Hi,

we've had some trouble finding a heating engineer locally with sufficient experience of solar thermal, so I'm hoping for some feedback on our proposed circuit (attached as PDF).

We've overspecified the collectors and the store in order to a) gain a greater winter contribution, and b) to offset the lower contribution on frequent cloudy days in western Perthshire.

We've been heating our well-insulated and air-tight 57sq.m 2-bedroom cabin with the occasional use of a 2kW electric radiator on the cold (sub-zero) nights - installing a 4kW solid fuel stove (c/w 1kW back-boiler) will a) reduce reliance on electricity, b) contribute to DHW, c) improve air-flow (via window trickle-vents), and d) be cosy!

Due to roof pitch and topography, we're intending to install the collectors on a shed some 25m from the store (see below); 12vDC thermal differential control, pump, expansion vesel, and pressure relief will be in the shed, with solar PV alongside the collectors. Thermal store, other valves, and heat exchanger for stagnation-preventing heat dump will be in the cabin.

Any comments are welcome, bur specific questions are:
1) can anyone recommend a H.E. in 50-mile radius of Crieff?
2) are there any safety concerns with the proposed schematic?
3) how does a 'filling loop' work? what's the procedure and hence, required plumbing?
4) for the 25m run, we thought to not use std s/s corrugated due to pressure loss; should we go for copper and just deal with thermal expansion somehow, or instead use s/s tube at extra expense?
5) suggestions for valves?

Links to kit:
Thermal store: http://jouleuk.co.uk/Cyclone_Thermal_Stores/299
Collectors: http://www.kingspansolar.co.uk/products/16/df100.aspx
Stove: http://shop.salamanderstoves.com/small-stoves/hobbit-stove
Pump: http://www.solarproject.co.uk/page2.html
Thermal differential controller: http://www.reuk.co.uk/buy-SOLAR-PUMP-CONTROLLER-CIRCUIT.htm or http://www.solarproject.co.uk/controller%20detail.html
Plate heat exchanger: http://www.ebay.co.uk/itm/Stainless-Steel-Heat-Exchanger-Plate-Heat-Exchanger-20-Plates-44-kW-/330867777183?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item4d09419e9f

In the longer term, I'm looking to investigate programming a dedicated 12v Linux 'intelligent' controller using Raspberry Pi (http://www.raspberrypi.org/) - see: http://www.raspberrypi.org/phpBB3/viewtopic.php?t=31455&p=272911

best wishes - Jeremy

The panels to which you refer say they are 'direct flow'? I take it these are the type where the fluid is circulated through the tubes, rather than the heat-pipe type. I appreciate your array will be pretty close to the ground, and thus good for access, but our experience with (Schott) 'through-flow' tubes at South Yorkshire Energy Centre was that when the local 'little dears' used the array for target practice, the entire system drained down (of course). Because we had them on a frame on the gable, the access was more tricky, and it cost over £500 just for the access equipment and 2 engineers. If we'd had heat-pipes the system would have carried on working, albeit a bit less efficiently.

This may not be an issue for you if access is easy and any labour DIY (and also if you have no vandals!), but others may find it worth noting. Out of interest, if I am right that they are the 'through-flow' type, why did you choose these against the heat-pipe type?

@Steamy: as mentioned, store is over-specified for reasons listed; also, in the schematic you'll see that we're hoping to use both the lower (solar) coil and the upper (primary) coil for the solar circuit, but diverted by hydraulic temperature valve (right name? see http://www.danfoss.com/United_Kingdom/Products/Categories/List/HE/Domestic-Control-Valves/Miscellaneous-Controls/RAVK/bcd81841-0aa9-417a-a06a-53552b1bae78.html - possibly the least helpful website?) depending on temperature, to maintain stratification.

Any failure to achieve ~50degC at top of tank results in extra heat being delivered by inline electric heater, i.e. only heating the water we use, not the whole tank (CEX-U from http://www.zipheaters.co.uk/zip-hot-water).

Nevertheless, point taken. Calculations suggest at least 7.5hrs at peak solar output, with no system losses, to heat whole tank from 10 to 60degC. However, once warmed above 25degC electric load is significantly reduced, and large volume should attenuate variation due to draw-off; during winter, the ~daily top up with solid fuel should further reduce electric load, by being introduced at top of tank.

@Nick: access is good; the only vandals I'd envisaged are home-grown and armed with footballs - I'd intended to screen the tubes with chicken wire, being concerned more about the glass than the actual pipes.

@Ed: point taken re: thermal mass - maybe an option is to introduce a short-circuit that diverts solar flow past store until fluid exceeds store temp. The long run is unavoidable. Price from Kingspan: £1500 for 3m2. (cf. £1880 for heat pipes). In short term heat 'dump' (on rare occasions - ~5x p.a.) will be to 'solar paddling pool'; longer term, we're considering massive (up to 20m3) inter-seasonal storage for UFH, i.e. low-grade heat, with attendant lower heat-loss over time.

6.5kWp seems high for 3m2 of panel. I'm not far away from you and have 60 47mm navitron tubes (around 4m2) and these work into 180 litres first and then 120 litres in a secondary tank (only on decent days does this get heated). I doubt theres much difference in panel efficiency and I have a shorter run (30m circuit), so I think you need more collector area for 500 litres, oversizing is needed around here!

You can fill the system with one of those weed/paint sprayer tank things you get at DIY stores, no need for a filling loop.
I wouldn't bother with an AAV if the panels are accessible, I just loosened one of the compression elbows on the panel to remove air although I did arrange pipework to have a constant rise to the panels. I've never had to remove air (or add water) to the system in over 6 years now - very much fit and forget. I did check a few times for air after installing but never found any.

I use a danfoss 3 way motorised valve for tank switching , from memory these have a higher temp rating , 95'C maybe.
I don't know any decent plumbers around here as I DIY everything, if you are practical and have the time you can do the same.

Can you use the solid fuel burner as a heat dump somehow? The long pipe run will help limit the output.

Hi,
updated plan.
I've ditched the 12V (solar PV powered) pump
Still intending to use Raspberry Pi to control, so all diverter valves are now actuated and hope to gradually develop optimised switching dTs, T thresholds, and pump cycle.

Important design change: heat upper coil until >60degC, and don't run through lower coil, i.e. don't try to extract maximum heat, but maintain transfer fluid temp in order to achieve maximum temp.

(WBS circuit removed for clarity - it's in and working well, although cool tank causes tar build-up on back boiler, which should be reduced once solar thermal is in place - PNG is illegible, and I can't upload SVG, so PDF is further down...)

Posted By: Jeremy S(seems PDF can't be uploaded any more...)

Dunno, but it's not a size limit is it?

Posted By: Jeremy SYou are not allowed to upload (140830CultyMS_SolarTS_12.pdf) the requested file type: application/.pdf

I don't know what's changed, and it seems stupid that it has, but it might be worth renaming the files with a .txt ending instead of .pdf - that's worked for some spreadsheets in the past. Simple-minded rules!

uploaded with a different browser - see fix for Firefox 29.0.9 here: http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12522&page=1#Item_8

Ref Q 5 - the AAVs (the solar rated ones) usually leak eventually - I think all do leak eventually. Leave the isolating valve open for 2 or 3 days after all 'work' and then close the valve until next time more air has a chance to get in. Think of it as a commissioning tool rather than as a full-time AAV. Once or twice a year I open the isolating valve and any air that has caught under the valve (it is still the highest point) can then escape.