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Water Heating
Solar Electricity
Humans use about 15 terawatts of power per
year. 89 petawatts of sunlight reaches the earth's surface each year
– 6000 times more than humans use.
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If 100% of this energy could be harnessed, an area of
85,000kmē would be sufficient. This is about the area of 1/2 the
size of The South Island of New Zealand- for the whole world’s
energy requirements. Many countries have large areas of unused land
i.e. desert, especially America. For a country of their size and power usage,
a relatively small area would be required.
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Solar electricity has
huge potential, but right now only 10-15% of the suns energy is put
into electrical energy, solar water heating is a lot more efficient.
By the end of 2007 the world will have
produced 9000MW of electricity by solar.
Solar is the
easiest energy source for you as an individual to harness. It isn't
intruding, they are flat and fit well on your roof. Solar is silent
and maintenance free.
The two ways you can apply this energy
to is Water heating and Electricity.
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Water heating is by far the most
popular for households and is very successful.
It can save a household
between $350-450 NZD a year. Hot water heating is around 30% of a household
power bill. It uses the suns energy to heat water. It is so
effective that electricity usually isn’t required during summer. Solar water heating has been used for many years. Many
swimming pools are solar heated. Systems vary a lot but the
principle is similar.
Newer solar heating uses a Passive systems
to circulate water or a heat transfer fluid by natural
circulation. These are also called thermosiphon systems. As the
liquid heats it rises, when it cools it drops, which is called
a convection current.
Most new household systems use a fluid which
is a good conductor of heat. The liquid flows through the panel
which faces the sun- directly north, at an angle where the panel is
perpendicular to the arc of the sun. The
liquid is passed through a heat exchanger which has water flowing
through it. The heat is transferred from the hot liquid to the water
by conduction in a heat exchanger. The hot water is stored in a well insulated tank for
later use. The liquid- now cooled flows back through the panel to be
reheated.
They are called Evacuated Tube
systems and are the best systems currently.
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Water can only be heated to 100’C,
where as the heat transfer liquid can go much higher, increasing
the efficiency. Solar Hot water systems don’t replace electrical
heating systems. Some are used in conjunction with electricity/gas.sufficient, so they are a
completely
separate unit.
If the solar system is hooked to the current hot water cylinder
then the water will only be heated as much as needed to get it
to the required temperature. New Zealand is an ideal
location for Solar Hot water, better than Asia and
Europe. We have nearly 2000 sunlight hours every year
and low pollution in the summer, which severely restricts Asia and
Europe.
(www.niwascience.co.nz/edu/resources/climate/sunshine/)
Because there are few moving parts,
solar hot water panels last a
very long time. You will
be able to cut you power bill by 30% and pay pack the installation
in 5-10 years. But you will also save greenhouse gas emissions – up
to 4.5 tonnes per year.
Solar is very efficient compared to electrical hot water
heating.
 Solar energy is also
widely used to generate electricity. The sun
generates about 1000 watts per square meter. The efficiency of
producing this into electricity is about 10-15%, but is improving
rapidly. In general this is the most expensive grid alternative
energy installation, especially in New
Zealand, where power is relatively
cheap. In America an installation can be paid
off within 5 years, New Zealand is
more like 50 years but this will change when electricity costs rise.
Solar systems are also used in off grid applications. An on grid
installation is very easy. The Solar panels put electricity through
an inverter (raise voltage to the mains voltage) back through your
power meter, which credits your account. No electricity storage is
required.
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Solar electricity is generated by
solar cells, or photovoltaic cells, which use the photovoltaic effect
of semiconductors to generate electricity from sunlight.
Photovoltaic is derived from Greek. Photo, meaning light, voltaic
meaning electrical. Solar panels contain Solar cells connected
and encased in a module. Solar cells are
connected in modules to create required voltage (most panels have
36). Photons in Sunlight hit the panel and are absorbed by
semiconducting materials, usually silicon. Negatively charged
electrons are separated loose from their atoms, allowing them to
flow through the material to produce electricity. Photovoltaic
panels last a very long time, as they have no moving parts. Most
manufactures have a 20-25 year output warranty - kyoceria are
introducing a lifetime warranty. |
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There are different means of
generating electricity from solar. Photovoltaic’s being the most
common. One method is to have a
parabolic mirror which directs sunlight to a point, which heats,
powering a Stirling engine, until recently
they had the record for producing solar energy in to electricity –
30%.
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Another quire interesting method is called a
solar updraft tower (also known as a solar chimney) It is quite
simple. It has a huge area – up to 8km wide which has a glass roof,
the air inside heats and it rises, going to the centre where it
rushed up a huge “chimney” – 1800 feet tall and through
turbines generating electricity. Such a system is proposed for
Australia, which has
large amounts of sunlight and huge desert.
A solar power
tower has many mirrors around a tower. The mirrors direct the light
to a point which heats a substance, such as molten sodium (which has
a very high heat capacity and will store heat overnight, water will
not) which can be used to heat water to drive a turbine. The one
pictured below has a massive capacity of 10MW (when the sun is
shining - annual generation will be restricted to day light hours
and weather conditions.
Click picture to see
bigger image
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Problems with solar is that it is only
available on average 12 hours a days, and not all of that can
produce peak output. Overcast/cloudy conditions severely lower
energy production. Solar electricity is currently very expensive,
usually about $7-10/watt; wind is about $1-3/watt for a small
installation. But I believe this will be a huge electricity supplier
for the future, when technology enables the Suns energy to be more
efficiently and cost effectively used.
 
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