GB2429044A

GB2429044A - Arrangement to reduce damage to Stirling machine from oxidation of regenerator

Info

Publication number: GB2429044A
Application number: GB0513179A
Authority: GB
Inventors: Jarlath Michael Mcentee; Laurence George Meade
Original assignee: Microgen Energy Ltd
Current assignee: Microgen Energy Ltd
Priority date: 2005-06-28
Filing date: 2005-06-28
Publication date: 2007-02-14
Anticipated expiration: 2025-06-28
Also published as: GB0513179D0; GB2429044B

Abstract

A Stirling engine with a regenerator 7 has an Oxygen getter 32 positioned in the hot space in the path of the inert working gas, to remove traces of oxygen from the gas and so prevent oxidation of the regenerator. The engine may also or alternatively have a filter 33 which absorbs particles which break off the regenerator due to oxidation. The filter may be made of PTFE and positioned at the cold end of the regenerator.

Description

A STIRLING MACHINE

The present invention relates to a Stirling machine.

More particularly, the invention relates to a Stirling engine, such as a linear free piston Stirling engine.

However, the invention is also applicable to other Stirling machines such as a Stirling motor or refrigerator.

In a Stirling machine an inert working gas, such as helium, is alternately displaced in opposite directions through a regenerator so that gas flowing from a hot space within the engine to a cold space gives up its heat to the regenerator as it passes through the regenerator in a first direction. Gas flowing in the opposite direction, namely from the cold space to the hot space absorbs heat from the regenerator as it passes through.

When the engine is filled with the inert gas, this displaces the air within the engine casing. However, the gas does not have 100% purity and it is very difficult to remove all of the residual air from within the engine during the fill process. There is therefore a very small amount of oxygen present in the working engine.

The regenerator is commonly made of a ferrous material such as stainless steel. The oxygen within the engine can oxidise the regenerator material which, over time, will result in a deterioration of the engine performance. Also, particles from the regenerator can break away during operation due to the fast moving gases in the regenerator and the thin nature of the material being used. These loose particles will move about within the engine potentially becoming trapped in narrow passages which feed gas bearings within the engine thereby causing damage to the bearings themselves. The particles can also clog the flow passages within the regenerator itself reducing its effectiveness of operation and therefore need to be removed from the gas as soon as possible. If the engine is designed to be "sealed- for-life" this presents a considerable challenge.

JP 2003-042577 discloses a regenerator with filter rings at the top and bottom of the regenerator. These are made of sintered porous metal or porous ceramic and therefore only act to trap particles on the surface. This, in itself, can lead to problems with clogging as the particles build up.

According to the present invention there is provided a Stirling machine comprising a displacer piston which reciprocates within the machine, a regenerator through which an inert gas is alternately pumped in opposite directions by the displacement of the displacer piston, and an oxygen getter positioned in the path of the inert gas.

The oxygen getter removes the oxygen from the inert gas, thereby minimising the oxidisation of the regenerator material. This helps to maintain the performance of the regenerator, and also minimises the number of particles which will break off of the regenerator.

The oxygen getter can be positioned anywhere within the path of the inert gas, for example, disposed on the surface of the displacer piston. Alternatively, the getter material may be incorporated into the material of the regenerator itself, either by being an additive to the material of the regenerator, or, in the case of a regenerator made up of a number of wires, a number of these wires may be made from the getter material. However, preferably, the oxygen getter is positioned such that the bulk flow of inert gas passes through the oxygen getter. For example the oxygen getter may cover an end portion of the regenerator.

The machine has a hot space associated with one side of the displacer piston and a cold space associated with the opposite side. The oxygen getter is preferably positioned in the hot space. This enhances the performance of the oxygen getter as it operates more effectively at higher temperatures. Thus, the optimum position is for the oxygen getter to cover the portion of the regenerator facing the hot space. In this position, the getter material may provide a ring which supports the end of the regenerator.

Such a ring is provided in known regenerators. This can be implemented by the substitution of a conventional component with an improved component which does not increase the complexity of the machine.

The invention is applicable to any regenerator material which is susceptible to oxidisation. For example, the regenerator may be a ferrous metal or copper.

In addition to the use of the getter material to minimise the number of particles being formed, a filter such as that disclosed in JP 2003-042577 may also be used. As the number of particles is reduced, the filter design of this document may be adequate to remove the reduced number of particles. However, preferably, the filter is made of an absorbent material positioned so that particles breaking off of the regenerator, in use, are embedded within the filter.

As the particles become embedded in the porous material, rather than being trapped on the surface, they are more effectively removed from the gas stream as they are held in such a way that they do not clog up the filter.

The use of the absorbent filter provides a second aspect of the present invention which may be used together with or independently of the first aspect. According to the second aspect of the invention there is provided a Stirling machine comprising a displacer piston which reciprocates within the machine, a regenerator through which an inert gas is alternately pumped in opposite directions by the displacement of the displacer piston, and a filter of absorbent material softer than the regenerator material positioned so that particles breaking off of the regenerator when in use, are embedded within the filter.

The filter may be any material capable of withstanding the temperatures within the engine and which can absorb particles from the regenerator, such as PTFE (as disclosed, for example, in US 4,110,392), nylon (6/6) , PEEK (polyetheretherketofle) or polyethersolphone. Most thermoplasticS would be suitable with only those with low melting points being unsuitable.

The filter may be positioned anywhere within the gas path, but it is preferably positioned in the cold space.

This is because the particles from the regenerator potentially cause more damage to the engine components at the cold end of the engine. Thus, by positioning the filter in the cold space, the particles can be prevented from reaching the downstream components. Of course, a second filter may be provided at the hot end to prevent particles from entering the hot space.

preferably, the filter covers the end of the regenerator facing the cold space. This provides an effective way of preventing the particles from interfering with the downstream components as referred to above. Also, in this position, the filter can be used in place of a conventional retaining ring which supports the lower end of the regenerator.

These preferred features of the second aspect of the present invention may equally be used in conjunction with the first aspect of the present invention.

An example of a machine constructed in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic view through a linear free piston Stirling engine; and Fig. 2 is a perspective view of the regenerator.

The basic 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 external fins 3 and internal fins 3A 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 beneath the internal fins 3A. 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 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 4 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.

A coolant circuit essentially comprises an annular coolant passage 22 which extends around the radially outermost region of the casing 1 at an axial location between the displacer 4 and power piston 10. A coolant inlet 26 and a coolant outlet 27 extend into and out of the coolant passage 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 will 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 present invention relates to an improved design of the regenerator 7. This operates as follows. The top end 7A of the regenerator is the hot end which is immediately adjacent to the internal fins 3A above the displacer 4. The hot end is heated by conduction of heat from an external burner through the casing 1 and fins 3, 3A. The lower end 7B of the regenerator is adjacent to a cold space 31 between the displacer 4 and power piston 10. Heat is removed from this cold space along the coolant passage 22. Thus, a substantial temperature gradient is maintained along the regenerator. As the displacer piston 4 moves upwardly, gas is displaced downwardly through the regenerator and gives up its heat as it flows from the top end 7A to the lower end TB. On the return stroke the gas passes in the opposite direction, namely up through the regenerator, absorbing heat from the regenerator as it flows.

As shown in Fig 2 the regenerator 7 is made up of three axially arranged annular components and is bounded at its outer extremity by the engine casing 1 and at its inner extremity by cylinder 6.

Each of the annular elements may be formed in a number of ways. They may be made up of a number of random wire segments which are compacted and/or sintered, a sintered powder of the correct porosity, a sheet material rolled into an annular configuration, a stack of perforate plates, or any other known configuration.

A ring 32 of getter material is provided above the top end 7A of the regenerator. This getter material may be any material having a high affinity for oxygen and a highly stable oxide layer (for example titanium, tantalum, zirconium or alloys of these) . As the hot gas passes from a hot space 30 into the regenerator 7 it initially passes through this getter ring which absorbs residual oxygen within the gas. This will reduce the oxygen available within the regenerator to cause oxidisation of the regenerator. At the lower end of the regenerator beneath the lower segment 7B is a filter ring 33. This is made of an absorbent material such as PTFE. This traps and embeds any loose particles from the regenerator and prevents them from is entering the cold space 31. The filter has a porous lisponge!! structure with passageways to allow the flow of gas. The porosity (i.e. the percentage volume of air to material) is 85-95%. PTFE will soften at the temperature to which it is subjected beneath the lower segment 7B so that particles are driven into the surface by the gas flow.

Larger particles tend to be trapped closer to the surface of the ring than smaller ones which are able to travel further through the ring. The ring should have a length of approximately 2 mm. A longer ring may be able to provide better absorption of the particles, but this needs to be balanced against resulting reduction in the volume of the regenerator.

Generally the porosity of the rings 32,33 should match or be greater than that of the regenerator to provide less resistance to flow. However, the ring porosity can be less than that of the regenerator as long as the ring is thin relative to the length of the regenerator (less than 1/30 of the overall regenerator length). This is balanced by the need to allow the filter to be long enough to absorb all particles before the gas passes through. The best design would therefore probably have a porosity roughly equal to the regenerator. Although the example described has a getter ring at its top end and a filter ring at its bottom end, it is possible that a getter ring could additionally or alternatively by used at the bottom end, or that a filter ring could additionally or alternatively be used at the top end.

Further, the getter ring 32 and/or filter ring 33 can also fulfil the function of end rings which are typically provided at the ends of the regenerator, in that they provide a course filter and physical support for the regenerator. Alternatively, the getter and filter rings 32, 33 may be used in addition to separate end rings.

Claims

- 10 - Claims 1. A Stirling machine comprising a displacer piston which

reciprocates within the machine, a regenerator through which an inert gas is alternately pumped in opposite directions by the displacement of the displacer piston, and an oxygen getter positioned in the path of the inert gas.
2. A machine according to claim 1, wherein the oxygen getter is positioned such that the bulk flow of inert gas passes through the oxygen getter.
3. A machine according to claim 2, wherein the oxygen getter covers an end portion of the regenerator.
4. A machine according to any one of the preceding claims, wherein the machine has a hot space associated with one side of the displacer piston and a cold space associated with the opposite side, and the oxygen getter is positioned in the hot space.
5. A machine according to claim 3 and 4, wherein the oxygen getter covers the portion of the regenerator facing the hot space.
6. A machine according to claim 5, wherein the oxygen getter is in the form of a ring which supports the end of the regenerator.
7. A machine according to any one of the preceding claims, further comprising a filter to trap particles from the regenerator.

- 11 -
8. A machine according to claim 7, wherein the filter is made of an absorbent material positioned so that particles breaking off of the regenerator, in use, are embedded within the filter.
9. A Stirling machine comprising a displacer piston which reciprocates within the machine, a regenerator through which an inert gas is alternately pumped in opposite directions by the displacement of the displacer piston, and a filter of absorbent material positioned so that particles breaking off of the regenerator, in use, are embedded within the filter.
10. A machine according to claim 9, wherein the filter is made of a thermoplastic.
11. A machine according to claim 10, wherein the filter is made of PTFE.
12. A machine according to claim 11 wherein the filter is in the form of a ring which supports the end of the regenerator.

12. A machine according to claim 9, 10 or 11, wherein the filter is positioned in the cold space.

13. A machine according to claim 12, wherein the filter covers the end of the regenerator facing the cold space.

14. A machine according to claim 13, wherein the filter is in the form of a ring which supports the end of the regenerator.

638739; MJD; CAR Amendments to the claims have been filed as follows

L

Claims 1. A Stirling machine comprising a displacer piston which reciprocates within the machine, a hot space associated with one side of the displacer piston, a cold space associated with the opposite side, a regenerator through which an inert gas is alternately pumped between the hot and cold spaces in opposite directions by the displacement of the displacer piston, and an oxygen getter positioned in the hot space in the path of the inert gas.

2. A machine according to claim 1, wherein the oxygen getter is positioned such that the bulk flow of inert gas passes through the oxygen getter.

3. A machine according to any preceding claim, wherein the oxygen getter covers the portion of the regenerator facing the hot space.

4. A machine according to claim 3, wherein the oxygen getter is in the form of a ring which supports the end of the regenerator. - 5. A machine according to any one of the preceding claims, further comprising a filter to trap particles from the regenerator.

6. A machine according to claim 5, wherein the filter is made of an absorbent material positioned so that particles breaking off of the regenerator, in use, are embedded within the filter.

7. A Stirling machine comprising a displacer piston which reciprocates within the machine, a regenerator through which an inert gas is alternately pumped in opposite directions by the displacement of the displacer piston, and a filter of absorbent material positioned so that particles breaking off of the regenerator, in use, are embedded within the filter.

8. A machine according to claim 7, wherein the filter is made of a thermoplastic.

9. A machine according to claim 8, wherein the filter is made of PTFE.

10. A machine according to claim 7, 8 or 9, wherein the filter is positioned in the cold space.

11. A machine according to claim 10, wherein the filter covers the end of the regenerator facing the cold space.