WO2000005769A1

WO2000005769A1 - A differential voltage cell

Info

Publication number: WO2000005769A1
Application number: PCT/GB1998/002172
Authority: WO
Inventors: Peter King
Original assignee: Btg International Ltd
Priority date: 1997-01-18
Filing date: 1998-07-21
Publication date: 2000-02-03
Also published as: AU8453098A

Abstract

A differential voltage cell includes one or more units each comprising a photovoltaic device or solar cell (2) and a thermoelectric Peltier device (4). The two devices are separated from one another by a layer (3) of thermally conducting but electrically insulating material and the Peltier device is thermally connected by a layer (5) to a heat sink or heat exchanger (6). A lens (1) serves to concentrate sunlight on the photovoltaic device. The temperature generated in the photovoltaic device (2), which would otherwise be wasted, creates a temperature difference which provides an electrical output from the thermoelectric device (4). The outputs from the two devices are electrically connected. Heat exchangers may be incorporated in the layer (3 or 5) or both, to control operation of the cell or to provide useful heat for other purposes.

Description

A DIFFERENTIAL VOLTAGE CELL

The invention relates to a differential voltage cell which is able to convert and direct solar radiation into electrical and thermal energy. Solar cells are well known devices which produce electrical energy from sunlight. Thermoelectric devices are also well known and can produce electrical energy from temperature differentials. In both devices the movement of holes and electrons produce the electrical output. The present invention relates to the combination of such devices in a unit cell, a number of which unit cells may be connected together.

Some prior art devices of the above kinds have disadvantages in their construction which limit their efficiency for practical energy conversion.

One of the applications of the present invention is in the supply of power for domestic use. There are a number of problems at present which affect roof-mounted solar panels which are increasingly used to provide part of a domestic electricity supply.

For example, heat produced by the solar cells must be conducted away, and some proposals use a 'cooling tower' type arrangement, so as not to compromise the efficiency of the cells. A large area of silicon solar cells is required on roofs to produce enough electricity, and this is not practical in every household, and also the available roof area may be insufficient.

Heating of any description, for example heating water, usually requires more energy than can be supplied by the solar cells, so that the extra energy must be obtained from other sources, normally the connected national electrical grid.

In countries with erratic sunlight, it is desirable to make as much energy as possible from such sunlight as is available.

Certain embodiments of the invention address the above problems by producing more electricity from a smaller area of semiconductor material than by conventional solar cells and, as a by-product, heating water or another liquid or gas (such as a refrigerant fluid) in a heat exchanger construction for directly useful purposes, such as hot water for showers. A further improvement in efficiency of these embodiments can be achieved by control from an "intelligent" device, such as a microprocessor, even to the extent of possibly cooling the internal rooms of a house during the summer.

Another application of this invention is on moving vehicles, such as electric or hydrogen powered cars. Electricity, produced and regulated by airflow or otherwise, could be used to supply direct power or to power electrolytic cells. Prior art devices which produce electricity through heat and light are mentioned in the literature, for example US 3,956,017, but have disadvantages.

Accordingly, in one aspect of the present invention there is provided a differential voltage cell in which a photovoltaic cell or solar cell is separated from a Peltier device by a thermally conductive but electrically insulating body. The Peltier device may consist of a series of p-n elements in the classical configuration. A plurality of stacked Peltier devices may be used. The Peltier device(s) and the solar cell are connected to each other by an electrically conductive means, such as wires, preferably in series, although parallel arrangements can also be considered.

The thermally conductive but electrically insulating body may be made in part, or wholly, of a ceramic or pipe or container. The container or pipe is preferably metal, preferably black, having water or some other liquid or gas passing through it.

Also, the electrically insulating but thermally conductive body may be made in part, or wholly, from semiconductor material, or otherwise, such as carbon, silicon, germanium, ZnS, CdS, CdSe, ZnO, GaP, CdTe, ZnTe, GaAs or a suitable form of carbon (such as diamond or C₆₀ structures). Examples of other materials that can be used, themselves or as compounds as the case may be, in the said material are Sn, Sb, Bi, Pb, Cd, Zn, S, O, Te, Ga. Plastics and polymers could also be used such as polyethylene, polyamide such as Nylon (6,6), polyacetylene, polypyrrole, polythiophene, polyaniline, sexithiophene, 8-hydroxyquinoline aluminium and poly-p- phenylenevinylene (P.P.V.). Mixtures, compounds or ceramics such as silicon carbide, boron nitride, NiO, MnO and CoO may be used, some of which are used for resistors.

Preferably a focusing lens, reflector, or parabolic dish is disposed to concentrate radiation onto the surfaces of a differential voltage cell according to the invention. Such a differential voltage cell can comprise a number of unit cells, comprising . solar cells and Peltier devices, connected such that only two electrical outputs exist. The unit cells can be connected in parallel or in series.

The invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a single unit cell comprising a combined Peltier and photovoltaic device according to the present invention,

Figure 2 shows embodiments diagrammatically represented, of a unit cell according to the present invention, Figure 3 illustrates diagrammatically the electrical connections internally in a unit cell according to Figure 1 , Figure 2 or Figure 4,

Figure 4 is a diagrammatic representation of a single unit cell with more than one combination of Peltier and photovoltaic devices according to the present invention,

Figure 5 and Figure 6 illustrate further embodiments and show how the differential voltage cells can be used in certain applications, described previously, according to the present invention, and

Figures 7, 8 and 9 show other embodiments of different internal constructions relating to cooling at the photovoltaic junction or junctions.

The unit cell of Figure 1 comprises a photovoltaic cell 2 with electrical outputs 8 and 9 and a thermoelectric (Peltier) device 4 with electrical outputs 10 and 11. The Peltier device 4 comprises n- and p- type materials arranged in sequence with junctions between them (as known). An electrically insulating and thermally conductive body in the form of a plate 3 is located between the cell 2 and the Peltier device 4. A further similar electrically insulating and thermally conducting plate 5 is located between the underside of the Peltier device 4 and finned cooling body 6. Airflow may be provided over the body 6 as indicated at 7. A lens 1 is used for concentrating sunlight on the upper surface of the photovoltaic cell 2.

In use, sunlight is directed and concentrated on to the cell 2 by the lens 1.

Provided the n- and p- type materials in the Peltier device 4 are in the correct sequence, the output 9 from the photovoltaic cell can be connected to the output 11 from the

Peltier device. An electrical output will then be obtained between 10 and 8. In operation, the temperature of the photovoltaic cell 2 will increase and heat will be transferred through the plate 3 to the Peltier device 4, the plate 5 and the fmned body 6. Only a small proportion of the output from the photovoltaic cell 2 will be used in the Peltier device 4 to transfer more heat energy from the plate to the plate 5, provided the cell 2 and device 4 are matched properly. This is due to the current produced in the Peltier device 4 from the temperature differential between the plates 3 and 5 (produced by the concentration of the lens 1) contributing to the overall effect.

A device, as described in Figure 1 , of two units was built and tested. Two solar cells of areas approximately 529mm² of output 0.4 V (open circuit), 70mA

(instantaneous current) at lkW/m² illumination, and two commercially available Peltier devices, were used. Focusing was applied by a lens of area 3837mm and focal length

133mm, which boosted the output of the cell.

The arrangement successfully powered a small motor when the sunlight was approximately 850 W/m². It would normally take eight solar cells of total area 7392mm to power the motor in normal sunlight. This constitutes a saving in area of 85% of the semi-conductor material and almost a 1 : 1 ratio in the overall sun capture area being used. The ambient temperature was 18°C and the maximum recorded temperature of the solar cell was 45°C.

In addition, this arrangement cooled the solar cell by 1.6°C when the motor was connected. When this arrangement was being tested, from cold, through a meter measuring 7 ohms, the temperature differential was held at 5°C lower than the open circuit condition.

The heat sink temperature was approximately 22°C at equilibrium and no extra cooling had been applied, either from water, liquid, gas or air (wind velocity ~ 0). If additional cooling had been used (tap water temperature on the day was 12°C) then a greater cooling figure and, in some circumstances, a larger output would have been expected.

An increase in the area of the solar cell (keeping the same W/m²) would result in a larger cooling effect.

This arrangement was not optimal, but it demonstrates the principle of the cell and that the normal operating temperature can be shifted upwards or downwards, depending on the environments and materials used. A further improvement to the tested device would be to choose a photovoltaic device and Peltier which would output the same current within the unit cell as implied by the description 'matched properly'.

Examples will now be described to illustrate this, using the relationship:

V V ^σN -^ΛN - _j ~ ^σp '^Λp ' _j ^LN ^Lp

where suffix N denotes N-arm characteristics suffix P denotes P-arm characteristics

A = Area L = Length σ = Conductivity

V = Voltage due to temperature differential across P and N arms. Other symbols which will be used are:

Nd = doping concentration of donors (electrons) in N-arm. Na = doping concentration of acceptors (holes) in P-arm. ni = intrinsic carrier concentration of material.

ΔT = voltage differential across P and N arms.

The above relationship is observed in order to maximise output from an element, consisting of an N and P arm, Peltier or thermoelectric device, and with the elements connected in series or otherwise.

In some circumstances Nd>Na, and the mobility of the electrons is greater than the mobility of holes. So it can be seen that if V and V_p are equal, then A_p must be increased or L increased to maintain this relationship.

Also in conjunction with this or otherwise,

V ^V N _ V ^V P

Δ7Λ, ΔΓ_P

to optimise output. This gives optimum doping concentrations with respect to ni. With reference to Figure 1 , if the lens 1 is omitted and the airflow 7 is replaced by water cooling, an array of these cells, which can be placed on a roof operating at normal temperatures, will have an overall output greater than that of the solar cell (photo-voltaic cell) array on its own. It has been calculated that a 0.4V 100mA type silicon cell could have its output increased in this type of configuration, when the water is cold (mains temperature), from 4.5% to 10% efficiency for the same photo absorption (collector) area.

So a configuration, even in its simplest form, as described, can yield significant advantages. Where water cooling is employed alternative semiconductor material, such as

CdTe operating up to 150°C, could be used as a photo-voltaic device in this configuration, allowing the cooling water to reach temperatures of up to 60°C.

Other materials which may be used in the N and P arms, and/or the photovoltaic device or otherwise (such as the conductor connecting the N and P arms) are, and note that a plurality of these materials may be used in the N and P arms:

C, Si, Ge, αSn, ZnTe, ZnSe, ZnS, CdTe, HgSe, HgS, HgTe, BP, AIP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, B-SiC, Ga₂Te₃, In₂Te₃, Pb, ZnO, ZnS, CdSe, ZnSiP₂, CdSiP₂, ZnSnAs₂, CdeGeAs₂ CdSnAs₂, Cu₂GeS₃, Cu₂GeSe₃, Cu₂SnS₃, CuSnSe₃, ZnIn₂Se₄, CdGa₂S₄, CdGa₂Se₄, CdIn₂Te₄, HgGa₂Se₄, HgInSe₄, HgIn₂Te₄, αSiC, Sn

HgIn₂Te₈, PbS, PbSe, PbTe, GdSe, NiD, Mg₂ Si Mg₂Ge, Mg₂S_n, Sb₂Te₃, Bi₂Se₃, Bi₂Te₃, B, Se, Te, SnO₂, α-In₂Te₂, B-In₂Te₃, InSb, CdO, CdS, ZnSb, CdSb, Bi₂S₂, Bi₂Se₂, Bi₂Te₂, Mg₂Sb₂, Zn₂As₂, Cd₂As₂, GaSe, GaTe, InSe, TlSe, Ga₂Te₃, CuInSe₂, CuS, Zn₃P₂, CuAlO₂, In₂O₃, SnO₂. Other materials which are relevant are polymers, molecular polymeric semiconductors and organic materials as discussed in patent specifications such as GB2297647A, GB2288181A and GB2296815A, and materials such as polyacetylene, polypyrrole, polythiophene, polyaniline, sexithiophene, 8-hydroxyquinoline aluminium and poly-p-phenylenevinylene (P.P.V.). All these materials could also be used for the layer or layers (such as the plate 3) between the photovoltaic cell and thermoelectric devices as described in Figures 1-3 and 5. Either one of the p and n materials could be omitted; in such a case, preferably the p material (P arm) is the one that is retained. The photovoltaic cell should then occupy a larger area, the device tapering inwardly downwardly to a smaller area of thermoelectric material. With reference to Figure 1 , a further possibility would be to connect the outputs

8 and 9 in parallel with the outputs 10 and 11 , such that the matching between the two cells, in terms of resistance or voltage, would be such as to give a significant electrical output. Output would then be taken between 10 and 11 or between 8 and 9. This type of device could be used in both applications described previously. Figure 2 shows another embodiment of the device containing a number of possible variations.

In Figure 2, the photovoltaic cell comprises n- and p-type semiconductors 14,

15 separated from the Peltier device 20 by electrically insulating but thermally conducting layers 16, 17 between which is sandwiched a conduit 18 along which flows a fluid material. A further conduit 22 containing fluid material 23 is separated from the underside of the Peltier device 20 by a further electrically insulating but thermally conducting layer 21.

Initially, consider the conduit 18 and its fluid 19 not being included. The layers

16 and 17 become one unitary piece. Solar radiation 12, focused or otherwise, impinges upon the n- and p-type semiconductors layers 14, 15 of the solar cell producing current at 8 and 9. Heat is then transferred via the layers 16/17 to the Peltier device 20 and then eventually to the conduit 22 and its fluid 23. The layers 16/17 and 21 are preferably made from a ceramic or polymer material having properties of high thermal conductivity but electrically insulating. By this process an electrical output is produced at 10 and 11 while the temperature at layer 17 is greater than the temperature at layer 21. Heat at 21 will then be transferred via the conduit 22 to the fluid 23. The rate of flow and initial temperature of the fluid 23, which can be water, Freon or another suitable liquid or gas, determine the final temperature of the layer 21 and the fluid 23. Once again, if output 9 is connected to 11 , and if the fluid is water, it can be seen that warm or hot water is produced while at the same time cooling the photovoltaic cell and assuring an electrical output. If the layer 21 approaches the temperature of the layer 17 then no useful electrical output is obtained from the Peltier device 20 so this can be disconnected or reconnected as shown in Figure 3.

In Figure 2, a chamber 13 may be used to insulate from heat or trap heat around the solar cell 14, 15 and also possibly the conduit 18 and layers 16, 17. The chamber 13 could also be used to contain the flow of water, or another liquid or gas, employed to cool the solar cell 14, 15.

A further modification of Figure 2 would be to replace the conduit 22 with a heat sink as in Figure 1, use air for cooling the lower surface 21 and allow heat from the solar cell 14, 15 to be absorbed by the fluid 19 flowing through the sandwiched between the layers 16, 17. 9 would be connected to 11 for the overall output. Once the fluid 19 had absorbed heat, electrical output and hot water could result as described previously.

If a back reflector (as described below in relation to Figure 5 or Figure 6) is used to direct radiation onto the layer 21, then connection 10 would be connected to 9, because the current is flowing in a different direction, so that in the beginning (e.g. in the morning) the layer 21 becomes the hot face and the layer 17 the cool face. The conduit 18 and its fluid 19 absorb the heat flowing from the Peltier device 20 and the solar cell 14, 15. Once the layer 21 approaches the temperature of the layer 17, a switch 28 may be employed, as shown in Figure 3, controlled by a device such as a microprocessor 27, to change the connections between 9, 10 and 11, i.e. to connect 9 to 11 in the event of the layer 17 becoming hotter than the layer 21.

A similar arrangement to the last description was tested except that fluid coolant

19 was omitted and a lens was disposed to concentrate the sun's rays 12 onto the layer 17. Output was 0.58V at 98mA from the device, the Peltier device and solar cell being combined as described. The normal output of the solar cell was 0.4V at 100mA in direct sunlight.

By taking Figure 2 in its entirety, another embodiment will be explained. Here fluids 19 and 23 flow and regulate the temperature in the layers 16, 17 and 21. One of the conduits 18 or 22 could be the cold inlet from a water mains supply and the other the heated water derived from the function of the cell (as explained previously) with connections 9, 10 and 1 1 being controlled as in Figure 3. Also for this embodiment some control of the flow of the fluids 19 and 23 is required. For example, when the conduit 18 and fluid 19 have reached the appropriate temperature, cold water can be directed to flow through the conduit 18 and the solar cell used to drive the Peltier device 20 such that the fluid 23 is cooled below its ambient temperature. In this way the device could possibly be used to cool the house during the summer. This device can be placed in the positions shown by Figures 5 (52) and 6 (61).

Figure 4 shows a further embodiment in block diagram form which illustrates that a number of Peltier devices 31, 47 and solar cell devices 30, 32 can be used within the same unit cell. Consider, for example, a modification of Figure 4 where fluid flows 33 and 35 are not included so that surfaces 40 and 41 and surfaces 36 and 37 touch while conducting heat. This device (used for example in the positions shown in Figure5 (52) and Figure 6 (61)) allows radiation to impinge on the solar cells 30 and 32 transferring the heat via Peltier devices 31 and 47 to a flowing fluid 34, which can be air, liquid or gas within a pipe or other suitable container or conduit.

42, 44, 45 and 46 are the outputs from the solar cells and Peltier devices connected via 43, so that currents flow in addition and can be controlled by a switch similar to that indicated at 27, 28 in Figure 3. Initially, say in the morning, the temperature of the upper and lower faces of 48, 49 the solar cells respectively are hotter than the surfaces 38, 39 and the fluid 34. Heat is then usefully transferred to the fluid 34 during the course of the day.

Consider another example of Figure 4 where the Peltier device 47 is omitted, so that the flow of fluid 34 is between the surface 38 of the Peltier device 31 and the surface 41 of the solar cell 32. The fluid flow 33, between the solar cell 30 and Peltier device 31, would be cold water whereas the fluid flow 34 would be hot water. Connections via 43 would be such as not to oppose current flows from the devices, being possibly controlled by a microprocessor 27 and switch 28 as in Figure 3.

Connections via 43 may be in parallel, series or as suits the circumstances of operation. Figure 5 shows at 52 a single unit cell as described in any of Figures 1-4. It gives an example of how the cell may be utilised (as previously described by some examples). Solar radiation can be concentrated by a lens or Fesnel lens 51 onto the surface of the cell 52.

Surrounding radiation 54 is reflected onto the lower surface 53 of the cell by reflectors 55, hence allowing radiation to impinge on more than one surface of the cell. Figure 6 is another variation on Figure 5 showing an exaggerated view of a cellόl, according to the invention, on a roof 58. Solar radiation 56 is reflected or concentrated by a reflector or other device 57 onto the cell 61 and also by a parabolic reflector 62 positioned away from the roof. The cell 61 is supported by a mounting bracket 60 and is supplied through a pipe 59 with water or other fluid from the house. Whether mounted on a roof or elsewhere, the cell may advantageously be disposed in a partially or wholly gas-tight chamber, for reasons of heat retention, keeping cool, general protection or otherwise enhancing its performance, as already mentioned in relation to Figure 2.

There are many different ways this device could be configured depending on the environment in which it is used.

Further embodiments of the invention will now be described with reference to Figures 7, 8 and 9, each of which shows a cell comprising a combination of photovoltaic and thermoelectric materials.

Light, focused or otherwise, is concentrated on to the upper surfaces 70, 77 and 85 penetrating into photovoltaic materials 71, 78, 80, 82, 86 and 89 which are thin (of the order of 100' s of μm) and produce the photoelectric current therein. Accordingly the majority of the heat differential is dropped across the thermoelectric materials 73, 75, 83, 87 and 90 which have a longer length (of the order of mm). A voltage and hence current is thus produced by the thermoelectric elements when the lower surfaces 88, 91, 84, 74, 76 are being cooled or having their temperatures controlled.

Intermediate layers 72, 79, 81, 82, 93 and 94 may or may not be incorporated in the constructions and are materials which are thin (<100's μm) and have different doping concentrations or are opposite doped designation semiconductor (p-type or n- type) when compared to their closest two neighbouring materials. Lower conductors, 74, 76, 84, 88, 91 may also be semiconductors.

In all these constructions, depending on the materials and configurations used, the thermoelectric material can be used either to aid in cooling the photovoltaic material and hence its junction or as an additional voltage source in series to boost the output of the unit cell. Materials that could be used in these constructions are described and listed above. An external current can be applied to the thermoelectric device to facilitate cooling, intermittently or to assist start-up of the cell, and control means (which may operate in response to information received from thermometers, clocks and other sources) may be provided to apply such external current when appropriate.

In the arrangement of Figure 7, the photovoltaic material 70 would be a thin, electrically conductive and possibly thermally insulating material such as a metal or high bandgap semiconductor. The layer 72 would be n-type material having a high bandgap, high electrical and thermal conductivity. The material 71 would have bandgap energies suitable for terrestrial radiation at the temperature of operation for the photovoltaic junction. In some cases it would be preferable if the material 71 were thermally insulating. Conducting surfaces 70, 77 and 85 will be opaque, transparent or partially transparent to radiation in different places. Figure 8 shows a p-type arm but the same could be applied to a n-type arm with the p and n positions swapped around. The intermediate layers 79, 81 and 92 are the same as layer 72 except that they are transparent, whereas layer 72 is not. The Eg of layer 78 is greater than that of layer 80 which is greater than that of layer 82 so that longer wavelength radiation is absorbed closer to the conductor 84. Layer 83 is again the same as layer 73. More or less of this segmentation (as with layers 78, 80 and 82) can be used in the other constructions if necessary.

Figure 9 is similar to Figure 7 except that the layer 89 is a N-type photovoltaic material. Layer 85 can be N or P-type or metal. A photovoltaic junction is formed between layers 89 and 85 which will, probably, have a lower junction voltage than the junction layers 85 and 86. Intermediate layers 94 and 93 are similar to layer 72. Layers 90 and 87 are similar to layers 75 and 73. Materials 85, 86 and 89 in some cases must be thermally insulating.

The construction in Figure 8 can be used in either Figure 7 or Figure 9 or both. It is envisaged during operation that some constructions, provided surfaces 85 and 70 and also 84 and 86 are thermally insulated in a chamber sealed or otherwise, will function with the lower surfaces at a higher temperature than the photovoltaic junctions between layers 85 and 86, 89 and 85, and 70 and 71. If these junction temperatures can be brought below -30°C materials with energy gaps such as germanium and GaSb could be used effectively. If this construction would further allow junction temperatures to be below -70°C then extremely efficient photovoltaic cells could be built using materials such as CdAs₂, InSb, InAs, CdSnAs₂, HgTe, HgSe, CdSb and Hg₅In₂Te₈ and the other materials mentioned previously.

Claims

1. A differential voltage cell including at least one unit comprising a photovoltaic device and a thermoelectric device separated from one another by means which are thermally conducting but electrically insulating, the outputs of the two devices being electrically connected to one another.

2. A differential voltage cell according to Claim 1, wherein the electrical outputs from the two devices are connected in parallel.

3. A differential voltage cell according to Claim 1, wherein the electrical outputs from the two devices are connected in series.

4. A differential voltage cell according to any of the preceding claims, including means to direct radiation on to one or more surfaces of the cell.

5. A differential voltage cell according to Claim 4, wherein the means to direct radiation on to the cell comprises a lens, Fresnel lens or reflector.

6. A differential voltage cell according to any of the preceding claims wherein the separating means between the photovoltaic device and the thermoelectric device includes one or more layers of thermally conducting but electrically insulating material.

7. A differential voltage cell according to any of the preceding claims wherein the separating means between the photovoltaic device and the thermoelectric device includes a heat exchanger.

8. A differential voltage cell according to Claim 7, wherein the heat exchanger includes at least one conduit for the transport of fluid through the separating means.

9. A differential voltage cell according to Claim 8, wherein the fluid is air or other gas.

10. A differential voltage cell according to Claim 8, wherein the fluid is water or other liquid.

11. A differential voltage cell according to any of the preceding claims wherein the thermoelectric device is thermally coupled to a heat exchanger at a location spaced from the photovoltaic device.

12. A differential voltage cell according to Claim 11, wherein the thermoelectric device is thermally coupled to the heat exchanger by a layer of thermally conducting material.

13. A differential voltage cell according to Claim 11 or Claim 12, wherein the heat exchanger includes at least one conduit for the transport of fluid through the heat exchanger.

14. A differential voltage cell according to any of the preceding claims, including switch means operable to vary selectively the manner in which the outputs of the photovoltaic device and thermoelectric device are electrically connected.

15. A differential voltage cell according to Claim 14, wherein said switch means are under the automatic control of a microprocessor in a manner to control the electrical output according to the conditions to which the cell is subjected.

16. A differential voltage cell according to any of the preceding claims wherein said unit comprises a plurality of photovoltaic devices.

17. A differential voltage cell according to Claim 16, wherein said unit comprises a single thermoelectric device located between two photovoltaic devices, thermally conducting and electrically insulating means separating each photovoltaic device from the thermoelectric device, and the outputs of all three devices being electrically connected to one another.

18. A differential voltage cell according to any preceding claim, wherein the photovoltaic device has generally planar configuration and is mounted flat on a face of the said thermally conducting means, the thermoelectric device being mounted with one side on the opposite face of said means, said side of the thermoelectric device being its 'hot' side.

19. A differential voltage cell according to any preceding claim, when mounted with the photovoltaic device exposed to light and with the thermoelectric device shaded by the thermally conducting means.

20. A differential voltage cell according to any preceding claim, wherein the photovoltaic device and the thermoelectric device are of such construction relative to each other as in use substantially to comply with the formula

VΓÇ₧ V_n

╬örΓÇ₧ ╬ö7 the terms being as defined herein.

21. A differential voltage cell according to any preceding claim, wherein the thermoelectric device is of such construction as substantially to comply with the formula

V V ╧â_N.A_N -^V^ = ╧â_p.A_p .f- the terms being as defined herein.

22. A differential voltage cell according to any preceding claim, when disposed in a partially or wholly gas-tight chamber.

23. A differential voltage cell according to any preceding claim, further comprising control means for applying occasional external current to the thermoelectric device.