Solar Energy: Collector Systems
Variation of Incident Sunshine with location in the US

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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