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factsheet
solar
electricity |
what
is it?
It is the generation of electricity from the power of the sun, via
photovoltaic (pv) cells. It is different from solar water heating,
where water passes through panels to be heated directly, and no
electricity is generated.
Photovoltaic cells are made from silicon; when particles of sunlight
(photons) fall on the cells, they dislodge the outer electrons of
their atoms, and push them along to the next atom; a chain of moving
electrons is produced, and if a wire is attached to the panels,
these electrons can be pushed down it to supply a useable electric
current. This current is measured in amps, and to give some idea
of the scale involved, one amp of current involves the movement
of 6 million million million electrons per second.
The electricity produced this way (and also from batteries) flows
in one direction only, and so is called direct current, whereas
electricity from the UK national grid is alternating current, as
the flow of electrons changes direction 50 times per second. Direct
current can be stored in batteries to power 12 volt appliances.
However, these are more expensive and less readily available than
ordinary domestic 240 volt appliances, so batteries and an inverter
can be used to convert the 12 volt direct current to 240 volt alternating
current, or the panels can be connected to the grid, with a meter
to see how much electricity is put into the grid and how much taken
from it. A grid-support system is one which charges batteries, and
re-directs any surplus into the grid if the batteries are full.
what are the benefits?
As a
renewable source of energy, the main environmental benefits of pv
is that it doesn't cause the problems that other types of electricity
generation do.
Burning fossil fuels in conventional power stations releases nitric
oxides, nitrogen dioxide and sulphur dioxide, causing acid rain
which damages forests, wildlife and human health; it also releases
carbon monoxide, nitrous oxides, lead, particulates and hydrocarbons,
which cause damage to plants, ecosystems, and human health, especially
respiratory problems. Also, burning fossil fuels releases 5 billion
tonnes of CO2 into the atmosphere each year.
CO2 is the most important of the ‘greenhouse
gases’ responsible for global warming.
With pv there are no emissions, no environmentally-damaging extraction
and transport of coal and oil to feed power stations, and no radioactive
waste, or the potential leaks and disasters associated with nuclear
power stations.
Pvs take around 5 years to generate the same amount of electricity
as is used in their manufacture - see this
research paper (pdf) from Pennsylvania State University.
Large-scale use of lead-acid batteries would cause environmental
problems in their manufacture and disposal, so connection to the
grid would be better unless in a remote location.
what
can I do?
You may think that pv is fine for the tropics, but can they work
in temperate countries? Well the answer is yes they can, and there
is enough south-facing roof space in the UK to provide all the country’s
electricity needs using pv.
A single panel can be used with a battery to power lights or animal
fencing in a remote location, or you can attempt to supply all or
most of your electricity with a large system.
The first thing to do is find out how much electricity (in kilowatt-hours,
kWh) you use in a year (check your bills), and think about ways
to reduce your usage: switch lights off when you leave the room;
don’t leave appliances on standby; use a laptop (c. 18 watts)
instead of a desktop (c. 180 watts); use Savaplugs and low-energy
light bulbs and appliances; don’t overfill kettles; think
about solar hot water and a wood-burning stove, and whether you
need so much electrical gear at all. A typical UK family will use
about 3000-4000kWh per year, but if you are single, have no children
and / or reduce your electricity use, we can work with a figure
of 2000kWh / year. Taking the annual amount of sunshine into consideration,
this will require a 2.4kW system, covering 20m², and costing
around £16,000 installed. There is now a £2500 government
grant in the UK for professionally-installed, grid-connected systems
(see below), which will reduce this to £13,500. With electricity
at around 11p per kWh this will save £220 per year, giving
a payback time of 60 years and saving about 2 tonnes of carbon.
The payback time is way too long at the moment, but will come down
as fossil fuel prices rise, and if the UK government start supporting
photovoltaics as much as the German govt. It could be cost-effective
for remote locations where the cost of grid connection is prohibitive.
The cost can’t be reduced by self-build (like solar hot water),
because the manufacturing process is too high-tech, but you could
self-install if you have the know-how. Of course you could use pv
in conjunction with a wind turbine to take advantage of all weather
conditions.
Other factors to consider are: is your roof south-facing? is it
big enough? (if not, panels could be located on a frame in the garden
or on a flat roof) is it shaded? (panels can be damaged if parts
of them are shaded) if you are using batteries they need to be deep-cycle
(able to be continuously drained and re-charged) with a charge regulator
to prevent overcharging.
resources
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more
- information, books, links, courses, online
shop etc. |
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printable version
of this factsheet (pdf)
solar roof
tiles: 1.56kW of solar tiles installed by Solar
Century on a roof in Nottingham
an 80-watt
‘polycrystalline’ panel: enough to power lights or animal
fencing
solar pv
is not just for the tropics; even with our clouds, an average of
over 1000kWh/m²
of solar radiation falls on the UK, which is about 60% of the solar
radiation found at the equator
a roof in
Germany with 16 80-watt photovoltaic panels, which will generate
around 1000kWhours of electricity per year – about one third
of a family’s needs. There are also 5 solar hot water panels
at the bottom of the roof, which do not produce electricity –
water passes through them and is heated directly
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