How much makes it through the atmosphere

1
How much makes it through the atmosphere
2
Why a seasonal variation?

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

3
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.

4
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

5
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

6
Active Solar heating

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

7
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

8
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

9
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.

10
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

11
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

12
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

13
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.

14
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)

15
Cost effectiveness

• Assume a 5000 system
• Pays itself off in 27 years if replacing a
  natural gas or oil hot water heating system
• 14 years if replacing or supplementing electric
  hot water heating
• Between 1980 and 1985 there were tax credits for
  installing these systems. You could install one
  up to 10,000 at no personal cost.
• Similar credits have been reinstated in 2005 and
  in the stimulus package