Quote:
Originally Posted by buffalobillpatrick
Each tank has about the same surface area as 300' of 3/4" pex.
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I think there is good and bad news.
The good news is that your calculation looks way off, and a tote has the same surface area as much longer pex. Can you check your figures?
The bad news is that the surface area to volume ratio of a tote is less than half as good as for the pex, so a tote would not be as good at replenishing the energy you extract - but should hold a lot more embedded energy at the start of your heating season.
The overall result should be good though, because the number of tanks (10) should more than make up for their lower efficiency.
Would you insulate the pipe runs between the tanks?
How would you plumb everything together? I think I would run a pipe from the top of tank 1 to the bottom of tank 2 and so on, so the water would get hotter as you go through the series of tanks. A sort of horizontal stratification. I would also try to promote vertical stratification within each tank.
The ideal feed to the solar panels would then be from the bottom of tank 1, which should be to coldest water in the tanks, but where would you feed back the heated water? It is not obvious to me what the best strategy would be, nor how to make the connections for keeping the levels constant.
Would you circulate the fluid in the tanks, or circulate fluid through coils in the tanks? Coils would probably be better.
Would you insulate tank 10 (if that is where you put the heat from the solar panels)? If not, you are going to have a lot of losses with heat flowing out of tank 10 to the ground.
If you use the solar panels to heat the final tank, feed water from that to your heat pump, then feed the cooled water back into tank 1, will tank 1 then be hotter than the surrounding ground? If so, you will have more losses. It depends what heat drop you expect through your heat pump, but I can envisage that being a problem.
It could be worth insulating the top of all your tanks, leaving the sides and bottom open, so that you don't lose so much of your solar heat, but still can gather heat from the ground when that is warmer than the tanks. This will of course make the surface area to volume ratio even worse, so it is swings and roundabouts.
Will you add some mechanism to top up the tanks? I would not rely on there being no fluid loss for decades and you don't want to have to dig to replenish any losses. In theory there should be no losses but I know someone who used a large number of plastic pop bottles as a heat store and he found there were losses over the years so I wouldn't rule it out.
How much energy do you need to extract during a heating season? You could calculate the energy in the tanks and so work out what replenishment rate you would need. My gut reaction is that this should be a good system but it is worth doing some detailed planning at this stage before committing to a design.