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

Eugene OR:

Summary Solar Radiation and Climate Data

Station = 24221

City: EUGENE * State: OR * WBAN No: 24221 *
Lat(N): 44.12 Long(W): 123.22 * Elev(m): 109 * Pres(mb): 1004 *
Stn Type:Primary

Flat-Plate Collector Facing South at Fixed Tilt=Lat+15


Solar Radiation, kWh/m^2/day Percentage Uncertainty: 9
         Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov   Dec   Year
Average   2.0   2.8   3.7   4.3   4.6   4.8   5.4   5.5   5.2   3.8   2.1   1.6   3.8

Minimum   1.3   1.7   2.7   3.2   3.8   4.1   4.1   4.6   4.0   2.5   1.5   1.0   3.3

Maximum   3.6   3.7   5.4   5.3   5.3   5.5   6.1   6.2   6.5   5.1   2.8   2.7   4.1

Summary Solar Radiation and Climate Data

Station = 23232

City: SACRAMENTO * State: CA * WBAN No: 23232 *
Lat(N): 38.52 Long(W): 121.50 * Elev(m): 8 * Pres(mb): 1015 *
Stn Type:Secondary

Flat-Plate Collector Facing South at Fixed Tilt=Lat+15


Solar Radiation, kWh/m^2/day Percentage Uncertainty: 9
         Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov   Dec   Year

Average   3.1   4.3   5.2   5.9   6.0   6.0   6.3   6.5   6.6   5.8   3.9   2.9   5.2

Minimum   2.0   2.9   3.8   4.4   5.2   5.4   5.8   5.8   5.7   4.8   2.5   1.8   4.7

Maximum   4.3   6.2   6.8   6.6   6.5   6.3   6.4   6.9   7.1   6.4   5.4   4.7   5.6

Summary Solar Radiation and Climate Data
Station = 13873
City:    ATHENS                
 * State:   GA
 * WBAN No:  13873
 * 

Lat(N):   33.95
Long(W):  83.32
 * Elev(m):    244
 * Pres(mb):   989
 * 

Stn Type:Secondary
Flat-Plate Collector Facing South at Fixed Tilt=Lat+15


Solar Radiation, kWh/m^2/day Percentage Uncertainty: 9
         Jan   Feb   Mar   Apr   May   Jun   Jul   Aug   Sep   Oct   Nov   Dec   Year

Average   4.2   4.8   5.2   5.4   5.1   5.0   4.9   5.1   5.1   5.3   4.5   3.9   4.9

Minimum   3.3   3.6   4.1   4.5   4.5   4.3   4.1   4.5   4.2   4.0   2.7   3.0   4.5

Maximum   5.7   6.3   6.1   6.3   5.8   5.5   5.7   5.8   6.0   6.4   5.6   5.0   5.1

Flat Plate Collector Systems:

Heat transfer to a circulating liquid (antifreeze) to be used as supplemental space heating source in the Winter

Basic System Design

On average, a house loses 1 BTU per cubic foot per degree day

A degree day = 65 - average 24 hour temperature

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 equivalent 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

Focusing Collectors:

Not practical for the homeowner somewhat dangerous due to high temperatures.

Parabolic reflectors (heliostats) are pretty expensive but as we will see are becoming more cost effective