3.2.1 PHOTOVOLTAIC / PV/ SYSTEMS

The light of the sun consists both of direct light and indirect or diffuse light, (which is the light that has been scattered by dust and water particles in the atmosphere). Photovoltaic cells not only use the direct component of the light, but also produce electricity when the sky is overcast. It is a common misconception that PV only operates in direct sunshine and are therefore not suitable for use in temperate climates. This is not correct: photovoltaics make use of diffuse solar radiation as well as direct sunlight. To determine the PV electricity generation potential for a particular site, it is important to assess the average total solar energy received over the year.

When sunlight strikes a photovoltaic cell, direct current (D.C.) is generated. By putting an electric load across the cell, this current can be utilised. The amount of useful electricity generated by a PV module is proportional to the intensity of light energy, which falls onto the conversion area. So, the greater the available solar resource, the greater the electricity generation potential. A PV system will not generate electricity at night but a system is able to store collected energy in a battery for use during non-daylight hours.

PV systems can be used for homes, offices, public buildings or remote sites where grid connection is either unavailable or too expensive. PV systems can be mounted on roofs or on building facades or operate as a stand alone system. The innovative PV array technology and mounting systems means that PV can be retrofitted on existing roofs or easily incorporated as part of the building envelope at construction stage. Modern PV technology has advanced rapidly and PV is no longer restricted to square and flat panel arrays but can be curved and shaped to the building design. In the northern hemisphere PV systems require south facing roofs and facades to capture the sun's light.

PV systems are very environmentally friendly and are virtually maintenance free. PV electricity is generated from a renewable source and is a renewable energy technology that is proven, available now and has no barriers to installation. The generating component produces electricity silently and does not emit any harmful gases during operation. The PV modules are made out of silicon which is entirely benign, and is available in abundance.

PV energy can be incorporated into all types of buildings , from domestic homes and offices through to public buildings and factories. It can be mounted on buildings either fully integrated with the building envelope or as a separate element. The system is usually linked to the mains electricity supply (grid) using an electronic 'inverter'. As a renewable energy technology, PV provides an ideal solution for a low energy, low carbon, low maintenance, sustainable design strategy.

PV cells are referred to in terms of the amount of energy they generate in full sunlight, know as kilowatt peak or kWp. PV cells are easy to install and they need low maintenance to keep the system running.

You can use PV systems for a building with a roof or wall that faces within 90 degrees of south, as long as no other buildings or large trees overshadow it. If the roof surface is in shadow for parts of the day, the output of the system decreases.

The householders must consult with their local authority regarding planning permission.

Market status and government policy

Prices for PV systems vary depending on the size of the system to be installed, type of PV cell used and the nature of the actual building on which the PV is mounted. The size of the system is dictated by the amount of electricity required. The average domestic systems are usually between 1.5 and 3 kWp. The cost is different in different countries.

Solar tiles cost more than conventional panels and panels that are integrated into a roof are more expensive than those that sit on top.

If you intend to have major roof repairs carried out it may be worth exploring PV tiles as they can offset the cost of roof tiles.

Grid connected systems require very little maintenance, generally limited to ensuring that the panels are kept relatively clean and that shade from trees has not become a problem. The wiring and components of the system should however be checked regularly by a qualified technician.

Stand-alone systems, i.e. those not connected to the grid, need maintenance on other system components, such as batteries.

Savings are dependent on the level of on-site consumption and/or value of export tariff. Assumes a 2.5kWp system with 50% - 100% on-site consumption with excess exported to grid on a typical export tariff.

The state subsidizing of the PV-systems in different countries is different. In Bulgaria , for example the produced electricity from PV is bought compulsory at a price of 0.40 Euro, whereas the average price of electricity is 0.07 Euro, i.e. at a price about 6 times higher than the average.

In most of the countries the construction of the PV installation itself is subsidized.

The exact energy payback for PV systems is obviously dependent on the available solar resource and on the degree to which the system is operational. High levels of solar irradiation and a high utilization factor will offer more rapid energy paybacks than if there is less sun and less usage, but typically energy payback will be realized within three to four years.