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Comment by bufferoverflow | Hacker News Reader

Creating hot water is much easier and much cheaper. You run a copper or stainless steel pipe with water, paint it matte black, and put it in an enclosure with a transparent side. The sun will hear it up. It's quite popular in Greece and Italy.

https://c8.alamy.com/comp/D24NWW/solar-hot-water-glass-tube-...

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Given the fallen prices of photovoltaic panels, it is frequently cheaper to use them rather than to build a passive collector, particularly if you have to deal with freezing weather and already have some photovoltaic infrastructure in place.

Photovoltaic panels still cost €0.19 per peak watt, wholesale (https://www.solarserver.de/service-tools/photovoltaik-preisi...). Lowe's lists 50 feet of 3-inch corrugated black plastic flex pipe at US$31.98 (https://www.lowes.com/pd/ADS-3-in-x-50-ft-Corrugated-Solid-P...), which works out to 1.16 m² and thus 1160 peak watts, which is 2.7¢ per peak watt — 9¢ per peak watt if you figure that the collector is only 25% efficient (this depends on the temperature). Currently US$1 ≈ €0.874, so that's €0.079 per peak watt.

That pipe is HDPE, and corrugated to boot, so it can go through quite a few freeze/thaw cycles before it starts leaking. It's true that it won't provide you hot water when it's freezing outside unless you put it under glass or at least plastic, though, and that drives the cost up.

I was expecting to find that you were ridiculously wrong, but actually it seems like the costs for a passive solar collector that can produce hot water in freezing weather actually might be about equal, or in any case no less than half the cost of photovoltaic. Bravo! And thank you for the insight!

I've seen it done in Northern Spain with just a long black PVC tube in a spiral pattern on top of a roof. Worked well enough for three showers/day, even during the winter.

I've seen this occasionally in the deserts of the United States, though usually in poorer communities and neighborhoods. Perhaps electricity is cheaper there, since that's where it's grown (solar farms).

PVC and ABS aren't recommended for hot water, nor are they particularly well suited to sustained UV exposure. It'll work in a pinch, but if building something permanent with the luxury of options I'd ignore those.

I don't know, I just know it's been working untouched for over a decade. They don't use the water for drinking or cooking, though.

I've just recently learned about parabolic reflectors concentrating solar energy on water pipes: https://en.m.wikipedia.org/wiki/Parabolic_trough

But I guess they must be rotated to follow the sun, which is more complicated than just pipes on the roof.

Using non-imaging optics, trough concentrators up to a factor of five or ten suns can work through a substantial part of the day with only seasonal tracking (i.e. twice a year you go out and change their angle by hand).

The very first solar power plant, in Egypt in I think about 1910, was actually a steam plant that worked on precisely this principle. Photovoltaic is cheaper in most of the world than a steam engine now, though.

this is how it's done in Israel for decades.

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