Method and Device for Converting Thermal Energy Into Mechanical Work

Abstract

The invention relates to a method for converting thermal energy into mechanical work. Said method comprises the following steps which are performed as a cycle: A liquid work medium is fed from a supply reservoir (1) to a work container (3); the work medium in the work container (3) is heated by a first heat exchanger (5); a sub-amount of the work medium flows from the work container (3) to a pneumatic-hydraulic-converter (8), a hydraulic medium from the pneumatic-hydraulic-converter (8) is compressed in a work machine (9) in order to convert the hydraulic work of the hydraulic medium into mechanical work; the work medium from the pneumatic-hydraulic-converter (8) is fed back into the supply reservoir (1) and the hydraulic medium is returned into the pneumatic-hydraulic-converter (8). The invention also relates to a device for carrying out said method.

Description

The present invention relates to a method and to an apparatus for converting thermal energy into mechanical work.

Many kinds of cyclic processes and apparatus for converting thermal energy into mechanical work and, where required, from there to electric power are known. These processes are for example steam power processes, Sterling processes or the like. One possibility of utilizing such methods is to increase the efficiency of internal combustion engines by making use of the waste heat. The problem here however is that the available temperature levels are quite disadvantageous since the cooling circuit of internal combustion engines usually operates at temperatures of about 100° C. A similar problem arises when heat from solar power plants is to be converted into mechanical work.

A special solution for such a thermal power process is shown in the document WO 03/081011 A. In this document, there is described a method by which a hydraulic fluid is pressurized by heating a working fluid in a plurality of bladder accumulator means, said hydraulic fluid being worked off in a working machine. Although such a method is working in principle, it has been found that its efficiency is moderate and that, compared to the amount of energy that can be generated, equipment expense is quite high.

A discontinuously operated method capable of generating work through heat conversion at moderate efficiency is further known from U.S. Pat. No. 3,803,847 A.

It is the object of the present invention to configure a method of the type mentioned herein above in such a manner that high efficiency is achievable even under thermally disadvantageous conditions, with the equipment expense being as low as possible.

In accordance with the invention, such a method consists of the following steps, which are performed as a cyclic process:

• • supplying a liquid working fluid from a storage reservoir to a work tank;
  • heating the working fluid in the work tank via a first heat exchanger;
  • allowing a fraction of the working fluid from the work tank to overflow into a pneumatic-hydraulic converter, this causing a hydraulic fluid to be urged from the pneumatic-hydraulic converter into a working machine for conversion of the hydraulic work of the hydraulic fluid into mechanical work;
  • returning the working fluid from the pneumatic-hydraulic converter into the storage reservoir by recirculating hydraulic fluid into the pneumatic-hydraulic converter

In the first step, a working fluid having an appropriate vapor pressure curve such as for example R134a, that is 1,1,1,2-tetrafluoroethane, is drawn from a storage reservoir. The working fluid in this storage reservoir is in an equilibrium state between a liquid phase and a gaseous phase. The pressure is hereby chosen such that this equilibrium is maintained. In the case of R134a and of an ambient temperature of about 20° C., this first-pressure will be about 6 bar. The working fluid is transferred to a work tank in which it is preferred that a second, higher pressure prevails. The second pressure is for example 40 bar. The energy expense for the transfer can be minimized if, in a preferred manner, only liquid working fluid is transferred to the work tank by pumping. In the second step, the working fluid is heated in the work tank. Heating causes the pressure to increase even more and the working fluid evaporates partially. Heating preferably occurs through waste heat, for example from an internal combustion engine. If the working fluid is heated to a temperature of 100° C., the waste heat can be optimally utilized.

In the third step, the working fluid is allowed to overflow into a pneumatic-hydraulic converter. This can occur after the second step, i.e., the heat is completely supplied first and the connection between the work tank and the pneumatic-hydraulic converter is established thereafter. These steps may however also be performed in part or in whole simultaneously, i.e., the fluid in the work tank is heated while it is flowing into the pneumatic-hydraulic converter. In this way, the efficiency can be optimized since the cooling effected by the expansion of the working fluid is immediately accommodated. Moreover, the cycle time is shortened. In the pneumatic-hydraulic converter, which is for example implemented as a bladder accumulator, the inflowing working fluid displaces a hydraulic fluid that is present in the hydraulic chamber and is being worked off in a suited working machine, for example a hydraulic motor, in order to produce mechanical work that may in turn be used to produce electrical energy.

In the fourth step, the pneumatic-hydraulic converter is re-filled with hydraulic fluid through a small pump, with the working fluid being displaced and recirculated into the storage reservoir. Where appropriate, the working fluid is thereby directed through a second heat exchanger, this making it possible to adapt the temperature to the ambient temperature.

After this fourth step, the cyclic process is continued with the first step.

The efficiency and the performance of the system can be optimized if the possible phase transitions are made use of accordingly. More specifically, in the first step, the working fluid should be moved in the liquid state only, whereas in the third step, only the gaseous phase will be transferred to the pneumatic-hydraulic converter.

Preferably, there is provided that during recirculation of the working fluid from the pneumatic-hydraulic converter into the storage reservoir the connection between the work tank and the pneumatic-hydraulic converter is interrupted. This permits to minimize overflow losses.

The efficiency may be optimized if the working fluid is cooled while being supplied from the storage reservoir to the work tank. Cooling can occur through an ambient heat exchanger, meaning through a current cooler, but it is also possible to use cold produced by the second heat exchanger provided it is not needed for some other purpose, for example for an air conditioning system or a cooling aggregate.

A particular effect of benefit is achieved if the hydraulic fluid is kept at a temperature that corresponds to the mean temperature of the working fluid in the pneumatic-hydraulic converter. This way, undesirable temperature compensating effects can be avoided.

As already explained, it is possible that the working fluid be directed from the pneumatic-hydraulic converter through a second heat exchanger. Depending on the way of conducting the method, low temperatures occasioned by the expansion of the working fluid may be generated in the second heat exchanger. These low temperatures can be used for cooling in order to economize the energy needed there.

Another improvement of the production of low temperatures can be achieved by causing the working fluid from the pneumatic-hydraulic converter to expand to an expansion pressure that is lower than the first pressure in the storage reservoir and is next compressed to the first pressure.

The invention further relates to an apparatus for converting thermal energy to mechanical work, said apparatus having a storage reservoir, a work tank and a working machine for converting hydraulic work into mechanical work.

In accordance with the invention, there is provided that the work tank is connected to a first heat exchanger for heating the working fluid, that the work tank is further connected to a pneumatic-hydraulic converter that transfers the pressure of the working fluid to a hydraulic fluid and that there is provided a recirculation line for recirculating the working fluid from the pneumatic-hydraulic converter into the storage reservoir.

In a particularly preferred implementation variant, there is provided that a plurality of work tanks and pneumatic-hydraulic converters are connected in parallel.

In practical implementation, five of the apparatus illustrated in FIG. 1 are for example arranged parallel to each other in a side-by-side relationship and operated in a time-staggered fashion as this is for example the case in a five-cylinder internal combustion engine. This permits to achieve continuous operation without noteworthy cyclic fluctuations.

The method of the invention and the apparatus of the invention will be discussed in greater detail herein after with reference to the circuit diagram of FIG. 1, which illustrates the major component parts of the system. FIG. 2 shows a typical vapor pressure curve of a working fluid.

A storage reservoir 1 holds a working fluid; a coolant such as R134a can be utilized for example. The working fluid in the storage reservoir 1 is in phase equilibrium at ambient temperature and at a pressure of about 6 bar. The storage reservoir 1 is connected to a work tank 3 through a feed pump 2, this connection being switchable through a valve 4. In the work tank 3 there is disposed a first heat exchanger 5 that serves to heat the working fluid in the work tank 3. Heat exchanger 5 is supplied with waste heat from an internal combustion engine that has not been illustrated herein via a booster pump 6, with water at 100° C. being directed through the first heat exchanger 5 for example. Through an overflow line 7, the work tank 5 communicates with a first working chamber 8a of a pneumatic-hydraulic converter 8 that is configured to be a bladder accumulator. The first working chamber 8a is separated from a second working chamber 8b by a flexible membrane 8c that separates the two working chambers 8a, 8b while allowing for pressure compensation. The second working chamber 8b of the pneumatic-hydraulic converter 8 communicates with a hydraulic circuit consisting of a working machine 9 having a generator 10 flanged thereon, an oil tank 20, a recirculating pump 17 and a third heat exchanger 11. The third heat exchanger 11 is supplied from a pump 12. Another work line 19 connects the first working chamber 8a of the pneumatic-hydraulic converter 8 to a second heat exchanger 16 that communicates through a booster pump 14 with the storage reservoir 1. For the rest, the lines 7, 19 may be closed selectively by valves 7a, 19a.

The mode of operation of the apparatus of the invention will be explained in closer detail herein after:

In a first step, liquid working fluid is transferred from the storage reservoir 1 into the work tank 3 via the feed pump 2, with the pressure being increased from 6 bar to 40 bar.

After the work tank 3 is completely filled with liquid working fluid, the valve 4 is closed and heating through the first heat exchanger 5 occurs. This heating constitutes the second step. Waste heat from another process can be used therefor.

By heating the working fluid to 100° C., part of said fluid evaporates in the work tank 3 and this vapor is transferred in a third step, through the line 7 with the valve 7a being open, into the first working chamber 8a of the pneumatic-hydraulic converter 8. The pressure drop is compensated by further heating through the first heat exchanger 5. Simultaneously, the membrane 8c of the pneumatic-hydraulic converter 8 is displaced toward the second working chamber 8b so that hydraulic fluid is urged through the working machine 9 driving the generator 10. The third step ends as soon as the second working chamber 8b of the pneumatic-hydraulic converter 8 has largely emptied.

In a fourth step, hydraulic fluid is recirculated via the pump 17 from the tank 20 into the second working chamber 8b of the pneumatic-hydraulic converter 8 and the working fluid is directed from the first working chamber 8a, through the valve 19a in the line 19, which has opened in the meantime, through the second heat exchanger 16 and is expanded. A booster pump 14 recirculates the working fluid back into the storage reservoir 1. As denoted by the arrow 21, the heat absorbed by the working fluid in the second heat exchanger 16 can be evacuated as cooling capacity for operating a cooling system or an air conditioning system. A partial flow through a heat exchanger 15 may also be used for cooling the working fluid during compression, though.

FIG. 2 illustrates a typical vapor pressure curve of a working fluid adapted for use in the cyclic process described herein above. Said working fluid is R134a, which is known to be a coolant, meaning 1,1,1,2-tetrafluoroethane. As can be seen, at ambient temperature and at a pressure of about 6 bar, the liquid phase is in equilibrium with the gaseous phase. At a temperature of 100° C., this equilibrium pressure is about 40 bar.

With simple equipment structure the present invention allows for optimal use of waste heat from other processes, like for example from the operation of an internal combustion engine.

Claims

1. A method for converting thermal energy into mechanical work, said method involving the following steps that are performed as a cyclic process:

supplying a liquid working fluid from a storage reservoir (1) to a work tank (3);

heating the working fluid in the work tank (3) via a first heat exchanger (5);

allowing a fraction of the working fluid from the work tank (3) to overflow into a pneumatic-hydraulic converter (8), this causing a hydraulic fluid to be urged from the pneumatic-hydraulic converter (8) into a working machine (9) for conversion of the hydraulic work of the hydraulic fluid into mechanical work; and

returning the working fluid from the pneumatic-hydraulic converter (8) into the storage reservoir (1) by recirculating hydraulic fluid into the pneumatic-hydraulic converter (8).

2. The method as set forth in claim 1, comprising compressing the working fluid from a first, lower pressure in the storage reservoir (1) to a second, higher pressure in the work tank (3).

3. The method as set forth in claim 2, comprising transferring the working fluid in a liquid form from the storage reservoir (1) to the work tank (3).

4. The method as set forth in claim 3, wherein the working fluid evaporates partially while being heated in the work tank (3) and is directed in the gaseous state from the work tank (3) into the pneumatic-hydraulic converter (8).

5. The method as set forth in claim 4, comprising heating isochorically the working fluid in the work tank (3).

6. The method as set forth in claim 5, wherein the connection between the work tank (3) and the pneumatic-hydraulic converter (8) is interrupted by a valve (7a) while the working fluid is being returned from the pneumatic-hydraulic converter (8) into the storage reservoir (1).

7. The method as set forth in claim 6, comprising cooling the working fluid by a heat exchanger (15) while it is being supplied from the storage reservoir (1) into the work tank (3).

8. The method as set forth in claim 7, wherein the hydraulic fluid is maintained by a heat exchanger at a temperature that corresponds to the mean temperature of the working fluid in the pneumatic-hydraulic converter (8).

9. The method as set forth in claim 8, comprising directing the working fluid from the pneumatic-hydraulic converter (8) through a second heat exchanger (16).

10. The method as set forth in claim 9, comprising expanding the working fluid coming from the pneumatic-hydraulic converter (8) to an expansion pressure that is lower than the first pressure in the storage reservoir (1) and compressing the working fluid to the first pressure thereafter.

11. An apparatus for converting thermal energy to mechanical work, said apparatus comprising a storage reservoir (1), a work tank (3), and a working machine (9) for converting hydraulic work into mechanical work, wherein the work tank (3) communicates with a first heat exchanger (5) in order to heat the working fluid, that the work tank (3) is further connected to a pneumatic-hydraulic converter (8) that transfers the pressure of the working fluid to a hydraulic fluid and that there is provided a recirculation line for recirculating the working fluid from the pneumatic-hydraulic converter (8) into the storage reservoir (1).

12. The apparatus as set forth in claim 11, including a feed pump (2) for pumping the working fluid from the storage reservoir (1) into the work tank (3).

13. The apparatus as set forth in claim 12, wherein the first heat exchanger (5) is mounted in the work tank (3).

14. The apparatus as set forth in claim 13, wherein the working machine (9) is a hydraulic motor.

15. The apparatus as set forth in claim 14, wherein the pneumatic-hydraulic converter (8) a bladder accumulator.

16. The apparatus as set forth in claim 15, including a second heat exchanger (16) interposed between the pneumatic-hydraulic converter (8) and the storage reservoir (1).

17. The apparatus as set forth in claim 16, wherein the second heat exchanger (16) is a condenser.

18. The apparatus as set forth in claim 17, including a booster pump downstream of the second heat exchanger (16).

19. The apparatus as set forth in claim 18, wherein the work tank (3) is an evaporator.

20. The apparatus as set forth in claim 19, including a third heat exchanger (11) in the circuit of the hydraulic fluid.

21. The apparatus as set forth in claim 20, including an internal combustion engine having a cooling system that communicates with the work tank (3).

22. The apparatus as set forth in claim 21, including a plurality of work tanks (3) and of pneumatic-hydraulic converters (8) connected in parallel.