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Help: Need Simplest possible solar water heater

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Posted by Frank Scrooby on February 19, 2008, 7:01 am
Hi all

In case you don't know (and I don't expect you to) South Africa is currently
undergoing somewhat of an energy crisis. Our government graciously continued
the previous government's habit of giving one company the monopoly in
producing and distributing electricity, while packing it's own people onto
the company board and management. Let's not discuss that.

The problem is South Africans are being pinched for electricity. We have
been experiencing blackouts routinely (although the last week or two hasn't
been that bad - but then again the electricity supply to the country's major
industries like mining have all been cut back to 90% of their needs).

Now South Africans have a very bad habit of using electricity to heat water.
There is no wide spread solar water heating. And if you look at the prices
you'll know why.

I did some research. The cheapest commercial unit is a camping module for +-
R 2000.00. R = Rands = South Africa's funny money which currently buys you a
dollar for R 7.50. That means a camping unit costs you $ 266.67. Given that
this unit can't be connected up to your existing water heater, it would seem
to be a pointless purchase unless you want to shower in the yard.

The cheapest 'proper' domestic installations start at about R 12,000.00 or
about 6 x as much. They (admittedly) claim to be able to keep your 120 to
150 liter water heater hot and avoid the need for electricity for at least
270 days a year. The average South African doesn't have the R 2000.00 to
shell out, let alone R 12,000.00.

Ok, so what is the simplest (most simple?) solar water heater that a small
South African firm could manufacture to replace or supplement their existing

Flat panels involving welding or braizing, which is why I'm looking for
something even simplier.

My idea is to go back to the 'original' solar water heaters of the early
1900s and put a metal tank painted black on the roof, facing north.

If it warms (for example) 50 liters to 45 degrees Celcius (about 25 degrees
up on muncipal water temp) every day that's 50 liters your electric heater
doesn't have to heat. For a household of four, assuming everyone takes quick
showers and doesn't take long hot baths this should amount to 1/4 to 1/6 of
your daily water heating requirements.

If someone feels they need more hot water they can install two tanks or four
or ten, or add a flat panel to this system.

Buying metal tanks, putting in two plumbing connections and painting them
black will no cost R 2000.00 a unit. It would be affordable to most South
Africans and possibly free up enough capacity that we can stop living under
the tyranny of possible power cuts.

Is this worth exploring or are flat panels the lowest common denominator for
solar water heating?

Thanks and regards
Frank Scrooby

Posted by nicksanspam on February 19, 2008, 11:22 am

I'd vote for a plywood tank on the ground with a polycarbonate lid with
a shallow north slope (if the north-south dimension is 2 meters, it might
have a 2tan(90-34-23.5) = 1.3 m reflective south wall) over a shallow pond
on a draindown foamboard cover during the day, over an EPDM liner, with
100 meters of 25 mm black plastic PE pressurized water pipe inside.

A small pump with 2 thermostats would move water up through the pond when
the cover temp is more than 60 C and the tank temp is less than 60 C.
(A differential thermostat would help...)

Energy Plus says Cape Town (S Lat 34 deg) gets 2373 Wh/m^2 of sun on
the ground on a 12.8 C average June day with a 26.0 max and a 16.3 ground
temp, so 1 m^2 of single polycarb (Paltough?) glazing with a greenhouse
polyethylene film layer beneath to protect it from warm water vapor might
gain 0.8x2373 = 1898 Wh and lose 6h(60-19.4)1m^2/R0.35, for a net gain of
1202 Wh, so heating 200 liters of water per day from 16.3 to 40 C would
take 200(40-16.3)4.186/3.6 = 5512 Wh, ie 4.6 m^2 (49 ft^2) of glazing.
With lots of insulation, a 4 m^2 tank could supply hot water for 5 cloudy
days in a row if 5x5.512 = 4d(60-40)1.163, with depth d = 0.3 meters.

IIRC, we've discussed this before.


Posted by J. Clarke on February 19, 2008, 1:35 pm
 nicksanspam@ece.villanova.edu wrote:

This sounds to me like gross overkill for the situation he's

to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Posted by nicksanspam on February 19, 2008, 6:08 pm
Something like this?

6'x8' Dynaglas polycarbonate     $8
6'x8' polyethylene film            3
4'x4' EPDM pond liner              5
4'x4'x4" polyiso foamboard cover  40
4'x4'x3'-tall plywood tank        60
10'x10' folded EPDM tank liner    30
300'x1" PE plastic pipe           60
10 watt pump                      20
2 140 F thermostats               20
      Total                     $46


Posted by J. Clarke on February 19, 2008, 8:27 pm
 nicksanspam@ece.villanova.edu wrote:

I don't see any labor or overhead there.

to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

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