GB2321338A

GB2321338A - A differential voltage cell

Info

Publication number: GB2321338A
Application number: GB9800968A
Authority: GB
Inventors: Peter King
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-01-18
Filing date: 1998-01-19
Publication date: 1998-07-22
Anticipated expiration: 2018-01-19
Also published as: GB2321338B; GB9800968D0

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. The outputs from the two devices are electrically connected. 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. This may enable the output and/or the efficiency of the cell to be increased as compared with the photovoltaic cell alone of similar area. Heat exchangers may be incorporated in the layer 3 and/or the layer 5 to control operation of the cell and/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. As is well known, 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.

Solo, 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 often 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 sources other than the solar cells. This 'extra' energy is normally supplied by the connected national electrical grid.

In countries such as England, where cloud cover and the period of time cells remain obscured or partially obscured from direct sunlight is significant, it is necessary to make as much energy as possible from the sunlight that is available.

Certain embodiments ofthe 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 Freon, in a heat exchanger construction, for directly useful purposes, such as hot water for showers.

A further improvement in efficiency of these embodi Lents can be achieved by the control of 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 cars or the cells of cars 'powered by hydrogen'. Electricity, produced and regulated by airflow or otherwise, could be used to supply direct power or to power the 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 pn elements in the classical configuration. Both the Peltier device 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, painted black, which has 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, ZnS, CdS, CdSe, ZnO, GaP, CdTe, ZnTe, Ge, Ga AS. Examples of other materials that can be used to make up the said material are Sn, Sb, Bi, Pb, Cd, Zn, S, O Te, Ga. Plastics and polymers could also be used such as Polyethylene, Nylon (6,6), polyacetylene, polypyrrole, polythiophene, polyaniline, sexithiophene, 8-hydroxyquinoline aluminium, Alq, poly-pphenylenerinylene (P.P.V.) Mixtures such as Silicon Carbide, Ni 0, Mn O and Co 0 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 the said differential voltage cell.

As described previously, 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 following is a description of embodiments of the invention, by way of example, reference being made 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. As is well known, the Peltier device 4 comprises n- and p-type materials arranged in sequence with junctions between them. An electrically insulating and thermally conductive body 3 is located between the cell 2 and the Peltier device 4. A further similar electrically insulating and thermally conducting body is located between the underside of the Peltier device 4 and a 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.

Providing the n- and ptype material in the Peltier device 4 is in the correct sequence, the output 9 from the photovoltaic cell can be connected to 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 to the body 3, the Peltier device 4, the body 5 and the 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 body 3 to the body 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 bodies 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 0.4v, 70mA (Voc.Isc) at 1KW/m2 illumination and two commercially available Peltier devices were used. Focusing was applied by a lens of area 3837mm2 and focal length 133mm which boosted the output of the cell.

The arrangement successfully powered a small motor when the sunlight was approximately 850W/m2. It would normally take eight solar cells of total area 7392mm2 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 and the maximum temperature of the solar cell recorded 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 50C 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 - O). If additional cooling had been used (tap water temperature on the day was 1 2 C) then a greater cooling figure and, in some circumstances, a larger output could be achieved.

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 device 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: ON. AN. VN = Op.Ap. ffi N P where suffix N notates N-arm characteristics.

suffix P notates P-arm characteristics.

A = Area.

L = Length.

a = Conductivity.

V = Voltage due to temperature differential across P and N arms.

Nd = doping concentration of donors (electrons) in N-arm.

Na = doping concentration of acceptors (holes) in P-arm.

ni = intrinsic carrier concentration of material.

AT = voltage differential across P and N arms.

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 VN and Vp are equal Ap must be increased or LN increased to maintain this relationship.

Also in conjunction with this or otherwise, V V N P AT AT N 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 loO/o 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 photo-voltaic device or otherwise (such as the conductor connecting the N and P arms) are: C, Si, Ge, dSn, AgBr, ZnTe, ZnSe, ZnS, CdTe, HgSe, HgS, HgTe, BP, AIP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs, InSb, B-SiC, Ga2T, In2Te3, Pb, ZnO, ZnS, CdSe, ZnSiP2, CdSiP2, ZnSnAs2, CdGeAs2, CdSnAs, Cu2GeS3, Cu2GeSe3, Cu2SnS3, CuSnSe3, ZnIn2Se4, CdGA2S4, CdGa2Se4, CdIn2Te4, HgGa2Se4, HgIn2Se4, Hgin2Te4, aSiC, Sn Hg2In2Ter, PbS, PbSe, PbTe, GdSe, NiD, Mg2 Si Mg2Ge, Mg2Sn, Sb2Te3, Bi2Se3, Bi2Te3, B, Se, Te, SnO2, cc-In2Te2, B-In2Te3, InSb, CdO, CdS, ZnSb, CdSb, Bi2S2, Bi2Se2, Bi2Te2, Mg2 Sb2, Zn2As2, Cd2As2, GaSe, GaTe, InSe, TISe, Ga2Te3, CuInSe2, CuS, Zn3P2, CuAl 02, In203, SnO2.

Other materials which are relevant are polymers, molecular polymeric semiconductors and organic materials as discussed in patent specifications such as GB2297647A, GB2288181 A and GB2296815A, and materials such as polyacetylene, polypyrrole, polythiophene, polyaniline, sexithiophene, 8-hydroxyquinoline aluminium, Alq, poly-p-phenyenerinylene (P.P.V.) All these materials could also be used for the layer or layers between the Photovoltaic cell and thermoelectric elements as described in Figures 1-3 and 5.

With reference to Figure 1, a fUrther embodiment 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 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 19. A further conduit 22 and 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 18 and 19 not being included such that the surface of 16 and 17 touch and become one piece. Solar radiation 12, focused or otherwise, impinges upon the layers 14, 15, the n- and p-type semiconductors 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 fluid 23. The layers 16, 17, 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 such that the temperature at layer 17 is greater than the temperature at layer 21. This heat 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, determines 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 giving a combined electrical output and cooling of the photovoltaic cell.

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.

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 conduit 18, sandwiched between the layers 16, 17, and the fluid 19 flowing through it. 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 is 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 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 some device, possibly 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 water was not used as a cooling agent and a lens was disposed to concentrate the sun's rays 12 onto the layer 17. Output was 0.58 V at 98 mA from the device, the Peltier device and solar cell being combined as described. The normal output of the solar cell was 0.4 V at 100 mA in direct sunlight.

By taking Figure 2 in its entirety, another embodiment will be explained. Here the essential feature is the fluids 19 and 23 flowing and regulating the temperature in the layers 16, 17 and 21. One ofthe 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 11 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 is another example of what can be placed in the position shown by Figures 5 and 6.

In Figure 2, 13 indicates diagrammatically a chamber which is preferably 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.

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 position shown in Figure 5 and Figure 6) 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 transferred to the fluid 34 during the course of the day achieving some of the objectives set out by the invention.

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, once again, being possibly controlled by switches such as 27, 28.

Connections via 43 may be in parallel, series or what suits the circumstances of operation.

Figure 5 shows at 52 a single unit cell as described in any of Figures 14. 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 Fresnel 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 the cell 61, 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 59 indicates a pipe which may contain water or another fluid leading from the house.

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 llm) and produce the photoelectric current therein. Accordingly the majority ofthe 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 clam) and have different doping concentrations or are opposite doped designation semiconductor (p-type or ntype) 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.

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 ptype 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 Hg5In2Tes 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 of the preceding claims wherein the cell comprises two or more of said units in combination, the outputs of the units being electrically connected to one another.

19. A differential voltage cell substantially as hereinbefore described with reference to any of the accompanying drawings.