Solar Diverter to Control GSHP

[Benguela]

Hello all,

 

I have a GSHP and am getting some solar panels installed soon.

 

To use my excess solar, I'm getting a solar diverter.

 

Now, most solar diverters divert your excess power to an immersion heater and immersion heaters have a CoP of 1. So I thinks to myself: 'why don't I divert my excess solar to switch on my GSHP whenever I have excess solar and with my CoP of 4, I turn 1kWh of excess solar into 4kWh of heat.'

 

Granted, when it comes to producing DHW, my CoP is probably between 2 and 3... but that's still more efficient than my immersion heater. And on the odd sunny winter's day, I might run my GSHP on free solar for an hour or two mid-day.

 

Problem is I can't find a diverter that is designed to switch a heat pump on and off... they can control immersion heaters, car chargers, even individual sockets, but nothing seems to be optimised to control your heat pump.

 

Any ideas why that is? Or could such a thing be made?

[severnside]

myenergi Eddie comes with a relay extention module. You can configure relay outputs based on certain conditions

e.g. Turn on relay 1 when solar output is more than 4kW. Also do it for a minimum of x mins and don't start again for y mins (for proper cycling)

https://myenergi.com/product/eddi-relay-sensor-board/

Edited March 15, 2022 by severnside

[Wil]

OR buy a Shelly EM for £50 which can monitor your export. If it reaches a level you're happy with, switch on the built in relay and enable the HP (assuming your HP can take a simple 'on-off' switch input). When the export drops below a certain level switch it off.

 

Probably add a timer so you don't flick on and off as the sun goes behind a cloud.

 

Your GSHP is unlikely to get the water as hot as an immersion, so depending on how much solar you have to divert, it can be a lot simpler to use a COP 1 Immersion to get the water really hot...

[billt]

Standard solar diverters are incompatible with heat pumps. The principle of the diverter is to switch the immersion on when generation exceeds local consumption and switch it off when consumption exceeds generation. Because local consumption changes from second to second, as does PV production, the immersion is switched on and of very frequently. An immersion heater can cope with this but it won't do a heat pump any good at all.

 

If you want to feed excess solar production to a heat pump you are going to need a more sophisticated solution, possibly battery storage, which will be expensive and inefficient or a delay system to make sure the heat pump runs for some time after it has been started, but that is likely to use mainly grid supplied power rather than locally generated power.

[ProDave]

Even if you were going to control a heat pump via the relay from an eddie I would not do it by turning the power on and off, instead it would be via a thermostat or call for heat input.

 

I just have the timer set so DHW heating only starts at 11AM on the hope the sun will be out by then.  A lot simpler.

[Ajn]

4 hours ago, Benguela said:

Any ideas why that is? Or could such a thing be made?

You can't directly switch the HP off and on because the solar is not constant. The diverters pulse the immersion heater with the solar surplus, you can't do that with a heat pump.

You need a battery system. Solar charges the battery with the surplus and the battery via an inverter provided the constant output required for the heat pump.

You would need to know how much surplus solar you will get. How much your HP needs including any starting currents. How long you want to run the pump on your solar, to size the battery requirement. Then you will need to switch to the battery at the required time. If only the HP was connected to the inverter then this could be done by the battery and inverter based on the power in the battery.

[Benguela]

Great answers, thanks guys.

 

I totally understand: the excess solar isn't constant and if I tried to divert the excess solar to my heat pump, I would in effect be forcing it to short cycle.

 

Severnside and Wil suggest some good ideas to smooth out the short cycles.

 

But I'm going to take away that this solution needs a bit of thought!

 

[SteamyTea]

You can take a statistical approach.

If you know the most likely times, which is based on your system angles and geographic location, you can make sure your HP is set to heat water at those times only.

There will be times when it will be more expensive, but overall you should get a lower running cost.  This does depend on how you set up your electricity tariff, and your other electrical loads.

So first things first, get some data, especially about your current usage.  If you think collecting data is expensive and hard work, the price of not doing it is much higher.