Cooling Photo-Voltaic Cells
S
This invention relates to cooling photovoltaic (PV) cells, and in particular so-called multi-junction PV cells.
PV cells have the ability to directly convert light energy into electrical energy, and are a common means of providing electricity where a conventional electrical supply is not available. PV cells are a popular retrofit in domestic dwellings, as a means of reducing imported elcctricity.
Conventional PV cells are generally arranged in multiples as an array, and mounted at an appropriate angle to maximize conversion of solar energy. In complex installations the array may be motorized, and track the sun.
Conventional PY cells comprise a single electrical junction, and are tuned to a single wavelength of light; the efficiency of energy conversion at other wavelengths is reduced. More recently multi-junction PV cells have been proposed, each junction being tuned to a different wavelength of light. Such muhi-j unction cells may double the efficiency of energy conversion as compared with single junction cells, and further increases are theoretically possible -thus a comparatively smaller multi-junction PV cell may be used to generate a standard amount of electrical energy.
However multi-junction PV cells are also comparatively expensive, and the high power to weight ratio cannot yet be economically used in conventional installations.
The economic factors determining the use of multi-junction PY cells can be affected by the use of optical systems for concentrating light energy. In particular optical and!or minor systems can be used to focus light energy on a relatively small area of multi-junction PY cell, and accordingly the energy conversion per unit PV area can be theoretically increased. Concentration factors of over 200 are possible, and optical concentration is relatively inexpensive as compared with the cost of PV cells.
However, a focused light source also results in increased heating of the PY cell and an increased range of operating temperatures. Unfortunately the operational efficiency of a PY cell tends to reduce with increasing temperature of the cell. Furthermore the life of a PV cell may be adversely affected by exposure to raised temperatures.
Means may be provided to reduce cell temperature, but these tend to further increase cost and complication, in particular by the use of pumps and/or fans. Such motorized devices require electrical energy, and are typically located in areas which are inaccessible for servicing and maintenance.
PY cells may have a design life of 30 years or more, and efficiency typically falls with age. A standard PV cell requires no maintenance, apart from cleaning. Accordingly maintenance of cooling devices after a period of service may not be justified, because the remaining generating capacity of the cell may not justify the economic cost of repair of motorized cooling systems.
What is required is a cost effective solution to the problem of increasing operational temperatures of multi-junction PV cells, which can avoid on-going maintenance and servicing for the life of such a cell.
According to the invention there is provided a photovoltaic cell assembly comprising a multi-junction photovoltaic cell, a heat absorber behind the cell, a thermosyphon cooling circuit for removing heat from said heat absorber, and a heat sink.
Such an assembly can be self-contained and self regulating in use. Furthermore such an assembly has no moving parts and may be designed for the life of the PV cell -typically 30 years -without any requirement for maintenance. All that is required is that the thermosyphon circuit include a gradient sufficient to induce thermosyphon flow in the desired conditions of use. Typically the assembly will be an integrated self-supporting unit, having only electrical connections to be coupled to an electrical consumer in use. The assembly may typically mount the heat sink at least partly above the PV cell in use.
In an embodiment, the PY cell assembly of the invention is used in combination with a light conccntrator, such as a Fresncl lcns. An incxpcnsive sourcc of Fresnel lenses is provided by redundailt rear projection televisions, which are rapidly being replaced by televisions with flat screen technology.
In an embodiment the heat sink may be a bulk tank of thermosyphon fluid, an air/water heat exchanger or a combination of both.
The heat absorber is typically a strong absorber of heat, such as copper, and has an integral or intimately connected part of the thermosyphon circuit, such as an array of copper tubes.
The appended claims define additional featurcs of the invcntion.
Other features of the invention will be apparent from the following description of an embodiment described by way of example oniy th the accompanying drawings, in which: Fig. 1 illustrates a concentrator for light falling on a PV cell.
Fig. 2 illustrates in end elevation a PV cell according to the invention.
Fig. 3 illustrates the cell of Fig. 2 in side elevation.
Fig. 4 illustrates a cell assembly according to thc invention.
Fig. 5 illustrates graphically PV cell output with increasing temperature.
The dimensions of elements depicted in the drawings are schematic, and will be selected according to conditions of use and conventional design considerations. In many cases dimensions are exaggerated in order to emphasize a feature of the invent ion.
With reference to Fig. 1, a multi-junction PY cell 11 receives incident light 12, typically sunlight, via a concentrator 13 of known type using, for example, optical or mirror elements. The PY cell has an electrical output, which is shown connected to a storage battery 14 by way of example.
A PV cell unit 20 for use in the invention is illustrated in Figs. 2 and 3.
The PV cell 21 has a backing of copper sheet 22, and an intermediate layer of insulating thermal paste 23. The copper sheet includes on the underside an array of individual copper tubes 24 connected at respective ends to an intake manifold 25 and an outtake manifold 26. Typically the backing sheet may be 0.1mm thick, and the tubes may have an internal diameter of about 2mm.
The arrangement of parallel copper tubes 24 is an example of a configuration that is straight forward to construct. Serpentine arrays are possible, and the tubes may also be constituted by one or more passages formed integrally with the sheet 22.
The important feature of the tubes and sheet is that together they make an efficient collector of heat for a thermosyphon, as will explained below. The thickness of copper sheet, and of the tubes is selected according to the required heat capacity, and the internal diameter of the tubes is selected according to the rate of fluid flow and the amount of heat to be extracted in use. These aspects are within the skill of a suitable trained technician, and can be determined by routine empirical testing.
The PY cell unit provides an integrated unit in which the structure of sheet and tubes 22, 24 provides rigidity and strength to the PY cell 21.
The insulating thermal paste 23 electrically insulates the electrical connections at the underside of the PY cell. whilst permitting effective transmission of heat to the backing sheet 22. Any suitable electrical insulator may be used, provided the rate of heat transmission is good, and the thickness of the layer of paste or other material should be no thicker than is required for the function of electrical insulation. The skilled man will select a suitable material and thickness, again according to design requirements and empirical testing. Heat transmission may be in the range 90-100%, whereas electrical insulation will be sufficient to eliminate a short-circuit at the operating voltage of the PV cell.
The PV ccli 21 and backing 22 may be secured together in any suitable manner, by for example adhesive and! or mechanical fasteners.
Although not illustrated, the tubes may be backed with a protective cap, which may itself be thermally insulated and may provide an undersurface for contact with a support, such as a roof The PV cell unit may be as large or small as necessary to suit the installation space and the amount of electricity to be produced. A plurality of smaller cells may be joined together in a larger sub-assembly so as to make a modular system adaptable to several sizes. The cell unit may further include attachment features to permit direct mounting to a support structure for holding the PV cell at an angle appropriate to the latitude thereof Fig. 4 shows a PY cell assembly including a heat sink 27 and manifold connections 28,29 defining a thcrmosyphon fluid circuit. The boundary 30 represents that the PV cell assembly is self-contained; that is to say that the thermosyphon fluid is sealed within for the life of the assembly, and that maintenance is not required. The assembly may be a self-contained, self-supporting integrated unit.
The heat sink 27 can be an air!liquid radiator, or a bulk tank of thermosyphon liquid or a combination of the two. The heat sink may be a liquid!liquid heat exchanger. The thermosyphon fluid is preferably water, and may contain an anti-freeze additive to prevent formation of ice at night.
Thermosyphons are well-known. In use the hot liquid rises, and the thermosyphon uses this phenomenon to cause liquid to circulate in a closed system between a heat collector and a heat emitter.
Thermosyphons generally rely upon a physical and a temperature gradient, so the practical embodiment places the heat sink abovc the PY ccli to an extent sufficient to promote the required rate of flow. The vertical separation (or high point of the fluid circuit) can be determined empirically by conventional testing; so that insufficient flow may bc countered by increasing the net vertical separation of the heat sink and PV ccli. In a variant of Fig. 4, the heat sink is placed at the side, but the fluid circuit includes a high point above the PY cell and immediately adjacent an inlet to the heat sink.
In the illustrated anangement the PV ecH unit will heat-up in use, due to the effect of the concentrator 13, and by conduction water in the tubes 24 wili increase in temperature. Water in the heat sink 27 will remain at a lower temperature, and as a resuit a flow of water from the PV cell unit to the heat sink 20 will commence. On reaching the heat sink 20, warm water wiil be cooled, and flow back to the intake manifold 25, whereupon it will again be warmed. Such a circuit is seif-starting.
It will be understood that the system of Fig. 4 can also be self-regulating, with certain design limits, so that flow will be zero when heat input is very low, and increase progressively to the system maximum, when heat input is very high. Circulation will fall in a corresponding manner as the heat input reduces.
The skilled man will determine flow capacity and fluid volume according to the expected maximum temperature to be reached, for example by adjusting the bore of the pipes. A manually adjustable throttle valve may be included for initial setting of maximum flow rate in the thermosyphon circuit. For exampic such a valve may be opened to improve flow if the assembiy becomes too hot in the installation location.
The ability of the heat sink to reject andior absorb heat will be specified to ensure that in hot conditions sufficient heat energy can be rejected from the system to ensure efficient operation. Necessarily the assembly wiii be adapted to the iatitude of installation, it being hotter as the equator is approached.
The heat sink may be a radiator, typically water/air for a roof mounted PY cell assembly, or a bulk tank of liquid which is allowed to increase temperature during the day, and cool down at night.
Tf required the manifold connections 28,29 may be insulated, and furthermore the thermosyphon fluid may be used to supply heat energy for other purposes, such as domestic hot water via a water/water heat exchanger.
Fig. 5 illustrates schematically the deterioration of PY cell output (P) with increasing temperature (t). The thcrmosyphon of the invention is capable of maintaining operation in a lower temperature range ti in circumstances where an uncooled PY cell may reach a temperature t2 in use. It is important to note that elevated operating temperature not only reduces cell output, but also results in premature cell failure due to heat degradation.