GB2460221A

GB2460221A - Free vane Stirling engine

Info

Publication number: GB2460221A
Application number: GB0808884A
Authority: GB
Inventors: Michael Bolwell
Original assignee: HYBRIDISE Ltd
Current assignee: HYBRIDISE Ltd
Priority date: 2008-05-13
Filing date: 2008-05-15
Publication date: 2009-11-25
Also published as: WO2009138724A2; GB0808666D0; WO2009138724A3; GB2481182A; GB0819117D0; GB0808884D0

Abstract

A Stirling cycle device 10 comprises a vessel 11 housing a displacer vane 14 and a power vane 24, in communication with each other and pivotable about an axis of the vessel. The vanes are each coupled to the vessel by magnets 15, 18 providing a resilient bias, and an electric means comprising magnets 18 and coils 16 allows the movement of the displacer can be electrically controlled. The magnets 18 and coils 16 of the power vane 24 allow power to be extracted (or supplied in heat pump mode). There may be two displacers, one at either axial end of the power vane cavity (figure 4). The power vane and its cavity may be replaced by a resonant tube (207, figure 8) and a turbine (210, figure 8).

Description

Free Vane Stirling Engine The present invention relates to a Stirling cycle device, specifically, but not exclusively to a free vane Stirling device suitable for use as an engine, generator, heat pump, or heat driven heat pump and which is designed to efficiently operate with low to moderate temperature differentials.

Since its invention in 1816, the Stirling engine has held much promise as a prime mover, being able to operate quietly and safely on any type of fuel or even solar heat. However, its widespread use has been held back by factors such as low power density and cost when compared to the Otto four stroke engine. It is however, becoming clear that the high grade fuel required for the Otto engine is in short supply, a situation that can only worsen with time.

The most successful prior art Stirling engines to date have been based on piston designs, using hydrogen, heJium or nitrogen at elevated pressures as the working fluid. A continuous combustion process can be used to supply heat, so most types of harmful emissions can be reduced. There are kinematic forms of piston Stirling engine, these use mechanical linkages to couple the displacer and power producing elements of the engine while maintaining a phase difference between their movements. These linkages introduce friction, complexity, weight and cost to the engine and also increase its physical size. These disadvantages become more apparent at low temperature differentials as the increased friction from the larger linkages required to move a large displacer consumes a greater portion of the energy available, making the engine inefficient. They can also suffer from leakage of the working fluid through shaft seals.

The free piston Stirling engine overcomes many of the problems with kinematic Stirling engines by eliminating the mechanical linkage between the displacer and power pistons. However, considerable skill in the design, elaborate construction means and close manufacturing tolerances are required to centre the pistons within their cylinders and prevent friction or internal damage resulting from overstroke of the pistons. These measures are reflected in the end cost of the engine and make it impractical for low temperature differential applications. There are diaphragm forms of free piston engine but these tend to be physically large in relation to the power they produce. The thermo-acoustic forms of Stirling engine also present considerable design challenges and additionally do not work at low temperature differentials.

There are also fluidyne Stirling engines. These make power generation possible from low temperature differentials but the nature of these engines makes them unsuitable for portable or mobile applications. They also tend to be physically large for the power they produce.

The substantial form of the pressure vessel required to contain the pressure of the working gas limits the power density and adds to the cost of prior art engines. The use of exotic materials has enabled prior art engines to be operated on very high hot end temperatures of over 600 degrees Celsius, so they can operate at high efficiency but this further adds to their cost and restricts their use to high grade fuels or highly concentrated solar power.

The theoretical Carnot efficiency available with lower grade heat sources is limited. For this reason, the cost of prior art engines has generally prevented their use on lower grade sources of heat, such as waste heat from internal combustion engines or industrial processes, heat from wood stoves, dung, wood-gas, geothermal or un-focused solar energy.

In the art, the power output of a Stirling engine tends to be constant and to adjust it requires careful design and additional mechanisms, often leading to increased complexity and cost. Typically, changes in output are achieved in the art by varying the displacement of the engine (often through use of a swashplate crankshaft arrangement), or by changing the quantity of working fluid, or by altering the piston/displacer phase angle, or in some cases simply by altering the engine load. This property is less of a drawback in hybrid electric propulsion or "base load" utility generation where constant power output is actually desirable.

In the art, there tends to be three different types of Stirling design, one type centres around a more traditional piston design, with a displacer, piston and heat exchanger, the second design uses vanes or moving baffles to displace the internal fluid across the heat exchanger, the third design is based on a rotary design. In the art, Stirling engines have not been effectively used as low temperature differential (LTD) generators or heat pumps.

US Patent number 3,460,344, granted in 1969, describes a Stirling cycle system with reciprocating vanes on a common shaft, with double acting versions within a single cylinder. The heat exchangers are external to the engine and the two vanes are mechanically linked. It is not suitable for use as an LTD Stirling engine or heat pump due to friction losses in the mechanical linkages.

US patent number 4,312,181 describes a variable volume vane displacer Stirling engine. The power output is regulated by varying the physical size of the displacer. It requires complex and elaborate mechanical means to vary the displacer size and the design of the outer housing does not lend itself to pressurisation of the working gas.

US patent number 4,455,841 describes a heat actuated heat pump operating on the Vuilleumier cycle, using mechanically or pneumatically driven vanes. The vanes are in separate cylinders and the engine does not operate on the Stirling cycle. The Vuilleumier cycle is less efficient than the Stirling cycle.

Patent 5,907,201, (WO 99/28685), describes an electrically driven displacer using a linear electric motor. It relates only to free piston engines.

US patent number 7,134,279 describes a double acting, thermodynamically resonant, free piston, multi-cylinder Stirling system which is complex, elaborate, space inefficient and relates only to free piston engines.

US patent 5,115,157 describes a liquid sealed vane oscillator where the vane oscillation is shaft driven from an external source. It generates power on the principle of electromagnetic conduction through the sealing liquid, the sealing liquid is also an integral part of the heat exchange process. The use of liquids in this manner restricts the orientation of the machine when in use and adds to the overall complexity of the machine, making it unsuitable for portable or low power applications. There are also potential problems of containing the liquid where the vane shaft exits the vane housing and toxicity of the liquid used.

US patent 5,337,562 discloses a vane Stirling engine, but it cannot be highly pressurised and the vane does not have a large angular displacement, making it impractical in small size applications. It uses mechanical linkages and is also not designed for operation at the higher frequencies used in mains generation. The heat exchanger design is large for the amount of heat throughput. These factors limit the efficiency and power density of this engine.

US 6,195,992 and Japanese patent number JP60212659 describe a rotary Stirling engine, with external heat exchangers. They are not reciprocating or oscillating engines.

US Patent 6,568,169, describes a fluidic piston LTD Stirling, for waste heat conversion in industrial processes, operating on the principle of the fluidyne Stirling engine mentioned above. It is not designed for use in applications where size or portability are important considerations.

The present invention therefore seeks to provide a Stirling cycle device, which overcomes, or at least reduces some of the above-mentioned

problems of the prior art.

Accordingly, in a first aspect, the invention provides a Stirling cycle device comprising a vessel, wherein the vessel houses a displacer means and power means, the displacer means and power means being in communication one with the other, the displacer means and power means being movable about an axis of the vessel, the displacer means further comprising a first resilient bias means coupling to the vessel; the power means further comprising a second resilient bias means coupling to the vessel; and a first electric means, wherein the first electric means controls the movement of the displacer means.

The free vane Stirling engine design of the present invention is optimised for low to moderate temperature differentials.

According to a second aspect, the invention provides a Stirling cycle device, the device comprising a vessel, wherein the vessel houses a displacer means in a displacer means cavity and a power means in a power means cavity; the displacer means and the power means cavities being in communication one with the other, the displacer means and the power means being pivotable about an axis of the vessel, the displacer means further comprising a first resilient bias means coupling to the vessel; the power means further comprising a second resilient bias means coupling to the vessel; a first electric means, wherein the first electric means controls the movement of the displacer means; and wherein the power means cavity further comprises a volume separating means and wherein the displacer means cavity further comprises two heat exchange means.

Preferably and according to first and second aspects, the displacer means and power means may be vanes.

Further preferably wherein the device is cylindrical and may be pressurised.

One advantage of the present invention is that the cylindrical engine design minimizes the external dimensions for a given internal working volume resulting in a very compact design. Further, the cylindrical design means it can be pressurized and sealed which gives great benefits to its performance.

Further preferably wherein the displacer means and! power means may be driven, or drives an output, using an integral or otherwise, magnet and coil assembly.

Further preferably wherein there are two displacer means.

One advantage of the present invention is that the double displacer design can help maintain efficiency at lower temperature differentials. This would normally add much cost and complexity to a Stirling engine, but the present invention is well suited to double acting operation.

The double displacer vane design means that the pressure can be doubled across the power vane, theoretically this allows significantly more power to be extracted from a given temperature difference.

According to a third aspect, the invention provides a Stirling cycle device comprising a vessel, wherein the vessel houses a displacer means, the displacer means being movable about an axis of the vessel, the displacer means further comprising a first resilient bias means coupling to the vessel; a power means, wherein the power means is a resonant tube which further comprises a turbine coupled to the displacer means and positioned within the resonant airflow; and wherein the power means is in fluid communication with the displacer means; and an electric means, wherein the first electric means controls the movement of the displacer means.

Further preferably and in accordance with aspects 1 to 3, wherein the operating mode and power of the device may be controlled electrically by changing the phase and amplitude of the displacer means.

According to a fourth aspect, the present invention provides a method of switching between different operating modes of a Stirling cycle device, the device comprising a displacer vane, the method comprising using electric means to change the phase of the oscillation of the displacer vane.

According to a fifth aspect, the present invention provides a method of varying the output power of a Stirling cycle device, the device comprising a displacer vane, the method comprising using electric means to change the amplitude of the oscillation of the displacer vane.

The device of the present invention is very versatile, the same unit can be used as a generator, heat pump, or heat driven heat pump, depending upon whether the unit is electrically driven to create a temperature differential or whether it is used as a low temperature differential converter, using solar power for example to produce cooling or electricity or both in inversely varying proportion. Further, the electrically driven displacer design of the present invention allows control of the power output and rapid stopping and starting of the device.

Other preferable features are cited in the remaining dependent claims.

Three embodiments of the invention will now be more fully described, by way of example, with reference to the drawings, of which: Figure 1 is a diagram showing a vane Stirling cycle device, according to one embodiment of the present invention; Figure 2 is a diagram showing a plan view of a displacer vane, taken along line Z, of the vane Stirling cycle device of Figure 1; Figure 3 is a diagram showing a plan view of a power vane, taken along line Y, of the vane Stirling cycle device of Figure 1; Figure 4 is a diagram showing a vane Stirling cycle device, according to a second embodiment of the present invention; Figure 5 is a diagram showing a plan view of a first displacer vane, taken along line Xl, of the vane Stirling cycle device of Figure 4; Figure 6 is a diagram showing a plan view of a power vane, taken along line Yl, of the vane Stirling cycle device of Figure 4; Figure 7 is a diagram showing a plan view of a second displacer vane, taken along line Zi, of the vane Stirling cycle device of Figure 4; Figure 8 is a diagram showing a vane Stirling cycle device, according to a third embodiment of the present invention; Figure 9 is a diagram showing a plan view along line A, of the vane Stirling cycle device of Figure 8; and Figure 10 is a diagram showing a second arrangement of the vane Stirling cycle device of Figure 8; In a brief overview of one embodiment of the present invention, there is shown in Figure 1 a vane Stirling cycle device 10. The first embodiment of the vane Stirling device 10 is described with reference to Figures 1, 2 and 3 following. The same numbering is used for the same features throughout these Figures, where applicable.

The Stirling cycle device 10 comprises a cylinder 11, which further comprises an axially pivoted displacer vane 14 and power vane 24, each within its own cylindrical cavity, 13 and 12 respectively. Portions of the cylinder 11 are held together by bolts 20.

There are also two heat exchangers and a regenerator (shown collectively as 17, described in further detail with reference to Figure 2 following) occupying a portion of the displacer cavity 13. The displacer vane 14 is pivoted around a first central pivot 27 (see Figure 2) and the power vane is pivoted centrally within the power cavity 12 by a second central pivot 26 (see Figure 3).

The displacer vane 14, as it is pivoted back and forth, sweeps a gas (in this case air) in the displacer cavity 13 back and forth, causing the gas to be alternately heated and cooled as it emerges from the heat exchangers (17).

The resulting cyclic pressure variation acts on the power vane 24 via a gas transfer ports 23 located between the two cavities 13, 12.

The displacer vane 14 is resiliently coupled to the cylinder 11 using fixed centering magnets 15 coupled to vane drive magnets 18, so it can oscillate either side of a rest position. The magnet coupling arrangement 15, 18 in conjunction with the mass of the displacer vane 14 have a resonant frequency.

The oscillation of the displacer vane 14 is electrically driven via vane drive magnets 18 and fixed coils 16 and controlled by an electric controller (not shown). Further, the amplitude and phase of the oscillations of the displacer vane 14 are electrically variable relative to the oscillation of the power vane 24. In this way the power output can be modulated and the device 10 easily stopped, started or its operating mode changed from generator to heat pump with the power vane 24 being electrically driven. The drive magnet assembly 18 is attached to an iron ring 28, to which the displacer vane 14 and vane shaft (see Figure 2) are attached.

The power vane 24 is also resiliently coupled to the cylinder 11 using a second set of fixed centering magnets 15' coupled to a second set of vane drive magnets 18', so it can also oscillate either side of a rest position. The second drive magnet assembly 18' is attached to an iron ring 28', to which the power vane 24 and vane shaft (see Figure 2) are attached. As the power vane 24 oscillates in reaction to the movement of the displacer vane 14, the power vane drive magnets 18' also interact with fixed generator coils 16' to generate electricity and charge a battery (not shown).

Both vanes 14, 24 are supported by a ball race 21 or similar low friction support at each end of the two vane shafts. The generator coils 17, 17' are fixed to iron stators 29, 29' which complete the magnetic circuit of the vane drive magnets 18, 18, locates the vane shaft ball race 21 and is attached to the cylinder 11 via the divider that separates the cavities 12, 13.

The power vane cavity 12 is separated into portions by a baffle 19. A bounce space 25 is also shown and is described further with reference to Figure 3.

Figure 2 is a diagram showing a plan view of a displacer vane, taken along line Z, of the vane Stirling cycle device of Figure 1. There is shown the axially pivoted displacer vane 14 within its own cylindrical cavity 13.

Located within the displacer cavity 13 is a hot heat exchanger 171 and an ambient heat exchanger 171' (although in use these could be the other way around). Also shown is a regenerator 172 which also occupies a portion of the displacer cavity 13 and which removes heat from, or returns heat to, a working fluid, in this case air, as the fluid passes back and forth across it (the hot heat exchanger 171, ambient heat exchanger 171' and regenerator 172 are shown collectively as 17).

In the present embodiment, the regenerator 172 is made from stainless steel ribbon loosely packed between the heat exchangers 171, 171'. The regenerator 172 could be of aluminium flat plate design and be integrated with the construction of the heat exchangers 171, 171', forming a one piece design with low gas flow resistance. The heat exchangers 171, 171' are comprised of pipes passing through an aluminium foil construction with fluid filling the pipes. The fluid filled pipes could contain oil, antifreeze, steam or any fluid with a high heat capacity at the desired temperature of operation.

The displacer vane 14, as it is pivoted back and forth around its central pivot point 27, sweeps the air in the displacer cavity 13 back and forth, causing the gas to be alternately heated and cooled as it emerges from the hot and ambient exchangers 171, 171'. The resulting cyclic pressure variation acts on the power vane (not shown) via a gas transfer ports 23. The outer cylinder 11 and a vane counterweight 22 are also shown. The vane counterweight 22 is sized to balance the weight of the displacer vane 14 about its pivot, minimizing the potential for vibration and allowing the device to operate in any orientation. Lead may be a suitable choice of material, but someone skilled in the art should understand that any other suitable material may be used.

Figure 3 is a diagram showing a plan view of a power vane, taken along line Y, of the vane Stirling cycle device of Figure 1. There is shown the power vane cavity 12 which is divided into two sectors by a fixed baffle 19 and the pivoting power vane 24 to allow the expansion and compression of a gas as it emerges from the displacer cavity (not shown) through the a gas ports 23.

The port 23 is arranged so the rising or falling pressure in the displacer cavity pushes or pulls on the power vane 24.

The power vane cavity 12 is separated from a resilient gas bounce volume (feature 25 in Figure 1) and is fluidly accessible by port 23'. The mass of the power vane 24 and the compliance of the bounce volume 25 and displacer cavity 13 have a resonant frequency, by suitable sizing of masses and compliances, the two vanes 14, 24 are given similar resonant frequencies. The work done on the power vane 24 by the pushing or pulling of the change in pressure caused by the motion of the displacer vane 14 is converted into electricity by a second coil 16' and magnet 18' arrangement to which the power vane 14 and the power vane shaft 26 are attached. A vane counterweight 22 is also shown.

The power vane 24 is resiliently held in place by a second set of centering magnets 15' which couple with the power vane drive magnets 18'. It should be clear to someone skilled in the art that the centering magnets 15, 15' vane drive magnets 18, 18' and generator coils 16, 16' can also be housed outside the displacer and/or power vane cavities, or in any other suitable arrangement. Further, in the place of centering magnets 15', a spring or other suitable resilient bias means could be used to keep the displacer and power vanes in a specific resting position.

The device 10 of the present embodiment can either act as a generator or heat pump. When acting as a generator, a heat differential (between hot and ambient or cold & ambient) drives the power vane 24 and moves the magnet 18' relative to the coil 16', causing the coil to generate electricity.

Specifically, the cyclic movement of the displacer vane 14 moves air alternately out of the hot, then ambient, heat exchangers 171, 171'. This causes a cyclical rise and fall in air temperature and volume/pressure in the displacer cavity 13. It's this cyclic pressure change that drives the power vane 24 and so generates electricity via the magnet 18' and coil 16' arrangement. Further, a portion of the generated power is taken to provide an input drive for the displacer vane 14 (the initial energy required to start the displacer vane 14 may be provided by a battery or other such device).

The power generated by the power vane coils 16' is as an alternating current and needs to be converted to direct current by the control circuitry (not shown). The control circuit senses the position (phase) of the power vane 24 by detecting the zero crossing point of the generated alternating current waveform (this is the point at which the vane magnets 18' pass directly over the coils 16'). The control circuit provides a current pulse to the displacer vane coil 16 to sustain the oscillation of the displacer vane 14 and can advance or retard the pulse timing as necessary to maintain the 900 phase difference between the vanes 14, 24. The displacer vane 14 is driven purely by the current from the control circuit, and so indirectly from the heat differential. The temperature difference needs to be great enough that the power generated by the power vane 24 exceeds the power consumed by the displacer vane 14 & control circuit, or the engine 10 won't run. The engine 10 is able to generate power from lower temperature differentials as compared to other prior art Stirling designs as there are fewer moving parts and less friction. Moreover, the displacer volume can be increased relative to the power vane without a significant friction penalty, this also assists with operation at low temperature differentials.

There is a further benefit with the electrically driven displacer. The 90° phase difference between the displacer and power components of a Stirling engine assumes perfect, instantaneous heat transfer between the working gas and the heat exchangers and regenerator. In practice, this is not the case and real life Stirling engines are often designed with a phase difference less than 90°. Somewhere between 60° and 90° may be best to compensate for the thermal time-lag between the air and the heat exchangers, particularly in high speed engines. Advancing the displacer phase can allow the engine to produce more power at higher speeds but can also make it harder to start. Many model and experimental Stirling engine designs allow for easy adjustment of the displacer phase by adjusting the crank positions when the engine is stationary, in this way the optimum phase difference can be found to suit the application. Further, some experimental dual swashplate design Stirling engines have been built where the displacer phase can be altered while the engine is running, this is also used as a way of controlling the power from the engine. With the device of the present invention, this feature is built in. The phase difference between the vanes can be adjusted and set easily while the engine is running to find the optimum setting.

Alternatively, the power vane 24 can be driven electrically to cause a heat differential in the displacer cavity 13 and so the device can be used as a heat pump. Specifically, the driven power vane 24 cyclically compresses and expands the gas, causing cyclic temperature changes. The driven disp'acer vane 14 cyclic&ly moves the hot compressed gas through one heat exchanger and the expanded cooled gas through the other. If the hot compressed gas is passed out of the ambient heat exchanger, heat is removed from it. The other heat exchanger becomes cold as the cooled, compressed gas is subsequently expanded, causing the gas temperature to drop betow ambient. Similarly by reversing the phase of the displacer vane 14, the expanded cooled gas is passed out of the ambient heat exchanger, taking heat from the heat transfer liquid within it to pass to the other heat exchanger on the compression part of the cycle, so heating it and the heat transfer liquid within. In both operating modes, electrically driven pumps (not shown) may be used to move the heat transfer liquid through the heat exchangers. The electricity for the pumps may be derived from the power vane coils in generator mode or from the external power supply when in heat pump mode.

Figures 4, 5, 6 & 7 are diagrams showing a vane Stirling cycle device, according to a second embodiment of the present invention. The same numbering is used for the same features throughout these Figures, where applicable.

There is shown in Figure 4 a vane Stirling cycJe device 100 which comprises a cylinder 101 which further comprises two displacer cavities 102, 102', each containing an axially pivoted displacer vane 103, 103'. Each displacer cavity 102, 102' also comprises a regenerator and two heat exchangers, shown here collectively as 104 (and described further with reference to Figures 5 and 7).

The displacer vanes 103, 103' have a rest position set by a compliant centering magnets 105 coupled to vane drive magnets 108 to the displacer cavities 102, 102'. There is also a vane drive arrangement comprising the vane drive magnets 108, attached to the vanes 103, 1031 and coils 106 fixed to the displacer cavities 102, 102'. The drive magnets 108 interact with the fixed coils 106 to provide drive to the displacer vanes 103, 103' via a control circuit and battery (not shown). The displacer vanes 103, 103' oscillate in unison and sweep the gas in each displacer cavity 102, 102' back and forth through the heat exchangers (104), creating a rise and fall in gas pressure as the gas is heated and cooled.

The heat exchangers (104) are arranged so the gas in one displacer cavity 102 is heated as the gas in the other cavity 102' is cooled. The two displacer cavities 102, 102' are connected through ports 110, 110' to a common power vane cavity 115. The power vane cavity 115 further comprises an axially pivoted, compliantly fixed vane 107, having a resonant frequency similar to the displacer vanes 103, 103'.

The displacer vane 103, 103' resonance is determined by the mass of each vane assembly 103, 103' and the compliance of the vane positioning magnet arrangement 105.

The power vane cavity 115 has a fixed baffle (feature 125 with reference to Figure 6), this and the power vane 107 form cavities for the expansion and contraction of gas flowing through the ports 110, 110'. The ports 110, 110' are arranged so the rising pressure in one displacer cavity 102 pushes on the power vane 107 as the falling pressure in the other displacer cavity 102' pulls the power vane 107. A drive magnet assembly 108' is attached to the power vane 107 and interacts with fixed generator coils 106' to generate electricity and charge a battery (not shown). All vanes 103, 103', 107 are supported by a ball race 109 or similar low friction support at each end of the vane shaft (112).

The power vane 107 has a rest position set by a second compliant centering magnets 105 coupled using the power vane drive magnets 108' to the power vane cavity 115. It should be noted that the centering magnets 105' and generator coils 106' on the power vane 107 are different compared to the displacer vanes 103, 103'. Smaller centering magnets 105' are needed for the power vane 107 as the displacer air volumes provide most of the spring effect and the power vane generator coils 106' carry more current than the displacer drive coils 106.

Figure 5 is a diagram showing a plan view of a first displacer vane, taken along line Xl, of the vane Stirling cycle device of Figure 4. There is shown a displacer cavity 102 which comprises a displacer vane 103 which can be pivoted around a central pivot 112 and a port 110. Further, there is shown a regenerator 112 and two heat exchangers 104, 104'. The outer cylinder 101, as well as the displacer vane centering magnets 105, drive magnets 108 and generator coils 106 are also shown. A displacer vane counterweight 111 is also shown.

Figure 6 is a diagram showing a plan view of a power vane, taken along line Yl, of the vane Stirling cycle device of Figure 4. There is shown a power vane 107 which is axially pivoted by a central pivot 112 and which is housed in a power vane cavity 115. The power vane cavity 115 also comprises a fixed baffles 125 which separates the power vane cavity 115 into portions.

Two ports 110 & 110' can also be seen, which connect the power vane cavity 115 fluidly with the upper and lower displacer cavities 102, 102'. The outer cylinder 10]., as well as the power vane centering magnets 105', drive magnets 108' and generator coils 106' are also shown. A power vane counterweight 111' is also shown.

Figure 7 is a diagram showing a plan view of a second displacer vane, taken along line Zi, of the vane Stirling cycle device of Figure 4. There is shown a displacer cavity 102' which comprises a displacer vane 103' which can be pivoted around a central pivot 112'. A port 110' is also shown. Further, there is shown a regenerator 112' and two heat exchangers 104, 104'. The outer cylinder 101, as well as the second displacer vane centering magnets 105, drive magnets 108 and generator coils 106 are also shown. A vane counterweight 111 is also shown.

In both of the previous two embodiments, the power vane oscillation needs to be in 90 degree phase shift to the displacer vane(s) in oscillation -this gives the Stirling cycle. The matching of the vanes resonant frequencies improves the efficiency of the engine.

Figures 8, 9 & 10 are diagrams showing a vane Stirling cycle device, according to a third embodiment of the present invention. The same numbering is used for the same features throughout these Figures, where applica ble.

There is shown a vane Stirling cycle device 200 which comprises two displacer cavities 201, 201' and two displacer vanes 203, 203'. The displacer vanes 203, 203' are given a rest position by two sets of centering magnets 205, 205'. Two sets of magnets 208 and generator coils 206 are also shown and operate as described with reference to previous figures.

The two displacer cavities 201, 201' are linked by a tube 207, the air mass within and the air volume in the displacer cavities 201, 201' form a double-ended Helmholtz resonator whose resonant frequency is made similar to that of the displacer vanes 203, 203' by appropriate sizing of the tube length and diameter.

The displacer cavities also house two heat exchangers and a regenerator (shown collectively as 204) which work in operation in the same manner as previously described.

Two turbines 210, 210' are coupled to and drive each of the displacer vanes 203, 203' from the oscillating air column in the tube 207. Only one turbine would be necessary to drive a single displacer vane or both displacer vanes if they were on a common shaft. Fixed air flow guides 212, 212' direct and straighten the air flow though the turbines 210, 210'. It should be clear to someone skilled in the art that the turbines 210, 210' and air flow guides 212, 212' could be located at any point within the resonant air flow.

The amplitude of the resulting displacer vane 203, 203' oscillation is controlled (damped) by drawing power from coils 206 to charge the battery (not shown) and generate power. The oscillation may be started initially by using a battery (not shown) to drive the displacer vanes 203, 203'.

Figure 9 is a diagram showing a plan view along line A, of the Stirling cycle device of Figure 8. There is shown a displacer cavity 201 which comprises a displacer vane 203 which can be pivoted around a central pivot 213.

Further, there is shown a regenerator 202 and two heat exchangers 204, 204'. The outer cylinder 201, as well as the displacer vane centering magnets 205, drive magnets 206 and generator coils 208 are also shown. A vane counterweight 211. is also shown.

The air in the Heimholtz tube 207 needs to be in 9odegree phase shift to the displacer vane in oscillation -this gives the Stirling cycle. The matching of the resonant frequencies improves the efficiency of the engine. In the present embodiment, the matching resonances are necessary to give a 90 degree phase difference between the oscillating air mass and the displacer oscillation required for Stirling cycle operation.

Figure 10 is a diagram showing a second arrangement of the vane Stirling cycle device of Figure 8. There is shown a different arrangement of parts as when compared to Figure 8, specifically there is shown a device 200, which comprises a tube 207 which interconnect two displacer cavities 203, 203'.

The displacer cavities each house a displacer vane 207, 207' and a heat exchanger and regenerator arrangement 204, 204'. Also shown is the location of the turbines 210, 210', vane centering magnets 205, 105', drive magnets 208, 208' and generator coils 206, 206'.

In all designs of engine in the three embodiments described, the amplitude of the displacer vane oscillation can be controlled electrically so as to modulate the power generated. The process is reversible, so by swapping the heat exchanger roles in one displacer cavity, electricity is used to drive the oscillation and create a temperature difference in the heat exchangers, so the engine is now a heat pump.

Further, with reference to embodiments 2 and 3, a temperature difference may be provided to just one displacer unit to drive the oscillation and provide a temperature difference in the other displacer unit. In this mode, the device is a heat driven heat pump and may be used for solar powered air conditioning or refrigeration. A small amount of electricity may also be generated at the expense of heat pumping capacity.

In embodiments 2 and 3, if both heat exchanges have the same temperature difference across them and the displacers move in the same direction to drive power vane and further, the hot and ambient exchangers are swapped at one end so the gas is heated by one displacer unit as it is cooled by the other, this results in twice the pressure across the power vane, twice the gas volume flow, therefore resulting in up to four times the power for a given temperature difference. The same result may be achieved by reversing the phase of the electrical drive to one displacer vane, this may be preferable as having the vanes oscillating in opposite directions would cancel the reaction forces from the displacer vane motion and minimise vibration of the engine.

If the displacer vanes are driven whilst the power vane oscillation is electrically driven, this results in a heat pump with the same temperature differential being created in both heat exchanger arrangements 204, 204'.

The amount of heat pumping is in proportion to the electrical input, and so the double acting design of embodiments 2 and 3 should improve the heat pumping capacity for a given size of unit.

If the displacers are driven whilst there is a temperature differential in one heat exchanger arrangement 204, a temperature differential will be created in the other heat exchanger arrangement 204'. This provides for a heat driven refrigeration unit (for example a solar powered heat driven refrigeration unit). Specifically, a temperature difference between hot and ambient in one displacer unit is converted to cyclic pressure & temperature changes in the gas. This is then transferred via the power vane to the other displacer unit creating another temperature difference in the second displacer cavity. If the exchanger that gets heated is held at ambient temperature, the heat is removed from that displacer unit, resulting in its other heat exchanger getting chilled. Sufficient electric power would need to be drawn from the power vane coils to drive the displacer vanes and the heat carrying liquid pumps if used, leaving less power for cooling, but if there is an abundance of heat and modest cooling requirements (as in a solar powered fridge) this shouldn't be an issue.

To change between generator and heat pump modes, the only thing that changes is the flow of electric current from/to the engine and the phase of the power vane relative to the displacer vanes.

To change from generator to heat driven chiller mode, remove the flow of heat to one heat exchanger and remove the electrical load on the power vane alternator. Chilled water could then be drawn from the formerly heated heat exchanger.

The embodiments of the present invention are compact, simple and elegant designs with few moving parts. The use of a cylinder maximises the internal working volume, whilst minimising the external dimensions of the engine.

Low tech materials and engineering may be used in its production and this should make the unit cheap to produce. The vane design frees the designer from considering the interaction of the moving masses caused by pressure waves, or the design of floating gas bearings, unlike free piston prior art designs of engine.

The main component of the engine of the present invention, the cylinder, can also act as a pressure vessel, allowing the engine to be charged with gas at several atmospheres. As the shape is naturally pressure resistant, lightweight pressed steel can be used. The generator/drive arrangement gives less drag as it moves through the gas compared with linear designs, it is also more compact. There are none of the friction, wear or sealing problems found on kinematic engines and many of the design difficulties and sensitivity to load changes of free piston engines are avoided.

Further, the ease in which the one engine can be switched between three operating modes is a unique feature; the different modes can be selected by simply changing the phase of the drive to a displacer vane. The large area of the heat exchangers helps reduce flow losses, making the embodiments of the present invention particularly suitable for lower temperature differentials.

The present invention lends itself to the double acting form of engine where there are no dead spaces or bounce spaces to waste energy or take up space. In this case the efficiency and power density at low temperature differentials are maximised. The design could easily accommodate a direct solar heated heat exchanger (as disclosed in US Patent 6,735,946 for solar free piston engine) through a quartz quadrant window in the displacer cylinder.

For small engine sizes, the entire surface area of the cylinder could be used as the ambient heat exchanger with suitable internal and external fins. Low weight, low heat conductivity materials such as carbon fibre could be used for the vanes and the resulting low inertia would allow the vane motion to be easily modified using a suitable arrangement of vane centering magnets to give a dwell period at the vane's extremes of travel, the resulting non-sinusoidal motion would more closely match the 4 stages of the ideal Stirling cycle. While this is known in the art, current mechanical means to introduce dwell periods tend to introduce additional friction and complexity, partially negating the efficiency gains.

The present invention is readily scaleable. For example, an array of displacer vanes could feed into a common central resonant tube or power vane in large installations, e.g. waste heat generation at power stations or industrial processes where steam needs to be condensed, the latent heat in steam being harnessed for electricity generation as it condenses within the engines heat exchangers. Small scale applications could include improving the efficiency of the internal combustion engine, the hot exchanger being a gas to gas heat exchanger, forming part of the internal combustion engine's exhaust system. In this way the efficiency of a hybrid car could be improved, particularly on steady speed cruising where hybrid cars currently don't show economy benefits over conventional cars.

As with all Stirling engines, there is vibration caused by the moving masses.

However, the lightweight vanes of the present invention can easily be counterbalanced about the pivot axis, the only remaining vibration would be a torsion-al oscillation of the cylinder about its axis. In the turbine driven displacer vane form of engine (embodiments 1 & 2), the vanes could counter oscillate, so this source of vibration would also be cancelled, resulting in a very quiet engine. In embodiments 1 or 2, the power vane oscillation may be cancelled out by the use of an axially mounted, electrically driven, counter-oscillating balance weight. Alternatively, the engine may be installed using resilient mountings on the cylinders axis to greatly reduce the transmission of vibration from the engine. Stopping the transmission of tortion-al oscillation in this way is far more effective than when dealing with the linearly induced vibration of other engine designs.

To enable a low temperature differential Stirling engine to produce a useable amount of power, it needs to move a large volume of air through efficient, low flow loss regenerator and heat exchangers with minimal friction losses.

It also needs to be double acting. To do this with conventional prior art designs of Stirling engine would result in a large, inefficient and costly machine. The present invention is a double acting, thermodynamically resonant, free vane, single cylinder Stirling system with electrically operated vanes. It can be used efficiently to provide a low temperature differential Stirling generator in a cost efficient and compact manner.

The present invention provides a Stirling engine design which is a simple, lightweight, low cost construction using commonly available materials, compact design and the ability to achieve moderate efficiencies and power densities from modest temperature differentials. Further the present invention has the added benefits of controllability, self starting and low noise & vibration.

The present invention also provides a design that readily allows the use of two engines back-to-back, as this makes possible double acting forms of engine to further improve efficiency and power density. Back-to-back operation also makes possible the generation of cold from heat (e.g. solar powered refrigerators, in-car air conditioners powered by the heat in the car's exhaust) or even additional heat from heat, such as a gas fired Stirling heater that pumps heat from outside for space heating. Such a system would yield far greater than 100°h heating efficiency as compared to the 85 -90% typical of conventional gas fired heating systems.

It will be appreciated that although only three particular embodiments of the invention has been described in detail, various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention.

Claims

Claims: 1. A Stirling cycle device comprising: a vessel, wherein the vessel houses a displacer means and power means, the displacer means and power means being in communication one with the other, the displacer means and power means being movable about an axis of the vessel, the displacer means further comprising a first resilient bias means coupling to the vessel; the power means further comprising a second resilient bias means coupling to the vessel; and a first electric means, wherein the first electric means controls the movement of the displacer means.
2. A device as claimed in claim 1, wherein the displacer means and power means are housed in cavities within the vessel.
3. A device as claimed in claim 2, wherein the power means cavity further comprises a volume separating means.
4. A device as claimed in any preceding claim, wherein the displacer means and the power means are pivotable about an axis of the vessel
5. A device according to any of claims 2 to 4, wherein the displacer means cavity further comprises a heat exchanger.
6. A Stirling cycle device comprising: a vessel, wherein the vessel houses a displacer means in a displacer means cavity and a power means in a power means cavity; the displacer means and the power means cavities being in communication one with the other, the displacer means and the power means being pivotable about an axis of the vessel, the displacer means further comprising a first resilient bias means coupling to the vessel; the power means further comprising a second resilient bias means coupling to the vessel; a first electric means, wherein the first electric means controls the movement of the displacer means; wherein the power means cavity further comprises a volume separating means; and wherein the displacer means cavity further comprises two heat exchange means.
7. A device as claimed in any preceding claim, wherein the displacer means and the power means are vanes.
8. A device as claimed in any preceding claim, wherein the displacer means and the power means are pivotable about the same axis.
9. A device according to any preceding claim, further comprising a second electric means, wherein the second electric means controls the movement of the power means.
10. A device according to any of claims 2 to 9, wherein the displacer means cavity further comprises a regenerator.
11. A device as claimed in any of claims 2 to 10, wherein the power means cavity further comprises a bump volume.
12. A device as claimed in any preceding claim wherein the displacer means and power means are coupled by bearings to the vessel.
13. A device as claimed in any preceding claim, further comprising a vane counterweight.
14. A device as claimed in any preceding claim, further comprising an axially mounted, electrically driven, counter-oscillating balance weight.
15. A device according to any preceding claims, wherein the first and second resilient bias means is arranged to allow the displacer and power means to move with a sinusoidal oscillatory motion.
16. A device according to any of claims 1 to 14, wherein the first and second resilient bias means is arranged to allow the displacer and power means to move with a non sinusoidal motion and increase the dwell time at the end of each sweep.
17. A device as claimed in any preceding claim wherein the displacer means is driven using a magnet and coil assembly.
18. A device as claimed in any of claims 1 to 16, wherein the displacer means drives an output means using a magnet and coil assembly.
19. A device as claimed in any of claims 1 to 17 wherein the power means is driven using a magnet and coil assembly.
20. A device as claimed in any of claims 1 to 17 wherein the power means drives an output means using a magnet and coil assembly.
21. A device as claimed in any of claims 17 to 20, wherein the magnet and coil assembly is integral.
22. A device according to any preceding claim, wherein there are two displacer means.
23. A device as claimed in any preceding claim, wherein the vessel is cylindrical.
24. A Stirling cycle device comprising: a vessel, wherein the vessel houses a displacer means, the displacer means being movable about an axis of the vessel, the displacer means further comprising a first resilient bias means coupling to the vessel; a power means, wherein the power means is a resonant tube which further comprises a turbine coupled to the displacer means and positioned within the resonant airflow; and wherein the power means is in fluid communication with the displacer means; and an electric means, wherein the first electric means controls the movement of the displacer means.
25. A device as claimed in claim 24, wherein the resonant tube further comprises fluid guides.
26. A device as claimed in any of claims 24 or 225, wherein the displacer means is housed in a cavity.
27. A device as claimed in any of claims 24 to 26, wherein the displacer means is coupled by bearings to the vessel.
28. A device according to any of claims 24 to 27, wherein the resilient bias means is arranged to allow the displacer means to move with a sinusoidal oscillatory motion.
29. A device according to any of claims 24 to 27, wherein the resilient bias means is arranged to allow the displacer means to move with a non sinusoidal motion and increase the dwell time at the end of each sweep.
30. A device as claimed in any of claims 24 to 29, wherein the displacer means is driven using a magnet and coil assembly.
31. A device as claimed in any of claims 24 to 29, wherein the displacer means drives an output means using a magnet and coil assembly
32. A device according to any of claims 24 to 31, wherein the device is driven or gives drive to an oscillating air mass.
33. A device according to any preceding claim wherein the operating mode and power of the device may be controlled electrically by changing the phase and amplitude of the displacer means.
34. A device according to any preceding claim wherein, the resonant frequency of all vanes and/or air columns are broadly matched.
35. A device according to any preceding claim, further comprising control circuitry.
36. A device according to any preceding claim, further comprising a battery.
37. A device according to any preceding claim, wherein the vessel is pressurised in use.
38. A device according to any preceding claim, wherein the device is suitable for use as a Stirling cycle engine, generator, heat pump, or heat driven heat pump.
39. A method of switching between different operating modes of a Stirling cycle device, the device comprising a displacer vane, the method comprising: using electric means to change the phase of the oscillation of the displacer vane.
40. A method according to claim 39, further comprising altering the temperature of the fluid in the heat exchangers.
41. A method of varying the output power of a Stirling cycle device, the device comprising a displacer vane, the method comprising: using electric means to change the amplitude of the oscillation of the displacer vane.
42. A device as substantially hereinbefore described with reference to Figures 1 to 3.
43. A device as substantially hereinbefore described with reference to Figures 4 to 7.
44. A device as substantially hereinbefore described with reference to Figures 8 to 10.