Re: Basic Choices and Constraints on Long-Term Energy Supplies

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

On Friday, Jul 30, 2004, at 13:00 America/New_York, Brian Whatcott
wrote:

> . . . Cost $40 plus Professorial labor to drill two holes in masonry
>   and hang garden hose or etc.,. Payback? Possibly 11 months....
> ..if you have folks who shower each day in warm water.

That can be turned into a physics problem on calorimetry.

Let me assume that each showering uses only 20 liters (`~5 gallons) of
water whose temperature must be increased by 40 C. That means that
Q=1*20*1000*40=8*10^5 calories (or ~32*10^5 J = 3.2 MJ) per person per
day. For 5 folks this amounts to 16 MJ per day. Compare this with the
amount of energy received in 24 hours by a solar heater whose area is 3
m^2. My reference shows that each square meter receives (on the
average) 230 J/s (watts) in Florida and Texas, 190 J/s in New Jersey
and 150 in Maine)

Not all heat "goes into"  water; I will assume water heating rate is
150 W. At this rate the average amount of heat received by circulating
water is 150*3*24*3600= ~4*10^7 J or 40MJ. This is about three times
more than what is needed. Thus a solar heater whose area is ~1 m^2
should be sufficient.
Ludwik Kowalski