Data and monitoring: PV+ASHP - dream team or what?

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- CommentAuthorSteamyTea
- CommentTimeJan 5th 2014
No actual data on it, but would think that it will benefit the PV a little, though probably not the ASHP as you will be shifting a lot of air. Do you have any idea how much air the ASHP needs? Tonnes I would think.

What have you measured so far, the temp behind the panels? Do you have high res PV data to go with it and is there a local Weatherunderground station with a solar meter on it?
- CommentAuthorfostertom
- CommentTimeJan 6th 2014
Posted By: ratmin... PV combined with ASHPs
Surely a complete non starter because
Posted By: ratminon grey cold days when heat is most needed PV produces nothing and COP of ASHP plummets
At any time when the ASHP is called upon to produce heat, it'll be running 95% on full-price on-peak mains electricity.

Solar-sourced space heating is possible (incl right thro UK Dec/Jan if the building is near-PH) but it won't involve either PV or heat pumps.
- CommentAuthorRosco_82
- CommentTimeJan 6th 2014
I have no idea about air volume req as yet.

so far I have measured temp behind panels and ambient external temperature. Results so far surprised me a little as colder behind panels than where I am taking the ambient reading. I have re-adjusted data loggers - one for PV a lot closer to the panels now as opposed to just sitting on roof tiles.

I have data of PV generation from the inverter available to download and local (5 miles) weather station data available.

I won't necessarily need ASHP in the winter or on grey days as we always have the wood burner as primary source - I'll keep going with investigations but maybe solar thermal into store for summer HW is simpler and keep PV separate.
- CommentAuthorSteamyTea
- CommentTimeJan 6th 2014 edited
Posted By: Rosco_82Results so far surprised me a little as colder behind panels than where I am taking the ambient reading
Probably location and air movement, but if you think about it, the PV module will take up to about 20% or so of the energy out of the sunlight.
A better test would be to disconnect a module and then measure the temperatures of that and one connected.
- CommentAuthordavidfreeborough
- CommentTimeJan 6th 2014 edited
In typical daytime conditions, a ventilated cavity will always be cooler than a surface exposed to the sun. The energy lost to the PV will only cool it further.

How that temperature compares to ambient air temperature will depend upon a lot of other variables.

David
- CommentAuthorSprocket
- CommentTimeJan 6th 2014
> Solar-sourced space heating is possible (incl right thro UK Dec/Jan if the building is near-PH)
>but it won't involve either PV or heat pumps

I don't want to be picky but I think it is working here with PV + 3x GSHP.
Though partly that is because the weather hasn't been too bad (PV wise). It may be different next month.

But we do also have quite a bit more PV than is typical... and more than a modest amount of insulation and airtightness... and there have certainly been a good few days where the PV was just not useable even so.

And we haven't rigged up anything to try to only run the GSHP when the PV is available (it's just on timeclocks)but my impression is that this would be doable with the exception of if snow settles in for more than a few days.

I agree with the sentiment. It is not easy/likely to work well and I doubt it would make sense for reasons of finances alone. But it is certainly possible. Just not one for a tight budget or normally limited roof space.
- CommentAuthorfostertom
- CommentTimeJan 6th 2014
Posted By: SprocketI don't want to be picky but I think it is working here with PV + 3x GSHP
Blimey - where is 'here' and what does '3x' mean?
- CommentAuthorSteamyTea
- CommentTimeJan 6th 2014 edited
Posted By: davidfreeboroughHow that temperature compares to ambient air temperature will depend upon a lot of other variables.
Mainly the windspeed and position of the sun. So look at night time temperatures as well, one variable is taken out.

I always think it is best to look at micro generation as just energy offsetting. If you can export more than you import then well done. If not, then still better than not having any at all.
- CommentAuthorSprocket
- CommentTimeJan 7th 2014
Here is Somerset.
A farm with one PV install (20kWp), three GSHP installs (4kW, 12kW, 16kW).
That is a proper in-roof domestic install, not a farmer install in a field or on a barn.

On the rare days when there is not almost-no-sunlight there is enough PV to run at least the two smaller heat pumps for long enough to top up the heat in those two buildings. Even our recent very rainy stormy spell has had spells of sunlight where we have seen 10kW for a few hours.

It helps that the buildings will hold heat for a couple of days without much of a temperature drop.

A prolonged spell of settled snow would stuff all this up though.
- CommentAuthorfostertom
- CommentTimeJan 7th 2014
And the two buildings aren't low-demand super-insulated? seems incredible. How do you know the heat pumps are using only PV power? I could just believe it in a cold bright spell tho.
- CommentAuthordavidfreeborough
- CommentTimeJan 7th 2014
Remember the figures quoted are the power delivered by the heat pump. The power consumed will be around a third of that.

David
- CommentAuthorSprocket
- CommentTimeJan 7th 2014
> And the two buildings aren't low-demand super-insulated?

Yes they are. Converted old rubble-stone agricultural buildings. Not quite PH but close I think.

I think that without decent heat retention it would be too difficult or you would at least have to put up with being a bit chilly in the morning. Still not impossible... I could swap some smaller numbers into our spreadsheet to get a better idea but gut feeling from our site is that 50% of this would be something like 10kW PV with 8-12kW GSHP and a reasonable COP should be reasonably comfortable in a fairly modest eco build.

This is GSHP though. And our ground loop runs a pretty constant 10-12C all year round.

The farmhouse is not part of this refurb (not yet anyway) although we did our best to fix draughts. But the new buildings are all kept warm 24-7-365 and add more floor area than the farmhouse but overall heating bills have not gone up at all vs previous years.
- CommentAuthorEd Davies
- CommentTimeJan 7th 2014
Posted By: fostertomSolar-sourced space heating is possible (incl right thro UK Dec/Jan if the building is near-PH) but it won't involve either PV or heat pumps.
My humble (but based on moderately detailed calculations) opinion is that PV beats water based solar thermal in terms of capital cost for power output even at PH UFH type temperatures (say 30 Â°C above outdoor air temperature).

http://edavies.me.uk/2012/11/pv-dhw/#Tout=30

At high insolation solar thermal wins, but a) not by much and b) who cares? - it's the dull days you need to optimize.

ET's have a small advantage in that they can make better use of sunlight from oblique angles but if you've designed for good winter collection (large steep south-facing roof) this doesn't matter as the sun is pretty much south all the time it's up in winter.

PV needs more roof area for the same amount of collection, though.

Adding an appropriate heat pump swings things in PV's favour.

I don't have an opinion on building-integrated solar warm air as I've no idea of the extra cost vs normal building or the power output, particularly under low-light conditions.
- CommentAuthorfostertom
- CommentTimeJan 7th 2014
Verry interesting - how times/opinion/prima facie calcs have changed! Only a few yrs ago PV was 'obviously' a non starter relative to solar thermal, because of much poorer collection/conversion efficiency. I remember arguing - forget conversion efficiency i.e. kW collected for given collector area, just look at Â£ per kW collected regardless of collector area. Back then, the answer was still 'no' because of cost of large PV areas. Now reducing PV cost has swung the equation, apparently - but still providing you have enough roof for much larger PV area, than ST wd need.

So Ed, you're saying that provided larger roof area is available, and PV costs low enough, a big PV installation will produce more lo-grade heat per Â£ for space heating, right thro deep winter, than a cold-optimised, ultra-lo-thermofluid-temp ST set-up?
Do you also say that that heat delivery from PV is more continuous, less interrupted by cloud, and insensitive to outside temp/re-radiation loss? That wd be a clincher, for me.
However, you're saying that the electricity has to be multiplied up by a heat pump, rather than used directly as u/floor resistance heating? That might kill it, for me.

I can see that electric tape resistance heating in place of the UFH pipes in the attached pic (within a tea cosy of EWI) wd vastly simplify and cheapen all downstream (heat storage and delivery) aspects of a Dec/Jan solar space heating scheme.
- IMG00265-20101210-1441smaller.jpg
- CommentAuthorEd Davies
- CommentTimeJan 7th 2014
Posted By: fostertomSo Ed, you're saying that provided larger roof area is available, and PV costs low enough, a big PV installation will produce more lo-grade heat per Â£ for space heating, right thro deep winter, than a cold-optimised, ultra-lo-thermofluid-temp ST set-up?
Yes. PV prices are volatile - e.g., Navitron seem to have run out of those particularly cheap Kinve panels but there seem to be cheaper Yinglis available by the pallet load (22 panels) elsewhere - but Â£/W PV can beat solar thermal.

Do you also say that that heat delivery from PV is more continuous, less interrupted by cloud, and insensitive to outside temp/re-radiation loss? That wd be a clincher, for me.
No, I don't see why that would be true. However, PV wouldn't suffer from the startup losses that ST does with intermittent sunshine.

However, you're saying that the electricity has to be multiplied up by a heat pump, rather than used directly as u/floor resistance heating? That might kill it, for me.
No, that calculator makes a direct comparison between PV with resistance heating vs ST. Heatpump COP would be a bonus but you'd have to balance the cost of the heat pump against just having more PV which is not a calculation I've done.
- CommentAuthorfostertom
- CommentTimeJan 7th 2014
I knew you were on this kick - much food for thought!
- CommentAuthorfostertom
- CommentTimeJan 8th 2014
Splitting Ed's reply into 2 separate issues:
Posted By: Ed Davies
Do you also say that that heat delivery from PV is more continuous, less interrupted by cloud ... ?
No, I don't see why that would be true
Can we say whether or not PV absorbs more advantageously than ST, in various non-direct-sun conditions. ET ST is supposed to absorb better than flat panel ST, in non-ideal solar conditions; where does PV stand on that spectrum?
Posted By: Ed Davies
Do you also say that that heat delivery from PV is ... insensitive to outside temp/re-radiation loss?
No, I don't see why that would be true
Surely PV must be unaffected by thermal re-radiation loss - in fact prefers cold?
- CommentAuthorbillt
- CommentTimeJan 8th 2014
PV is linear, that is it will generate whenever the light level is high enough and increase production as the light level rises (with a slight loss due to temperature in bright sunshine).

EV tubes are not linear. They won't produce anything until the energy intensity is high enough to start the evaporation cycle in the tubes, that seems to need at least 100w/sq.m. of energy. Then the amount of energy harvested is complicated by ambient to collector fluid temperature difference and other heat losses.

In my experience PV will produce more effectively than ET until the light level is quite high.

PV needs about 4 times the area to collect the same amount of energy, but PV systems have fallen in price by more than a factor of 4 in the last three years, while ET systems have become more expensive.

It still makes absolutely no sense to try and heat a building with indirect solar energy, unless it's a building that doesn't need heat!
- CommentAuthorRedDoor
- CommentTimeJan 8th 2014
I have been waiting for this company to get to market as it may have a development that could work in this situation. http://v3solar.com/
Slow progress though.
- CommentAuthorRedDoor
- CommentTimeJan 8th 2014
And for SteamyTea, here's a little video of a direct ac generator.
www.youtube.com/watch?v=9ogD_kHPciY
I've seen your video on the heat recovery unit so you've prob had a go at this already.
- CommentAuthorfostertom
- CommentTimeJan 8th 2014 edited
Posted By: billtIt still makes absolutely no sense to try and heat a building with indirect solar energy ...
Why so self-evident? Explain.
Posted By: billt... unless it's a building that doesn't need heat!
Short of that, agree only poss for buildings that need little heat i.e. near-PH standard.
Having said that, under favourable circs, space heating by indirect solar incl thro UK Dec/Jan, can even allow some fabric relaxation to below PH standard.
- CommentAuthorSprocket
- CommentTimeJan 8th 2014
> It still makes absolutely no sense to try and heat a building with indirect solar energy,
> unless it's a building that doesn't need heat!

Have to agree with that. It is generally the wrong way to go about heating.
But there are always going to be crazy exceptions.

If it is an old building (not a newbuild with designed-in solar spaces), and heating system selected is a heat pump... and if you just happen to have PV anyway.

I wouldn't tell anyone they were crazy to do it. It's just a matter of expectations and what they think they are achieving or trying to.

A while ago I saw a big Grade-I listed manor house heated by two 40kW GSHPs. It's a worthwhile upgrade from the oil heating they had before but still pretty cold and draughty IMHO and now with a big electricity bill. They didn't have PV but if they decided to add lots of it... well why not?
- CommentAuthorEd Davies
- CommentTimeJan 8th 2014
Posted By: fostertomCan we say whether or not PV absorbs more advantageously than ST, in various non-direct-sun conditions. ET ST is supposed to absorb better than flat panel ST, in non-ideal solar conditions; where does PV stand on that spectrum?
It's complicated but in general I think PV tends to win in less than ideal conditions. On a hot day in bright sunshine flat panels win, ETs come second and PV third for Â£/W but who cares - you don't need much energy then and any old bodged collector will harvest enough. As you say:
Surely PV must be unaffected by thermal re-radiation loss - in fact prefers cold?
which I missed in replying to your more general points in the original paragraph - yes, PV effectively has no re-radiation losses. It also works a bit better in the cold than it does in warmer conditions which is the opposite of ST. In particular, PV has no (or technically very tiny?) disadvantages when delivering joules to water which is already hot (e.g., DHW).

Posted By: billtPV is linear, that is it will generate whenever the light level is high enough and increase production as the light level rises (with a slight loss due to temperature in bright sunshine).
Yes, this is a good first approximation but I don't think it's strictly true. Though people have measured output from PV in moonlight (as a joke) the datasheets typically show a minimum insolation level to generate any power at all. The one for those Kinve panels says 40 W/mÂ² so that's what my calculator page uses by default (parameter G0pv).

Posted By: billtIt still makes absolutely no sense to try and heat a building with indirect solar energy, unless it's a building that doesn't need heat!
All cold-climate residential buildings without effective interseasonal stores need some heat in the winter. Providing some energy for some purposes from indirect solar makes sense. Why, particularly, does it not make sense to try to provide the relatively small amount of energy needed to heat a well-insulated airtight house from indirect solar? I need to know now as I plan to get the planning application for my small well-insulated airtight house heated primarily by indirect solar, plus probably a small wind-turbine, in next week.
- CommentAuthorfostertom
- CommentTimeJan 8th 2014
Posted By: RedDoora development that could work in this situation.http://v3solar.com/" >http://v3solar.com/
Hm - so there's much redundancy in the mirror area (because most of it will be turning away from optimum clear-sky orientation at any given time) but the payoff is that some part of it will be near-optimum at all times. So just add more reflector area; in any case the PV module will get well-sustained irradiation. It's just a cost-effective tracking system. Doesn't help to sustain output with non-clear skies. But does turn into a windmill!

As for 'nature works on spin' - true, but not this kind of spin - that bit is just spin-doctory!
- CommentAuthorfostertom
- CommentTimeJan 8th 2014 edited
Posted By: SprocketIt is generally the wrong way to go about heating
I say the same - why so self-evident? Explain.

Maybe you guys are thinking of standard set-ups that are optimised for whole-year maximum production, or at least you're thinking of the dregs of production that still remain in mild-weather. A whole system (incl but not only the collectors) that's optimised in several ways for cold-weather production, storage and input is a different animal - talking 60% end-to-end capture/conversion efficiency.
- CommentAuthorbillt
- CommentTimeJan 8th 2014
Photo cells are pretty linear over most of their operating range. They probably drop off slightly at very low light levels and inverters inefficiencies will drop the output as well, but observation shows that output is generated even in very low light levels, well below that at which ETs have stopped operating. It's a bit academic anyway, as the incident energy levels are too low to be useful.

It's self evident, because the capital cost of a PV system to generate usable energy in the winter is enormously greater than the cost of burning something else (gas, wood, coal, oil) directly. Bear in mind that for much of the UK winter there isn't any useful solar energy to be had, so, in addition to the collectors you'll need some sort of store, which will also be expensive. Of course, the area of roof or grounds needed to install a suitably large array is also incompatible with any house with any sort of environmental pretensions (i.e. small).
- CommentAuthorfostertom
- CommentTimeJan 8th 2014 edited
Posted By: billtcapital cost of a PV system to generate usable energy in the winter is enormously greater than the cost of burning something else (gas, wood, coal, oil) directly
Sure, but we're aiming to not burn stuff, and like anything costs come down both as new principles are dreamed up (at present/prototype stage) and as production ramps up (at commercialisation stage).
Posted By: billtfor much of the UK winter there isn't any useful solar energy to be had, so, in addition to the collectors you'll need some sort of store, which will also be expensive
Look a bit deeper, refer to previous stuff on GBF - yes storage is key but don't assume that means expensive tanks, plumbing, super insulation etc - much neater ways are being envisaged.
Posted By: billtOf course, the area of roof or grounds needed to install a suitably large array is also incompatible with any house with any sort of environmental pretensions (i.e. small)
Not so in case of ultra-lo-temp 'wet' collection, but wd be an issue if substituting PV collection, as discussed above, balanced against the much cheaper/simpler whole-system set-up that PV wd enable.

BTW Ed's investigations on that score are exactly the kind of cost-reducing prototype-stage re-dreaming that I mentioned in 1st para above.
- CommentAuthorEd Davies
- CommentTimeJan 8th 2014
PV in winter is about Â£20/W (panels at Â£0.60/W operating for effectively 45 minutes a day on average). That's a lot more than most other forms of generation (about double nuclear and wind which are the next best) but given the low running costs, lack of need for capital investment in grid and local network infrastructure not self-evidently so enormously greater than burning stuff as to put it totally out of the question.

My 60 mÂ² house design has room for about 6 kW of PV and 8 off 20x47mm tube solar thermal panels on the south roof with room for a bit more as funds and experience indicate. It's an A-frame design which probably uses a bit more material and insulation than that floor area would normally indicate but, given that I have good uses for the bits at the bottom, not by enough to make me give up on â€œenvironmental pretensionsâ€.
- CommentAuthorSteamyTea
- CommentTimeJan 8th 2014
I think this is all getting a bit confusing.
Ask simpler questions.
So:

What is the best method of heating
What is the best method of self generation
What are the alternatives

(by best I really mean most appropriate)

If we had electrical generation from non polluting sources then we would not be worrying about fitting domestic PV.
- CommentAuthorbillt
- CommentTimeJan 8th 2014
Figures from December last year, first column 3.8kW PV system about 27 sq.m. area, second column 120 x 58mm tube ET collectors about 20 sq.m gross area. Both arrays face SSW with no significant shading. IOW they are very nearly optimally sited. The PV array is larger than it would be possible to install on any reasonably sized house. The ET array is much larger than any rational person would install.

01/12/13 0.9 0.0
02/12/13 0.6 0.0
03/12/13 1.0 0.0
04/12/13 7.7 5.9
05/12/13 1.2 0.0
06/12/13 2.0 0.9
07/12/13 3.3 1.0
08/12/13 6.2 6.6
09/12/13 4.3 2.7
10/12/13 1.2 0.0
11/12/13 2.4 0.0
12/12/13 0.5 0.0
13/12/13 0.8 0.0
14/12/13 3.2 1.7
15/12/13 1.1 0.0
16/12/13 0.8 0.0
17/12/13 6.5 7.9
18/12/13 0.5 0.0
19/12/13 3.6 2.2
20/12/13 2.9 2.0
21/12/13 2.6 0.3
22/12/13 3.4 1.1
23/12/13 0.3 0.0
24/12/13 4.4 3.3
25/12/13 4.4 3.1
26/12/13 6.1 6.2
27/12/13 4.2 2.1
28/12/13 6.4 3.0
29/12/13 7.8 7.1
30/12/13 3.8 3.6
31/12/13 3.5 1.7

Total 97.7 62.4
Average 3.2 2.0

In other words, not enough generation to meet the demand for electrical power, let alone any heating requirements, even for a PH. Yes, if PV gets up to it's theoretical maximum efficiency, you might get 3 times the electrical output, but 3x nothing much is still nothing much.

People seem to be missing the fact that it's cold in winter largely because there is very little solar energy. If there's no solar energy you can't harvest it, no matter how much wishful thinking is applied.

ST, as you no doubt know, there is no best. If you have a PH, then mains electricity is undoubtedly best. Low capital cost and running costs should be trivial, as not much energy will be needed.

If you have a more normal house and access to mains gas, then mains gas is best. Cheapest energy costs, low local pollution and fairly low capital cost.

If you have a normal house and can't get mains gas, then toss a coin, it all depends on circumstances

Self generation is silly if you have a mains connection, so there isn't a "best". If you're off grid and have the resources, hydro is best followed at some considerable distance by PV.

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Data and monitoring: PV+ASHP - dream team or what?