Low temp solar water reservoir?
I am currently designing the master bathroom for my future home. Energy efficiency is a prime goal of the overall construction (small size, tight construction with ICF and SIPs, solar design, etc) and I would like to keep the theme with the master bath.
The bathroom will have dual sinks, a toilet, a shower, and a custom square ofuro. The ofuro is essentially a japanese soaking tub - about 3' x 3' x 3' (deep, for more upright soaking than most western equivalents). It will be heavily insulated (including a removable lid), and is supposed to be kept filled between uses (you shower and clean before soaking), and hot - 40C or about 104F.
This basically gives me about 150 gallons of hot water. I'd like to heat this economically of course, but I'd also like to use it as a heat reservoir if possible.
My idea is to wrap two copper tubes around the inside surface of the tub (not in contact with the water, but in contact with the tub, which would be stainless steel):
- One would form a closed loop solar heating circuit with a pump and a solar collector. During the day, the solar panel would heat the tub and hence the water. :-).
- The second loop would come from the cold water supply and be preheated by the tub and water before going to a tankless water heater and then to the hot taps throughout the room (even for the ofuro itself, or I could incorporate a circulating heater just in case the sun wasn't bright enough that day).
Theoretically this seems like a good idea - it allows me to incorporate a large thermal reservoir right where the heat will be used without giving up extra space to it. The solar collector requirements should be modest as well.
My main question is whether it would be reasonable to go through all this trouble to keep the reservoir at only 40C or 104F? Most solar water heating systems seem to use much higher temperatures for the reservoir - I've even seen warnings about overheating PVC pipe at around 200F. Would a lower temperature system still be a good idea? Should I consider solar to heat the water but not bother trying to use the tub to preheat the water going to the heater?
Could I expect to take a hot shower without the heater even kicking on if the ofuro starts out at 104F? This seems possible, since you can get quite a lot of contact with 3+3+3+3=12' of copper tubing per loop around the tub - and you could put quite a few loops around the tub! And you could set the system to keep the ofuro even hotter if you assume you're going to take some heat out with a cleansing shower before soaking.
The water in the tub itself will be changed regularly, just not as often as normal tubs (ie not on every use). It will also likely have a hot-tub type non-chemical treatment system (UV? ionizer? ozone?) to inhibit growth of nasties. Only getting in after showering clean helps a lot - no soaps, oils, etc for the most part - and with the lid on except when soaking there should be no light for algae.
But I welcome "you'll be growing an disease farm" criticisms too - I've never seen this done, and modern japanese ofuros don't keep the water hot all the time any more either (for efficiency).