(Not affiliated, no idea if that particular one is any good, it was just one of the first results)
A solar thermal heater typically lasts 10 years, its a pretty harsh environment and they have pumps and expansion vessels and water in the heat range of -25 through +80 C is just damaging over time. Where as a solar panel has a rated 80% output at 25 years. That mismatch is problematic for Solar panel payoff, 10 years is only just past break even point. Whereas the solar thermal heaters typically pay off within a few years so the reduced lifetime isn't such a big problem (but both are driven by local gas and electrical prices).
Companies are trying to do this better and products exist to do it. The added installation complexity and reduced expected lifetime are all a bit of an issue. Currently I suspect most people are better off with a dedicated solar heater and solar panels separately, at least for now until the water channels in the panels at least are reliable enough that they meet the panel lifetime and then the pump is installed in the roof cavity so its cheap and easy to replace. Work still needs to be done to work out the details and make the parts that fail easily swappable.
Right now most people are better off (from a total cost of ownership perspective) with PV panels + heat pump water heater. The latter can achieve a COP of 3.0 (300% "efficiency"), and also can just use grid electricity on a cloudy day. Unless you are off-grid, the additional cost/complexity of running plumbing to the roof for a solar water heater doesn't pencil out.
Some premium hot water heaters like the SanCO2 can achieve COP of up to 5.0 [2], so high that they don't need backup resistance elements, even in northern climates.
1. https://www.energystar.gov/productfinder/product/certified-w...
2. https://www.smallplanetsupply.com/small-planet-blog/2021/1/7...
It would be really nice if manufacturers published delta-T vs COP curves.
They use concentrated PV to generate electricity directly, and the cooling water goes into a pond (with insulation on top) that reaches ~90C. They also have a “cool pond” cooled using a heat pump to just above freezing.
The combination of the heat and cold reservoirs are used to run a turbine with a low boiling point fluid.
Obviously with the low temperature differentials it’s not terribly thermally efficient, but the proponents claimed the cost of electricity combined with the storage will make it viable in sunny locations.
If heating water is the goal then going directly to heating the water is more than 4x the power capture of a solar panel. Dropping one solar panel on your roof for a water heater pays off its just that the solar heaters are kind of expensive dealing with all the other aspects, like if the pump fails or its too cold and the water would freeze etc etc. Those other aspects are probably going to dominate the complexity and cost of the devices.