Solar Resources & Photovoltaics

Solar Resources & Photovoltaics

Key Facts

Estimated that 1000TW of recoverable solar resource available; in comparison human use is approximately 1.3TW. (1)
Where Sun directly overhead, 1000Wm² strikes the ground available for collection.
Power output of a solar cell directly correlated to irradiance.
Irradiance varies due to annual variations, diurnal (daily) variations, and the effects of geographical position due to latitude and local weather conditions.
Solar cell will collect much less than 100% of available radiation because of conversion inefficiencies in the cell, the tilt and orientation of the cell, and by any shading issues.
Photovoltaics convert light to electricity in a solid state device. By 2002, World production exceeded 525MW/annum.
Photovoltaics only convert a maximum of 20% of available irradiance, but even so achieve excellent energy payback.

All solar cells are tested under a series of standard conditions including: i) 1000Wm² (air mass 1), ii) air mass 1.5, which is equivalent to the Sun at 45°, iii) temperature at 25°C.

Daily solar radiation varies greatly in the UK. Typical maximum summer values are 1100KWh/m²/day in Cornwall and 700KWh/m²/day in north Scotland.(2) In UK conditions (and somewhat unusually), as much radiation is available as diffuse radiation as beam / direct radiation. Diffuse light still contains usable energy.

Ideally, solar panels would follow the path of the Sun to maximum efficiency of converting the available radiance. (3)However, this is not practicable, except for small remote system which can have a small number of seasonal settings.

Shading for a particular location can be determined by using a diagram showing the sun paths at various points of the year, and adding buildings and other obstructions to this diagram. Direct obstructions are less than an issue in winter as there is more diffuse light. (4)

A solar cell typically consists of phosphorous and boron wafer slices stuck together. As light hits the wafers, electrons move between the layers and then onwards down the connecting wires. The current (amps) is approximately proportional to irradiance. Volts is determined by the cell configuration, and temperature. The higher the temperature, the lower the voltage, which reduces the total power output.

Solar cell technology is still developing. There are a number of competing technologies, silicon-based technologies include single crystalline, poly crystalling and amorphous silicon. Other non-silicon technologies are cadmium telluride, copper indium diselenide and gallium arsenide. (5)None have achieved greater than 20% maximum efficiency, with an expected life of 20 years.

Batteries are required to smooth the output of photovoltaic cells, therefore the battery design is critical. Applications include remote supply for monitoring equipment and lighting, communications equipment and for water pumping where the energy is stored in the body of water rather than as electricity.

Further Information