Discussion ... ASHP(Air/Air) with Solar pv ....
zeupater
Forumite 5.2K Posts
Hi All
The combination of a small ASHP(Air/Air) with Solar pv seems to be a valid pairing of complementary technologies which has been raised a number of times on this forum.
The idea of this thread is to encourage discussion of this combination in order to validate what an appropriate equipment specification would look like in order to maximise performance benefits in a typical domestic environment.
Obviously, it is well understood that a typical domestic-scale solar pv system will not generate sufficient energy to provide winter heating, but should be considered as being a 'shoulder' months supplementary heat source for well insulated properties ... it is therefore best to concentrate on Spring & Autumn as being the target periods for heat provision, with an additional benefit of provision of cooling in heat-wave conditions such as we've been experiencing recently.
HTH
Z
The combination of a small ASHP(Air/Air) with Solar pv seems to be a valid pairing of complementary technologies which has been raised a number of times on this forum.
The idea of this thread is to encourage discussion of this combination in order to validate what an appropriate equipment specification would look like in order to maximise performance benefits in a typical domestic environment.
Obviously, it is well understood that a typical domestic-scale solar pv system will not generate sufficient energy to provide winter heating, but should be considered as being a 'shoulder' months supplementary heat source for well insulated properties ... it is therefore best to concentrate on Spring & Autumn as being the target periods for heat provision, with an additional benefit of provision of cooling in heat-wave conditions such as we've been experiencing recently.
HTH
Z
"We are what we repeatedly do, excellence then is not an act, but a habit. " ...... Aristotle
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We have a ~4kWp pv system and currently use some power to provide heating to living areas using a small oil radiator (~400W/600W/1100W) and/or fan heater (~1kW/2kW) at times, just to take the chill off in Autumn/Spring when either lighting the log-burner or turning the GCH isn't justified ....
During the recent hot weather we've used a portable air-conditioning system (hose through window type) which provides a theoretical ~2kW of cooling whilst using ~1kW of electricity .... this is a pretty old unit which isn't used much, but has proven to be extremely useful when elderly relatives have been visiting over the past few weeks ...
Over the last few years I've looked at a split system Air/Air heat-pump system and have a number of ideas on what we would require from the system, but hopefully these will also be requirements which others would raise as part of the on-going discussion ... so I'll leave it there for the moment ...
Enjoy the learning process, I'm sure I will ....
Z
Maybe its also time for me to look at getting around to purchasing a Wattson to compliment things as most of these systems are plug and play types. At least then I wouldn't be using any more power than I was generating.
They're probably going to be a better option than oil fired heaters. At least these units will use 1kWh and turn it into 3-4kWh of heat. I can honestly say I haven't looked into portable units so probably should do a little more research(skulks off to do a bit more net nerding)
The portable unit we have is about 10 years old (possibly more) & cooling only, although there was a reversible version available according to the manual. The overall efficiency of the heat exchanger is (/was) probably much better than 2:1, but venting hot air out through the window means that there's always warm external air being drawn into the house to replace it thus reducing overall cooling .... there are portable split units available, I bought one for a specific application once, but they're not as efficient as wall mounted inverter systems.
To pair a heat-pump to solar pv so as to ensure that they are well matched, the HP would need to be pretty controllable in terms of manual variability of power consumed to match pv generation rather than overall heating/cooling duty .... all of the specifications I've seen never detail the consumption and efficiencies at various speed settings, just a general range, if that !! ..... talk to an installation company and they have no idea other than the manufacturers' specifications so it would be good if someone has any actual comparative test data to share ! ... it's my opinion that it would be better to 'right-size' a heat-pump to the pv system's capabilities than to 'over-size' or 'under-size' it - that way it becomes a more useful addition to the household.
HTH
Z
So to keep things contained, here's what I pondered:
T - your link:
http://ecoairdirect.com/split-air-conditioning/bravo-non-inverter/inverter-air-conditioning-12000btu-bravo-inverter-eco1216sd.html
is pretty much what I was considering. Thanks very much for the link, I didn't realise how possible this actually was.
Ideally need a compressor that will run 2 units, but luckily they would be two rooms above/below each other, so roughly same pipe run. Also, having experienced what a small amount of heating can do in marginal/shoulder months, I'm reasonably confident that 3+kW's of heating (or indeed cooling) would have a noticeable effect.
Zeup, I'm a pretty positive person, and willing to give things a go, but feel a little sceptical about the COP figures I've read. Is a COP of 3+ for temps of 7C+ reasonable? I really don't know much about this at all, only at toe dipping stage so far.
Following on from COP - another thought, that has led me here, is that a COP of 3 (or more) 'balances' leccy v's gas costs. So whilst I would expect some or all leccy from PV, it would not be a disaster if I failed to keep an eye on pesky clouds etc. Ok, not a disaster on running costs, but still a disaster in duplication of heating systems, but that gets balanced out by having AC as a luxury, and definitely 'free' leccy.
Lastly, whilst this is only a personal issue, I have approx 700kWh's of PV to spare (export about 2,800, get paid for 2,100) so no guilt felt, if that is turned into 2,000+kWh's of heat.
2nd last. Shortly after mentioning this a week or so back, the Jeremy Vine show ran a piece on AC. Didn't catch much of it as I was transistioning (gyming, showering, cycling) but little snippets all mentioned 'efficient heating as a bonus'.
So, whose going to jump first, and get some quotes for an installed system - should we wait till December when the companies are quiet?
Mart.
For general PV advice please see the PV FAQ thread on the Green & Ethical Board.
This is a toshiba top-of-the-line heatpump costing £899.
For the 'shoulder' months - I looked up the performance at various temperatures.
15C 3000W 6.1
10C 2700W 5.8
7C 2500W 5.7
0C 1700W 4.6
As you can see - for this sort of heat pump, for the 'shoulder' months - you get about 5 times the output per watt as a resistive heater.
Assuming for the moment your heat pump throttles according to the output from the solar panel.
Taking for no particular reason a 4kW panel in london.
Total generation over these 7 months is about 1500kWh.
Assuming that heat is always desired - which is probably a bit optimistic - and you can run the heatpump using the whole output of the panels - you'd come out at something like 7500kWh.
In reality, due to (amongst other things) the limited capacity of the heatpump, you'd be at more like 3500kWh.
If you put this in terms of peak-rate electricity resistive heating - you come out at £350 or so.
Which really isn't bad, and would indicate payback in under 3 years.
Using the numbers for gas - the gain would come out as something like £120ish.
This is still a modest payback.
The first year in the house was that of a stunning summer followed by the worst winter in years which was a terrible experience. LPG cost a fortune, we were not exeprienced/prepared for the one new log burner fitted in Oct so ended up using oil rads to supplement.
The priority then was a new heating system to fit in with our renovations... Going through all the options I gave serious consideration to air to air with electric underfloor in the bathroom. Given the cost to do our house then revamping DHW with a new tank and solar thermal/immersion it was quite an expensive solution when compared to an ASHP with proposed RHI.
I sometimes think of our ASHP which is coupled to fancoils not disimilar to air to air. With hindsight it was a good choice and has transformed the house despite the non existant RHI.
Now that we have added a second log burner, PV and solar immersion we are sorted. Just need to build a log store now... The one I fancy costs £616 from the posh shed company so I'll make a couple just like it
If we built on a conservatory air to air and another log burner would be a definite. But that would be expensive as would have to match the style of a 1906 station....
My thoughts on this are based on only running the HP when there is sufficient power being generated by the pv system in Spring & Autumn ... that really means that the ambient temperature whilst operating should be considerably higher than 7C.
Here's a link to hadcet data for maximum daily temperatures for 2012 as averaged across a number of weatherstations .... http://www.metoffice.gov.uk/hadobs/hadcet/cet_max_2012 .... take care when reading the values as decimal points aren't shown - the first two lines show anomaly from 30 year average (1961-1990) and 2012 monthly average to two decimal places, the daily readings are to one decimal. Readings of -32768 simply describe there being no data for that particular date. As can be seen, there are plenty of days in March/April/Sept/Oct when daytime max temperatures of 10C-15C should be lifting the COP above the 7C standardardised performance test.
A secondary consideration which would influence the COP would be the house's interior temperature, however, it's likely that anyone interested in a pv/HP combination wouldn't be looking to achieve tropical indoor temperatures ...
HTH
Z
For modern top-of-the-line heatpumps - there is little difference between 15/7/0C.
The heat output goes down somewhat, but the efficiency stays over around 5.
I question the sense of using a low-efficiency heatpump - while they may be around half the price, they use lots more electricity for a given heat output.
My thoughts exactly ... by the time you've had the HP installed the difference in price between a top performing unit and a cheap unit should soon be recovered in terms of heating/cooling delivery.
On Air/Air units, I have seen some manufacturers/models claiming a COP of 5+, but have noticed that this is very size/model specific ... even within the same model range the COP can vary between ~3 and ~5 depending on model heating capacity, however, like products in many other areas, marketing material seems to concentrate on the best possible performance data even if it mixes figures from different models ....
I suppose that one of the main discussion points on this thread will revolve around the difference between claimed specification performance and what is achievable in the UK.
HTH
Z
Hiya, 100% spot on what I had in mind. To try to keep the house heat above 18C (ideally 19 to 20C) when temps outside are probably 10C to 15C during the day (PV window).
Tried to dip a second toe in, not so easy. Just had a chat with a company about 1/2 mile away, really helpful, but when they asked the application and I said 1 or 2 rooms in the house, they said they don't do domestic installs anymore as people's expectations are too high.
He said I (we?) were pretty spot on, with expectations for a small system, perhaps 4 months questionable/too cold if GCH available, 4 months very suitable, and 4 months no heating needed but AC a nice luxury. But still didn't want to know. Has given me another company details, and waiting for call back from a third, but they did say that they are busy and priority to commercial jobs at the moment - which was pretty honest of them.
Critically, I need to know what those £400/£500 internet supply only prices turn into, when they look around the house and start sucking air through their teeth!
Another question, you know we were talking about boiler efficiencies for DHW of around 80%, what would that be for heating as some losses (pipework, boiler) would still be beneficial. So would 90% be a fair figure when comparing gas to leccy? Taking that a step further, if a COP of 3.6 could be achieved, would that mean 1kWh of leccy replacing 4kWhs of gas (4*90%), or am I not understanding this correctly?
Cheers.
Mart.
For general PV advice please see the PV FAQ thread on the Green & Ethical Board.