Solar Radiation
Data Base
Flat Plate Collector Systems:
Heat transfer to a circulating liquid (antifreeze) to be
used as supplmental space heating source in the Winter
Basic System Design
On average, a house loses 1 BTU per cubic foot per degree day
Requirements:
- 1500 sq. feet and 8 ft ceilings --> 12,000 BTU per degree day to
maintain interior heat at 65 degrees
- Consider a winter day with average 24 hour temperature = 15 F (
50 degree days) --> need 600,000 BTUs to keep house at 65 degrees
- 100,000 Btus comes from internal sources (lighting, cooking,
families, TV, etc) --> need 500,000 more BTUs
- Assume incident radiation is 600 Watts per sq. meter on a slanted
collector surface averaged over 8 hours.
- There are 3.41 BTUs per Watt so this is equvalient to 600 x 8hrs x
3.41 = 16,400 BTUs per sq. meter per day (8 hours)
- Assume collector efficiency is 50% (this is not a photovoltaic
device; we are merely heating something up and transferring that
heat to a working circulating fluid)
- The net yield is then 8200 BTU per day per sq. meter
- We need 500,000 BTUs --> how many sq. meters are required?
- 500,000/8200 = 60 sq meters --> 648 sq feet (25x25 feet of collectors)
Heat Storage:
- Water --> stores heat at the rate of 62.4 BTUs per cubic foot per
degree F
- Stones --> 20 BTUs per cubic foot per degree F (lower efficiency
due to voids in the storage unit)
- Heat water to 130 F and extract heat from it until it cools to
80 F. Total volume of water required is:
500,000/(62.4*50) = 160 cubic feet = 1250 gallons
- For Stones:
500,000/20*50 = 500 cubic feet = 25 tons of stones !
Energy losses in a flat plate collector system:
- Not all solar photons penetrate black collector surface
- Once the collector becomes hotter than the environment --> heat
loss by conduction, convection (aided by wind) and radiation
- Transmission/reflectance depends on incident angle
- efficiency starts to rapidly go down if the collector becomes
too hot as the thermal radiation (which goes as temperature to
the fourth power) dramatically increases
- Overall effect is that the collector system has an efficiency
which decreases with higher temperatures. But the effect is not
dramatic:
- Temp. difference between collector and surroundings = 20
F --> efficiency is 74%
- Temp. difference between collector and surroundings = 100
F --> efficiency is 48%
Single glass cover --> admits most sunlight but also most re-radiated
infrared radiation. Best at low temp differences
Three layers of glass -> admits fewer photons but also inhibits
radiative losses --> best at high temperature differences
Can fiddle with number of glass covers on the front of the collector:
Orientation of collector with
respect to sun is crucial part of overall efficiency
Focussing Collectors:
Not practical for the homeowner --> somewhat dangerous due to
high
temperatures.
Parabolic reflectors (heliostats) are pretty expensive
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