Report 4 (of 5): All Done With Mirrors - Solar Power

Introduction

Concerns for the environment, coupled with the heavy reliance of conventional power plants on fossil fuels, have encouraged research and development into sources of renewable energy supplies. Electricity demand is growing all the time due to population growth and the increasingly rapid industrialisation of developing countries. Solar thermal power is an appropriate energy source for countries located in the "Sunbelt", that is, countries that are thirty degrees or more north or south of the equator, where there is high direct solar radiation all year round.

There are two main types of technology for converting energy from the sun into electricity. One is known as solar electricity - photovoltaic - where sunlight is directly converted into electricity via solar cells. This technology is most appropriate for small scale applications. Solar radiation is the largest renewable energy resource and has greatest potential in the Sunbelt.

Solar Thermal Power

The two major players for large scale application of solar thermal power technology are the Parabolic Trough and Power Towers . There is also the Parabolic Dish system which has a great deal of development potential because the applications are suitable for remote power supply and smaller energy needs. Solar thermal power uses different media to create heat. The heat is then used to convert water to steam which will power a conventional steam turbine to produce electricity. Fossil fuel is sometimes used as a back up to heat the water in the boiler, so that the plant can still produce energy on demand when the sun is not shining (figure 1).

The Parabolic Trough

The Parabolic Trough, or "solar farm", uses long parallel rows of identical "concentrator modules" which are glass mirrors in the shape of troughs (figure 2).

The trough shape design ensures that energy from the sun is maximised and concentrated to reflect on to the absorber tube. The trough collector is placed on an axis to allow it to track the sun from east to west. A heat transfer medium, usually oil, is in the absorber tube. The concentrated energy from the mirrors will heat the oil up to temperatures of four hundred degrees centigrade. The hot oil is then able to heat the water in the boiler and convert it to steam which will drive a steam turbine to produce electricity.

A typical solar thermal plant is able to provide 2,000 - 2,500 full load hours per year.

The Power Tower

The principle of harnessing and concentrating sunshine with mirrors to a receiver is applied using the Power Tower (figure 3). In this case, rather than being in rows, the mirrors, referred to as heliostats, are placed in a circular pattern, at the centre of which is the tower. At the top of the tower is the receiver, which contains a fluid, such as water, air, liquid metal or molten salt. The heated fluid from the receiver then goes to a power block which is used to power a steam turbine.

The power tower is still at an earlier stage of development than the trough system. Tests with different heat transfer media show the power tower system is able to produce higher temperatures.

The Benefits of Solar Thermal Technology

The key benefit of solar thermal technology is for the environment. The carbon dioxide emmisions produced from energy production alone account for 50% of the harmful gases contributing to the green house effect. The hybrid solar thermal plants in operation in California where fossil fuel is used as back up, help to reduce the emissions of carbon dioxide, nitrogen oxide and sulphur dioxide because the plant reduces fossil fuel usage. A typical 80 Megawatt solar trough power plant saves 4.7 million tonnes of carbon dioxide emissions from being released into the atmosphere. It also saves about 2 million tonnes of coal from being used during its 25 years of useful life.

Solar power is sometimes considered to be a land intensive technology because of the area of land needed for the sites. However, the amount of energy a solar thermal plant produces from a given area is more than would be produced by a large hydro-electric scheme on a similar sized site. Furthermore, desert land which is otherwise redundant is the most suitable for solar thermal plants and it is unlikely that using such a location will have a negative impact on settlements and habitats.

Diversifying energy resources means there is less reliance on fossil fuels whose prices fluctuate and are subject to increase due to their depleting supplies.

The Costs of Solar Thermal Power

The initial investment costs of installing a solar thermal power facility are high, with the "solar field" accounting for 50% of the total cost of the plant. Current solar thermal systems in commercial operations are designed to integrate solar power into conventional fossil powered plants which can significantly reduce fuel bills as the majority of the power supply is free from the sunshine.

Future Outlook for Solar Thermal Power

Investment to encourage growth in the solar industry is demotivated by the larger scale but lower cost fossil fuel plants. Despite this obstacle, conventional coal or oil fired steam plants supported by small solar fields are able to compete with large scale fossil fuel power plants. The strong environmental advantages of integrating solar thermal power to conventional coal fired plants are cost-efficiency and environmental friendliness.

Financial investment prospects are improving as the international commitment to the environment becomes firmer. Schemes set up by bodies such as the World Bank, The Global Environmental Facility set up by UNCED (United Nations Commission on Environmental Development), the European Union’s JOULE and THERMIE programme, all aim to provide financial incentives and earmark funding for solar thermal projects for developing countries in the Sunbelt.

Developments in parabolic trough technology have lead to new methods and designs of integrating the solar technology to gas fired plants which give lower investment costs, better conversion efficiency and lower electricity costs.

Whilst power tower technology is still in an earlier stage of development, its prospects for successful commercial growth look increasingly positive. A project in California using molten salt as a heat transfer medium would be able to replace fossil fuel back up as thermal energy storage is built in to the design to run the plant on demand. Power towers are considered to have good long term prospects because of the high conversion efficiencies and low electricity costs, particularly in large unit sizes (100-200 Megawatts).

For further information, please contact:

Intermediate Technology would like to thank Pilkington Solar International for the original material and pictures on Solar Thermal Technology.

TVE/ITDG gratefully acknowledge support for the HANDS ON programmes from the UK's Department for International Development (DFID), the European Commission (EC), the UN Foundation and UNDP/The Equator Initiative in collaboration with the Government of Canada, IDRC, IUCN, BrasilConnects and the Nature Conservancy.