The present invention relates to a thermal energy storage plant. More specifically, the invention relates to a thermal energy storage plant, suitable for effective conversion of thermal energy into electricity as well as for other types of applications of the stored energy, such as drying of humid products.

Background art

Energy in the form of electricity and heat is vital for the modern society. The majority of all energy today comes from fossil primary energy sources such as coal, oil, and gas. Emissions from fossil energy ultimately lead to global warming and other negative environmental effects. The world has now slowly started the transition towards renewable energy sources such as solar and wind energy. The inherent nature of these energy sources is that they are dependent on sun, weather and climatic conditions, which ultimately leads to intermittent and low reliability of energy supply. Most countries now have the ambition of increasing the share of renewable energy in their total energy mix, phasing out polluting fossil power plants. Unfortunately, it has proven to be very difficult to shut down such plants simply because most renewable energy source are unreliable and cannot guarantee delivery of power when it is needed; thus, to some extent conventional power production capacity has to be maintained rather than being phased out. For this reason, efficient, large-scale energy storage has been identified as the key enabler to facilitate transition to reliance on renewable energy and making energy from renewables predictable and reliable.

Thermal energy storage (TES) will have a key role in this future, especially in concentrating solar power (CSP) plants where heat from the solar field can be stored during the day and released for use during the late evening and night. TES can also be used to store surplus energy from wind or photovoltaic plants by converting surplus electricity to heat, which may be reconverted to electricity later. TES may also find applications in conventional fossil based or nuclear power plants, enabling increased operational flexibility, which is becoming more and more urgent in areas with high penetration of intermittent renewable energy sources. Despite provisions with support from governments, overall cost performance is and will be the main driver for the shift towards renewable and sustainable energy on a large scale. In the end, the crucial challenge is to come up with sustainable energy technology and, in particular new energy storage technology that can facilitate the much-wanted shift towards environmentally friendly power delivery.

Storage of energy allows for better match between when energy is produced and when it needs to be used; in short this means the ability to "time shift" energy delivery. This is particularly important for facilitating higher pen- etration of renewable energy. Typically, there is no delivery of wind energy when the wind does not blow and there is no solar energy available when the sun has gone down. Beyond this, traditional power units such as coal fired and nuclear power plants operate most efficiently with a constant power production whereas the market demands typically varies strongly through a 24 hours day cycle.

Large scale and "grid scale" energy storage is clearly a key component in an environmentally friendly, efficient future power system making the most out of renewable and other energy sources. In patent publication WO 2012/169900 Al, a Thermal Energy Storage (TES) is described, having bene- ficial properties over prior art storages. More specifically, a practical and cost effective solution is provided for a thermal storage using solid-state material as the main storage medium, allowing for storage of energy in the form of high temperature heat, which means thermal energy at sufficiently steam turbine- generator set or equivalent means.

In international patent application PCT/NO2013/050120, it is described how thermal energy storages like those according to the teaching of WO 2012/169900 Al are beneficial in order to simplify and increase efficiency of energy plants of various types, such as for concentrated solar power plants and nuclear power plants. Some relevant art has been developed by Deutches Zentrum fur Luft- und Raumfarth e.V. (DLR), such as described in patent publication DE 10 2009 036 550 Al . However, the thermal energy storages of DLR are rather difficult and expensive to build for a large-scale storage.

WO2015093980 discloses an element for use in an easily scalable thermal energy storage. WO2016099290 discloses a thermal energy storage using concrete heat storage elements arranged in a housing and a heat transfer medium for exchanging energy. WO2016099289 discloses a method that includes the stacking of a number of modules. The individual stacks may be connected so that they may obtain different temperatures during operation. US20130111904 discloses a thermal energy storage which includes a heat exchanger arrangement configured for guiding airflow offer heat transfer medium between a first and a second end. The heat exchanger arrangement includes a compressor circuit and an expander circuit. In the compressor circuit hot air exiting the heat exchanger is used in an expander to partly drive a compressor for delivering hot air to heat exchanger. It is not suggested to use other gas types, such as water vapor, in the compressor circuit and it is not suggested to use a condenser in the compressor circuit.

Despite the significant steps forward, provided by the technology described above, there is still a need for further improvements. The effect/cost ratio is always in demand for improvement, as well as the versatility and flexibility of the thermal energy storage with respect to sources of energy. Further, providing better reliability of delivery from renewable energy sources such as sun and wind is clearly another goal. The objective of the present invention is to meet growing needs and demands with new technology.

The present invention provides an improved plant and process for storage of energy in time periods of excess production and delivery of energy in other time periods in which power is in shortage.

Summary of the invention

The present invention relates to a plant for thermal energy storage plant comprising a thermal energy storage, a compressor circuit and an expander circuit, wherein

a. the compressor circuit comprises

i. a compressor for compressing and heating a vapor, ii. a heat exchanger for delivering heat to a thermal energy storage,

iii. a condenser for condensing the vapor to a liquid, and iv. an apparatus for producing a vapor for the compressor, b. the expander circuit comprises

i. an expander for decompressing a vapor, ii. a condenser for condensing vapor to liquid,

iii. a boiler receiving heat from the thermal heat storage, iv. an apparatus for generating a superheated vapor for the expander.

The plant according to the invention provides a cost effective method of storing energy when it is available at a low cost and subsequently retrieve the energy when the cost is high . The plant comprises three main elements : a thermal energy storage, a compressor circuit and an expander circuit.

As used in the context of the present invention as disclosed in the present description and claims, the term "saturated steam" or any derived forms thereof is to be interpreted as a steam having a relative humidity at or above 90%, such as above 95% and suitably above 98%. In some occasions, the steam may be super-saturated, i.e. the steam is a mixture of steam and water droplets. In most instances, however, the term "saturated steam" is to be understood as steam having a relative humidity close to 100%.

As used in the context of the present invention as disclosed in the present description and claims, the term "superheated steam" or any derived forms thereof is to be interpreted as a steam being at a temperature higher than its vaporization point (dew point) at the pressure, where the temperature is measured . Thus, a superheated steam can therefore evaporate water in a humid product without condensing to water.

The compressor is usually of the centrifugal type. Centrifugal compressors use a rotating disk or impeller in a shaped housing to force the steam to the rim of the impeller, thus increasing the velocity of the gas. A diffuser (di- vergent duct) section converts the velocity energy to pressure energy. Generally, axial-flow compressors are preferred . They are dynamic rotating compressors that use arrays of fan-like airfoils to progressively compress a fluid .

In a certain embodiment of the invention, one or more additional compressors) are used for step-wise increase of the pressure, wherein the outlet of a preceding compressor is connected to the inlet of a subsequent compressor. The application of one or more additional compressor may be referred to herein as multiple staging, i.e. two or more compressors are used consecutively to increase the pressure of the steam .

In an axial flow compressor, the arrays of airfoils are usually set in rows, usually as pairs : one rotating and one stationary. The rotating airfoils, also known as blades or rotors, accelerate the fluid. The stationary airfoils, also known as stators or vanes, decelerate and redirect the flow direction of the fluid, preparing it for the rotor blades of the next stage. Axial compressors are usually multi-staged, with the cross-sectional area of the gas passage dimin- ishing along the compressor to maintain an optimum axial Mach number. The compressor may be fitted with features such as stationary vanes with variable angles (known as variable inlet guide vanes and variable stators), to allow some air to escape part-way along the compressor (known as inter-stage bleed) and being split into more than one rotating assembly (known as twin spools, for example).

In a certain embodiment of the invention, a heat exchanger is provided between the outlet of a preceding compressor and inlet of a subsequent compressor for cooling the partly compressed steam and heating an expander circuit stream. The step-wise cooling in more than one step ensures a higher energy efficiency.

The heat exchanger provided between compressing stages or a heat exchanger provided after the final or the sole compressor is/are thermally connected to the thermal energy storage for transfer of heat energy to the thermal energy storage. The transfer of heat energy may occur directly by placing a heat exchanger inside the thermal energy storage or indirectly by using a heat transfer medium, i.e. the heat exchange medium is heated by the heat exchanger connected to the compressor and subsequently delivers the heat energy in a closed circuit to the thermal energy storage. Examples of heat exchange media include molten salt, such as aluminium and silicon, and eutectic mixtures of metals, such as potassium nitrate, calcium nitrate, sodium nitrate and lithium nitrate. A number of heat exchanger are available for the purpose, including shell and tube heat exchangers, plate and shell heat exchanger, plate fin heat exchanger etc.

After the delivery of heat to the thermal energy storage, the vapor is condensed. Typically, the condenser is of the shell-and-tube type. The medium to be heated flows through the tube side and the vapor enters the shell side where the condensation occurs on the outside of the heat transfer tubes. The condensate drips down and collects at the bottom, often in a built-in pan called a hotwell. The shell side often operates at a vacuum or partial vacuum, pro- duced by the difference in specific volume between the steam and condensate. Conversely, the vapor can be fed through the tubes with the first medium flowing around the outside. Other types of condensers may also be used, such as the Liebig condenser, Graham condenser, or Allihn condenser.

The medium extracting the heat from the condensation process may be used for various purposes, including district heating systems, process water for industrial processes, including diary processes, etc. In a certain embodiment of the invention the medium is a vapor, which is superheated in the condenser.

The apparatus producing vapor for the compressor may be selected in accordance to the specific purpose of the plant. In a first aspect, the apparatus for producing vapor is an evaporator, such as a vacuum evaporator. The condensate from the condenser may be transferred directly to the evaporator or alternatively, the condensate may be cooled in a heat exchanger before the evaporator. Thus, the present invention encompasses an embodiment in which a heat exchanger is configured to cool a liquid stream leaving the compressor circuit condenser and heating a vapor stream entering the compressor.

In another aspect of the invention, the apparatus for producing a vapor for the compressor is a drying apparatus. The drying apparatus is supplied superheated vapor from the condenser. The superheated vapor is allowed to interact with a humid product. The interaction results in an evaporation of water from the humid product and a cooling of the superheated vapor. After the interaction the vapor exits the drying apparatus close to its saturation point. At least a part of the vapor leaving the drying apparatus is transferred back to the compressor.

The drying apparatus used in the present invention may be selected among a variety of known apparatuses, i.e. the drying apparatus may be selected among the group comprising a spray drying apparatus, a fluid bed dryer, a tray dryer, a roller dryer, a drum dryer, a pneumatic dryer, a rotary drier, a bin drying apparatus, a tunnel dryer, and belt drying apparatus.

In tray dryers, the humid product is spread out, generally quite thinly, on trays in which the drying takes place. Heating is performed by a steam current sweeping across the trays or through perforation of the bottom of the trays.

Tunnel dryers may be regarded as developments of the tray dryer, in which the trays on trolleys move through a tunnel where the heat is applied and the vapors removed . In most cases, the humid product can move through the dryer either parallel or counter current to the steam flow. The tunnel dryer may be compartmented and cross-flow may be used .

In roller or drum dryers the humid product is spread over the surface of a heated drum. The drum rotates, with the humid product being applied to the drum at one part of the cycle. The humid product remains on the drum surface for the greater part of the rotation, during which time the drying takes place, and is then scraped off.

In a fluid bed dryer, the humid product is maintained suspended against gravity in an upward-flowing stream of steam. There may also be a horizontal steam flow helping to convey the food through the dryer. Heat is transferred from the steam to the humid product mostly by convection.

In a spray dryer, liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of steam. Steam and solids may move in parallel or counterflow. Drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high.

In a pneumatic dryer, the humid particles are conveyed rapidly in a steam stream, the velocity and turbulence of the stream maintaining the particles in suspension. The steam accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried product is separated, the dry material passes out as product and the moist remainder is recirculated for further drying.

In rotary dryers the humid product is contained in a horizontal inclined cylinder through which it travels, being heated either by steam flow through the cylinder, or by conduction of heat from the cylinder walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle or screw rotates within the cylinder conveying the material through.

In trough dryers the materials to be dried are contained in a trough- shaped conveyor belt, made from mesh, and air is blown through the bed of material. The movement of the conveyor continually turns over the material, exposing fresh surfaces to the hot steam. In bin dryers, the humid material is contained in a bin with a perforated bottom through which warm steam is blown vertically upwards, passing through the material and so drying it.

In belt dryers the humid product is spread as a thin layer on a hori- zontal mesh or solid belt and air passes through or over the material . In most cases the belt is moving, though in some designs the belt is stationary and the material is transported by scrapers.

In a certain embodiment of the invention the drying apparatus is divided into two or more sections, wherein a first drying section has an inlet for superheated steam connected to the outlet for superheated steam of the first condenser and a second drying section has an inlet for superheated steam connected to the outlet for superheated steam of the second condenser.

The application of two or more sections in the drying apparatus allows for heating of the humid product in a steps with varying exposure to the su- perheated steam .

When drying a product, it is often suitable to dry at different temperatures depending on the content of water in the product to be dried . In one way of using the present invention it is generally preferred to dry at a higher temperature when the product contains the highest amount of water. When the product is partially dried it is subjected to a lower drying temperature. The lower drying temperature in later drying stages may be used to prevent heat damages. The higher temperature in the first section may be obtained by directing the superheated steam from the expander circuit condenser to the inlet for the first section and the relative lower temperature in the second section may be obtained by directing part of the stream of superheated steam from the compressor circuit condenser to the inlet of the second section .

In another embodiment it is desired to use a steam with a lower temperature in the first section and a steam with a higher temperature in the second section . Thus, in the first section when the humid product has the highest humidity, a stream of steam with a relative low heating capacity is used . In a certain aspect of the invention, the steam for the first section is obtained from the outlet of the compressor circuit condenser. In the second section the product is partly dried and it may advantageous to use a stream of steam with a higher heating capacity for obtaining a product with a low residual humidity. In a certain aspect of the invention, the steam for the second section is obtained from the outlet of the expander circuit condenser.

The invention also comprises the use of one or more intermediate drying sections. These intermediate sections may be fed with superheated steam from either the expander circuit, the compressor circuit, and any intermediate condensers. Optionally, the streams of the first, the second, and any intermediate condensers may be mixed before entering the first, second or intermediate drying sections.

In a preferred aspect of the invention, the drying apparatus is a con- tinuous mesh-belt drying apparatus divided into a first, a second and optionally intermediate drying zones. The humid product may be applied on the mesh- belt in an even layer and transported by the mesh-belt through the various drying zones. In each drying zone, steam is directed through the mesh and allowed to interact with the humid product. During the interaction, the humidity of the product to be dried is decreased and the relative humidity of the steam is increased . To obtain a high efficiency it is desired that the steam leaving each drying zone is saturated with humidity, i.e. the relative humidity (RH) is at or close to 100%. However, in some instances it may be desired to operate at a lower relative humidity of the leaving steam, such as relative humidity of 90% . In another aspect of the invention, the drying apparatus is a fluid bed divided into a first, a second and optionally intermediate drying zones.

The dried product may be potato starch, grains, minerals, phosphates, peat, wood chips, timber, cellulose pulp, hay, veneer, foodstuff, sludge, bark, enzymes, pharmaceuticals, instant coffee, etc.

The thermal energy storage may be a single compartment having a uniform temperature or it may be divided into two or more compartments designed to obtain different temperatures. Thermal energy storages are available on the market and include apparatuses that elevates or lowers the temperature of a substance, i.e. altering its sensible heat, apparatuses that changes the phase of a substance, i.e. altering the latent heat, and a combination of these. In a preferred aspect of the invention, the thermal energy storage use solid substances like stones, bricks, concrete elements etc. for absorbing and de- sorbing the heat. The operation of a thermal energy storage involves three modes : charging, storing and discharging . Suitable heat energy storage appa- ratuses are available from EnergyNest, Calmac, Theiss, etc. The expander circuit comprises an expander, which is a centrifugal or axial flow turbine through which a high pressure gas is expanded to produce work. The expander may also be referred to as turbo-expander or expansion turbine and comprises an expander wheel coupled to a shaft. The expander extracts work from the expanding high pressure vapor. More than a single expander may be used, such as two, three, four, etc. expanders in succession. Each of the expanders used in the present invention may be coupled to a separate shaft, which may be combined through a suitable gearing. In a preferred aspect, however, the expanders are sharing a common shaft.

The expanded vapor leaving the expander is usually at or close to its saturation point. The vapor is subsequently directed to the condenser, which condenses the vapor to a liquid . The condenser may be of the same type as that used for the compressor circuit or may be of different type. The condensation energy is transferred to a medium to be heated, which may be the same or different from the medium used in the compressor circuit condenser. In the event the same medium is used, the medium to be heated is generally first treated in the compressor circuit condenser and subsequently in the expander circuit condenser. As an example the medium to be heated may be return water from a district heating system, which in the compressor circuit condenser is heated 10 to 30 degrees Celcius and subsequently in the expander circuit condenser is heated further 10 to 30 degrees Celcius to obtain a temperature suitable for entering into the district heating system.

The condensate obtained by condensing the vapor is transferred to a boiler either directly or indirectly. The transport is generally conducted by a pump, which is able to provide a pressure above the normal ambient pressure, such as 4-20 bar. The pump may be selected according to the specific need of the pressure in the boiler. Typically, the pump if of the positive displacement type, however, centrifugal pumps may also be applicable. Specific types of positive displacement types include gear pumps, screw pumps, and rotary vane pumps.

The boiler is in thermal correspondence with the thermal energy storage and capable of receiving energy from the thermal energy storage to evaporate the condensate. When the boiler receives the energy directly from the thermal energy storage the boiler may be provided inside the thermal energy storage. When the boiler is heated indirectly a heat transfer medium is usually used for transporting heat from the thermal energy storage to the boiler.

The vapor leaving the boiler is close to its saturation point. The vapor is subsequently conveyed to an apparatus for superheating the vapor. The apparatus for superheating may be a combustion furnace heated by a fossil en- ergy source, an electrical heating device, a heat exchanger in a higher temperature compartment of the thermal energy storage, etc.

In a certain embodiment, the apparatus for generating a superheated vapor for the expander is a high temperature compartment of the thermal energy storage. The use of a high temperature compartment for superheating the vapor has the advantage that the high temperature compartment may be heated with an energy source at a point in time and then the vapor may be superheated at another. In a preferred aspect, the high temperature compartment of the thermal energy storage is supplied thermal energy from an electrical heater. The use of electricity to heat the high temperature compartment makes it possible to store electrical energy as thermal energy.

The superheated vapor is subsequently supplied to the expander for closing the circuit.

The thermal heat storage may be heated by available heat sources in concert with the other heating sources disclosed herein. Thus, exhaust gas from a combustion engine may be used to heat the thermal energy storage. Combustion engines are readily available on the market and may be selected as internal or external combustion engines. Generally, internal combustion engines are preferred to the present invention, such as piston engines, Wankel rotary engines, or gas turbines. The combustion engine is generally connected to a generator for converting power to electricity. By the combustion an exhaust gas is generated, which is used in the present plant and process.

The piston engines may be driven by a suitable combustible fuel, such as natural gas, gasoline, diesel fuel, fuel oil, biodiesel fuel, (bio)ethanol, methane, propane, hydrogen etc. and are generally preferred because the cooling water needed for cooling the engine may be used in the present apparatus for heating various streams. Furthermore, piston engines tend to be more energy effective than gas turbines. A preferred piston engine is the gas engine, which runs on e.g . coal gas, producer gas, biogas, landfill gas, liquid natural gas (LNG), or compressed natural gas (CNG). Compared to diesel engines, the gas engine exhaust gases are much hotter for a given output, which is considered an advantage for the present invention. The term gas engine refers to a heavy- duty industrial engine capable of running continuously at full load for periods approaching a high fraction of per year. Typical power ranges from 10 kW to 18 MW.

Manufacturers of applicable gas engines include Hyundai Heavy Industries, Rolls-Royce with the Bergen-Engines AS, Kawasaki Heavy Industries, Liebherr, MTU Friedrichshafen, GE Jenbacher, Caterpillar Inc., Perkins Engines, MWM, Cummins, Wartsila, GE Energy Waukesha, Guascor Power, Deutz, MTU, MAN, Fairbanks Morse, Doosan, and Yanmar.

Other thermal energy sources which may be used for delivering heat to the thermal energy storage include incineration plants.

The compressor may be driven by an electrical motor, an expander, or both. The expander may drive a generator, a compressor or both. In a certain embodiment of the invention, the compressor and the expander are con- nected through a common shaft, optionally interrupted by a gearing or a clutch. Such arrangement assists an electrical motor in driving the compressor, thereby avoiding a transformation step for the energy. Preferably, the common shaft is also connected to a hybrid electrical motor generator capable of supplying power to the arrangement, when the assistance of the expander is not sufficient for driving the compressor. On the other hand, when the supply of shaft power from the expander exceeds the power needed for driving the compressor, the power is converted to electrical energy by the hybrid electrical motor generator.

The plant according to the invention may be driven in three modes: a charging, a storing and a discharging mode. In a charging mode, the plant as described above receives energy from an external source, which may the electrical grid, a wind turbine, solar panel etc. The control unit directs the power from the external source to the electrical motor M driving the compressor. The compressor circuit will in the charging mode store heat energy in the Thermal Energy Storage. The activity of the expander circuit will be reduced during charging to avoid a high production of energy from the generator connected to the expander.

In a discharging mode, the control unit will reduce the supply of energy to the electrical motor, that drives the compressor. The activity of the expander is increased in the discharging mode thereby withdrawing energy from the thermal energy storage and transferring it to electrical energy in the generator coupled to the expander. It will be understood by the person skilled in the art, that the plant may also be operated in a combined mode applying both a charging and a discharging mode. Furthermore, the plant may be operated in a stor- age or passive mode, without supply or production of energy.

The present invention also relates to process for storing and production of energy, wherein the process comprises a compressing circuit and a turbo-expanding circuit,

a. said compressing circuit comprising the steps of:

i. compressing and heating a vapor by a compressor, ii. cooling the vapor in a heat exchanger thermally communicating with a thermal energy storage, iii. condensing the vapor to a liquid in a condenser, and iv. evaporating a liquid for production of a vapor and convey- ing the vapor to compressing step, and b. said turbo-expanding circuit comprising the steps of:

i. expanding a superheated vapor in a turbo-expander, ii. condensing the expanded vapor to a liquid in a condenser, iii. evaporating a liquid to a vapor in a boiler thermally con- nected to the thermal energy storage, and

iv. superheating the vapor and conveying the superheated vapor to the expanding step.

In certain aspects the process relates to :

the compressor and the expander being connected through a common shaft, optionally interrupted by a gearing or a clutch,

the vapor being superheated in a high temperature compartment of the thermal energy storage, and

the high temperature compartment of the thermal energy storage being supplied thermal energy from an electrical heater.

The process of the invention may further comprise the step of cooling a liquid stream leaving the condenser of the compressing circuit and heating a vapor stream entering the compressor.

The process may further comprise the step of cooling a vapor stream entering the condenser of the compressing circuit and heating a stream of the turbo-expander circuit. In a certain embodiment, the process of the invention includes that the vapor for the compressor has a pressure below the ambient pressure. Furthermore, the process of the invention includes that the evaporation of the liquid for the production of vapor for the compressor occurs in a drying appa- ratus. In addition, the invention includes the embodiment that the vapor is superheated in one or more condensers before it is entered into the drying apparatus.

While the expander and the compressor circuits have been described using water and vapor as the working fluid, the person skilled in the art will understand that other working fluids can be used instead . Alternative working fluids for the expander and compressor circuit includes ammonia, propane, butane, pentane, cyclopentane, toluene, chlorofluorocarbons, hydrochlorofluoro- carbons, fluorocarbons, propane, butane, isobutane, ammonia, and sulfur dioxide.

The invention also relates to the use of the plant and the process according to the present invention for storing and production of energy. Furthermore, the invention relates to the use of the plant and the process according to the present invention for drying woodchips. Furthermore, the invention relates to the use of the plant and the process according to the present invention for heating water in a district heating system.

Summary of the drawings

The present invention will now be described in greater detail based on preferred embodiments with reference to the drawings in which :

Figure 1 discloses a thermal storage plant comprising a thermal energy storage, a compressor circuit and a turbo-expander circuit,

Figure 2 shows a modified version of the embodiment shown in Figure 1, in which the turbo-expander and the compressor has been connect via a common shaft.

Figure 3 shows a variant for the drying of a humid product (HP) to a dry product (DP) in a drying chamber.

Figure 4 disclose a variant of the embodiment shown in figure 3, in which both condensers are used to superheat steam for drying the humid product. Detailed description

Figure 1 discloses a depiction of an embodiment of the invention according to which a compressor 10 receives a vapor at inlet 11 to the compressor. The compressor compresses and heats the vapor using mechanical energy received from an engine or electrical motor M. The heated and compressed vapor exits at outlet 12 and is transferred to inlet 13 of the heat exchanger 14. The heat exchanger is placed in a low temperature compartment 15 of the thermal energy storage. In the heat exchanger 14, the compressed vapor is cooled by delivering heat to the low temperature department of the TES.

The cooled vapor leaves the heat exchanger at outlet 16 and is transferred to inlet 17 condenser 18. In the condenser the vapor is condensed to a liquid, which leaves the condenser at outlet 19. The condensation evolves heat, which is transferred to a first medium to be heated, such as water running in a district heating system. The medium to be heated enters at IN and exits at OUT.

The condensed liquid from the condenser is transferred to inlet 20 for the evaporator 21. The evaporator heats the liquid and transforms it so vapor, which leaves the evaporator at outlet 22. The vapor is directed to the inlet 11 of the compressor to close the circuit. The evaporation requires energy, which may be delivered by a low quality source like cooling water from a power plant, seawater, etc.

Turbo-expander circuit comprises a turbo-expander or turbine 30, which expands a superheated vapor. The shaft power developed by the turbo- expander drives a generator G for the production of electrical energy. After decompression in the turbo-expander, the vapor leaves the turbo-expander at outlet 31 and the vapor is conveyed to inlet 32 of condenser 33. In the condenser, the vapor is condensed to a liquid under the development of heat. The heat is absorbed by a second media to be heated, which may be the same as the first medium to be heated . In particular, the compressor circuit condenser 20 and the expander circuit condenser 33 may be operated so that they work in succession, thereby heating a medium to be heated in two steps.

The condensed liquid leaving the condenser at outlet 34 is conveyed to a boiler 35 by pump 43. The boiler is provided heating from the low temperature compartment of the TES, to evaporate the liquid. The evaporated liquid is at or close to its saturation point. The vapor leaves the boiler at outlet 36 and is transferred to the inlet 37 of a heat exchanger 38 in thermal connection with a high temperature compartment 39 of the TES. In the heat exchanger 38 the vapor is superheated, i.e. the temperature of the vapor is raised significantly above the condensation temperature.

The superheated vapor exits the heat exchanger 38 at outlet 40 and is transferred to the inlet 41 of the turbo-expander 30 to close the circuit. The high temperature compartment of the TES may be heated with an electrical heater 42, the receives power in time periods when it is abundant or available in excess or low price from the electrical grid.

The embodiment of figure 1 will now be further explained with reference to specific operation conditions. In the compressor circuit, the compressor 10 receives a vapor at 0.05 bar from the evaporator 21. The vapor is compressed to obtain a temperature of 450°C. The compressed vapor is cooled in the heat exchanger 14 to a temperature of 155°C. The vapor is subsequently condensed in condenser 20. In exchange a medium to be heated, such as return water from a district heating system, is heated from 40°C to 60°C. The temperature of the condensate leaving the condenser at outlet 19 is about 43°C. In the evaporator 21 the condensate is evaporated at 0.05 bar and the vapor is returned to the compressor 10. The medium to be cooled in the con- denser 21 is typically process or cooling water from an industrial process, sea water, groundwater, air etc. If groundwater is used the temperature at the inlet of the evaporator will typically be 8°C.

In the turbo-expander circuit, the turbo-expander 30 receives superheated vapor at a temperature of around 400°C. The turbo-expander decom- presses the superheated vapor and transfers the shaft energy obtained to a generator G for the production of electricity. The temperature of the vapor after the decompression in the turbo-expander is typically 85°C. The vapor from the expander is condensed in condenser 33 to obtain a liquid of 65°C at the outlet. The medium to be heated may be the water from the compressor circuit con- denser 20, which is received at the inlet of the turbo-expander circuit condenser 33 at a temperature of 60°C. In the condenser 33, the medium is heated and leaves the condenser at the outlet having obtained a temperature of 80°C.

The condensate from outlet 34 is heated in a boiler 35 at pressure of approx. 5 bar to obtain a vapor having a temperature of around 150°C. The saturated vapor is transferred to heat exchanger 38 for superheating the vapor to 400°C. The superheated vapor is returned to the turbo-expander.

The heating of the second compartment 39 of the TES may be done by an electrical heater, which may heat the compartment to a temperature of 400°C to 800°C.

In a charging mode, the plant as described above receives energy from an external source, which may the grid, a wind turbine, solar panel etc. The control unit directs the power from the external source to the electrical heater 42 and/or the electrical motor M driving the compressor 10. The compressor circuit will in the charging mode store heat energy in the low temperature com- partment of the TES, whereas the electrical heater will store heat energy in the high temperature compartment of the TES. The activity of the turbo-expander circuit will be reduced during charging to avoid a high production of energy from the generator connected to the turbo-expander 30.

In a discharging mode, the control unit will reduce the supply of energy to the electrical heater 42 and/or the electrical motor, that drives the compressor 10. The activity of the turbo-expander is increased in the discharging mode thereby withdrawing energy from the TES and transferring it to electrical energy in the generator coupled to the turbo-expander 30. It will be understood by the person skilled in the art, that the plant may also be operated in a com- bined mode applying both a charging and a discharging mode.

Figure 2 shows a modified version of the embodiment shown in Figure 1. The modification includes a shaft connection 44 between the turbo-expander 30 and the compressor 10. A shaft furthermore connects the expander and the compressor to a hybrid electrical motor generator M/G. During the charging mode, the shaft power provided by the turbo-expander will assist the electrical motor in supplying energy to the compressor. In the discharging mode, the expander will supply shaft energy to the hybrid electrical motor generator for production of electrical energy and to the compressor. The combining of the compressor and the expander in a single entity omit conversion of energy, thereby making the process more efficient.

Figure 3 shows a variant of the embodiment of figure 1 for the drying of a humid product (HP) to a dry product (DP) in a drying chamber 45. A steam at or close to its saturation point is entered the compressor circuit condenser 20 at inlet 46. The steam is superheated in the condenser 20 and leaves at the exit 47. The superheated steam is introduced in the drying apparatus at inlet 48 and allowed to interact with the humid product. The interaction result in an evaporation of the humidity in the product for the production of a dried product and a cooled steam enriched in relative humidity. For an efficient process the steam leaving the treated product is normally close to the saturation point. The steam exiting the evaporator is divided in two streams. A first steam is directed back to the inlet 46 of the condenser 20, whereas a second stream is conveyed to the inlet 11 of the compressor 10. The compressor compresses and heats the vapor using mechanical energy received from an engine or electrical motor M. The heated and compressed vapor exits at outlet 12 and is transferred to inlet 13 of the heat exchanger 14. The heat exchanger is placed in a low temperature compartment 15 of the thermal energy storage. In the heat exchanger 14, the compressed vapor is cooled by delivering heat to the low temperature department of the TES.

The cooled vapor leaves the heat exchanger at outlet 16 and is trans- ferred to inlet 17 condenser 18. In the condenser the vapor is condensed to a liquid, which leaves the condenser at outlet 19. The condensation evolves heat, which is transferred to the steam entering at inlet 46.

In an exemplary operation of the embodiment of figure 3, the steam entering at inlet 46 has a temperature of about 100°C. In the condenser 20 the steam is superheated to a temperature of 150°C and transported to the inlet 48 of the drying apparatus 45. In the drying apparatus the superheated steam interacts with the product to be dried. The interaction results in a lowering of the temperature and an increase of the relative humidity. At the exit

49 the steam is at or close to its saturation point and has obtained temperature of 100°C. A first part is returned to the condenser 20 for introduction at inlet

46 and a second part is directed to the condenser 10, which the steam is heated to 450°C. The heated steam is cooled in heat exchanger 14 to 155°C. The steam exiting the heat exchanger 14 is conveyed to inlet 17 of the condenser 20 for condensation thereof at a pressure slightly above the ambient pressure. The hot liquid condensate may be transferred to the boiler 35 or heat exchanged with the fluid entering the boiler 35.

Figure 4 discloses a variant of the embodiment shown in figure 3, in which both condensers are used to superheat steam for drying the humid product. According to this embodiment, a part of the steam close its saturation point leaving the drying apparatus at outlet 49 is conveyed to the inlet 51 of the expander circuit condenser 33 for superheating thereof. The superheated steam exits the condenser 33 at outlet 50 and is transferred to inlet 48 of the drying apparatus 45.

In a charging mode, the plant as described above receives energy from an external source, which may the grid, a wind turbine, solar panel etc. The control unit directs the power from the external source to the electrical heater 42 and/or the electrical motor M driving the compressor 10. The compressor circuit will in the charging mode store heat energy in the low temperature compartment of the TES, whereas the electrical heater will store heat energy in the high temperature compartment of the TES. The activity of the turbo-expander circuit will be reduced during charging to avoid a high production of energy from the generator connected to the turbo-expander 30.

In a discharging mode, the control unit will reduce the supply of energy to the electrical heater 42 and/or the electrical motor, that drives the compres- sor 10. The activity of the turbo-expander is increased in the discharging mode thereby withdrawing energy from the TES and transferring it to electrical energy in the generator coupled to the turbo-expander 30. It will be understood by the person skilled in the art, that the plant may also be operated in a combined mode applying both a charging and a discharging mode.