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Sadly, I read often that at this point, PV is so good that it makes more sense to install PV panels and power a water heater via electricity. I don't want to be

Sadly, I read often that at this point, PV is so good that it makes more sense to install PV panels and power a water heater via electricity. I don't want to believe it, but it's been said often enough that I am starting to do so.

nerpderp82 3y ago
I think electric solar heating makes sense for a lot of installations. Wires are easier than pipes, we still need electricity, make the existing system larger rather than split between the two. Heat water during the midday when output is high. I use almost no hot water, so the system would have a long payback period. But I can heat water in a 15A kettle. I love solar thermal, Trombe Walls should be integrated into the construction directly or added after the fact. They are for building heat, not water. https://en.wikipedia.org/wiki/Trombe_wall https://en.wikipedia.org/wiki/Trombe_wall
thsksbd 3y ago
PV can beat solar thermal once a heat pump is used. However that ignores the externalities of PV and heat pumps manufacture, replacement and disposal so I think the argument is BS. Solar thermal is an aimed aluminum trough and a stainless steel pipe painted black on its focal point.
- 15457345234 3y ago
  > Solar thermal is an aimed aluminum trough and a stainless steel pipe painted black on its focal point. No, it's actually these fairly sophisticated phase-change evacuated tubes that have a heatsink at one end immersed in the water, from what I remember reading.
  - thsksbd 3y ago
    Evacuated tube? Like the heat pipes on you CPU? Sure, but completely unnecessary unless you want to get rid of the pump [1]. And why get rid of the pump to substitute it with an evacuated tube? [1] which you don't even need if you route the system well or use valves.
  - mlyle 3y ago
    It can be either. There's fancy thermosiphons, etc, but you can also just preheat water going into a water heater by pretty dumb means. You can run circulating pumps. You can have a separate working loop. Etc, etc, etc. The Carter panels were pretty simple boxes with pipes and water circulated through them, I think.
  - BoiledCabbage 3y ago
    It sounds like you're talking about an "air to water" heat pump. Or some other source to water heat pump. Phase changes, and pressurized tubes screams heat pump to me. This is different than solar thermal which is essentially leaving a bottle of water sitting in your car on a sunny day. Nothing more complex needed.
    - PaulDavisThe1st 3y ago
      There are two kinds of water-heating systems based on heat-pump related ideas. One circulates a non-water liquid between the panels and the hot water storage tank, and it exchanges heat obtained from the panel with the water in the tank. This has the downside of being complex (pumps, weird liquids etc.), but has the upside of keeping the fluid system associated with the panels distinct, which can reduce issues with freezing and can boost efficiency. The other system doesn't use thermal panels at all, but just uses electricity to drive an air-source heat pump that exchanges with the storage tank.
kevin_thibedeau 3y ago
It's an easy intermittent power sink for supplemental hot water if you don't want to be grid tied or bother with a battery bank.
the_third_wave 3y ago
The best would be too use water-cooled PV panels with a heat pump to cool down the coolant and transform the heat. Cool panels produce quite a bit more electricity while the heat can be used elsewhere. Water-cooled panels are available commercially (with promises of far too high efficiencies, salesdroids gonna salesdroid) so this is not a pipe dream, especially not there where a water/water heat pump is available anyway - just install some valves to re-route the coolant through the panels.
- pstuart 3y ago
  I've had this notion too. The kicker is, as always, cost.

nerpderp82 3y ago

I think electric solar heating makes sense for a lot of installations. Wires are easier than pipes, we still need electricity, make the existing system larger rather than split between the two. Heat water during the midday when output is high. I use almost no hot water, so the system would have a long payback period. But I can heat water in a 15A kettle. I love solar thermal, Trombe Walls should be integrated into the construction directly or added after the fact. They are for building heat, not water. https://en.wikipedia.org/wiki/Trombe_wall https://en.wikipedia.org/wiki/Trombe_wall

thsksbd 3y ago

PV can beat solar thermal once a heat pump is used. However that ignores the externalities of PV and heat pumps manufacture, replacement and disposal so I think the argument is BS. Solar thermal is an aimed aluminum trough and a stainless steel pipe painted black on its focal point.

15457345234 3y ago
> Solar thermal is an aimed aluminum trough and a stainless steel pipe painted black on its focal point. No, it's actually these fairly sophisticated phase-change evacuated tubes that have a heatsink at one end immersed in the water, from what I remember reading.
- thsksbd 3y ago
  Evacuated tube? Like the heat pipes on you CPU? Sure, but completely unnecessary unless you want to get rid of the pump [1]. And why get rid of the pump to substitute it with an evacuated tube? [1] which you don't even need if you route the system well or use valves.
- mlyle 3y ago
  It can be either. There's fancy thermosiphons, etc, but you can also just preheat water going into a water heater by pretty dumb means. You can run circulating pumps. You can have a separate working loop. Etc, etc, etc. The Carter panels were pretty simple boxes with pipes and water circulated through them, I think.
- BoiledCabbage 3y ago
  It sounds like you're talking about an "air to water" heat pump. Or some other source to water heat pump. Phase changes, and pressurized tubes screams heat pump to me. This is different than solar thermal which is essentially leaving a bottle of water sitting in your car on a sunny day. Nothing more complex needed.
  - PaulDavisThe1st 3y ago
    There are two kinds of water-heating systems based on heat-pump related ideas. One circulates a non-water liquid between the panels and the hot water storage tank, and it exchanges heat obtained from the panel with the water in the tank. This has the downside of being complex (pumps, weird liquids etc.), but has the upside of keeping the fluid system associated with the panels distinct, which can reduce issues with freezing and can boost efficiency. The other system doesn't use thermal panels at all, but just uses electricity to drive an air-source heat pump that exchanges with the storage tank.

kevin_thibedeau 3y ago

It's an easy intermittent power sink for supplemental hot water if you don't want to be grid tied or bother with a battery bank.

the_third_wave 3y ago

The best would be too use water-cooled PV panels with a heat pump to cool down the coolant and transform the heat. Cool panels produce quite a bit more electricity while the heat can be used elsewhere. Water-cooled panels are available commercially (with promises of far too high efficiencies, salesdroids gonna salesdroid) so this is not a pipe dream, especially not there where a water/water heat pump is available anyway - just install some valves to re-route the coolant through the panels.

pstuart 3y ago
I've had this notion too. The kicker is, as always, cost.