GB2426553A

GB2426553A - Stirling machine cooling circuit

Info

Publication number: GB2426553A
Application number: GB0510939A
Authority: GB
Inventors: Jariath Michael Mcentee; Andrew Nicholas Hill
Original assignee: Microgen Energy Ltd
Current assignee: Microgen Energy Ltd
Priority date: 2005-05-27
Filing date: 2005-05-27
Publication date: 2006-11-29
Also published as: GB0510939D0

Abstract

A Stirling machine has a coolant circuit 20 for extracting heat from the space between a displacer 4 and a power piston 10. The coolant circuit comprises an annular channel 22 through which coolant fluid is circulated. A number of heat-absorbing fins 30 project outwardly from the annular channel 22 to enhance heat transfer into the cooling channel 22 of heat generated by an electromagnetic converter 17. The fins 30 may extend axially, as shown, at an angle to the axis, or radially inwardly (figs.4,5). The cooling channel 22 may be made of copper and brazed to the wall of the casing 1. The cooling channel 22 may have at least one internal fin of steel to resist buckling under the pressure of helium in the casing 1.

Description

A STIRLING MACHINE The present invention relates to a Stirling machine. Stirling engines are particularly suited to use in the domestic combined heat and power (DCHP) field. The applicant is developing a linear free piston Stirling engine for use in such an application. However, the present invention has applications to other types of Stirling machine (e.g. Stirling cycle refrigerators and cryocoolers). The Stirling cycle requires a hot space heated by a heater and a cold space cooled by a cooling circuit. A working fluid is moved from one space to the other and back cyclically via a regenerator. The movement is effected by a displacer displacing gas from the hot space and a power piston which reciprocates out of phase with the displacer. The power piston moves a reciprocating component of an alternator and cooperates with a stator to generate electricity as the power piston reciprocates. During operation, the alternator produces a small amount of heat due to ohmic losses. However, it has been found that this can build up over time causing elevated temperatures within the alternator. This has a detrimental effect on the performance of the system and the like of magnets. In a Stirling refrigerator, the lower casing houses a motor to drive the piston. This system will suffer performance degradation at elevated temperatures. Thus the need to remove heat from the lower casing is also present in Stirling refrigerators. According to a first aspect of the present invention there is provided a Stirling machine comprising a head, a displacer reciprocably mounted within the head, an electromagnetic converter spaced from the head, a power piston reciprocably mounted proximate to the electromagnetic converter, a coolant circuit for the extraction of heat from the space between the displacer and power piston, the coolant circuit comprising an annular channel through which coolant fluid is circulated, in use, wherein a heat absorbing member projects outwardly from the annular channel to enhance heat transfer into the channel of heat generated by the electromagnetic converter. Thus, the present invention provides an enhanced design of the coolant circuit which is now designed to perform an additional function, namely to cool the electromagnetic converter. This will be an alternator for a Stirling engine and a motor for a Stirling refrigerator. Not only does this extend the useful life of the magnets in the electromagnetic converter, but it also improves the overall efficiency of the machine as the amount of heat recovered is enhanced. The heat absorbing member may, for example, be formed by a number of recesses or grooves formed in the wall of the channel. Alternatively, the heat absorbing member may be at least one fin. The coolant channel preferably has an axially facing wall which directly faces the electromagnetic converter. The at least one fin may then project from this axial face. This could either be in the form of a plurality of concentrically mounted fins, or a single spirally wound fin. Alternatively, the fins may extend from a radially inwardly facing wall of the channel. In this case, the fins may either be annular, or may be a plurality of circumferentially spaced fins each extending in a generally axial direction. As a further alternative the or each fin may be positioned directly below the axially facing wall of the channel and extend in a radial direction. Such a fin or fins may either be in a radial plane, or there may be a plurality of such fins spaced circumferentially and each extending in an axial plane. The coolant channel is generally made of copper as this provides good heat transfer characteristics. However, in use, the channel is subjected to the relatively high pressures of the working gas between the displacer and power piston which fluctuate periodically during the operation of the engine. In order to increase the strength of the channel, we have increased the thickness of the wall of the channel. However, this increased thickness has a detrimental effect on heat transfer. According to a second aspect of the present invention there is provided a channel member for the cooler of a Stirling machine, the member comprising a copper annular channel element at least partially defining the wall of the channel member and at least one copper fin having an annular configuration and extending radially outwardly from a radially outwardly facing wall of the channel element, wherein the channel member further comprises a non-copper annular strengthening fin, the strengthening fin extending radially outwardly from a radially outwardly facing wall of the channel element. By providing a non-copper strengthening fin the ability of the channel member to resist deformation caused by the high internal pressure is enhanced. The use of a non-copper material will have a detrimental effect on the heat transfer properties. However, this is less significant than the detrimental effect of increasing the wall thickness of the channel element. Preferably, the strengthening fin is a steel fin, but may alternatively be a high strength copper-based alloy such as silicon bronze. Preferably there are a plurality of copper fins and a single strengthening fin. The first and second aspects of the present invention may be used independently of one another or in combination. An example of a Stirling machine constructed in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is an axial cross-section through a linear free piston Stirling engine; Fig. 2 is an enlarged cross-section through a portion of the coolant channel; Fig. 3 is a perspective view of a segment of the radially inner wall of the water channel; Fig. 4 is an enlarged cross-section through a portion of the coolant channel and casing wall showing a second example; and Fig. 5 is an enlarged cross-section through a portion of the coolant channel and engine casing in accordance with a third example. The basis Stirling engine design will now be described with reference to Fig. 1. The engine comprises a casing 1 in which all of the engine components are housed. The engine has a head 2 from which a plurality of fins 3 project in order to enhance heat transfer into the head. A displacer 4 is arranged to reciprocate along a main central axis 5 within a cylinder 6. A regenerator 7 is positioned between the cylinder 6 and the engine casing 1 at the head 2. The displacer 4 is coupled by a flexible rod 8 to a pair of leaf springs 9 which are fixed with respect to the engine casing and provide a restoring force on the reciprocating movement of displacer 4. A power piston 10 is positioned beneath the displacer 4 and reciprocates within a cylinder 11. The power piston 10 has an annular configuration with a central space 12 through which the flexible rod 8 and an annular sleeve 13 of the displacer 14 extend. This arrangement maintains the coaxiality of alignment between the displacer 4 and power piston 10. At the opposite end to the displacer 4, the power piston 10 is coupled to an annular magnet cage 15 on which a plurality of magnets 16 are mounted so as to reciprocate together with the power piston 10. The magnets 16 reciprocate within a stator 17. The reciprocating magnets 16 and stator 17 comprise an alternator 18. Electrical wires (not shown) lead from the casing 1 for the transfer of electrical power. The engine coolant circuit 20 will now be described with reference to Figs. 1 to 3. The coolant circuit 20 essentially comprises a copper annular coolant channel 22 which extends around the radially outermost region of the casing 1 at an axial location between the displacer 4 and power piston 10. In fact, the channel 22 is divided into a number of annular passages 23 by a plurality of internal annular fins 24 each in a radial plane as shown in Figs 2 and 3. The channel 22 is bounded on the radially outermost side by the engine casing 1 and on the radially innermost side by an annular plate 25. The casing acts as the sealing surface at the longitudinal end regions of the channel 22. To achieve this, the channel 22, is brazed to the casing wall after, or at the same time as the internal head components. A coolant inlet 26 and a coolant outlet 27 extend into and out of the channel 22 respectively through the wall of the casing 1.In Fig. 1 the inlet 26 and outlet 27 are shown opposite one another. However, in practice, they may be positioned differently, for example adjacent to one another so that the coolant liquid has to flow circumferentially almost all of the way around the engine when flowing from inlet to outlet. The annular wall 25 provides the interface between the coolant channel 22 and the gas space 28 through which, in use, gas is progressively cycled from the space between the displacer 4 and power piston 10 to the regenerator 7 and back. As described thus far, the Stirling engine is of a known design. The improvement resides in the arrangement for cooling the alternator 18 which will now be described with reference to Figs. 1 to 3. The whole of the engine casing 1 is filled with a working gas, typically helium. Thus, in use, any heat generated by the alternator 18 is transferred into the surrounding gas. A plurality of external annular fins 30, typically 5 or 6 as shown in the drawings extend from the lower axially facing wall of the coolant channel 22. These fins increase the effective surface area of the lower wall and increase the residence time of the gas surrounding the alternator close to the channel wall thereby enhancing the transfer of heat from the gas into the coolant channel 22. Such an arrangement has been found, in use, to provide a significant improvement to the performance of the engine. The coolant channel 22 could be manufactured, for example, by casting as a single component or by brazing or soldering the annular fin(s) to a cylindrical housing, or by a combination of the two processes where fins are brazed or soldered onto a copper casting. The fins may also be copper. However, it may be advantageous to include at least one internal steel fin. This prevents the copper channel from buckling under the internal operating helium pressure and allows the channel to be thinner thereby improving heat transfer and hence increasing thermal efficiency. Alternatively the fins could be formed as part of the cylindrical housing by machining for example by removal of annular slots of material to form annular fin(s). The external fins 20 are shown in Fig. 1 extending in the direction of the main axis of reciprocation 5. However, it will be appreciated that the fins could be angled with respect to this axis, for example to follow the slope of the engine casing 1 at this point. Alternative configurations of external fins are shown in Figs. 4 and 5. In Fig. 4, the lower and outermost edge of the channel 22 extends downwardly and at least one radially inwardly extending fins 40 is provided. Fig. 4 shows a pair of axially spaced annular fins. However, more fins could be provided if necessary. Alternatively, there could be a plurality of circumferentially spaced fins each of which is positioned in a radial plane passing through the main central axis 5. Alternatively, the circumferentially spaced fins may be inclined with respect to this axis. Fig. 5 has a plurality of circumferentially spaced fins 50 (only one of which is shown in Fig. 5) each of which extends in a plane passing through the main central axis 5. In this case, the fins 50 extend from the annular plates 25 and their radially inward extent is limited by the cylinder 11. The heat from alternator 18 travels upwardly within the casing 1 to the fins 50. The fins 50 could alternatively be a number of annular fins extending in a radial plane. However, openings would have to be provided in at least the lowermost fins to allow access to the uppermost fins. Stirling refrigerators are well known in the art, and it will be appreciated that the same fins can be employed on the water channel cooling the refrigerator below the head.

Claims

1. A Stirling machine comprising a head, a displacer reciprocably mounted within the head, an electromagnetic converter spaced from the head, a power piston reciprocably mounted proximate to the electromagnetic converter, a coolant circuit for the extraction of heat from the space between the displacer and power piston, the coolant circuit comprising an annular channel through which coolant fluid is circulated, in use, wherein a heat absorbing member projects outwardly from the annular channel to enhance heat transfer into the channel of heat generated by the electromagnetic converter.

2. A machine according to claim 1, wherein the heat absorbing member is provided by at least one fin.

3. A machine according to claim 2, wherein the at least one fin projects from the annular channel towards the electromagnetic converter.

4. A machine according to claim 3, wherein the at least one fin extends axially from an axially facing portion of the channel.

5. A machine according to claim 3, wherein the at least one fin extends radially inwardly from a radially inwardly facing portion of the channel.

6. A machine according to claim 3, wherein the at least one fin is positioned below the channel and extends radially inwardly.

7. A machine according to any of claims 2 to 6, wherein the at least one fin is annular.

8. A machine according to any of claims 2 to 6, wherein a plurality of discrete fins are circumferentially spaced from one another.

9. A machine according to any one of the preceding claims further comprising a channel member for the coolant circuit, the member comprising a copper annular channel element at least partially defining the wall of the coolant circuit and at least one copper fin having an annular configuration and extending radially outwardly from a radially outwardly facing wall of the channel element, wherein the channel member further comprises a non-copper annular strengthening fin, the strengthening fin extending radially outwardly from a radially outwardly facing wall of the channel element.

10. A machine according to any one of the preceding claims, wherein the machine is an engine and the electromagnetic converter is an alternator.

11. A machine according to any one of claims 1 to 3, wherein the machine is a Stirling refrigerator, and the electromagnetic converter is a motor.

12. A channel member for the cooler of a Stirling machine, the member comprising a copper annular channel element at least partially defining the wall of the channel member and at least one copper fin having an annular configuration and extending radially outwardly from a radially outwardly facing wall of the channel element, wherein the channel member further comprises a non-copper annular strengthening fin, the strengthening fin extending radially outwardly from a radially outwardly facing wall of the channel element.

13. A channel member according to claim 12, wherein a strengthening fin is a steel fin.

14. A member according to claim 12 or 13 comprising a plurality of copper fins and a single strengthening fin.