WO2012076847A1

WO2012076847A1 - Solar energy apparatus with a combined photovoltaic and thermal power generation system

Info

Publication number: WO2012076847A1
Application number: PCT/GB2011/001697
Authority: WO
Inventors: Nicholas Simon Walker
Original assignee: Microsharp Corporation Limited
Priority date: 2010-12-07
Filing date: 2011-12-07
Publication date: 2012-06-14
Also published as: GB201020717D0

Abstract

A solar power generation system (1) comprises a solar concentrator (2) and a photovoltaic cell (4, 104, 204) at the focus of the solar concentrator (2). The system (1) also comprises a thermal receiver (6, 106, 206) adjacent to, but thermally isolated from, the photovoltaic cell (4, 104, 204). The thermal receiver (6, 106, 206) is arranged to be illuminated by solar radiation (8) incident upon the solar concentrator (2). The system (1) further comprises a first heat transfer mechanism (12, 112, 212) in good thermal contact with, and arranged to dissipate heat from, the photovoltaic cell (4, 104, 204). The system (1) further comprises a second heat transfer mechanism (14, 114, 214), independent of the first heat transfer mechanism (12, 112, 212) and in good thermal contact with, and arranged to dissipate heat from, the thermal receiver (6, 106, 206). The first (12, 112, 212) and second (14, 114, 214) heat transfer mechanisms may comprise a solid thermal conductor (a heat sink) or a heat pipe, connected to a heat exchanger (16, 18) to transfer heat to a working fluid (e.g. water). The working fluid may also be arranged simply to pass through the thermal receiver (6, 106, 206). The first (12, 112, 212) and second (14, 114, 214) heat transfer mechanisms may be separate from one another or the first (12, 112, 212) and second (14, 114, 214) heat transfer mechanisms may be in thermal contact with a common working fluid. The heated fluid can be used e.g. for heating, thermoelectricity generation or in cooling systems. The system (1) may also comprise one or more secondary solar concentrators (228) to further concentrate the incident solar radiation onto the photovoltaic cell (204) and/or the thermal receiver (206).

Description

SOLAR ENERGY APPARATUS WITH A COMBINED PHOTOVOLTAIC AND

THERMAL POWER GENERATION SYSTEM

This invention relates to apparatus and systems for harnessing solar energy. In particular this invention relates to a combined photovoltaic and thermal power generation system.

Solar concentrator systems seek to reduce the cost of generating solar power by using large area, low cost optics (e.g. a concentrating Fresnel lens) to focus light onto expensive, but high efficiency, photovoltaic (PV) cells, e.g. triple junction gallium arsenide cells. These PV cells operate efficiently at solar concentrations between 100 suns to 1000 suns or more. Increasing the concentration of the incident solar radiation reduces the relative cost of the PV cells with respect to the other components in the system as fewer PV cells are needed to provide the power generation requirements of the system, and this can result in an overall cost reduction as high efficiency PV cells are generally very expensive.

In such systems with a refractive Fresnel lens as the primary concentrator, concentrations of greater than 1000 suns are difficult to achieve at low cost and high optical transmission efficiency, even with a suitably designed secondary optical concentrator. This is because Fresnel lenses are generally manufactured out of an acrylic plastic, e.g. polymethyl methacrylate (P MA), which suffers from

imperfections of manufacture, chromatic aberration, etc., all of which reduce the ability of the lens to focus light onto a very small spot and therefore support these very high levels of concentration.

Even expensive PV cells, e.g. triple junction cells, only have conversion efficiencies as high as 40%, so 60% of the incident solar radiation is converted to heat. This heat must be removed rapidly from the PV cell to avoid it getting too hot, as this results in a reduction in the conversion efficiency and the potential for damage to the cell. Hybrid systems have been tried, e.g. concentrator photovoltaics and thermal (CPVT) power systems, which use the additional heat to provide a low grade heat output which can be used, e.g. for hot water, or thermally driven cooling, water purification, etc, if the water is slightly hotter. The Applicant has appreciated, however, that hybrid systems have not prospered commercially because of the problem that the PV cell requires a low operating temperature to work at maximum efficiency which is incompatible with having a high temperature thermal component in the same system that provides a useful thermal output, e.g. a high temperature working fluid. For example, PV cells operate efficiently at temperatures below 60 °C while a useful working fluid would ideally exit the system at at least 100 °C.

It is an aim of the present invention to provide an improved arrangement for a concentrated photovoltaic and thermal power system.

When viewed from a first aspect the invention provides a solar power generation system comprising:

a solar concentrator,

a photovoltaic cell at the focus of the solar concentrator,

a thermal receiver adjacent to but thermally isolated from the photovoltaic cell and also arranged to be illuminated by solar radiation incident upon the solar concentrator,

a first heat transfer mechanism in good thermal contact with, and arranged to dissipate heat from, the photovoltaic cell,

a second heat transfer mechanism, independent of the first and in good thermal contact with, and arranged to dissipate heat from, the thermal receiver.

As will be appreciated by those skilled in the art, placing a photovoltaic (PV) cell at the focus of a solar concentrator locates the expensive component of the system, i.e. the PV cell, at the point where the solar concentration has its highest intensity which therefore maximises the efficiency of the power generation from the PV cell. As the light intensity from a solar concentrator generally has a Gaussian profile, the thermal receiver adjacent to the PV cell collects the remaining, less intense light which does not fall onto the PV cell and therefore maximises the efficiency of energy captured from the solar radiation incident upon the solar concentrator without having to make improvements to other components of the system, e.g. the solar concentrator, or provide expensive PV cells across the whole area onto which the solar concentrator directs the incident solar radiation. By thermally isolating the thermal receiver from the PV cell and providing a second, independent heat transfer mechanism, the PV cell can be maintained at its relatively cool operating temperature by dissipating heat through the first heat transfer mechanism, while the second heat transfer mechanism is used to dissipate heat from the thermal receiver, thus enabling the second heat transfer mechanism to be heated to a higher temperature than the PV cell, i.e. the thermal isolation achieved by providing independent heat transfer mechanisms allows a significant temperature difference to be obtained between the PV cell and the thermal receiver so that they can both be operated in temperature regimes best matched for their operation. The greater thermal energy in the second heat transfer mechanism, can therefore be transferred into a working fluid which can be used for useful purposes, e.g. secondary power generation, heating, as an energy source for cooling systems, etc. The focus of the solar concentrator could be linear, i.e. the PV cell, or a plurality of PV cells, is arranged along the line onto which a linear solar concentrator focusses the incident solar radiation. However, in a preferred set of embodiments the focus of the solar concentrator is a point focus, i.e. the PV cell is arranged at the focal point of the solar concentrator.

In one set of embodiments the thermal receiver at least partially surrounds the PV cell. The thermal receiver may not be continuous, i.e. it could comprise two or more discrete parts which together at least partially surround the PV cell. In this set of embodiments each discrete part of the thermal receiver could be in good thermal contact with a separate heat transfer mechanism. In a preferred set of

embodiments the thermal receiver fully surrounds the PV cell. The thermal receiver therefore acts to maximise the energy efficiency of the system by collecting as much of the solar radiation incident upon the solar concentrator that is not focussed onto the PV cell. In this set of embodiments a plurality of second heat transfer mechanism could be in good thermal contact with different points of the thermal receiver.

In one set of embodiments the first heat transfer mechanism could be provided by a solid thermal conductor, i.e. a heat sink to dissipate heat from the PV cell. The heat sink could, for example, comprise fins to be cooled by air. In another set of embodiments the first heat transfer mechanism could comprise a heat exchanger to transfer heat from the PV cell to a working fluid. In a preferred set of embodiments the first heat transfer mechanism for transferring heat from the PV cell comprises a heat pipe. A heat pipe is a device, typically a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminium, which contains a liquid which is in thermal contact with a heat source (in this case the PV cell), from which it absorbs heat. This turns the liquid into a vapour which condenses back into a liquid at the other end of the heat pipe, distal from the heat source, thus releasing its latent heat. The liquid then returns to the end of the heat pipe proximal to the heat source either through capillary action or under gravity where it evaporates once more and repeats the cycle.

The end of the heat pipe distal from the PV cell could be cooled by a working fluid, e.g. via a heat exchanger. Thus in some embodiments the first heat transfer mechanism is in good thermal contact with a working fluid, i.e. a first fluid path.

This enables the heat dissipated from the PV cell to be usefully used in the same or similar ways to the heat from the thermal receiver. This can therefore increase the efficiency of the system in collecting the energy from the solar radiation incident upon the solar concentrator.

Similar arrangements could be provided for the second heat transfer mechanism, i.e. in one set of embodiments the second heat transfer mechanism could be provided by a solid thermal conductor, e.g. a heat sink. In another set of embodiments the second heat transfer mechanism could comprise a heat exchanger to transfer heat from the thermal receiver to a working fluid. Thus in some embodiments the second heat transfer mechanism is in good thermal contact with a working fluid, i.e. a second fluid path. However, in one set of embodiments a working fluid could be arranged simply to pass through the thermal receiver, i.e. the second heat transfer mechanism could simply comprise the working fluid. This is advantageous in keeping the system compact and reducing its thermal mass which in turn increases the efficiency of heat transfer through the system. In another set of embodiments the second heat transfer mechanism comprises a heat pipe. Again the end of the heat pipe distal from the thermal receiver could be connected to a heat exchanger to transfer heat to a working fluid. ln a set of embodiments the first heat transfer mechanism and the second heat transfer mechanism are thermally connected to a common heat exchanger. This arrangement could be used even when there are multiple first and/or second heat transfer mechanism, i.e. they could all be connected to the same heat exchanger.

In the set of embodiments in which the first and second heat transfer mechanisms are both in good thermal contact with a working fluid, the fluids could be separate from one another. This would allow different types of fluid to be used in separate thermal paths, e.g. a gas to cool the PV cell and a liquid to transfer heat from the thermal receiver. However, in a set of embodiments the first and second heat transfer mechanisms are in thermal contact with a common working fluid. This allows a common fluid path to efficiently collect heat from both the first and second heat transfer mechanisms, whilst allowing the PV cell and thermal receiver to be thermally isolated from one another in order to maintain the temperature differential therebetween. By allowing the thermal energy from the PV cell, albeit at a lower temperature, to be added to the thermal energy from the thermal receiver, further increases in the energy efficiency of the system can be achieved.

Where the first and second heat transfer mechanisms comprise connected separate fluid paths, there are a number of arrangements in which the two fluid paths can be connected. In one set of embodiments the second fluid path is a continuation of the first fluid path, e.g. the first fluid path cools the PV cell, with the fluid subsequently being heated further in the second fluid path by the thermal energy from the thermal receiver. The advantage of this arrangement is that the fluid flowing into the first fluid path is pre-heated, e.g. from ambient temperature to around 60 °C, with the second fluid path being used as an "after heater". This preheating enables the fluid in the second fluid path to be heated to particularly high temperatures, increasing the usefulness of the fluid for its subsequent use. In another set of embodiments the first and second fluid paths converge at a point away from the PV cell and the thermal receiver. In these embodiments separate fluid paths are provided to dissipate heat from the PV cell and the thermal receiver respectively but then these fluid paths converge to provide a single fluid path which exits the system for the thermal energy in the fluid to be used. This is particularly advantageous because combining the fluid paths increases the ratio of the cross section of the fluid path to the surface area at the boundary of the fluid path (compared to separate fluid paths) and therefore decreases the overall fluid resistance of the system. The first and second fluid paths could have a different source, but in a preferred set of embodiments a common source is provided for the first fluid path and the second fluid path, e.g. the fluid flow from the common source splits into the first fluid path and the second fluid path which then subsequently re- converge.

In one set of embodiments a plurality of first fluid paths are provided to cool the PV cell. In a set of embodiments a plurality of second fluid paths are provided to dissipate heat from the thermal receiver. This is particularly convenient in the set of embodiments which comprises a plurality of second heat transfer mechanisms, e.g. in the set of embodiments in which the thermal receiver at least partially or fully surrounds the PV cell, as it may be more practical to have a separate fluid path for different parts of the thermal receiver and/or different second heat transfer mechanism. The plurality of first and/or second fluid paths could then subsequently converge to remove the fluid from the system.

In a preferred set of embodiments the fluid in the first fluid path has a temperature of less than 70 °C, preferably less than 60 °C, to keep the PV cell at an optimum operating temperature to maximise its efficiency. In a preferred set of embodiments the fluid in the second fluid path has a temperature of at least 100 °C, preferably at least 20 °C, more preferably at least 140 °C. It will therefore be appreciated that this results in a large temperature difference between the PV cell and the thermal receiver which maximises the efficiency of the system both in terms of power generation in the PV cell and the collection of heat energy from the thermal receiver.

The fluid used in the first and second fluid paths could be any suitable fluid, and, as has been mentioned previously, different fluids could be used for the first and second fluid paths if they are separate. It is advantageous for a fluid with a relatively high specific heat capacity to be used and therefore in a preferred set of embodiments the working fluid comprises water support other. The high temperatures in the thermal receiver may lead to the fluid in the first fluid path being evaporated, e.g. water being heated into steam. This could then be usefully used to extract energy from the heated fluid. The system could comprise a pump or pumps to pump the working fluid through the one or more fluid paths. Alternatively the working fluid may not need to be driven by a pump as convection or evaporation creating a pressure differential may suffice.

The heated fluid, in the one or more fluid paths, could be used for many different purposes, e.g. for heating, thermo-electricity generation, in cooling systems, etc. There are many different ways in which the heated working fluid could be used to generate electricity, e.g. by driving a gas turbine, used in a Stirling or Rankine engine, etc. The electricity generated could at least partly be used in the system itself, e.g. to power a pump for pumping the working fluid around the system.

In one set of embodiments the PV cell and the thermal receiver are provided in a single module. Thus while being thermally isolated from each other, this provides a single component onto which the solar radiation can be concentrated, facilitating straightforward assembly of the system, in particular in such a way that allows flexibility in the number of modules employed in a system.

In one set of embodiments thermal isolation of the PV cell from the thermal receiver and the second heat transfer mechanism is provided by an insulating material, e.g. a suitable temperature resistant rubber or plastic. In another set of embodiments the thermal isolation is provided by a physical gap between the PV cell and the thermal receiver, preferably which is evacuated. The insulating material or physical gap helps to keep the PV cell cool, e.g. below 60 °C, whilst allowing high temperatures in the rest of the thermal system, e.g. above 100 °C for the thermal receiver, which results in efficient power generation.

In a set of embodiments the thermal receiver is enclosed in an evacuated enclosure, e.g. an evacuated glass enclosure. This maximises the efficiency of the heat captured from the incident solar radiation and minimises radiation being reflected or re-emitted. The thermal receiver could also comprise a solar absorbing surface; this also maximises the capture of solar radiation. The PV cell can also be enclosed in an evacuated enclosure, e.g. an evacuated glass enclosure. In one set of preferred embodiments the PV cell and the thermal receiver are enclosed in a single evacuated enclosure, e.g. an evacuated glass enclosure. This is particularly convenient in the set of embodiments in which the PV cell and the thermal receiver are provided in a single integrated module.

In one set of embodiments the solar concentrator is arranged to produce a concentration of greater than 100 suns at its focus, preferably greater than 200, preferably greater than 500, e.g. 1000 suns. This intense concentration of solar radiation at the focus of the solar concentrator maximises the efficiency of the PV cell which receives this concentrated radiation. The solar concentrator could comprise refractive and/or reflective optics, e.g. it could comprise a lens, a reflective mirror or set of mirrors, e.g. a parabolic trough, a Fresnel reflector, a parabolic dish, an array of mirrors, e.g. an array of heliostats. In a preferred set of embodiments the solar concentrator comprises a Fresnel lens, e.g. the Applicant's thin film point focus Fresnel lens as described in WO

2009/106798.

As well as the primary solar concentrator, in one set of embodiments the system comprises one or more secondary solar concentrators. The one or more secondary solar concentrators could be provided to further concentrate the incident solar radiation onto the PV cell and/or the thermal receiver. More than one secondary solar concentrator could be provided for each of the PV cell and the thermal receiver. In one set of embodiments the system comprises a secondary solar concentrator arranged to concentrate the outer portion of the central focus of the solar radiation focused by the primary solar concentrator onto the thermal receiver. This could be used as an alternative way of thermally isolating the thermal receiver from the PV cell, and could also be used in the set of embodiments in which the system comprises a physical gap between the PV cell and the thermal receiver to thermally isolate these components. For example, the secondary solar

concentrator could comprise a reflector to reflect part of the incident solar radiation onto the thermal receiver.

In one set of embodiments the electrical contacts of the PV cell are arranged to be shielded from the incident solar radiation. In one set of embodiments the wires connected to the electrical contacts of the PV cell are arranged to run down the sides of the PV cell. This avoids the wires shadowing any of the PV cell from the incident radiation and keeps the wires at a cool temperature, thus avoiding a direct thermal connection to the PV cell and thereby keeping the PV cell from being unnecessarily heated. The skilled person will appreciate that there are many different types of PV cells that could be used in the system of the present invention. As has been discussed, one of the aims of the invention is to maximise the efficiency of power generation from the incident solar radiation and therefore in one set of embodiments the PV cell has an energy conversion efficiency of greater than 20%, preferably greater than 30%, e.g. greater than 40%. In set of preferred embodiments the PV cell comprises a triple junction PV cell, e.g. a triple junction gallium arsenide PV cell. In some embodiments a single PV cell is provided, in other embodiments a plurality of PV cells are provided at the focus of the solar concentrator. Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a schematic view of a combined photovoltaic and thermal power generation system in accordance with an embodiment of the invention;

Fig. 2 is a more detailed view of certain elements of a different embodiment;

Fig. 3 is a view of the same elements as in Fig. 2 but for a different embodiment of the invention; and

Fig. 4 is a view of the underside of the elements shown in Fig. 3. Fig. 1 shows a schematic view of a combined photovoltaic and thermal power generation system 1. The system 1 comprises a Fresnel lens solar concentrator 2 and a high efficiency photovoltaic (PV) cell 4, e.g. a triple junction gallium arsenide cell, arranged at the focus of the solar concentrator 2. At this point the incident solar radiation 8 is at its most intense. Arranged spaced from and below the PV cell 4 is a thermal receiver 6 which is located so that the less intense solar radiation 10 surrounding the main focus is incident upon it. The spacing of the thermal receiver 6 from the PV cell 4 acts to thermally isolate these components.

Attached to the underside of and in good thermal contact with the PV cell 4 is a first heat pipe 12 and attached to the underside of and in good thermal contact with the thermal receiver 6 is a second heat pipe 14. The first and second heat pipes each comprises a sealed copper tube which is filled, after the air inside is removed using a vacuum pump, with a fraction of a percent by volume of a coolant, e.g. water or ethanol. Owing to the partial vacuum that is near or below the vapour pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase.

The first and second heat pipes 12, 14 are connected via respective heat exchangers 16, 18 to a pipe 20 through which flows a working fluid. The pipe 20 is connected to an external system (not shown) which uses the heat from the working fluid.

Fig. 2 shows a more detailed view of the PV cell 104, the thermal receiver 106 and their associated components of a second embodiment. Fig. 2 does not show a solar concentrator which is provided, e.g. in the same arrangement as shown in Fig. 1. In this embodiment the thermal receiver 106 is arranged surrounding the PV cell 104, i.e. the PV cell 104 and the thermal receiver 106 are concentric.

A first heat pipe 112 is attached to a heat conductor 124 which in turn is attached to the underside of the PV cell 104 so that the first heat pipe 112 is in good thermal contact with the PV cell 104. A second heat pipe 114 is attached to the underside of and in good thermal contact with the thermal receiver 106. Although not shown, the first and second heat pipes 112, 114 will be thermally connected to a working fluid system as shown in Fig. 1.

Between the PV cell 104 and the thermal receiver 106, and acting to thermally isolate these components, is a thermal insulator 122. The thermal insulator also thermally isolates the first and second heat pipes 112, 114 from each other. Also shown between the PV cell 104 and the thermal receiver 106 are the wires 126 attached to the PV cell 104. The wires 126 are fed down between the PV cell 04 and the thermal receiver 106 so that they are not illuminated by the incident solar radiation. The wires 126 then lead out to a load that the system is delivering power to (not shown). Operation of the embodiments of the system shown in Figs. 1 and 2 will now be described. The system 1 is arranged so that solar radiation is incident upon the Fresnel lens solar concentrator 2. The solar concentrator 2 focuses the solar radiation onto the PV cell 4, 104 at the focus of the solar concentrator 2 such that the most intense concentration of the solar radiation 8 is incident upon the PV cell 4, 04. The less intense solar radiation 10, i.e. in the outer edges of the Gaussian distribution of the intensity of the solar radiation, is incident upon the thermal receiver 6, 106. The solar radiation 8 that is incident upon the PV cell 4, 104 is converted into electrical power which is conducted away from the PV cell 4, 104 through the wires 126. As the wires 126 pass down the sides of the PV cell 4, 104 they are not illuminated with solar radiation and therefore do not cast a shadow over the PV cell 4, 104 or the thermal receiver 6, 106. Owing to the efficiency of the PV cell 4, 104 being much less than 100%, e.g. 40%, the PV cell 4, 104 is heated by the incident solar radiation that is not converted into electrical energy. The resultant thermal energy in the PV cell 4, 104 is dissipated into the first heat pipe 12, 112 (via a heat conductor 124 in the embodiment shown in Fig. 2). The thermal energy transferred into the heat pipe 12, 112 is subsequently transferred into the working fluid in the pipe 20. The transfer of thermal energy away from the PV cell 4, 104 keeps the temperature of the PV cell 4, 104 below its maximum operating temperature of 60 °C.

The solar radiation 10 that is incident upon the thermal receiver 6, 06 is converted into thermal energy which is transferred to the second heat pipe 14, 114. The thermal energy transferred into the heat pipe 12, 112 is subsequently transferred into the working fluid in the pipe 20, i.e. the working fluid first flows through the heat exchanger 16 connected to the first heat pipe 14, 114 to transfer thermal energy into it from the PV cell 4, 104 and then flows through the heat exchanger 18 connected to the second heat pipe 16, 116 to transfer thermal energy into it from the thermal receiver 6, 106.

The working fluid is therefore first heated by the thermal energy from the PV cell 4, 104 to a first temperature, e.g. 60 °C and subsequently by the thermal energy from the thermal receiver 6, 106 to a higher temperature, e.g. 150 °C. Depending on the type of working fluid used, it may be boiled to produce a vapour, e.g. if water is used. This working fluid is then removed from the system to be used in an external system, e.g. for heating, hot water or secondary electricity generation. A further embodiment is shown in Figs. 3 and 4. Fig. 3 shows a side view of the arrangement of the inner components of a combined photovoltaic and thermal power generation system, i.e. similar to those shown in Fig. 2. In this embodiment, as in that shown in Fig. 2, a thermal receiver 206 surrounds a PV cell 204, but here the thermal receiver 206 is displaced below the PV cell 204. This is to

accommodate a reflective secondary solar concentrator 228 above the thermal receiver 206. As in Fig. 2 a first heat pipe 212 is attached to a heat conductor 224 which in turn is attached to the underside of the PV cell 204 so that the first heat pipe 212 is in good thermal contact with the PV cell 204. A second heat pipe 214 is attached to the underside of and in good thermal contact with the thermal receiver 206. Although not shown, the first and second heat pipes 212, 214 will be thermally connected to a working fluid system as shown in Fig. .

Fig. 4 shows a view from the underside of Fig. 3 showing the plan arrangement of the components. It can be seen that the PV cell 204 is disposed in the centre of the system, surrounded by the secondary solar concentrator 228 and the thermal receiver 206. The second heat pipe 214 is in good thermal contact with the thermal receiver 206 all the way round the outside of the PV cell 204, with one portion 230 projecting away from the thermal receiver 206 to transfer heat the working fluid (not shown).

Operation of the embodiment shown in Figs. 3 and 4 is very similar to that of the embodiments shown in Figs. 1 and 2. The main difference is due to the provision of the reflective secondary solar concentrator 228 which focuses the outer portion 10 of the incident solar radiation, initially concentrated by the primary solar

concentrator 2, onto the thermal receiver 206. The solar radiation is therefore further concentrated to maximise the intensity incident on the thermal receiver 206 which in turn maximises the efficiency of energy collection of the incident solar radiation. It will be appreciated by those skilled in the art that many variations and modifications to the embodiments described above may be made within the scope of the various aspects of the invention set out herein. For example the system could be part of an array of solar concentrators, each with their own PV cell and thermal receiver. However in this arrangement, the fluid paths of the separate modules could be linked so to use the working fluid on a larger scale.

Claims

1. A solar power generation system comprising:

a solar concentrator,

a photovoltaic cell at the focus of the solar concentrator,

2. A system as claimed in claim 1 wherein the PV cell is arranged at the focal point of the solar concentrator.

3. A system as claimed in claim 1 or 2 wherein the thermal receiver at least partially surrounds the photovoltaic cell.

4. A system as claimed in claim 1 , 2 or 3 wherein the thermal receiver fully surrounds the photovoltaic cell.

5. A system as claimed in any preceding claim wherein the first heat transfer mechanism comprises a heat pipe.

6. A system as claimed any preceding claim wherein the second heat transfer mechanism comprises a heat pipe.

7. A system as claimed in any preceding claim wherein the first heat transfer mechanism is in good thermal contact with a working fluid in a first fluid path.

8. A system as claimed in claim 7 wherein the working fluid has a temperature of less than 70 °C, preferably less than 60 °C

9. A system as claimed in any preceding claim wherein the second heat transfer mechanism is in good thermal contact with a working fluid in a second fluid path.

10. A system as claimed in any of claims 1 to 8 wherein a working fluid is arranged to pass through the thermal receiver.

11. A system as claimed in claim 9 or 10 wherein the working fluid has a temperature of at least 100 °C, preferably at least 120 °C, more preferably at least 140 °C.

12. A system as claimed in any preceding claim wherein the first heat transfer mechanism and the second heat transfer mechanism are thermally connected to a common heat exchanger.

13. A system as claimed in any preceding claim wherein the first heat transfer mechanism and the second heat transfer mechanism are in thermal contact with a common working fluid.

14. A system as claimed in claim 13 comprising a first fluid path in thermal contact with the first heat transfer mechanism and a second fluid path in thermal contact with the second heat transfer mechanism, wherein the second fluid path is a continuation of the first fluid path.

15. A system as claimed in claim 13 comprising a first fluid path in thermal contact with the first heat transfer mechanism and a second fluid path in thermal contact with the second heat transfer mechanism, wherein the first and second fluid paths converge at a point away from the photovoltaic cell and the thermal receiver.

16. A system as claimed in claim 15 wherein a common source is provided for the first fluid path and the second fluid path.

17. A system as claimed in any preceding claim comprising a plurality of first fluid paths in thermal contact with the first heat transfer mechanism.

18. A system as claimed in any preceding claim comprising a plurality of second heat transfer mechanisms.

19. A system as claimed in any preceding claim comprising a plurality of second fluid paths in thermal contact with the second heat transfer mechanism(s).

20. A system as claimed in any of claims 7 to 1 1 or 13 to 19 wherein the working fluid comprises water.

21. A system as claimed in any of claims 7 to 11 or 13 to 20 comprising means for generating electricity from heated working fluid.

22. A system as claimed in any preceding claim, wherein the photovoltaic cell and the thermal receiver are provided in a single module.

23. A system as claimed in any preceding claim, wherein the thermal receiver is enclosed in an evacuated enclosure.

24. A system as claimed in any preceding claim, wherein the photovoltaic cell is enclosed in an evacuated enclosure.

25. A system as claimed in any preceding claim, wherein the photovoltaic cell and the thermal receiver are enclosed in a single evacuated enclosure.

26. A system as claimed in any preceding claim, wherein the solar concentrator is arranged to produce a concentration of greater than 100 suns at its focus, preferably greater than 200, preferably greater than 500, e.g. greater than 1000 suns.

27. A system as claimed in any preceding claim, further comprising one or more secondary solar concentrators.

28. A system as claimed in claim 27 wherein the one or more secondary solar concentrators are arranged to further concentrate the incident solar radiation onto the photovoltaic cell and/or the thermal receiver.

29. A system as claimed in claim 28 wherein the photovoltaic cell comprises electrical contacts arranged to be shielded from the incident solar radiation.

30. A system as claimed in any preceding claim wherein the photovoltaic cell has an energy conversion efficiency of greater than 20%, preferably greater than 30%.

31. A system as claimed in any preceding claim wherein photovoltaic cell comprises a triple junction photovoltaic cell.