US20160069218A1

US20160069218A1 - Power plant system having a thermochemical accumulator

Info

Publication number: US20160069218A1
Application number: US14/785,931
Authority: US
Inventors: Uwe Lenk; Alexander Tremel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2013-04-26
Filing date: 2014-03-03
Publication date: 2016-03-10
Also published as: EP2796671A1; WO2014173572A2; EP2976509A2; WO2014173572A3

Abstract

A power plant system has a heat-generating unit for providing thermal energy; a water-steam circuit, which is connected to the heat-generating unit for heat transfer; an electricity-generating device, which can be energised by the thermal energy conducted in the water-steam circuit to generate electricity; and a thermochemical accumulator, which is connected to the water-steam circuit. The thermochemical accumulator is connected to an exhaust gas line of the heat-generating unit for heat transfer, and the thermochemical accumulator has two tanks, which are connected to each other for fluid flow. A thermochemical storage material is arranged in a first tank, and the first tank is supplied with heat by thermally conditioned exhaust gas of the heat-generating unit, and the second tank is connected for heat flow to a heat exchanger, by which the second tank is supplied with low-temperature heat at a temperature level of at most 150° C.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2014/054089 filed Mar. 3, 2014, and claims the benefit thereof. The International Application claims the benefit of European Application No. EP13165605 filed Apr. 26, 2013. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The present invention relates to a power plant system having a heat generating unit for providing thermal energy and an associated water-steam circuit, which is interconnected in regard to heat transfer with the heat generating unit. Furthermore, the present invention relates to a method for operating such a power plant system.

BACKGROUND OF INVENTION

The performance flexibility of power plants is determined, inter alia, by the chronological and performance behavior in the event of load change operations. These are typically startup and shutdown operations, and also changes for switching into part load operation. In particular, for example, in the case of combined gas and steam turbine power plants (COGAS), the achievable load change behavior for flexible operation is a defining technical variable, which additionally decides whether and to what extent the power plant can be used for network stabilization measures. In such gas and steam turbine power plants, the gas turbine can typically be started up comparatively rapidly or rapid load change switches are executable (in a chronological order of magnitude range of minutes). However, above all basic thermal requirements, for example, maintaining thermal and chronological gradients in the water-steam circuit, which can contribute to undesired mechanical stresses, restrict the speed of load change switches in the steam part of the power plant. Comparable restrictions can also result, for example, in other power plant technologies (for example, fossil fuel combustion power plants, biomass power plants, etc.), in which the load change speeds are restricted by the basic thermal requirements of a water-steam circuit.
To remedy this disadvantage from the prior art, the use of thermal accumulators is sometimes provided, by means of which thermal energy can be temporarily buffered, wherein the energy can then be made available again on short notice upon increased demand.
One example of such a technical approach is described, for example, in patent application WO 2012/150969 A1, in which a thermal accumulator having a gas turbine for improved flexible use is described. In this case, for example, the thermal accumulator is charged with heat at times at which only low power generation performance is demanded from a gas turbine. At another point in time, however, at which, for example, an increased demand for electrical energy exists, this energy which is buffered in the thermal accumulator can be retrieved at short notice, in order to convert it back into electrical energy in a reconversion method.
Such thermal accumulators are suitable in particular for thermal assistance of the startup of a combined gas and steam turbine power plant, after cooling has already taken place in the region of the water-steam circuit. If electrical power again has to be provided by the power plant after such cooling, for example, additional heat can be supplied to the steam part from the thermal energy accumulator, to enable faster startup of the overall power plant. Such thermal accumulators are embodied on the basis of the technology of sensitive thermal accumulators or latent thermal accumulators.
However, these approaches known from the prior art have the disadvantage that only comparatively low thermal energy densities can be supplied to or dissipated from the power plant by means of these accumulator technologies in the case of a load change. To nonetheless achieve high energy densities, comparatively large sensitive thermal accumulators or latent thermal accumulators would be necessary, which require undesirably large amounts of space and high investments.
Furthermore, the solution approaches known from the prior art have the disadvantage that because of a high power loss, thermal energy is lost during longer shutdown times of the power plant, for instance, without use. In particular because of the high temperature level at which the thermal energy is typically buffered, the thermal storage results in a high power loss.
Only inadequate monitoring of the amount of energy taken from the thermal accumulator has also proven to be disadvantageous in the conventional thermal energy accumulators used in the power plant field. They usually require a complicated flow control to be able to fulfill the chronological temperature requirements.
To avoid these and similar disadvantages, U.S. Pat. No. 5,127,470 proposes introducing waste heat into a set of chemical thermal accumulators, to be able to temporarily buffer the waste heat after conversion into chemical energy in the thermal accumulators. A set of the chemical thermal accumulators comprises in each case two accumulator containers, wherein each set is interconnected in regard to heat transfer with the respective other set.
Such chemical thermal accumulators are also described in US 2010/0263832 A1, wherein only the operation using a set comprising two accumulator containers is described here.
However, these embodiments known from the prior art have the disadvantage that the heat supply of the chemical thermal accumulator is performed via a central heat source. In the case of U.S. Pat. No. 5,127,470, for example, this heat supply occurs via the waste heat of an internal combustion engine, in the case of US 2010/0263832 A1, the heat is provided by a solar thermal system, for example.
However, because of a heat supply at a high temperature level, a high heat loss power is also a concern during operation, in particular during the charging procedure. Furthermore, undesired heat losses occur during the heat transport between the individual accumulator containers of the chemical thermal accumulators.
It has therefore been shown to be technically desirable therefrom to avoid these disadvantages and propose efficient heat utilization. Furthermore, it is desirable to enable improved monitored withdrawal of heat as needed.

SUMMARY OF INVENTION

These objects on which aspects of the present invention is based are achieved according to the invention by a power plant system as claimed and by a method for operating such a power plant system as claimed.
In particular, these objects on which the invention are based are achieved by a power plant system, which comprises a heat generating unit for providing thermal energy, a water-steam circuit, which is interconnected with regard to heat transfer with the heat generating unit, a power generating unit, which can be energized for power generation by the thermal energy guided in the water-steam circuit, and a thermochemical accumulator, which is also interconnected with regard to heat transfer with the water-steam circuit, wherein the thermochemical accumulator is interconnected with regard to heat transfer with an exhaust gas line of the heat generating unit, wherein the thermochemical accumulator has two containers, which are fluidically interconnected with one another, wherein a thermochemical storage material is arranged in a first container, and wherein the first container can be supplied with heat by thermally conditioned exhaust gas of the heat generating unit, and wherein the second container is interconnected with regard to heat transfer with a heat exchanger, via which the second container can be supplied with low-temperature heat at a temperature level of at most 150° C.
Furthermore, these objects on which the invention is based are achieved by a method for operating such a power plant system, as also described hereafter, which comprises the following steps: —charging the thermochemical accumulator by supplying heat at a first temperature level T1 to the thermochemical accumulator; —temporarily storing at least a part of this heat in the thermochemical accumulator in chemical form; —discharging the thermochemical accumulator at a second temperature level T2 and dissipating heat therefrom with transfer of heat to the water-steam circuit, wherein the discharge of the thermochemical accumulator is performed with supply of low-temperature heat at a temperature level of at most 150° C.
According to an embodiment, the heat generating unit is typically designed as a gas turbine, as a furnace chamber of a power plant operated using synthetic or also fossil fuels, or also as an atomic reactor of an atomic power plant.
In the present case, the water-steam circuit is additionally to be understood in its general meaning. In particular, the water-steam circuit therefore comprises all components, for example, of a waste heat steam generator, the associated pipelines, and also fittings and discharge lines to the power generating unit.
The power generating unit according to the invention is particularly designed as a steam turbine provided with a generator, which can be operated (=can be energized) by means of the thermal energy from the steam in the water-steam circuit.
The present invention provides integrating a thermochemical accumulator for buffering thermal energy into a power plant. In comparison to conventional thermal accumulators, latent thermal accumulators, and sensitive thermal accumulators used in power plant technology, a thermochemical thermal accumulator has a significantly higher storage density. For this purpose, thermochemical accumulators typically use a physical and/or chemical sorption procedure or a chemical reaction (usually between solid and gas or between liquid and gas) to be able to temporarily store thermal energy. In this case, for example, a quantity of heat introduced into the thermochemical accumulator can be used to promote a desorption procedure between two reactants, for example, to separate the two again. The free reaction products obtained therefrom are now at a higher energy level, for example, whereby the quantity of heat thus introduced can be buffered in the form of chemical energy. If, as a result of a requested withdrawal of thermal energy, for example, a renewed recombination of the free reaction products is triggered in the thermochemical accumulator, thermal energy is released. This thermal energy released in this case can now be provided for dissipation from the thermochemical accumulator and for further use in the power plant system. The chemical or physical reactions occurring in the scope of the storage procedures are reversible in this case, so that no consumption of reaction partners occurs.
In the case of thermochemical heat storage, the reactions of reaction partners occurring in this case are normally subject to temperature and pressure dependence. These influences also determine the side on which the chemical equilibrium of the reactions lies. Such a chemical equilibrium may be formulated as follows, for example, according to equation I:
F+vG
FG _v (Equation I)
In this case, F represents a solid, G represents a gas, and FG represents a reaction product of the two. Alternatively, F can also be a liquid, which reacts with a gas, for example.
The free reaction enthalpy ΔG and the equilibrium constant of the chemical equilibrium reaction K have the following relationship according to equation II:
ΔG=−RT ln K (Equation II)
In this case, R is the general gas constant and T is the temperature. The free reaction enthalpy ΔG can be determined as a function of the reaction enthalpy ΔH and the change of the entropy ΔS according to equation III:
ΔG=ΔH−TΔS (Equation III)
If one expresses the equilibrium constant K in accordance with the law of mass action in the meaning of equation IV, the equilibrium constant K may be approximately determined as follows from the relative partial pressures of the equilibrium state of the reaction according to equation I:
$\begin{matrix} K = \frac{p_{G}^{v}}{p_{0}^{v}} & (Equation IV) \end{matrix}$
In this case, the activities of the gas were assumed for simplification as the partial pressures thereof, wherein the activity of a solid is 1. Furthermore, p₀is the normal pressure of the reaction gas. As a result of the above equations, the following relationship therefore results according to equation V:
$\begin{matrix} \ln \frac{p_{G}^{v}}{p_{0}^{v}} = \frac{Δ S}{R} - \frac{Δ H}{RT} & (Equation V) \end{matrix}$
Since the variables ΔS and ΔH are generally only weakly temperature dependent, plotting the logarithmic partial pressure of the reaction gas p_gagainst
$- (\frac{1}{T})$
in a graph therefore results approximately in a straight line.
An exemplary plot of two different reactions, which are coupled with one another, however (LT reaction, for low-temperature reaction, and HT reaction, for high-temperature reaction) is illustrated in FIG. 1, for example. According to the reaction curves illustrated therein, for example, by supplying heat at a moderate temperature level T_m, the dissociation of a starting material is achieved according to the HT reaction (point A). The gaseous reaction product released in this case can now react again in another reaction (LT reaction). Since heat is again released during this second reaction, heat dissipation is required to keep the temperature level at a value T₁, which enables the second reaction (point B). Both reactions take place at the pressure level p₁and result in storage of energy, since the reaction enthalpy ΔH of the HT reaction is typically higher than the reaction enthalpy ΔH of the LT reaction. The heat supply at point A according to FIG. 1 is greater in absolute value in this case than the required heat dissipation at point B.
The withdrawal of thermal energy from this coupled system is essentially performed in reverse. If heat is again supplied to the reaction product of the LT reaction at the temperature level T_m, a dissociation of this reaction product occurs (point C). The pressure level p_hresults in this case. The reaction of the gaseous reaction product released in this case from the LT dissociation reaction together with the starting product of the HT reaction (for example, a solid which remains after the dissociation at point A) results in a release of heat at a higher temperature level T_h(point 4). The temperature release according to this reaction thus occurs at a temperature level T_h, which is higher than the above-described temperature levels T_mand T₁. Since the reaction enthalpy ΔH of the HT reaction is again typically greater than that of the LT reaction, the released quantity of heat at the point D is greater than the required heat supply at point C of the LT reaction.
According to the reaction sequence illustrated in FIG. 1, the heat supply at points A and C takes place at a comparatively moderate temperature level T_m. This represents a special case, since generally this heat supply can also take place at different temperature levels.
A selection of suitable reaction partners can be performed on the basis of the calculation of the equilibrium location and the respective reaction enthalpies ΔH. For example, possible selections of suitable reaction pairs are listed hereafter in detail, for example, in Table I.
Due to the interconnection of a thermochemical accumulator in the power plant system according to the invention, as already stated above, heat can be incorporated at different temperature levels in the power plant processes. This also enables greater flexibility of the power plant system.
Furthermore, a thermochemical accumulator is distinguished, for example, in comparison to sensitive storage or to latent heat storage, by a comparatively higher energy density. A thermochemical accumulator can thus be formed comparatively smaller, i.e., with a smaller space requirement. With comparable space requirement, the stored quantity of energy of a thermochemical accumulator is therefore higher in inverse conclusion.
Since the heat storage in a thermochemical accumulator additionally takes place in the form of chemical materials, the quantity of heat stored in a thermochemical accumulator can also be stored nearly without loss over a comparatively long period of time. Thus, even after relatively long shutdown times of the power plant, the stored materials can be made available again essentially completely for improved startup.
Furthermore, the incorporation of a thermochemical accumulator into the power plant system also proves to be advantageous in that waste heat from other processes (for example, return flow from the district heating network, waste heat from industrial processes) can also be incorporated. In particular in the case of physical desorption processes, heat supply at a comparatively low temperature level can also result in a sufficient dissociation rate. The incorporation of heat sources at lower temperature levels and the release of the heat at a higher temperature level (as explained for FIG. 1), can also be understood as a chemical heat pump, which operates particularly energy efficiently.
Furthermore, it is provided according to the invention that the thermochemical accumulator is interconnected with regard to heat transfer with an exhaust gas line of the heat generating unit, wherein the thermochemical accumulator has two containers which are fluidically interconnected with one another. In a first container, a thermochemical storage material is to be arranged, wherein the first container can be supplied with heat by thermally conditioned exhaust gas of the heat generating unit. In contrast, the second container is interconnected with regard to heat transfer with a heat exchanger, via which the second container can be supplied with low-temperature heat having a temperature level of at most 150° C. The incorporation of low-temperature heat can therefore be used in the meaning of a thermochemical heat pump by the thermochemical accumulator. This is possible in particular at a temperature level at which other usages of the heat in power plants may no longer be economically advantageous.
According to one advantageous embodiment of the invention, the thermochemical accumulator is connected downstream from the heat generating unit with respect to an exhaust gas stream therefrom. The thermochemical accumulator thus absorbs waste heat from the exhaust gas stream for storage and can buffer it in chemical or physical form (i.e., in the meaning of materials which are released). The thermochemical accumulator is advantageously interconnected with an exhaust gas line, which is also coupled to the heat generating unit. According to the embodiment, the thermochemical accumulator can also be interconnected in parallel with a partial line of the exhaust gas line in this exhaust gas line, i.e., in a further partial line of the exhaust gas line. Such an interconnection enables the storage of thermal energy in the thermochemical accumulator on demand, but also enables thermal energy to be guided past the thermochemical accumulator. Such embodiments increase the flexibility of the power plant system.
According to the power plant system according to the invention, it is provided that the heat generating unit is also interconnected with the thermochemical accumulator with regard to heat transfer. According to the embodiment, the interconnection can also be performed in a fluidic manner. By way of the interconnection, heat of the heat generating unit can be used to charge the thermochemical accumulator, so that it can again be available for heat emission at a later point in time. In particular, during operation of the power plant system, a supply of the thermochemical accumulator for charging with heat takes place at a temperature level of at least 200° C., particularly of at least 350° C., and very particularly at a temperature level of at least 500° C. Such temperature levels are suitable for promoting typically occurring reactions in a thermochemical accumulator. For a more precise depiction of the specific temperatures, reference is made to the respective reactions in Table I, which are also claimed in the present case according to the embodiment.

	TABLE 1

	reaction	temperature

	Cu(OH)₂ CuO + H₂O	10-200
	MgSO₄•7H₂O MgSO₄+ 7H₂O	50-150
	CuSO₄•5H₂O CuSO₄+ 5H₂O	120-160
	ZnCO₃ ZnO + CO₂	100-150
	MgCO₃ MgO + CO₂	250-550
	CaCO₃ CaO + CO₂	800-950
	Mg(OH)₂ MgO + H₂O	150-350
	Ca(OH)₂ CaO + H₂O	250-550
	Ba(OH)₂ BaO + H₂O	700-800
	BaO₂ BaO + ½ O₂	750-850
	2KO₂ K₂O + ½ O₂	600-800

It is furthermore provided according to the invention that the thermochemical accumulator has at least two containers, which are fluidically interconnected with one another, wherein a thermochemical storage material is arranged in a first container. Such storage materials are again listed, for example, in Table I. Advantageously, both containers are fluidically interconnected with one another via at least one pipeline, especially via a circuit. Furthermore, an inert gas can also be provided in the containers to assist the circulation in such an interconnection, which can be moved by a flow generator, for example, a fan, for example, between the containers. The first container can also, according to a further embodiment of the invention, be interconnected with a waste heat steam generator, which is comprised by the water-steam circuit. According to this embodiment, an assistance of the circulation between the two containers by a flow generator between the two containers can be omitted. The structure of the thermochemical accumulator with provision of two containers enables controlled charging and discharging of the accumulator. In particular, the containers enable, with interruption of the fluidic connection or with the provision of suitable reaction conditions, the long-term buffering of thermal energy in chemical or physical form.
According to a further embodiment of the invention, it is provided that the thermochemical storage material arranged in the first container comprises a metal oxide and/or a metal hydroxide. In particular, the metal oxide is to be formed as magnesium oxide, calcium oxide, barium oxide, copper oxide or the metal hydroxide is to be formed as magnesium hydroxide, calcium hydroxide, barium hydroxide, or copper hydroxide. Alternatively or also additionally, metal carbonates or metal sulfates can also be provided as storage materials. Hydrates, in particular of chlorides, are also conceivable as the storage material (see also Table 1), which can be dehydrated by supplying heat. The oxidation and reduction of metal oxides is also suitable for providing a suitable storage material. For example, the reaction of manganese-IV oxide to form manganese-III oxide and vice versa by (air) oxygen would be conceivable here. Further suitable materials can be inferred from Table I, for example. Such materials enable the provision of a storage material which is substantially harmless to handle and is also cost-effective.
According to a refinement of this concept, it can also be provided that a thermochemical storage material is also arranged in the second container, wherein in particular the thermochemical storage material arranged in the second container is not identical to the thermochemical storage material arranged in the first container. In particular, the thermochemical storage material arranged in the second container also again comprises a metal oxide and/or a metal hydroxide. The metal oxide is particularly to be formed as magnesium oxide, calcium oxide, barium oxide, copper oxide or the metal hydroxide is to be formed as magnesium hydroxide, calcium hydroxide, barium hydroxide, or copper hydroxide. Alternatively or also additionally, materials can also be provided as were described above, for example, as a thermochemical storage material for the first container.
In a further advantageous refinement of this embodiment, it is provided that a cold sink is provided in the second container, the refrigerating capacity of which during operation of the thermochemical accumulator enables vaporized water to be condensed in particular. In particular, this cold sink is designed as a heat exchanger or is interconnected with regard to heat transfer with such a heat exchanger, so that the condensation heat can be dissipated, for example. Alternatively, reaction heat can also be dissipated, if a chemical or physical reaction in the second container results in the release of heat. In a further embodiment, the cold sink can also be interconnected with regard to heat transfer with a low-temperature thermal accumulator, so that, for example, the released heat can be temporarily buffered therein. To use this released heat, it can also be coupled into the region of a low-pressure part of the water-steam circuit.
In addition, it can also be provided that a flow generator, in particular a fan, is fluidically interconnected between the two containers. The flow generator is capable of promoting the reaction rate by circulation of a gas or fluid participating in the reaction and therefore contributing to the release of larger quantities of heat.
During the time of the buffering, according to one embodiment, condensed water is typically located in the second container, which again has to be at least partially vaporized to discharge the thermochemical accumulator, to subsequently be able to be supplied to the first container. Heat can in turn be withdrawn from a thermal accumulator, or from a low-temperature source such as the district heating network, to vaporize this water. This embodiment provides that no thermochemical storage material is provided in the second container. Alternatively, of course, thermochemical storage material can also be provided in this second container, which would then also result in a release of water steam with the supply of heat.
In addition to the gaseous reactant water steam described here, other gaseous materials, such as ammonia NH₃, hydrogen H₂, or carbon dioxide CO₂can also be used.
In a further alternative embodiment, a cold sink can also be provided in the first container, which is designed comparably to the cold sink in the second container. In particular if thermochemical storage materials are provided in both containers, such a cold sink in the first container can be used for advantageous heat dissipation.
It is provided according to the invention that the second container is interconnected with regard to heat transfer with a heat exchanger, via which the second container can be supplied with low-temperature heat. Such a supply with low-temperature heat is, for example, a supply with heat from industrial cooling water from power plants, or waste water from industrial processes, and also heat from a district heating network (return flow from district heating water). Alternatively or additionally thereto, of course, the first container can also be provided with a heat exchanger. In particular if thermochemical storage material is provided in both containers, such an embodiment is advantageous. Low-temperature heat typically has a temperature level of at most 150° C. in this case. The heat exchanger according to the embodiment can sometimes also be provided to operate as a cold sink. The heat exchanger is to be supplied with corresponding refrigerating capacity for this purpose.
According to a further advantageous embodiment of the power plant system according to the invention, the thermochemical accumulator is interconnected with the water-steam circuit not only with regard to heat transfer but rather also fluidically. In particular, it is provided that water or steam can be withdrawn from the water-steam circuit, to supply it to the thermochemical accumulator. An interconnection solely with regard to heat transfer does not provide withdrawal of water or steam from the water-steam circuit, in contrast to the fluidic interconnection according to the embodiment. The water thus withdrawn can additionally also be provided as a reaction educt, in that, for example, the water steam is brought into direct contact with a thermochemical storage material for the release of heat. Since this water also hardly has any contaminants because of the purity of the water guided in the water-steam circuit, this steam is particularly well suited for supplying the thermochemical accumulator with water steam.
According to a further advantageous embodiment of the power plant system according to the invention, the thermochemical accumulator is interconnected with the water-steam circuit not only with regard to heat transfer but rather also indirectly fluidically. In this configuration, for example, water steam is withdrawn from the water-steam circuit. Heat is released by the condensation of this water steam, which results in a reaction (dissociation) in the first container. Heat is thus stored in chemical and/or physical form by the system of the thermochemical heat accumulator. The condensed water is returned back into the water-steam circuit.
According to a further advantageous embodiment of the invention, it is provided that the thermochemical accumulator has an air supply and an air exhaust, via which air supply the thermochemical accumulator can be supplied with air, and via which air exhaust thermally conditioned air can be emitted from the thermochemical accumulator. Such an emission is performed in particular for the heat transfer to the water-steam circuit. The absorbed and also emitted thermally conditioned air is therefore used as a heat carrier medium, which transfers the heat to be dissipated from the thermochemical accumulator to the water-steam circuit. Such a heat transfer by means of air is substantially harmless and can also be controlled well in an industrial scale.
Air is referred to here and in the entire description as a gas mixture, which contains nitrogen as a main component. Therefore, both fresh air having a high oxygen content can be used, and also exhaust gas after a combustion process, in which the oxygen concentration is typically lower and it additionally contains combustion products such as CO₂.
According to a particular embodiment of the invention, it is provided that the thermochemical accumulator is connected between the water-steam circuit and the heat generating unit. This intermediate connection enables comparatively short paths during the heat transfer, whereby the power losses are low. In addition, the waste heat of the heat generating unit can furthermore, after interaction with the thermochemical accumulator, also additionally still be used for the thermal conditioning of the water in the water-steam circuit.
According to a further advantageous embodiment, it is provided that the power plant system has positioning means, which are designed to supply a heat stream originating from the heat generating unit, in particular an exhaust gas stream of the heat generating unit, to the thermochemical accumulator, or to supply it to the water-steam circuit while bypassing the thermochemical accumulator. This interconnection is particularly flexible, since if needed heat can be supplied to the thermochemical accumulator but, for example, if the thermochemical accumulator is already completely charged, the heat can be supplied directly to the water-steam circuit while bypassing the thermochemical accumulator.
Furthermore, according to another advantageous embodiment of the invention, it can be provided that the power plant system is designed such that a heat stream originating from the heat generating unit, in particular an exhaust gas stream of the heat generating unit, can be supplied to the thermochemical accumulator, wherein after heat-transfer interaction, the residual heat stream or accordingly the exhaust gas stream is supplied to the water-steam circuit. Only a small fraction of heat is typically withdrawn from the exhaust gas stream to supply it to the thermochemical accumulator for storage. The remaining fraction in the exhaust gas stream can thus be supplied to the water-steam circuit for preparing steam. It is thus possible, for example, to charge the thermochemical accumulator while simultaneously operating the water-steam circuit. Chronological staggering between the two power plant industrial processes is thus not required.
According to a first particular embodiment of the method according to the invention, it is provided that during the thermal discharge of the thermochemical accumulator at a second temperature level T2, no generation of electrical energy is performed by means of the power generating unit. In other words, this means that the heat withdrawn from the thermochemical accumulator is primarily used for keeping the water-steam circuit warm. According to the embodiment, the water-steam circuit is thus kept at a temperature level, which is significantly above the temperature level in the case of shutdown or standby of the power plant system, by continuous heat supply from the thermochemical accumulator. If it is necessary as a result of increased electrical energy demand to start the power plant system for power generation or to ramp up the power plant system, which is located in part load, for increased power delivery, this can be performed in a shorter time, since the thermal gradients within the water-steam circuit are significantly less than if the water-steam circuit had not been supplied with heat from the thermochemical accumulator. More rapid load changes of the power plant system are thus possible in this way.
According to a further advantageous embodiment of the method according to the invention, it is provided that the charging of the thermochemical accumulator is performed by supplying heat from an exhaust gas of the heat generating unit and/or by supplying heat from the water-steam circuit. By withdrawing steam from a moderate pressure part of the waste heat steam generator, heat can advantageously be withdrawn from the water-steam circuit and supplied to the thermochemical accumulator. The thermochemical accumulator can therefore be supplied with heat from different sources flexibly, depending on the operating state of the power plant system. This increases the flexibility of the power plant system.
According to a further embodiment of the invention, it is provided that the discharge of the thermochemical accumulator is performed with supply of heat, in particular of low-temperature heat at a temperature level of less than 150° C. The incorporation of further heat, in particular of low-temperature heat, can therefore be used in the meaning of a thermochemical heat pump by the thermochemical accumulator. This is possible in particular at a temperature level at which other usages of the heat in power plants are no longer economically advantageous.
The invention will be described in detail hereafter on the basis of figures. It is to be noted in this case that the figures are to be understood as solely schematic and do not permit a restriction with respect to the ability to embody the invention.
Furthermore, it is to be noted that in the present case, the technical features listed in detail in the figures are to be claimed both alone and also in any arbitrary combination with one another, if the invention resulting therefrom can achieve the objects on which the invention is based.
In addition, it is to be noted that the technical features provided with identical reference signs in the figures have identical technical effects or functions.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures:

FIG. 1 shows a thermodynamic phase diagram to illustrate the reactions in a thermochemical accumulator, which operates as a heat pump, during charging and discharging;

FIG. 2 shows a first embodiment of the power plant system 1 according to the invention in a schematic circuit diagram;

FIG. 3 shows a further embodiment of the power plant system 1 according to the invention in a schematic circuit diagram;

FIG. 4 shows a further embodiment of the power plant system 1 according to the invention in a schematic circuit diagram;

FIG. 5 shows a flow chart depiction of an embodiment of the method according to the invention for operating a power plant system 1.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows an illustration in a diagram of the relationships depicted in equation V (see above!) of temperature (T) and pressure (p) during the charging and discharging of a thermochemical accumulator 40 (as shown in FIG. 2, for example), which operates as a heat pump. In this case, the thermochemical accumulator 40, as already stated above, has two containers 41 and 42, which are each provided with thermochemical storage material 45 and 46. The charging of the thermochemical accumulator 40 is performed, for example, at a pressure level p₁, wherein the first container 41 is supplied with heat at a temperature level T_m(point A). A storage of thermal energy is performed due to the fluid quantity (gas quantity) released as a result of this heat supply from the thermochemical storage material 45 and the supply to the second container 42, and also the subsequent reaction in the second container 42 (point B). The discharge of the thermochemical accumulator 40 is performed at a comparatively higher pressure p_h, wherein heat is supplied to the second container 42 at a temperature level T_m(point C). The free fluid (gas) resulting in this case is supplied to the first container 41. Because of the chemical and physical reactions in the first container 41 occurring as a result, a release of heat takes place at a comparatively higher temperature level T_h(point D). Further detailed statements can be inferred from the statements above.
FIG. 2 shows a schematic illustration of an embodiment of a power plant system 1 according to the invention, which has, in addition to a heat generating unit 10 designed as a gas turbine, a power generating unit 30, which is interconnected with regard to heat transfer via a water-steam circuit 20 with a waste heat steam generator 21. The power generating unit 30 can be embodied as a steam part having a steam turbine for the electrical energy generation. In this regard, the present case relates to a combined gas and steam turbine power plant system.
During correct operation of the power plant system 1, air 100 is sucked in, compressed, and supplied to the combustion chamber 12 by means of the compressor 11, to fire the combustion chamber together with fuel 110. The exhaust gas arising during this combustion is relayed to the expander 13 to perform mechanical work with simultaneous depressurization, wherein the exhaust gas 120 is supplied after depressurization via a suitable line guide to the waste heat steam generator 21 for the heat exchange. The waste heat steam generator 21 is designed to use the waste heat remaining in the exhaust gas 120 for the steam preparation in the water-steam circuit 20, to be able to convert the thermal energy into electrical energy by means of the power generating unit 30. At the same time, generation of electrical energy can be performed by means of the heat generating unit 10. In this case, for example, a generator (G) can be suitably interconnected with the heat generating unit 10, which is designed as a gas turbine.
Furthermore, the power plant system 1 according to the embodiment has a thermochemical accumulator 40, which is also interconnected with regard to heat transfer with the water-steam circuit 20.
Specifically, in the present case, the thermochemical accumulator 40 is interconnected with regard to heat transfer in the line guide between heat generating unit 10 and water-steam circuit 20. The thermochemical accumulator 40 thus also has an influence on the heat transfer rate of the exhaust gas 120 in the waste heat steam generator 21.
It is to be noted here that an interconnection with regard to heat transfer is generally always to be assumed if the operation of one component of the power plant system 1 directly or indirectly has an influence on the heat balance of a further component.
The thermochemical accumulator 40 has a first container 41 and a second container 42, which are both fluidically interconnected with one another. To assist the fluid guiding between the two containers 41, 42, a flow generator (fan) is provided, which is not provided with further reference signs. According to the embodiment, the first container 41 has a thermochemical storage material 45, which chemically or physically dissociates upon the transfer of heat. A solid or liquid dissociation product in particular remains in the first container 41, while in contrast a gaseous dissociation product is conducted by means of the fluid connection to the second container 42. According to the embodiment, the second container 42 does not have a further thermochemical storage material, but is interconnected with a heat exchanger 44 operating as a cold sink 43. Alternatively, the second container 42 can also be provided with a thermochemical storage material. The heat exchanger 44 is designed to transfer the thermal heat in the second container 42, after transfer of the dissociation product from the first container 41 to the second container 42, to a thermal accumulator 50. If the correspondingly transferred dissociation product is water steam, for example, a condensation of the water steam can occur, so that liquid water is collected in the second container 42.
According to the embodiment, the first container 41 of the thermochemical accumulator 40 is supplied with corresponding heat for charging by thermally conditioned exhaust gas 120. For example, to set the temperature level suitably, the exhaust gas 120 can be mixed with a suitable quantity of air 100, for example, to achieve a reduction of the temperature level of the exhaust gas 120. Sometimes, the thermochemical accumulator 40 has for this purpose an air supply 47 and an air exhaust 48, via which the thermochemical accumulator 40 can correspondingly be supplied with exhaust gas 120 mixed with air 100.
It is to be noted here that the air supply 47 can fundamentally also be capable of exclusive supply with exhaust gas 120.
The thermochemical accumulator 40 is fluidically connected in the present case between the heat generating unit 10 and the water-steam circuit 20. In this case, the exhaust gas stream can be guided both partially through the thermochemical accumulator 40 for heat transfer, or also past it in a partial line extending parallel thereto, so that a heat supply of the thermochemical accumulator 40 can be omitted. Such a parallel connection of two partial lines increases the flexibility of the use of the thermochemical accumulator 40.
If, for example, the heat generating unit 10 is not in operation because of low power demand, the waste heat steam generator 21 can thus furthermore be supplied by means of the heat decoupled from the thermochemical accumulator 40, to keep the thermal gradient in the water-steam circuit 20 substantially low. For this purpose, the second container 42 can be supplied with thermal energy from the thermal accumulator 50, wherein the heat exchanger 44 is used as a heat source. The supply of this thermal energy is sufficient, for example, to again vaporize the liquid water collected in the second container 42, so that the generated steam can be supplied by means of the flow generator to the first container 41 again. By way of the recombination reaction of the thermochemical storage material 45 in the first container 41 with the water steam, a chemical or physical reaction is maintained, which generates a large quantity of heat. This heat can be transferred to air 100, which is supplied to the thermochemical accumulator 40 via the air supply 47. The air is conveyed in this case by the fan 100. The air which is thus thermally conditioned is discharged via the air exhaust 48 from the thermochemical accumulator 40 and supplied to the waste heat steam generator 21. Accordingly, the water-steam circuit 20 interconnected with the waste heat steam generator 21 can be supplied with heat to such an extent that cooling of the water-steam circuit 20 below a predetermined temperature limit does not occur. It can therefore be ensured that the thermal gradients in the water-steam circuit 20 remain comparatively low, so that a rapid startup of the power generating unit 30, which is interconnected with the water-steam circuit 20, can also be enabled. This significantly increases the flexibility of the present power plant system 1.
It is to be noted here that the water-steam circuit 20 also comprises the components of the waste heat steam generator 21, but both are separated as concepts for improved illustration.
FIG. 3 shows a further embodiment of the power plant system 1 according to the invention in a schematic circuit diagram, which only differs from the power plant system 1 shown in FIG. 2 in that the heat supplied to the thermochemical accumulator 40 is not withdrawn from the exhaust gas 120 of the heat generating unit 10, which is designed as a gas turbine, but rather steam, which is withdrawn from the water-steam circuit 20 in a moderate pressure part. The waste heat steam generator 21 shown in the present case comprises, in addition to an economizer 25, a vaporizer 26 and also two superheaters 27, which are interconnected sequentially in the water-steam circuit 20. This embodiment of the water-steam circuit only represents one example, arbitrary further embodiments are conceivable. The power generating unit 30 which is interconnected with the water-steam circuit is designed as a steam turbine provided with a generator G. A condenser 29 is used for condensation of the steam guided in the water-steam circuit 20.
Steam is now withdrawn from the water-steam circuit 20 between the vaporizer 26 and the first superheater 27 during operation of the power plant system 1 to charge the thermochemical accumulator 40. This steam which is thus withdrawn is supplied to the first container 41, in which the thermochemical storage material 45 is arranged. By way of the condensation of the steam, heat is released, which is emitted to the storage material 45. After completed heat transfer, the water is again supplied to the water-steam circuit 20 in the region between economizer 25 and vaporizer 26.
The further functions of the thermochemical accumulator 40 or the power plant system 1 essentially correspond to those of the thermochemical accumulator 40 or the power plant system 1, respectively, shown in FIG. 2.
The mode of operation of the power plant systems shown in FIGS. 2 and 3 will be explained once again hereafter in detail on the basis of further specific embodiments. Both embodiments are designed such that the thermochemical accumulator 40 is connected downstream from the heat generating units 10 designed as gas turbines. In normal operation, for example, exhaust gas 120 does not flow through the thermochemical accumulator 40. If the thermochemical accumulator 40 is to be charged, the exhaust gas 120 of the heat generating unit 10 is entirely or partially conducted through the first container 41. In this case, heat absorption or slight cooling of the exhaust gas 120 typically occurs, for example, by approximately 50° C. The exhaust gas exiting from the thermochemical accumulator 40 can furthermore be thermally used in the waste heat steam generator 21, whereby the generating power of the power plant system 1 is only slightly reduced.
According to one specific embodiment, calcium hydroxide is located in the first container 41, which is not in direct contact with the exhaust gas, however. Because of the heat transfer, the dissociation reaction according to equation VI occurs.
Ca(OH)₂→CaO+H₂O (Equation VI)
In this case, water steam (H₂O) is formed. This water steam is conducted into the second container 42. The conduction can be performed by assistance of a flow generator. In the second container 42, the heat is discharged to the heat exchanger 44 or cold sink 43, whereby the water steam condenses. The introduction of heat into the first container 41 is performed at a temperature level of approximately 550° C. (temperature level of the exhaust gas 120). The condensation of water in the second container 42 can take place under pressure at approximately 120° C. According to internal estimations of the applicant, for each megawatt hour of thermal energy which is introduced into the first container 41 by heat transfer, approximately 0.4 MWh of heat has to be dissipated from the second container 42. This thermal energy can be buffered in the second container 42, for example, in a thermal accumulator 50 designed as a low-temperature accumulator. The storage of this heat is not only cost-effective, but rather also large storage capacities may be implemented. Alternatively, this heat can also be coupled back into the low-pressure part of the water-steam circuit 20 again. The losses due to the charging of the thermochemical accumulator system 40 can thus be reduced.
After complete dissociation of calcium hydroxide in the first container 41, calcium oxide is present and the thermochemical accumulator 40 is completely charged. If a gas exchange is now avoided between the two containers 41 and 42, the chemical energy in the first container 41 can be stored for an essentially unlimited length of time without further heat losses.
If the heat generating unit 10 designed as a gas turbine is not operated further, the energy content of the thermochemical accumulator 40 can be used to keep the water-steam circuit 20 at an elevated temperature level, particularly at operating temperature. For this purpose, for example, as described above, water in the second container 42 is vaporized and supplied to the first container 41 by means of a flow generator. Alternatively, it is also possible to integrate the first container 41 directly into the waste heat steam generator 21, so that the flow generator could be omitted. The water condensed in the second container 42 can be vaporized by means of the heat discharged from the thermal accumulator 50, wherein the vaporization occurs in a temperature range between 60 and 150° C. It is thus also possible to withdraw heat from low-temperature waste heat sources for this purpose (for example, return flow of the district pipeline heating network, waste heat of an industrial process). If further waste heat sources are used, a thermal accumulator 50 can sometimes also be omitted. After reaction of the water steam transferred into the first container 41 with the calcium oxide present therein as a thermochemical storage material 45, the following reaction occurs according to equation VII:
CaO+H₂O→Ca(OH)₂ (Equation VII)
The reaction according to equation VII is strongly exothermic, whereby heat is released. This heat can furthermore now be used to supply the water-steam circuit 20 with heat. At an assumed vaporization temperature of approximately 90° C. in the second container 42, according to internal estimations of the applicant, heat is generated at a temperature level of up to 500° C. in the first container 41. For each megawatt hour of energy which is released in the first container 41, approximately 0.4 MWh have to be supplied to the second container 42. After complete reaction of the calcium oxide in the first container 41, the discharge procedure is completed.
According to one special embodiment of the method according to the invention for operating the power plant system 1, the withdrawal of thermal energy from the first container 41 by discharging the thermochemical accumulator 40 is only performed when the heat generating unit 10 designed as a gas turbine is not operated.
According to a further specific embodiment of the power plant system 1 according to the invention, magnesium hydroxide or magnesium oxide can be provided as a thermochemical storage material 45 in the first container 41. According to such an embodiment, it is advantageous to withdraw heat from the water-steam circuit 20 at a lower temperature level. By way of the removal of steam in the region of a moderate pressure part of a waste heat steam generator 21, for example, saturated steam can be withdrawn in a temperature range of 200° C. to 230° C. Sufficient condensation heat is released by condensation of this saturated steam, which can promote the decomposition of magnesium hydroxide according to equation VIII in the first container 41:
Mg(OH)₂→MgO+H₂O (Equation VIII)
According to the internal estimations of the applicant, for each megawatt hour which is stored in the first container 41, approximately 0.5 MWh accordingly has to be dissipated from the second container 42.
To keep the water-steam circuit 20 warm, the thermochemical accumulator filled with magnesium oxide can be discharged by supply of low-temperature heat to the second container 42. This heat in turn causes the vaporization of water condensed therein, which is conducted into the first container 41 to maintain an exothermic reaction according to equation IX. The reaction temperature is approximately 230° C.
MgO+H₂O→Mg(OH)₂ (Equation IX)
Upon discharge of the thermochemical accumulator 40, a heat transfer to pressurized water from the water-steam circuit 20 occurs, wherein the flow direction can be reversed in comparison to the flow direction shown in FIG. 3. This enables keeping the water-steam circuit 20 warm in a temperature range of approximately up to 230° C.
FIG. 4 shows a further embodiment of the power plant system 1 according to the invention, which is fluidically interconnected with the water-steam circuit 20 comparably to the embodiment shown in FIG. 3. In contrast to the preceding specific embodiments, however, not only is the first container 41 filled with a thermochemical storage material 45, but rather also the second container 42 is filled with a second thermochemical storage material 46. Such an embodiment is particularly advantageous if, for example, waste heat exists in a low temperature range, which can be used to operate the thermochemical accumulator 40 in the meaning of a thermochemical heat pump.
For example, copper hydroxide is located in the second container 42, which dissociates due to the supply of heat with the formation of water steam. For example, magnesium oxide is located in the first container 41, which can again absorb the water thus released. The reaction running in this case in the first container 41 is strongly exothermic, as already stated above. The embodiment is thus capable of raising waste heat from a comparatively low temperature level between 100 and 160° C., which is supplied to the second container 42, to a higher temperature level, which is at approximately 230° C. in the first container 41. In addition, for each megawatt hour which is provided as waste heat at the second container 42, up to 1.5 MWh are released at the higher temperature level in the first container 41.
After the reaction has run completely during the discharge of the thermochemical accumulator 40, essentially exclusively magnesium hydroxide is located in the first container 41 and copper oxide is located in the second container 42. Heat can again be withdrawn from the water-steam circuit 20 to charge the thermochemical accumulator 40. At the same time, it is necessary to cool the second container 42 by way of corresponding heat dissipation. In this case, a temperature of approximately 30° C. is advantageous. During the charging, a decomposition of the magnesium hydroxide occurs in the first container 41 as a result of the heat transfer at approximately 140° C. The water released in this case is bound after transport into the second container 42 by the reaction according to equation X:
CuO+H₂O→Cu(OH)₂ (Equation X)
For each megawatt hour of waste heat which is supplied to the first container, a quantity of heat of approximately 0.6 MWh is to be absorbed by the cooling. The heat dissipated in this case can in turn be buffered suitably in a thermal accumulator (not shown in greater detail). The charging procedure is completed when the water bound in the first container 41 is essentially completely expelled.
FIG. 5 shows an embodiment of the method according to the invention for operating an above-described power plant system 1, which comprises the following steps: —charging the thermochemical accumulator 40 by supplying heat at a first temperature level T1 to the thermochemical accumulator 40 (first method step 200);—temporarily storing at least a part of this heat in the thermochemical accumulator 40 in chemical and/or physical form (second method step 210);—discharging the thermochemical accumulator 40 at a second temperature level T2 and dissipating heat therefrom with transfer of heat to the water-steam circuit 20, wherein the discharge of the thermochemical accumulator 40 is performed with supply of low-temperature heat at a temperature level of at most 150° C. (third method step 220);
Further embodiments result from the dependent claims.

Claims

1.-12. (canceled)

13. A power plant system, comprising

a heat generating unit for providing thermal energy,

a water-steam circuit, which is interconnected with regard to heat transfer with the heat generating unit,

a power generating unit, adapted to be energized for power generation by the thermal energy guided in the water-steam circuit, and

a thermochemical accumulator, which is also interconnected with regard to heat transfer with the water-steam circuit, wherein the thermochemical accumulator is interconnected with regard to heat transfer with an exhaust gas line of the heat generating unit, wherein the thermochemical accumulator has two containers, which are fluidically interconnected with one another,

a thermochemical storage material arranged in a first container, wherein the first container is adapted to be supplied with heat by thermally conditioned exhaust gas of the heat generating unit, and wherein the second container is interconnected with regard to heat transfer with a heat exchanger, via which the second container is adapted to be supplied with low-temperature heat at a temperature level of at most 150° C., and

a second thermochemical storage material arranged in the second container.

14. The power plant system as claimed in claim 13, further comprising

a flow generator between the containers, and

an inert gas in the containers, adapted to be moved by the flow generator.

15. The power plant system as claimed in claim 13,

wherein the thermochemical storage material arranged in the first container comprises a metal oxide and/or a metal hydroxide.

16. The power plant system as claimed in claim 13, further comprising

a cold sink in the second container, the refrigerating capacity of which during operation of the thermochemical accumulator enables vaporized water to be condensed.

17. The power plant system as claimed claim 13,

wherein the thermochemical accumulator is interconnected with the water-steam circuit with regard to heat transfer and fluidically.

18. The power plant system as claimed in claim 13,

wherein the thermochemical accumulator comprises an air supply and an air exhaust,

wherein the thermochemical accumulator is adapted to be supplied with air via the air supply, and

wherein thermally conditioned air is adapted to be emitted from the thermochemical accumulator via the air exhaust.

19. The power plant system as claimed in claim 13,

wherein the thermochemical accumulator is connected between the water-steam circuit and the heat generating unit.

20. The power plant system as claimed in claim 13,

wherein the power plant system comprises a positioner designed to supply a heat stream originating from the heat generating unit to the thermochemical accumulator, or to supply the heat stream to the water-steam circuit while bypassing the thermochemical accumulator.

21. The power plant system as claimed in claim 13,

wherein the power plant system is adapted such that a heat stream originating from the heat generating unit is adapted to be supplied to the thermochemical accumulator, wherein after heat-transfer interaction, the residual heat stream or accordingly the exhaust gas stream is supplied to the water-steam circuit.

22. A method for operating a power plant system as claimed in claim 13, comprising:

charging the thermochemical accumulator by supplying heat at a first temperature level (T1) to the thermochemical accumulator;

temporarily storing at least a part of this heat in the thermochemical accumulator in chemical form;

discharging the thermochemical accumulator at a second temperature level (T2) and dissipating heat therefrom with transfer of heat to the water-steam circuit, wherein the discharge of the thermochemical accumulator is performed with supply of low-temperature heat at a temperature level of at most 150° C.

23. The method as claimed in claim 22,

wherein, during the thermal discharge of the thermochemical accumulator at a second temperature level (T2), no generation of electrical energy is performed by the power generating unit.

24. The power plant system as claimed in claim 13,

wherein the second thermochemical storage material arranged in the second container is not identical to the thermochemical storage material arranged in the first container.

25. The power plant system as claimed in claim 20,

wherein the heat stream originating from the heat generating unit comprises an exhaust gas stream of the heat generating unit.

26. The power plant system as claimed in claim 21,