Energy from the Sun

Earth intercepts a large amount of energy radiated by the sun. It has been estimated that after reflection and absorption in the earth's atmosphere, the total radiation reaching the land area of South Africa amounts to roughly 1 kilowatt per square meter at noon on a sunny day. Direct solar energy can be used to heat water or buildings, or to produce electricity in solar cells. Energy from wind, water or plants is considered indirect solar energy. South Africa has one of the world's best climates for solar power, but not many people are making use of this form of energy. The difficulty is not so much with the energy source (the sun) but rather with the cost and availability of the solar technologies and appliances.

Photovoltaic (PV) applications

Solar photovoltaic (PV) cells convert sunlight, the world's most abundant and widespread renewable energy source, directly into electricity. The first major use of solar cells occurred in the American space programme and satellites in orbit around the earth are still powered by solar electricity. Solar cells are used to generate electricity for water pumping, vaccine refrigeration, security lighting, electric fencing, household power, meteorological monitoring, radio repeaters, railway line switches, street lighting, highway telephones, calculators and watches.

Technology description

Photovoltaic cells (also called solar cells) are made from semiconductor material, usually silicon. The silicon is chemically treated so that the upper and lower layers are oppositely charged. Solar cells operate according to what is called the photovoltaic effect, ('photon - light, 'Voltaic' - electricity). In the photovoltaic effect, sunlight or photons, strike the surface of semi-conductor material such as silicon, energy penetrates the cell and dislodges electrons from the material's atoms. Certain chemicals added to the material's composition help establish a path for the freed electrons. This creates an electrical current. Metallic contacts placed in a grid on the top and bottom of the cell enable current to flow through an external circuit. Most commonly the cells are electrically connected and mounted on a flat surface to make a flat plate PV module or panel. Concentrating modules use lenses, for example fresnel lenses that concentrate the sunlight onto the solar cells. Because light is concentrated on to the cells, it is necessary to track the sun to keep sunlight focussed on the cells. Flat plate panels are commercially available and more commonly used than concentrating modules.

The main components of a PV system include a module, a battery a battery/charge controller, and DC-AC inverter (where applicable), and the load (appliances). A typical module is rated between 10 and 80 watts of power and produces in excess of 12 volts of direct current (DC). Modules are often connected together in series or parallel in an array to provide higher power outputs and higher DC voltages. Normally the electricity generated through the panels is stored in a battery or a bank of batteries.

Battery protection regulators are usually included to prevent excessive discharge or overcharging which could damage the battery. Because PV panels produce direct current (DC) and most appliances and equipment are designed to be powered by alternating current (AC), an inverter is used to convert the direct current from the PV panels or battery into alternating current. A 50Wp module (typically one panel) would provide enough energy to power a few lights (12 volt DC or 220volt AC compact fluorescent lights), a radio, a black and white television and a cell phone battery charger for a few hours per day. It is important to note that ordinary incandescent bulbs should not be used with a PV system as it drains power from the batteries much faster than energy efficient compact fluorescent lights will.

Modules should be properly orientated to collect maximum energy. The amount of tilt of towards the sun depends on the latitude and what time of year most solar collection is required. It is important to remember when choosing a suitable site for the solar panel that it should be sited as far as possible from shades and shadows, as it reduces the panel's overall output. Batteries are used mainly as a back-up system that stores energy collected during sunny days for use at night or during cloudy days. PV systems can provide power to a household (stand-alone systems) or be linked to the national power grid and feed power directly into the electricity network (grid connected systems).

The wide scale use of PV technology has been boosted by a dramatic drop in production costs over the past 20 years. Recent estimates indicate the capital cost of PV ectricity is in the region of US$ 7000 - 8000 per installed kWp, which translates to about US$ 25c/kWh.

Can Solar PV work for me?

Yes, if you are living far away from the existing electricity network, for example, calculations show that If you are more than 5 kilometres away from the grid, PV becomes a cost-effective option. Also, if you want to be independent in terms of your electricity supply, if you only require power occasionally. Remember that PV systems will only supply a limited amount of electricity - one 50 W panel produces approximately 250Wh/day. The more panels you have in your system, the more electricity can be generated, but due to the cost of a single panel, it can become expensive to have many panels. It is generally recommended that PV is not suitable for thermal applications, i.e. you should not power an electric stove, a geyser, a welding machine or such like with a PV system (mainly due to the number of panels that would be required to provide the energy). If you are considering PV to provide power for lighting and entertainment, it would be advisable to look at alternative options such as other renewable energy technologies or LPG to supply energy for thermal applications.

Benefits of PV

1. Solar power is a renewable form of energy, i.e. it won't run out like oil, coal and gas. It is therefore sustainable to use;
2. Once you have paid forfhe equipment the energy is free;
3. Solar PV can provide electricity for lighting and entertainment purposes where grid electricity unobtainable;
4. It provides a safer lighting option than candles and paraffin;
5. No harmful emissions are caused either through generating electricity or inside the home;
6. The photovoltaic industry can provide up to 400% more jobs than the conventional energy industry

If you are considering a PV system, a professional supplier and installer will discuss your requirement, analyse your energy needs and advise you on the optimal system.