Solar thermal hottub.

Not sure if this is the right spot but I have had a theory for a while that I cant seem to find a ton of information about.
I have an 8 person hottub (roughly 600 gallons) and live in northern Ontario zone 3b. It gets down to -40  a few times a year so winter is cold but often very sunny. I have often thought that solar thermal water heaters would likely work for keeping the hottub at manageable temperatures. I not talking about heating it up from tap temperature as that is extreme. More just thinking the daily fluctuations.

The biggest issue is how to get the heat from the thermal panels to the hottub. I dont want to do an open circuit as I fear that any trapped water could freeze causing a big issue.

I could do a closed loop with some sort of heat exchange and glycol but not sure how to work that. With a closed loop, I worry about the sun heating the glycol too much and causing pressure issues. The other issue would be when the sun is not out, a closed loop would actually cool the hottub.

I'm thinking a simple pump with a temperature sensor will be what turns it on and off.

Any suggestions?

With a glycol system, in addition to a collector and pump, you'll need a heat exchanger for the bath water, and an expansion tank - though I don't know that you need a bladder-type like in a pressurized system? maybe you could have/use an automotive expansion tank.
With a drain-back system (what you referred to as "open circuit") you could directly use the bath water. A vacuum breaker at the high point of the system, and careful plumbing with a continuous downward slope should get it drained.

A solar pump might cover most of the control, by default, NO sun = NO heat = NO pumping, but some thermostatic control would probably be in order.

If you haven't found it yet, builditsolar.com is a great resource.

Volume = 600gallon
Temp = 30F (70F to 100F)
Effective Winter Sunlight Hours = 2hours
Rate = 3WHr/gallon/F (or 10BTU/gal/F)

Energy Needed = 54KWHr = 600 x 30 x 3
Solar Array Size = Energy Needed / Sunlight Hours = 54KWHr / 2hr = 27KW

A 27,000W solar array is going to cost alot of money, and cover alot of roof space.

if you use a hot water heat pump you could reduce the needed electricity demands but it is quite better the use of the a rocket stove

My solar thermal drainback system worked beautifully and saved thousands of dollars over the course of many years. I simply followed the instructions of a Home Power article, which are now available for free online.

I dedicated two smaller panels know that the hot tub water would eventually corrode them, but for me in this application is was not a problem. I also had a lot of roof space to work and the drainback feature worked fine.

I had an electric heater/pump on a timer that would turn on around 6pm and run for 2-3 hours. The solar hot water was technically a "pre-heat system," augmented by a conventional backup. The solar got so hot during the day the only thing preventing it from overheating was a differential controller, which worked like a charm.

Don't expect the solar hot water system to provide 100% of your heat unless you have the capacity and want to only soak during the day.

Drainback insures non-freezing.

My monthly bill for the hot tub averaged about $7, compared to what would have been over $100 to heat conventionally. When the pump and heater turned on at 6pm, the heater would turn on for a few minutes and then turn off because it has a sensor that prevents overheating...so the only draw was for the pump.

I have a snorkel wood fired hot tub. Last week, I got the solar thermal flowing. It will be a drain down system. It might work best to heat the tub with wood and try to maintain the heat with solar. Here in the high desert of Southern California, by far I use it as a cold tub in the summer. The solar powered swamp cooler works best after the sun is behind the hill. Some of the oak trees have died. So, I like to burn some of the oak in the winter/ rainy? season. Spring and Fall I would rather use solar. One time, years ago, I made too much smoke. Someone called in and a fire truck showed up. I was recognized by one of the guys who asked if I knew a firefighter. I did know him and got off with a warning that next time I would have to pay for the fire truck to come out. That's how I got motivated for a solar thermal hot tub.