Stirling engines are well-known in the prior art and normally comprise two reciprocating pistons, one called a displacer piston and one called a working piston. A significant feature of a Stirling engine is that a working fluid, typically a gas, receives heat through the wall of a cylinder (rather than internally as in an internal combustion engine) and heat is removed from the working gas as it passes through a cooler of the engine. As a consequence of the resulting temperature difference between the heated and cooled states of the working gas, some of the thermal energy is converted into mechanical energy which can be applied to a mechanical load.

The majority of Stirling engines comprise a two piston arrangement of a displacer piston and a working piston. Stirling engines are commonly arranged such that the displacer piston has a relatively long stroke, the stroke being larger than the diameter of the piston. This is the accepted practice for Stirling engines.

Many different configurations of linear motors and alternators have been tried with Stirling engines including moving coil, moving iron and various moving magnet configurations. Moving coil configurations tend to be more expensive than the other types, requiring a much greater volume of magnetic material to generate a comparable magnetic field to a moving magnet configuration. Different variations of moving magnet configurations have been tried including ones with radial laminations and axially stacked

laminations.

Many magnet configurations, including most utilising radial laminations, are inefficient due to the magnets leaving the air gap between the magnetic material during the stroke of the working piston.

This causes strong fringing fields to be set up outside the linear alternator which. induce wasteful eddy currents in neighbouring components.

One of the most commonly used alternators is supplied by Sunpower, Inc. This alternator comprises a substantially annular stack of inner laminations located co-axially inside a stationary coil and outer lamination, with an annular air gap defined therebetween. An annular magnet assembly reciprocates within the air gap, driven by the working piston.

Whilst the Sunpower, Inc. engine works perfectly satisfactory in most applications, there are situations where the size of the alternator can cause problems. Another Sunpower, Inc. linear alternator is described in US4602174. However, in this alternator at least some of the curved or annular magnets move axially out of the air gap during the stroke of the working piston leading to unwanted fringing losses as described above.

In addition, curved or annular magnets are expensive to manufacture. Also, assembly of large curved or annular magnets is very difficult since the forces are large with the result that specialised constructions rigs are necessary to hold the magnets in place during assembly.

The present invention provides a linear alternator for use with a free piston Stirling engine comprising: an annular stationary coil assembly; and an annular movable magnet assembly; the annular stationary coil assembly comprising a plurality of magnetic material assemblies

circumferentially arranged about the annular stationary coil assembly, each magnetic material assembly comprising a first section and a second section of magnetic material separated by an air gap, the plurality of first sections defining at least one annular coil bed in which there is received a coil of electrically conducting material; the annular movable magnet assembly comprising at least one series of magnets circumferentially arranged around the annular movable magnet assembly; wherein the annular movable magnet assembly is movable in the air gap along a stroke directed substantially perpendicularly to the annular coil of electrically conducting material such that during the stroke of the annular movable magnet assembly the magnets always remain within the air gap between the first and second sections of the annular stationary coil assembly, and wherein each magnet is shaped as a rectangular parallelepiped.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which :- Fig. 1 is a schematic side elevation of a linear alternator; Fig. 2 is a schematic plan view of the linear alternator of Fig. 1; Fig. 3 is a schematic side elevation of an alternative linear alternator; Fig. 4 is a schematic plan view of the linear alternator of Fig. 3; Fig. 5 is a cross-sectional view of a linear alternator according to the present invention; and Fig. 6 is a plan view of the linear alternator of Fig. 5, with the inner and outer support frames omitted.

Figures 1 and 2 illustrate schematically the working of a linear alternator embodying some of the principles of the present invention. The linear alternator comprises a stationary coil assembly 1 and a movable magnet assembly 2.

The coil assembly 1 comprises outer and inner sections 3,4 of laminated magnetic material defining therebetween an air gap 5. The outer section 3 of magnetic material is planar and of uniform thickness.

The inner section 4 of magnetic material has a body portion 6 aligned parallel to the outer section 3. and three arm portions 7-9 dependant from the body portion 6 and arranged substantially perpendicularly thereto.

The upper and middle arm portions 7,8 define therebetween an upper coil bed 10. The middle and lower arm portions 8,9 define therebetween a lower coil bed 11.

Two substantially planar coils 12, 13 of electrically conducting wire or similar are seated in the coil beds 10,11 with the plane of the coils 12, 13 being aligned with the axes of the arm portions 7- 9. Consequently, the coils 12, 13 circle the inner section 4 of magnetic material.

The magnet assembly 2 comprises two magnets 14, 15 and three sections of non-magnetic material 17-19 attached to a connecting member 16 which couples the magnets 14,15 and non-magnetic material 17-19 to the working piston of the Stirling engine. The magnets 14,15 and non-magnetic material 17-19 are arranged in an alternating pattern such that magnet 14 is bordered by non-magnetic material sections 17 and 18, and magnet 15 is bordered by non-magnetic material sections 18 and 19. In this way, short-circuiting of the flux of the magnets 14,15 is prevented.

Any suitable magnet type may be used. Typical examples include Neodymium-Iron-Boron, Samarium-Cobalt and Ferrite.

The sections of non-magnetic material 17-19 are preferably electrically non-conducting. However, the sections may be electrically conducting with little loss in efficiency being incurred.

The magnet assembly 2 is located within the air gap 5 and aligned parallel to the faces of the outer section 3 and arm portions 7-9 of magnetic material.

The connecting member 16 constrains the magnet assembly 2 to move only axially within the air gap 5 so as not to contact the magnetic material 3,4.

The number and positioning of the magnets in the magnet assembly 2 may be adjusted to meet the required power output from the alternator.

Operation of the Stirling engine causes the working piston to reciprocate, which in turn causes the connecting member 16 and magnet assembly 2 to reciprocate in air gap 5. Relative movement between the magnets 14,15 and the non-magnetic material 3,4 induces an electrical voltage in coils 12,13. The stroke of the connecting member 16 is such that no part of the magnets 14,15 leaves the air gap 5 between the inner and outer sections 3,4 of magnetic material during the stroke. Rather, at one extreme of the stroke magnet 14 is aligned with arm portion 7 and at the other extreme of the stroke magnet 15 is aligned with arm portion 9. Only the non-magnetic material sections 17 and 19 ever move out of alignment with the stationary coil assembly 1. Thus, the flux path is well defined and there is little or no flux leakage or fringing. This results in an increased efficiency of the alternator.

The use of two coils 12,13 results in a more compact design. As a result the alternator may be used in situations where a larger alternator could not be accommodated. For example, an engine, incorporating the alternator of the present invention may be used in a domestic micro Combined Heat and

Power (CHP) system.

It has also been found that this type of alternator produces a high degree of linearity of the alternator's output voltage during the piston stroke.

This is important as it allows a user to determine, and control, the engine's stroke using the output voltage.

Figures 3 and 4 illustrate an alternative linear alternator, again embodying some of the principles of the present invention wherein two coil assemblies 1 and two magnet assemblies 2 are located back to back.

Both magnet assemblies 2 are coupled to a common connecting member 16. The coils 12,13 pass through the coil beds 10,11 of both coil assemblies 1.

Figures 5 and 6 show a linear alternator according to the present invention. The coil assembly 1 and moveable magnet assembly 2 have been shaped to be annular having a rotational symmetry about a longitudinal axis 20. Similar components to those described previously have been referenced with similar numbers.

The annular linear alternator comprises a plurality of laminated structures 30 each of which has an inner section 4 and an outer section 3 substantially as described above with reference to Figures 1 to 4, defining an annular air gap 5 therebetween. For example, in the embodiment shown, the inner sections 4 define two coil beds 10,11 which each receive an annular coil of electrically conducting material, such as wire turnings.

Alternatively, the coil beds 10,11 may be defined by the outer sections 3.

The laminated structures 30, which may be thought of as magnetic material assemblies, are circumferentially arranged around the longitudinal axis 20 in an annular, or ring, formation. Preferably the circumferential spacing of the laminated

structures 30 is uniform.

The moveable magnet assembly 2 is formed into an annular structure having a plurality of magnets 14,15 circumferentially arranged therein. The magnet assembly 2 comprises a holder or connecting member 16 which is preferably formed from aluminium or other non-magnetic material. Preferably, the magnets 14,15 are fixedly located in. the holder 16 by means of a glue such as epoxy or'superglue'.

The magnets 14,15 are shaped as rectangular parallelepipeds, in this embodiment, having a length to depth to width ratio of 4: 2: 1. In one particular embodiment the length of the magnets is 20mm, the depth of the magnets measured in the direction of the longitudinal axis 20 is 10mm and the thickness measured in the radial direction perpendicular to the longitudinal axis 20 is 5mm. Other length ratios may be used.

The linear alternator as shown in Figure 5 is shown housed in a support structure comprising an inner lamination support frame 31 and an outer lamination support frame 32. The outer lamination support frame 32 is connected by means of bolts 34 to a base support 35.

Operation of the annular linear alternator works on the same basic principles as that of the alternators shown in Figures 1 to 4. Advantageously, the use of a plurality of discrete magnets 14,15 circumferentially inter-arranged with non-magnetic material (such as aluminium) causes the movable magnet assembly 2 to move reciprocally along axis 20 without rotating about axis 20.

In the above embodiments the electrically conducting coils 12, 13 are located in the inner section or sections 4 of the stationary coil assembly 1. However, the arrangement may be reversed such that the coils 12,13 are located on the outside of the

movable magnet assemblies 2.

In this embodiment the coils 12,13 are almost entirely enclosed by the magnetic material assemblies 30 leading to a tolerable degree of flux leakage and voltage retention.

Advantageously, the rectangular parallelepipeds magnets 14,15 are relatively small compared to magnets conventionally used in annular linear alternators and can be easily manipulated and assembled by hand. In addition, the magnets 14,15 are inexpensive and do not produce large magnetic interactive forces during assembly such that the linear alternator may be assembled without the need for a specialised construction rig.