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Photovoltaics for dwellings - an update

In the search for reduced carbon emissions, one of the most useful tools in the low carbon building programme is the photovoltaic (PV) electricity generation system. Consulting technical author, Dave Elsy, explores some of the issues, in a supplement to the new NBS Engineering Services work section.

Photovoltaics have a number of advantages over the other sustainable energy sources. They are ‘fit and forget', have no moving parts, generate no noise and require no maintenance. Each square metre of installed PV saves the emission to the atmosphere of more than two tonnes of carbon dioxide over its lifetime (at least 25 years), which would otherwise have been produced by a fossil fuel burning power station.

The basic PV module is made up from about 30 to 40 cells, depending on size and shape, which are interconnected and encapsulated in a robust panel with a translucent face. Light shining onto the surface of the PV module generates electricity which is then collected via a connection (usually on the back of the module) and converted into a voltage suitable for use. Even in cloudy northern latitudes PV panels can generate sufficient power to meet all or most of the electricity demand of a dwelling.

If insufficient electricity is generated the building takes the balance from the electricity network through the normal mains electricity connection. Conversely, if more electricity is generated than the building is using, the surplus is exported (sold) to the electricity supplier.

The electricity output varies with the amount of light striking the surface of the PV modules and, in the UK, is highest where the modules are south facing and inclined at 30-40° to the horizontal.

Photovoltaics can be applied to every type of dwelling from small cottages to large mansions. Because each array is made up from several interconnected modules, arrays can be sized to suit the area available (usually the roof) and the power requirements of the premises. There are some installations in the UK where the PV array has been erected at ground level in the garden.

Installation

PV modules were historically constructed as framed free-standing panels which were separately fixed by specialist installers over the roof covering. However, whilst this type is still predominant, modules are increasingly being manufactured in the form of tiles (and slates) to match many of the major UK roofing manufacturers' products. When used in this form they present less visual impact and integrate closely with the remainder of the roof covering. The tiles can usually be incorporated in a traditional roof construction (either retrofit or new-build) without any planning implications. There is also a cost offset when these products are fitted, as the PV tiles replace a portion of the main roof covering.

Installation of the PV modules can be undertaken by traditional roofing contractors as they are fixed to the tiling battens in the normal manner, and the electrical installation completed by the electrician within the loft space.

Output

PV modules are rated in peak kilowatts (kWp), which is the output of the module under standard conditions. As these standard conditions are rarely achieved at UK latitudes and the intensity of the sunlight varies widely over the seasons, a more useful comparative measurement is the total amount (kW·h or ‘units') of electricity generated over a year. In the UK, depending on the efficiency of the PV material incorporated in the modules, around 8 m² are required to generate each kWp, which will typically produce 750 units of electricity per annum.

The overall efficiency of a PV system is low (12-15%) but, as the primary fuel costs nothing, there are no running costs to consider (thermal electricity generating plant in the UK is about 35% efficient). Also, as the energy is generated local to the point of use, there are no transportation losses.

PV efficiencies have increased by over a third in the last 10 years and are expected to exceed 20% by 2010. By then, in southern Europe, it is predicted that the cost of PV-generated electricity will be competitive with electricity produced from fossil fuels.

Costs have reduced by over 50% in the last 6 years. As part of the government's low carbon building programme, grants are available to encourage the installation of PV systems. The grants are administered by the Energy Saving Trust and currently pay up to £3,000 per kWp with a maximum of £15,000 or 50% of the total installed cost. To qualify, however, the system must be installed by an accredited contractor.

Written November 2006

 

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