2

3 The present invention relates to a heat engine and method of manufacture. In particular,

4 the described heat engine utilises a phase change of a fluid to convert thermal energy to

5 mechanical energy.

6

7 Background to the Invention

8

9 A heat engine is a cyclic device which converts heat into work, or in other words, thermal0 energy into mechanical energy. In general, a heat engine contains a working substance,1 such as a gas or fluid, that absorbs heat from a high temperature reservoir, does work on2 its surrounding and releases heat as it returns to its initial state. There exist numerous3 different types of heat engines known in the art which operate on this basic principle, such4 as an internal combustion engine. 5 6 The working substance of a heat engine cyclically undergoes changes in pressure,7 temperature, and volume as well as the addition and removal of heat. For example, within8 an internal combustion engine, a gas comprising a fuel-air mixture is compressed and then9 ignited causing the gas to subsequently expand and drive a piston. The motion of the 1 piston if configured to expel the ignited gas and draw in unignited gas for the cycle to

2 continue.

3

4 Despite their ubiquitous use, there are numerous disadvantages to an internal combustion

5 engine. An internal combustion engine requires a fuel to operate and cannot operate on

6 waste heat from an external high temperature (TH) source. It is necessary to ignite the fuel

7 to drive a piston which creates noise and requires numerous moving components. These

8 components can degrade and fail with use over time, requiring regular maintenance and

9 ultimately limiting the lifetime of the engine. Furthermore, a suitable fuel for an internal0 combustion engine is typically limited to expensive, refined gaseous or liquid hydrocarbon1 compounds. In addition, the combustion of the fuel results in undesirable toxic and2 environmentally unfriendly gases. Internal combustion engines are also not scalable and3 so are not suitable for large scale power generation. 4 5 An external combustion engine operates by an external high temperature (TH) source6 heating a working fluid through a heat exchanger or engine wall. The heat causes the7 working fluid to expand driving a piston. External combustion engines, such as steam8 engines, can exploit numerous types of heat sources and such engines are widely used. 9 Nevertheless, these engines are typically suited to large scale power production so are0 large, heavy, expensive devices, which can be unsafe and relatively inefficient. An1 external combustion engine also comprises moving components which creates noise and2 requires maintenance. 3

1 Summary of the Invention

2

3 It is an object of an aspect of the present invention to provide a heat engine that obviates

4 or at least mitigates one or more of the aforesaid disadvantages of the heat engines

5 known in the art.

6

7 According to a first aspect of the present invention there is provided a heat engine

8 comprising:

9 a housing; 0 a first liquid and a second liquid located within the housing, the first liquid having a1 higher density and lower boiling point than the second liquid; 2 a heat exchanger to transfer heat to the first liquid to evaporate the first liquid to form3 a first liquid vapour; and 4 at least one fluid flow member to move in response to a fluid flow created by the5 interaction of the first liquid vapour and the second liquid. 6 7 Most preferably, the housing is sealable. The heat engine is a closed heat engine. In this8 arrangement the first and or second liquids are not added and or removed during 9 operation. 0 1 Preferably, the first and second liquids occupy an interior volume of the housing. The first2 and second liquids may mix within the interior volume of the housing. 3 4 Preferably, the first liquid is located within a first portion of the housing. The second liquid5 is located within a second portion of the housing. 6 7 Most preferably, the first liquid is de-mineralised water and the second liquid is Xylene.8 Alternatively, the first liquid is de-mineralised water and the second liquid is kerosene.9 Alternatively, the first liquid is decafluoropentane and the second liquid is de-mineralised0 water. Alternatively, the first liquid is chloroform and the second liquid is de-mineralised1 water. 2 3 Preferably, an operating temperate range of the heat engine is between 1 10 to 150 °C. 4 Alternatively, the operating temperature range of the heat engine is between 70 to 90 °C. 1 Preferably, the heat exchanger transfers heat from an external high temperature heat

2 source to the first liquid.

3

4 Preferably, the heat exchanger is the first portion of the housing. Alternatively, the heat

5 exchanger is a pipe. The pipe may pass through the first portion of the housing.

6

7 Optionally, the heat engine may further comprise one or more pellets. The one or more

8 pellets are located within the interior volume of the heat engine. The one or more pellets

9 are suspended within the first liquid and or second liquid. The density of the one or more0 pellets is between the density of the first liquid and second liquid. The pellets are 1 chemically unreactive with the first liquid, second liquid, and or first liquid vapour. 2 Preferably, the pellets are magnetically neutral. Alternatively, the pellets are magnetic.3 4 Most preferably, the at least one fluid flow member may take the form of one or more rods. 5 The one or more rods may comprise a first end and a second end. The first ends of the6 one or more rods are preferably mounted to an interior surface of the housing. The one or7 more rods may extend into the interior volume of the housing. The second ends of the one8 or more rods are preferably free to move. The second ends of the one or more rods are9 preferably located towards a central axis of the housing. 0 1 Preferably, the one or more rods are uniformly distributed about the interior surface. 2 Alternatively, the one or more rods are non-uniformly distributed about the interior surface. 3 4 Preferably, the one or more rods are orientated perpendicular to the interior surface. 5 Alternatively, the one or more rods are orientated non-perpendicular to the interior surface. 6 7 Preferably, the one or more rods are uniformly dimensioned. Alternatively, the one or8 more rods are non-uniformly dimensioned. 9 0 Preferably, the one or more rods comprise the same material composition. The one or1 more rods may comprise brass. Alternatively, the one or more rods comprise different2 material compositions. 3 4 Optionally, the at least one fluid flow member may take the form of one or more plates.5 The one or more plates preferably comprise one or more perforations. The one or more 1 plates are preferably dimensioned in the form of a circular cross-section of the housing.

2 The one or more plates may be mounted to the interior surface of the housing. The one or

3 more plates may intersect the central axis of the housing.

4

5 Optionally, the at least one fluid flow member may take the form of one or more

6 diaphragms. The one or more diaphragms may comprise one or more perforations.

7

8 Optionally, the at least one fluid flow member may take the form of one or more pellets.

9 The one or more pellets are magnetic. 0 1 Preferably, the housing comprises an inlet port and an outlet port. The inlet and outlet2 ports are preferably sealable. 3 4 Optionally, the heat engine further comprises a condensing loop. The condensing loop5 transfers heat to an external low temperature heat sink from the first liquid vapour. The6 condensing loop preferably condenses the first liquid vapour and returns the first liquid to7 the first portion of the housing. 8 9 Optionally, the heat engine further comprises a sink. The sink may comprise the first0 liquid. The sink is preferably connected to the housing. The sink maintains the level of the1 first liquid within the first portion of the housing. 2 3 According to a second aspect of the present invention there is provided an energy 4 harvesting system comprising a heat engine in accordance with the first aspect of the5 present invention, an energy conversion means and an external high temperature heat6 source. 7 8 Optionally, the energy harvesting system may further comprise an external low 9 temperature heat sink. 0 1 Most preferably, the energy harvesting system may further comprise a vibrational lens.2 3 Preferably, the vibrational lens comprises at least two focusing members, each of the at4 least two focusing members having a first end for attachment to a source of vibration and a5 second end, wherein the at least two focusing members are arranged such that the 1 separation between the focusing members decreases from the first ends towards the

2 second ends.

3

4 Most preferably, the at least two focusing members each comprise a first portion located

5 between the first end and second end. The first portions of the at least two focusing

6 members are angled relative to each other such that the at least two focusing members

7 converge at the second ends.

8

9 Preferably, the at least two focusing members each comprise a second portion located at0 the first end. Preferably, the second portions of the at least two focusing members are1 substantially parallel. 2 3 Most preferably, the vibrational lens further comprises a backplate. The first ends of the at4 least two focusing members may be fixed to the backplate. The second portions of the at5 least two focusing members may be fixed to the backplate. 6 7 Preferably, the at least two focusing members each comprise a third portion located at the8 second end. The third portions of the at least two focusing members are substantially9 parallel. The third portions of the at least two focusing members define a focal point of the0 vibrational lens. 1 2 Preferably, the at least two focusing members comprise brass. 3 4 Optionally, the at least two focusing members comprise two or more layers and or5 coatings. The two or more layers and or coatings may exhibit different vibrational and or6 thermal characteristics. The at least two layers and or coatings may comprise different7 dimensions, materials, densities and or grain structures. 8 9 Optionally, the at least two focusing members comprise a first layer and a second layer. 0 The first layer is fixed to the second layer. The first layer may comprise brass. The1 second layer may comprise steel. 2 3 Optionally, the vibrational lens further comprises one or more springs. The one or more4 springs connect the at least two focusing members. 1 Optionally, the vibrational lens further comprises one or more weights attached to one or

2 more of the at least two focusing members.

3

4 Optionally, the vibrational lens further comprises a dynamic control system. The dynamic

5 control system changes the vibrational characteristics of the vibrational lens during

6 operation. The dynamic control system may adjust the stiffness of the spring. The

7 dynamic control system may adjust the location and or magnitude of the weights.

8

9 Optionally, the vibrational lens may comprise three focusing members. 0 1 Most preferably, the focusing members are focusing plates. 2 3 Alternatively, the focusing members are focusing rods. 4 5 Most preferably, the first end of the vibrational lens is fixed to the heat engine. 6 7 Most preferably, the energy conversion means is located at the second end of the8 vibrational lens. Preferably, the energy conversion means is located between the third9 portions of the at least two focusing members. 0 1 Optionally, the housing of the heat engine further comprises sealable openings. The rods2 of the heat engine are directly connected to the focusing members of the vibrational lens. 3 The rods pass through the sealable openings. 4 5 Preferably, the energy conversion means is one or more piezoelectric crystals. 6 Alternatively, the energy conversion means is one or more coils. 7 8 Alternatively, the energy conversion means is a coil. The coil may be wound around the9 housing of the heat engine. 0 1 Embodiments of the second aspect of the invention may comprise features to implement2 the preferred or optional features of the first aspect of the invention or vice versa. 3 4 According to a third aspect of the present invention there is provided a method of5 manufacturing a heat engine comprising, 1 - providing a housing,

2 - providing a first liquid and a second liquid located within the housing, the first liquid

3 having a higher density and lower boiling point than the second liquid;

4 - providing a heat exchanger to evaporate the first liquid to form a first liquid vapour;

5 and

6 providing at least one fluid flow member that moves in response to a fluid flow

7 created by the interaction of the first liquid vapour and the second liquid.

8

9 Preferably, the method of manufacturing a heat engine may further comprise determining0 the characteristics of an external high temperature heat source. 1 2 Preferably, determining the characteristics of the external high temperature heat source3 may include determining the temperature, energy, power, variability and or duration of the4 external high temperature heat source. 5 6 Preferably, the method of manufacturing a heat engine may further comprises determining7 optimum parameters of a heat engine for use with the external high temperature heat8 source. 9 0 Preferably, determining the optimum parameters of a heat engine for use with the external1 high temperature heat source may further comprise utilising the characteristics of the2 external high temperature heat source. 3 4 Preferably, determining the optimum parameters of a heat engine may comprise 5 determining: the dimensions of the heat engine; the volume, relative ratio and chemical6 composition of the first and second liquids; the distribution, orientation, dimensions and or7 material composition of the at least one fluid flow member; the operational proximity of the8 heat engine to the high temperature (TH) heat source; if a condensing loop is required; and9 if a sink is required. 0 1 Embodiments of the third aspect of the invention may comprise features to implement the2 preferred or optional features of the first and or second aspect of the invention or vice3 versa. 4 1 According to a fourth aspect of the present invention there is provided a method of

2 manufacturing an energy harvesting system comprising,

3 - providing a heat engine in accordance with third aspect of the present invention;

4 - providing an external high temperature heat source; and

5 providing an energy conversion means.

6

7 Preferably, the method of manufacturing an energy harvesting system comprises providing

8 an external low temperature heat sink.

9 0 Preferably, the method of manufacturing an energy harvesting system may comprise1 providing a vibrational lens. 2 3 Preferably, providing a vibrational lens comprises, 4 - providing at least two focusing members, each having a first end and a second5 end; and 6 - arranging the at least two focusing members such that the separation between the7 at least two focusing members decreases from the first ends towards the second8 ends. 9 0 Preferably, providing a vibrational lens further comprises determining the characteristics of1 the heat engine. 2 3 Preferably, determining the characteristics of the heat engine comprises quantifying any4 one of the following parameters: the dimensions of the heat engine, the dimensions of at5 least one fluid flow member and the frequency characteristics of any mechanical 6 vibrations. 7 8 Preferably, providing a vibrational lens may further comprise determining the optimum9 parameters of the vibrational lens for use with the heat engine. 0 1 Preferably, determining the optimum parameters of a vibrational lens comprises 2 determining an optimum length, width and or depth of the at least two focusing members;3 and or the optimum separation of the first ends of the at least two focusing members; and4 or the optimum separation of the second ends of the at least two focusing members; and5 or the optimum distance for the at least two focusing members to converge; and or the 1 optimum material or materials for the at least two focusing members; and or the optimum

2 coefficient of thermal expansion of the material or materials of the at least two focusing

3 members.

4

5 Optionally, determining the optimum parameters may also include: determining the depth

6 of a first layer and a second layer of the at least two focusing plates; the material of the

7 first layer; and the material of the second layer. The first layer may comprise brass. The

8 second layer may comprise steel.

9 0 Preferably, providing the heat engine is performed before providing vibration lens. 1 2 Optionally, the method of manufacturing a vibrational energy harvesting system may be3 iterative. The heat engine may be optimised following providing the vibrational lens. 4 5 Embodiments of the fourth aspect of the invention may comprise features to implement the6 preferred or optional features of the first, second and or third aspects of the invention or7 vice versa. 8 9 According to a fifth aspect of the present invention there is provided a heat engine0 comprising: 1 a housing; 2 a first liquid and a second liquid located within the housing, the first liquid having a3 higher density and lower boiling point than the second liquid; 4 a heat exchanger to transfer heat to the first liquid to evaporate the first liquid to form5 a first liquid vapour; and 6 at least one fluid flow member to move in response to a fluid flow created by the7 interaction of the first liquid vapour and the second liquid, 8 wherein the heat exchanger is adapted to receive heat from thermal radiation and or9 one or more geothermal heat sources. 0 1 Most preferably, the heat exchanger is adapted to receive heat from thermal radiation2 wherein the thermal radiation comprises solar radiation. 3 4 Most preferably, the first and second liquids occupy an interior volume of the housing. 5 1 Preferably, the first liquid is located within a first portion of the housing and the second

2 liquid is located within a second portion of the housing.

3

4 Optionally, the heat engine further comprises one or more filament portions. The one or

5 more filament portions are an extension of the first portion of the housing. The first liquid

6 is located within both the first portion of the housing and the one or more filament portions.

7

8 Preferably, the heat exchanger comprises the first portion of the housing. Additionally, or

9 alternatively, the heat exchanger comprises the one or more filament portions. 0 Additionally, or alternatively, the heat exchanger comprises a pipe and or a portion of a1 pipe. Additionally, or alternatively, the heat exchanger comprises a conductive plate2 thermally connected to a conductive coil, wherein the conductive plate is located on the3 exterior of the housing and the conductive coil extends into the interior volume of the4 housing. 5 6 Embodiments of the fifth aspect of the invention may comprise features to implement the7 preferred or optional features of the first, second third and or fourth aspects of the 8 invention or vice versa. 9 0 According to a sixth aspect of the present invention there is provided an energy harvesting1 system comprising a heat engine in accordance with fifth aspect of the present invention,2 an energy conversion means and an external high temperature heat source. 3 4 Optionally, the energy harvesting system further comprises a fluid circulation system5 configured to transfer heat. 6 7 Preferably, the external high temperature heat source comprises thermal radiation. Most8 preferably, the external high temperature heat source comprises solar radiation. 9 0 Preferably, the energy harvesting system further comprises one or more optical lenses and1 or one or more mirrors suitable for focusing thermal radiation. The one or more optical2 lenses and or one or more mirrors are mounted on one or more stands. The one or more3 stands each comprise a pivot arrangement suitable for adjusting and or optimising the4 angle and or orientation of the one or more optical lenses and or one or more mirrors. 5 1 Optionally, the one or more optical lenses and or one or more mirrors are configured to

2 focus thermal radiation towards the heat exchanger of the heat engine.

3

4 Optionally, the fluid circulation system comprises a solar fluid circulation system with a

5 fluid within a vessel, pipes connecting the vessel to the housing of the heat engine and a

6 pump the circulate the fluid along the pipe between the vessel and the heat engine.

7

8 Optionally, the one or more optical lenses and or one or more mirrors are configured to

9 focus thermal radiation towards the fluid and or vessel or the solar fluid circulation system. 0 1 Most preferably, the external high temperature heat source comprises one or more 2 geothermal heat sources. 3 4 Optionally, the one or more filament portions of the heat engine are located and or5 orientated to extend towards the one or more geothermal heat sources. 6 7 Optionally, the fluid circulation system comprises a geothermal fluid circulation system with8 a fluid contained within pipes and a pump to circulate the fluid around the pipes, wherein9 the pipes extend between the geothermal heat source and the heat engine. 0 1 Preferably, the energy harvesting system further comprises one or more vibrational lenses. 2 3 Embodiments of the sixth aspect of the invention may comprise features to implement the4 preferred or optional features of the first, second third, fourth and or fifth aspects of the5 invention or vice versa. 6 7 According to a seventh aspect of the present invention there is provided a method of8 manufacturing a heat engine comprising, 9 - providing a housing, 0 - providing a first liquid and a second liquid located within the housing, the first liquid1 having a higher density and lower boiling point than the second liquid; 2 - providing a heat exchanger to evaporate the first liquid to form a first liquid vapour;3 and 4 providing at least one fluid flow member that moves in response to a fluid flow5 created by the interaction of the first liquid vapour and the second liquid, 1 wherein the heat exchanger is adapted to receive heat from thermal radiation and

2 or one or more geothermal heat sources.

3

4 Embodiments of the seventh aspect of the invention may comprise features to implement

5 the preferred or optional features of the first, second third, fourth, fifth and or sixth aspects

6 of the invention or vice versa.

7

8 According to an eighth aspect of the present invention there is provided a method of

9 manufacturing an energy harvesting system comprising, 0 - providing a heat engine in accordance with seventh aspect of the present 1 invention; 2 - providing an external high temperature heat source; and 3 providing an energy conversion means. 4 5 Embodiments of the eighth aspect of the invention may comprise features to implement6 the preferred or optional features of the first, second third, fourth, fifth, sixth and or seventh7 aspects of the invention or vice versa. 8 9 Brief Description of Drawings 0 1 There will now be described, by way of example only, various embodiments of the2 invention with reference to the drawings, of which: 3 4 Figure 1 presents a schematic cross-sectional view of a heat engine in accordance with an5 embodiment of the present invention; 6 7 Figure 2 presents a cutaway perspective view of the heat engine of Figure 1 ; 8 9 Figure 3 presents a schematic cross-sectional view of the heat engine of Figure 1 in0 operation; 1 2 Figure 4 presents a schematic cross-sectional view of an alternative embodiment of the3 heat engine of Figure 1 in operation; 4 1 Figure 5 presents a cutaway perspective view of an alternative embodiment of the heat

2 engine of Figure 1 ;

3

4 Figure 6 presents a schematic cross-sectional view of an energy harvesting system

5 comprising the heat engine of Figure 1 ;

6

7 Figure 7 presents a perspective view of a vibrational lens employed within the vibrational

8 energy harvesting system of Figure 6;

9 0 Figure 8 presents a schematic cross-sectional view of the vibrational lens of Figure 7;1 2 Figure 9 presents a plot of (a) a voltage generated by a piezoelectric crystal located at a3 second end of the vibrational lens of Figure 7, when the vibrational lens is attached to an4 internal combustion engine and (b) a voltage generated by a reference piezoelectric5 crystal; 6 7 Figure 10 presents a schematic cross-sectional view of an alternative embodiment of the8 vibrational lens of Figure 7; 9 0 Figure 11 presents a schematic cross-sectional view of a further alternative embodiment of1 the vibrational lens of Figure 7; 2 3 Figure 12 presents a schematic cross-sectional view of yet another alternative 4 embodiment of the vibrational lens of Figure 7; 5 6 Figure 13 presents a schematic cross-sectional view an alternative embodiment of the7 energy harvesting system of Figure 6; 8 9 Figure 14 presents a flow chart of the method of manufacturing the heat engine of Figure0 1 ; 1 2 Figure 15 presents a schematic cross-sectional view of an alternative energy harvesting3 system of Figure 6; 4 1 Figure 16 presents a schematic cross-sectional view of a further alternative energy

2 harvesting system of Figure 6;

3

4 Figure 17 presents a schematic cross-sectional view of another alternative energy

5 harvesting system of Figure 6;

6

7 Figure 18 presents a schematic cross-sectional view of an alternative energy harvesting

8 system of Figure 6; and

9 0 Figure 19 presents a schematic cross-sectional view of a further alternative energy1 harvesting system of Figure 6. 2 3 In the description which follows, like parts are marked throughout the specification and4 drawings with the same reference numerals. The drawings are not necessarily to scale5 and the proportions of certain parts have been exaggerated to better illustrate details and6 features of embodiments of the invention. 7 8 Detailed Description of the Preferred Embodiments 9 0 An explanation of the present invention will now be described with reference to Figures 11 to 19. 2 3 Heat Engine 4 5 Figure 1 depicts a heat engine 1a comprising a substantially cylindrical, sealable housing6 2. The housing 2 comprises stainless steel, specifically, SA516 GR.65. For ease of7 understanding, Figures 1 also depicts a cylindrical coordinate system with r, 0, and z axes. 8 9 The heat engine 1a can be seen to comprise a first liquid 3 and a second liquid 4 both of0 which are located within the housing 2. The first and second liquids 3, 4 occupy an interior1 volume 5 of the housing 2. The first liquid 3 has a higher density but lower boiling point in2 comparison to the second liquid 4. As such, whilst the first and second liquids 3, 4 are free3 to mix within the housing 2, the first liquid 3 locates within a first portion 6 of the housing 2,4 at the base of the housing 2, and the second liquid 4 locates within a second portion 7 of5 the housing 2, above the first liquid 3. 1

2 By way of example, the first liquid 3 may be de-mineralised water and the second liquid 4

3 may be Xylene. The density of de-mineralised water is approximately 1 .2 times that of

4 Xylene and demineralised water has a boiling point of 100 °C which is lower than the

5 boiling point of Xylene, 138.5 °C. De-mineralised water and Xylene are both in a liquid

6 state at room temperature (20 °C) and pressure. A heat engine 1a comprising de¬

7 mineralised water and Xylene as the first and second liquids 3, 4 is suitable for operation

8 at a temperature between 110 °C and 150 °C.

9 0 Further examples of the first and second liquids 3, 4 are provided in Table I along with an1 operating temperature range of a heat engine 1a comprising the first and second liquids 3,2 4. All of the first and second liquids 3, 4 in Table I are in a liquid state at room temperature3 (20 °C) and pressure. Furthermore, it will be appreciated that different operating 4 temperature ranges to those detailed in Table I, such as an operating temperature range5 lower than 70 - 90 °C, could be achieved by using different first and second liquids 3, 46 and different combinations of the first and second liquids 3, 4 beyond the disclosed liquids7 and combinations in Table I. 8 9 Table I: Examples of the first liquid, second liquid and an operating temperature range of a0 heat engine comprising the first and second liquids 1 2 The heat engine 1a also comprises a heat exchanger which transfers heat from an3 external high temperature (TH) heat source 8 to the first liquid 3 in order to evaporate a4 quantity of the first liquid 3. The first liquid 3 is not directly exposed to the external high5 temperature (TH) heat source or any external fluid carrying heat from the external high6 temperature (TH) heat source 8. In the embodiment of Figure 1 , the heat exchanger takes7 the form of the first portion 6 of the housing 2. 8 1 The heat engine 1a further comprises at least one fluid flow member 9. As can be clearly

2 seen in Figure 2, the at least one fluid flow member takes the form of rods 10. Each rod

3 10 has a first end 11 and a second end 12. The first ends 11 of the rods 10 are mounted

4 to an interior surface 13 of the housing 2. The rods 10 extend into the interior volume 5 of

5 the housing 2. The second ends 12 of the rods 10 are free to move and are located

6 towards a central axis 14 of the housing 2. The rods 10 are distributed across the interior

7 surface 13 of the housing 2 in both 0 and z directions. The rods 10 are located in the

8 second portion 7 of the housing 2. Figures 1 to 3 depict the rods 10 as being uniformly

9 distributed about the interior surface 13, orientated perpendicular to the interior surface 130 and all of uniform dimensions such as length. The rods 10 may be made from bronze and1 or brass as the relatively high density effectively transmits any movement or mechanical2 vibrations. 3 4 The housing 2 comprises a sealable inlet port 15 and a sealable outlet port 16. The5 sealable inlet port 15 is located at a top end 17 of the housing 2, through the second6 portion 7 of the housing 2 and provides a means for adding the first and second liquids 3,7 4 into the housing 2. Similarly, the sealable outlet port 16 is located, at a base end 18 of8 the housing 2, through the first portion 6 of the housing 2 and provides a means for9 draining the first and second liquids 3, 4 from the housing 2. In order to fill and maintain0 the housing 2 at a positive pressure, the first and second liquids 3, 4 may be pumped to1 and from the housing 2 by a pumping system 19. 2 3 Figure 3 shows the heat engine 1a of Figure 1 in operation, in other words converting4 thermal energy into mechanical energy. The heat engine 1a is a closed engine such that5 first and second liquids 3, 4 are not added or removed during operation. The first portion 66 of the housing 2 is exposed to the external high temperature (TH) heat source 8 resulting in7 thermal energy being transferred through the housing 2, to the first liquid 3. As such, a8 portion of the first liquid 3 evaporates to form a first liquid vapour. The first liquid vapour9 takes the form of gaseous bubbles 20. The gaseous bubbles 20 have a lower density than0 both the first liquid 3 and the second liquid 4. As such, the gaseous bubbles 20 move in1 the positive z-direction, into the second portion 7 of the housing 2 and through the second2 liquid 4. The thermal energy from the external high temperature (TH) heat source 8 is3 converted into kinetic energy in the form of the motion of the gaseous bubbles 20. 4 1 The interaction, in the form of relative motion and or thermal gradients, of the gaseous

2 bubbles 20 and the second liquid 4 creates a fluid flow. More specifically, the fluid flow

3 includes the flow of the first liquid 3, second liquid 4 and gaseous bubbles 20. For

4 example, the fluid flow is depicted by the arrows in Figure 3. This fluid flow may be

5 Laminar and or turbulent. The fluid flow induces movement in the rods 10, or more

6 specifically, the fluid flow induces mechanical vibrations within the rods 10. As such, the

7 kinetic energy of the gaseous bubbles 20 is converted into mechanical vibrational energy.

8 For example, the Laminar fluid flow of the gaseous bubbles 20 may result in the gaseous

9 bubbles 20 directly colliding with the rods 10, deflecting the rods 10. Furthermore, the0 turbulent fluid flow of the gaseous bubbles 20 and second liquid 4 may induce movement1 and or mechanical vibrations within the rods 10. 2 3 Each gaseous bubble 20 dissipates kinetic and thermal energy. As a result, each gaseous4 bubble 20 will eventually condense to form a liquid bubble 21 of the first liquid 3. The5 liquid bubbles 21 sink back towards the base end 18, into the first portion 6 of the housing6 2 as the density of the liquid bubbles 21 is greater than the density of the second liquid 4. 7 An advantage of the liquid bubbles 21 sinking back through the second portion 7 of the8 housing 2, is the liquid bubbles 21 may further create fluid flows and induce movement9 and or mechanical vibrations within the rods 10. 0 1 As an alternative embodiment, instead of being cylindrical, it will be appreciated that the2 housing 2 could take any regular or non-regular three-dimensional shape. 3 4 As an additional or alternative embodiment, the heat exchanger may take the form of a5 pipe 22 which passes through the first portion 6 of the housing 2, see the heat engine 1 b6 of Figure 4. An external fluid carrying the heat from the external high temperature (TH)7 heat source 8 passes through the pipe indirectly transferring the heat to the first liquid 3. 8 The pipe 22 is more efficient at transferring heat to the first liquid 3 than through the first9 portion 6 of the housing 2, as the pipe 22 has greater thermal contact with the first liquid 3. 0 1 As an additional or alternative embodiment, the distribution of the rods 10 may be non-2 uniform. As another additional or alternative embodiment, the rods 10 may be orientated3 non-perpendicular to the interior surface 13. As a further additional or alternative 4 embodiment, the dimensions of the rods 10, such as the rods length, may vary. As yet5 another further additional or alternative embodiment, the material composition of the rods 1 10 may vary. Furthermore, the distribution, orientation, dimensions and material

2 composition of the rods 10 may be computationally optimised.

3

4 As an additional or alternative embodiment, the heat engine 1b of Figure 4, further

5 comprises pellets 23a. The pellets 23a are located within the interior volume 5 of the heat

6 engine 1b, suspended within the first and second liquids 3, 4. The pellets 23a move about

7 the interior volume 5 of the housing 2 in response to the fluid flow created by the

8 interaction of the gaseous bubbles 20 and the second liquid 4. The pellets 23a collide with

9 the rods 10 inducing further movement, or more specifically, mechanical vibrations within0 the rods 10, in addition to the movement induced directly by the fluid flow. The density of1 the pellets 23a is between the density of the first and second liquids 3, 4 such that the2 pellets 23a are not too heavy or buoyant when suspended within the first and second3 liquids 3, 4. Furthermore, the pellets 23a are chemically unreactive with the first liquid 3,4 second liquid 4 and gaseous bubbles 20. The pellets 23a are also magnetically neutral. 5 The dimensions and material composition of the pellets 23a may be optimised to achieve6 the desired interaction with the fluid flow. As a further additional or alternative 7 embodiment, the pellets 23b may be magnetic, as discussed further below in the context8 of Figure 15. 9 0 As an additional or alternative embodiment, the heat engine 1b of Figure 4, further1 comprises a condensing loop 24. Instead of the gaseous bubbles 20 passively 2 condensing once they have lost sufficient energy within the housing 2, the condensing3 loop 24 actively condenses the gaseous bubbles 20. More specifically, once the gaseous4 bubbles 20 have traversed through the second portion 7 of the housing 2, the gaseous5 bubbles 16 pass through the condensing loop 24 where an external low temperature (T _L )6 heat sink 25 actively cools the gaseous bubbles 20 such that they condense to liquid7 bubbles 21 . The liquid bubbles 21 are returned to the first portion 6 of the housing 2. A8 condensing loop 24 may be advantageous if, for example, the gaseous bubbles 209 accumulate at the top end 17 of the housing 2. 0 1 As another additional or alternative feature, the heat engine 1 b of Figure 4, further2 comprises a sink 26 of the first liquid 3. The sink 26 is connected to the housing 2 and3 maintains the level of the first liquid 3 within the first portion 6 of the housing 2. As the first4 liquid 3 evaporates within the heat engine 1b, this may induce non negligible changes in 1 pressure and or volume within the heat engine 1b. The sink 26 minimises any changes in

2 pressure and or volume.

3

4 As an additional or alternative embodiment, instead of the rods 10, the at least one fluid

5 flow member may take the form of a plate 27 comprising perforations 28, as depicted by

6 the heat engine 1c of Figure 5. The plate 27 is dimensioned in the form of a circular cross¬

7 section of the housing 2, mounted to the interior surface 13 of the housing 2 and orientated

8 to intersect the central axis 14. The fluid flow induces movement and or mechanical

9 vibrations within the plate 27. For example, the fluid flow of the gaseous and liquid0 bubbles 20, 21 are blocked by the plate 27 and redirected through the perforations 281 inducing movement and or mechanical vibrations in the plate 27. The size, distribution2 and relative location of the perforations 28 can be optimised to enhance the turbulent fluid3 flows within the heat engine 1c. As a further additional or alternative feature, the plate 274 may be flexible, in other words, the fluid flow member takes the form of a diaphragm with5 perforations. 6 7 The process of heat transfer to the first liquid 3, evaporation of the first liquid 3 to form8 gaseous bubbles 20, energy transfer from the gaseous bubbles 20 to the fluid flow 9 member (in other words the rods 10, plate 27 and or diaphragm) and condensation of the0 gaseous bubbles 20 to form liquid bubbles 21 is repeated forming a cycle. The 1 mechanical energy (in other words the movement and or vibrations) can be further2 converted into electrical energy. 3 4 Energy Harvesting System 5 6 Figure 6 depicts the heat engine 1 and the external high temperature (TH) heat source 8 as7 part of an energy harvesting system 29a, more specifically, a vibrational energy harvesting8 system. The vibrational energy harvesting system 29a further comprises an energy9 conversion means 30. The energy harvesting system may optionally comprise the0 external low temperature (T _L ) heat sink 25 if required to condense the gaseous bubbles1 20. Furthermore, the vibrational energy harvesting system 29a may optionally comprise a2 vibrational lens 31 . 3 4 Figures 7 and 8 depict a suitable vibrational lens 31a for use in the energy harvesting5 system 29a. The vibrational lens 31a may be of a type as described in the applicant’s co- 1 pending UK patent application number GB1911017.0. As such, the vibrational lens 31 a

2 comprises a backplate 32 and two focusing members. The focusing members take the

3 form of a first focusing plate 33 and a second focusing plate 34. The first and second

4 focusing plates 33, 34 each have a first end 35 and a second end 36. The first and

5 second focusing plates 33, 34 each comprise a first portion 37, having a length y, located

6 between a second portion 38, at the first end 35, and a third portion 39, at the second end

7 36.

8

9 The second portion 38 of the first and second focusing plates 33, 34 is fixed to the 0 backplate 32. As shown in Figure 7, the second portion 38 is angled to be substantially1 parallel and in contact with the backplate 32 such that the second portion 38 is fixed to the2 backplate 32 by welding. In addition to or as an alternative to welding, the fixture means3 may take the form of an adhesive, a nut and a bolt, rivets, a combination thereof or any4 other suitable alternative. 5 6 The second portions 38 of the first and second focusing plates 33, 34 are fixed to the7 backplate 32 at substantially the same orientation and separated by distance a, as can be8 seen in Figure 8. 9 0 As can also be seen in Figure 8, the first portions 37 of the first and second focusing plates1 33, 34, are angled relative to the backplate 32 such that they converge towards each2 other. In the presently described embodiment, the first portions 37 of the first and second3 focusing plates 33, 34, are angled relative to the backplate 32 such that they converge4 towards a point at a distance along a normal to the backplate 32 located midway (a/2)5 between the second portions 38 of the first and second focusing plates 33, 34. 6 7 The third portions 39 at the second end 36 of the first and second focusing plates 33, 348 are angled to be substantially parallel, and preferably perpendicular to the backplate 32,9 and act as the focal point of the vibrational lens 31a. 0 1 As depicted in Figures 6, the vibrational lens 31 a is attached to the heat engine 1 . The2 backplate 32 of the vibrational lens 31a is fixed to the heat engine 1 , by for example nuts3 and bolts, welding and or any other appropriate, equivalent means or combination thereof. 4 Mechanical vibrations induced in the rods 10 of the heat engine 1 are transmitted through5 the housing 2 of the heat engine 1 to the vibrational lens 31a. 1

2 As can clearly be seen in Figure 6, located between the third portions 39 of the first and

3 second focusing plates 33, 34 is the energy conversion means 30 which takes the form of

4 one or more piezoelectric crystals 40. The piezoelectric crystals 40 are connected to

5 electrical components 41 and directed to, for example, an appropriate electrical load (not

6 shown) by cables 42. The one or more piezoelectric crystals 40 convert vibrational

7 mechanical energy originating from the heat engine 1 into useful electrical energy. An

8 alternative energy conversion means could take the form of nano-coils and magnets.

9 0 It will be appreciated that in an additional or alternative embodiment of the energy 1 harvesting system 29a, the piezoelectric crystals 40 may be attached directly to the heat2 engine 1 . However, in the embodiment as depicted by Figure 6, the piezoelectric crystals3 40 are attached to the vibrational lens 31 as more electrical energy can be generated, as4 generically demonstrated by Figure 9. 5 6 Figure 9a shows the voltage as a function of time, generated by a piezoelectric crystal 407 located between the third portions 39 of the first and second focusing plates 33, 34 of the8 vibrational lens 31a when the vibrational lens 31 a is attached to an internal combustion9 engine which acts as a vibrational source, taking the place of the heat engine 1 . Figure 9a0 depicts a root mean-square voltage of 0.743 V. Figure 9b shows the voltage as a function1 of time, generated by a reference piezoelectric crystal (not shown in the Figures) directly2 attached the internal combustion engine. Figure 9b depicts a root mean-square voltage3 0.003 V. The piezoelectric crystal 40 between the third portions 39 generates a voltage4 approximately 248 times greater than the voltage of the reference piezoelectric crystal.5 6 The reason for this is that vibrational lens 31a transmits, converges and or focuses7 vibrations from the first end 35 to the second end 36 of the focusing plates 33, 34. As8 such, the focusing plates 33, 34 could be considered equivalent to a cantilever as the first9 end 35 of each focusing plate 33, 34 is fixed to the backplate 32, and the second end 36 is0 free to move, actuating the piezoelectric crystals 40. 1 2 The focusing plates 33, 34 are substantially triangular, as can clearly be seen in Figure 7. 3 The first end 35 of the focusing plates 33, 34 are equivalent to the base of a triangle and4 the second end 36 equivalent to the (truncated) tip of a triangle. The triangular shape of 1 the focusing plates 33, 34 minimises the space required to house the vibrational lens 31a

2 at the perpendicular distance p from backplate 32 whilst maintaining functionality.

3

4 The vibration lens 31a as depicted in Figures 6 to 8 is made from brass due to the

5 relatively high density of brass which facilitates efficient transmission of vibrational

6 mechanical energy through the vibrational lens 31a. The vibrational lens 31 a may

7 alternatively be made from other metals, alloys or even non-metallic materials, such as

8 ceramics, suitable for transmitting vibrational energy.

9 0 As an additional or alternative feature, the vibrational lens 31b of Figure 10, further1 comprises a spring 43 between the first and second focusing plates 33, 34. It will be2 appreciated that the vibrational lens 31b could comprise multiple springs 43. Similarly, as3 a further additional or alternative feature the vibrational lens 31c of Figure 11 , further4 comprises a weight 44 attached to the first focusing plates 33. Again, it will be appreciated5 that the vibrational lens 31c may comprise multiple weights 44 of equal or non-equal6 weight located on both or just one of the first and second focusing plates 33, 34. As a7 further alternative the vibrational lens 31 may comprise both a spring 43 and a weight 44. 8 Both the spring 43 and the weight 44 modify the vibrational characteristics of the 9 vibrational lens 31b, 31c by damping and or changing the resonant frequency of the0 vibrational lens 31b, 31c, which provides a mechanism to optimise the characteristics of1 the vibrational lens 31 b, 31c. Figures 10 and 11 show the vibrational lens 31 b, 31c may2 additionally comprise a dynamic control system 45 to dynamically adjust the stiffness of3 the spring 43 and or location of the weight 44 on the first and or second focusing plates 33,4 34 and or the magnitude of the weight 44 on the first and or second focusing plates 33, 34. 5 For example, the weight 44 may take the form of a container into which water may be6 pumped in and or out of by means of the dynamic control system 45. The dynamic control7 system 45 facilitates modifying the vibrational characteristics of the vibrational lens 31 b,8 31c during operation. 9 0 As another additional or alternative feature, the focusing members may comprise multiple1 layers and or coatings. The different layers and or coatings may exhibit different 2 vibrational and or thermal characteristics due to comprising, for example, different 3 dimensions, materials, densities and or grain structures. 4 1 For example, Figure 12 depicts focusing plates 33, 34 comprising a first, outer layer 46

2 and a second, inner 47 layer. The second, inner layer 47 may be less dense than the first,

3 outer layer 46. It is found this arrangement improves the transmission of vibrations

4 through the vibrational lens 31 d. As another example, the grain structure of the first, outer

5 layer 46 may be more aligned in comparison to the grain structure of the second, inner

6 layer 47. Again, this arrangement improves the transmission of vibrations through the

7 vibrational lens 31 d. As another example, the first layer 46 may be made from brass and

8 the second layer 47 may be made from steel.

9 0 In addition, it is further noted the relative physical properties of the first, outer layer 46 and1 the second, inner layer 47 may be reversed such that, for example, the second, inner layer2 47 may be more dense than the first, outer layer 46. As a further alternative, the grain3 structure of the first, outer layer 46 may be less aligned in comparison to the grain 4 structure of the second, inner layer 47. The physical properties of the different layers such5 as the dimensions, materials, densities and or grain structures are optimised according to6 the desired vibrational and or thermal characteristics which ultimately depends on 7 frequency characteristics of the vibrational source, in other words, the heat engine 1 .8 9 As a further alternative, the vibrational lens 31 a, 31 b, 31c, 31 d may comprise more or less0 than two focusing plates 33, 34. For example, a vibration lens 31 a, 31 b, 31 c, 31 d with just1 a first focusing plate 33 could actuate piezoelectric crystals 40 located at the second end2 36 of the first focusing plate 33 against the heat engine 1 , more specifically, a protruding3 portion of the housing 2. Conversely, a vibrational lens, 31 a, 31 b, 31c, 31 d with three4 focusing plates 33, 34 may comprise two sets of piezoelectric crystals 40, one set of5 piezoelectric crystals 40 between the second end 36 of a first and a second focusing6 plates, and the other set of piezoelectric crystals between the second 34 and third 487 focusing plates, as shown in Figure 13. 8 9 As yet another alternative, instead of the vibrational lens 31 a, 31 b, 31c, 31 d comprising a0 backplate 32, the focusing plates 33, 34 may be fixed directly to the heat engine 1 . 1 2 Figure 13 shows another additional or alternative embodiment of an energy harvesting3 system 29b, where the housing 2 of the heat engine 1 may comprises sealable openings4 49 such that the rods 10 pass through the housing 2 and directly connect to the focusing5 plates 33, 34 of the vibrational lens 31 a, 31 b, 31 c, 31 d. As such, the mechanical 1 vibrations induced in the rods 10 can propagate along the rods 10 and directly along the

2 focusing plates 33, 34 of the vibrational lens 31a, 31 b, 31c, 31 d. In this embodiment, as

3 the rods 10 connect directly to the focusing plates 33, 34, a backplate 32 is not required.

4 The openings 49 are sealable, with or without the rods 10 passing through the openings

5 49, to ensure the housing 2 does not leak.

6

7 As a further alternative, instead of the vibrational lens 31 a, 31 b, 31c, 31 d comprising

8 focusing plates 33, 34, the focusing members could take the form of focusing rods. The

9 focusing rods may just be an extension of the rods 10 of the heat engine 1 . Furthermore,0 the planar layers 46, 47 of the focusing plates 33, 34 as depicted in Figure 12 are 1 equivalent to concentric layers and or coatings of a focusing rod. Advantageously,2 focusing rods take up less space than the focusing plates 33, 34. 3 4 Method of Manufacturing a Heat Engine 5 6 Figure 14 shows a flow chart for a method of manufacturing the heat engine 1 . The7 method comprises: providing a housing (S1001 ); providing a first and second liquid located8 within the housing, the first liquid having a higher density and lower boiling point than the9 second liquid (S1002); providing a heat exchanger to transfer heat to the first liquid to0 evaporate the first liquid to form a first liquid vapour (S1003); and providing at least one1 fluid flow member to move in response to a fluid flow created by the interaction of the first2 liquid vapour and second liquid (S1004). 3 4 In addition, the method of manufacturing the heat engine 1 may optionally comprise5 characterising the external high temperature (TH) heat source 8. For example, this may6 include characterising the temperature, energy, power, variability and or duration of the7 external high temperature (TH) heat source 8. In the context of the present invention, the8 term high temperature (TH) broadly refers to any temperature above ambient temperature. 9 0 As a further addition, the method of manufacturing the heat engine 1 may optionally1 comprise utilising the characteristics of the high temperature (TH) heat source 8 to 2 determine the optimum parameters of a heat engine 1 . For example, this optimisation3 process may include determining: the dimensions of the heat engine 1 ; the volume,4 relative ratio and chemical composition of the first and second liquids 3, 4; the distribution,5 orientation, dimensions and material composition of the rods 10; the operational proximity 1 of the heat engine 1 to the high temperature (TH) heat source 8; if a condensing loop 24 is

2 required; and if a sink 26 is required. As an example of the parameter dependency, the

3 higher the temperature and power of the external high temperature (TH) heat source 8, the

4 greater the maximum viable size (i.e. dimensions, volume) of the heat engine 1 . When

5 choosing the first and second liquids 3, 4 factors such as the heat capacity, relative density

6 and relative boiling points are key considerations. It is advantageous to optimise the heat

7 engine 1 as this ensures the heat engine 1 can operate, for example, the external high

8 temperature (TH) heat source 8 will provide enough heat to evaporate any quantity of the

9 first liquid 3. Furthermore, the optimisation ensures the heat engine 1 can operate0 efficiently. 1 2 Method of Manufacturing a Vibrational Energy Harvesting System 3 4 A method of manufacturing an energy harvesting system 29 comprises providing a heat5 engine 1 in accordance with the flow chart depicted in Figure 14 and as described above,6 providing an external high temperature (TH) heat source 8 and providing an energy7 conversion means 30. 8 9 As an additional or alternative feature, the method of manufacturing an energy harvesting0 system 29 may optionally comprise providing an external low temperature (T _L ) heat sink1 25. 2 3 As a further additional or alternative feature, the method of manufacturing an energy4 harvesting system 29 may optionally comprise providing a vibrational lens 31 a, 31b, 31c5 31 d. The vibrational lens 31a, 31b, 31c 31 d is manufactured such that it is optimised for a6 specific heat engine 1. Providing a vibration lens 31 a, 31 b, 31c 31 d may comprise,7 determining the characteristics of the heat engine 1 such as the dimensions of the heat8 engine 1 , the dimensions of the fluid flow member (i.e. rods 10) and most significantly the9 frequency characteristics of the mechanical vibrations induced within the rods 10. 0 1 In addition, providing a vibrational lens 31a, 31b, 31c 31 d may optionally comprise2 determining the optimum parameters for a vibrational lens 31a, 31b, 31c 31 d for 3 harvesting the mechanical vibrational energy from the heat engine 1 . This includes4 determining the shape and dimensions of the vibrational lens 31 a, 31 b, 31c 31 d such as,5 distances a, and y. More specifically, the optimisation may include dimensioning the 1 length y of the focusing plates 33, 34, to match an average resonant frequency across the

2 operational range of the heat engine 1 .

3

4 Furthermore, providing a vibrational lens may optionally comprise providing a vibrational

5 lens 31a, 31b, 31c 31 d according to the optimum parameters. More specifically, the

6 focusing plates 33, 34 of the vibrational lens 31a, 31 b, 31c 31 d are provided by water jet

7 cutting brass plates to the required dimensions and introducing appropriate bends in

8 focusing plates 33, 34. The focusing plates 33, 34 are welded to the backplate 32.

9 0 Providing a vibrational lens may optionally comprise further optimising the parameters of1 the vibrational lens 31 a, 31b, 31c 31 d according to factors such as: the type of energy2 conversion means located at the second end 36 of the focusing plates 33, 34; the number3 of focusing plates 33, 34 the vibrational lens 31a, 31 b, 31c 31 d comprises; the space4 available to house the vibrational lens 31a, 31b, 31c 31 d; and more generally the 5 operational constraints and desired performance characteristics. For example, the first6 portions 37 of the first and second focusing plates 33, 34 are not limited to converging7 midway between the second portions 38 of the first and second focusing plates 33, 34. In8 other words, the first portions 37 of the focusing plates 33, 34 may be asymmetrically9 angled relative to the backplate 32 to fit within the available space and or for a desired0 performance of the vibrational lens 31a, 31 b, 31c 31 d. 1 2 As describe above, the heat engine 1 is optimised for a specific external high temperature3 (TH) heat source 8. Therefore, when manufacturing an energy harvesting system 29 it4 may be suboptimal to provide the vibrational lens 31a, 31 b, 31c 31 d without first 5 manufacturing and characterising the heat engine 1 . However, it is noted that this method6 may be iterative. For example, parameters of the heat engine 1 may be altered to 7 optimise the vibrational lens 31 a, 31 b, 31c 31 d and energy harvesting system 29. 8 9 Alternative Heat Engine and Energy Harvesting System 0 1 Figure 15 depicts an alternative heat engine 1 as part of an alternative energy harvesting2 system 29c. The heat engine 1 and energy harvesting system 29c depicted in Figure 153 may comprise the same preferable and optional features as the heat engines 1 and energy4 harvesting systems 29 depicted in any of Figures 1 to 14. 5 1 Instead of the at least one fluid flow member 9 taking the form of rods 10, a plate 27 and or

2 a diaphragm, the at least one fluid flow member 9 of the heat engine 1 of Figure 15 takes

3 the form of at least one magnetic pellet 23b located within the interior volume 5 of the heat

4 engine 1 and suspended within the first and or second liquids 3, 4. The magnetic pellets

5 23b move about the interior volume 5 of the housing 2 in response to the fluid flow created

6 by the interaction of the gaseous bubbles 20 and the second liquid 4. The thermal energy

7 of the external high temperature (TH) heat source 8 is converted into mechanical energy in

8 the form of motion of the magnetic pellets 23b. In this embodiment it may be preferably for

9 the housing 2 to comprise a non-magnetic material such as Aluminium. 0 1 As well as the heat engine 1 , the alternative energy harvesting system 29 comprises an2 external high temperature (TH) heat source 8 and an energy conversion means 30. 3 Instead of piezoelectric crystals 40, the energy conversion means 30 takes the form of a4 coil 50, wound around the housing 2 of the heat engine 1 . The coil 50 may comprise5 copper although other alternative magnetically inductive materials may be employed. It6 will also be appreciated by the skilled reader that the location the coil 50 may vary from7 that shown in Figure 15. For example the coil 50, or at least a portion of the coil 50, may8 be located within the housing 2. 9 0 The motion of the magnetic pellets 23b within the heat engine 1 induces useful electrical1 energy within the coil 50. This energy harvesting system 29 relies on magnetic induction2 instead of mechanical vibrations to harvest the thermal energy originating from the 3 external high temperature (TH) heat source 8. 4 5 As an additional or alternative embodiment, the at least one fluid flow member 9 of a heat6 engine 1 may take the form of both rods 10 and magnetic pellets 23b. The fluid flow7 created by the interaction of the gaseous bubbles 20 and the second liquid 4, induces both8 mechanical vibrations within the rods 10 and the motion of the magnetic pellets 23b. 9 Correspondingly, the energy conversion means 30 of an energy harvesting system 29 may0 be both piezoelectric crystals 40 and a coil 50. The piezoelectric crystals 40 convert the1 mechanical vibrational energy into useful electrical energy and the motion of the magnetic2 pellets 23b induces useful electrical energy within the coil 50. As well as inducing 3 electrical energy, the motion of the magnetic pellets 23b may advantageously also collide4 with the rods 10 inducing further mechanical vibrations. 5 1 Figure 16 depicts an alternative energy harvesting system 29d comprising a heat engine 1

2 which may comprise the same preferable and optional features as the heat engines 1 and

3 energy harvesting devices 29a, 29b, 29c of Figures 1 to 15.

4

5 As can be seen in Figure 16, the energy harvesting system 29d further comprises optical

6 lenses 51 . In operation, the optical lenses 51 focus thermal radiation in the form of solar

7 radiation 52 towards the heat engine 1 . In particular, the optical lenses 51 focus the solar

8 radiation 52 towards the heat exchanger of the heat engine 1 . The heat exchanger

9 transfers heat from the solar radiation 52 to the first liquid 3. As such, the solar radiation0 52 acts as the external high temperature (TH) heat source 8. 1 2 As with previous embodiments, the heat exchanger can take the form of the first portion 63 of the housing 2. As an additional or alternative feature, the heat exchange can take the4 form of a conductive plate 53 thermally connected to a conductive coil 54. The conductive5 plate 53 is located on the exterior of the housing 2 and the conductive coil 54 extends into6 the housing 2. The solar radiation 52 is focused on the conductive plate 53, heat is7 transmitted along the conductive coil 54 and then the heat is transferred to first liquid 3. 8 Advantageously, in comparison to the heat exchanger taking the form of the first portion 69 of the housing 2, the conductive plate 53 and conductive coil 54 arrangement can more0 efficiently and evenly transfer heat from the solar radiation 52 to the first liquid 3. 1 2 The optical lenses 51 are mounted in position by a stand 55. The stand 55 comprises a3 pivot arrangement 56 which facilitates adjusting and or optimising the angle and or4 orientation of the optical lenses 51 relative to the heat engine 1 . 5 6 It will be appreciated that as an additional or alternative feature the optical lenses could7 take the form of a mirror, more specifically, a parabolic mirror array. 8 9 Figure 17 depicts an alternative energy harvesting system 29e comprising a heat engine 10 which may comprise the same preferable and optional features as the heat engines 1 and1 energy harvesting devices 29a, 29b, 29c, 29d of Figures 1 to 16. 2 3 Similar to the embodiment of Figure 16, the energy harvesting system 29e of Figure 174 comprises optical lenses 51 suitable for focusing solar radiation 52. However, in contrast5 to the embodiment of Figure 16, the energy harvesting system 29e of Figure 17 comprises 1 a fluid circulation system 57e. The fluid circulation system 57e comprises a fluid 58 within

2 a vessel 59, pipes 60 connecting the vessel 59 to the housing 2 of the heat engine 1 and a

3 pump 61 to circulate the fluid 58 along the pipes 60 between the vessel 59 and the heat

4 engine 1 . Similar to the embodiment of Figure 4, the heat exchanger of the heat engine 1

5 of Figure 17 takes the form of the portion of the pipe 60 which passes through the first

6 portion 6 of the housing 2.

7

8 In operation, optical lenses 51 focus solar radiation 52 upon the fluid 58 and or the vessel

9 59 of the fluid circulation system 57e. The fluid 58 directly and or indirectly, through the0 vessel 59, absorbs heat from the solar radiation 52. The heated fluid 58 is then circulated1 about the pipes 60 by the pump 61 to transfer the heat to the first liquid 3 within the heat2 engine 1 . The fluid 58 does not mix with the first liquid 3. 3 4 As an example, the fluid 58 of the fluid circulation system 57e may take the form of liquid5 sodium which can hold heat for a relatively long time as has a relatively high specific heat6 capacity. In this embodiment, the liquid sodium can be considered a thermal battery as7 can retain heat from the solar radiation 52, and continue to transfer heat to the first liquid 38 within the heat engine 1 , even when the solar radiation 52 is no longer focused upon the9 fluid 58 and or the vessel 59. 0 1 To summarise, the energy harvesting systems 29d, 29e of Figures 16 and 17 are 2 configured to generate electricity by harnessing heat from thermal radiation, for example,3 solar radiation. In other words, the external high temperature (TH) heat source which4 drives the heat engines 1 depicted in Figures 16 and 17 is solar radiation. 5 6 Figures 18 depicts an alternative energy harvesting system 29f comprising a heat engine 17 which may comprise the same preferable and optional features as the heat engines 1 and8 energy harvesting devices 29a, 29b, 29c, 29d, 29e of Figures 1 to 17. 9 0 As can be seen from Figure 18, the energy harvesting system 29f comprises a fluid1 circulation system 57f configured to transfer heat from a subterranean hotspot 62 to the2 first liquid 3 within the heat engine 1 . In this embodiment, the fluid circulation system 57f3 comprises fluid 58 contained within pipes 60 and a pump 61 to circulate the fluid 584 around the pipes 60. Similar to the embodiment of Figures 4 and 17, the heat exchanger 1 of the heat engine 1 of Figure 18 takes the form of the portion of the pipe 60 which passes

2 through the first portion 6 of the housing 2.

3

4 In operation, fluid 58 in close proximity to the subterranean hotspot 62 absorbs heat. The

5 heated fluid 58 is then circulated by the pump 61 to the heat engine 1 . The heat is

6 transferred from the fluid 58 to the first liquid 3 within the heat engine 1 .

7

8 Figures 19 depicts an alternative energy harvesting system 29g comprising a heat engine

9 1 which may comprise the same preferable and optional features as the heat engines 10 and energy harvesting devices 29a, 29b, 29c, 29d, 29e, 29e of Figures 1 to 18. 1 2 Similar to the embodiment of Figure 18, the energy harvesting system 29g of Figure 193 transfers heat from a subterranean hotspot 62 to the heat engine 1 . However, instead of4 employing a fluid 58 to carry heat between the subterranean hotspot 62 and the heat5 engine 1 , the housing 2 of the heat engine 1 comprises a filament portion 63 originating6 from the base end 18 of the housing 2. The filament portion 63 is located and orientated7 to extend towards the subterranean hotspot 62. The filament portion 63 could be 8 considered an extension of the first portion 6 of the housing 2. The first liquid 3 locates9 within both the first portion 6 and filament portion 63 of the housing 2. The heat exchanger0 of the heat engine 1 of Figure 19 takes the form of the filament portion 63 of the housing 2,1 and specifically, the region of the filament portion 63 closest to the subterranean hotspot2 62. 3 4 In operation, heat is transferred from the subterranean hotspot 62, through the filament5 portion 63 of the housing 2, to the first liquid 3 within the filament portion 63. It will be6 appreciated that the heat engine 1 may comprise two or more filament portions 63 each7 extending towards one or more subterranean hotspots 62. 8 9 The energy harvesting systems 29f, 29g of Figures 18 and 19 are configured to generate0 electricity by harnessing heat from a subterranean hotspot. In other words, the external1 high temperature (TH) heat source which drives the heat engine 1 is the subterranean2 hotspot, also termed a geothermal heat source. It will be appreciated that the terms3 subterranean hotspot and geothermal heat source may comprise a radioactive decay heat4 source. 5 1 It will be appreciated that the external high temperature (TH) heat source which drives a

2 heat engine 1 may be a combination of heat sources. For example, the external high

3 temperature (TH) heat source may comprise, solar radiation 52, one or more subterranean

4 hotspots 62 and any other alternative or additional source of heat, such as waste heat

5 from a data centre, or combination thereof. As such, the corresponding energy harvesting

6 system 29 may comprise a combination of features from Figures 16 to 19, such as, optical

7 lenses 51 in combination with a filament portion 63, as well as the features from Figures 1

8 to 15.

9 0 The heat engine 1 has numerous advantages. The heat engine 1 does not rely on 1 conventional thermodynamic cycles, but instead provides an alternative mechanism of2 converting heat into work by utilising a phase change of the first liquid 3 to create fluid3 flows and the subsequent interaction with the rods 10. 4 5 The heat engine 1 operates primarily on changes in temperature as well as the addition6 and removal of heat. Changes in pressure and volume, whilst might be present due to the7 intrinsic relationship to temperature, are not fundamental to the operation of the heat8 engine 1 . In other words, the heat engine 1 does not reply on the expansion of a gas to9 perform work. As such, the heat engine 1 has minimal moving components, reducing the0 amount of maintenance that may be required and maximising the lifetime of the device.1 Also, as there are minimal moving components, the heat engine 1 is relatively quiet. 2 3 The heat engine 1 is not limited to a specific type of fuel so can utilise a variety of external4 high temperature (TH) heat sources 8 ranging in temperature and power. Depending on5 the origin of the external high temperature (TH) heat source 8, the heat engine 1 does not6 result in the release of toxic and un-environmentally friendly gases. 7 8 Furthermore, the heat engine 1 is scalable as can be adapted for different external high9 temperature (TH) heat sources 8 ranging in temperature and power. As such, the 0 dimensions of the heat engine 1 can be adapted to the desired size and resulting expense. 1 The heat engine 1 is a sealed device with minimal moving components so is relatively2 safe. 3 4 The heat engine 1 is customisable as the rods 10 can be optimised for a specific external5 high temperature (TH) heat source 8. 1

2 A heat engine is disclosed. The heat engine comprises a housing, a first liquid and a

3 second liquid located within the housing. The first liquid has a higher density and lower

4 boiling point than the second liquid. The heat engine further comprises a heat exchanger

5 which transfers heat to the first liquid to evaporate the first liquid to form a first liquid

6 vapour. The heat engine also comprises at least one fluid flow member which to moves in

7 response to a fluid flow created by the interaction of the first liquid vapour and the second

8 liquid. The heat exchanger is adapted to receive heat from thermal radiation and or one or

9 more geothermal heat sources. The liquid-gas phase change of the first fluid provides an0 alternative mechanism for converting heat into work with numerous advantages. The heat1 engine has minimal moving parts, a relatively long lifetime, does not require a specific fuel,2 does not directly release toxic or un-environmentally friendly gases, and can be adapted to3 a specific source of waste heat. 4 5 Throughout the specification, unless the context demands otherwise, the terms “comprise”6 or “include”, or variations such as “comprises” or “comprising”, “includes” or “including” will7 be understood to imply the inclusion of a stated integer or group of integers, but not the8 exclusion of any other integer or group of integers. Furthermore, unless the context clearly9 demands otherwise, the term “or” will be interpreted as being inclusive not exclusive.0 1 The foregoing description of the invention has been presented for purposes of illustration2 and description and is not intended to be exhaustive or to limit the invention to the precise3 form disclosed. The described embodiments were chosen and described in order to best4 explain the principles of the invention and its practical application to thereby enable others5 skilled in the art to best utilise the invention in various embodiments and with various6 modifications as are suited to the particular use contemplated. Therefore, further 7 modifications or improvements may be incorporated without departing from the scope of8 the invention as defined by the appended claims.