Solar Powered Electrics

Today photovoltaic systems are recognized by governments, environmental organizations and commercial organizations as a technology with the potential to supply a significant part of the worlds energy needs in a sustainable and renewable manner. The UK government has recently committed £10 million towards encouraging the installation of photovoltaic systems on buildings in the UK. Organizations such as Shell and BP have set up large photovoltaic manufacturing plants and environmental organizations such as Greenpeace strongly support the use of solar energy.* Photovoltaic means electricity from light. Photovoltaic systems use daylight and not sunshine to power ordinary electrical equipment. The photovoltaic (PV) process converts free solar energy - the most abundant energy source on the planet - directly into electricity. Note that this is not the same as  solar thermal technology used for heating and hot water. A PV cell consists of two or more thin layers of semi-conducting material, most commonly silicon. When the silicon is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (DC). The electrical output from a single cell is small, so multiple cells are connected together and encapsulated (usually behind glass) to form a module (sometimes referred to as a "panel"). The PV module is the principle building block of a PV system and any number of modules can be connected together to give the desired electrical output. The main area of interest in the UK today is grid connect PV systems. These systems are connected to the local electricity network. This means that during the day, the electricity generated by the PV system can either be used immediately (which is normal for systems installed on offices and other commercial buildings), or can be sold to one of the electricity supply companies (which is more common for domestic systems where the occupier may be out during the day). In the evening, when the solar system is unable to provide the electricity required, power can be bought back from the network. In effect, the grid is acting as an energy storage system, which means the PV system does not need to include battery storage. PV systems can be incorporated into buildings in various ways. Sloping rooftops are an ideal site, where modules can simply be mounted using frames. Photovoltaic systems can also be incorporated into the actual building fabric, for example PV roof tiles are now available which can be fitted as would standard tiles. The components typically required in a grid-connected PV system are illustrated below. The PV array consists of a number of individual photovoltaic modules connected together to give the required power with a suitable current and voltage output. Typical modules have a rated power output of around 75 - 120 Watts peak (Wp) each. A typical domestic system of 1.5 - 2 kWp may therefore comprise some 12 - 24 modules covering an area of between 12 - 40 m2, depending on the technology used and the orientation of the array with respect to the sun. Most PV modules deliver direct current (DC) electricity at 12 volts (V), whereas most common household appliances in the UK run off alternating current (AC) at 230 V. An inverter is used to convert the low voltage DC to higher voltage AC. Other components in a typical grid-connected PV system are the array mounting structure and the various cables and switches needed to ensure that the PV generator can be isolated both from the building and from the mains. Finally, a meter will be required to ensure that the system owner can be credited for any PV power fed into the mains supply. Can a PV System be installed on my building? The most important questions to consider in deciding whether or not a PV system can be installed on a building and what type of system should be installed are:     - is there a suitable place on the building where the solar array could be mounted (taking into account orientation, shade, and available area)     - what type of photovoltaic system would be suitable     - is planning permission required Photovoltaic modules can be placed on almost any building surface which receives sunshine for most of the day. Roofs are the usual location for PV systems on houses but photovoltaic modules can also be placed on facades, conservatory or atrium roofs, sun shades, etc. The surface on which the PV array is mounted should receive as much light as possible. The more light the solar array receives the more electricity will be generated. The three issues which affect how much light a surface receives are: 1. Orientation: Due south is the best possible orientation. If the PV is to be mounted on a vertical façade the orientation should preferably be between South East and South West. If the PV is to be mounted at a tilt a wider range of orientations will still give a reasonable energy yield. North facing orientations should be avoided. 2. Tilt: A tilted array will receive more light than a vertical array. Any angle between vertical and 15o off horizontal can be used. A minimum tilt of 15o off horizontal is recommended to allow the rain to wash dust off the array. The optimal tilt angle is 30o - 40o for a south facing array in the UK. Shallower tilt angles are better for east or west facing arrays. 3. Shadowing: Shadows cast by tall trees and neighbouring buildings must also be considered. Even minor shading can result in significant loss of energy. The area required for mounting a PV array depends on the output power desired and the type of module used. An area of around 8 m2 will be required to mount an array with a rated power output of 1kW, if monocrystalline modules are used (the most efficient modules type). If multicrystalline modules are used an area of around 10 m2 will be required for a 1kWp system and if amorphous modules are used an area of about 20 m2 will be required. These areas can be scaled up or down depending on the output power desired. 1 - 3 kWp is a typical power output for a domestic system, although smaller or larger systems can be installed. There are various ways in which a PV array can be mounted on a building. The various options offer different appearances and vary in cost. The commonest way of mounting an array on a house is to place it on the roof either with modules mounted in a frame above the existing roof tiles or integrated into the roof. If the array is to be integrated into the roof PV roof tiles may be used instead of modules. PV roofs do not usually require planning permission unless the building is listed or in a conservation area. However you should call your council to check on local policy.