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re: solar power efficiency
19 jun 1997
stephen johnson   wrote:

>dave wrote:
>> would much energy be lost using solar power cells to generate
>> electricity to heat water as against using solar powered water heating
>> panels and do these panels need the water pumped through them?
>using solar electric (or even wind) to heat water is extremely inefficient.

to clarify this, electric water heating is almost 100% efficient, but pvs are 
15% or so. wind electricity might be close to 30%, but cost-effectiveness
seems more important. 

> 20 amps and 12v it would take 2 weeks to heat 50 gallons to a
>decent temperature. 

heating 50 gallons of water from 60->120 f takes 50x8(120-60) = 24,000 btu or
24k/3.41 = 7038 watt-hours, ie 20ax12v = 240 watts for 7038/240 = 29 hours. 

>...a watt of solar power is only about 3.4 btus.

so many people still confuse power and energy...
a watt-hour is 3.41 btu of energy.
>water is best heated by direct solar gain on a tank holding the water...

direct gain systems lose lots of heat at night and on cloudy days. 

how about putting 4 2x8' amorphous pv panels in a sunspace, with an extra
layer of polycarbonate glazing, between reflecting 2:1 solar troughs, and 
allowing some water to thermosyphon up through a gravity-feed conventional
water heater or a few 55 gallon drums in an insulated box above?

the water at 120 f in an 80 f sunspace would lose about 6h(120-80)64ft^2/r1
= 15k btu on a 6 hour winter day, and gain 1,000btu/ft^2/dayx0.9x0.9x128ft^2
= 104k of heat, for a heating efficiency of 85%, while making 104k/3.41x.05
= 1.5 kwh of electrical energy using 5% pvs at $2/peak watt. the "lost heat" 
could warm sunspace air to circulate through an attached house in the winter. 


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