Using an UVC as a buffer for ASHP & PV heating

[Brian Paul]

I am in the process of designing (with architects!) and commissioning a small low-energy house which will rely mainly on solar pv for energy. There is no gas at the property.  I have over 15 years experience of solar thermal and solar pv.  My current house uses solar pv exclusively for hot water for more than 7 months per year, uses gas for ch & backup hw for colder months. My current 3 immersion design cylinder has proved very effective. 

I have trawled through various heating and dhw designs including thermal stores and multi-tank solutions but cannot reconcile the different requirements (eg thermal stores need a good temperature gradient to provide dhw - heat pumps provide 55º  as a working maximum).  I have also noted comments on this forum. (“Hot water and heating are different – keep them separate!”)

However I’m still looking for an efficient & effective solution.

I am considering using a large unvented cylinder (say 500 litres) which will also act as a buffer for a heat pump and a thermal store for underfloor heating.  For at least 8 months of the year the total input would be from solar pv through immersion heaters.  For the colder months there would be the possibility of input from a heat pump. Primary heating would be through 3 immersion heaters. I have included a diagram.

I see the following as advantages :

1. A single tank with straight-forward plumbing. No complex valves or diversions.

2. Separation of circuits for input/output.  Antifreeze needed only in the heat pump circuit. - Better heat transfer to UFH with water than with antifreeze mixture. (and much less of it)

3. Ability to maximise use of solar pv (8kW) throughout the year.

4. Possibility of running as a fully electric system without heat pump - under consideration for at least one year to assess relative costs. Also provides alternative heating if heat pump fails.

5. Typically : we boost heat in the mornings (before 9.00) and in the late afternoon/early evening. Background temperature maintained at other times (although with a good store in the slab this will become less pronounced).  Best efficiency from an ASHP will be when the outside temperature is highest and the best use of solar pv will be between 9.00 and 17.00. So having a facility to store heat at these times will maximise both efficiencies.

I recognise one potential problem:

In our current location, with excessive water hardness, cylinders frequently deposit more than 0.5kg calcium salts per year. We have completely avoided this by quality water softening and would do so in the new location. 

Although the physical design is simple the efficiency will depend on the control circuits. I propose to design & program these myself rather than rely on the current commercial provisions which seem to be over-complicated but still limit options to those provided by manufacturers.

 All of the plumbing would be installed by G3 qualified tradespeople.

So I would welcome constructive guidance …

 

1. Is there any commercial system which already follows these lines which I’ve missed?

2. Has anyone on this forum taken a similar line – with what results?

3. Are there any genuine down-sides which I’ve missed?

 

[Brian Paul]

Sorry - illustration in first post got caught out by file size. Correct one to follow

[PeterW]

2 minutes ago, Brian Paul said:

Although the physical design is simple the efficiency will depend on the control circuits. I propose to design & program these myself rather than rely on the current commercial provisions which seem to be over-complicated but still limit options to those provided by manufacturers.

Ask yourself the question who will maintain this when you can’t ..? A number of members here started down the custom controls track, and some have removed them as the need for such granular control is not necessary. 
 

Your issue will be one of stratification in a UVC (or lack of) and where your coils go. Most tank manufacturers will only give you two coils at best that will be enough to get heat in and assuming you use the solar coil, heat out for UFH. 
 

A better design would be to mirror in some way what @TerryE has done and use the slab as the “store” for heating and load shift to an overnight tariff. You could if you wanted use a pair of 250 slimline UVCs side by side and use one as DHW but the other as the buffer. Use the UVC coil in the buffer tank as a preheat circuit to your DHW UVC and you’ve pretty much got a standard set up. 
 

UFH then would be on the buffer side, and would give you a tank for secondary PV diversion and also you can decide to either use PV, E7 direct, or a small ASHP to heat the tanks.

 

9 minutes ago, Brian Paul said:

Best efficiency from an ASHP will be when the outside temperature is highest

That isn’t actually true ...! Best efficiency is when the RH is at its lowest for the temperature. 
 

10 minutes ago, Brian Paul said:

Antifreeze needed only in the heat pump circuit. - Better heat transfer to UFH with water than with antifreeze mixture. (and much less of it)

Again - not sure where you have got this but a 25% antifreeze mix will be virtually identical for heat transfer at the temperatures you’re talking about. Yes it’s better for having limited volume because it’s cheaper but not much else. 
 

 

[ProDave]

My experience has been 4kW of solar PV is dead easy to self use all year round.  8kW will not be so easy, even with a lot of water to heat to store energy.  This is the point where I think it makes sense to look at battery storage.  You do have permission from your DNO for 8kW PV don't you?

 

My approach is much simpler, I run the UFH direct from the ASHP and use the ASHP to heat the UVC for the DHW.  I time the ASHP to do it's DHW heating after 10AM when I can be reasonably sure there will be a good level of PV output by then, so for at least half the year most of the DHW power comes from the PV.

 

Assuming you get an UVC with a high capacity heat pump input coil, I am not sure it will have much extra room for other coils?

 

Regarding G3, if you know the plumber you can actually do a lot yourself.  Run all the pipework to the cylinder and just get him to do a couple of hours work to make the connections, test it and sign it off.  All the output plumbing to the various taps in the kitchens and bathrooms anyone can do.  I even installed the D2 discharge pipe because that needed to be done early in the build, and the p;umber was happy just to see photographs of the hidden bits and connect to it.

[SteamyTea]

17 minutes ago, PeterW said:

Your issue will be one of stratification in a UVC (or lack of) and where your coils go

Yes, you get 'clumping'.  Stratification does not really happen, more a case of a spatial-temporal changing temperature gradient.

There is probably good reasons they are not common.

[Brian Paul]

Thanks to PeterW, ProDave and SteamyTea for these comments. Perhaps a more conventional arrangement would be better - now looking at 2 tanks again.

 

ProDave : 4kW or 8kW? I agree that it would be difficult to use 8kW easily in the summer (possible production up to 80kWH per day) but I do plan to provide at least 1 charger for EV - 2 if it seems justified. If the network supplier limits output to 3.6kW I would use a limiting inverter to ensure that happens.  The proposal for 8kW is more to do with getting best use in the darker months where a doubling of input can make a much bigger difference and provide enough for ashp even at reduced sunlight levels.  Batteries are possible - but not until they are more cost effective.

[SteamyTea]

21 minutes ago, Brian Paul said:

Batteries are possible - but not until they are more cost effective.

You could dedicate part of your PV to DHW.  That could reduce the need to limit exports.

You should be able to work out, for your orientation (that's panels, not long girly showers) and location the best proportions to get you maximum efficiency.

[Nickfromwales]

35 minutes ago, Brian Paul said:

If the network supplier limits output to 3.6kW I would use a limiting inverter to ensure that happens.

That will be a decision the DNO will make based on the local infrastructure, and may be refused. Eg they may say 5kWp and export limitation at 3.96 etc, instead of limit at 3.96 and you can have the full 8kWp. Don't assume they will give you this, and then go and base all your assumptions on that for the synergy of the M&E side. Also, if you box clever and plan for the retrospective addition of a DC battery, then the DNO application will ignore your batteries altogether. If you go for an AC battery system you will need DNO approval and the two figures ( PV + Batteries ( total kW size combined )), will be the number they use. DC batteries are designed to mothball themselves and hibernate when there is no current / no long term excess PV available to store, and are warrantied to operate that way too vs AC which need babysitting and constant topping up to stay within acceptable state of charge. 

[Brian Paul]

On 01/03/2021 at 13:27, Nickfromwales said:

That will be a decision the DNO will make based on the local infrastructure, and may be refused. Eg they may say 5kWp and export limitation at 3.96 etc, instead of limit at 3.96 and you can have the full 8kWp. Don't assume they will give you this, and then go and base all your assumptions on that for the synergy of the M&E side.

 

I made an initial enquiry to the DNO (Western Power) They came back very quickly ...and said - where available - all new connections are now 3 phase - and would be in this case.  I was advised that they would allow 11kW export over 3 phases with no additional restrictions.  

 

It does change the thinking a bit but welcome news anyway.  Many thanks to NickfromWales for this advice

 

[ProDave]

24 minutes ago, Brian Paul said:

 

I made an initial enquiry to the DNO (Western Power) They came back very quickly ...and said - where available - all new connections are now 3 phase - and would be in this case.  I was advised that they would allow 11kW export over 3 phases with no additional restrictions.  

 

It does change the thinking a bit but welcome news anyway.  Many thanks to NickfromWales for this advice

 

That's just the standard G98 16A per phase limit that they cannot refuse.  So it sounds like they WOULD play hard ball if you want to go above that.

[Trw144]

1 hour ago, Brian Paul said:

 

I made an initial enquiry to the DNO (Western Power) They came back very quickly ...and said - where available - all new connections are now 3 phase - and would be in this case.  I was advised that they would allow 11kW export over 3 phases with no additional restrictions.  

 

It does change the thinking a bit but welcome news anyway.  Many thanks to NickfromWales for this advice

 

Useful to know - I've just finished submitting a request for a new connection to Western Power this evening and requested three phase if possible. 

[Bramco]

On 28/02/2021 at 09:45, Brian Paul said:

My current 3 immersion design cylinder has proved very effective. 

 

Just out of interest who supplied the 3 immersion heater cylinder - we're looking for a 2 immersion heater indirect cylinder with the immersion heaters at the middle and towards the bottom, so excess solar PV can go in the middle until it is up to heat and then at the bottom.

 

Simon