WO2007025517A1

WO2007025517A1 - Device and method for the conversion of thermal energy into electric power

Info

Publication number: WO2007025517A1
Application number: PCT/DE2006/001502
Authority: WO
Inventors: Günter NOETZEL
Original assignee: Webasto Ag
Priority date: 2005-08-29
Filing date: 2006-08-29
Publication date: 2007-03-08
Also published as: DE102005040866B3

Abstract

The invention relates to a device and a method for converting thermal energy into electric power. The heat generated during operation of a parking heater is used in part to evaporate a working medium (12). Expansion work that is converted into electric power in a generator (26) is performed as a result of the working medium interacting with a movable element (18, 20, 22, 24). Disclosed is a cyclic process which is performed without renewing the working medium as suitable measures are taken, according to preferred embodiments, to allow the working medium to overflow between partial volumes of a housing (10) in the different aggregation states thereof.

Description

Apparatus and method for converting thermal energy into electrical energy

The invention relates to an apparatus and a method for converting thermal energy into electrical energy.

In Kraftfahrzeugbereiσh win working independent of the internal combustion engine heaters increasingly important. Such auxiliary heaters are designed as auxiliary heaters for operation with a stationary internal combustion engine and / or auxiliary heater for operation with the internal combustion engine running.

For the start and maintenance of the operation of such additional heaters electrical energy is needed, for example, a hot air blower, the circulation of a water cycle in water heaters, a fuel pump, a combustion air blower and an electrically heated glow plug for the start of the heater. This required electrical energy additionally loads the electrical energy budget of the motor vehicle. In particular, in the case of a parking heater operation, this is critical, since a discharge of the battery takes place. Although this discharge of the battery can be limited by a suitable control of the auxiliary heater by the stoppage heating operation is interrupted upon reaching a critical discharge state, but this then leads in most cases to an insufficient service life of the heater. Furthermore, even despite the termination of the heating operation due to a low state of charge of the battery is not ensured that the battery-dependent starting process of the engine can take place reliably, as this crucially depends on the rest of the state of the motor vehicle and the prevailing ambient temperatures. in the In the worst case, one is confronted with a cold and icy vehicle, whose internal combustion engine can no longer be started. Achieving such a condition is all the more likely, the shorter the distances that are covered between the individual operating phases of the auxiliary heating with the motor vehicle, since short distances are not sufficient to bring about a satisfactory state of charge of the battery.

To remedy this disadvantage, it has been proposed to partially convert the heat energy generated by the additional heating into electrical energy, and to use this electrical energy for the operation of the auxiliary heater. In this way, unwanted discharge of the vehicle battery would fail. An example of such a concept is described in DE 102 35 601 A1. In the context of this concept, a thermoelectric device is used which is arranged between a "hot side" forming housing section and a "cold side" forming housing section. During operation of the heater can thus be tapped at the thermoelectric device, a voltage. The problem with this solution, however, the maintenance of the highest possible temperature differences, which are required for the efficient operation of thermocouples, thermal expansions in the field of thermoelectric device and a poor thermal conductivity between the thermoelectric device and the housing sections. Ultimately, these problems can lead to the fact that the required for the operation of the additional heating electrical power can not be provided in full.

The object of the invention is to provide a concept for converting heat energy into electrical energy so as to ensure in each case is that a sufficient power for the operation of the auxiliary heater is available.

This task is solved with the features of the independent claims.

Advantageous embodiments of the invention are indicated in the dependent claims.

According to the invention, the object is achieved by a device for converting heat energy into electrical energy with a housing, the free inner volume is at least partially filled with a working fluid, a heat source for delivering heat energy to the working medium, a heat sink for receiving Heat energy from the working medium, a movable element arranged in the housing, which is movable by a heat-related phase transition of the working medium with a change in volume, whereby due to a movement of the movable element, working medium heated beforehand by the heat source can be cooled by the heat sink, previously cooled by the heat sink working medium from the heat source can be heated and an electric power generating generator can be driven. An essential feature of the invention is thus the utilization of a phase transition in a working medium. The invention will be explained by way of example with reference to the solid-liquid phase transition, wherein other phase transitions associated with a sufficient change in volume may also form the basis for the embodiment of the invention; For example, subliming substances can be used, that is, a phase transition solid-gaseous can be used. Due to the presence of a movable element, a heat-induced phase transition can take place with an increase in volume. The kinetic energy of the movable element can then be converted into electrical energy via a generator. Furthermore, due to the movement of the element, it is possible for the working medium previously evaporated by heating to come into contact with a region for cooling the working medium, that is to say a heat sink, and thus again carry out a phase transition from the gaseous to the solid state of aggregation. As a result, a pressure reduction and thus an oppositely directed movement of the movable element is brought about directly, or indirectly the prerequisite for a pressure reduction and the movement of the movable element brought about thereby is created. In any case, the movable element returns to a position in the sense of a circular process, which allows the renewed phase transition while increasing the volume.

According to a preferred embodiment of the present invention, it is provided that the movable element divides the inner volume of the housing into a first and a second variable partial volume, wherein the first partial volume is connected to the heat source and the second partial volume is connected to the heat sink and in a first position of the movable element, a transfer of working medium between the first and the second sub-volume and an entry of working medium in the first and the second sub-volume is inhibited or prevented, in a second position of the movable element an entry of previously in the second sub-volume cooled working medium in the first sub-volume is made possible and a transfer of working fluid between the first and the second sub-volume is prevented and in a third position of the movable element, a passage of previously heated in the first sub-volume working medium in the second sub-volume n is enabled. In the first position of the movable element, starting from the liquid state of the working medium an evaporation take place with heat. Due to the movement of the movable element caused thereby, the volume required for the phase transition is provided, and furthermore the movable element is brought into a second position, in which liquid working medium can enter the first partial volume. This liquid medium has passed from the gaseous to the liquid state during a preceding period of the cycle. The movable element is then moved further from the second layer to a third layer, which then additionally allows the gaseous working fluid to pass from the first sub-volume to the second sub-volume. This transition will take place because the pressure in the first partial volume is significantly higher than the pressure in the second partial volume associated with the heat sink. The transferred working medium can then condense in the second partial volume, and the movable element is moved in an opposite direction, for example due to the weight or due to some other force, such as an elastic force. During or at the end of this movement, liquid working medium can then be provided from the second partial volume for the passage into the first volume, this transfer then taking place when the movable element again assumes the second position during the next cycle of the cycle.

Usefully, it is provided that the movable element is a piston.

The backflow of the liquid working medium from the second partial volume into the first partial volume can then be realized in such a way that at least one return flow channel is provided in a wall section of the housing leading to the piston, which in the first position of the piston with the second Partial volume connected and not connected to the first subvolume that in the second position of the piston, the at least one return flow channel is connected to the first subvolume and not connected to the second subvolume and that between the first and the second layer no intermediate layer exists in the both the first and the second subvolume are connected to the at least one return flow channel. The arrangement of return flow channels in the wall of the housing is useful because on this basis the piston can be made simple. But it is also possible to integrate the return flow channels based on suitable mechanisms in the piston. It is essential for the invention that a return flow can take place at a suitable time, so that the cyclic process takes place continuously without renewing the working medium. The fact that there is no intermediate position between the first and the second position, in which both the first and the second sub-volume are connected to the at least one return flow, ensures that no liquid working medium from the first partial volume and the second partial volume due to the high pressure difference first sub-volume is pressed into the second sub-volume.

The invention is furthermore preferably formed by providing at least one overflow channel in a wall section of the housing leading to the piston, which permits the transfer of previously heated working medium in the first partial volume into the second partial volume in the third position of the piston. An overflow channel arranged in this way makes it possible in a simple manner to equalize the pressure between the sub-volumes, with its arrangement in the housing wall again ensuring that the piston can be designed in a simple manner. But it is also possible that a closable overflow channel is provided in the piston, which is open in the third position of the piston and thus allows the passage of previously heated in the first part volume working fluid in the second part volume. This can be done by a

Mechanism can be achieved, which opens one or more channels in the piston at the moment when the piston has reached a 'certain position. In particular, this is the upper "dead center" well defined.

According to a further embodiment of the present invention it can be provided that the movable element is a bellows. Such a bellows can be efficiently molded from a plastic. With a suitable design of the molding tool, the bellows can be equipped in a uniform operation with flaps, valves or similar means, so that they can provide the return flow channels and / or the overflow channels. In a solution with a bellows, therefore, the housing can be formed without any channels.

Of course, however, it is also possible in connection with a bellows to arrange the transfer channels and / or the return flow channels in the housing. A bellows also makes it possible to choose the ratio of the first partial volume to the second partial volume. This is advantageous since, as a result, small quantities of vapor passing from the first subvolume into the second subvolume are sufficient relative to the total vapor volume in order to bring about a pressure equalization. In this way, the risk of a sealing of the liquid in the first partial volume during the overflow phase of the vapor into the second partial volume can be reduced, and the operating frequency of the piston can be increased due to the small excess amount of steam. An increased operating frequency then in turn allows a simplification of the generator. According to another embodiment of the present invention, it can be provided that the movable element is a membrane. Such a solution has similar features and advantages as the solution with a bellows, in particular as regards the arrangement of the channels and the ease of manufacture.

The invention may further be realized in the form that the movable element has at least one barrier for the working medium and the movable element divides the inner volume of the housing into a first and a second variable partial volume, wherein upon movement of the movable element into a first Direction of the first sub-volume is increased, the second sub-volume is reduced, the first sub-volume receives an enlarged contact area with the heat sink and the second sub-volume receives a reduced contact area with the heat sink and wherein upon movement of the movable member in a second direction, the first sub-volume decreases is, the second sub-volume is increased, the first sub-volume receives a reduced contact area with the heat sink and the second sub-volume receives an enlarged contact area with the heat sink. In contrast to the previously mentioned embodiments, both the evaporation and the condensation of the working medium take place on the same "side" of the movable element in the latter solution. In order to realize a movement of the movable element in the opposite direction, which is a prerequisite for the continuous

Operation of the generator, two such sub-volumes are provided, in which identical or comparable processes take place. The invention further relates to a method for converting heat energy into electrical energy with the steps:

Providing heat energy,

Heating a working medium using the heat energy, whereby a first phase transition, an increase in pressure in a partial volume of a housing and an enlargement of the partial volume are brought about by moving a movable element in a first direction,

Cooling of the working medium, whereby a second phase transition opposed to the first phase transition, a pressure reduction in the previously heated working medium and a reduction of the partial volume by moving a movable element in a second direction are brought about, and

Converting the kinetic energy of the moving element into electrical energy.

In this way, the advantages and special features of the device according to the invention are also implemented in a method.

With regard to the application of the method according to the invention this unfolds its particular advantages in that the heat energy is at least partially provided by a parking heater of a motor vehicle available and the electrical energy is at least partially used for the operation of the heater. The invention is based on the finding that under all circumstances sufficient supply of electrical power can be made available on the basis of a comparatively simple thermodynamic process. A medium undergoes a large volume expansion through heating and a phase transition, which is made possible by the movement of a movable element. The resulting volume work can be converted by a generator into electrical energy. By the liquid and gaseous amounts of the working medium in a suitable

Be guided, a cycle can take place, which provides continuously, in particular without consumption of the working medium, an electrical power that is in concrete embodiments of the present invention, for example in the range of 50 to 100 W, that is generally sufficient to the Maintain operation of the auxiliary heater with sufficient reserve. Excess electrical power can be fed into the on-board network of the vehicle and used, for example, to charge the battery.

The invention will now be described by way of example with reference to the accompanying drawings by way of particularly preferred embodiments.

Show it:

Figure 1 is a sectional view of an embodiment of a device according to the invention in a first working position;

Figure 2 shows the embodiment of Figure 1 in a second working position; Figure 3 shows the embodiment of Figure 1 in a third working position;

FIG. 4 is a sectional view of a piston for use in a device according to the invention in a first working position;

FIG. 5 shows the piston according to FIG. 4 in a second working position;

Figure 6 is a sectional view of another piston for use in a device according to the invention in a first working position;

FIG. 7 shows the piston according to FIG. 6 in a second working position;

Figure 8 is a sectional view of another embodiment of a device according to the invention in a first working position and

FIG. 9 shows the embodiment according to FIG. 8 in a second working position.

In the following description of preferred embodiments of the present invention, like reference characters designate like or similar components.

Figures 1 to 3 show sectional views of an advantageous embodiment of a device according to the invention in three different working positions. A housing 10, which is preferably cylindrical, accommodates a piston 18 and a generator 26. The piston 18 is connected via a plunger 44 to the generator 26 for the purpose of driving the generator 26 in connection. The piston 18 divides the inner volume of the housing 10 in a first sub-volume 28 below the piston 18 and a second sub-volume 30 above the piston 18. Due to a possibility of movement of the piston 18 in the housing 10, the sub-volumes 28, 30 are variable. The free inner volume of the housing 10 is filled with a working medium 12, wherein the present in the liquid state of aggregate state working medium 12 is shown. The remaining free volume of the housing 10 is filled with the working medium in gaseous phase corresponding to the respective vapor pressure of the liquid working medium 12 in the partial volumes 28, 30. As a working medium, for example, water or alcohols come into question. The piston 18 has a smooth cylindrical outer wall, over which it is guided in the housing 10. The substantially cylindrical inner wall of the housing 10 is provided with channels 32, 34. There are Rückströmkanäle 32 and overflow 34 provided, this designation of the channels 32, 34 is explained in the context of the following functional description of the device according to the invention. The housing 10 provides a heat source 14 and a

Heat sink 16 is available, wherein the heat source 14 deliver heat energy generated by a heater to the sub-volume 28 and the heat sink 16 can be supplied by the cooling system of the vehicle.

The inventive device according to Figures 1 to 3 operates as follows. In the illustration according to FIG. 1, the piston 18 is in a first position, its lower end position. In this state, the overflow channels 34 are closed by the piston 18. Likewise by the

Piston 18 prevents present in the return flow channels 32 working medium 12 can enter the first sub-volume 28. Since the first partial volume 28, that is to say the working medium 12 present in the first partial volume 28, is connected to a heat source 14, it can be connected to the heat exchanger 14 Heat source 14 are heated. The mechanical and thermodynamic properties of the system are now designed so that the working medium 12 in the first sub-volume 28 undergoes a large volume change due to a phase transition, this volume change being made possible by the mobility of the piston 18. Due to the heating of the working medium 12 in the first partial volume 28, the piston is thus converted into its position shown in Figure 2. With regard to the overflow channels 34, the position of the piston 18 shown in FIG. 2 is equivalent to the position shown in FIG. However, in order to achieve the position shown in FIG. 2, the upper end of the return flow channels 32 has been crossed over by the upper boundary of the piston, before it has then released the lower end of the return flow channels 32, that is to say has connected it to the first partial volume. Accordingly, the liquid working medium present in the return flow channels 32 could flow back into the first partial volume 28. In this situation, the working medium 12 continues to be heated in the first partial volume 28, so that a further movement of the piston 18 takes place in its position shown in FIG. With regard to the return flow channels 32, the position of the piston shown in FIG. 3 is equivalent to the position shown in FIG. However, the overflow 34 are no longer closed by the piston 18 now. Consequently, gaseous working medium can flow over the overflow channels 34 from the first partial volume 28 into the second partial volume 30 while reducing the pressure in the first partial volume 28 and increasing the pressure in the second partial volume 30. In this way, a pressure equalization between the sub-volumes 28, 30, and the piston 18 can move back into its position shown in Figure 1. The downward movement of the piston may be provided by weight and / or the force of a spring not shown during the upward movement, and / or by the generator become. Meanwhile, the working medium transferred into the second partial volume 30 condenses by heat emission to the heat sink 16, which is provided by the second partial volume 30 delimiting colder housing sections available. This condensed working medium 12 then collects again at least partially in the return flow channels 32, so that it is available for the next partial cycle 28 for the next working cycle.

FIGS. 4 and 5 show a piston 20 with an overflow channel 36 provided therein. A housing which accommodates such a piston 20 therefore does not have to have the overflow channels 34 shown in FIGS. 1 to 3. In the piston 20, a bistable spring 46 is arranged and fixedly connected to the piston 20. The two ^' stable positions of the bistable spring 46 are shown in Figures 4 and 5, respectively. The bistable spring 46 is connected centrally with a slot 48 having a rod 50. On its other side, the bistable spring 46 is connected via a further rod 52 with a cover 54 which is suitable for closing or opening the overflow channel 36. The closed position of the overflow channel 36 is shown in Figure 4, while the open position of the overflow 36 is shown in Figure 5. Furthermore, a pin 56 can be seen, which is firmly connected to the housing, not shown here, and thus moves in a movement of the cylinder 20 in the slot 48.

In Figure 4, the position of the pin 56 with respect to the elongated hole 48 is shown in a lower position of the piston. In this state, the bistable spring 46 is curved upward, and thus the cover 54 closes the overflow channel 36. Now moves the piston 20 due to the phase transformation of the working medium upwards, so remains the overflow channel 36 initially closed, and the pin 56 moves relative to the slot 48 down. If the pin 56 reaches the lower end of the elongated hole, this results in a downwardly directed, centrally acting force being exerted on the bistable spring 56, with the result that the bistable spring 46 assumes its downwardly curved position. Consequently, the overflow channel 36 is opened by lifting the lid 54 from the piston 20.

On the basis of this piston 20 shown in FIGS. 4 and 5, a more favorable operating behavior of the device according to the invention sometimes results, since the overflow channel 36 is opened only shortly before top dead center and remains open until the lower end position is reached.

FIGS. 6 and 7 show a variant of the embodiments according to FIGS. 4 and 5. In order to make the first partial volume 28 (see FIGS. 1 to 3) as large as possible, which entails the advantages already described during operation, the closable region of the piston 22 is made very short, and below the lid 54 there is an additional volume that can be attributed to the first partial volume.

A comparable construction with a large first partial volume also results in the case where a bellows is used instead of a piston. Also, this embodiment can be seen with reference to Figures 6 and 7, assuming that the free volume below the lid limiting area is constructed bellows. The lower end portion of this bellows is then firmly connected to the housing, not shown here, for example, via a circumferential in the radial direction ^■ extending collar, which by a lower and an o- Beren housing part is fully clamped. Similarly, a membrane may be arranged as a movable element in the housing.

Figures 8 and 9 show a further embodiment of a device according to the invention, wherein two successive operating states are shown. The device has a housing 10 in which a liquid working medium 12 is present. A heat source 14 and a heat sink 16 are provided. The heat source extends along the underside of the housing 10 which communicates with the liquid working medium 12. The heat sink 16 communicates with other areas of the free interior volume. A movable element 24 arranged in the housing 10 has two barriers 38, which subdivide the volume in the housing 10 into three sub-volumes 40, 42, 58. The movable member 24 is rotatably supported so that it can drive a generator via a shaft 60.

The invention shown in Figures 8 and 9

Device works as follows. If one starts from the state shown in FIG. 8 and looks at the partial volume 40, one recognizes a larger amount of liquid working medium 12. This liquid working medium 12 is heated via the heat source 14, so that a phase transition takes place with an increase in volume. This increase in volume is made possible by a movement of the barrier 38 together with the movable element 24. As a result of this movement, the evaporated working medium comes into contact with cooler regions of the housing 10, that is, with the heat sink 16. Meanwhile, however, gaseous working medium already present in the partial volume 42 is condensed, so that now the partial volume 42 assumes the initial role of the partial volume 40. The liquid working medium 12 present in the partial volume 42 is consequently heated, while in the partial volume 42 the liquid working medium 12 is heated. Lumen 40, a pressure reduction due to the condensation of the working medium takes place. Ultimately, then, a movement of the movable element 24 can take place in the opposite direction to again reach the state shown in Figure 8.

With regard to the surfaces involved within the embodiments described in the context of the present invention, the following circumstances, in particular with regard to the heat transfer, play a role. The first

Partial volumes of the devices described in connection with FIGS. 1 to 7 are wetted by the liquid working medium. In addition, since a relatively high temperature difference between the burner flame, which can act, for example, directly on the housing portion forming the heat source, and the liquid working medium, the heat transfer is unproblematic here. However, the temperature difference between the condensing liquid and the cooling water, which preferably provides the conditions for a heat sink, is low. Therefore, the heat transfer is less favorable here despite wetting the housing wall on both sides. In order that the outer housing dimensions do not become too large in connection with the provision of a sufficient surface, it is possible to provide the condensation surface in the second partial volume with a structure, for example with truncated pyramids, the surface of which may optionally be further enlarged by etching ,

Another aspect related to the present invention and to all described embodiments relates to the implementation of the generator. For example, a ring magnet can be used for the magnet of the generator, as used in loudspeakers. A magnetic ring from a speaker with a Power of 100 W, for example, has an outer diameter of about 100 mm, an inner hole of about 57 mm and a height of about 14 mm. On the top and bottom of the magnet sheets can be attached to the field guide. The coils can be arranged in the inner ring, that is, the magnetic ring encloses the coil package. In the case of the up and down movement of the piston, which is relevant in connection with FIGS. 1 to 7, it can then be transmitted to the magnetic ring, which moves up and down on the longitudinal axis of the coils. Since the generator can be arranged in the second partial volume, a movement of the magnet can move the gaseous working medium in the second partial volume and thus generate a flow at the condensation surface. This also improves the heat transfer. As coils, two coil groups can be provided, which deliver by offset in height two in a phase mutually shifted voltages. Each coil group can consist of three individual coils distributed evenly around the circumference of the ring. Thus, the reaction forces between the magnet and the coil core are evenly distributed, and tilting of the magnet is prevented. So that no "cogging forces" lead to a jerky movement of the magnet during the transition between the two phases, the cores in the height projection should overlap. By changing the cross sections, a sliding transition of the magnetic field can be achieved as a result of the saturation of the coil core. The coil cores themselves may consist of ferrite material or vertical transformer sheets.

The features of the invention disclosed in the foregoing description, in the drawings and in the claims may be essential to the realization of the invention both individually and in any combination. List of reference numbers:

10 housing

12 working medium

14 heat source

16 heat sink

18 pistons

20 pistons

22 pistons

24 movable element

26 generator

28 first partial volume

30 second partial volume

32 return flow channel

34 overflow channel

36 overflow channel

38 lock

40 partial volumes

42 partial volumes

44 pestles

46 bistable spring

48 slot

50 pole

52 pole

54 Lid

56 cones

58 partial volumes

60 wave

Claims

1. Device for converting heat energy into electrical energy with

a housing (10) whose free inner volume is at least partially filled with a working medium (12),

a heat source (14) for delivering heat energy to the working medium,

a heat sink (16) for absorbing heat energy from the working medium,

a movable element (18, 20, 22, 24) arranged in the housing, which is movable by a heat-related with a volume change phase transition of the working medium, wherein due to a movement of the movable member

previously heated by the heat source working fluid from the heat sink can be cooled,

previously cooled by the heat sink working fluid from the heat source is heated and

- An electric power generating generator

(26) is drivable.

2. Device according to claim 1, characterized in that the movable element, the inner volume of the housing in a first (28) and a second (30) variable Teilvolu- divided, wherein the first sub-volume with the heat source (14) is connected and the second sub-volume with the heat sink (16) is connected and wherein

in a first position of the movable element (18, 20, 22), a transfer of working medium between the first and the second partial volume and an entry of working medium into the first and the second partial volume is inhibited or prevented,

in a second position of the movable member, an entry of previously cooled in the second partial volume working medium in the first sub-volume is made possible and a transition of working medium between see the first and the second sub-volume is prevented and

in a third position of the movable element, a passage of working medium previously heated in the first partial volume into the second partial volume is made possible.

3. Apparatus according to claim 1 or 2, characterized in that the movable element is a piston (18, 20, 22).

4. Apparatus according to claim 3, characterized in that

in that at least one return flow channel (32) is provided in a wall section of the housing (10) leading to the pistons (18, 20, 22), which is connected in the first position of the piston to the second subvolume and is not connected to the first subvolume . that in the second position of the piston, the at least one return flow channel is connected to the first subvolume and is not connected to the second subvolume, and

in that no intermediate layer exists between the first and the second layer, in which both the first and the second partial volume are connected to the at least one return flow channel.

5. Apparatus according to claim 3 or 4, characterized in that in a piston (18) leading wall portion of the housing at least one overflow channel (34) is provided, the Ü in the third position of the piston before the Ü in the first part volume heated working medium in the second partial volume allows.

6. Device according to one of claims 3 to 5, characterized in that in the piston (20, 22) a closable overflow channel (36) is provided, which is open in the third position of the piston and thus the passage of previously in allows the first sub-volume of heated working fluid in the second sub-volume.

7. The device according to claim 1 or ^'2, characterized in that the movable member is a bellows.

8. Apparatus according to claim 1 or 2, characterized in that the movable element is a membrane.

9. Device according to claim 1, characterized in that the movable element (24) has at least one barrier

(38) for the working medium and the movable element divides the inner volume of the housing into a first (40) and a second (42) variable partial volume, wherein during a movement of the movable element (24) in a first direction the first partial volume (40) is increased, the second partial volume (42) is reduced, the first partial volume is an enlarged one

Receives contact area with the heat sink (16) and the second sub-volume receives a reduced contact area with the heat sink and

- In a movement of the movable member (24) in a second direction, the first partial volume (40) is reduced, the second partial volume (42) is increased, the first partial volume receives a reduced contact area with the heat sink (16) and the second partial volume receives an enlarged contact area with the heat sink.

10. A method of converting thermal energy into electrical energy, comprising the steps of:

Providing heat energy,

Heating a working medium (12) using the thermal energy, whereby a first phase transition, an increase in pressure in a partial volume (28, 40, 42) of a housing (10) and an enlargement of the partial volume by moving a movable element (18, 20, 22, 24) in a first direction,

Cooling of the working medium, whereby a second phase transition opposite to the first phase transition, a pressure reduction in the previously heated working medium and a reduction of the partial volume (28, 40, 42) are brought about by moving a movable element in a second direction, and

Converting the kinetic energy of the movable element into electrical energy.

11. The method according to claim 10, characterized in that the heat energy is at least partially provided by a parking heater of a motor vehicle available and the electrical energy is used at least partially for the operation of the heater.