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PHOTOVOLTAICS

The pitched roof is the ideal position for systems designed to harness solar energy. In the past, these systems have had to be mounted on relatively clumsy frames positioned on top of the tiles. Lafarge Roofing has now developed a technology that can be integrated totally into a tiled pitched roof, maintaining a clean roof line. This system, known as PV700, is based on photovoltaics.

Photovoltaic technology generates electricity from light.

As it is the light energy and not the heat of the sun that matters, it is particularly appropriate for the UK climate. The PV700 solar system consists of a series of solar panels, each of which has 18 photovoltaic cells embedded in it. The cells incorporate layers of semi-conductors, one positively and one negatively charged; when light shines on them, the electric field across the junction between the two layers causes electricity to flow. The electricity is converted from DC to AC power by an inverter; from there it can be connected to the local electricity grid, stored in a battery or used with another source of power, such as a wind generator.

The PV700 solar panels are relatively simple to install.

Each panel replaces four adjacent roof tiles and weighs no more than the tiles it replaces. The panels are fixed to the roof structure using a system of trays and concealed metal brackets. Solar panels can be combined with Mini Stonewold, Grovebury, Double Roman and Landmark Double Pantile tiles.

Photovoltaic systems are generally measured in terms of their peak output. For example, a PV700 system with 20 solar panels will have a peak output of 700Wp (Watts peak output). A typical domestic installation might consist of two PV700 systems connected together, with a total of 40 panels. In the UK, the annual yield from 40 panels is expected to be about 1,100kWh (kilowatt-hours), equivalent to a third of the average electricity consumption of a four-person household (excluding central heating and hot water).

Larger systems may be able to achieve higher solar-energy yields. The average cost for 20 panels would be about £5,500 including the inverter but excluding installation.

The PV700 system has a 15-year weather-tightness guarantee. It is installed exclusively by SunDog Energy, which will also negotiate grants from the Energy Saving Trust. Several new housing schemes, in Liverpool and in Newcastle upon Tyne, are being erected with pitched roofs incorporating the PV700 system.

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