Hi Rudi
For the system to be donated to a less fortunate individual, I think that as much of the material as possible should have little resale / scrap value. For this reason, I would be looking at various plastic drums and pipes that could withstand the higher temperatures. I think that as long as the insulation is effective, there shouldn't be any pros / cons between the metal and plastic units.
For my home system, I am thinking of using a standard pressure geyser about 200 / 250 liters and a number of home made panels using copper tubing. The geyser will be in the garage on a robust stand ( 3 m from the house ) and the solar panels either side of a east / west facing wall which gets sun literally from sunrise to about 1 hour before sunset.
My logic is that as we are at home all day, the east facing panels will capture plenty of heat for the daily washing, dishes, etc. The west facing then would top-up the heat for the later afternoon and evening baths / shower / dishes. The length of the wall is about 8 meters, so I have plenty of space to add additional panels. Also, if I can insulate the geyser sufficiently to keep some heat overnight, it may well be worth it to add a second geyser ( and possibly more panels ) to be used in the winter months so that I can do away with the eskom powered geyser completely.
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