Patent Application
20150059342
The invention relates to a plant for storing thermal energy which has a circuit for a working gas. In the circuit in this case the following units are interconnected in the specified sequence by means of a line for the working gas: a first thermal fluid energy machine, a heat accumulator, a second thermal fluid energy machine and a cold accumulator.
The first thermal fluid energy machine is operated as a working machine and the second thermal fluid energy machine is operated as a power machine, as seen in the flow direction of the working gas from the heat accumulator to the cold accumulator.
Furthermore, the invention relates to two methods for operating this plant. In one method for storing thermal energy, passage through the circuit is in the direction of the heat accumulator to the cold accumulator, which corresponds to the sequence of the modular units specified above. According to a further method, to which the invention also refers, stored thermal energy from the plant can also be converted into mechanical energy, for example. In this case, passage through the units is in the reverse sequence, in other words the flow direction of the working gas is reversed. This working gas then passes first of all through the cold accumulator and then through the heat accumulator, wherein in this case the first thermal fluid energy machine is operated as a power machine and the second thermal fluid energy machine is operated as a working machine.
The terms power machine and working machine are used within the scope of this application so that a working machine absorbs mechanical work in order to fulfill its purpose. A thermal fluid energy machine which is used as a working machine is therefore used as a compressor or fluid compression machine. Compared with this, a power machine performs work, wherein a thermal fluid energy machine for performing the work converts the thermal energy which is made available in the working gas. In this case, the thermal fluid energy machine is therefore operated as a motor.
The term “thermal fluid energy machine” constitutes a generic term for machines which can extract thermal energy from a working fluid—a working gas within the context of this application—or can feed thermal energy to this. Both heat energy and cold energy are to be understood by thermal energy. Thermal fluid energy machines can be designed as piston machines, for example. Hydrodynamic thermal fluid energy machines, the impellers of which allow a continuous flow of the working gas, can preferably also be used. Axially acting turbines or compressors are preferably used.
The principle specified in the introduction is described according to US 2010/0257862 A1, for example. In this case, piston machines are used in order to implement the method which is described above. According to U.S. Pat. No. 5,436,508, moreover, it is known that by means of the plants for storing thermal energy which are specified in the introduction over-capacities can also be temporarily stored when wind energy is being utilized for producing electric current in order to retrieve this again when required.
An object of the invention is to disclose a plant for storing thermal energy of the type specified in the introduction and methods for conversion of thermal energy (for example conversion of mechanical energy into thermal energy with subsequent storage or conversion of the stored thermal energy into mechanical energy) by means of which a utilization of the stored thermal energy which is as efficient as possible is enabled.
This object is achieved according to the invention by means of the plant specified in the introduction by the cold accumulator being able to be connected into a circuit for a cooling medium, which differs from the aforesaid circuit, by the following units being interconnected in the specified sequence by means of a line for a cooling medium: the cold accumulator, a cooling unit and a cold consumer which is to be cooled. The cooling medium is normally different from the working gas which accounts for the fact that the cooling circuit is different from the circuit. In order to avoid cleaning during a change of the operating state, it is particularly advantageous if in the cold accumulator the passages used for heat transfer also form two passage systems and if in this respect each of the passage systems can be connected to one of the circuits. The cooling circuit therefore uses the one passage system, whereas the charging circuit uses the other passage system. The charging circuit, however, can share the passage system in the cold accumulator—and possibly also other parts of the line associated with this—with a discharging circuit (more about this below). Whereas the charging circuit is responsible for storing the thermal energy, the thermal energy can be released again to the working gas via a discharging circuit.
The cooling unit is required in order to be able to set the necessary temperature level for the cold consumer which is to be cooled because the storage temperature of the cold accumulator is higher than the necessary temperature level. However, by means of the cold accumulator precooling of the cooling medium can be carried out so that a smaller temperature difference has to be overcome in the cooling unit. This also advantageously reduces the power demand for the cooling unit. Process cold, which accumulates anyway in the plant for storing thermal energy, can be utilized. This is admittedly no longer available during discharging for the purpose of releasing thermal energy, for which this does not have to be produced separately, however, for operation of the cold consumer which is to be cooled. The overall energy balance of the plant for storing and releasing thermal energy and of the cold consumer is advantageously improved as a result.
The charging circuit (and discharging circuit) can be operated as an open or closed circuit (more about this below). In the case of an open circuit, air forms the working gas which can be extracted from the atmosphere and then fed again to this. As a cooling unit, any form of unit can be used. The use of a thermosiphon, which advantageously achieves comparatively low temperature levels, is particularly advantageous.
The cold consumer is equipped according to a specific embodiment of the invention with a superconducting component. As cooling medium, nitrogen can be used here, especially when high-temperature superconductors, for example Bi2223 or YBCO, are being used. This has to be brought to a temperature level of approximately 50 to 60 K. A precooling via the cold accumulator to approximately 180 K simplifies the cooling process and reduces the power consumption on the cooling unit.
According to another embodiment of the invention, it is provided that the electric machine is a generator which in particular can be installed in a wind power plant. This application offers particular advantages since electric machines can be constructed with superconducting components (especially the winding of an electrically excited rotor in a synchronous generator) with a lower mass. The mass of the generator, however, constitutes the limiting factor in the design of wind power plants since the generators have to be installed at great height in the nacelle of the wind power plant. In the case of conventional generators, the mass of the applied generators increases more quickly, however, than the output, wherein approximately a power of 1.6 lies within this ratio. Therefore, at present the economical limit for an increase of the generator output in wind power plants is approximately 6 MW. On the other hand, the construction of wind power plants exposed to strong winds requires the installation of a larger generator capacity in the nacelle. This can be achieved according to the invention by using generators with superconducting components. If the wind power plant is coupled to the plant according to the invention of storing and releasing thermal energy, then this has the advantage that the cold accumulator can be expediently used in order to minimize the losses which become necessary on account of the required cooling of the superconducting generator windings. At the same time, this plant for storing thermal energy can also be used in order to temporarily store, in a known manner per se, over-capacities in the electricity network and to convert them again into electric current, by releasing the thermal energy, in the event of consumption peaks in electric energy. It therefore involves the utilization of a synergy effect which altogether increases the efficiency during operation of the plant especially wind power plants. However, the plant can also be operated for example with pump storage power plants or with conventional power plants, such gas-turbine power plants.
It can furthermore be advantageously provided that the electric machine is a motor which is mechanically coupled to the first thermal fluid energy machine. This fluid energy machine has to be operated specifically during the charging process of the cold accumulator and of the heat accumulator (possibly also of an additional low-temperature heat accumulator) in order to bring the thermodynamic charging process into operation. It is particularly advantageous to also construct this motor as an electric machine with a superconducting winding if the infrastructure for cooling this machine is available on account of using a superconducting generator, for example in the wind power plant. With this, a further efficiency increase for the plant is possible.
It can have an equally efficiency-increasing, advantageous effect if a further generator with superconducting components (e.g. the winding) is used as an electric machine. This is then coupled to the first fluid energy machine and used if in times of increased energy demand the heat accumulator and the cold accumulator are to be discharged. It is alternatively also possible that the generator is connected to a third thermal fluid energy machine, wherein the third thermal fluid energy machine is connected in parallel with the first thermal fluid energy machine and a fourth thermal fluid energy machine is connected in parallel with the second thermal fluid energy machine in the charging and discharging circuit. In this case, a valve mechanism is provided in each case between the first and the third thermal fluid energy machines and/or between the second and the fourth thermal fluid energy machines. By switching of the valve mechanism, the one or the other fluid energy machine can now be advantageously selected in each case, depending on the flow direction of the working gas. This has the advantage that the respective fluid energy machine being used can be optimized to the operating state which is to be selected. Since when using only two fluid energy machines both have to be used both as a working machine and as a power machine, depending on the flow direction, only one constructional compromise can be selected without the provision of additional fluid energy machines. Since, however, both in the thermal charging operation and in thermal discharging operation the aim is a highest possible level of efficiency, the parallel connection of fluid energy machines allows both the method for storing the thermal energy and the method for conversion of the thermal energy to be undertaken with optimum efficiency. Instead of using valves, separate lines can also be provided for the charging circuit and the discharging circuit. The configuration has the advantage that the fluid energy machines being used in each case can be optimized to the respective operating state during the charging process and the discharging process. As a result of this, an increase of efficiency of the system is achieved. This, however, is at the cost of higher investment costs of the plant. An economical assessment has to be made in this case.
By choice, the working gas can be conducted in a closed circuit or an open circuit (this applies both to the charging circuit and to the discharging circuit, but not to the cooling circuit). An open circuit always uses the ambient air as working gas. This is drawn from the environment and at the end of the process is also released into this again so that the environment closes the open circuit. A closed circuit also allows the use of a working gas which is other than ambient air. This working gas is conducted in the closed circuit. Since an expansion into the environment with simultaneous adjustment of the ambient pressure and the ambient temperature does not apply, the working gas, in the case of a closed circuit, has to be conducted through a heat exchanger which allows a release or absorption of the heat of the working gas into or from the environment.
In addition, it can be provided that a low-temperature heat accumulator is additionally provided in the circuit upstream of the first fluid energy machine. This heat accumulator is referred to as a low-temperature heat accumulator because the temperature level which is reached as a result of storing the heat principally lies below the temperature level of the heat accumulator. Heat is defined in relation to the ambient temperature of the plant. Everything above ambient temperature is heat, whereas everything below the ambient temperature is cold. Therefore, it also becomes clear that the temperature level of the cold accumulator lies below the ambient temperature.
The use of the low-temperature heat accumulator has the following advantages. If the plant for storing thermal energy is used, then the low-temperature heat accumulator is passed through before passage through the first fluid energy machine which in this case works as a working machine (compressor). As a result of this, the working gas is already heated above ambient temperature. This has the advantage that the working machine has to absorb a lower level of power in order to achieve the required temperature of the working gas. In concrete terms, the heat accumulator is to be heated to above 500° C., which can advantageously also be carried out, subsequent to the preheating of the working gas, using technically available thermodynamic compressors which allow compression of the working gas to 15 bar. Therefore, recourse may advantageously be had to components for the modular units of the plant which are obtainable on the market without costly modifications.
The achieving of an object is additionally managed by means of the method referred to in the introduction for storing and releasing thermal energy by the cold accumulator being connected, when required, into a cooling circuit which differs from said circuit, wherein in the cooling circuit cooling medium flows through the following units in the specified sequence: the cold accumulator, a cooling unit and a cold consumer which is to be cooled. By means of this method, the advantages explained above for the plant according to the invention are achieved, wherein the method can be implemented with the aforesaid plant. As a particularly suitable cooling medium, especially for superconducting components, nitrogen can be used. This exists in liquid form at the temperatures which are required for cooling these superconducting components and can be brought to the required temperature level in a thermosiphon, for example, as the cooling unit. The precooling via the cold accumulator reduces the energy expenditure in this case during operation of the cooling unit. This can also be of smaller dimensions. This make this solution particularly economical.
Further details of the invention are described below with reference to the drawing. The same or corresponding elements of the drawing are provided with the same designations in each case in this and are repeatedly explained only insofar as differences arise between the individual figures.
In the drawing:
A plant for storing thermal energy according to
In
Also provided in
The construction of the low-temperature heat accumulator 12, of the heat accumulator 14 and of the cold accumulator 16 in the plant according to
The cold accumulator 16 also supplies a further line 31 with the stored cold. For this line 31, a passage system—not shown in more detail—is provided in the cold accumulator 16 and is independent of another passage system (not shown either) which is connected to the line 11. The line 31 is part of a cooling circuit by means of which a cooling medium, such as nitrogen, can be precooled. By means of a pump 32, this cooling medium is circulated in the cooling circuit and also pumped through a cooling unit in the form of a thermosiphon, which is not shown in more detail. Via different valves 34, bypass lines 35, which are connected to heat exchangers 36 in each case, can be connected into the cooling circuit. The heat exchangers 36 in each case lead to the motor M, to the generator G1 and to a generator G2 in the wind power plant 22. These generators are provided with superconducting components, especially windings, comprising high-temperature superconductors. The cooling medium is sufficient to hold these windings at a temperature level which the superconducting properties maintain.
Shown in
With reference to the plant according to
In the model calculation the working gas at 1 bar and 20° C. makes its way into the (previously charged) low-temperature heat accumulator and leaves this at a temperature of 80° C. As a result of compression by means of the first fluid energy machine 13, working as a compressor, a pressure increase to 15 bar takes place and consequently also a temperature increase to 547° C. This calculation is based on the following formula
T
2
=T
1+(T2−T1)/ηc;T2s=T1π(K-1)/K,
wherein
T2 is the temperature at the compressor exit,
T1 is the temperature at the compressor inlet,
ηc is the isentropic efficiency of the compressor,
π is the pressure ratio (in this case 15:1) and
K is the compressibility, which in the case of air is 1.4.
The isentropic efficiency ηc can be assumed to be 0.85 in the case of a compressor.
The heated working gas now passes through the heat accumulator 14 where the main part of the available thermal energy is stored. While being stored, the working gas is cooled to 20° C., whereas the pressure (apart from flow-inducted pressure losses) is maintained at 15 bar. The working gas is then expanded in two series-connected stages 15a, 15b of a second fluid energy machine so that it arrives at a pressure level of one bar. In the process, the working gas is cooled to 5° C. after the first stage and cooled to −114° C. after the second stage. The basis for this calculation is also the formula specified above.
In the part of the line 11 which connects the two stages 15a, 15b of the second fluid energy machine in the form of a high-pressure turbine and a low-pressure turbine, provision is additionally made for a water separator 29. After a first expansion, this enables the air to be dried so that the air moisture which is contained in this in the second stage 15b of the second fluid energy machine 15 does not lead to icing of the turbine blades (necessary only for the case of an open circuit).
In the further process, the expanded and therefore cooled working gas extracts heat from the cold accumulator 16 and is heated to 0° C. as a result. In this way, cold energy is stored in the cold accumulator 16 and can be utilized during a subsequent energy generation. If the temperature of the working gas at the outlet of the cold accumulator 16 and at the inlet of the heat accumulator 12 is compared, then it becomes clear why the heat exchanger 17 has to be provided for the case of a closed charging circuit. In this case, the working gas can be reheated to an ambient temperature of 20° C., as a result of which heat is extracted from the environment and made available to the process. Such a measure can naturally be dispensed with if the working gas is drawn directly from the environment since this already has ambient temperature.
For the cooling, an embodiment which deviates from the variant in
By means of
The working gas is directed through the cold accumulator 16. In the process, it is cooled from 20° C. to −92° C. This measure serves for reducing the power consumption in order to operate the second fluid energy machine which works as a compressor. The power consumption is reduced correspondingly by the factor of the temperature difference in Kelvin, that is to say 293K/181K=1.62. In the example, the compressor compresses the working gas to 10 bar. During this, the temperature rises to 100° C. A compression of up to 15 bar would also be technically acceptable. The compressed working gas passes through the heat accumulator 14 and is consequently heated to 500° C., wherein the pressure reduces slightly to 9.8 bar. The working gas is then expanded by means of the first fluid energy machine which therefore works as a turbine in this operating state. An expansion to 1 bar is carried out, wherein at the outlet of the first fluid energy machine a temperature of 183° C. still prevails in the working gas.
In order to be able to also utilize this residual heat, the working gas is then directed through the low-temperature heat accumulator and additionally cooled to 130° C. as a result. This heat has to be stored in order to serve for preheating of the working gas to 80° C. in a subsequent charging process of the heat accumulator 14 and of the cold accumulator 16 (as already described above). The low-temperature heat accumulator therefore works as a temporary store and is always charged especially when the two other accumulators, i.e. the heat accumulator 14 and the cold accumulator 16, are discharged, and vice versa. As already mentioned, the functioning principle of the plant and of the method is not limited, however, if the low-temperature heat accumulator is omitted.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 206 296.3 | Apr 2012 | DE | national |
This application is the US National Stage of International Application No. PCT/EP2013/056735 filed Mar. 28, 2013, and claims the benefit thereof. The International application claims the benefit of German Application No. DE 102012206296.3 filed Apr. 17, 2012. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/056735 | 3/28/2013 | WO | 00 |
