Thermal power station and method for generating electric power in a thermal power station

The invention relates to a thermal power station and a method for generating electric power in a thermal power station.

A thermal power station in which a Rankine cycle steam engine is coupled with a thermal energy storage is known from the state of the art. However, the roundtrip efficiency of such a thermal power station is limited and it is desired to have a greater roundtrip efficiency.

Therefore, it is an object of the invention to provide a thermal power station having a greater roundtrip efficiency.

This object is solved by the subject-matter of the claims. In particular, the object is solved by a thermal power station according to claim 1 and a method for generating electric power according to claim 14. Further details of the invention unfold from the other claims as well as the description and the drawings. Thereby, the features and details described in connection with the thermal power station of the invention apply in connection with the method for generating electric power of the invention, so that regarding the disclosure of the individual aspects of the invention it is or can be re ferred to one another.

According to a first aspect of the invention, the object is solved by a thermal power station comprising (a) at least one thermal energy storage having a housing, a storage chamber with heat storage material inside the storage chamber and a fluid inlet port fluidically connected to the storage chamber and a fluid outlet port fluidically connected to the storage chamber, and (b) a Brayton cycle heat engine comprising a gas turbine, a heat exchanger, a cooler and a compressor connect ed with each other by means of a closed cycle containing a second working fluid, whereby (c) the Brayton cycle heat en gine further comprises a control unit arranged for operating the Brayton cycle heat engine according to a Brayton cycle,

(d) the gas turbine is thermally coupled to the at least one thermal energy storage by means of a first heat exchanger and a first working fluid, the first working fluid being differ ent from the second working fluid, and (d) the gas turbine is connected to a generator for producing electrical power by means of the thermal energy from the thermal energy storage.

The inventor has found, that the thermal energy storage may store and release heat at higher temperatures than it is cur rently being used in a Rankine steam cycle, for example.

Based on this, the inventor has found that the heat stored in a thermal energy storage may be particularly well utilized in a Brayton cycle heat engine. This is because the Brayton cy cle may operate at much higher temperatures than the Rankine steam cycle, which is limited to operating temperatures of approximately T = 630 to 650 °C. In fact, the Brayton cycle may be operated at temperatures of up to T = 1000 °C to 1500 °C. Thereby, a greater roundtrip efficiency is achieved in the thermal power station according to the first aspect of the invention. By means of the invention the thermal energy storage is pro vided with an improved discharging cycle to increase overall efficiency, because it has been found that the discharging efficiency is a limiting factor for the overall efficiency.

According to the invention, the generator produces electrical power by means of the thermal energy from the thermal energy storage. This means that the thermal energy from the thermal energy storage is used for producing the electrical power. However, it may be possible to provide measures to increase the production rate of electrical power. For example, an ad ditional heater, e.g. an electrical heater, may be provided to heat the first working fluid and/or the second working fluid even further than possible only by means of the thermal energy storage. In such a case, the electrical power is pro duced by means of the thermal energy from the thermal energy storage and by means of the further measure, such as the ad ditional energy supplied to the additional heater.

In the invention, the thermal energy storage is used for storing heat or in other words thermal energy. In the follow ing a preferred design of the thermal energy storage is in troduced in more detail.

Preferably the thermal energy storage may be a horizontal storage with the main fluid flow direction in horizontal di rection. It comprises at least one fluid inlet port for re ceiving the first working fluid, which may be water, hot or cold steam, air, nitrogen or argon, for example, and at least one fluid outlet port for ejecting the first working fluid. The thermal energy storage further comprises a housing, pref erably with insulation, comprising a storage chamber with heat storage material inside the housing.

The storage chamber may be substantially a space, cavity, ex cavation or - as previously said - a housing in which the heat storage material is located. Within the storage chamber a heat exchange between the working fluid and the heat stor age material takes place. In order to provide an efficient heat exchange, the heat exchange chamber is preferably ther mally insulated against the surroundings. The loss of thermal energy is reduced by the thermal insulation.

For a modified distribution of the working fluid within the storage, instead of a single inlet port or a single outlet port, a plurality of inlet ports and/or a plurality of outlet ports may be arranged in the thermal energy storage.

The housing of the thermal energy storage may be substantial ly in cuboid or cylindrical form. The storage may form a hor izontal or vertical heat exchange chamber. The term "horizon tal heat exchange chamber" implies a horizontal main (aver- age) flow of the working fluid or heat transfer fluid through the chamber interior. The flow direction of the horizontal main flow is essentially parallel to the average surface of the earth. The horizontal direction is essentially a perpen dicular direction to the direction of the gravity force which affects the heat transfer fluid. A horizontally oriented di rection of the heat exchange flow can be achieved by lateral inlet openings and/or lateral outlet openings. The horizontal heat exchange chamber may comprise these openings in its side chamber boundaries.

In one first mode of operation, a charging mode and in par ticular a charging cycle, hot charging mode first working fluid will be provided via the fluid inlet port. After pass ing through the thermal energy storage and passing along the heat storage materials and thereby heating these heat storage materials, a cooler charging mode first working fluid is ex hausted via the fluid outlet port.

In a second mode of operation, a discharging mode and in par ticular a discharging cycle, the direction of the first work ing fluid flow may be reverted, so that a cool discharging mode first working fluid is supplied to the opening which was introduced as fluid outlet port, now acting as a fluid inlet port. At the other end of the storage end, i.e. its hot end, hot discharging mode first working fluid is exhausted via the port that was previously introduced as fluid inlet port, therefore now acting as fluid outlet port.

Thus, in the charging mode the thermal energy storage may be charged with thermal energy by feeding a hot charging mode first working fluid, such as hot air, to the fluid inlet port. The hot charging mode first working fluid will flow through the thermal energy storage and thereby heat up the heat storage materials. The thereby cooled charging mode first working fluid leaves the storage via the fluid outlet port. After the charging is completed, the thermal heat stor age may be left in a standstill period of hours or even days until the stored thermal energy is needed and discharged by feeding a cold discharging mode first working fluid, such as air, to the fluid inlet port or as explained before, in a re verse mode, to the port previously mentioned as fluid outlet port. After flowing through the thermal energy storage, the heated discharging mode first working fluid is ejected via the second port previously mentioned as fluid inlet port.

Preferably, the thermal insulation may comprise at least one, preferably at least two thermal insulation layers. The ther mal insulation layer may comprise at least one thermal insu lation material selected from the group consisting of ceram ics, concrete, bricks, vermiculite, perlite, calcium sili cate, microporous insulation material, chamotte, sinter, stones, foamed clay, mineral wool, mineral foam, mineral fi bers, foam glass, foil, in particular plastic foil, and soil layer with filled ground or sand.

Thereby it is advantageous that the thermal insulation mate rial comprises a density between 300 kg/m3 and 1.500 kg/m3, even though lower densities are possible, too. The function of the insulation is to prevent heat losses to the exterior and to prevent working fluid from exiting the storage at lo cations other than the inlet/outlet section.

The thermal energy storage is especially adapted for opera tion at high temperatures. Therefore, in a preferred embodi ment, an operating temperature of the operating mode is se lected from the range between 300 °C and 1500 °C, preferably selected from the range between 500 °C and 1300 °C, more preferably selected from the range between 600 °C and 1000 °C, 650 °C to 1000 °C and most preferably between 700 °C and 1000 °C. The operating temperature is the maximum temperature in the thermal energy storage achieved after charging of the thermal energy storage. A deviation of the temperature ranges is possible. In this context, very advantageous is an upper limit of the temperature range of 900 °C and most preferably an upper limit of the temperature range of 800 °C. Preferably, the thermal energy storage is a sensible heat storage, a latent heat storage or a thermo-chemical heat storage. In a sensible heat storage, heat storage material such as concrete, sand, stones, slag, steel elements or liq- uids, for example molten salt, may be used for storing ther mal energy. In a latent heat storage, heat storage material such as metal, metal alloys or silicon may be used, whereby the phase change is for storage of thermal energy. In a ther mo-chemical heat storage, energy is stored in a thermo- chemical energy storage material via an endothermic reaction whereas energy can be released via an exothermic reaction.

Generally, the heat storage material may comprise sand and/or stones. The heat storage chamber may comprise multiple dif- ferent heat storage materials. The stones can be natural stones or artificial stones. Mixtures thereof are possible, too. Artificial stones can consist of containers which are filled with heat storage material. Preferably, the stones comprise gravels (pebbles), rubbles and/or grit (splits). The artificial material comprises preferably clinkers, ceramics, steel or steel slack pellets. The stones may in particular be selected from the group of bricks, volcanic rocks, granites, basalts or ceramics provided as bulk material, for example. This can also be called pebble bed.

It is preferred, that the heat storage material consists of magmatic rock (also commonly referred to as igneous rock).

The magmatic rock may be vulcanite and/or plutonite, for ex ample. Magmatic rock is rock that is formed by cooling- conditioned solidification of a rock melt (magma). Magmatites are one of three main groups of rocks, along with sedimentary rocks (sedimentites) and metamorphites. The magmatite rock may preferably not contain any crystalline quartz portion of S1O2 (modal 0 %), since this is already present at atmospher ic pressure and at approx. 575°C would change its crystal structure. At correspondingly higher temperatures in the thermal energy storage, this prevents stresses in the rock- forming quartz grain and thus the occurrence of very fine cracks or chipping.

Further preferably, the heat storage material forms a channel system of heat exchange channels within the storage chamber. The thermal energy storage may form inside a kind of mesh network or a channel system of heat exchange channels embed ded into the storage chamber such that the heat exchange flow of the working fluid or heat transfer fluid through the heat exchange channels causes the heat exchange between the heat storage elements and the first working fluid. The heat ex change channels can be formed by interspaces (gaps) of the heat storage material, e.g. between the stones. In addition, or alternatively, the heat storage material may be porous. Open pores of the heat storage material form the heat ex change channels. It is also possible to have an indirect heat exchange by providing additional heat exchange channels run ning through the heat storage material, e.g. piping.

Moreover, preferably, the fluid inlet port is connected to a diffusor section of the thermal energy storage and/or the fluid outlet port is connected to a nozzle section of the thermal energy storage. The diffusor section evenly distrib utes the first working fluid into the thermal storage and re duces the flow speed of the first working fluid. The nozzle section increases flow speed and pressure of the first work ing fluid leaving the thermal energy storage in the housing and forwards it to the fluid outlet port for ejection from the thermal energy storage.

Therein, preferably, the diffusor section and/or the nozzle section may be formed by the housing. In particular, the dif fusor section and/or the nozzle section may be integrally formed with the housing. This allows for optimal distribution of the first working fluid inside the thermal energy storage and consequently greater roundtrip efficiency of the thermal power station. It is also preferred, that the thermal energy storage com prises at least two fluid inlet ports and/or at least two fluid outlet ports. Of course, all of the fluid inlet ports may be fluidically connected to the storage chamber and all of the fluid outlet ports may be fluidically connected to the storage chamber. The thermal energy storage may also comprise at least three fluid inlet ports and/or at least three fluid outlet ports. The number of fluid inlet ports may be equal to or different from the number of fluid outlet ports. The in crease of fluid inlet ports and/or fluid outlet ports at the thermal energy storage allows for better distribution of flow of first working fluid therethrough and ultimately a further increase in roundtrip efficiency of the thermal power plant. Each one of the fluid inlet ports may be connected to the same diffusor section of the thermal energy storage or sepa rate diffusor sections of the thermal energy storage. Also, each one of the fluid outlet ports may be connected to the same nozzle section of the thermal energy storage or separate nozzle sections of the thermal energy storage. The diffusor section (s) and/or nozzle section (s) may be formed by the housing of the thermal energy storage.

Moreover, it may be provided, that the thermal energy storage is provided with at least one electric heater. The at least one electric heater may be positioned before and/or after the thermal energy storage for heating the first working fluid. The at least one electric heater can further increase the roundtrip efficiency of the thermal power station.

In the invention, the gas turbine and generator of the ther mal power station are used for generating electric power us ing the heat stored in the thermal energy storage. In the following a preferred design of the thermal power station is introduced in more detail.

It is preferred, that the first working fluid is air and the second working fluid is CO2. CO2has been found to be particu- larly suitable for the high temperatures of the heat storable in the thermal energy storage. Air has sufficient heat- conductivity and may be provided at low costs such that a cost-effective thermal power station having a high roundtrip efficiency may be operated in combination with the CO2 as the second working fluid.

It is further preferred, that the second working fluid, in particular CO2, is trans- or supercritical in the Brayton cy cle. A working fluid is supercritical or in other words has a supercritical state when it is above its critical point. For CO2 this means that it is above its critical temperature of T = 30,980 °C and above its critical pressure of p = 73,74 bar. In supercritical working fluids, distinct liquid and gas phases do not exist. A Brayton cycle operated with super critical working fluid may be called a supercritical Brayton cycle. A working fluid is transcritical when it goes through subcritical and supercritical states in its thermodynamic cy cle. A working fluid is in a subcritical state when it is kept below its critical temperature, yet kept in the liquid state and above its boiling point. A Brayton cycle operated with transcritical working fluid may be called a transcriti cal Brayton cycle. It has been found that such second working fluid is particularly suitable for the high temperatures of the heat storable in the thermal energy storage.

Further, it is preferred, that the control unit is arranged to control the Brayton cycle heat engine in a way such that the second working fluid at the gas turbine is provided with a temperature of at least T = 700 °C, in particular at least T = 800 °C and moreover in particular at least T = 900 °C. At these high temperatures, which are made possible by means of the Brayton cycle heat engine, the roundtrip efficiency is particularly high. In this case it is particularly preferred and has been found to provide particularly great roundtrip efficiency, when CO2, in particular transcritical or super critical CO2, is used as the second working fluid. It is particularly preferred, that the Brayton cycle heat en gine further comprises a second heat exchanger arranged be tween the turbine and the cooler in the closed cycle to heat the second working fluid after passing through the cooler by means of residual heat in the second working fluid after passing through the gas turbine. Thereby, the residual heat of the second working fluid may be efficiently cooled before passing through the cooler by means of the second heat ex changer and the cooled second working fluid may be preheated before entering the first heat exchanger and the gas turbine, whereby the roundtrip efficiency is further increased.

It is further preferred, that the Brayton cycle heat engine comprises at least two second heat exchangers and at least two compressors of the at least one compressor, whereby they are arranged such that the second working fluid after passing through the cooler is alternatingly compressed by means of one of the at least two compressors and heated by means of one of the at least two second heat exchangers. Thereby, the roundtrip efficiency is increased even further.

Moreover, it is preferred, that the thermal power station further comprises a Rankine cycle heat engine having a steam turbine or a further Brayton cycle heat engine being thermal ly coupled with the Brayton cycle heat engine such that they form a combined cycle. By connecting the Brayton cycle heat engine with the Rankine cycle heat engine or a further Bray ton cycle heat engine, the roundtrip efficiency may further be improved.

Also, it is preferred, that the at least one thermal energy storage is connected to a renewable energy source. The renew able energy source may be a wind turbine, a solar energy plant or similar. Thereby, renewable energy may be stored as thermal energy in the thermal energy storage and efficiently obtained at a later time as electric power by means of the Brayton cycle heat engine. According to a second aspect of the invention, the object mentioned in the beginning is solved by a method for generat ing electric power in a thermal power station according to the first aspect of the invention, whereby the method com prises the steps of: (a) heating the first working fluid in a charging mode, so that a heated charging mode first working fluid is obtained, (b) transporting the heated charging mode first working fluid to the fluid inlet port of the thermal energy storage, whereby thermal energy from the heated charg ing mode first working fluid is transferred to the heat stor age material of the storage chamber, so that thermal energy is stored in the heat storage material, (c) transporting dis charging mode first working fluid of a discharging mode to the fluid inlet port of the thermal energy storage, whereby the stored thermal energy from the heat storage material of the storage chamber is transferred to the discharging mode first working fluid, so that a heated discharging mode first working fluid is obtained, which exits the fluid outlet port of the thermal energy storage and the heat from the heated discharging mode first working fluid is transferred to the second working fluid by means of the first heat exchanger, and (d) producing electric power in the generator by means of driving the gas turbine with the second working fluid.

Thereby, at least the advantages of the thermal power station according to the first aspect of the invention are achieved by the method according to the second aspect of the invention as well.

It is preferred, that the second working fluid flows through the closed cycle according to the Brayton cycle. The Brayton cycle comprises a first step of compressing the second work ing fluid. This is performed by means of the at least one compressor and is an adiabatic process. The Brayton cycle further comprises a second step of heating the second working fluid. This is performed by means of the first heat exchanger and is an isobaric process. The Brayton cycle moreover com prises a third step of expanding the second working fluid. This is performed by means of the gas turbine and is an adia batic process. Finally, the Brayton cycle comprises the fourth step of heat rejection. This is performed by means of the cooler in a closed cycle and is an isobaric process.

These four steps form the Brayton cycle, whereby the first step is repeated after the fourth step and all the steps are repeated in the order of their enumeration.

Further advantages, features and details of the invention un fold from the following description, in which by reference to drawings FIGS. 1 to 2 embodiments of the present invention are described in detail. Thereby, the features from the claims as well as the features mentioned in the description can be essential for the invention as taken alone or in an arbitrary combination. In the drawings, there is schematical ly shown:

FIG. 1 a sectional cut through a thermal energy storage as can be used in a thermal power station according to the invention, and

FIG. 2 a circuit diagram of a thermal power station ac cording to an embodiment of the invention.

Same objects in FIGS. 1 to 2 are denominated with the same reference number. If there is more than one object of the same kind in one of the figures, the objects are numbered in ascending order with the ascending number of the object being separated from its reference number by a dot. The specific dimensions of features and parts in the figures are exemplary and may be enlarged for ease of reference only.

FIGURE 1 shows a sectional cut through a thermal energy stor age 10 as can be used in a thermal power station 1 (see FIG. 2) according to the invention.

The thermal energy storage 10 comprises a housing 11, in which a storage chamber 12 filled with heat storage material 13 is located. A first working fluid A (see FIG. 2) may enter a fluid inlet port 14 of the housing 11 in the direction in dicated by an arrow. The fluid inlet port 14 is connected to a diffusor section 15. The fluid inlet port 14 and the diffu- sor section 15 are formed by the housing 11. Further, the first working fluid A may exit the housing 11 in the direc tion indicated by a further arrow through a fluid outlet port

16. The fluid outlet port 16 is connected to a nozzle section

17. The fluid outlet port 16 and the nozzle section 17 are formed by the housing 11.

FIGURE 2 shows a circuit diagram of a thermal power station 1 according to an embodiment of the invention.

The thermal power station 1 comprises the thermal energy storage 10 and the Brayton cycle heat engine 20 coupled ther mally with each other by means of the first heat exchanger 25.

A first working fluid A flows through a pipe taking in the heat from the thermal energy storage 10 and through the first heat exchanger 25, thereby transporting the heat from the thermal energy storage 10 to the first heat exchanger 25. For this purpose, a pipe from the first heat exchanger 25 is flu- idically connected to the fluid inlet port 14 of the thermal energy storage 10 and a further pipe from the first heat ex changer 25 is fluidically connected to the fluid outlet port 16 of the thermal energy storage 10. In this particular em bodiment, the first working fluid A may be air, for example.

A second working fluid B flows within a closed cycle 26 of the Brayton cycle heat engine 20 having several pipes 26.1, 26.2, 26.3, 26.4, 26.5, 26.6, 26.7, 26.8, 26.9, 26.10 and through the first heat exchanger 25. Thereby, the heat from the first working fluid A is exchanged with the second work ing fluid B being in a compressed state, whereby the second working fluid B becomes heated. The second working fluid B is supercritical CO2 in this particular embodiment. The heating of the second working fluid B is a second step within the closed Brayton cycle of the Brayton cycle heat engine 20.

The second working fluid B is transported by means of the pipe 26.1 of the closed cycle 26 to a gas turbine 21 of the Brayton cycle heat engine 20. In the gas turbine 21, the sec ond working fluid B being in the compressed state is expand ed. This is a third step within the closed Brayton cycle. The gas turbine 21 is connected to a generator 30, whereby the gas turbine 21 by means of expanding the compressed and heat ed second working fluid B drives the generator 30, which in turn produces electric power.

The second working fluid B in the expanded state still has residual heat. Therefore, the second working fluid B passes through two second heat exchangers 22.1, 22.2 by means of the pipes 26.2, 26.3 of the closed cycle 26.

After the two second heat exchangers 22.1, 22.2, which are arranged in series, the second working fluid B is relatively cold and is further cooled in cooler 23 to which it is passed through by means of pipe 26.4. This is a fourth step of the Brayton cycle.

In the first step of the Brayton cycle, the cold second work ing fluid B is compressed by means of two compressors 24.1, 24.2 of the Brayton cycle heat engine 20 being arranged in series. For this purpose, a pipe 26.5 connects the cooler 23 with the compressor 24.1 and a pipe 26.7 connects the second heat exchanger 22.2 with the compressor 24.2. Thereby, the second working fluid B is passed before and after the cooler 23 to different compressors 24.1, 24.2, in which it is com pressed. Thereby, the same compression of the second working fluid B may be achieved in every one of the two compressors 24.1, 24.2.

However, before passing through the first heat exchanger 25, the second working fluid B from pipe 26.6 coming from the compressor 24.1 is heated by means of the second heat ex changer 22.2 and by means of the second heat exchanger 22.1 and the second working fluid B from pipe 26.8 coming from the compressor 24.2 is heated by means of the second heat ex- changer 22.1. Thereby a type of two-stage-preheating is pro vided. In pipe 26.9 arranged between the second heat exchang ers 22.1, 22.2, the second working fluid B coming from the different compressors 24.1, 24.2 is mixed together. The pre heated and compressed second working fluid B then passes through pipe 26.10 and to the first heat exchanger 25, where the second working fluid B once again undergoes the second step of the closed Brayton cycle.