Patent Description:
In <CIT> the heat absorbing and releasing structure comprises bent fins fixed to the base of a cylinder. The fins move in and out of the liquid. Fixed baffles in the bottom of the cylinder can be used to reduce the amount of liquid splashing and so help stabilize the liquid. A stable liquid is required to prevent gas liquid mixing and stop the potential for liquid being carried with the gas stream leaving the cylinder.

While baffles in <CIT> help stabilise the liquid and reduce splashing, their effectiveness is limited at increasing speeds.

Due to surface tension and liquid viscosity, a layer of liquid adheres to the fins as they are moved in and out. Not all the liquid adhering to the fins remains attached, some of the liquid falls away from the fins as liquid droplets. This is particularly the case with increasing speed as the higher acceleration forces applied to the fins encourages the liquid layer to separate from the fins.

As the fins move in and out of the liquid, the liquid is displaced by the fin volume, so the liquid surface level moves in the opposite direction to the fins. By design the fins are thin to limit this effect, however as there is a film of liquid attached to both sides of the fin, this increases the apparent thickness of the fins. This increase in apparent thickness can often be as much as <NUM> or <NUM> times the fin thickness dependent on liquid viscosity (fins are typically <NUM> thick). The liquid surface displacement is typically <NUM> to <NUM>% of the piston displacement. With increasing speeds, the liquid surface will experience increasing acceleration forces. Once the peak liquid acceleration approaches or exceeds the gravitational acceleration (<NUM>/s<NUM>) the liquid will separate, as the gravitational acceleration is not sufficient to return the liquid as fast as the liquid is being moved by the withdrawing fins. During these periods of high negative acceleration, the effective gravitational field in the liquid is reversed so that gas then migrates below the liquid surface for part of the cycle. Once this occurs the gas and liquid will mix and form a foam in the bottom of the cylinder and further liquid loss to the exported gas is likely to occur. For example, if the liquid displacement is only <NUM> and the speed is <NUM> then the peak negative acceleration exceeds the gravitational acceleration for part of the cycle.

As the fins move downwards into the liquid, in addition to the liquid acceleration effect described above, it is likely that small gas bubbles will be drawn down due to viscosity and surface tension effects at the gas/ liquid interface.

Gas/liquid mixing in the bottom of the cylinder is detrimental to the operation of the near isothermal technology because:.

<CIT> also describes how heat can be transfer between the internal and external environment with a heat exchanger jacket wrapped around the cylinder or heat exchanger coils inside the cylinder. For this type of heat exchanger to be useful and efficient a large surface area (coils or jacket) is required. This heat exchanger area must be accommodated in or around the cylinder. If high power transfer is required, then larger heat exchangers are needed to make this possible. So, power density of this type of near isothermal machine is limited by the size of heat exchanger it can accommodate.

One further issue in some devices described in the prior art is maintenance of the liquid levels when there is liquid loss, for example, through leakage.

Two machines according to the invention may be mounted together to form a near isothermal Stirling heat pump. In one arrangement the machines are driven with a <NUM>° phase difference.

When compared to existing machines using a heat absorbing and releasing structure, reciprocating into and out of a liquid, the liquid stability in the bottom of the main cylinder is significantly improved. As a result, the isothermal efficiency and output from a machine of a given size are improved. Manufacturability of the heat absorbing and releasing structure is simplified so that the fins that make up the heat absorbing and releasing structure can be more closely spaced.

The invention improves the liquid level (volume) control in main cylinder. With better level control, the minimum ullage volume when the main cylinder is at bottom dead centre can be reduced. This can improve the pressure ratio of the machine and so the potential power density. Any small excess volumes of liquid drawn into the lower chamber can be ejected through the level control port.

<FIG> shows a schematic vertical section of a near isothermal machine <NUM> according to the invention.

A main piston <NUM> compresses or expands a gas in chamber <NUM> as it reciprocates in main cylinder <NUM>. Attached to the bottom of the piston and extending orthogonally therefrom are a plurality of fins <NUM> which forms the heat absorbing and releasing structure of the machine. The fins <NUM> reciprocate in and out of slots 20A in a slotted plate <NUM> which is part of a slotted plate assembly <NUM>. The slotted plate assembly <NUM> is cylindrical shape with the slotted plate <NUM> mounted at atop the slotted plate assembly <NUM> and with the cylindrical wall <NUM> of the assembly <NUM> extending down within the main cylinder <NUM> close to its inner wall. (<FIG> show an illustrative slotted plate assembly <NUM> in greater detail). The bottom of the chamber <NUM> is filled with hydraulic liquid <NUM>. The slotted plate assembly <NUM> is mostly submerged below the liquid level <NUM>, however, the top face of the slotted plate <NUM> is just above the liquid level.

The gas is compressed or expanded between the piston <NUM>, main cylinder <NUM> and liquid <NUM>. As the gas is compressed or expanded it is mainly located between the fins <NUM>. The fins <NUM> provides a large surface area for heat transfer such that the mean distance for heat transfer between the gas and the fins <NUM> is small. As the gas is compressed or expanded at speed, its temperature is held at substantially the same temperature as the fins and gas compression and expansion occurs at near isothermal conditions. The fins' temperature is stabilised by the liquid below it, into which the fins <NUM> are inserted into on every cylinder stroke. The liquid temperature itself stabilised by external heat transfer.

The present invention provides a method to compensate for the liquid displacement that occurs as the fins <NUM> are inserted and withdrawn from the liquid <NUM>. This compensation system ensures the liquid level is maintained at near constant level. The larger volume of the working liquid is enclosed in a slotted plate assembly <NUM> which helps control the thickness of the liquid film that attaches to the fins. Compared to the prior art, the reduced thickness of the liquid film attached to the fins <NUM> in the upward stroke of the piston in this invention helps to reduce the amount of compensation required.

The main cylinder <NUM> has an upper chamber <NUM> and lower chamber <NUM> which are split from one another by the slotted plate <NUM>. Hydraulic and gas links between the chambers are described below.

Liquid level compensation is provided by a compensator piston <NUM>. The compensator piston <NUM> is rigidly connected to the main piston <NUM> via a connection rod <NUM>. The compensator piston <NUM> always operates below the liquid level <NUM>. The compensator piston <NUM> moves in and out of a compensator chamber <NUM> below the lower chamber <NUM> through an aperture 25A in a bush or seal <NUM>, the bush or seal <NUM> sealing the bottom of the lower chamber <NUM> and separating it from the compensator chamber <NUM>. The cross-sectional area of the compensator piston <NUM> or the bore of bush or seal <NUM> is approximately equal to the effective sectional area of fins <NUM> (that is the fin cross sectional area plus the cross-sectional area of the attached liquid film) plus the cross-sectional area of connecting rod <NUM> and the small film thickness attached to rod <NUM>. As the reduced volume of liquid displaced by the fins and connecting rod is the same as the volume gained by the compensator piston <NUM> moving through the bush or seal <NUM> into the lower chamber <NUM>, the liquid level remains constant.

Although a cylindrical bush or seal <NUM> is shown in the drawings mounted in the aperture 25A, the bush or seal can be mounted within a spherical bearing within aperture 25A, to allow for minor relative rotational movement of the compensator piston with respect to the aperture 25A.

The liquid film thickness attached to the fins is difficult to calculate because it depends on a number of factors, the most important ones being the width of slots 20A, liquid viscosity (which can vary with temperature) and operating speed. As a first order approximation, the total film thickness (both sides of the fins <NUM>) is ½ x (width of slot 20A - fin <NUM> thickness). This is reasonably correct calculation when slot widths are no more than <NUM> wider than the fin <NUM> thickness.

The film thickness attached to the connecting rod <NUM> is not very significant in calculating the optimal compensator <NUM> area because the surface area of the rod <NUM> is very small compared to the area of the fins <NUM> so this term can be excluded from the calculation without making much difference.

This calculation can be used for initial sizing of compensator <NUM> area. But the final area should be found or confirmed by testing.

The compensator <NUM> has optional openings <NUM> in its surface in the form of pockets between separators <NUM> along its length. The separators <NUM> have cylindrical lands 48A at their extremities, the lands being co-axial with the compensator <NUM>. As the openings <NUM> move between chambers <NUM> and <NUM>, they transfer liquid between the chambers and so mix the liquids between chambers <NUM> and <NUM>. As the compensator <NUM> moves through bush or seal <NUM> at least one land 48A of a pocket separator <NUM> makes a hydraulic seal between chambers <NUM> and <NUM>.

As the compensator <NUM> reciprocates it is subjected to acceleration forces which it in turn applies to the liquid in the openings <NUM>. The acceleration forces on the liquid in the openings <NUM> makes it circulate with liquid outside the opening <NUM>. This circulating flow can be encouraged by optional large radii <NUM> between the base and sides of the openings <NUM> or by scalloping. This circulating flow ensures effective transfer of the liquid in the openings <NUM> to the adjacent liquid.

The liquid mixing between chambers <NUM> and <NUM> also ensures rapid heat transfer between the chambers as good liquid mixing ensures the temperature in both chambers approach the same value. If the size of compensator <NUM> is not sufficient to provide the required heat transfer its cross-sectional area can be increased by increasing the size of the connecting rod <NUM>. Normally rod <NUM> is sized to safely carry the loads between the main piston <NUM> and the compensator <NUM>, but if needed its cross-sectional area can be increased to facilitate a larger compensator <NUM>, so liquid mixing and heat transfer between the chambers can be increased.

The liquid in compensator chamber <NUM> is pumped through an external heat exchanger <NUM>. As this is an external heat exchanger it can be any size needed to make the required heat transfer. It is not limited by the physical size of the near isothermal compressor or expander as in the machine of <CIT>.

As the compensator <NUM> moves downwards into compensator chamber <NUM> it pumps liquid through check valve <NUM>, then through the external heat exchanger <NUM> and back into a liquid container <NUM>. When the compensator <NUM> moves upwards, liquid <NUM> is drawn into compensator chamber <NUM> via check valve <NUM>. The pumping process then repeats. The preference is for the inlet port, (via check valve <NUM>) to be towards the bottom of compensator chamber <NUM> and the outlet port (via check valve <NUM>) to be towards the top. This arrangement helps with efficient heat transfer because as compensator <NUM> is moving downwards the flow in compensator chamber <NUM> is moving upwards, this helps sweep the liquid transferred from lower chamber <NUM> into compensator chamber <NUM> and then into the external heat exchanger circuit, and in the process ensuring fresh liquid from container <NUM> is moved into lower chamber <NUM> as quickly as possible.

There is a channel <NUM> wholly or partially around the slotted plate <NUM>, between the slotted plate <NUM> and the inner wall of the main cylinder <NUM>. For optimal performance of the near isothermal compressor or expander the liquid level needs to be maintained as close as possible to a predefined level <NUM>, just below the top of the slotted plate assembly <NUM>. The liquid level is set by the level control port <NUM> passing through the wall of the main cylinder from channel <NUM>.

The level control system uses the pressure changes inside the main cylinder <NUM> relative to the external pressure to maintain the correct liquid level. The specific application of near isothermal compressor and expander will influence the relative pressure differences inside the main cylinder compared to the external pressure, so for different applications a slightly different approach is required.

If used in a Stirling cycle the mean internal pressure is approximately the same as the external pressure and the pressure difference will occur around this mean pressure. If used in a gas compressor, the mean internal pressure will be well above the external pressure and will only drop slightly below the external pressure on the suction inlet stroke.

The machine design is such that there is a small amount of net liquid <NUM> flow into the bottom of the lower chamber <NUM>. The net inflow volume is typically between <NUM> and <NUM>% of main cylinder displacement per cycle. This provides a slow continuous filling of the chamber. Once the liquid level reaches the control port <NUM> the excess liquid is expelled from the main cylinder.

<FIG> shows a configuration which would be used if the machine <NUM> was operating in a Stirling cycle. In <FIG> excess liquid from the port <NUM> passes through a restrictor <NUM> and check valve <NUM> back into container <NUM>. The restrictor or orifice <NUM> is size so that the flow rate is slightly more than the net liquid inflow into the bottom of the main cylinder. Check valve <NUM> prevents reverse flow. If the invention is used in a gas compressor, an example arrangement is described below with reference to <FIG>.

In a Stirling cycle (but not a gas compressor) when the liquid level is below port <NUM>, on the compression stroke when the internal pressure is higher than the external pressure, gas is vented via restrictor <NUM> and check valve <NUM>. Gas port <NUM> is connected via a regenerative heat exchanger <NUM> (see <FIG>) to the main cylinder of another near isothermal gas compressor and expander, so some of the working gas is lost from the cycle, this reduces the mean pressure inside the main cylinders to a pressure lower than the external pressure. As the mean pressure is now lower than the external pressure there is a net gas leakage flow into the main cylinders. The gas leakage paths are between the piston <NUM> and bush or seal <NUM> and via optional restrictor <NUM> and optional check valve <NUM>. The system should be designed so that when the liquid level is below control port <NUM> the mean pressure in the main cylinder is depressed but the amount is limited by the leakage of gas back into the main cylinder. Accurately controlling the gas leakage flow between the piston <NUM> and bush or seal <NUM> can be difficult. In many cases it is better to try and reduce the leakage between the piston <NUM> and bush or seal <NUM> as close to zero as possible and then use an alternative leakage flow path through restrictor <NUM> and port <NUM>. Check valve <NUM> is optional to ensure the gas leak is only in one direction. However, there are risks in using check valve <NUM> because in some circumstances the net inflow of gas via port <NUM> may be too high so making the mean operating pressure inside the main cylinder increase and then the level control system will not work. Check valve <NUM> is not a preferred configuration. For a Stirling cycle, restrictor <NUM> only needs to be fitted to one of the main cylinders, or it could possibly be fitted anywhere in the connecting gas flow passages. It should be noted that bush or seal <NUM> acts also as a seal preventing the passage of liquid.

Schematic <FIG> shows two optional flow paths <NUM> or <NUM> from check valve <NUM>. The gas and liquid output from the level control port can be fed directly back via <NUM> into container <NUM> but an advantageous alternative is to feed the gas liquid mixture via <NUM> into a small piston wetting pool10 on top of the bush or seal <NUM>, so that liquid pools in piston wetting pool <NUM>. Any excess liquid simply overflows and drains back into the container <NUM>. This pooled liquid wets the piston <NUM>. It is much better for gas sealing and friction reduction for there to be a wetted piston bush or seal interface.

For the reasons explained above when the liquid level is below the level control port <NUM> the mean internal pressure is reduced. This reduced mean pressure causes a net leakage inflow of liquid into the bottom of the main cylinder. There are two potential paths for this leakage flow, either via an optional control restrictor <NUM> and optional check valve <NUM> (this is not the preferred option) or via the annular clearance gap between the lands 48A of the separators and the bush or seal <NUM>. Unlike the gas seal between the piston <NUM> and its bush or seal <NUM>, the liquid leakage between the compensator and its bush or seal are much easier to control because the liquid has much higher density and viscosity and the diameter of the compensator is smaller than the piston. Typically, the radial gap between the separator lands 48A and bush or seal <NUM> is about <NUM> to <NUM>.

The advantage of not using flow restrictor <NUM> is that it reduces the component count but if leakage flow needs to be increased it can be used with or without check valve <NUM>.

When the liquid level is below level control port <NUM> the mean pressure inside the main cylinder is below the external pressure so there is a net liquid leakage flow into the main cylinder raising the liquid level (for part of the cycle the leakage is outwards, but the net flow is inwards). Once the liquid level covers the level control port <NUM>, gas will no longer leave the main cylinder via this route. As gas is still leaking into the main cylinder (via restrictor <NUM> and between piston <NUM> and bush or seal <NUM>), the mean pressure inside the main cylinder will slowly rise. The rising gas pressure will slowly reduce the net leakage flow of liquid into the main cylinder. This combined with liquid being pumped out via the level control port <NUM> will reverse the situation and the liquid level inside the main cylinder will start to drop. The process will then repeat keeping the liquid level at about the control port <NUM> level. When the liquid level is around the control port level <NUM>, there is often a mixed gas liquid flow being expelled from port <NUM>.

<FIG> shows check valve <NUM> below the level of outlets <NUM> or <NUM>. It can be advantageous to trap some liquid at the check valve outlet, as this keeps the check valve seat wet. If the check valve is working dry it can be difficult to prevent reverse flow when working with gas only.

The preferred configuration is for two main cylinders in a Stirling cycle machine to have individual level control ports <NUM>, restrictors <NUM> and check valve <NUM>. When the liquid level in one main cylinder reaches control port <NUM>, it stops ejecting gas, but gas is still being ejected at the other main cylinder so there is still a reduction in mean pressure but not as much as when both main cylinders were ejecting gas. This situation will continue until the level in the second main cylinder catches up. Generally, both main cylinders will have the same restrictor and liquid leakage rates so their liquid levels will closely match. Tests have shown this works very well.

An alternative is to link the two liquid chambers <NUM> of a Stirling machine together in a similar way to that shown in <FIG> of <CIT>. Then both main cylinders have the same liquid level and a single level control port <NUM> can be used to control the levels in both main cylinders.

When a machine of <FIG> system is stopped, the level in lower chamber <NUM> will gradually drop to the external level <NUM> in container <NUM> because of the designed leakage flow. To ensure the machine primes when the machine is started, the liquid level <NUM> needs to be at or above the level of bush or seal <NUM> but lower than port <NUM> to avoid overfilling the upper chamber <NUM>. Overfilling upper chamber <NUM> could create a hydraulic lock preventing piston <NUM> moving to its bottom dead centre position which may be catastrophic.

In normal operation the liquid transferred between chambers <NUM> and <NUM> via the openings <NUM> in the compensator does not results in a net transfer of liquid. But when the level is low there can be significant splashing in lower chamber <NUM> with some gas being transferred via the openings <NUM> into chamber <NUM>. Some of this transferred gas is then vented through the heat transfer circuit <NUM> and heat exchanger <NUM>. Thus, when the internal liquid level is low the compensator <NUM> can help with initial priming. As the main priming process relies on a small liquid leakage flow it may take some minutes for the machine to prime fully.

In <FIG>, rather than the openings <NUM> being pockets, they are in the form of slots, flat top and bottom and rounded at their sides, which pass diametrically through the compensator <NUM>. Separators <NUM> separate the slots. The separators <NUM> have cylindrical lands 48A, the lands 48A being co-axial with the compensator <NUM>. As the openings <NUM> move between chambers <NUM> and <NUM>, they transfer liquid between the chambers and so mix the liquids between chambers <NUM> and <NUM>. As the compensator <NUM> moves through bush or seal <NUM> at least one land 48A of a separator <NUM> makes a hydraulic seal between chambers <NUM> and <NUM>. At the top of the compensator 34A is a threaded aperture 34B into which a thread extending from the end of connecting rod <NUM> is fitted.

For a gas compressor the mean pressure inside the upper chamber <NUM> will be greater than the external pressure so a different approach to level control is required. <FIG> shows the preferred arrangement for a gas compressor. The gas compressor <NUM> differs from the structure in <FIG> in that the restrictor <NUM> and check valve <NUM> are removed. Gas leakage between piston <NUM> and bush or seal <NUM> should be reduced as far as practical. Any radial gap between the separators <NUM> and bush or seal <NUM> should also be reduced as far as practical to reduce liquid leakage across the lands 48A.

The pressure in upper chamber <NUM> will be below the external pressure during the gas suction stroke, as piston <NUM> moves upwards. During the suction stroke some liquid is drawn into lower chamber <NUM> via check valve <NUM> and restrictor <NUM>. In this case check valve <NUM> is required to stop reverse flow on the compression stroke.

The level control is still via port <NUM> but in this application, it does not vent any gas. When the liquid level is at or above the level control port <NUM> the liquid flows into float vent valve <NUM>. When there is sufficient liquid volume in the float vent chamber the float lifts and allows excess liquid to flow back into container <NUM>.

An alternative to the float vent valve <NUM> could be a level sensor, such that when a predefined level is reached a control valve is opened which allows flow to drain back into container <NUM>.

Part of the slotted plate assembly's <NUM> function is to control the thickness of the liquid film attached to the fins; it also ensures liquid stability in the bottom of the main cylinder. Without the slotted plate assembly <NUM>, liquid splashing and gas liquid mixing would occur as the speed increases. Ultimately, liquid is transferred with the gas through port <NUM>. Once liquid transfer starts the isothermal compressor or expander is not working effectively. The slotted plate assembly <NUM> improves the liquid stability significantly over the interleaved baffles shown in <CIT>.

The slotted plate assembly <NUM> can also be used to support and guide the fins <NUM> allowing the fins to be flat rather than the arcuate or bent fins in <CIT> used to improve structural stability, flat fins being prone to bending.

<FIG> show the slotted plates assembly <NUM> used in the machines of <FIG>, <FIG> and <FIG> and the Stirling cycle machine of <FIG>. These parts can be made by 3D printing or by machining including the use of wire erosion or electrical discharge machining.

The slots 20A are sized to accommodate the fins <NUM> so that they can move up and down freely through the slots without friction. There is a central hole <NUM> to accommodate the connection rod <NUM>. The widths of slots 20A need to be sized to accommodate any tolerancing issues that may arise during manufacture. In the figures the slots 20A are shown as straight to accommodate flat fins, but they could be curved. With curved slots and fins, getting accurate tolerance control between the fins and slots is more difficult, so the slot width may need to increase to accommodate this tolerance issue, this is not advantageous.

Typically, the slot width for a flat fin <NUM> needs to be about <NUM> to <NUM> bigger than the fin thickness. The narrower the slot can be made without inducing any friction the more advantageous it is.

As the fins <NUM> move upwards out of the liquid <NUM>, the fins are wet with a surface layer of liquid. The thickness of this surface layer is limited by slot width, obviously the smaller the slot width the thinner the surface layer. As it is not possible to be assured that each fin <NUM> is central in the slot the liquid layer may be thicker on one side compared to the other.

The less liquid that is attached to the fins as it moves out of the liquid the less likely it is to separate from the fin due to the acceleration forces, as piston <NUM> rapidly moves up and down. If liquid separates it is likely to form liquid droplets which can be carried out with the gas at port <NUM>, this is very undesirable.

When the fins <NUM> moves downwards most of the liquid attached to the fins goes back through the slots into lower chamber <NUM>. However, some of the liquid may be removed or scraped off the fin as it moves down. A chamfer or radius lead into the slots may help reduce the amount of liquid removed as the fins are reinserted into lower chamber <NUM>. The liquid removed during reinsertion will initially sit on top of the slotted plate assembly <NUM> and it then drains into the channel <NUM> between the main cylinder <NUM> and the slotted plate assembly. From here the liquid then drains back through passage <NUM> between the cylindrical wall of the slotted plate assembly and the wall of the main cylinder <NUM> to the bottom of lower chamber <NUM>.

There can be a small difference between the volume of liquid drawn out of chamber <NUM> as the fins are extracted compared to the amount returned when the fins are reinserted. This volume difference allows gas on top of the slotted plate to be drawn down below the slots as the fins are reinserted.

This unwanted gas can accumulate under the slots if provision is not made for its venting. It is difficult for the gas to vent in the narrow gap between the fin <NUM> and the walls of slots 20A in the slotted plate <NUM>. In <FIG> two routes for gas venting are shown. Gas vent slot <NUM> is orthogonal to the fin slots and is typically <NUM> to <NUM> wide. Any gas below the slotted plate can vent up through this larger slot. Additionally, the diameter of the central hole <NUM> which accommodates connecting rod <NUM> is larger than the connecting rod so it can also vent gas.

Adjacent fins below the slotted plate, and the bottom of the slots form a series of potentially isolated gas pockets. Each of these gas pockets needs to be vented, that is why gas vent <NUM> and central hole <NUM> break into every potential gas pocket.

As some gas can be below the slotted plate assembly there may be turbulent gas/liquid mixing when operating at speed circa <NUM>, but the slotted plate assembly will contain this turbulence. At higher speeds increased volumes of liquid <NUM> are returned to lower chamber <NUM> via a passage <NUM>. A small head difference is required to move the liquid through passage <NUM>. At the inlet to the passage <NUM> the head is set by the level of the level control port <NUM>. Inside lower chamber <NUM>, there will be some small volume of gas this will lower the effective head in lower chamber <NUM>. This small head difference of possibly only a few millimetres will drive the returning liquid through passage <NUM>.

In <FIG>, <FIG> and <FIG> the passage <NUM> is the gap between the cylindrical wall <NUM> of the slotted plate assembly and the inner wall of the main cylinder <NUM>. In <FIG> it is small duct between the channel <NUM> at least partially around the slotted plate <NUM> between apertures <NUM> at the bottom of the cylindrical wall <NUM>.

By design, passage <NUM> provides a limited amount of friction and liquid flow inertia. This is required because as the fins are inserted into the liquid in lower chamber <NUM>, the liquid friction between the fins and liquid pushes the liquid downwards in lower chamber <NUM> and then up into passage <NUM>. Conversely as the fins are withdrawn from the liquid the liquid friction pulls the liquid back into lower chamber <NUM> and down in passage <NUM>. This liquid friction effect could potentially cause liquid sloshing in passage <NUM> and the liquid level <NUM> at control port <NUM> would not be stable. The flow resistance and liquid inertia of passage <NUM> needs to be designed to prevent any significant liquid sloshing while at the same time not so much flow resistance that it prevents the easy return of the liquid. It should also be noted that the surface area of channel <NUM> between the slotted plate assembly <NUM> and main cylinder <NUM> is much greater than the area of passage <NUM>, so the amplitude of the small amount of sloshing that does occur in passage <NUM> is reduced at the control port <NUM>.

It is important that the system design keeps the liquid level <NUM> just below the top of slotted plate <NUM>. Once liquid gets permanently on top of the slotted plate <NUM>, gas liquid instability and mixing in the upper chamber <NUM> may occur. This liquid gas mixture can then be transferred through port <NUM> which is very undesirable.

The support fins <NUM> below the slotted plate <NUM> are used to guide and support the fins <NUM>. They are particularly useful when the fins are flat. If the fins <NUM> are curved, the support fins <NUM> can be omitted as the fins <NUM> will be structurally stiffer.

There should be plenty of clearance between the support fins <NUM> and the compensator <NUM> so hydraulic liquid can flow easily between them as the compensator is moved up and down. The support fins are stepped <NUM> so that the central aperture, in which the compensator moves, between the fins is of greater diameter further below the slotted plate <NUM>. This can be seen in <FIG> where a passage 55A is created to allow liquid to move freely.

Compensator piston <NUM> and the support fins <NUM> allow liquid to flow around the compensator, but as the main piston approaches its top dead centre and the compensator approaches the slots, its velocity is also approaching zero. As the flow rate between the compensator <NUM> and support fins <NUM> reduces, the space between the support fins <NUM> and compensator <NUM> can be reduced closer to the slots. This allows the support fins <NUM> immediately below the slotted plate <NUM> to increase the strength of the slotted plate <NUM>.

It can be seen that the upper chamber <NUM> and the lower chamber <NUM> remain linked by the gas vent <NUM> and, hydraulically, by the slots 20A and the passage <NUM>.

<FIG> is a vertical section view through a near isothermal Stirling heat pump comprising a near isothermal compressors and expanders according to the invention. Pinion <NUM> is driven by an electric motor, which drives a Ross Yoke linkage <NUM> which is then connected to connecting rods <NUM>, which in turn drive the two pistons <NUM> of a near isothermal compressors and expanders with a phase angle of about <NUM>° between them.

The output ports <NUM> of the two main cylinders are connected via regenerative heat exchanger <NUM>.

The near isothermal Stirling heat pump is contained in a pressurised container <NUM>. The internal gas could be compressed air or preferably a gas with high thermal conductivity such as helium or hydrogen.

One of the external flow ports is labelled <NUM> in <FIG> but there would be four ports, two output ports (hot and cold flow) and two return ports. The other ports are not shown in this section.

<FIG> shows a detailed section view of a possible alternative external flow circulation system which may be used. The inlet check valve <NUM> is a reed valve. When compensator <NUM> moves upwards check valve <NUM> opens and allows flow into chamber <NUM>. When the compensator <NUM> moves downwards the liquid in openings <NUM> mixes with the liquid in the chamber <NUM> so providing the heat transfer. The liquid then is pumped through chambers <NUM> and duct <NUM> and then out via reed check valve <NUM>.

Most of the heat transfer occurs due to the mixing of liquids in chamber <NUM> but main cylinder bottom has a plug <NUM> made from a thin wall metallic material such as aluminium, this provides an additional thermally conductive heat transfer path between the liquids in chambers <NUM> and <NUM>. While this heat transfer is not significant it does provide some benefit at little extra cost.

<FIG> shows the level control and piston liquid lubrication system. The level control is as previously described using level control port <NUM>, flow restrictor <NUM> and check valve <NUM>. The ejected liquid from the level control system is fed via passage <NUM> into piston wetting pool <NUM>. Passage <NUM> will always retain some liquid to keep the seat of the check valve <NUM> wet, even when gas is being ejected via level control port <NUM>.

<FIG> shows the gas leakage restrictor <NUM>. In this case only one of the main cylinder pair has a gas leak restrictor <NUM>. This near isothermal Stirling heat pump does not use a check valve <NUM> as shown in <FIG> so small quantities of gas can leak in and out, this is the preferred arrangement for a near isothermal Stirling heat pump.

In <FIG>, the piston <NUM> is longer than its bush or seal <NUM>. In <FIG> and <FIG>, it is the other way around, there is a relatively long main cylindrical bush or seal <NUM> which forms a gas seal with the relatively short piston <NUM> yet allows the piston <NUM> to reciprocate within the main cylinder <NUM>. The advantage of a short piston and long bush or seal <NUM> is that it reduces the weight of the moving parts, which can be advantageous. This arrangement is shown in more detail in <FIG>.

A liquid retainer <NUM> moves up and down with the piston. It is made of plastic or some other lightweight material. There is a small annular gap <NUM> (seen in <FIG>) between the liquid retainer <NUM> and the bush or seal <NUM>. This annular gap between the liquid retainer <NUM> and bush or seal <NUM> can fill with liquid. The liquid provides lubrication and helps reduce gas leakage between the piston <NUM> and bush or seal <NUM>.

Claim 1:
A machine for compressing or expanding gas which comprises: a piston (<NUM>); a vertical main cylinder (<NUM>) or main cylinder inclined to the vertical; a bush or seal (<NUM>) inside the main cylinder through which the piston moves; a heat absorbing and releasing structure comprising a plurality of fins (<NUM>) attached to and disposed orthogonally to a bottom of the piston (<NUM>); wherein the piston (<NUM>) moves downwards in a compression stroke with respect to the main cylinder (<NUM>) and upwards with respect to the main cylinder (<NUM>) in an expansion stroke, the main cylinder (<NUM>) containing a substantially constant volume of liquid (<NUM>) maintained at a substantially constant temperature and a variable volume of gas, wherein the gas temperature is controlled to substantially the same temperature as the liquid (<NUM>) by the movement with the piston (<NUM>) of the heat absorbing and releasing structure (<NUM>) between the variable gas volume and the liquid; characterised in that a connector rod (<NUM>) is orthogonally attached to the base of the piston (<NUM>) and to a compensator (<NUM>), the compensator oscillating, in use, upwards into the main cylinder and downwards into a compensator chamber (<NUM>) mounted below the main cylinder and containing the same liquid, the volume of the compensator (<NUM>) entering the main cylinder on an upward movement at least partially compensating for the drop in liquid level (<NUM>) in the main cylinder (<NUM>) on an upward movement of the heat absorbing and releasing structure (<NUM>) and the volume of the compensator (<NUM>) leaving the main cylinder on a downward movement at least partially compensating for liquid level gain in the main cylinder (<NUM>) on downward movement of the heat absorbing and releasing structure (<NUM>).