Solar PV to heat water for central heating

Freepost

Just an idea because I don't use all that I generate. I have underfloor heating with a system boiler (gas) and a hotwater tank that is heated by both the system boiler and the solar PV. So the question is could I have a 2nd hotwater tank installed, heated by the solar PV that would feed into the underfloor heating system?

70sbudgie

I am going to follow this with interest, as it is something I have wondered.

We also have a system boiler, but I invested in one with a dual coil (I was future proofing with a view to solar thermal, but haven't ever been able to make the financials stack up vs a PV immersion divertor). I wondered whether I could use the immersion (from PV) to heat the tank and perhaps the second coil to feed into the heating?

My husband used to live in a house where the heating controls seemed to only control the pump (no room thermostats), so the hot water had to be on at the same time otherwise the radiators never got warm. It seemed a weird set up to me, but got me wondering whether something similar could be plumbed to take advantage of solar heating.

QrizB

The problem you'll have is that the months with surplus solar aren't generally the ones where you need space heating.

There are a handful of forum members who have fitted air-to-air heat pumps specifically for those months, but the advantage ther is that you get 3-4x as much heat out as you would with a resistance heater.

Solarchaser

Can it be done, sure.
Would it be worth it, well due to there only being maybe 4 months of the year that are both bright and cool is the only times it will likely help.
Also the water in the second tank may have to be temperature controlled if its for floor heating (I don't have floor heating by water, but note most sources suggest a lower temperature than radiators)

ccbrowning

Our home is fully electric, but we do have two separate large tanks - one for heating and one for dhw. Temperature-wise, the dhw is normally a higher temperature plus it has to go even higher on occasion due to some regulation related to legionella. 

We don't have a diverter (yet), so I've just been using timers or manual intervention to take advantage of the solar pv on sunny days, which is great for half the year at least. That said, winters are often dark and rainy with not so many potential sunlight hours...

70sbudgie

I obviously can't talk for the op, but my interest is whether it can be done to reduce the imported energy to a property. I am well aware that most PV generation doesn't tie up with most heating demand.

But I am also aware that heat pumps are also significantly less efficient at these times (probably only about 150% efficient), so a I believe it is worth exploring all possibilities.

I don't know if anyone else is aware, but the efficiencies quoted about heat pumps has changed recently. Previously heat pump efficiency was quoted as the theoretical max for that pump. Which is great for grabbing headlines. However, to make them less misleading, they now have to quote a "seasonal average". This takes into account, but still hides to a certain extent, the significant reduction in efficiency at the time when it is most likely to be required. 

I can't remember the exact figure, but if the temperature differential is 25°C or more, (between demand temp and source temp) the unit efficiency is seriously compromised. This is why heat pumps struggle to provide dhw which typically needs heating to a min of 55°C and why they are so much more efficient with underfloor heating which typically only needs heating to 40°C.

Reed_Richards

70sbudgie said:

I can't remember the exact figure, but if the temperature differential is 25°C or more, (between demand temp and source temp) the unit efficiency is seriously compromised. This is why heat pumps struggle to provide dhw which typically needs heating to a min of 55°C and why they are so much more efficient with underfloor heating which typically only needs heating to 40°C.

I don't believe its a "cliff edge" situation.  The theoretical maximum efficiency is inversely proportional to the temperature difference and I think that will be broadly true of practical efficiencies.  There are High Temperature heat pumps that can achieve an output of 60 C and more but I'm afraid I don't know the technical difference between heat pumps that can do this and those that can't.  They cost more.   

ccbrowning

How do you tell what is optimal for your heat pump? The people who installed ours aren't very good on this sort of question - they know how much it helps with the speed of hot water availability, how quickly it makes the house warm up, etc., but now the impact of the settings on efficiency. The only number I was told is that if you use the element in the dhw immersion tank to heat it is 1:1 but the GSHP doing the same thing is 3:1, so 3x more efficient (in theory?).

70sbudgie

Reed_Richards said:

70sbudgie said:

I can't remember the exact figure, but if the temperature differential is 25°C or more, (between demand temp and source temp) the unit efficiency is seriously compromised. This is why heat pumps struggle to provide dhw which typically needs heating to a min of 55°C and why they are so much more efficient with underfloor heating which typically only needs heating to 40°C.

I don't believe its a "cliff edge" situation.  The theoretical maximum efficiency is inversely proportional to the temperature difference and I think that will be broadly true of practical efficiencies.  There are High Temperature heat pumps that can achieve an output of 60 C and more but I'm afraid I don't know the technical difference between heat pumps that can do this and those that can't.  They cost more.   

I agree, I don't think it is a cliff edge. But I think there is a threshold where the reduction in efficiency is suddenly noticeable. I will try to locate the article I read about heat pump efficiency.

70sbudgie

This is it. 

https://www.isoenergy.co.uk/latest-news/renewable-energy-news-from-isoenergy/what-size-ground-source-heat-pump-do-i-need-for-my-home

Although, I believe the point of the article is to sell heat pumps and therefore I haven't paid too much attention to the standard figures given, I did find the explanation of CoPs interesting and informative.

QrizB

70sbudgie said:

But I am also aware that heat pumps are also significantly less efficient at these times (probably only about 150% efficient), so a I believe it is worth exploring all possibilities.

I think you';ve got some duff info there. Yes, there will be circumstances where heat pumps are only 150% efficient but for a typical domestic heat pump these will be few and far between.

All major heat puimp manufacturers publish data books which show the performance of their heat pumps under various conditions. For example here is a recent Mitsubishi data book; this link will take you to the page for a range of air-to-water heat pumps:

https://library.mitsubishielectric.co.uk/pdf/book/ATW_Databook_R32_2020#page-60-61

Looking at the top-left table, you'll see that with an ambient temperature of -10C and a water flow temperature of 50C you're still getting a COP of 2.06. The number of times a year when the ambient temperature is -10C but there's also a surplus of electricity from a solar PV system is pretty low, even in the frozen North of the UK. Here is the subtropical South I don't recall it ever happening.

At more common temperatures, as you might expect in the shoulder months where you have surplus PV, COPs are much higher.