Air source heat pump and solar PV

[cjcoffin]

I have a 3.68 kW PV system and would like to use it to power a modulating heat pump of about 5 - 10kW output in such a way that during the day it only draws as much power as would otherwise be exported.

One installer has said you can do this with an Eddi. However from example 14 here https://myenergi.com/wiki/eddi/application-examples/ it is clear this is in an on-off fashion so does not modulate the heat pump output at all.

I already have a PV diverter box which could be easily modified to produce e.g. a 0-10V demand signal or possibly a digital output for a suitably receptive heat pump.

Can anyone tell me if there are heat pumps that are controllable by an external signal in this way and/or a suitable commercial PV controller?

TIA, Chris

 

[Swwils]

Isn't the modulation from the heat demand of the house not the available (which will be low when you need heat... Winter....) PV?

 

[cjcoffin]

Not quite that simple. I am thinking in terms of a large buffer/thermal store say 300 litres because we mostly need heat in the mornings and evenings and there is little heat demand in the house during the period when there is most solar PV available. The plan is based on doing most of the recharging at night using Economy 7, the solar as a daytime top-up is merely the icing on the cake.

Yesterday for example we got nearly 12kWh of PV (which was I agree hardly typical for January), which if multiplied by a COP of 3 say would fully heat the 300 l tank and the entirely separate 200 l domestic hot water tank and leave 15kWh left over as buckshee heat into the house or for running appliances.

Separately I have looked into OpenTherm as a digital option, and although it is fully specified with features for interfacing with heat pumps I have not been able to find a single ASHP mfr who claims OpenTherm capability, and only Kensa amongst GSHPs.

 

[Swwils]

Using the unused solar to heat water directly is a great idea, since the transfer is very efficient. No heat pump involved, just minimal inversion losses on what would have already been "wasted" or sent to grid at a pitiful rate. Effectively "free" (no taking into account PV install costs) hot water compared to using electrical price kWh or gas price kWh.

Do you use that much hot water a day though? 300L?

One other key thing to consider is that your heat pump will modulate output based on the external temperature. If it's 18C outside then no matter how much solar energy you can deliver to it it's maximum power demand will not be large.

 

[cjcoffin]

I already divert all surplus solar power to the hot water tank, but using a heat pump with a Coefficient of Performance of 3 would mean I would effectively get 3kWh into the tank(s) for every 1kWh the solar panels generate, leaving more for other purposes.

Anyway what I am proposing above is to heat a separate 300 l tank of water purely as a thermal store to supply the heating circuit when there is no solar output in the evenings. For this I would obviously need to run the HP at its highest available output temperature and turn off the weather compensation, the easiest way of doing this is to switch it to domestic hot water mode so it thinks it is heating a water tank (which of course it is).

The problem I have not been able to solve is how to modulate the HP according to the available solar power, as per my OP.

 

[Swwils]

I think you are forgetting that heat transfer in these situations are not spontaneous.

Deliberately using a heat pump like this will likely never recoup costs compared to a simple electric element.