Hi again,
Please a little advice, does any one know if any solar thermal panel is suitable for use in drainback system?
My seraches so far have informed me that evacuated tubes are not ideal but can be used. Flat panels are beter but are restricted to a parallel?? internal pipe configuration as opposed to a meandering layout. Meandering layouts do not drain back fully so present a risk...is this so?
Are there any other things I should be looking for?
I have already purchased some worcester bosch greenskies fkt panels off ebay but i believe these meander and the man at bosch said that they have not been tested in a drainback system and wouold therefore be unsuitable.
If I wish to use a drainback system will I have to buy some different panels?
I am looking second hand, in ebay, preloved etc.

Many thank
tim

Imagination Solar (no connection, etc) do a drain-back system.

Posted By: SteamyTeaI was under the impression that EV can be damaged by the high temperatures they can get to without cooling.
How high can the refrigerant gas go? Think I read once that it was 275°C

ETs, presumably. Nope, don't think it's a problem - they should stand stagnation though the glycol could be affected by the heat and, I think, become acid and generally horrid. Still, you don't want to overheat them too often but that shouldn't be a problem with a drain-down system if you mostly use the drain-down for freeze protection rather than overheat protection.

There a lots of different designs with different trade-offs. I think some people even have a belt and braces scheme with glycol solution and drainback (e.g., in parts of the US which get very cold) but the normal reason for using drainback is that you can avoid glycol. A minor advantage is that water alone has a lower viscosity and higher heat capacity so the amount of pumping required to transfer a given amount of heat is less.

The main reason, though, for using water (or water + inhibitor) is that you can directly heat the thermal store water without a separate heat exchanger (e.g., an external plate heat exchanger or a solar coil in the store). Any heat exchanger will have a temperature drop across it meaning that the panels have to run at a higher temperature and so slightly less efficiently.

In my case, I plan to have multiple thermal stores at different temperatures. Having to have separate exchangers for each or whatever just adds to the complexity and/or removes redundancy.

Yes, in many simple systems the fluid drains back every time the pump switches off. This has the advantage of simplicity but does mean that the pump is pumping against the head from the top of the panels - you get less advantage from the syphon on the down side “pulling” the fluid round.

Alternatively, you could have a valve at the top of the system to let air in and cause a drainback. Re-priming might be a little awkward to sequence properly. You'd probably want to close off the output of the panels until they're full, close the air in/outlet valve then immediately open the output valve.

There are alternatives to drainback without having glycol freeze protection. With ETs the manifolds are well enough insulated that it is feasible to simply pump a warm slug of water round the system every time the temperature drops towards freezing (say, gets down to 4 °C). Some Navipeeps use this system with a backup of some glycol in the liquid. You need to pump a couple of litres per panel every hour or two so the losses are not huge.

For a simple system where drainback happens whenever the pump stops I think you can use a standard controller. Some controllers also have a “freeze protection” function which will pump slugs of water round to keep the panels warm.

I don't know of any controllers which will deal with the valve type system but I haven't looked at all carefully as I'll do my own control for the solar as part of a wider control system.

A few points need clarifying:

SolarTwin aside, I haven't come across a 'direct' ST system in this country. Not saying there aren't any, but they are not thick on the ground like the Ozzie direct setups, and there is usually a good reason for that. With a 'normal' - ie pressurised system - you would have antifreeze and hence could not connect directly unless the store itself had the same mix.

Having seen the state of much of the sludge which passes for water in systems a few years old (passing through rads/boilers) me myself, I would always go the ST route via heat exchanger to avoid baking that same sludge onto the innards of the ST exchanger, possibly reducing its efficiency much more than any heat exchanger.

A solar panel is OK for drainback provided it is accredited as such. That accreditation will come via the manufacturer and can vary even WITH THE SAME PANEL depending on whether it is installed in portrait (vertical) or landscape (hor) mode. Harp being a better drainback than Meander, as mightyworthit suggests. Panels with twin top fittings will not drain back effectively in portrait, but aceptably in landscape. Filsol, now demised of course, had the ideal drainback panel with TBOE connections. Always check with mfr.

From my own observations, ETs are not considered as suitable as FPs for drainback if they are 'wet state' - ie have a live dip pipe within the tube rather than heatpipe. Also, systems like Navitron's ETs would, according to Phil at Navitron, be damaged by draining back as the fluid, while hot, still provides a mode of cooling more than no fluid at all.

My ideal is layered, oversize coils, so minimal efficiency loss, drainback if possible, but antifreeze belt with my braces, albeit at a lesser percentage than a full pressurised system with pressure vessel. An oversize storage capacity means more sun is stored, and the money-saving and effectiveness of the whole system - rather than somewhat specious spot efficencies - is measurably improved.

It is worth noting, from a recent thread, that some manufacturers are having issues with reliability of their ETs - that is nothign to do with drainback but appears to be due to the long extended hours of excessive temperature. Approved installations all, and some big names, but clearly if any panel is stagnating for a long time, it is not going to do it any good. And means that the useful energy it is absorbing is not being released for storage. Ergo longer payback and, it appears, shorter life.

Posted By: crusoeHaving seen the state of much of the sludge which passes for water in systems a few years old (passing through rads/boilers) me myself, I would always go the ST route via heat exchanger to avoid baking that same sludge onto the innards of the ST exchanger,

Fair point, but only applies if you're using metal radiators or boilers, of course.

A solar panel is OK for drainback provided it is accredited as such.

For a DIY system the question is does it work or not, manufacturer testing for the purpose is not overly relevant. It could easily be that they just haven't bothered to test it.

Also, systems like Navitron's ETs would, according to Phil at Navitron, be damaged by draining back as the fluid, while hot, still provides a mode of cooling more than no fluid at all.

Yes, I don't think a one-off loss of coolant event is likely to do much harm but it'll always be better for the life of any equipment not to bake it more than necessary. Using drainback for freeze protection is, I think, fine; using it for overheat protection much less so.

Posted By: Viking HouseThe highest boiling glycol is from Staub & Co I gather, its steady state temperature is 230°C and short term temperature is 280°C.

Wasn't thinking of the glycol, more the refrigerant gas/liquid in the tubes. When overheated does this cause problems i.e. leakage, damaged seals, loss of performance. Not something I have looked at but, generally speaking, if you want to test long term reliability of something quickly, you raise the operating temperature up and increase the cycle time.

As Ed says. when DIYing it is a matter of it working, my response to that is for how long?

djh: Search under Replacement ETs - referring to SMART Energy, an early, 'premium' (price anyway) supplier.

Ed saId: " Fair point, but only applies if you're using metal radiators or boilers, of course." Most boilers and radiators are metallic by nature. As are most thermal stores.

Ed said: "For a DIY system the question is does it work or not, manufacturer testing for the purpose is not overly relevant. It could easily be that they just haven't bothered to test it."

Can't agree. Your comment implies ignorance on the manufacturer's part. Testing for fitness for purpose - and stating of same to potential purchasers - is absolutely relevant. EG: I recently fitted some Wagner German ST flate plates. They were stated as accredited for drain back - but only in landscape installation. As the OP's post made the point, WB's representative told him NOT to use them for drainback, which indicates that they kNOW their meander pattern does not drain back well. To know that, they must have tested it.

Posted By: Ed DaviesSteamy is more worried about what temperature the collectors would get to if left uncooled.

Yes, never actually measured one, but could put a black plate under some glass and point the lazer thermometer onto it on a sunny day.
People claim that the transfer fluid can get to 165°C when working properly in a pressurised system.

I have some vague memory that that when water is heated to over 385°C, no matter what pressure it is at, it will remain a gas. Not that I think they would get that hot or you would have a mini Windscale, Three Mile Island, Chernobyl or Fukushima (take your pick depending on memory) mishap on your hands, or in airing cupboard, loft, roof.

Just wondered if anyone knew if there was an upper limit to the temperatures that refrigerant gasses in ETs can stand.

May be a wait for a sunny day up your way, shall see if I can rig up something down here before the wind changes direction.

A few minutes testing with a roof slate in an insulated box under a double glazed panel and angled to the sun gave this.

Time, Ambient Temp, Slate Temp, Solar Power
10:20,4.0,18.4
10:28,4.0,21.6,318
10:35,4.0,34,283
10:41,4.1,42,547
10:47,4.3,49,534
10:53,4.5,55,455

Think that would give a maximum temp of about 90°C if:
Slate Temp [°C] = 31 x Ln(Solar [W.m^-2]) - 150

Not really enough data to really claim that with any real accuracy though.

Posted By: SteamyTea

Posted By: Ed DaviesThe weather here has been much less extreme than in England

No idea about England, but Cornwall is much colder than it should be and been so for many weeks now. Should be about 11°C this time of year and been 4°C.

From what I have seen Cornwall has been much warmer than the rest of the UK!!!

Just did a little science this afternoon. Had two tubes out in the sun - one full of water with a bit of washing cloth as a bung, the other with a heat pipe in it wrapped in a couple of work gloves and a plastic bag for insulation. Ambient temperature was pretty low, about 5 °C. Mostly bright sun with thin cloud from time to time. Not much wind.

Started at 11:30Z. Highest temperature seen on the heat pipe was 194 °C at 14:00Z - intermittent thin clouds after that left the temperature around 170 to 180 °C:

Time (UTC)
11:30 Start
12:20 145 °C
12:55 169 °C
13:30 187 °C
14:00 194 °C
14:30 183 °C
15:10 170 °C
15:40 180 °C

The tube with water in was bubbling a bit when I looked at 15:10 and seriously so, spitting out water when I removed the bung, at 15:40 when I put everything away in case it spat boiling water over anybody when I wasn't looking.

The tube with the pipe in looks fine except that the bottom of the bung in the top (looks like nylon) is browned a bit.

Trouble is, I haven't got any ground I can leave anything unattended on without fear of somebody fiddling.

(Actually, might have quite a lot of ground in a month or two - things moving again on my plot purchase after a rather frustrating winter of legal wrangles.)

AO Smith do a commercial system which has a reservoir mounted just below the panels. Reservoir has the capacity to hold all the fluid in the panels when the pump shuts off. So. Temp in cylinder reaches design, stat switches off the pump, gravity drains down the solar panel fluid into the reservoir, solar panels are empty of fluid, so no overheat.

Steamy - solar radiation received in the UK (peak) is about 1000w/m2. 700 is pretty good, must have been a bright clear day with a good solar azimuth too.