Solar | The Ashden Awards for Sustainable Energy

Summary

Solar systems collect energy from sunlight; thermal systems convert it to heat, while photovoltaic (PV) systems convert it to electricity. The amount of energy produced varies according to the system’s location, the time of year and the weather, although some energy is produced even on cloudy days.

Solar thermal systems can be used to heat water, dry crops or cook food. Solar PV systems can be connected directly to an electricity grid when one is present, or can be used to charge batteries to store power until it is needed. Typical applications for off-grid solar PV include home lighting, mobile phone charging, computers, vaccine refrigerators, portable lanterns and irrigation.

The capital cost of solar PV is high compared to other renewable energy sources, although the running costs are minimal. The rapid uptake of solar PV in developed countries has been made possible by subsidies, but in developing countries, where the alternative is often kerosene or dry cell batteries, solar PV is very competitive.

The benefits of off-grid solar PV in developing countries include the avoidance of fire risk and pollution from kerosene lamps, the ability to charge mobile phones, and the provision of radio, television and computer services.

Technology background

Solar systems collect energy directly from sunlight. The energy can be converted either to heat (solar thermal) or electricity (photovoltaics, or PV). Solar thermal technology can be used to heat water, dry crops or to cook food. PV systems use solar cells that convert light into electricity. Cells are usually linked together and arranged into weatherproof modules, so they can be mounted in a place where they can be in sunlight for as much of the day as possible.

Solar PV systems can be made in a wide range of sizes, from a few small cells to run a calculator to a large solar farm with hundreds of large modules. The rating of a solar PV module is given in watts-peak (Wp), which is the power generated by the module in direct sunlight of an intensity of 1 kW per square meter. Although in most places and over much of the day, the solar intensity is well below this, the efficiency of PV cells is virtually independent of solar intensity, so they continue to supply some power even at low light levels when the sky is overcast.

Most of the PV in operation uses thin wafers or layers of crystalline silicon as the active material, and these technologies accounted for about 93% of the global market in 2006. The production of the silicon cells, and the silicon feedstock to make them, is growing rapidly, with new factories opening, particularly in China. There is growing interest in the use of very thin films of silicon and other materials for PV. Although these currently account for only 7% of the global market, their use is increasing rapidly, and they offer the potential for significant cost reduction as production scales up.

Off-grid systems

Solar energy, since it derives from the sun, is available only during the day and varies as the sun follows its daily and yearly cycles, as well as being affected by cloud cover. An off-grid solar PV system, therefore, usually includes batteries to store excess energy when it is available and deliver it when it is needed. An electronic charge controller prevents the batteries from being overcharged, or over-discharged, which increases their useable lifetime.

Grid-connected systems

Grid-connected PV systems do not need batteries, as they are mainly used to off-set the energy supplied by the mains electricity grid to a building, or to supply power into the grid. The electronic control for a grid-connected system includes an inverter to convert the dc power generated by the solar panels to the ac power, usually at higher voltages, required by the grid.

Use in developing countries

Solar home systems

The most commonly used PV system in developing countries is the solar home system. This consists of a solar module (at a typical rating of 35 Wp) connected to a charge controller and a battery. The system usually includes one or more lights and a socket to which other electrical equipment, such as a radio, TV or mobile phone charger can be connected. Efficient lights are always recommended for PV systems, to make best use of the limited supply of electricity. Small dc fluorescent light are available down to 3 or 4W power rating, in both tube and compact forms, and LED lights are becoming sufficiently cheap and reliable to be used as well. Although the amount of electricity supplied by a solar home system is small, typically below 0.1 kWh per day, the benefits can be significant:

— Electric light avoids the fumes and the fire-risk of kerosene lamps, and gives opportunities for study, income-generating work and recreation after dark.

— Radio and television provide information and entertainment.

— The use of mobile phones is expanding rapidly in developing countries, and gives the possibility of communications in very remote areas without mains supply. PV phone chargers are ideal to support this communications growth, and both individuals and phone-charging businesses are using them.

Large programmes have provided solar home systems in many countries, including India, Bangladesh, Sri Lanka, China and Indonesia. The most successful programmes usually run on a commercial or near-commercial basis, and programmes with substantial government funding have often been less successful.

Vaccine refrigerators

Another common use for PV is for running refrigerators, especially for vaccine storage in remote clinics. Vaccine refrigerators are designed to be very efficient, but still need at least 0.3kWh of electricity per day, so larger arrays of modules (typically 240 Wp) are used. Increased battery storage capacity is usually provided, to make sure that vaccines remain cold even if the sky is overcast for several days.

Lanterns

The solar lantern is another approach to lighting. It is a small self-contained light with a rechargeable battery that can be charged each day from a small module (typically 3 Wp), ready to be used at night. Lanterns are attractive to small traders and market stall holders in developing countries, who have good-quality light by which to display and sell their produce after dark. They are also used by midwives and traditional birth attendants, to deliver babies more safely. The lanterns can be charged by their owners during the day, or rented daily from a central charging site.

Irrigation

The use of PV power to run irrigation systems was encouraged several years ago by various governments and international agencies, but this was less successful than expected, partly because of high costs. As solar home systems are becoming more established and there is increasing local technical support for PV technology, the use of PV for irrigation is increasing as well.

Economics

The cost of electricity from PV is high compared to other renewable energy sources. The capital cost per Wp installed is about £4 for large, grid-connected systems, which – despite low running costs - gives a cost for the electricity of about 20-50p per kWh. There is currently enormous demand for PV to supply government-supported programmes in Europe, Japan and the USA, and, despite rapid growth, the supply industry cannot meet demand and as a result prices have not decreased. However, the economics of off-grid PV is very different, and in developing countries the cost of a solar home system can usually be paid back in three to five years from savings made on kerosene or dry cell batteries (which cost £30 per kWh!) for lighting. Even where a mains grid exists, it is often cost-effective to use off-grid PV for very low power applications like traffic and weather monitoring, as it avoids the cost of connecting to the grid.

The installed global capacity of solar PV was about 7,500 MWp in 2006, supplying about 6 TWh of electricity (about 0.2% of global electricity supply). About 64% is grid-connected applications in developed countries, in which people offset part of their electricity use with power from building-mounted PV modules. Despite the huge social benefit of off-grid PV in developing countries, these systems account for only about 6% of the world market.