Using PV energy to run a heat pump?

Roseybug

Hi, I'm unfamiliar with all the technology but exploring options to update my house and replace a last-century combi gas boiler (not condensing) with ASHP + HW tank. 

Will also need a re-roof (+ insulation) as currently between my bed and the stars there is a layer of plasterboard, 10cm-ish of air, then some slate. (Rooms in roof, uninsulated). External wall insulation also planned, subject to affordability.

I have a south-facing roof, not huge because it has dormers in it, but think I could fit about 2 kW of PV onto it, which according to PVGIS modeller would give me c. 6-7 kWh per day for April-Sept, less in the winter.

In order to use summer PV power cost-effectively - most of the stuff I've read seems to suggest using what you can in the day and shunting any excess to a hot water tank via an immersion heater, with any surplus sold back to the grid (or stored in a battery). I currently use about 4 kWh/day of gas for hot water heating. Current daily electricity use is c. 4 kWh in summer. Doesn't look like I will generate sufficient excess for a battery to be economic.

My question - if my normal hot water heating source is the ASHP, wouldn't it be more efficient to get the ASHP to use the PV electricity to heat the water at a COP of 3ish, rather than an immersion? Or are there voltage/technical reasons that make this impossible?

Thanks for all the discussion threads so far; I've read many of them

QrizB

Immersions are cheap and mostly foolproof, and if you get an ASHP you'll almost certainly have an immersion heater too.

Yes, in principle it would be more efficient to use the surplus PV in the ASHP rather than the immersion heater. The problem will be matching the ASHP to PV output; unless you've got a perfect summer's day PV output goes up and down with every cloud, so you risk running the ASHP on partly-imported power.

Immersion heater diverters monitor the meter tails looking for export and adjust the power of the immersion up-and-down automatically, so you don't get these problems.

Roseybug

Ah, I get it (I think), thanks for explaining. The ASHP is either completely off or full-on at (e.g.) 3 kW, which might be 2kW from PV and 1 kW being drawn from grid, then 3 kW from grid when a cloud goes over.

Whereas a 3kW immersion will just take the 2kW from the PV and run at 2/3 of its max power when it's sunny, then just stop for a bit when the cloud passes by?

Roseybug

Sorry, just thought of another question - in the shoulder months like March and October when I am likely to need some heating anyway - at that point then presumably it's sensible to keep the heat pump on during sunshine hours, and it will automatically suck up the PV electrons and take whatever it needs from the grid?

Then I can turn off the ASHP for the evening peak hours, and back on overnight if needed, when grid energy may be cheaper.

Reed_Richards

Roseybug said:

My question - if my normal hot water heating source is the ASHP, wouldn't it be more efficient to get the ASHP to use the PV electricity to heat the water at a COP of 3ish, rather than an immersion? Or are there voltage/technical reasons that make this impossible?

Yes it would, however:

1. An immersion heater takes 3 kW (or the vast majority do).  Whereas to heat your hot water with the heat pump might require 6 kW for a short time.  Many PV systems can generate more than 3 kW at peak output.  Therefore you can possibly heat your water entirely from solar electricity via the immersion heater but using the ASHP requires you to import some electricity from the grid.  
2. If you installed solar panels prior to April 2019 you are probably registered under the Feed-in Tariff scheme.  Normally under this scheme you don't get paid for the electricity you actually export.  So the solar electricity you generate is free if you use it but has no value if you don't.  That gives you a strong incentive to use only your solar electricity wherever possible. 

Neither of these considerations would apply in your case so just go ahead and use the ASHP to heat your water.  

Screwdriva

A couple questions:

1. Is your installation Heat Geek assured? They appear to enjoy the best reputation for consistency of installations. 

2. Have you decided which heat pump brand to get? I believe Vaillant AroTherm is best available option when achieving the highest possible SCOP is the goal.

I ask these questions because a well designed and installed heat pump solution will not require the immersion element for a weekly 65 degree legionella cycle (in theory 75 degrees is achievable) for a hot water cylinder.  Same goes for a SCOP of 4 for central heating. 

PV will always help reduce costs but it just doesn't work well during the winter months (think 5-10% of summer month's generation). However, you will earn quite the balance selling energy in the summer to offset against the winter month's bills. 

Roseybug

Thank you both.

Haven't got as far as choosing installer or heat pump brand yet. Will look into Heat Geek. 

I have a bit of a chicken-and-egg problem with timing of heat pump and roof installation, for two reasons - the BUS grant needs you to have loft/?roof insulation in place to be eligible. But I can't put roof insulation in place until I replace the roof. And as soon as I replace the roof I will have to ditch the gas boiler (because its flue exits through the roof and I don't want an unnecessary hole in the new insulated roof). 

Therefore I need an installer who will assess the house as it is planned to be, rather than as it is now. I did contact Octopus, but the guy seemed to be reading from a script and said they couldn't start the process unless there was loft/roof insulation in situ, and an EPC (I don't have one, have lived here too long, but it would probably be an F). Really it all needs doing at the same time as otherwise I will be without hot water. But I also need the BUS grant for it to be affordable. Struggling to get my head around it all.

70sbudgie

Have you considered PV roof tiles? They are more expensive, but might not be if you need to replace your roof anyway. I don't know what the grants are, but it might bring you roof replacemei VAT down?

Only thing is you said you have slate tiles, so you might need to check planning. I think a PV roof is considered more permanent than panels. 

Reed_Richards

@Screwdriva had the opportunity to install a heat pump but instead chose a fancy gas boiler https://forums.moneysavingexpert.com/discussion/6345707/going-green-er-with-a-viessmann-gas-boiler-wet-underfloor-heating/p1 .  Yet they have some romantic ideas about what a heat pump can do.  Most heat pumps have a maximum output water temperature of 55 C.  Some go higher for a price premium but why would you want to?  The only reason is for a one hour a week Legionella cycle of your hot water cylinder.  Instead use the immersion heater to raise the temperature from 50 C to 60 C.

The most important thing to understand about running a heat pump is to minimise the running cost you keep the output water temperature as low as you can get away with.  You cannot quote an SCoP figure out of the air without the context of a maximum output water temperature.  To achieve an SCoP approaching 4 you would need underfloor heating throughout.  @Screwdriva bought a Vaillant gas boiler and seemingly still likes the company despite the problems their installer experienced fitting the boiler.

I recommend you get your heat pump installed to MCS standards so you get an MCS certificate at the end.  Heat Geek are a gimmicky outfit who post a lot of YouTube videos, have a look at some and form your own opinion.      

ccbrowning

Our heat pump just has some weekly cycle to push it to 65c. That said, I have just been using the immersion tank's heating since our heat pump has some fault and at least it doesn't seem to draw more than the PV generates during the summer anyway.

When the heat pump broke and the immersion wasn't switched on it still took us a long time to notice since we were only using the hot water for showers anyway. 

Screwdriva

Reed_Richards said:

@Screwdriva had the opportunity to install a heat pump but instead chose a fancy gas boiler https://forums.moneysavingexpert.com/discussion/6345707/going-green-er-with-a-viessmann-gas-boiler-wet-underfloor-heating/p1 .  Yet they have some romantic ideas about what a heat pump can do.  Most heat pumps have a maximum output water temperature of 55 C.  Some go higher for a price premium but why would you want to?  The only reason is for a one hour a week Legionella cycle of your hot water cylinder.  Instead use the immersion heater to raise the temperature from 50 C to 60 C.

The most important thing to understand about running a heat pump is to minimise the running cost you keep the output water temperature as low as you can get away with.  You cannot quote an SCoP figure out of the air without the context of a maximum output water temperature.  To achieve an SCoP approaching 4 you would need underfloor heating throughout.  @Screwdriva bought a Vaillant gas boiler and seemingly still likes the company despite the problems their installer experienced fitting the boiler.

I recommend you get your heat pump installed to MCS standards so you get an MCS certificate at the end.  Heat Geek are a gimmicky outfit who post a lot of YouTube videos, have a look at some and form your own opinion.      

My "ideas" are based solely on what my installer has  shared - in this video example, that DHW at 65 degrees is achievable on the Vaillant AroTherm heat pump without immersion element use. My romance is reserved for my responses to any incorrect information shared on these forums. @Reed_Richards above post is a great example.

I purchased a Viessmann V-200W gas boiler (not a Vaillant), as it is the most efficient boiler sold. I did so for two reasons: 1) the installation cost for the boiler was £6K lower than the cost for a heat pump install and projected heat pump savings weren't enough to recover this cost over the following decade. 2) The supply of Arotherm pumps were extremely limited - I was quoted a 3 month wait. 

I stand by my recommendation for the Heat Geek assured installation and would suggest you reach out to Urban Plumbers incase you're in the Greater London Area.

70sbudgie said:

Have you considered PV roof tiles? They are more expensive, but might not be if you need to replace your roof anyway. I don't know what the grants are, but it might bring you roof replacemei VAT down?

Only thing is you said you have slate tiles, so you might need to check planning. I think a PV roof is considered more permanent than panels. 

Wise recommendation. GBSol will offer you a free quote for solar slate tiles which will be much cheaper than PV + new tiles. And much better looking!