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Active Solar heating

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Active Solar heating Used for space and or water heating Flat plate collector system – PowerPoint PPT presentation

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Title: 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

6
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

7
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

8
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.

9
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

11
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.

12
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

13
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

14
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).

15
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.

16
Chicago City Hall green roof
17
Storage
  • Direct Gain method
  • South facing windows with thermal mass in the
    floor and opposite wall to regulate and store
    heat.

18
Potential
  • Based on sun angle, this figure shows the
    potential for passive solar use across the US
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