You can buy such things. Eg, https://dualsun.com/en/products/dualsun-spring/

(Not affiliated, no idea if that particular one is any good, it was just one of the first results)

The problem is you want your hot water much hotter than your solar panels so there may be limited use for this compared to a traditional solar water heater.

Heat pump water heaters exist. I would think running a warm water source past one would increase the efficiency since it lowers the delta-T

The main problem is the expected lifetime (and payout periods) of the two parts. Cooling solar panels brings more efficiency so it feels like a reasonable fit, cool the panels and get hot water. Its less efficient water heating because the panels aren't the ideal coating but they do still get quite hot and a few of them can definitely do the job. The big mismatch is in life.

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.

> Currently I suspect most people are better off with a dedicated solar heater and solar panels separately

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.

Where are you finding COP for water heaters? The best I found was a field study by the Canadian government where it seemed like most units operating in ambient temperatures relevant for me (in central California) get about 2.5.

The COP is roughly equivalent (but more forgiving) than the UEF rating. Many new heat pump water heaters have a UEF of 4 [1], but this accounts for things like standby losses, so the COP of the heat pump itself is higher than that.

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...

Here's the definition of UEF: https://www.govinfo.gov/content/pkg/CFR-2016-title10-vol3/pd...

It would be really nice if manufacturers published delta-T vs COP curves.

There’s a company called RayGen trying something along these lines at grid scale.

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.

https://raygen.com/