Transcript Active Solar heating

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 coatings)
• Insulation on the bottom and sides to reduce heat
losses.
• 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 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
How does this work?
• The energy emitted from a hot surface is
described by Stefan’s Law:
P/A = εσT4
Where ε is the emissivity (describes the degree
to which a source emits radiation, ranges from
0 (no emission) to 1 (a perfect emitter) and σ
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:
λmax(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 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
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 (170°F to
350°F), making them more appropriate for
cooling applications and commercial and
industrial application. However, evacuatedtube collectors are more expensive than flatplate collectors, with unit area costs about
twice that of flat-plate collectors.
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
Gore’s troubles with installing solar panels)
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
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.
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
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
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).
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 that’s 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.
Chicago City Hall green roof
Storage
• Direct Gain method
– South facing windows with thermal mass in the
floor and opposite wall to regulate and store heat.
Potential
• Based on sun angle, this figure shows the
potential for passive solar use across the US