Heat pump water heaters already exist. These are hybrid things where a traditional electric water heater is fitted with a heat pump. The heat pump can increase the water temp but cannot deliver enough, so heating elements are still needed to reach a usable temp.
I’m wondering if that design can be improved on this way: instead of powering the heat pump from the wall, the heat pump can be connected directly to a PV. I think that would be more efficient and cheaper because PV output is not normally directly usable. IIUC, it’s variable D/C which must be regulated and/or inverted to A/C involving more hardware, conversion, and waste. But exceptionally, I’ve heard that a PV can directly power a compressor with no middleware. Any reasons this would be infeasible or uninteresting?
Of course the tank still needs wall power for the heating elements, but would use less wall power and entail less conversion loss.
I think the cost of all the additional wiring and components would outweight the benefits.
What if the sun isn’t shining? You would need a heat pump that can use AC and DC. What if the sun is shining, and the water is hot enough? Your PV would need DC cabling and a converter to AC to send power to the grid. And all these components would need some switches etc.
It’s just easier to have all your home run on AC.
Nice point.
If all appliances had a DC mode, or even an AC+DC mode [1] by default, this would be something worth working up for.
But if you are doing it just for a single appliance, guess it’s better to just connect the Solar power output to your AC grid, the way others do.
Also, solar water heaters seem to be more efficient in this regard [2], so if your use case is only water heating, just add that next to your overhead tank, and let the heat pump work separately.
smells of inefficiency, but I didn’t do the maths ↩︎
no PV, larger spectrum used (even IR) and directly converted to heat, instead of light -> electric -> heat ↩︎
Shit… sounds about right.
Although the /water is hot enough/ scenario could be addressed mechanically: bigger water tank, lower heating element raised and the heat pump heating the bottom exclusively where it could /always/ add heat because it would never be hot enough at the bottom.
(edit) after some thought, it would superficially make sense to get a factory water heater (tank) and not tamper with it at all. Just have a PV-powered HP heat water before it enters the stock water heater (in a tank or coil). Thus there could be 2 heat pumps (but for economy the stock tank would just be a simple non-heat pump type thus 1 HP). I guess this is still a dead idea anyway if it’s true that a PV cannot simply directly connect to a compressor.