8

We have long carried a backpacker's solar water heater on our trips. The bliss of warm -- nearly hot -- water in quantity for washing one's filthy self is worth the small extra weight -- especially when a packer is carrying all our gear up the first day -- the day of maximum elevation gain and minimum fitness.

Our heater is similar to this, but is a different, earlier brand.

The heater works very well when the ambient temperature is roughly 50 degrees F (10 degrees C) and heats cold stream water to about 100 degrees F after a few hours lying on a sunny rock.

We have never used the heater in cold weather, and, looking ahead to a major power failure this winter (we are overdue), we wonder if we could get warm water when the ambient T is in the 20s (F) by laying the heater on our sunny patio table (cleared of snow), and on top of an insulating sleeping pad. Assume no wind. The only time we used the heater at home was in the derecho of June/July 2012, when the ambient temperature was in the 90s; it worked well then, obviously.

If you think a patio table makes this Q off topic, assume a winter trip, a flattish rock brushed clear of snow, insulated sleeping pad, and water taken from a flowing stream. No wind.

7

The water temperature will warm up until the heat gained is identical to the heat lost. At 20F, with a thin layer of plastic between the water and outside air, its unlikely the equilibrium will be much above freezing, certainly it would be optimistic to expect a warm shower.

Keep in mind the reason it is 20F is there is not enough energy in the sun to make it warmer, so you will not get anywhere near the temperature increase you see ( 50 degrees ) in summer .

  • What about putting a clear plastic tarp over it to create a green house? – Jonathan Landrum Apr 26 '18 at 4:32
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    A greenhouse structure will certainly help a bit, and if you're making something, a double "glazed" structure would be better still. You can also reflect extra sunlight onto the reservoir. You may still be more along the lines of a chilly sponge bath – Chris H Apr 26 '18 at 7:04
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    @Chris H I didn't ask about another possibility because it wasn't about the outdoors. That would be putting the solar water heater on an indoor table next to a very sunny window. – ab2 ReinstateMonicaNow Apr 26 '18 at 14:30
  • @ab2 if the solar heater is indoors, there would certainly be a higher ambient temperature around it, even if the structure isn't heated. – Jonathan Landrum Apr 26 '18 at 14:33
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    @ab2 car camping could allow a situation with glass windows that's on topic. A bottle of water on the dashboard or parcel shelf (depending on how the vehicle is facing with respect to the sun) warms up quite a lot even though it's not optimised. Again, add foil reflectors for extra collection. – Chris H Apr 26 '18 at 14:40

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