Active Solar heating

1
Active Solar heating

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

2
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

3
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

4
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.

5
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

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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

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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

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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.

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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)

10
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

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Passive Solar

• Makes use of natural solar heating
• Requires buildings be designed to maximize the
  suns heating
• Most important element face south (toward the
  sun)!
• Requires 3 design elements insulation,
  collection, storage
• Passive because it does not involve pumps, fans,
  fuel, electricity etc.

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Insulation

• Keep the heat in!
• Walls, floors, ceilings must make use of
  materials that help hold in the heat.
• Doors and windows must also be designed to
  maximize heat retention in the building
• Most modern buildings ignore these ideas

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Collection

• Need a way to collect the suns energy
• One way is large windows on the south face of the
  building
• Another way is a passive solar collector on the
  south wall
• In the collector, the heated air rises and flows
  into the structure, while the cool air from
  inside sinks and flows back into the collector.
  No need for fans, this air flow sets itself up
  naturally

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Storage

• Need a thermal mass inside the house
• Thermal mass-any material that can absorb solar
  energy then cool down later giving its energy
  back to its surroundings.
• Example Why is it always warmer in cities than
  in the country in the summer, especially at
  night?
• Buildings and roads act as a thermal mass,
  heating up during the day and releasing that heat
  at night
• In our building the material has to hold enough
  heat to keep the temperature constant at night or
  over a cloudy day(s).

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Storage

• The heat stored in the thermal massive is not
  much greater than the usual temperature of the
  structure, thus a lot of it is needed.
• Water is an excellent thermal mass. Tanks of
  water could be stored just inside the windows,
  but thats not very aesthetic. Another way to use
  water is a roof pond (yes a pond on your roof!)
  or green roof (yes your garden on your roof).
• Example-Since Chicago installed a 20,000 square
  foot "green roof" atop City Hall five years
  ago(2006 report), the city has saved about
  25,000 in energy costs.
• Trombe Wall A massive concrete
• wall on the south side of the structure,
• with a space between it and the windows.
• The concrete wall acts as the thermal mass.
• Not only does the wall heat the air in the
• space and convection sets up a natural flow
• to warm the room on the other side, but the
• concrete itself will radiate into the room.

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Chicago City Hall green roof
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Storage

• Direct Gain method
• South facing windows with thermal mass in the
  floor and opposite wall to regulate and store
  heat.

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Potential

• Based on sun angle, this figure shows the
  potential for passive solar use across the US