Solar Panels and Air Source Heat Pumps

Redlander

We have thought about getting an air source heat pump when our combi boiler packs up. We have also thought about getting solar panels, perhaps with a battery.

What I'm wondering is whether these two installations interact with each other. For instance, supposing we got solar panels first and waited until the boiler failed before getting a heat pump, would it be advisable to make some kind of provision for the heat pump in the installation of the solar panels?

ComicGeek

You generally need more space heating when the sun isn't shining, so very rare that the running of the heat pump would coincide with the best yield from the solar panels. You might get a few days in March and October when it might work for a few hours, but not enough to design a whole system around.

The exemption is production of hot water in the summer, but you will need to find space for a hot water cylinder for that - one major advantage of your current combi is that this isn't needed.

Batteries work great with solar panels, particularly if the solar installer has massively oversized your system (as most do). But a heat pump system is going to drain a battery very quickly in the winter, even if you've charged it up every night on a cheap tariff.

Personally I always view solar and batteries as doing everything else apart from heating - if it works for a few hours each year to run the heat pump then that's a bonus. But don't be fooled into thinking that you can use 100% of the solar panel generation throughout the year - the peak yield will come during the summer months when most people's electricity use drops significantly, and will be tiny in the winter months when you have the highest usage.  

007apm

I am trying to work out heat pump economics, already having a 6KWP solar array and 14KWh usable battery.  We are considering getting an EV/PHEV and qualifying for very cheap overnight electricity (7.5p/9p per unit) but 31p/unit day time.  We charge the house batteries overnight and guess that the car will also fit in to the cheap electric window without exceeding the single phase capacity.  In December, 60% of the time we had some spare battery capacity and could add a further 2.4KWh for about £1k.  A panacea solution would use cheap rate electric + heat pump to heat a tank of hot water overnight and then use spare battery capacity to power the 'central heating element' of the system / electric hob during the day. If that works then the 6k investment has potential.  If not and the heat pump draws most of the energy during peak hours  and/or we need significantly more battery capacity then it is uneconomic.

Mutton_Geoff

I worked out the costs using just a battery charged overnight at cheap rate (10p) feeding a 4kw ASHP for 3 hours a day when it would then consume 29p electricity. At best you'd get one battery charge per day. Payback was >10 years and that was without the cost of PV panels.

007apm

Thanks, this suggests that the ASHP just needs 2.9 KWh per day which is about 1/4 of what i expected?

Mutton_Geoff

007apm said:

Thanks, this suggests that the ASHP just needs 2.9 KWh per day which is about 1/4 of what i expected?

Don't forget the COP.

ComicGeek

007apm said:

Thanks, this suggests that the ASHP just needs 2.9 KWh per day which is about 1/4 of what i expected?

What is your current heating system? You need to work out how much delivered heat you need for an average day, to see whether it's feasible. Big difference between a mid terrace house with a 5 kW heat pump and a large detached house with a 14 kW heat pump. It's definitely not going to be as low as 2.9 kWh per day. 

007apm said:

I am trying to work out heat pump economics, already having a 6KWP solar array and 14KWh usable battery.  We are considering getting an EV/PHEV and qualifying for very cheap overnight electricity (7.5p/9p per unit) but 31p/unit day time.  We charge the house batteries overnight and guess that the car will also fit in to the cheap electric window without exceeding the single phase capacity.  

Do you currently get a cheap overnight rate when charging the house batteries? If not, then that is actually costing you more money than not charging from the grid at all, due to conversion losses into and out of the battery. 

I've got a 17.5 kWh usable battery system, and use around 15% of this per hour of space heating in our house - that's without every room being heated, so can easily drain the battery running the heating for 6 hours on a cold day. That's before I use 10% on cooking each day, 30% on appliances/power/lighting, 30% for laundry etc.