Dear Jeff,
I’m hoping to develop a low tech, low cost solar water heater for use in a camper van. As such, it needs to be capable of supplying sufficient hot water for showering and dish washing for a family of four.
Assuming two daily wash-ups, and each family member showering perhaps every other day (we are roughing it in the great outdoors after all), daily water usage should be fairly small. I’ve seen the commercially available ‘Solar Shower’ bags, and have read about water-filled truck inner tubes and the like, but I’d like to develop something a little more sophisticated, if only to impress my (slightly more smelly) campsite neighbours!
Initially, I planned to make an open thermosiphon type system, utilising an insulated ground level collector, incorporating a secondhand refrigerator matrix, with a suitably sized storage tank sited on the van roof. This would be fine assuming we’re pitched up for a day or two- more often than not, we’re on the move for much of the day, and pitch camp in the evening- so any system would need to be permanently installed in/on the van and capable of making use of the sunshine on the move during the day.
Due consideration would have to be given to reducing the effect of cooling air passing over the collector while driving, but would a closed, pumped glycol based system, with a heat exchanger and on-board storage tank be feasible?
What precautions would I have to take regarding temperature and pressure build up, and how would one calculate the various capacities involved?
As you may have guessed, this idea hasn’t even reached the drawing board yet, but I’d be keen to hear your views, even if it’s to dismiss it out of hand!
Thanks for your time.
Jonathan
Jonathan:
You may want to check out the next issue of Backwoods Home Magazine (#108) as I show how to mount solar electric modules on a trailer which can tilt up.
Back in the 1920′s and 1930′s Florida did not have any piped natural gas or propane, and those homes with electricity back then usually only had one or two 15 amp fuses for lights and the radio. The did not wire homes for large electric hot water heaters. Therefore, almost every family home in Florida during this period had commercially made solar hot water heaters. They located the solar panel at the lowest part of the roof just up from the roof eve, then they cut a hole in the roof at the peak and stood up a hot water storage tank. They piled insulation around this tank which stuck up above the roof eve then fabricated a fake chimney around it to hid it. They did this so the tank would be much higher up than the solar hot water panels which allowed thermo-syphening and did not require a pump. Back in the late 1970′s we removed one of these old solar hot water heaters from an older Florida home that was still operational and replaced with a new system.
If you also want to use a thermo-syphen system, you must have the solar module as low as possible and the storage tank as high as possible. If you operate in colder locations you will need some type of anti-freeze. However, commercial solar hot water heaters are required to use a double-wall heat exchanger due to the danger of the heat exchanger getting a leak inside the storage tank and letting anti-freeze leak into water you could be drinking. Ethylene-glycol (automobile anti-freeze) is very poisionous, but propolene-glycol (red food coloring) is not poisionous and this is what they sell in RV stores for winterizing an RV. The problem with any system that uses anti-freeze is this reduces the heat transfer efficiency of the water mixture, so there is a down-side if you must prevent freezing.
If you can utilize a small DC powered pump, you will not need any anti-freeze. You have a small insulated non-pressurized holding tank just below the elevation of the solar panels. When the sun starts heating the panel, the pump is turned on and moves water up into the solar panels flowing from the bottom to top. When the sun goes down the pump shuts off and all the water drains back into this small holding tank, which is NOT the hot water storage tank. Your hot water storage tank will need a heat exchanger for this heated water to pass through as it moves back down from the top of the solar panels through the heat exchanger then into the small drain-back tank.
They also make special controllers that you connect a temperature sensor on the solar panel and the other in the hot water storage tank and it will turn on the pump anytime the solar panel is hotter than the water in the tank.
Since the solar panels are empty when the pump shuts off, there is nothing to freeze.
There are lots more to the design of these systems, but this should get you started.
Good luck,
Jeff Yago
