There was a manufacturer doing this a few years back when we did solar on our house and I looked deeply into this problem. The issue they had was that they had

There was a manufacturer doing this a few years back when we did solar on our house and I looked deeply into this problem. The issue they had was that they had two manufacturers with different guarantees — one for the water and one for the panels. Love the idea — works like a charm (I got my hands on their unit and tested it out — glorious), but when you’re deploying capital to prepuchase an offset to alternative expenses it needs the warranty or it’s foolish to do the deal. Now there is another manufacturer (dualsun.com) claiming they own the global patents to the idea (don’t know how, they weren’t the first to commercialize it). The issue is definitely the incompatibility of materials. The engineering required to make a plastic water system last 25-30 years is so much more expensive than what it takes to make a pv unit last that long, so it gets expensive. Further, the warranty is dependent on the small volume manufacturer still being in business in 20 years to have any warranty claim at all, and with the turnover in solar a lot of homeowners aren’t willing to take that chance. So you get a double edged disincentive. It’s expensive to buy, and the warranty uncertainty is high. It’s all the classic problems of something that needs a mass market success to get to scale, but needs that scale to deliver mass market success. If someone could bond the warranty support or similar solution, it could take off. I wanted to use it to heat a hot tub or a pool and that had added complexity as the chlorine degraded the system and it wasn't advised. Another twist. :) Hopefully advances in materials and manufacturing will make it viable because it’s a GREAT idea and it WORKS! It definitely kept the panels cooler and it definitely heated water well. Edit: For those interested this is the product I know of… no affiliation with them though. https://dualsun.com/en/products/dualsun-spring https://dualsun.com/en/products/dualsun-spring