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ASHP - to run directly off DC?


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Hi - I'm looking to install a PV/ASHP system but all systems seem to convert the DC power from the PV to AC and the ASHP then converts the AC power back to DC to run the pump!

Does anyone know of a 'joined up' system where the ASHP can run directly off DC from the PV/Battery and avoid all the energy losses in conversion? 

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3 minutes ago, GFT said:

Hi - I'm looking to install a PV/ASHP system but all systems seem to convert the DC power from the PV to AC and the ASHP then converts the AC power back to DC to run the pump!

Does anyone know of a 'joined up' system where the ASHP can run directly off DC from the PV/Battery and avoid all the energy losses in conversion? 

No, but it would run in the summer when the sun's out and that would be it. Unlikely to get enough power to start in winter, most the time, especially when you need it.

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14 minutes ago, GFT said:

Hi - I'm looking to install a PV/ASHP system but all systems seem to convert the DC power from the PV to AC and the ASHP then converts the AC power back to DC to run the pump!

Does anyone know of a 'joined up' system where the ASHP can run directly off DC from the PV/Battery and avoid all the energy losses in conversion? 

Most convert the 50Hz ac to variable frequency ac to power the compressor.

 

I take it you are trying to get something working off grid with PV and dc batteries?  Try looking at units made for caravans?

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Don't think of it as a "PV-ASHP system", but two entirely separate entities that do totally different things at different times. Then life will be simpler and happier. 

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One challenge with this is it will require high voltage DC (i.e. higher than 50V so not SELV), which while not a problem in theory, is a bit of a grey area for most installers as they just won't know what regs apply.

 

Camper/Caravan gear seems most likely place this would emerge but it will be a challenge as they tend to be only 12V or mains AC. I found this: https://coolmach.com/roadmap/ - if it gets beyond vapourware stage, then delivering 4kW output would imply around 100A @ 12V input (COP of 3.3), which just doesn't seem practical.  It's unfortunate camper leisure batteries don't typically get installed at 24V or even 48V. Or perhaps camper architecture will change to collocate leisure batteries with the aircon (and water heating) gear which would enable this better.

 

 

 

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5 hours ago, GFT said:

Does anyone know of a 'joined up' system where the ASHP can run directly off DC from the PV/Battery and avoid all the energy losses in conversion? 

 

It would be a bit of a project, but Heat Pumps in EVs are DC, although the ones I'm aware of are 400v - 800v, matched to the high voltage architecture on the vehicle.

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I think by the time the variable power from PV has been though a voltage regulator, a battery management system, then out to a controller for a heat pump fan and pumps, you have lost any efficiency gains and are probably worse off.

Modern inverters are very efficient and the lost efficient is mostly thermal, so can be useful (but at 98% efficient at 2000W delivery, you are only talking about 40W).

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  • 2 months later...
On 19/08/2024 at 09:22, GFT said:

Hi - I'm looking to install a PV/ASHP system but all systems seem to convert the DC power from the PV to AC and the ASHP then converts the AC power back to DC to run the pump!

Does anyone know of a 'joined up' system where the ASHP can run directly off DC from the PV/Battery and avoid all the energy losses in conversion? 

Thanks for the input but perhaps I need to be clearer. The proposal is for a new house and to charge a 12+Kw/hr battery (e.g. Tesla Power Wall) from a 2.5Kw solar tracking array. The battery will then run an ASHP and 12V LED lighting.

The current coming from the solar array is DC. The battery stores energy as DC and the lighting circuits are DC. Most ASHP's are AC but most have a transformer in them because the pumps themselves are....DC.

Essentially, I'm looking for an (circa 21Kw) ASHP which runs off DC to avoid energy losses in conversion.

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25 minutes ago, GFT said:

Thanks for the input but perhaps I need to be clearer. The proposal is for a new house and to charge a 12+Kw/hr battery (e.g. Tesla Power Wall) from a 2.5Kw solar tracking array. The battery will then run an ASHP and 12V LED lighting.

The current coming from the solar array is DC. The battery stores energy as DC and the lighting circuits are DC. Most ASHP's are AC but most have a transformer in them because the pumps themselves are....DC.

Essentially, I'm looking for an (circa 21Kw) ASHP which runs off DC to avoid energy losses in conversion.

21kW ???

You could try one of these in each room! https://www.hotspotenergy.com/DC-air-conditioner/#:~:text=An all-DC system means,for many years without maintenance.

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58 minutes ago, GFT said:

Thanks for the input but perhaps I need to be clearer.

I think you need to spend a few hours researching basic electrical and heating systems.

 

Then you will find out that what you are proposing is not viable.

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The pumps aren't DC in any real sense. 

 

They are variable frequency AC, created by an inverter chopping up DC. Chopping up DC created from rectifying AC is no major problem. 

 

240v AC is a a useful "universal" form of power - you power wall will output it efficiently, and your HP will accept it readily. Your electrician will be able to work with it and components like breakers, monitors, switches etc are easily availble. 

 

I doubt the efficency gains (if any) would be worth the extra hassle of a non standard setup. 

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It's not an unreasonable question. But the problem is that DC commonly exists at many different voltages and unlike AC it is not possible to convert between them with the low cost, simplicity, robustness and high efficiency of an AC transformer.

 

12V - cars, caravans, leisure boats. Actually 14V when engine is running.

24V - trucks, buses commercial boats, fishing vessels

48V - de facto standard for domestic battery systems. Highly variable 46 - 54V or more depending on # of cells/state of charge. Some systems now going to higher voltages to reduce the currents involved. Also historically used for telephone exchanges and telecoms equipment.

30 - 40V per solar panel depending on # of cells in it.

<120 - 450V or more per solar PV string depending on # of panels in string, load and temperature. Solar and hybrid inverters have to cope with this wide voltage range.

325V +6 -10% if you rectify 230V nom AC mains (as in an inverter-drive HP)

400 - 800V standards for batteries in EVs

400kV or more for long-distance underwater power transmission e.g. UK-France or Norway

 

48V would be a good choice for this as it is the highest nominal voltage reckoned to be safe for human contact. (DC is more dangerous than AC at the same voltage bc there isn't the natural muscle recoil.) A 12kW (thermal) HP would however draw about 100A which would require really heavy section cabling everywhere. Also the variable-speed inverter in the HP would be more expensive as it would need larger, higher current semiconductors. There is no saving to be gained there from the lower voltage.

 

Once you start trying to interface to anything solar you encounter wide voltage ranges as above which also make the inverter design more expensive.

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2.5kW solar, even tracking, won’t do diddly-squat for heating, and if you need a 21kW heat pump you’ve got far better places to look at for practical energy/efficiency gains I assure you!

Leave it all A/C, and keep it simple. 
Installers and repair/service agents will take one look at a Frankenstein setup, realise it’s completely illegal (ergo unsafe to work on), and walk away.

It’s a “no” for this one I’m afraid, but also can you get more solar? That’s where I’d put any funds from “avoided tomfoolery” tbh. 

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On 30/10/2024 at 16:01, GFT said:

Thanks for the input but perhaps I need to be clearer. The proposal is for a new house and to charge a 12+Kw/hr battery (e.g. Tesla Power Wall) from a 2.5Kw solar tracking array. The battery will then run an ASHP and 12V LED lighting.

I'm fairly sure a Tesla PowerWall is AC coupled and is connected at 240VAC? Within the "battery package" is an inverter/charger that takes in AC to charge the batteries and supplies AC to power loads. If you want to connect  a PW at DC you'd likely need to open the casing and bypass the inverter/charger which would void your warranty. You'd also need a DC charge controller to regulate the charge from the PV. Plenty of DC battery packs available but don't think a PW is what you're after

 

A 2.5kw array is gonna supply next to nothing in the winter when going to need the ASHP the most. Have a look at PVGIS to see what you'll get.

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13 hours ago, Dillsue said:

I'm fairly sure a Tesla PowerWall is AC coupled and is connected at 240VAC? Within the "battery package" is an inverter/charger that takes in AC to charge the batteries and supplies AC to power loads. If you want to connect  a PW at DC you'd likely need to open the casing and bypass the inverter/charger which would void your warranty. You'd also need a DC charge controller to regulate the charge from the PV. Plenty of DC battery packs available but don't think a PW is what you're after

 

A 2.5kw array is gonna supply next to nothing in the winter when going to need the ASHP the most. Have a look at PVGIS to see what you'll get.

Yup. +1. 

Would need to all be made up from

an off grid setup and mostly featuring Chinesium kit.

First thing to break would wipe out the assumed savings (but I doubt there would ever have been a saving here more of an expensive failed experiment, tbf).

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On 30/10/2024 at 15:29, PhilT said:

 

Missed this link earlier. Looks a serious product and close to what the OP proposed. There is also this which provides heating/cooling down to OAT of 5F from solar input, seems to include its own MPPT circuitry as well, which is even closer.

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That was always the Achilles heel of the original concept, short of interseasonal storage.

 

This morning the sun was out so I dried a load of washing and topped up the charge in my car. ATM it is in the middle of a Cosy cheap period so the HP is charging the thermal store, but it is now dull so I would have been fractionally better off doing the drying etc now having exported the surplus in the morning.

 

These things are what is just about feasible to do with PV in winter - but even with 6.9 kW installed I have no illusions about running  the HP off it until the clocks go forward again.

 

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