Excess PV to drive heat pump DHW not immersion heater.

[Jwenty]

Hi

We are looking to install  an ASHP (vaillent arotherm plus 7kw.) in our mid terraced house (significantly renovated). We are also looking to install a small (3kw) PV system. 

 

I wont have the cash for PV + batteries and as they don't add any CO2 reduction overall I'm not that interested. But we will have the hot water cylinder as a good place to use excess solar. 

 

But it seems most systems just dump the PV into the immersion which is OK, but with a heat pump with with COPs of 2+ for DHW is seems it would be better to use the solar to drive the HP? 

 

Are there systems that do this?  or is it just better to set a timer on the DHW on the HP to coincide with the solar peak? 

 

Thoughts welcome. 

 

Many thanks 

 

Joe 

 

 

[Conor]

Just now, Jwenty said:

 

 

But it seems most systems just dump the PV into the immersion which is OK, but with a heat pump with with COPs of 2+ for DHW is seems it would be better to use the solar to drive the HP?

 

 

 

No. For the simple reason your heatpump will have a min draw of something like 1.5kW. Your excess PV generation could be something as low as 200W... Which would mean if your heat pump is on, it'll be drawing much more power from the grid, negating any COP advantage over a immersion coil.

 

The solar diverter will control the power to the immersion to match the excess generation, and can do this from a few Watts up to 3kW.

 

We'll be setting our heatpump if for cooling and time for during the peak afternoon... So if cooling is needed in theory most of the power will come from the 4.5kW array, and only some from the grid. At this time of year we're not producing enough to power the heatpump, so still on the immersion

 

[ProDave]

What you can do, is set the timers on the heating system so the ASHP does not turn on to heat DHW until 11AM.  By then solar PV production should be good, so it is likely the ASHP will be using up a good chunk of your solar generation.

 

It's about the best you can do, other than (if you are in all day) manually turning on the ASHP when the sun comes out.

 

That, and shifting all the big appliances to the middle of the day and the immersion diverter means I don't export much of my PV.

[SteamyTea]

There is power, kW, and energy, kWh.

Power is the rate at which you use the energy up.

 

A PV system is a relatively low power generator, usually a few hundred watts. It is only when you multiply that by the time it runs i.e. 8 hours of daylight, that the numbers start to look impressive.

So if you get 100W of power 2 hours a day (0.2 kWh), 400W for 3 hours (1.2 kWh), 2 hours at 1.7 kW (3.4 kWh) and an hour at 2.8 kW (2.8 kWh).

When you add that up, 7.6 kWh, it sounds pretty good. But you could only run a 2.8 kW load, purely off solar for an hour.

Luckily, because PV systems inject power (VxA) at a higher voltage than the grid, the nearest loads use that power.

So say you run a 6 kW load for an hour, and during that same hour your PV generated an average of 2 kW, you are actually only importing at 4 kW, so paying for 4 kWh of energy.

[Dan F]

 

1 minute ago, ProDave said:

It's about the best you can do, other than (if you are in all day) manually turning on the ASHP when the sun comes out.

This works, but isn't ideal either given the sun comes and goes a lot in the U.K climate and is of course manual.

 

If you want to use excess PV to drive a ASHP, while avoiding importing electricitiy, you really need a buffer and and some logic.  What you'd do is fire up the ASHP (for a minumum amount of time, x) based on i) Average PV generation over a period of time ii) Battery state of charge.  This condition would then be revaluated every x to determine to determine if the ASHP should stay on or turn off.  It's not trivial to set up, but this would allow you to consume smaller/intermittant amount of PV generation and still leverage your ASHP COP.

[SteamyTea]

7 minutes ago, Dan F said:

you want to use excess PV to drive a ASHP, while avoiding importing electricitiy, you really need a buffer and and some logic

Comes down to marginal losses and gains.

Heating a DHW cylinder or a buffer tank us cheap and easy.

And for 8 months if the year, will probably supply 80% if your needs.

 

[Dillsue]

1 hour ago, Jwenty said:

or is it just better to set a timer on the DHW on the HP to coincide with the solar peak? 

 

With a relatively small PV array, thats probably the best strategy with an easily accessible timer to so can tweak times easily if theres a bright/dull period forecast. Remember that in the winter when you need the HP most youll be generating the least and vice versa in the summer. Nice weather in spring and autumn should see usage vs generation a bit more balanced

[Jwenty]

Thanks all this makes sense and I see this issue with the ASHP min load. As none of its installed yet, i will  see what the actual loads and generation look like and start to have a play around. 

 

Thanks again. 

 

Joe 

 

[joe90]

4 hours ago, ProDave said:

What you can do, is set the timers on the heating system so the ASHP does not turn on to heat DHW until 11AM.  By then solar PV production should be good, so it is likely the ASHP will be using up a good chunk of your solar generation.

This is exactly what I thought of doing, also still diverting excess load (when the ASHP etc are not on) to an immersion to store “energy” in the form of hot water above the temp of the ASHP (48’ in my case) 🤔🤔

[ReedRichards]

For me the issue is not the ASHP min load  but the ASHP max load.  To heat my water to the required 50 C my heat pump works its way up to using 6 kW of power for around 10 minutes.  My PV array can maintain over 4 kW of output for a few hours during the middle of a sunny day but that is not enough to prevent the heat pump drawing power from the grid also.  But my immersion heater draws only 3 kW so its power requirement is better matched to the output from my solar array.  I can also use my immersion heater to heat the cylinder to beyond the 50 C achieved by my heat pump alone.

Edited April 8, 2022 by ReedRichards

[gmarshall]

There are solar diverters that can, in principle do this- I believe the Eddi with the optional relay board installed is pretty much the only option at present. The issues above all stand about available power from solar being low, and you'd need a big solar array to fully supply the heat pump, especially when the water is getting up to temperature, but technically, it can be done. 

Practically, I think almost everyone just uses the immersion as it's better matched power wise and simpler/cheaper to do, and supplies many peoples DHW demand for much of the year. But if you had a big DHW demand I could see the advantage of using the added efficiency of the heat pump.

 

Eddi manual - see page 60

[Jwenty]

thanks ReedRichards another interesting point. I think my case the PV would meet max ASHP load but will check. 

 

Thanks also Gmarshall, i will take a look. but for not it does seem the timing ASHP DHW demand and using immersion for the excess is likely the simplest. I guess it will be some experimentations with the actual loads and generation i can achieve. 

 

Thanks again. 

 

Joe 

 

[m0deller]

We have the aerotherm+ and use an immersion diverter (Apollo GEM). This diverts every excess watt into the HW, to typically we don't need the heat pump running May to Sept. 

What I've found however is that the immersion is mid tank and the Vaillant temp sensor is below the immersion , so getting an accurate reading is difficult. I schedule the HW circulation pump to run around mid day to mix up the water (get more energy from the solar PV) .. but this only works if the HW is enabled on the Vaillant controller. It's too smart for its own good!

[Jwenty]

Another cleaver work around. I'll be sure to have a look at the immersion location. 

 

Thanks 

 

[Nickfromwales]

Why not heat the cylinder fully from a ToU tariff such as Octopus Go overnight, via the HP, and then pump any excess during the day into the immersion to hyper-inflate the energy storage capability of the UVC. Nighttime charge to 50oC then reduced by whatever was collected during the day, with the boost from the immersion going as high as 70-80oC. You will get the better CoP with running the HP, but only at the lower temp plus you'll be cycling the HP on and off with the pockets of solar gain, which it will NOT thank you for.

Once you calculate the wear and tear on the HP, PLUS those attributed losse of getting all of the HP components up to temp for each event, same with all of the interconnecting pipework etc, eg before it collectively then delivers premium temp hot water to give the stated recovery times, .......then the immersion route soon becomes a more attractive choice afaic.