Unit conversion oversight

Dabbler · March 2008

Hi, trying to compare output from passive solar water heaters for a pool to the wattage I get from pv panels. Conversions like this used to be simple for me, but now I could use some oversight if anyone has interest or time. Critisism welcome, stop me where I'm wrong.....

For the passive heaters... I measure pool water flow = 8 gal/min.

Temp gain from pool to return from panels = 5 degrees F

1 btu = 1 degree F per 1 lb water

8 gpm x 8 lbs/gal = 64 lbs water/min

64 lbs water x 5 degrees F = 320 btu/min

320 btu/min x 60 min/hr = 19,200 btu/h

Btu (thermo)/hr x 0.2928751 = watts (from http://www.simetric.co.uk/sibtu.htm, first google I found)

19,200 btu/h x 0.2928751 = 5,623 watts

Could I be collecting that much?

Collectors: 4'x10' X 6 = 240 sq. ft.

5,623 watts/ 240 sq. ft. = 23.43 watts/sq ft

Compared to my Kyocera 125w panels,

pv panel = 25 1/2" x 56" = 2.125' x 4.67' = 9.92 sq ft. (my measurements)

125w/9.92 sq ft = 12.6 watts/sq ft

Does this sound about right, or do I have my math wrong?

Many thanks

crewzer · March 2008

Re: Unit conversion oversight

Dabbler,

Efficiency calculations are missing from your calculations. Full solar irradiation at sea level under clear skies is ~1,000 W / M^2. Typical "normal" irradiation is ~800 W/m^2. PV module efficiency is ~14% or so. My recollection is that solar water heater efficiency is 2X to 4 X that, or ~30% to ~60%.

Try factoring in those numbers and see what you get. Also, check out this site for more info: http://www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=13230

HTH,
Jim / crewzer

Dabbler · April 2008

Re: Unit conversion oversight

Crewzer,

I took your suggestion and here's what I get:

Based on 800-1000 W / M^2 irradiation,
solar water heating panels: 42-53% efficiency,
PV panels: 14-17%.

I think these numbers match your predictions.

Thanks for having a look and the info and suggestion.

Unit conversion oversight

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