Sorry, no idea on specific products. I've been doing some noodling about evacuated tube vs flat plate (and PV) for water heating and come to, to me, slightly surprising conclusions. I still prefer ET for a variety of reasons but it's not as clear cut as I once thought:

http://edavies.me.uk/2012/01/pv-et-flat/

Well, v interesting. PV+immersion beats ST in poor sunshine!

What about PV+HP?

WHat about PV/T (eg Volther Hybrid PV/T) with immersion or with heat-pump (either circulating through the panels as per Newform Energy's scheme or just as before?

Rgds

Damon

JSH - is this on or off grid?

JSH you only need a few degreesC temp delta to start getting ST heat. Really all you need to worry about when modelling is the power output from the panels, which is known, and the pipe losses on the way to the tank, which are minimised by short runs, lots of insulation and thin pipe. There is a simulator for modelling this on Navitron wiki. (perl by ericw). And a spreadsheet for the pipe/pump calcs.

Ed you did a graph for insolation vs power for the PV, but not for the ST. Be nice to have that for comparison (for normalised roof area).

<blockquote><cite>Posted By: Gavin_A</cite>JSH - is this on or off grid?</blockquote>

Gavin, it's on grid, but with a limited maximum power connection (but more than enough to run a small (sub-4 kW output) heatpump. There are no other services on site.

<blockquote><cite>Posted By: Ed Davies</cite>

Posted By: wookey:<span style="background-color: #CFC">If you want it to do anything useful in winter you'll need to make it 'quite large' and you _will_ need a summer heat-dump.</span>

I see that as a slight advantage to having more PV in the mix - you can leave the electrons in the panel if it comes to it. Another advantage of electric heating is that you can throw some wind energy into the mix which tends to help more in the winter.

(P.S., assuming their prices are VAT inclusive solarproject.co.uk are marginally cheaper than eco-nomical. They also responded (and promptly) to my question a few weeks back about minimum temperatures for operation.)</blockquote>

Thanks for the tip about another good value supplier.

Like you, I can now see the potential advantage of having more PV and perhaps not having ST at all. I still need to sit down later today and do some what-if stuff to see how the two options stack up throughout the year, but my initial thoughts are that PV with a HP could be close to the same efficiency as an ST system, but with the added advantage of still being able to provide, or partially offset, the energy needed for useful DHW temperatures whenever there was enough sunlight.

My guess is that there are plenty of days in winter where a PV panel will be producing some useful energy, whereas an ST panel might not be able to produce anything useful at all, because of the need to reach a threshold temperature to start working and to effectively transfer heat.

The downside of the PV and HP option may be the relative cost, it may be cheaper to use ST in terms of capital outlay, depending on whether or not a HP might be required for other reasons.

One of the really frustrating things about planning the systems for this new build is that just about every time I think I've finalised the way to do things, either something untoward happens or someone comes up with a good idea for an alternative and makes me rethink the whole approach!

<blockquote><cite>Posted By: DamonHD</cite>JSH: if you think you're suffering indecision, try my mental meanderings here where I've gone round in circles and done nothing about solar thermal for at least 3 years, filling my roof with PV in the meantime:<a rel="nofollow" href="http://www.earth.org.uk/note-on-solar-DHW-for-16WW.html">http://www.earth.org.uk/note-on-solar-DHW-for-16WW.html</a>

The PV gets me carbon-neutral or negative overall whatever I do about my heating, but I'd still like energy imports from the electricity/gas grids to drop to zero in summer by putting (Li) batteries into my PV system (so becoming export only for 6 months of the year) and solar pre-heat for my gas combi. (Would also have to get my SO to switch to an electric cooker, but one battle at a time...)

But each would be very expensive for only small carbon (and minimal utility-bill) saving, though with significant satisfaction and some minor ergonomic benefits (like DHW getting to the bathroom taps much faster in summer).

Rgds

Damon</blockquote>

Thanks, it's frustrating going around in circles, perhaps more so for me because I need to finalise the systems in the next four or five weeks, I think. Like you I would like to reduce electricity import, hence the reason for thinking about adding ST to the mix.

The mention of lithium batteries made me chuckle, as in part that's the reason for the pickle I found myself in yesterday! Having a fair bit of experience with big lithium cells of various types (in electric vehicles) and having used high capacity nickel iron cells in the past, caused me to make a comment in another place about them.

FWIW, I'm not convinced that lithium cells are a good off-grid or grid-backup system yet, as they have a fairly limited calendar life. They start to lose capacity from the day they are manufactured and although newer chemistries have got this loss down to around 5% per year (if the cells are kept cool) it still amounts to a fairly short life when compared to either off-grid type lead acid or NiFe cells. I won't take this thread off topic with a debate on batteries, but it would be worth doing some serious research into design life (versus cycle life) if your thinking of going down this route (and taking any Chinese manufacturers figures with a pinch of salt, if my experience is anything to go by).

I'm sitting here with a spreadsheet open trying to find a way to make a valid comparison between PV and HP (bearing in mind I'll already have the HP for winter supplementary heating) and adding a sizeable ST array. My heart tells me that adding solar ST is the right thing to do, but the logic of using PV to offset the HP energy seem inescapable. It seems far from an easy conundrum to solve and I can fully understand why some suppliers sometimes end up in a pickle when trying to determine which system is "best" for a given client. With a boiler it must have been easy - just fit one a bit bigger than you think might be needed and be reasonably certain the customer will have as much hot water or heat as they need.

<blockquote><cite>Posted By: borpin</cite>One of the important aspects of this decision, especially on cost, is the storage tank / thermal store. Have you any thoughts on that? I have been looking at the Akvaterm Solar Plus; sophisticated but having had a 'simple' thermal store before I think the system makes sense.</blockquote>

I looked at them a while ago, but then realised (based on something posted elsewhere) that the Ecocent unit was the same sort of price (although a simpler thermal store)and included an ASHP as well as extra heat exchanger coils. After that I didn't go back and look at the Akvaterm units again, although perhaps I should.

Just to make things even more complicated, I could still opt to use the Ecocent but not fit the solar thermal system at all. It seems wasteful at first sight, but the Ecocent could provide summer hot water using the high solar gain I'll get in the upstairs rooms, and help to keep them cool by recirculating the cool exhaust air up there. I'm worried about those rooms over-heating, especially as they have a relatively large roof window area that faces south. Around 700 W of PV would keep the Ecocent delivering hot water, and if it works with a COP of 3 (which it should with a hot air input) then I would only need to generate around 2.6 kWh per day from the PV to supply all the DHW.

As the Ecocent is a similar price to a thermal store on its own, there seems to be no real economic reason not to use it. The cool air output isn't massive, but is probably enough to lower the temperature slightly in a well insulated house in summer. I'd just have to make sure it didn't pull in any "smelly" air from the kitchen, so would probably just restrict it to pulling air from the top of the bedrooms via the upstairs bathroom and blowing cool air back to the opposite end of the bedrooms.

Winter space heating was already going to be a small GSHP feeding UFH downstairs and also providing DHW to a thermal store/buffer. I could still use the Ecocent tank for this, and leave the ASHP turned off, or, perhaps, having it turned on if the bedroom temperature goes over a set limit (there may be a possibility of that happening, I guess, with the combination of heat rising from downstairs and winter solar gain through the roof lights).

The more I think about it the less convinced I'm becoming about the wisdom of including solar thermal. Either just the PV, GSHP and thermal store, or the use of an Ecocent instead of the thermal store, seems to make more sense. In effect I would have indirect solar thermal by virtue of having over 4 m² of south facing roof lights.

Posted By: JSHarrisI'm worried about those rooms over-heating, especially as they have a relatively large roof window area that faces south.

Based near Edinburgh, my previous house had 2 rooms upstairs (1 1/2 storey) and both got excessively hot in the summer with 2 Velux type wndows south facing. Insulation was pretty poor by today's standards (built 14 years ago) but I suspect most of the heat was solar generated.

I've just been trying to work out the solar gain through the year that will arise from the 4 m² of roof windows. I've extracted the global irradiance data for each month, converted it to kWh per day per m² and have found that there is a significant amount of heating even in the winter months from the roof windows.

The lowest figures I have are for January, at around 1.5 kWh per m² per day on average falling on the windows, which is about 6 kWh per day in total. If only 1/2 of that makes its way into the house that is still more than the heat energy loss from those rooms so they will heat up a bit.

The highest figures are for August, at around 4.5 kWh per m² per day, or around 18 kWh per day in total. This is massively more than the room heat loss at that time of year, so unless I do something those rooms will seriously overheat. The snag is that even pulling all the excess heat from those rooms with a HP into DHW that I can I'm still might end up with an excess, so I will need to reduce summer heating upstairs with blinds.

The PV requirement for DHW will be the same the year around, at about 2.7 kWh per day (assuming a HP COP of 3). I'm not sure how much of the capacity of my 2.8 kWp PV array will take, a fair hunk I think and maybe more than it can deliver during the winter (maybe one of the PV gurus might care to comment?).

Posted By: JSHarrisI've extracted the global irradiance data for each month, converted it to kWh per day per m²

Where did you get the data from? Did / do you need to take into account angle of incidence?

Posted By: JSHarris
J per degree C = 150 x 1000 x 4.18 = 627,000 J/deg C

if incoming water temp is 10 deg C and DHW temp is 55 deg C, then delta T = 45 deg C

Energy required for a 45 deg C temperature increase = 6127,000 x 45 = 28,215,000 J = 7.8375 kWh (seems a bit high to me, maybe we should use less DHW)

Maybe you will use 150 l of hot water but most of it will be diluted down from 55 °C so the amount used at that temperature will be noticeably less.

Posted By: owlmanLate last year I completed my Solar power ( PV and ST ) with the installation of a 30tube/180l cylinder set up. The location is only just OK in winter, ( 2.5 months ) with a couple of hours midday shading from my neighbours house. Still the performance has been very impressive. The merest hint of sunshine and the manifold temperature shoots up, and on a bright day my 180l cylinder is pushing 25°C top to bottom. It doesn't sound much but this winter pre-heat for my PHx is great and all considering the mains is coming in some days at 7°C or less. The array oversize is deliberate in order to take advantage of this winter gain.

My experience of playing with some small PV panels and a home made solar warm air collector is that if you can see where the sun is they begin producing something vaguely interesting.

What size tubes do you have? The commonly quoted rules of thumb are 7 litres/tube for 58 mm and 5 litres/tube for 47 mm so it doesn't seem like you're vastly oversized, even if it's 58 mm.

<blockquote><cite>Posted By: qeipl</cite>Jeremy,

I went through a similar process to yourself (though not in such detail) but in the end my choice was dictated by cost (a budget of £40k for a 100m^2 house).

When I discovered that the Ecocent would do the job of ST, MVHR, and UFH boiler all for less than £2k, the decision was made.

Yes, it's less efficient than pukka ST (when the sun shines) and MVHR (in winter) but I offset that inefficiency by generating 2MWh/y via my hydro. You can do the same with your PV.

I like the thrust of GreenPaddy's £100 heating challenge thread - minimise combined capital and running costs.</blockquote>

Malcolm, it was your story over on the forbidden forum that got me started on this! It is funny, but having started off thinking that your solution, adapted in my case to have a different secondary heat source, then having swung away from that when I realised a borehole made more sense for water and could also drive a GSHP well, I'm now swinging back around to your system again.

I like the inherent simplicity of it, but the cost is another attractive feature. I like the way that I can also use it to shift heat from the upstairs rooms, which I now know for sure are going to get overheated, to the ground floor where it will be more useful.

I may yet end up with a hybrid solution, using perhaps the Ecocent for hot water and as a thermal store and feeding the lower (solar) coil in the Ecocent via a small GSHP for space heating/DHW augmentation in the depths of winter. If I recirculate the Ecocent air, so the cool output feeds back to the opposite side of the over-heated rooms, I could still use a normal balanced MVHR for the rest of the house. The layout is such that I could pull air from the upstairs bathroom, with ducts feeding warm air from the apex (they are in the roof) of the upstairs bedrooms to the bathroom. That way I also get a measure of better heat recovery from the moist air after showers etc, provided I set the timing on the Ecocent correctly.

The only snag is finding a small GSHP, ideally one that's around 2 kW. So far I've not had any luck, but I shall carry on looking.

Posted By: Ed DaviesSome people prefer FP for aesthetic reasons but I think it's just a matter of getting used to whatever.

Our neighbour had some ET put on their roof a couple of months ago. From a surprisingly wide range of angles it just looks like a large window in the roof (i.e. like a flat surace rather than obviously being tubes).