GB2506110A

GB2506110A - Solar Energy Apparatus for Preventing Overheating of Concentrating Photovoltaic System

Info

Publication number: GB2506110A
Application number: GB1216599.9A
Authority: GB
Inventors: Gerardo David Gomez-Sainz Garcia
Original assignee: Renergy SARL
Current assignee: Renergy SARL
Priority date: 2012-09-18
Filing date: 2012-09-18
Publication date: 2014-03-26
Also published as: GB201216599D0

Abstract

Apparatus for protecting a concentrated photovoltaic system comprising photovoltaic (PV) cells 71 from overheating, the system being one in which sunlight is concentrated primarily by reflection surface 310, the apparatus comprising a detector 352 adapted to sense the temperature of the PV cells and to send a signal indicative of said temperature to a controller 350, wherein the controller is adapted, when said signal is indicative of a first threshold temperature in the PV cells, to close a shutter 342 which, when activated, substantially obscures the PV cells from the sunlight. The shutter may have a mirrored surface. The shutter may be biased towards a closed position. The shutter may be part of the reflective surface when open. In one embodiment an auxiliary PV cell which has no sun light concentrated onto it and is not obscured by the reflector or shutters is also included. Auxiliary fluid cooling systems are also disclosed.

Description

Solar Energy Apparatus

S

FIELD OF THE INVENTION

The present invention relates to solar power systems, particularly those which collect and concentrate sunlight, and particularly but not exclusively to concentrated photovoltaic (CPV) systems.

BACKGROUND ART

Solar, or photovoltaic (PV), cells are devices which convert light into electricity using the photoelectric effect; the first solar cell was constructed in the 1880s, but only since the creation of the more efficient silicon solar cell in the 1950s has it been possible to utilise solar cells for generating electricity in a commercially-viable way. Modern multiple junction CPV cells are capable of efficiencies above 4O%. Concentrated photovoltaic (CPV) technology uses optics such as lenses or curved mirrors to concentrate a large amount of sunlight onto a small area of solar photovoltaic (PV) cells (or "PV array") to generate electricity. Compared to non-concentrated photovoltaic systems, CPV systems can save money on the cost of the solar cells, since a smaller area of photovoltaic material is required. Because a smaller PV area is required, CPV5 can use the more expensive, high-efficiency triple junction solar cells. To get the sunlight focused on the small PV area, CPV systems require the expense of concentrating optics (lenses or mirrors), solar trackers, and cooling systems. Even at conversion efficiencies above 4O%, the remaining incident solar energy produces a large amount of heat in the PV cells which needs to be conducted away so it does not overheat and damage the PV cells -and this heat is often usefully employed, for the generation of additional electricity, or for heating or cooling (using an accessory absorption chiller)

SUMMARY OF THE INVENTION

A problem with CPV systems is due to the intense concentration of solar energy on the PV array. Whilst in normal operation, the system can be designed to operate in such a way that excess heat can be conducted away and the PV cells kept at a suitably cool operating temperature, if there is an interruption in the cooling system, the PV cells can rapidly overheat and be damaged. Also, if there is a rapid change in the solar radiation, such as when the sun comes out from behind a cloud, the immediate heating effect on the PV cells can be so great that the cooling system cannot react quickly enough -again with the result that the expensive PV cells are damaged through overheating.

The present invention provides an apparatus for protecting photovoltaic (PV) cells in a concentrated photovoltaic system from overheating, the system being one in which sunlight is concentrated primarily by reflection surfaces, the apparatus comprising a detector adapted to sense the temperature of the PV cells and to send a signal indicative of said temperature to a controller, wherein the controller is adapted, when said signal is indicative of a first threshold temperature in the PV cells, to close at least one shutter which, when activated, substantially obscures the PV cells from the sunlight.

With such an arrangement, if the system cooling the PV cells fails or is interrupted, the PV cells are automatically protected by the closing of the shutter. Because the same system can also protect the PV cells from any risk of overheating by a transient increase in solar intensity, the primary PV cell cooling system does not have to be "over engineered" in order to cope with such transient increases, meaning that the system as a whole can be made less expensive.

The shutter may have a reflective surface, such as a mirrored surface, so as to reflect concentrated solar radiation away from the PV cells when the shutter is in the closed position. The shutter may be biased or spring-loaded into the open position, so that the controller need only activate a simple latch, which ordinarily holds the shutter in the open position but which, when activated by the controller, allows the shutter to spring into the closed position. The shutter, when in the open position, may usefully form part of the reflection surfaces and, if it is arranged to close so that its reflective surface faces the incident solar radiation, this saves the expense of silvering a second surface so as to reflect sunlight away from the PV cells.

Embodiments of the invention may also incorporate auxiliary cooling systems which may operate independently of or in combination with each other and/or the shutter arrangement. The controller may be adapted, when said signal is indicative of a second threshold temperature, to activate fluid cooling means arranged to conduct heat away from the PV cells. The cooling means may be a liquid cooling circuit, and/or a gas or ambient air cooling circuit, and the arrangement may be such that a gas auxiliary cooling circuit is activated at a first temperature, a liquid auxiliary cooling circuit is activated at a second, higher temperature, and the shutter is operated at a third, higher temperature. This provides various levels of protection for the PV cells.

In a further aspect of this invention, there is provided an auxiliary power apparatus for a concentrated photovoltaic system comprising photovoltaic (PV) cells on which sunlight is concentrated primarily by reflection surfaces wherein the reflection surfaces comprise one or more mirrors arranged around a single curvature so as to collect solar radiation and to concentrate the collected radiation onto a front face of the PV cells, which face away from the sun, the apparatus comprising a secondary PV cell arrangement oriented towards the sun and disposed behind the front face of the PV cells. Alternatively, where the reflection surfaces comprise a plurality of mirrors, arranged around two separate curvatures, the mirror(s) on a first curvature being adapted to collect solar radiation and to concentrate it and direct it towards the mirror(s) on a second curvature, said mirror(s) on the second curvature being adapted to reflect and concentrate the solar radiation onto the PV cells, the arrangement being such that the PV cells face towards the sun but are substantially in the shadow of the mirror(s) on the second curvature, the auxiliary power apparatus may comprise a secondary PV cell arrangement oriented towards the sun and disposed behind the mirror(s) on the second curvature. Such an auxiliary power supply can be used to power the controller, the shutter, and the auxiliary cooling systems.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figures la and lB are schematic diagrams of two types of prior art solar energy concentrated PV systems; Figure 2 is a schematic diagram of the collector face of a prior art CPV system; Figure 3a is a schematic view of an embodiment of an apparatus in accordance with the invention, illustrating auxiliary cooling arrangements; Figure 35 is a schematic view of the embodiment of Figure 3A, but illustrating the control arrangements for the auxiliary cooling arrangements, and Figures 4A and 4B are views similar to those in Figures 1A and 1B, but illustrating the disposition of the auxiliary power supply of Figures 3A and 35 relative to the remainder of the apparatus.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1A shows a prior art concentrating solar energy receiver 20 to illustrate its mounting structure. The concentrating solar energy receiver 20 has a primary parabolic reflector 2, shown in cross section, having a circular rim 32 which defines the circular outer perimeter of the primary parabolic reflector 2, and an array of densely packed, multiple junction photovoltaic cells 4 mounted on a conversion module 6. The PV cells include one or more triple junction solar cells, such as triple junction GaInP2fGaAs/Ge solar cells. Such solar cells currently available are capable of operating with intensities of solar radiation of up to several hundred suns, where one sun equals 0.1368 wafts per centimetre squared (Wfcrri2). Sunlight incident on the primary parabolic reflector 2 is reflected and concentrated towards the focal point of the parabola (not shown). The cells 4 are offset from this focal point by the distance necessary to ensure that the concentrated sunlight is dispersed over the entire surface of the cells. The conversion module 6 may comprise the solar cell array alone or it may also be a combination of a thermal cycle engine and an electric generator unit as is known in the art. As will be described in more detail below, the conversion module 6 contains circuitry for dealing with the electricity generated by the PV cells, and also means for cooling the PV cells 4, as these only convert a proportion of the incident solar energy into electricity (typically about 40%). The remaining energy is largely thermal, which heats the PV cells 4; it is often beneficial to use this thermal energy to generate more electricity, and this is done by extracting heat from the coolant which cools the PV cells and turning that into electricity.

The primary parabolic reflector 2 and the conversion module 6, including the cells 4, are held in a fixed relationship by a frame member 8. The frame member 8 is connected to the primary parabolic reflector 2 near its centre and extends therefrom to connect with and support the conversion module 6 along the principal axis of the primary parabolic reflector 2. The cells 4 are thus positioned to directly face the centre portion of the primary parabolic reflector 2 such that it receives all of the solar energy radiation being reflected from the primary parabolic reflector 2. The frame member 8 is connected to a rotatable vertical post 14 at a pivoting joint 10 which permits the frame member 8 to rock in a vertical plane about a horizontal axis so that the primary parabolic reflector 2 may be positioned at any required elevation angle while pivoting about the axis of the pivoting joint 10. The rocking motion of the frame member 8 is provided by a control actuator 18 which consists of a variable length strut whose length may be varied under the action of a motor or linear actuator in the longitudinal axis of the vertical control actuator 18. The rotating post 14 is rotatably secured to a horizontal control motor 16 which in tum is supported by a vertically oriented stationary base 12 anchored upon the ground, a building or other structure. The vertical control actuator 18 provides for adjusting the elevation of the concentrating solar energy receiver assembly 20. The horizontal control motor permits the adjustment of the azimuth of the concentrating solar energy conversion receiver 20. Thus the primary parabolic reflector 2 of a concentrating solar energy receiver 20 may be aimed directly at the sun and enabled to track the sun as it proceeds across the sky during daylight hours.

Figure lB shows an alternative form of a prior art concentrating solar energy receiver 40. In this arrangement, the primary parabolic reflector 42, shown in cross section and having a circular rim 52, includes a secondary parabolic reflector 44 disposed along the principal focal axis of the primary reflector 42 and at the near focal area for reflecting radiant energy towards Pv cells 46 located at the surface of the centre portion of the primary parabolic reflector 42. Also located in the centre portion of the primary parabolic reflector 42 and behind the PV cells 46 is the conversion module 22. The secondary parabolic reflector 44 is shown supported on struts 48 which may be attached to the rim 52 or, as shown, to the concave side of the primary parabolic reflector 42. It will be appreciated that the focal axis of the secondary reflector 44 lies along the focal axis of the primary reflector 42, that is, their principal axes are coincident.

With the distribution of masses of the various components of the concentrating solar energy receiver 40 as shown in Figure 15, the centre of gravity is located approximately at the centre of and just behind the primary parabolic reflector 42. This location of the centre of gravity 24 considerably simplifies the supporting structure needed to support the concentrating solar energy receiver 40 and provide for its movement in both the elevation and azimuth directions. The concentrating solar energy receiver 40 is supported at the top of a rotating vertical post 26. Rotating vertical post 26 is controlled by a horizontal control motor 28 which is supported at the upper end of a vertically oriented stationary base 34.

Also attached to the rotating vertical post 26 is a vertical control motor 30, which is a variable length strut controlled by a linear actuator or motor disposed along the longitudinal axis of the variable length strut and is provided to control the elevation of the concentrating solar energy receiver 40. The azimuth orientation of the concentrating solar energy receiver is controlled by the horizontal control motor 28. It will be appreciated that in both FIGS. 1A and 1B, the respective control motors for the vertical (elevation) and horizontal (azimuth) may be controlled by suitable electronics which are not shown in the diagrams, but are readily available and known to persons skilled in the art.

Arrangements such as that of Figure 15 are sometimes preferred over those of Figure 1A because locating the most massive components together positions the centre of gravity in such a way that the responsiveness of the control system is maximized and the size of the actuating units and motors is minimized, thus increasing performance and reducing the cost of the assemblies required. This is particularly so if the conversion module 22 contains all the subsystems for cooling the PV cells 46, and/or if the primary parabolic reflector 42 concentrates medium or high concentrations of solar energy on the PV cells, above about 100 suns. It is common to utilise both the Figure 1A and lB solar receivers in multiple arrays, as this enables greater amounts of electricity to be produced, and enables efficiencies of scale to be achieved in relation to the conversion of thermal energy into electricity, as the fluid used to cool the various arrays of PV cells can be directed to a single heat engine or turbine for generating electricity.

Figure 2 shows the usual arrangement at the solar collecting face of the PV cells. The arrangement 50 comprises the PV cells, 51 mounted on a cooler assembly 54; this has an inlet manifold 56 and an outlet manifold 58, for the flow of coolant through the assembly 54 for the purpose of conducting heat away from the PV cells 54. The assembly also has mounted on it a reflective, or mirrored, collector surface 60, which reflects the concentrated rays of solar radiation S towards the PV cells 51. Collector surface 60 is shown as an open ended structure consisting of four planar surfaces which converge in the direction towards the PV cells, however other configurations may be used, such as a truncated conical shape, provided these serve to concentrate or funnel the solar radiation S towards the PV cells 51 whilst absorbing the minimum of solar energy.

One of the problems with CPV systems is that the concentration of solar energy onto a relatively small area of PV cells produces a large amount of thermal energy, which is not converted into electricity by the PV cells. This heat needs to be conducted away from the PV cells, to prevent them from overheating and being damaged. This is why the PV cells 51 are normally mounted on a cooling assembly 51; the coolant flowing through the assembly 54 conducts heat away from the PV cells, maintaining them at an optimum working temperature (the flow of coolant is usually controlled, in response to the temperature of the PV cells, in order to keep the PV cells at the right temperature, or within an appropriate temperature range; the heat can usefully be removed from the coolant once it has passed through the outlet manifold 58 and converted into electricity using a suitable heat engine/alternator arrangement (not shown) or similar device, to be added to that generated by the PV cells 51). The coolant may be water, or any other suitable liquid.

If there is an interruption in the flow of coolant, and/or if the solar radiation suddenly increases (such as when the sun emerges from behind clouds), the solar energy concentrated on the PV cells can quickly overheat the PV cells and damage them, before the coolant flow can be restarted, and/or increased to account for the additional solar radiation incident on the PV cells. Embodiments in accordance with the present invention address this problem.

Figure 3A is a view, similar to that of Figure 2, of an arrangement 300 at the solar collecting face of the RI cells 71 but showing auxiliary cooling and PV cell protection arrangements. As in Figure 2, the arrangement 300 has a reflective collector surface 310 which converges from an outer rim 311 towards the PV cell array 71. The PV cells, or array, are/is mounted on a cooler assembly 314, and coolant is pumped around a circuit from a coolant reservoir 320 via conduits 316, 318 and through the assembly 314 by pump 322.

Electricity generated by the PV cells 71 from the concentrated rays of solar radiation S is conducted away via line 344 to a converter, storage device or ring main, shown schematically at 346. Thus far therefore, the arrangement is conventional.

The cooler assembly 314 is mounted on an auxiliary cooler assembly 324, which is adapted for the flow of liquid coolant through it, pumped by pump 330 through different channels (not shown, though the square shapes seen in assembly 314 are indicative of a plurality of coolant channels running through the assembly 314) in assembly 314 to those used by the primary coolant and in a circuit via conduit 328. In the event of the instantaneous temperature of the PV cells reaching a threshold temperature approaching the temperature at which they risk thermal damage, pump 330 is initiated so that coolant flows through the cooler assembly 314 and the auxiliary cooler assembly 324; the auxiliary cooler assembly 324 acts as a radiator, emitting excess heat conducted away from the assembly 314 and the PV cells 71. Additionally or alternatively, the auxiliary coolant may be returned to a reservoir (not shown) and/or treated to remove heat energy for conversion to electricity as with the primary coolant (not shown).

The reflective collector surface 310 of the arrangement has, at its narrow end near to the PV cells 71, a closure 332, which is substantially transparent to solar radiation. This, in combination with baffles 336 which surround the PV cells 71 creates a narrow channel between the closure 332 and the PV cells 71. Inlet and exhaust fans 333, 334 (only one of each is shown, there could be any number of each), and these are operated when the temperature of the PV cells rises too high in order to pass ambient air over the PV cells 71 and thus cool them. Air filters 340 (only one is shown) are provided upstream of each inlet fan 333 to remove any dust from the ambient air which might otherwise settle on the surface of the PV cells 71 and cause damage and/or inhibit their efficiency. His ambient air cooling system may operate separately from the auxiliary cooling system described above, at a different temperature, or the two could operate in combination for maximum cooling.

In the event the auxiliary coolant and ambient air systems described above cannot cope with the heat load on the PV cells 71, part of the collector surface 310 is hinged at 344 and can be driven through the arc shown by the dotted line in Figure 3A to act as a shutter 342, preventing solar radiation S from reaching the PV cells 71, thus completely protecting them. The surface part 342 is of course shaped so as substantially to cover the closure 332, and this is conveniently achieved if the surface part 342 is planar. This arrangement may be triggered independently of the auxiliary coolant and ambient air systems described above, at a separate temperature. One particularly suitable arrangement would be for the ambient air system to begin to operate at a first temperature, approaching that where the PV cells 71 risk thermal damage, with the auxiliary coolant system beginning to operate at a second, higher temperature and the shutter 342 being moved into place to cut off solar radiation from the PV cells at a third, even higher temperature. The outer surface of the shutter 342, i.e. that face on which solar radiation is incident when the shutter 342 is covering closure 332, is preferably reflective, or mirrored, to reflect the solar radiation away from the PV cells 71. Although the shutter 342 is shown hinged at 344, close to the PV cells 71, so that it closes in a downward direction, it could alternatively be hinged closer to the rim 311 and close in an upward direction (not shown); with such an arrangement the reflective collector surface would form the upper surface when the shutter was "closed", so there would be no need to make the obverse face of part of the collector surface reflective, or mirrored, but the geometry of the collector surface 310 would mean that a single shutter would not totally obscure the closure 332 and would leave part of the PV array 71 unprotected from incident solar radiation S. This could be addressed by having a second shutter (not shown), or further shutters, opening from the other side(s) of the collector surface 310 so as completely to obscure the closure 332 from incident radiation S. Referring now to Figure 3B, where like numerals denote like features in Figure 3A, a suitable control mechanism is shown (and certain features from Figure 3A have been omitted for clarity). Controller 350, which can be a suitably programmed integrated circuit, microprocessor or mini-computer, is connected via line 354 to temperature sensor 352, which is located to accurately sense the temperature of the PV cells 71 (only one temperature sensor 352 is shown, but there could be several in an array). Controller 350 is connected via line 360 to inlet fan 333, via line 362 to coolant pump 322, via line 364 to auxiliary coolant pump 330, and via line 366 to exhaust fan 334; controller 350 operates all these as described above. Controller 350 is also connected via line 358 to latching mechanism 356; hinge 344 is spring-biased to move the shutter 342 into the closed position, so that to close the shutter controller 350 simply actuates latch 356 to permit the shutter to close under the operation of the spring (not shown) Alternatively, there may be no biasing, and instead line 358 leads to a rotary actuator adapted to rotate the shutter 342; the controller 350 could then both close the shutter 342 when necessary, and open it again when the need to obscure the PV cells 71 to protect them has passed.

Controller 350 is also connected via line 318 to auxiliary PV cells 100; these PV cells 100 provide power to the controller 350, pumps 322 and 330 fans 333, 334 and latching mechanism 356 (and/or to any actuator, as described above). It is advantageous to have the R/ cells 100 located as shown, as they can be cooled by the auxiliary cooler assembly 324 if necessary. It is desirable to have an auxiliary power supply, particularly if the PV ells 71 are either damaged or not producing electricity (because the shutter 342 has been deployed, for example), and the positioning of the PV cells 100 is advantageous, as will be described with reference to Figures 4A and 4B.

Figures 4A and 4B are in many ways similar to the arrangements illustrated in Figures 1A and 1B; and like references in Figures 4A and 4B, and in Figures 1A and lB denote like features. As will be seen, Figure 4A shows an arrangement 80 where the conversion module 6 faces away from the solar radiation, so that the main PV cells 4 face the parabolic reflector 2; in this case, the PV cells 100 are located on the obverse face of the conversion module 6, and the arrangement is exactly as also illustrated in Figure 3B.

In Figure 4B an alternative arrangement 90 is shown, in which the conversion module 22 is facing directly towards the solar radiation, however the primary PV cells 46 are substantially in the shadow cast by secondary parabolic reflector 44; in this case, the auxiliary PV cells 100 are located on the rear of the secondary reflector 44, with any necessary circuitry, cooling conduits (not shown) being located in or alongside the struts 48.

These locations for the auxiliary PV cells 100 are advantageous, as they use only the solar radiation which would otherwise be unused because it would strike either the rear of the primary PV cells 4, 46, 71 or the rear face of the secondary reflector 44. As will be clear, the arrangements of the auxiliary PV cells 100 could only be as shown as in Figure 4A or 4B, depending on the overall arrangement of the system, including whether or not a heliostat array (not shown) forms part of the overall system. The parabolic dishes shown in Figures 1 and 4 are typically about 3m in diameter, however the present invention is capable of being adapted for any size solar energy system, including a heliostat array where a plurality of mirror gather and direct solar energy towards a tower (in which case the auxiliary PV cells described herein would be located on the rear face of the tower (i.e. facing towards the sun), and operate a shutter on the opposite face of the tower.

It will of course be understood that many variations may be made to the above-described embodiments without departing from the scope of the present invention. For example, the shutters have all been described as rotating between closed and open positions, but the shutter could be any form of shutter, such as a sliding planar form or iris-type shutter. When auxiliary cooling is not required, any electricity generated by the auxiliary PV cells can be added to that generated by the primary PV cells, or it could be used to power the various motors driving the reflector so that it tracks the sun's movements, leaving all the electricity generated by the primary PV cells to be distributed for use.

Although the gas cooling system described uses ambient air, a source of compressed gas, such as carbon dioxide, could be used to provide a more intense cooling effect.

Claims

CLAIMS1. Apparatus for protecting photovoltaic (PV) cells in a concentrated photovoltaic system from overheating, the system being one in which sunlight is concentrated primarily by reflection surfaces, the apparatus comprising a detector adapted to sense the temperature of the PV cells and to send a signal indicative of said temperature to a controller, wherein the controller is adapted, when said signal is indicative of a first threshold temperature in the PV cells, to close a shutter which, when activated, substantially obscures the PV cells from the sunlight.
2. Apparatus according to Claim 1 wherein the shutter has a mirrored surface, to reflect sunlight away from the PV cells when closed.
3. Apparatus according to Claim 1 or 2 wherein the shutter is biased or spring-loaded into a closed position, and the controller is adapted to activate a latch adapted to hold the shutter in an open position.
4. Apparatus according to Claim 3 when dependent on Claim 2 wherein the shutter in the open position provides a part of the reflection surfaces.
5. Apparatus according to any preceding Claim wherein the controller is adapted, when said signal is indicative of a second threshold temperature, to activate fluid cooling means arranged to conduct heat away from the PV cells.
6. Apparatus according to Claim 5 wherein the fluid cooling means comprises a liquid cooling circuit.
7. Apparatus according to Claim 5 or 6 wherein the fluid cooling means comprises a gas cooling circuit.
8. Apparatus according to any preceding Claim wherein the reflection surfaces comprise one or more mirrors arranged around a single curvature so as to collect solar radiation and to concentrate the collected radiation onto a front face of the PV cells, which face away from the sun, the apparatus further comprising a secondary PV cell arrangement oriented towards the sun and disposed behind the front face of the PV cells.
9. Apparatus according to any of Claims ito 7 wherein the reflection surfaces comprise a plurality of mirrors, arranged around two separate curvatures, the mirror(s) on a first curvature adapted to collect solar radiation and to concentrate it and direct it towards the mirror(s) on a second curvature, said mirror(s) on the second curvature being adapted to reflect and concentrate the solar radiation onto the PV cells, the arrangement being such that the PV cells face towards the sun but are substantially in the shadow of the mirror(s) on the second curvature, the apparatus further comprising a secondary PV cell arrangement oriented towards the sun and disposed behind the mirror(s) on the second curvature.
10. Apparatus according to Claim 8 or 9 wherein the or each curvature is substantially parabolic.
11. Auxiliary power apparatus for a concentrated photovoltaic system comprising photovoltaic (PV) cells on which sunlight is concentrated primarily by reflection surfaces wherein the reflection surfaces comprise one or more mirrors arranged around a single curvature so as to collect solar radiation and to concentrate the collected radiation onto a front face of the PV cells, which face away from the sun, the apparatus comprising a secondary PV cell arrangement oriented towards the sun and disposed behind the front face of the PV cells.
12. Auxiliary power apparatus for a concentrated photovoltaic system comprising photovoltaic (PV) cells on which sunlight is concentrated primarily by reflection surfaces wherein the reflection surfaces comprise a plurality of mirrors, arranged around two separate curvatures, the mirror(s) on a first curvature adapted to collect solar radiation and to concentrate it and direct it towards the mirror(s) on a second curvature, said mirror(s) on the second curvature being adapted to reflect and concentrate the solar radiation onto the PV cells, the arrangement being such that the PV cells face towards the sun but are substantially in the shadow of the mirror(s) on the second curvature, the apparatus comprising a secondary PV cell arrangement oriented towards the sun and disposed behind the mirror(s) on the second curvature.
13. Apparatus substantially as hereinbefore described and with reference to Figures 3A, 35, 4A and/or 45 of the accompanying drawings.Amendments to the claims have been filed as follows r AT 1. Apparatus for protecting photovoftaic (PV) cells in a concentrated DflOtOVOtaC system from overheating, the system being one in which sunllght is concentrated on the PV cells prirnarlly by reflection surfaces, the apparatus comprising a detector adapted to sense the temperature of the PV cells and to send a signal indicative of said temperature to a controller, the controller being adapted, when said signal is indicative of a first threshold temperature in the PV cells, to close a shutter which, when activated, substantially obscures the PV cells from the sunlight, wherein the "elect;cr sjraces compnse at least a nrnar curved surace ddapted in use to present a concave surface towards the sun, and wherein, the appantus further comprises an auxiliary PV cell arrangement mounted so as to face in the same o general thr.ecflon as the primary curved surface so as to provide auxifiary power when the shutter is activated.2. Apparatus according to Claim 1 wherein the shutter has a mirrored surface, to reflect sunlight away from the PV cells when closed»= 3. Apparatus according to Claim 3 when dependent on Claim 2 wherein the shutter in the open position provides a part of the reflection surfaces.4. Apparatus according to Claim 1,2 or 3 wherein the reflection surfaces comprise one or more mirrors arranged around a single curvature so as to collect solar radiation and to concentrate the collected radiation onto a front face of the PV cells, which face away from the sun., the apparatus further comprising a secondary PV cell arrangement oriented towards th.e sun and disposed behind the front face of the PV cells.5. Apparatus according to Claim 1, 2 or 3 wherein the reflection surfaces comprise a plurality of mirrors, arranged around two separate curvatures, the mirror(s) on a first curvature adapted to collect solar radiation and to concentrate it and direct it towards the mirror(s) on a second curvature, said mirror(s) on the second curvature being adapted to reflect and concentrate the solar radiation onto the PV cells, the arrangement being such that the PV ceHs face towards the sun but are substantially in the shadow of the mirror(s) On the second curvature, the apparatus further comprising a secondary PY cell arrangement oriented towards the sun and disposed behind the mirror(s) on the second curvature, 6. Apparatus according to any preceding Claim wherein the shutter is biased or spring-oaded into a closed position, and the *controlier is adapted to activate a latch adapted to hold the shutter in an open position flU 7, Apparatus according to any preceding Claim wherein the controHer is adapted, when said signal is indicative of a secon.d threshold temperature, to activate fluid cooling 0 means arranged to conduct heat away from the PV cells. (0r 8. Apparatus according, to Claim 7 wherein the Puid cooling means comprises a liquid cooling circuit, 9. Apparatus according to Claim 7 or 8 wherein the fluid cooling means comprises a gas cooling circuit.10. Apparatus according to Claim 4 or 5. or any Claim dependent on Claim 4 or 5, wherein the or each curvature is substantially parabolic.11. Auxiliary power apparatus for a concentrated photovoltaic system comprising photovoltaic (PY) cells on' which sunlight is concentrated primarily by reflection surfaces wneren the reflection surraces compnse one or more mirrors arranged around a snge' curvature so as to coliect solar radabon anc to concentrate the collected radiation onto a front face of th.e PV cells, which face away from the sun, the apparatus comprising a secondary PV cell arrangement oriented towards ftc sun and disposed behind the front face of the PV ceUs.12. Auxiiiary power apparatus for a concentrated photovoitaic system comprising photovoitaic (PV) ceUs on which. sunlight is concentrated primarDy by reflection surfaces wherein the reflection surfaces comprise a plurality of mirrors, arranged around two separate curvatures, the mirror(s) on a first curvature adapted to coUect S solar radiation and to concentrate ft and direct it toward.s the mirror(s) on a second curvature, said mirror(s) on the second curvature being adapted to reflect and concentrate the solar radiation onto the PV cells, the arrangement being such that the PV celis face towards the sun but are substantiaHy in the shadow of the mirror(s) on the second curvature1 the apparatus comprising a secondary PV cell arrangement lO oriented towards the sun and disposed behind the mirror(s) on the second curvature. r13. Apparatus substantially as hereinbethre described and with reference to Figures 3k o 3B, 4A and/or 4B of the accompanying drawings. (0 r