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Solar Power


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 ... – PowerPoint PPT presentation

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Title: Solar Power

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

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

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)

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.

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.

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.

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

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

Solar Spectrum
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?).

How much makes it through the atmosphere
Why a seasonal variation?
  • First, why do we have seasons?
  • Earths axis is tilted 23.5 to the plane of its

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.

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

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

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

Elements of a flat plate collector
  • Cover (also called glazing) protects the system
    and keeps heat in.
  • Absorber plate-absorbs solar energy. Usually made
    of a metal that is a good conductor of heat such
    as aluminum or copper and painted with a coating
    that helps absorb and retain the heat (black
    paint is the lowest order of these types of
  • Insulation on the bottom and sides to reduce heat
  • Flow tubes air or fluid to be heated flows
    though these tubes

How does this work?
  • Cover is transparent to sunlight, so the light
    passes through the cover to the absorber.
  • The absorber will absorb energy from the sunlight
    and then try to re-emit it to come into thermal
    equilibrium with its surroundings. But the
    absorber re-emits the energy at infrared
  • Glass allows visible but not infrared radiation
    to pass through, so the energy emitted by the
    absorber is absorbed by the glass.
  • The glass re-emits this energy to the outside air
    and back into the collector.
  • The energy trapped in the collector heats the
    inside of the collector, and this energy is
    transferred to the air or fluid in the tubes via

How does this work?
  • The energy emitted from a hot surface is
    described by Stefans Law
  • P/A esT4
  • Where e is the emissivity (describes the
    degree to which a source emits radiation, ranges
    from 0 (no emission) to 1 (a perfect emitter) and
    s is the Stephan-Boltzman constant 5.67 x 10-8
    W/m2 K4. P/A is the power emitted per unit area,
    T is the temperature in Kelvin.

How does this work?
  • The wavelength at which this energy is emitted
    from the surface is described by the Wien
    Displacement Law
  • ?max(µm) 2898
  • T(K)

  • This gives the wavelength at which an object
    emits the maximum amount of energy

Types of flat plate collectors
  • Liquid flat-plate collectors heat liquid as it
    flows through tubes in or adjacent to the
    absorber plate.
  • Often unglazed

Types of Flat plate collectors
  • Air flat-plate collectors used for solar space
  • The absorber plates in air collectors can be
    metal sheets, layers of screen, or non-metallic
  • The air flows past the absorber by using natural
    convection or a fan.
  • air conducts heat much less readily than liquid
    does, less heat is transferred from an air
    collector's absorber than from a liquid
    collector's absorber, and air collectors are
    typically less efficient than liquid collectors

Types of Flat Plate Collectors
  • Evacuated Tube collectors -usually made of
    parallel rows of transparent glass tubes. Each
    tube contains a glass outer tube and metal
    absorber tube attached to a fin. The fin is
    covered with a coating that absorbs solar energy
    well, but which inhibits radiative heat loss.
  • Air is removed, or evacuated, from the space
    between the two glass tubes to form a vacuum,
    which eliminates conductive and convective heat
  • Evacuated-tube collectors can achieve extremely
    high temperatures (170F to 350F), making them
    more appropriate for cooling applications and
    commercial and industrial application. However,
    evacuated-tube collectors are more expensive than
    flat-plate collectors, with unit area costs about
    twice that of flat-plate collectors.

  • Need a storage system for cloudy days and nights.
  • Amount of solar energy that is usefully collected
    is 50.
  • To heat 100 gallons of water a day from a
    temperature of 50 to 120 you need a collector
    with a surface area of 112 square feet. That is
    one panel 9 ft x 14 ft. This would fill a good
    portion of our classroom
  • Where do you put it? In the back yard, on the
  • Are there structural, aesthetic considerations?
    (Al Gores troubles with installing solar panels)
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