Heat engine and method for utilizing waste heat The invention relates to a heat engine according to the preamble of claim 1 and to a method for utilizing waste heat according to the preamble of claim 11.

Heat engines based on the so called organic Rankine cycle (ORC) are utilized for exploiting shallow thermal gradients common with industrial waste heat usage, geothermal or solar thermal energy. They employ a volatile organic working fluid such as low chain- length alkanes, halogenated hydrocarbons or carbon dioxide, which is isobarically evaporated, isentropi- cally expanded driving a turbine and isobarically condensed after passage through the turbine.

While such heat engines excel at extracting mechanical energy from low power heat sources, they do not offer particularly high thermal efficiency. In typical applications, about 10% to 15% of the thermal energy can be converted to mechanical and further to electrical energy, while 85%- 90% are discarded as waste heat and dissipated to the environment via the condenser .

The object of the present invention is therefor to provide a heat engine according to the preamble of claim 1 and a method according to the preamble of claim 11 which provide an improved total efficiency in the usage of low-energy heat sources such as industrial waste heat.

This object is achieved by a heat engine according to claim 1 and a method according to claim 11. In such a heat engine for utilizing waste heat from a technical process, a working fluid is cycleable in a first liquid closed loop through a first heat exchanger thermally coupled to a fluid carrying the waste heat for transferring the waste heat to the working fluid, a turbine for generating mechanical energy, a second heat exchanger for condensing the working fluid and a pump for transporting the working fluid back to the first heat exchanger. According to the present inven- tion, the second heat exchanger is thermally coupled to a water treatment apparatus.

Such a thermally driven water treatment apparatus can for example be used for evaporative purification of salty or brack- ish water. While the temperature gradient between the condenser of the heat engine and the environment is usually too low to allow further conversion to mechanical energy, the invention enables the usage of waste heat for water treatment, thereby increasing the total efficiency of waste heat utili- zation.

In a preferred embodiment of the invention, the working fluid is preheatable by means of a regenerator thermally coupling a turbine outlet port with an inlet port of the first heat ex- changer. By employing such a regenerative heat exchanger, the internal thermal efficiency of the heat engine can be increased, since part of the waste heat is transferred back to the working fluid before the fluid enters the first heat exchanger. The internal efficiency can thus be raised to about 20%.

However, the use of regenerative heat exchangers does for practical applications not significantly raise the total thermal efficiency, since the preheating of the working fluid leads to a reduction in thermal energy transferred from the heat source to the working fluid within the first heat exchanger .

It is therefore advantageous to thermally couple the fluid carrying the waste heat to a third heat exchanger incorporated within a second liquid closed loop comprising the third heat exchanger for transferring the waste heat to the working fluid, a second turbine for generating mechanical energy, a fourth heat exchanger for condensing the working fluid and a pump for transporting the working fluid back to the third heat exchanger. The addition of a second closed liquid loop - essentially a second heat engine - allows for greater utilization of the waste heat. The second liquid loop can be coupled to the waste heat stream upstream or downstream from the first liquid loop. Furthermore, it is possible to also couple the fourth heat exchanger, that is, the condenser of the second liquid loop, to the same or to an additional water treatment apparatus to further improve heat utilization and thereby efficiency . The second liquid closed loop can also comprise a regenerator which thermally couples the working fluid at the outlet port of the second turbine to the working fluid at the inlet port of the third heat exchanger, in order to raise the internal thermal efficiency as described above.

Utilization of two liquid closed loops allows for choosing different working fluids within the first and second loop. Since part of the heat energy of the waste heat stream is removed by the closed liquid loop situated upstream with regard to the waste heat stream, the closed liquid loop located downstream necessarily has to operate at a lower process temperature. Choosing a different working fluid for the latter loop can increase the overall thermal efficiency by optimizing working parameters such as boiling point, saturation va- por curve, heat of vaporization and the like separately for the different loops.

It is, however, also possible to use the same working fluids for both liquid closed loops. This allows a more simple con- struction, since now the loops can be ^" coupled and certain components, in particular the condenser, can be jointly used. In this embodiment, the second closed liquid loop does not comprise its own condenser. Rather, the inlet port of the se- cond heat exchanger, that is, the condenser of the first liquid loop, is coupled on its inlet side to the first and second turbine outlet, or, in case regenerators are used, to the respective regenerator outlets. The outlet of the second heat exchanger is coupled to the pumps of the first and second liquid loop. This improves the thermal efficiency, since the waste heat of both liquid loops is transferred via the second heat exchanger to the water treatment apparatus. As an alternative to coupling two liquid closed loops sequentially to the waste heat stream, the second loop can be coupled directly to the first by means of a fifth heat exchanger located between the first turbine and the second heat exchanger within the first loop. Via this fifth heat exchanger, waste heat is transferred from the first loop's working fluid after its passage through the first turbine to the working fluid of the second loop, which then sequentially cycles through the second turbine, fourth heat exchanger and second pump back to the fifth heat exchanger. In this embodiment, the water treatment apparatus can either be directly coupled to the first heat exchanger or alternatively be indirectly coupled to the first loop via the second loop. In the latter case, waste heat is transferred to the water treatment apparatus via the fourth heat exchanger, i.e. the condenser of the second loop.

The invention further relates to a method for utilizing waste heat from a technical process, in which the waste heat is transferred to a working fluid within a first liquid closed loop by means of a first heat exchanger, said working fluid being evaporated in the first heat exchanger, expanded via a turbine, condensed in a second heat exchanger and pumped back to the first heat exchanger. According to the invention, heat from the second heat exchanger is transferred to a water treatment apparatus. As elaborated above, this allows for improved utilization of waste heat, thereby increasing the overall efficiency of the process . Further embodiments of the inventive method relate to the corresponding embodiments of the above-described heat engine. The advantages described for the respective designs apply likewise to the corresponding implementations of the method. In particular, the method according to the invention can be performed utilizing any form of the heat engine described above .

Further advantages, features and details of the invention appear from the following description of an embodiment as well as based on the drawings, which show in:

FIG 1 a schematic representation of a nonregenerative organic Rankine cycle heat engine according to the state of the art;

FIG 2 a schematic representation of a nonregenerative organic Rankine cycle heat engine according to the state of the art;

FIG 3 a schematic representation of a combined regenerative - non- regenerative organic Rankine cycle heat engine integrated with a thermally driven water treatment system according to an embodiment of the invention;

FIG 4 a schematic representation of a combined regener tive - non- regenerative organic Rankine cycle he engine integrated with a thermally driven water treatment system and a joint condenser according to an embodiment of the inven ion;

FIG 5 a schematic representation of a combined nonregenerative organic Rankine cycle heat engine in- tegrated with a thermally driven water treatment system according to an embodiment of the invention.

A conventional organic Rankine cycle heat engine 10 as de- picted in FIG 1 comprises an evaporator 12 to transfer heat from a waste heat stream 14 to a working fluid, for example a low chain-length alkane, thereby evaporating the working fluid. The working fluid is subsequently expanded via a turbine 16 driving a generator 18 to generate electrical energy. Af- ter passage through the turbine 16, the working fluid is condensed in a condenser 20 and pumped back to the evaporator 12 by means of a pump 22. The residual heat of the working fluid after passage through the turbine 16 is transferred via a cooling circuit 24 to a heat sink 26 and ultimately dissipat- ed into the environment.

While such an organic Rankine cycle is useful for converting thermal energy to electrical energy in case of only mild thermal gradients, the method only reaches rather small effi- ciency quotients of about 10%. At least the internal thermal efficiency can be improved by incorporating a regenerator 28, as shown in FIG 2.

Such a regenerator 28 is a heat exchanger thermally coupling the working fluid at the turbine outlet 30 with the working fluid at the evaporator inlet 32. This achieves preheating of the working fluid before it enters the evaporator. In practice, no large gains in total thermal efficiency are possible, however, since the preheated working fluid extracts less energy from the waste heat stream 14.

An improved heat engine 34 according to the present invention is shown in FIG 3. The heat engine 34 consists of a first liquid closed loop 36, working on the basis of a non- regenerative organic Rankine cycle, and a second liquid closed loop 38, working on the basis of a regenerative organic Rankine cycle. Instead of only dissipating waste heat via a heat sink 26, the condenser 20 of the first liquid closed loop 36 is coupled to a thermal water treatment apparatus 40, which utilizes the waste heat to evaporate salty or brackish water and separating it into fresh water and brine, which can be reclaimed via ports 42 and 44, respectively. Excess heat in the cooling circuit can be dissipated by means of the heat sink 26. Since the residual heat of the working fluid after passage through the turbine 16 is not wasted, but rather used for water purification, the total efficiency of the heat engine 34 is significantly improved.

A further improvement is gained by incorporation of the second liquid closed loop 38, which is coupled to the waste heat stream upstream from the first loop's 36 evaporator 12 by means of an evaporator 46. Like in the first liquid closed loop 36, heat is transferred in the evaporator 46 to a working fluid, which is evaporated and subsequently expanded over a turbine 48 coupled to an electrical generator 50. The tur- bine outlet port 52 is thermally coupled to the evaporator inlet port 54 by means of a regenerator 56, so that residual heat of the working fluid after passage through the turbine 50 is used to preheat the working fluid before it enters the evaporator 46. The working fluid is then channeled through a condenser 58 coupled to a cooling circuit 60 and subsequently pumped back through the regenerator 56 to the evaporator 46.

Since the second liquid closed loop 38 works on the basis of a regenerative organic Rankine cycle, only small amounts of heat have to be removed from the working fluid in the evaporator 58. Coupling the cooling circuit 60 to the water treatment apparatus 40 is therefore not essential - residual heat can be transferred to the environment via heat sink 62. The utilization of two liquid closed loops, essentially two distinct heat engines, allows the extraction of a higher amount of thermal energy from the waste heat stream 14. In combination with the transfer of residual waste heat to the water treatment apparatus 40, a particularly high efficiency can be achieved.

The complete separation of the liquid closed loops 36, 38 furthermore makes it possible to employ different working fluids in the loops 36, 38, which can be optimized separately regarding their boiling point and saturation vapor curve.

Should the heat difference in the waste heat stream 14 be- tween the evaporators 12, 46 be rather small, however, it can be advantageous to use the same working fluid in both loops 36, 38. In that case, it is possible to use only one condenser for both loops 36, 38, as shown in FIG 4. In this embodiment, the outlet 30 of the first turbine 16 and the outlet 66 of the regenerator 56 are coupled to the inlet 68 of the first condenser 20. The outlet 70 of the condenser 20 is in turn coupled to the inlets 72, 74 of both pumps 22, 64. All the residual excess heat of the working fluid can therefore be transferred to the water treatment apparatus 40 via the cooling circuit 24.

An alternative embodiment, as shown in FIG 5, is based on indirectly coupling the second liquid closed loop 38 to the waste heat stream 14. Heat from the waste heat stream 14 is first transferred to the working fluid of the first liquid closed loop 36 via the evaporator 12. After passage through and expansion in the turbine 16, the working fluid passes through an additional heat exchanger 76, transferring heat to the working fluid of the second liquid closed loop 38, thereby evaporating the latter.

The working fluid of the first liquid closed loop subsequently is channeled through the condenser 20, where waste heat is transferred to the cooling circuit 60 and dissipated via heat sink 62, and then pumped back to the evaporator 12 via the pump 22. The evaporated working fluid of the second liquid closed loop 38 is expanded in the turbine 48 and condensed in the condenser 58. Waste heat is transferred from the condenser 58 to the water treatment apparatus 40 via the cooling circuit 24 and used for evaporating and purifying salty or brackish water. The working fluid is then pumped back to the heat exchanger 76, closing the loop.

List of reference signs

10 heat engine

12 evaporator

14 waste heat stream

16 turbine

18 generator

20 condenser

22 pump

24 cooling circuit

26 heat sink

28 regenerator

30 turbine outlet

32 evaporator inlet

34 heat engine

36 first liquid closed loop

38 second liquid closed loop

40 water treatment apparatus

42 port

44 port

46 evaporator

48 turbine

50 generator

52 turbine outlet port

54 evaporator inlet port

56 regenerator

58 condenser

60 cooling circuit

62 heat sink

64 pump

66 regenerator outlet

68 condenser inlet

70 condenser outlet

72 pump inlet pump inlet heat exchange