I just bought a replacement hot tub cover, and was musing on how effective it would be to keep the tub warm in the winter, and yet how totally counter-productive it would be when the surface temperature of the cover was warmer than the tub!
So I propose a cover that would sense temperature differential and circulate a suitable fluid through coils above and below the cover when the controls indicated that the water should be warmed, and stop circulation when the reverse condition exists.
Sort of a macro Maxwell's demon.-- csea, Jun 30 2012 HB references to MD http://www.halfbake...l%27s+demon&ok=+OK+No offence intended to MB [csea, Jun 30 2012] A secondary idea (not clever enough for a self-standing idea) would be to have multiple reflective "wings" that would fold out when it is desirable to heat the tub, allowing for more efficient concentration of sunlight from various directions.
These would fold in (origami-like) when the cover was removed for use.-- csea, Jul 04 2012 Would make sense if the energy required to operate the pump and controls is less than the heat energy accumulated by the surface of the cover, or completely powered by that energy.-- rcarty, Jul 04 2012 Solar water heaters are pretty well understood, so there's no reason this shouldn't work. The only question I see is if there is an advantage to mounting it on the the hot tub cover.
It seems like the biggest advantage is that you can have a few standard cover sizes that can be sold and used without installation.
Often a solar water heater will use vacuum insulated tubes to maximize efficiecy in cold weather. It seems like a lower tech solution as you propose might be better for this situation because of cost, weight, and potential damage while throwing the cover around.
You could have an electric pump with temperature sensors, but then you have to plug it in. Alternately you could try to be clever and use some arrangement of bimetalic strips to bring two metal heat sinks together or sepate them with an insulator.
I also looked into using heat pipes. They work best to move heat upwards, using gravity to return condensed fluid, but can use capillary action as well. I didn't see any information about whether they woud be effective moving heat down several inches needed in this case.
I did find that heat pipes with no capillary action can be used as a thermal diode, conducting heat rapidly when the bottom is hotter than the top, but conducting heat slowly when the top end is hotter. Unfortunately that is the wrong direction for this. You could use aluminum to conduct the heat from the top surface to the bottom of one or more heat pipes embedded in the cover, then have aluminum from the top of the heat pipe down to the bottom surface of the cover (or a probe going into the water for faster heating), but then your cover would probably have to be thicker to have the same insulating value during cold weather.-- scad mientist, Jul 06 2012 [scad_mientist], Your heat pipe ideas are good, thanks!-- csea, Jul 07 2012 A series of cover-mounted freznel lenses focusesd on a dark bottom may have the same effect.-- 2 fries shy of a happy meal, Jul 07 2012 Lenses won't help. Heating depends on the quantity of light, not how concentrated it is.
Thermosyphoning would work, as long as you don't mind heating to at most 4ºC. The best solution might be an insulating but translucent cover - like a pool blanket.-- spidermother, Jul 08 2012 random, halfbakery