solar PV → heat pump → water heater; direct, no A/C or intermediate components. Practical? Feasible?

Submitted ⁨⁨4⁩ ⁨days⁩ ago⁩ by ⁨activistPnk@slrpnk.net⁩ to ⁨energy@slrpnk.net⁩

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?

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poVoq@slrpnk.net ⁨4⁩ ⁨days⁩ ago
I have such an air to water heat-pump. It reaches 55°C without the supplementary electric water heater, which is more than sufficient if you have a hot water tank. The supplementary electric heater is really only used when too many people want to shower in a short timeframe or to occasionally heat the tank up to 70°C+ to kill off any bacteria that might be growing in it.

I don’t think you will find a heat-pump with a variable drive compressor like you are proposing, but a DIY solution might be feasible.

But why not just install a vacuum tube solar-thermal water heater? They are more efficient anyways.

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- evasive_chimpanzee@lemmy.world ⁨3⁩ ⁨days⁩ ago
  
  occasionally heat the tank up to 70°C+ to kill off any bacteria that might be growing in it.
  
  Is that a built in function of your fancy water heater, or is that something you just go do periodically as part of maintenance?
  
  I’d love to get one of those heat pump water heaters. Seems like a win-win to dehumidify the space.
  
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  - poVoq@slrpnk.net ⁨3⁩ ⁨days⁩ ago
    It can be programmed to do so, but I disabled it as I am not particularly concerned about it, and the solar thermal vacuum tubes also connected to the same tank will occasionally push it to near boiling temperatures.
    
    I originally also thought it would be useful to dehumidify the air, but in reality the times when you need to heat water and the same time have a humidity problem rarely overlap in my case. Maybe if you have a perpetually humid cellar that you don’t use much otherwise (as the exhaust air is quite cold) it would be a good addition.
    
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GBU_28@lemm.ee ⁨4⁩ ⁨days⁩ ago
Why not thermal solar panels wired to a water heater? Seems to be the least energy transfers, each of which incura efficiency loses

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- BastingChemina@slrpnk.net ⁨2⁩ ⁨days⁩ ago
  Yes, thermal solar panels would be way more appropriate.
  
  While trying to gain a few % through the transformation process OP is not addressing the elephant in the room: PV panels turn around 20% of the sun energy into electricity, thermal panels turn around 80% of the same energy into heat, They are way more efficient.
  
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perestroika@slrpnk.net ⁨4⁩ ⁨days⁩ ago

instead of powering the heat pump from the wall, the heat pump can be connected directly to a PV

I have no experience with this exact combination. I know that “batteryless” inverters exist, but most of them are on-grid inverters. In that scenario, all that matters is monitoring your production: if you don’t want grid energy, you only run your system when your PV produces enough.

Another type of batteryless inverters are “pump inverters”. Farmers seem to like them for pumping water from wells into water towers. A pump inverter can be configured to run at 50 Hz (or 60 Hz for North Americans) and 230…240 V alright, but it is not designed to power electronic devices, but dumb agricultural motors. There is considerable risk involved with powering a heat pump from a pump inverter, unless you find an exceptionally simple and dumb heat pump with very limited or resilient steering electronics.

Efficiency losses are small anyway, but mostly happen during battery storage or when voltage needs to rise or drop considerably (e.g. a transition of 700 -> 24 V or 24 -> 240 V would cause a small efficiency loss).

I’ve heard that a PV can directly power a compressor

This seems unlikely as the compressor would have to be a brushed DC motor. That kind of motors don’t last long, they wear out their brushes. But a brushless DC motor is essentially a three-phased AC motor already, just the controller (full of smartness and MOSFET transistors) accepts DC input.

If you have a good technical overview of your heat pump system, maybe you can locate a point where regulated DC can be fed into the system, but that would be hacking. Alternatively, maybe a niche market already exists for DC-powered heat pumps, e.g. for caravans, trucks or ships? But on niche markets, prices typically aren’t good for you. :(

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- perestroika@slrpnk.net ⁨4⁩ ⁨days⁩ ago
  P.S. I have once used DC to power a pump “directly”. I use quotation marks because the pump (a water pump) was a brushless DC motor with an integrated controller. I used it on a field for removing water after a spring flood. Its controller accepted 24…48 V input, and it was powered from a 40 V solar panel brought on a wheelbarrow. :)
  
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gandalf_der_12te@slrpnk.net ⁨4⁩ ⁨days⁩ ago

IIUC, it’s variable D/C which must be regulated and/or inverted to A/C involving more hardware, conversion, and waste.

Actually, I had a discussion about this who really knows this stuff recently, and we figured that the inversion process isn’t actually that inefficient. The efficiencies achieved are often >95%, so there isn’t actually a lot of loss.

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cron@feddit.org ⁨4⁩ ⁨days⁩ ago
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.

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- ulterno@lemmy.kde.social ⁨4⁩ ⁨days⁩ ago
  
  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 ^[smells of inefficiency, but I didn’t do the maths] 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.
  
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- activistPnk@slrpnk.net ⁨4⁩ ⁨days⁩ ago
  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.
  
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