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UK National Section of the International Solar Energy Society


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Basic Facts about Renewables

Given are 5 extracts from the introductory sections of the Solar Energy Society/Solar Trust Factsheets. The full version (inc a number of diagrams and a resources sheet, minimum 4 pages per factsheet) can be obtained for a nominal charge of 5 p&p per set, from the Solar Energy Society Secretariat.


In Britain each square metre of a south-facing roof receives around 1000 kWh of solar radiation during a year. This means that the roofs of many of our homes receive more energy from the sun in a year than we need to provide both the space heating and hot water; by using solar collectors it is possible to capture some of this solar radiation and reduce our consumption of fossil fuels like gas, coal and oil. The sun is used to provide hot water for a house in many countries. In Britain it is possible to use the sun to provide most of an average families hot water requirements from about May to September and to obtain some 'pre-heating' of the cold water supply during the other months. In principle it is possible to scale up the size of a solar water heater to provide central heating, in general it is not cost effective. However, solar water heater can be used in a preheating arrangement if the produced hot water is not used elsewhere. Hot water is normally produced by heating the cold mains water to the required temperature with a gas or oil fired boiler or an immersion heater. By slightly modifying the conventional heating system, solar collectors may be introduced.


Solar cells convert energy from the sun directly into electricity by what is known as the photovoltaic effect. This effect was first observed by Edmund Becquerel in 1839, when he noticed that light directed on to the electrodes of a battery cell increased the voltage. This led in the 1930s to the development of photocells, which were used in photographic exposure meters and other light sensors. It was not until 1954 that devices with a performance good enough for electrical power generation came on the scene. In that year, a team at Bell Laboratories reported that they had made a solar cell with a conversion efficiency of 6%. Solar cells soon found an application in space programmes, where there was a need for a light, reliable, long-lasting power source for satellites. The first solar-powered satellite, Vanguard I, was launched in 1958. Since then, solar cells have been used as almost the sole source of power for satellites and have a proven record of reliability. Conversion efficiencies have increased to over 24% and power levels from a few milliwatts to tens of kilowatts. The sudden quadrupling of oil prices by OPEC in 1973 stimulated interest in the possibilities of solar electricity on the ground. Today, photovoltaics (PV) is a growing industry serving a wide range of terrestrial applications.


Solar radiation and Energy FLOWS ON EARTH - Solar radiation is the heat and light and other radiation given off by the Sun. Nuclear reactions in the interior of the Sun maintain a central temperature of 16 million C, and a surface temperature of 5700C. Like all hot objects, the Sun's surface radiates energy at a rate and a colour (wavelength range) which depends on its temperature. The Sun emits radiation at a rate of 3.8 x 1011 Watt, of which only two parts in a thousand million arrive at the Earth, with the rest disappearing into space or warming the other planets in our solar system. At the outer edge of the Earth's atmosphere, the solar radiation on each square metre amounts to 1350 Watt, mainly of visible radiation and heat plus a little ultraviolet. Nearly one third of this is reflected back into space by clouds, water, ice etc. The rest of the solar radiation is absorbed and provides the energy for almost all the natural processes on Earth, as shown in Figure 1. The energy is shown in units of GigaWatt (GW) - about equal to the output of a large electricity generating station.


PHOTOSYNTHESIS - Photosynthesis by green plants converts large amounts of sunlight into energy-rich biological material called biomass, for example: trees, crop and forest residues, cow dung, grains and products derived from these such as wood, sugar, alcohol, etc. Most of man's energy is at present provided by burning coal, oil and gas. These are products of photosynthesis from millions of years ago. Firewood is also widely used: this is the product of more recent and present day photosynthesis. The size of the world annual biomass production is often not realised.

BIOMASS FACTS - The world's total energy use is 1/10 of the biomass stored annually. The world's stored biomass energy (90% of this in trees) is as large as the proven fossil fuel reserves. The total fossil fuel store equals about 100 years of photosynthesis. The amount of carbon stored in biomass is equal to the atmospheric carbon in the form of carbon dioxide (CO2) and the CO2 in the oceans surface layer; this is important for the cycling of extra CO2 produced from burning fossil fuels.

BIOMASS TODAY - About 13% of the world's primary energy comes from biomass (equivalent to 25 million barrels of oil per day). This is ten times Britain's North Sea oil production. Most of this biomass is used in the rural areas of the Third World where over HALF of the earth's population lives.


The sun can meet the entire annual space heating needs of buildings in sunnier parts of the world. However, in much cloudier and cold climates, the sun can still make a very useful contribution. It may seem surprising, but solar energy can actually save more units of energy needed for space heating the further a building is away from the equator. This is partly because ordinary windows can capture more solar energy from low-angle sun-rays, and partly because temperatures tend to drop relatively more rapidly than the solar supply. Low temperatures generate a demand for heat - even in July if you live in some northern parts of Europe. Thus a house in Shetland, if appropriately designed, can save more fuel than the same house located in Cornwall; or one in Norway more than one in the south of France.