WO2009135485A9

WO2009135485A9 - Device for converting thermal energy into electric energy

Info

Publication number: WO2009135485A9
Application number: PCT/DE2009/000657
Authority: WO
Inventors: Daniel Wolf
Original assignee: Daniel Wolf
Priority date: 2008-05-08
Filing date: 2009-05-08
Publication date: 2009-12-30
Also published as: WO2009135485A3; WO2009135485A2; DE112009001670A5

Abstract

The invention relates to a method and a device for converting thermal energy into electric energy. An electrochemical concentration cell is operated at a high temperature while supplying heat. Using a mechanical compressor, hydrogen or oxygen is compressed at a low temperature and subsequently supplied to the concentration cell by way of a heat exchanger. The partial pressure difference present produces a flow of ions, which generates an electrical current that can be tapped at the electrodes. The mechanical compressor is operated with a portion of the electric energy. The benefits of fuel cells and Stirling engines are merged in said new cycle.

Description

Device for converting thermal energy into electrical energy

description

The invention deals with the conversion of heat energy into electrical energy, in particular the generation of electricity by solar energy, by a thermoelectric cycle.

Numerous ways to convert thermal energy into electrical energy are known and are sometimes successfully applied. Frequently, steam power plants are used. By heat, steam is generated which drives turbines that are coupled to generators. Also, the Stirling engine is a long-known way to convert heat into mechanical energy and then into electricity. Theoretically, this also offers the potential to achieve the efficiency of the Carnot cycle. In practice, however, there are many limitations. Thus, the Stirling engine in particular, the necessarily high temperatures to which the moving parts are exposed, a problem dar. In addition, the ideal process in the Stirling engine due to design, including through the circular motion and the fast clock rate, which allows only an incomplete heat transfer into the working gas , far from achieved. For the production of electrical energy, fuel cells have been increasingly used for some time. These do not require heat, but convert chemical energy sources to electricity relatively efficiently and directly in an electrochemical cell. Unfortunately, conventional fuel cell systems are not suitable for converting heat into electricity. They require a chemical energy source, for example hydrogen, to promote the transport of charge carriers in an electrolyte through reduction and oxidation. Thus, these systems are not suitable for the direct extraction of electrical energy from solar radiation. Furthermore, many types of fuel cell in operation rely on a special and relatively pure supply of the fuel as a gas. In order to use in fuel cells other energy sources, such as biogas or even coal, energy-consuming process and reforming steps are necessary.

As a direct possibility, any heat source for the production of electrical energy has been the use of a pressure gradient which drives an electrochemical cell, in conjunction with a cyclic process proposed. So is in the alkali-metal Thermoelectric converter sodium or potassium converted by supplying heat in vapor form and built up a pressure which leads to a flow of ions over a solid electrolyte to the low pressure compartment, where condensation takes place. By oxidation and reduction at the two electrodes of the ceramic electrolyte, a current is generated. One of the disadvantages of this type of thermoelectric converter is that the metals are relatively corrosive. Also, the converter, due to the high boiling point of the metals, must be operated at high temperatures, which also includes a high lower temperature level, and thus leads to a relatively low efficiency in terms of the Carnot process. Furthermore, the limited conductivity of the ionic conductors in this case poses a problem. Another development in this field is the patent of Robert, E. (US 4,677,038, 1987) in which two electrochemical cells are connected in a cyclic process. A cell produces a pressure gradient by supplying current by transporting a gas through an electrolyte in ionized form associated with reduction and oxidation. A second cell is operated at a high temperature and uses the established gradient to generate electricity. In this case, the higher temperature of the second cell releases a greater voltage and thus greater amount of electrical work than is ideally required to build up the pressure gradient in the first cell. A meaningful continuation of this idea is the patent of Johnnson, L. (WO 02/11220), wherein between the two electrochemical cells in the transport of the gas additionally heat exchange takes place, which allows an approximation of the efficiency of the theoretical Ericsson process. Despite the merits of this latter invention, there are some disadvantages. The electrochemical cell for building up the pressure gradient is operated at low temperatures. In this area, efficient cells are usually only possible by using special and elaborate electrodes and membrane materials.

The aim of the present invention is a device which allows the simplest possible means an efficient generation of electric power from heat. In particular, the generation of electrical energy from solar radiation with a high efficiency is the aim of the invention.

This is achieved by a closed system in which an increased partial pressure of hydrogen or oxygen is generated by a mechanical compressor in a room, at the same time having a reduced pressure space. The mechanical Compression takes place with dissipation of heat by liquid or gas cooling at a low temperature. At a high temperature, a concentration cell consisting of a solid ion conductor is operated while supplying heat. The electrolyte forms a boundary between the room with high and the space with lower partial pressure. At the electrolyte electrodes are each attached to the surfaces which face the two rooms. This electrochemical concentration cell generates electric energy by supplying heat at a high temperature level by an ion flux caused by the partial pressure difference. The heat of the hydrogen or oxygen removed from the cell in the low pressure chamber is used to heat the compressed hydrogen or oxygen supplied in the high pressure chamber of the cell.

The advantages achieved by the invention are, in particular, that in comparison to the pure mechanical heat engine no moving parts are exposed to high temperatures. Furthermore, the process runs continuously at high temperatures. Since, in contrast to the so-called Amtec converter, hydrogen or oxygen is used instead of alkali metals, there are advantages in terms of electrolyte properties, thermodynamics, phase change, corrosion and lifetime of the electrodes. Furthermore, the use of simple, yet efficient, electromechanical components for compression presents a potential for reducing costs over the construction of a partial pressure differential by electrolysis cells. Also, using efficient electric machines and intensive near-isothermal cooling in compression can provide very good overall efficiency be achieved and which can expect even better values compared to expensive polymer electrolyte cells. The invention thus forms a combination of the proven simple mechanical principle of the Stirling engine, with the benefits of direct, known from fuel cells, electrochemical energy conversion, which requires no moving parts at high temperature.

In the following the invention is illustrated by means of an exemplary embodiment and with reference to the two drawings in which

Fig. 1 shows the schematic structure of the device Fig. 2 shows an embodiment of the device, wherein solar heat is used to generate electricity shows.

Fig.3 shows the device corresponding to the theoretical Ercisson cycle.

As can be seen from the drawings, the device according to the invention for the conversion of heat into electrical energy comprises a solid electrolyte, which gas-tightly separates two chambers 1, 2 in a closed system. In this case, a pressure difference between the two spaces is generated within the system by a mechanical compressor 3. At the electrolyte 4 electrodes 5, 6 are mounted. At the electrodes there is a reduction and oxidation. The working gas 7, which is hydrogen or oxygen, or contains portions of hydrogen and oxygen, is ionized and can pass through the electrolyte 4 at high temperature. The driving force of this process is the partial pressure difference on both sides. The compressor 3 is operated by cooling 8 at a temperature T2 which is below the operating temperature Tl of the electrochemical concentration cell. The process in the electrochemical cell is carried out with the supply of heat energy QTl. This step in the cycle corresponds to the isothermal expansion in a Stirling or exact Ericsson engine. The heat of the gas 2, which leaves the electrochemical cell in the hot and relaxed state, is transferred in a heat exchanger 9 to the compressed gas 1, which is supplied to the cell. This process corresponds to the internal isobaric heating and isobaric cooling in the Ericsson process. The open circuit voltage of the cell is described by the Nernst equation:

No-load voltage = R * Tl / z * F In (pl / p2)

The work necessary for the isothermal compaction is described by the following equation:

Compaction work (heat dissipation) = n * R * T2 * ln Vl / V2

The maximum efficiency of the device is dictated by the Camot process. Efficiency = 1 - T2 / T1

The mechanical work of the compressor under isothermal conditions, at the lower temperature, corresponds to the amount of heat to be dissipated by the cooling. An increase in the pressure difference leads to a higher cell voltage. The current correlates with the amount of gas to be compressed and circulated through the electrolyte. The greater the temperature difference between the operating temperatures of the electrochemical cell and the compressor, the greater the maximum efficiency. The electrochemical cell must be continuously supplied with thermal energy during operation. The passage of the ions in the electrolyte from the high pressure side to the low partial pressure side corresponds to the isothermal expansion of the gas in the Stirling engine. In both cases, energy is supplied in the form of heat.

Advantageous embodiments of the invention are specified in claims 2-10. In order to achieve high efficiency, internal exchange of heat in the device is necessary. A heat exchanger according to the countercurrent principle transfers the heat of the hydrogen or oxygen which flows out to the gas flow under pressure to the electrochemical cell. This allows a good approximation to the theoretical Carnot process. The further embodiment of the invention according to claim 3, wherein the compressor is designed as a driven by an electric machine reciprocating compressor, allows an efficient construction of the compressor stage with proven means. In order to achieve a high efficiency by a possible isothermal compression, uses an advantageous embodiment of the invention in an efficient cooling, which is accomplished by directly introducing the coolant into the water or oxygen gas to be compressed. For example, water can be injected into the cylinder chamber of the compressor at a temperature of 3O ⁰ C. In claim 3, a very advantageous development is mentioned, wherein the compression process is carried out very slowly. An operating frequency below 6 Hz achieves the most effective and reversible heat dissipation and thus compression. This positively influences the overall efficiency. It is also conceivable that more compressor stages are used. The use of hydrogen as a working gas in conjunction with a proton conductor as the electrolyte membrane is mentioned in claim 6. Advantages of hydrogen include the reducing effect and the high thermal conductivity. In the Scientific literature has been discussed in particular proton conductors of the perovskite type. In the case of oxygen as the working gas, the use of yttrium-stabilized zirconia, which has a high ionic conductivity, is suitable. As a further embodiment, the use of a variable in the speed electronic control of the electric motor of the compressor is given in claim 7. Whereby the electronics ideally also have the possibility to buffer the kinetic energy of the piston during its braking in capacitors and again to use for the acceleration.

The drive of the compressor by a linear motor has the advantage that no gear is necessary. Ideally, a linear motor is used in synchronous design. As an advantageous embodiment of the invention, the generation of a water vapor pressure in the system is called by the cooling water in claim 10. Numerous proton-conductive materials, such as yttrium-doped barium zirconate, which may contain small amounts of cerium oxide and are considered suitable, generally require some water vapor pressure in the environment to achieve conductivity. The cooling water with a corresponding temperature provides this vapor pressure.

There now follows an explanation of the invention with reference to an exemplary embodiment which is illustrated by Fig. 2.

By nachfuhrbare mirror 10 solar radiation is focused on a ceramic receiver 11, which is modeled on the optimal absorption of a cavity. In the ceramic structure, two separate cavity systems 12, 13 are introduced with a large surface area. The partitions in between completely or partly consist of an ion conductor 14, for example of yttrium-doped barium zirconate, which is densely sintered and has no gas permeability. On the surface electrodes 15 of ceramic, metal or a mixture derselbigen are applied, such as a nickel cermet. Via a metallic or ceramic conductor 16, the respective electrodes are in communication and the current is thus collected and removed. The materials used ideally have a similar thermal expansion coefficient as well as high thermal conductivity to avoid damage due to temperature fluctuations. In a mechanical compressor 17, which is controlled by a synchronous linear motor 18, the working gas hydrogen is compressed from 0.5 bar to 30 bar. The Working frequency is about one stroke per second which makes an ideal heat dissipation through the cooling water 19, which is sprayed into the piston chamber with a temperature of 30 ⁰ C possible. The cooling water is then taken out at about 40 ° and circulated to a cooling tower or the like. The cooling water also provides a corresponding vapor pressure in the system, which maintains the proton conductivity of the electrolyte. Accumulator 19, 20 at ambient temperature are the compressor 17 upstream and downstream and take over a Windkesselfunktion. The compressed hydrogen gas 18 is heated by a countercurrent heat exchanger 19, by the flowing out of the cell at about 1000 ° C gas 21, also approximately 1000 °, while cools the low-pressure hydrogen from. The cell is operated as constant as possible at a temperature of 1000 ⁰ C. The pressure difference leads to a flow of ionized gas particles over the electrolyte membrane 14 and thus to a voltage which can be dissipated and used via the electrodes 15 and current collector 16. Part of the energy is needed for the operation of the compressor. However, the energy required for compaction is significantly lower than the energy released by the ion current at high temperature, corresponding to the isothermal expansion. The compressor is controlled by a computer-controlled power electronics 22. This also has the task to temporarily store the kinetic energy of the piston by electromagnetic braking and the charging of capacitors and to use for re-acceleration of the piston. To save costs, the compressor piston can alternatively be driven by a crankshaft with flywheel and a conventional electric motor with reduction.

The invention can also be used to convert parabolic mirrored solar energy into electricity. In large power plants, a central compression and cooling of the working gas offers. This can then be forwarded to numerous individual parabolic mirrors, which are equipped at the focal point with ceramic concentration cells for generating electricity. Also, the use of the invention in parabolic trough power plants is possible, in which case ideally an elongated shaped cell is used in the focal point. Because of the smaller concentration ratio in this case, the operation at slightly lower temperatures offers. Beyond solar use, the present invention also enables the efficient use of combustion heat of any kind.

Claims

Apparatus for generating electrical energy from thermal energy claims

1. A device for generating electrical energy from heat consisting of a closed system in which two spaces with different partial pressure of hydrogen or oxygen, said pressure difference is obtained by mechanical compression with removal of heat by liquid or gas cooling at a low temperature level, a Concentration cell consisting of a solid ion conductor, which forms a boundary between the spaces and wherein at its respective surfaces facing the surfaces electrodes are mounted, said concentration cell generates heat while supplying heat at a high temperature level by the flow of ions electrical energy, wherein heat from hydrogen or oxygen which escapes from the concentration cell in the space with low partial pressure and is led to the mechanical compression, on the gas flow, which is supplied in the space high partial pressure of the concentration cell practice will bear.

2. Apparatus according to claim 1, characterized in that the heat transfer from the hydrogen or oxygen, which flows from the cell at low pressure, takes place on the incoming compressed gas flow in a heat exchanger according to the countercurrent principle.

3. Apparatus according to claim 1, characterized in that the compressor is a reciprocating compressor and is driven by an electric motor.

4. Apparatus according to claim 1, characterized in that in the compressor, a liquid coolant is introduced, which is in direct contact with hydrogen or oxygen.

5. Apparatus according to claim 3, characterized in that the reciprocating compressor has a working frequency less than 6 sec ^'1

6. The device according to claim 1, characterized in that the solid ion conductor is a proton conductor and hydrogen is used as gas.

7. The device according to claim 2, characterized in that the electric motor is a synchronous motor, which is controlled by a programmable electronic control, wherein a storage of the kinetic energy of the piston in an energy storage is possible.

8. The device according to claim 2, characterized in that the electric motor is a synchronous linear motor.

9. The device according to claim 1, characterized in that the electrolyte consists of more than 90% of 2 - 25% yttrium-doped barium zirconate or stabilized zirconium oxide.

10. The device according to claim 7, characterized in that a water vapor pressure in the range of 0.01 bar to 0.5 bar is present within the system by cooling water introduced into the compressor.