That is so cool, thanks for posting it! I have been heating my domestic hot water with solar collectors..since the last energy crisis..in the early '80's. I know it is possible to heat your tub because even here in the East, in the middle of winter..if it is clear (air temperature doesn't matter) I can get between 140 and 150 degrees in my panels..
My house faces dead south so in the winter I get solar gain..to heat the entire house.
Solar is the future and the sooner we refine the technology, the sooner we become independent from foreign fuel sources...
Wouldn't that be wonderful!