There is a company called heliatos ( Heliatos Solar
) that makes 2 ft square solar water heaters that can plug
directly into your engine/waterheater glycol loop. They come with a solar panel to run the circulation pump which is included in the kit for a little over $400. The solar panel only runs the pump when the sun is shining so there is no need for any kind of solar controller. The PVC or copper coil will work within limits, but if the thing is not covered with an infrared blocking glazing it will start loosing heat to the environment
as soon as the water starts getting hot. This is also a problem with a metal container. It conducts heat so well that the side facing away from the sun looses heat almost as fast as as the sunny side absorbs it. I could not connect to the article on solar electric, but if they were suggesting that solar electric was efficient, they don't know what they are talking about. Resistance heaters are almost 100% efficient at converting electricity into heat, but the best solar panels
today are only about 22% efficient and most are in the mid teens. Flat plate solar thermal on the other hand can be up to 80% efficient. You'll get a lot more hot water from a solar thermal system than by heating your water with the output of a photovoltaic panel.
The only problem I've had with the sun showers is that they cool off very quickly after the sun gets low in the sky and you don't always have hot water when you want it. The garden sprayer lasts a bit longer because the plastic is thicker and hold the heat in longer. Oddly enough the most efficient way to set up the garden sprayer is to put the black paint on the inside as that puts the heat absorbing layer in contact with the water. It probably doesn't matter that much since I have noted that simply leaving a 5 gallon blue plastic paint bucket full of water on the back deck gets it plenty hot enough for a shower by the end of the day.
There is a company in Turkey
that makes hybrid solar panels
. They have photo
voltaic cells mounted on a flat plate solar collector and achieve very high efficiencies. They are oriented towards building heating and not appropriate for mobile applications.
I have thought about the possibility of taking some of those aluminum
radiant heating plates and mounting them on the back of a photovoltaic panel. They get quite hot and normally radiate this excess heat through the back of the panel. In theory one could remove the excess heat by conduction and capture it. I'm sure it would blow the warranty, but I am planning to replace my old 75 watt panels
with modern efficient panels and it might be worth experimenting with one of them.
Good luck with what ever you decide to do.