UKH

Does anyone have any advice or experience of using a thermal store for domestic hot water, where the store is about 20 meters away from the house?  

Context - I’m thinking of having a thermal store in an outbuilding with a heat pump cooling rooftop solar-PV arrays (which get very hot in the sun, to the detriment of their electrical efficiency) into the thermal store.  I’m looking to get ~9 months / year of hot water from this. 

My main worry is that the length of pipe run means a stupid lag on domestic hot water from turning a tap on in the house.  Some Googling didn’t return any great advice.  I was thinking about having a small thermal store in the house, and a closed loop between a coil in that and a coil in the large, outbuilding store, with a pump on some control loop that circulates fluid to heat the small one off the big one, and the small one heats the domestic hot water.  With lots of lagging I wouldn’t expect much efficiency losses from this, but the small tank is going to run a bit cooler than the hot one.

All thoughts welcome!  

I think you'll get much greater efficiency losses than you are expecting...

Sounds like a good hot tub option?

Yeah, local heat exchanger and store seems sensible. Unless the lagging really is spectacular...

Then you need to deal with the losses from store to exchanger, and you still need good lagging. I did wonder about permanently circulating this store to exchanger loop, but that would only encourage losses, and pumping.

The smaller the bore, the lower the latency? But possibly quicker cooling, due to reduced mass.

Dual heat exchanger sounds like reduced efficiency to me.

No practical experience, but when I saw samples of insulated pipe for district heating the pipe looked surprisingly normal, less than 10cm diameter including insulation. Presumably that's branch pipes for individual house.

https://www.isoplus.co.uk/district-heating/

If they were chained like I’d suggested, the big one would run hotter than the small one, driving greater losses from the big one, which I think is the efficiency loss of that setup - as well as increased surface area vs one thermal store and losses in the pipes, but the later two are addressable through more lagging.

I just wondered instead about pumping the storage fluid directly around the two tanks rather than moving heat via a loop with heat exchange coils in each.  This way there would be less of a temperature difference due to the decreased thermal resistance between the tanks. Edit: although that’s probably not so different from circulating the heat pump’s output to a store in the house.  Trying to figure out the most efficient solution is probably on a hiding to nowhere.  One constraint is that there’s not much space in the house for a chunky thermal store.

A bit of smarts should be able to do the pumping in bursts to reduce transfer losses.

In reply to elsewhere:

I like that way of looking at it - a mini district heating system. How very un British!

I couldn't quite find the one I was looking for, but this is close.

http://mibec.co.uk/products/pre-insulated-pipe/rehau-systems/

The pipe spec is 0.0225 W/m°C

Mass 1 m of water 25mm pipe with pi=3
3*(0.025/2) = 0.04 (40g)

Heat capacity 1m of water
0.04*4000=160J/m°C

Time constant for to cooling down
160/0.0225 = 7000s = 2hrs

That seems a very long time, needs a sanity check.

The first 20m (800g) of burst pumping may be cold, although if you pump 20 litres, who cares?  

Do you prefer to waste water or heat or both?

I could waste some heat, I’d rather not waste the water but the key thing is I don’t want to waste time waiting for hot water to come out.

Nice, thanks.  

The rest of the world just runs a recirculating system, there used to be one available using a small-bore tube threaded down an existing pipe for retro-fit. Alternatively they just pump back down the cold feed.

I can't add anything to the actual thread, but I've often wondered what is manufactured at the Rehau factory in Tan y grisiau. 

UPVC window frames and doors, as far as I know..

Run the heat pump between store#1 and store#2 instead?

Put the water tank on the roof, next to the pannels. That's the setup that is used everywhere I've seen solar thermal, it saves on material costs, it means the hot water supply can be gravity-fed, it cuts down the lag in getting hot water to a minimum... The solution you suggested just sounds very complicated, expensive and un-optimised to implement.

Several problems with that.

1. 0% chance of getting planning for a rooftop tank in our conservation area.

2. A rooftop tank would still have a long run and I’m not keen on waiting for the long line to run through to get hot water.

3. I’m talking about solar-PV that is cooled by a heat pump, not a solar thermal water heater.  The idea is you stop them getting very hot in sunny weather, increasing their efficiency and use them as a large collecting area for low grade heat in non-sunny weather - which requires a heat pump.  Cheaper than a GSHP and not going to make all the annoying noise of an ASHP.  

Eh? Solar thermal is fairly standard for new builds in Germany and not one has an external tank. Freezing is a real hassle!

Interesting. Benefits to running the refrigerant loop between buildings instead of a water loop.  Not helping narrow down the choices!  

Figuring out how to minimise them is the name of the game. I was planning to fit solar PV anyhow, so thermal energy harvested from it is a bonus and I can afford to spill some and the cooling of the PV increases their efficiency so it’s not all losses.

Running a small store in the house, heated by the big store in the outbuilding, makes good sense to me - you then lose all the latency problems and you just need as much lagging as you can fit.  For the sake of plumbing simplicity I wouldn't have any coils at all in the big store - just send and return ports at appropriate heights to directly circulate its fluid on one loop to the heat pump and another to the house store.  

As long as you keep the larger store hot enough at all times, you can probably get away with a surprisingly small internal store.  Greater flow rates in the inter-store loop will increase the effective power to the small store (allowing it to be smaller and still cope) and also reduce losses in that loop - but will mess up the stratification in the big store.  Not sure where your compromise is going to be there.  Having a smaller internal store also means that the effect of your emergency back-up immersion heater will be more immediate.

That internal store then only needs one coil (with dhw in it) - so you don't have to worry about legionnaire protection as you never store any significant volume of potable water at temperature.

What's your plan for the coldest three months?

Why make it simple when you can complicate it! Mine just has a coil in the thermal store, a hot feed and return with a tiny recirculating pump in the house and a cold water inlet/one way valve for when you draw water off. You need a hot/cold mixer where it joins the house system to keep the temp under control (the store gets over 95°) sometimes and there's a through-flow heater if the temp is too low.

This is Wintertree we're talking about...

Is a constantly circulating loop with non-return valve, mixer and backup heater much simpler?   More space efficient than a second small thermal store for sure.  Do you heat yours with the biogas?  Store and loop is going to run much hotter than one using a heat pump; I expect legionnaires is more of a concern with a constantly circulating DHW loop with a heat pump.

I was picturing something like that. Does your system constantly run the circulation loop so that there's near instant hot water at the tap? 

Well you need all/most of the junk somewhere in the system anyway and why move the heat from one store to another to use it instead of direct from source, it's just more complication, space and expense.

Mine is heated by my central heating system (wood gasifier) which is 30m from my house, at the biogas plant we have guaranteed 80° water so no heat store, just a heat exchanger at the house with a mixer valve.

Mine is set to keep the pipes at 65°, the pump only operates if the temp drops below this and if needed the inline heater comes on.

The water at the tap feed is always 65°, that's the idea. The recirculating systems are widely used especially in the USA and larger/higher buildings, there is/was a system which uses a thin 3mm bore tube you can thread through an existing water pipe, it doesn't take a lot to bleed the cold water away and the pump is tiny.

Good question.  I’m trying to find out what the real word output of heat pump cooled solar PV is in winter; it’s a large collecting area. I really don’t want an ASHP.  

In reply to jimtitt:

I think you’ve almost convinced me; I’ve not heard of a system like that before.  Do you know what kind of annual power usage the inline heater draws?

I assumed the OP was in the Middle East or North Africa or somewhere like that. They did say there were expecting to get 9 months of hot water just from cooling solar pv, not even from having a fully functional thermal solar setup. I'm used to seeing tanks on roofs everywhere in Greece but those things _do_ use a proper solar thermal setup so I was trying to imagine what sort of conditions you would need to heat water on the PV panels alone.

Fair enough.

How much electricity are you actually losing when your panels heat up? I know it's a problem for PV panels but I still don't know what sort of place you are living, Jimtitt made it sound like it was Northern Europe, in which case I would be surprised if you were suffering a huge loss and on the flip side of the coin, I'd be worried that there wasn't enough excess heat to make a significant change to the cost of your main method of water heating... And there is infrastructure cost to take into account too. ASHPs are going to become standard on a lot of houses so I guess it is entirely likely that even if you don't have one, your neighbour will get one at some point... I did a bit of youtubing to get an idea and it really doesn't sound like there is much sound at all coming from them. I think when you consider the noise of a washing machine, dish washer, tumble dryer, bathroom fan, tv, etc. etc. that are common in a house, the ASHP, on the outside of the house, is very much going to go un-noticed.

You heat pump cool the PV panels, gives ~600 W/m^2 thermal in sunlight and apparently still a decent amount without sunlight - the idea is to use them as a very large area thermal collector for the heat pump, rather than a small one with an annoyingly noisy and ugly fan pushing lots of air through the small area.  In theory it could do an appreciable amount of central heating as well.

About 60% of the light falling on solar-PV ends up as heat.  PV array areas are much larger than typical solar thermal arrays - our current small PV array has probably collected and then wasted 40 kWh thermal today, many times our daily thermal usage at this time of year.  So I’m not sure about that “not even”.

10%-20%, so about 500 kWh/year.  The plan is to triple solar-PV capacity when the outbuilding roof is rebuilt, so about 1,500 kWh/year at that point.  

When the sun is out, there's gobs of excess heat available from panels, and it sounds like they work reasonable well as a sub-ambient collector for heat pumps when it's not sunny.  

Yes, this is a great worry.   Cross that bridge when time comes.   

I notice that kind of noise a lot, and as they age they can get wobbly bearings etc.

Depends how much you like your garden.  I'd like mine an awful lot less with an ASHP in it.

Ah, I didn't realise the setup was using the pipes behind the solar panels as a source _for_ the heatpump, I thought it was two separate systems complementing each other. I guess the tubes would also be filled with some sort of coolant rather than just pure water so they wouldn't freeze in a cold snap.

During cooler temperatures I guess there could be a problem with ice building up on the pipes where you can't access them behind the pannels, and any snow would exacerbate that problem by insulating the pipes... But then I guess there would be an electric-thermal top-up for situations like that and for topping off the fresh hot water from the pump in the tap/shower circuit.

I guess either you have a seriously big roof PV array or we are back to hot places again. I give up

Northern England.  If planning will let me, the plan is in roof-arrays over both sides of the outbuilding.  If I can get electrify and heat from it and reduce the number of slates needed (recessed in roof arrays) it’ll save me having to buy 50% new/matching slates.  

Even on a somewhat cloudy day a lot of waste heat is generated on roofs.

Re: snow/ice 

At least one manufacturer has a reversible system so air conditioning heat can be rejected from the solar PV, handy at night when they’re not generating (although process is being made with panels that generate from radiating waste heat to a clear night sky…).  I figure you can use that reversibility to melt snow off the panels in winter.

One of the pros of a small in house thermal store as Jamie noted is that it’ll be responsive to immersion heating.  A 10 kW in line electric heater would plug any gaps I think.  

Initially it was disastrous! The heater was in the recirculating line so when the thermal store temp dropped the heater worked to reverse heat my house, I moved it to after all the stuff so now it's at the entry to the domestic water system. Half the year it's turned off anyway along with the heating and there is a normal immersion heater.

This doesn't sound right. How have you calculated a 10 - 20% loss due to high temperature? What ambient temperature are you using as your year-round baseline assumption?

A bit of reading suggests the method for calculating PV efficiency uses 25 degree C as the standard test condition. Efficiency losses at high temperatures vary per PV panel but the info suggests PV panels have a temp coefficient of between minus 0.2 to minus 0.5. So a loss of 0.2% to 0.5% in generation per 1 degree C increase above 25 degrees C. Or a gain in generation efficiency of 0.2% to 0.5% per 1 degree below 25 degree C.

To see a 20% loss of generation due to high temperatures for a PV panel with a temp coefficient of minus 0.5 (i.e. the worst case), you'd need to be making your baseline assumption for normal operating conditions a clear sunny day @ 0 degrees C and your peak high temperature day (max loss of efficiency) as 40 degrees C.   

Without knowing exactly which part of Northern England you're in, I think those aren't typical conditions. 

The use of excess heat for hot water sounds like it could make sense depending on costs. But I wouldn't be comparing the costs against a 20% loss of PV efficiency.

https://greentumble.com/effect-of-temperature-on-solar-panel-efficiency

Thanks for the detailed post.

Thanks. I've seen figures for the panels running at around 60°C in direct sunlight which would give that kind of loss.  I've only seen talkative pages rather than hard data on it though.  I wish I'd put some thermocouples on the back of my current panels before they were installed...  The efficiency gain is an aside really; it mostly applies on very sunny days, at which point the array of the size I have planned is going to be export limited anyhow.

I'm coming round to this idea.   Especially as the outbuilding store wouldn't be expected to get that hot, so it could be a large, more agricultural tank with less insulation and more volume.  

We have space for perhaps 1/3rd of the GSHP collectors we'd need for winter heating so I'd always assumed it was not a goer.  But...  If the large outbuilding store was a source for the heat pump, it could be use fluid loops to both roof-source and ground-source elements.  Which opens up the possibility of using the ground array to dump excess heat from the panels in the summer, building a heat bank in the ground and getting some of it back in the winter.  

I dread ASHP noise. One time after a trip to the US, returning to urban Glasgow was blissfully peaceful without the air conditioning noise of US cities.

Someone in the next valley to us (Black Mountains) has a thermal store about that distance from his house and he says it is extremely effective. No worries about excessive losses at all.