Solar Power

1
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

3
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)

4
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.

6
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.

7
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

9
Solar Spectrum
10
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?).

11
How much makes it through the atmosphere
12
Why a seasonal variation?

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

13
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.

14
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

15
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

16
Active Solar heating

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

17
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
  coatings)
• Insulation on the bottom and sides to reduce heat
  losses.
• Flow tubes air or fluid to be heated flows
  though these tubes

18
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
  wavelengths.
• 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
  conduction

19
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.

20
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

21
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

22
Types of Flat plate collectors

• Air flat-plate collectors used for solar space
  heating.
• The absorber plates in air collectors can be
  metal sheets, layers of screen, or non-metallic
  materials.
• 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

23
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
  loss.
• 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.

24
Limitations

• 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
  roof?
• Are there structural, aesthetic considerations?
  (Al Gores troubles with installing solar panels)