Hi,

Interesting idea.

Some comments embedded below:

...

"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. :-)."

As an alternative, you could circulate water directly from the tub through the collector when the collector is hot. When the collector goes cool, the pump turns off, and the water in the collector drains back to the tub for freeze protection. This is called a drain back system, and is simpler and more efficient than a closed loop system.
My understanding is that one of the challenges of solar heating hot tubs is maintaining the temperature of the tub just where you want it -- people are very picky about this temperature being spot on. Maybe this could be taken care of by just adding a bit of hot or cold water as needed?

"- 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)."

Getting a good thermal bond between the pipe loop and the tub would be critical to how well this would work. Not sure how you could do this between stainless and copper, but there may be a good way. Copper tub?

All practical heat exchangers (e.g. your pipe coil) have some temperature drop, so the water going to the shower from the pipe coil won't arrive at the shower at 104F -- just a guess, but I'd say more like 10F less. So, you would probably need the tankless to boost the temperature.

"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?"

104F is not so low as to not be useful, but the problem to me is that as soon as you start drawing hot water in the house, heat will be extracted from the tub, and the temperature will start dropping from 104F. This means if you take a shower, by the time you are ready to go in the tub which was at 104F, it will no longer be at 104F because the shower took heat out of it.
It you take a 5 minute shower at 2gpm and your ground water temp is 50F, then you remove about (104F - 50F)(10 gal)(8.3lb/gal)(1 BTU/lb-F) = 4480 BTU from the tub -- this would lower its temp by (4480 BTU)/((150 gal)(8.3 lb/gal)(1 BTU/lb-F)) = 3.6F -- enough to make it too cool I think.

"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! "

I'd say no for the reason mentioned above.

"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."

Maybe.
But, it seems like the basic problem you have is that the tub is kind of a fixed temperature device -- its not useful if it gets much hotter or cooler than 104F. For thermal storage to give back its heat, it has to go down in temperature -- thats where the stored heat comes from.

Its a neat idea, but I am wondering if you would be better off doing a conventional solar water heater, and just using that to supply both the tub, shower, etc. ??

Gary