Solar Power

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Solar Power

• Power derived directly from sunlight
• Seen elsewhere in nature (plants)
• We are tapping electromagnetic energy and want to
  use it for heating or convert it to a useful
  form, usually electricity
• Renewable-we wont run out of sunlight (in its
  current form) for another 4-4.5 billion years

2
Solar Energy

• Sun derives its energy from nuclear fusion deep
  in its core
• In the core Hydrogen atoms are combining (fusing)
  to produce helium and energy.
• Physicists refer to this as Hydrogen burning,
  though be careful, it is not burning in the usual
  (chemical) sense.
• The supply of H in the suns core is sufficient
  to sustain its current rate of H burning for
  another 4-4.5 billion years

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Solar Energy

• The energy is released in the H burning deep in
  the sun in the form of photons.
• Here we use the particle description of light,
  where light is considered a packet of energy
  called a photon.
• Photons have energy Eh? or E hc/? where ? is
  the frequency of the light, ? is the wavelength
  of the light, c is the speed of light
  (c3.00x108m/s) and h is Plancks constant
  (h6.626068 10-34 m2 kg / s)

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Solar energy

• The photons take a long time to reach the surface
  of the sun, about 1 million years.
• Why? Deep in the sun, the density is very high.
  The photons travel a very short distance before
  they are absorbed by an electron in an atom.
• Normally in an atom, the electrons occupy
  specific positions relative to the nucleus called
  energy levels.
• When the electrons are in the lowest energy
  levels possible, they are said to be in the
  ground state.
• When an electron in an atom absorbs a photon, it
  gains more energy and moves to a new (higher)
  energy level.
• It can only gain a photon with the correct energy
  to change energy levels. The photon energy must
  equal the energy difference between two energy
  levels in the atom.

5
Solar energy

• But electrons dont like to be in these higher
  energy states, so they will emit energy in the
  form of a photon to drop to a lower energy level.

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Solar energy

• So in the sun, the photons emitted by the H
  burning travel a short distance before they are
  absorbed by an atom.
• The atom quickly re-emits the photon, but not
  necessarily in the same direction it came from.
  The atom can re-emit the photon in any direction.
• The photon follows a random looking path on its
  way out of the sun, called a random walk.

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Random walk

• So the photons take this random walk form the
  core to the surface of the sun.
• On average, it takes 1 million years before a
  photon generated in the core leave the surface of
  the sun.
• It then takes another 9 minutes to reach the Earth

8
Solar spectrum

• The photons emitted from the sun have a range of
  energies, and therefore via Plancks law a range
  of frequencies and wavelengths.
• The distribution of the number of photons
  (intensity) as a function of wavelength( or
  frequency or energy) we call a spectrum.
• The maximum energy is at optical wavelengths

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Solar Spectrum
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Energy from the sun

• We can measure the amount of incoming energy from
  the sun by something called the solar constant
• 1,366 watts/m2 with fluctuations of almost 7
  during the year.
• This measures the energy at all electromagnetic
  wavelengths at the top of the atmosphere
• What reaches the ground (where a solar device
  would be ) is less
• By the time we take into account the effect of
  the Earths rotation, the different angles of
  sunlight at different latitudes, we find that the
  average intensity of sunlight is reduced by ¾.
• Then you have to consider how much is absorbed in
  the Earths atmosphere, which reduces it further,
  so only 47 of the average makes it to the
  surface of the earth, or about 160 watts/m2
• This is for a 24 hour day, averaging over an 8
  hour day gets you about 600 Watts/m2 or 1520
  BTU/ft2. This is often referred to to as the
  solar insolation (varies from 300 in the winter
  months to 1000 in the summer-why?).

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How much makes it through the atmosphere
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Why a seasonal variation?

• First, why do we have seasons?
• Earths axis is tilted 23.5 to the plane of its
  orbit

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Why such a large seasonal variation

• In the Northern hemisphere, the suns rays fall
  more directly on the earth than in the winter.
• Heating is most efficient when the suns rays
  strike the surface ay 90 (right)angles.
• So a solar energy device should be oriented so
  that the suns rays hit it at right angles.

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How is energy transferred

• Convection-Energy is carried by blobs of material
  that are moving in a medium for example -hot air
  rises, cold air sinks
• Conduction-energy transfer between two objects
  that are in contact
• Radiative transfer-energy transferred through
  the successive absorptions and emission of photons

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Types of solar heating and cooling

• Active
• Use a fluid forced through a collector
• Need an external energy source to drive a pump

• Passive
• Design the structure to make use of the incident
  solar radiation for heating and cooling
• No external energy source

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Active Solar heating

• Used for space and or water heating
• Flat plate collector system