, if waste heat is available, it can be used for pre-heating the tanks, in such a way as to start at a higher temperature.

As already indicated, the plant disclosed may be installed in multiple contexts.

First, it may for example be installed in a power plant for producing electric energy.

If that plant for producing electric energy is powered with traditional sources such as natural gas, coal, fuel oil, biomass, biofuels in general, etc., the plant according to this invention may be inserted close by (so as to reduce losses from conversion and transmission of the electric energy) in order to be able to store the surplus electric energy produced or as an emergency system able to produce energy that makes up for a power plant deficit. In these cases the invention allows, during the storage process, the storage of surplus energy produced by the plant for the production of electric energy, so that the load of the later is not changed too quickly (sudden changes stress plant components, reduce their lifetime and overall output efficiency). In the same way, if the plant according to this invention is even only partly charged, it can supply the energy that the plant with traditional source is either unable to produce because it is already at maximum capacity or because there are loading ramps that the plant with traditional source must comply with. In use, this invention helps the traditional plant to reduce the load variations and the number of start-ups/shut-downs, events that stress the components and reduce their lifetime.

In contrast, if the electric energy production plant uses renewable sources (photovoltaic solar, wind, etc.), the system described in this invention can again be positioned close by the power generating plant (to reduce losses from conversion and transmission of the electric energy) and it can be used mainly for storing the surplus electric energy produced. When necessary, this energy stored in the form of thermal energy can be re-converted into electric energy in order, for example, to make up for a production deficit linked to a momentary lack of solar radiation or wind.

Second, the plant according to this invention may be applied in any installation where a high temperature fluid is available.

In this case, the plant is always used for recovering heat that would otherwise be released into the environment. Moreover, if the outflow from the production process has an energy level such that it can appropriately heat the tank 1 , the plant may be configured to also allow the production of electric energy from the waste flow when required. In particular, if two tanks are to be installed, with a continuous hot flow available from the production process, a management strategy may be implemented which stores energy in one tank 1 (if discharged) with simultaneous use of the other tank 1 for producing electric energy (obviously, if electric energy is required by the network and if the tank 1 is charged).

Alternatively, if the outflow from the production process has a low energy level, it can be used for pre-heating the thermal storage bodies 5. Then, when there is energy available (whether electric or in another form), the thermal storage bodies 5 can be further heated in such a way as to reach the design energy levels. In this case, it is therefore possible to also use waste energy. This application was thought of partly because, at some industrial sites, different companies with different targets operate, which can be integrated using this invention with the aim of saving and making the best possible use of the energy available.

This invention is advantageously applied for the recovery of industrial sites that have been abandoned or are being abandoned.

In fact, since this invention requires one or more tanks and machines such as compressors, turbines and heat exchangers, components easily found in industrial applications such as chemical industries or plants for producing energy, it can be used for re-converting the industrial site, reducing the cost of dismantling existing plants, but above all reducing the costs of implementing the invention. It should also be noticed that at industrial sites, in addition to the main active components of the invention, there are already present all works, devices (electric lines, energy transformation systems, territorial safety and safeguarding systems, etc.) and permits that allow a reduction in the times needed to produce and put into service a plant according to this invention. Finally, it should be emphasised that the subject matter of this invention has undergone various preliminary checks, in particular with finite element simulation tools, which have shown excellent results.

For example, the data for the following simulation is provided. The simulation was performed considering the use of air as the carrier fluid and using the storing and generating layouts illustrated in Figures 1 and 2.

Geometric characteristics of the tank 1 :

- cylindrical shape;

- height 10 m;

- volume 150 m ³ ;

- superposed layers of thermal storage bodies 5 considered to have uniform behaviour in the tank 1 : 20 (layer discretization);

- diameter of infeed and outfeed duct 0.5 m;

- tank 1 thermal transmittance 0.7 W/(m ^2* K);

Characteristics of the thermal storage bodies 5:

- material: aluminium oxide (alumina);

- shape: spheres with diameter 0.05 m;

- void volume/total volume ratio (void fraction) equal to 0.4;

- density of aluminium oxide 3900 kg/m ³ ;

- specific heat of the aluminium oxide 840 J/kg K;

Other significant characteristics:

- specific heat of the air at constant pressure 1008 J/(kg K);

- air density 1 .21 kg/m ³ ;

- compression ratio = 8;

- air flow rate = 15 kg/s;

- efficiency of charging heat exchanger 18: 0.8;

- polytropic efficiency of turbine and compressor 6: 0.85

- polytropic efficiency of the ventilator 15 (fan): 0.8;

- external ambient temperature: 300 K; and

- maximum temperature that the tank 1 can reach 600 °C. Thermal storage was simulated with a control logic that makes the sequence of storage steps continue until the temperature at point F reaches 100 K higher than the tank 1 initial temperature (that is to say, 400 K). In contrast the generating control logic makes the discharging transient stop when the temperature at point P is equal to 550 °C, that it to say, 50 °C less than the design temperature (notice that during the discharging step use is preferred of a tolerance that is less than the charging step, in order to keep the output power more stable).

With the preceding specifications, the charging period lasted for 3.8 hours, and the energy expended for charging the system, which is the sum of power absorbed by the blower and by the heating element 17, was equal to 24.4 MWh and was supplied by means of system power input variable over time between 4.5 and 9.5 MW.

In contrast, the discharging process lasted for 3.2 hours and supplied an initial power of 5.26 MW, which over time dropped to a low of 4.8 MW. Overall, the energy fed into the network was 16.5 MWh, with a storage system efficiency of around 67.6%.

It should be noticed that the system efficiency (round-trip efficiency) obtained was greater than that calculated in the article by Desrues et al. (Ref. [01 ]) despite using a much smaller tank 1 , taking into account system pressure drops but, above all, using a cycle maximum temperature 400 °C lower (Desrues et al. assumed the value to be 1000 °C).

Moreover, the efficiency obtained by the system proposed is comparable to that of traditional storage systems such as PHS and CAES, but without the need to have available a special morphology of the installation site (a basin or an underground cavern).

Therefore, there is a clear economic advantage compared with both the PTES proposed by Desrues et al. (ref. [01 ]) and PHS and CAES, with fewer components being used, a smaller plant volume but, above all, reduced civil engineering works (for example, in PHS, all intake civil engineering works are needed).

Therefore, this invention brings many advantages compared with prior art solutions.

In addition to what has already been discussed, first, whilst in the systems previously proposed achieving acceptable efficiency often required large thermal storage systems, this invention can be applied both to high power plants and to low power plants. The thermal energy stored depends substantially on the dimensions of the tank 1 and the cycle maximum temperature and pressure. The versatility of the plant allows it to be installed where required, depending on the spaces available. For example, it could be installed in an electricity generating plant for reducing the load variations of the latter, or near a photovoltaic field or a wind farm for storing the surplus electric energy produced by renewable sources.

If the thermal energy stored is generated starting with electric energy, then according to this invention electric heating means must be used which are separate from a compression process. In this way it is possible, depending on the electric energy available, to easily heat a variable flow rate of fluid or to heat the fluid to a temperature greater than or less than the system design temperature. Moreover, the storing process can occur at low pressure, that is to say with pressures only slightly greater than atmospheric pressure, that is to say, such that they guarantee the flow of the working fluid through the plant components. This results in a clear reduction in the dimensions of the air movement system and in the costs of that machine compared with prior art plants in which the working fluid is heated by means of the compression step. Moreover, for the thermal storage step no turbine or similar machine is needed. This aspect further reduces dimensions and costs. In contrast, for the generating step only one pair of machines is required: a compressor 6 and a driving machine 10.

In the prior art solutions, since during the storage step the heat was supplied to the fluid mainly by means of compression, there also had to be high compression ratios: of around 4.6 in the case of Desrues et al. (Ref.

[01 ]) and 10.5 in the case of McTigue et al. (Ref. [03]). In contrast, for this invention, during the storing process the pressure obviously only has to be enough to make up for the pressure drops in the various devices and to circulate the fluid between the system infeed and outfeed, with consequent savings in terms of installation costs.

In the plant configurations proposed in the literature, which use a compressor 6 for moving the storage fluid (see for example Desrues et al. Ref. [01 ]), a "hot" heat exchanger is also needed during the storing process, for keeping the compressor 6 inlet temperature constant. However, this invention does not need a heat exchanger to keep the temperature constant at the inlet of the machine used for moving the fluid. This also allows plant installation and operating costs to be reduced.

Similarly, in the plant configurations proposed in the literature, a "cold" heat exchanger is necessary during the generating process for rejecting heat to the environment and keeping the hot tank 1 inlet temperature constant. In this invention that device is not necessary. As well as guaranteeing lower installation and operating costs, there is therefore the advantage of keeping the entire tank 1 at a high temperature level, thereby limiting axial temperature gradients (lower irreversibilities for mixing hot fluids with cold surfaces) and being faster in the subsequent storage step.

In the preceding solutions cycle minimum temperatures of between -70 and -150 °C were used. In contrast, this invention, being without a cold tank 1 , does not require such low cycle minimum temperatures, and in the preferred embodiment can operate with a minimum temperature equal to the ambient temperature.

A further advantage of this invention is the fact that, thanks to its limited technical/performance requirements, it can be implemented even by adapting existing plants or disused sites, without strictly needing purpose- designed plants. In particular, the ventilator 15 may even be a machine already present at the installation site. In contrast, for the generating process it is possible to use "purpose-designed" and built machines, but it is also possible to use, for example, the compressor 6 and the turbine of a traditional gas turbine, bypassing the combustion chamber 2 (that is to say, the machines available in turbogas cycle plants).

Moreover, this invention allows the use for the tank 1 of any container able to withstand an internal pressure that is not particularly high (the highest pressure is that generated with the generating working fluid) and able to be filled with particles of solid material or packed fibres. This means that, although even the tank 1 may be "purpose-designed", it is in any case possible to re-use a tank 1 present at the installation site. It should be remembered, for example, that at industrial sites or in plants for producing electric energy, there are usually various types of tanks available, which are able to withstand the pressures required by this invention. Therefore, all of these may be considered potential storage tanks. A further interesting application is constituted, for example, of furnaces or boilers of thermoelectric plants, devices that with a few modifications can be converted into tanks. This recovery of pressurised containers, or the conversion of containers into pressurised containers, allows a considerable reduction in the installation costs of this invention.

In addition, as already indicated, in the preferred embodiments the plant operates as an open cycle both during the storage step and the generating step. In particular the storage fluid may be air or any process fluid or product of combustion. It should be noticed that, in some industrial applications, the hot fluids expelled have a high enough pressure and therefore can be carried directly to the tank 1 without the need to use one or more ventilators.

In the preferred case of use of ambient air as the sole working fluid both for storing and generating, the invention only needs one tank 1 ; in particular a cold tank 1 is not necessary. This fact allows a reduction both in plant volumes and in its costs compared with traditional plants which use two tanks.

Finally, it should be noticed that this invention is relatively easy to produce and that even the cost linked to implementing the invention is not very high. The invention described above may be modified and adapted in several ways without thereby departing from the scope of the inventive concept. All details may be substituted with other technically equivalent elements and the materials used, as well as the shapes and dimensions of the various components, may vary according to requirements.