Energy output

The energy output from a monocrystalline cell varies with insolation level in an almost linear fashion across its operating range. Output is adversely affected by high operating temperature with a drop in efficiency from about 12 per cent at 20oC to about 10 per cent at 50oC.

Photovoltaic panels would need active cooling in many building situations to maintain maximum output during summer months. Clearly this is impractical and costly and at present the drop in efficiency has to be accepted. An alternative is to encourage ventilation of the panels by suitable design of their location and position in order to permit air flow and natural ventilation cooling to front and, if possible, rear of the array.

Since most uses of electricity require alternating current (AC), as stated earlier, an inverter must be employed. However, in the US PVs are now available from the Applied Power Corporation which deliver AC electricity which means they can be connected directly to the grid. An AC inverter is integrated with the cells (CADDET Renewable Energy Newsletter, March 2000).

It is often the case that the supply of electrical energy is not concurrent with demand, perhaps because of occupancy and use patterns. In such situations two alternatives exist: either the excess power can be stored in some form of battery or used to heat water to be stored in an insulated tank to provide space heating. Alternatively it can simply be offloaded to the electricity grid. The former of these options causes an energy loss in the conversion process and additionally requires the provision of a suitable and substantial battery store. The preferred option in most urban situations at the present time is the grid-connected system, though a sophisticated control system is required to ensure the output matches the grid phase. This also provides a backup supply when PV generation is insufficient. A major drawback at present is the price at which the utility companies purchase the excess PV production.

Pressure is mounting for the adoption of reversible meters that accumulate credit units from a renewable on-site installation but this is being resisted by some energy companies. The UK has some of the worst buy-back rates in Europe, currently about 5 p per unit as against the utility price of approximately 15 p. A combination of high capital cost and miserly buy-in rates is seriously undermining the adoption of this technology by householders in the UK in contrast to Germany where subsidised demand is outstripping manufacturing capacity.

It may be easier to justify the cost of PV cladding materials for commercial buildings where occupancy patterns coincide with peak production levels.

PV cladding materials can now be obtained in different patterns and colours depending upon the nature of the cells and the backing material to which they are applied. This offers an increasing range of facade options which might be exploited by architects to create particular aesthetic effects. Thin film photovoltaic systems, which basically have a layer of a coating layer applied to glass, look particularly promising.

In the Netherlands PV cells are being mounted on motorway sound barriers. The UK Highways Agency gave approval in 2004 for PVs to be mounted in panels alongside motorways. A pilot project array has been installed on the M27 in Hampshire to feed directly into the national grid.

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