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) 1000Wm
2 (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/m
2/day in Cornwall and
700KWh/m
2/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
- Total primary
energy supply
-
PVGIS UK Solar Radiation Map
-
PV Potential Estimation Utility (PVGIS)
-
Commercial Software - VisualSunChart
- Solar Cell
Technologies
