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Boiling solar panel in stagnation bursts the pressure vessel of a conventional solar water heating system – Q&A
Q – I own a conventional solar thermal system which has two solar panels and a pressurised, sealed, indirect, antifreeze based system, which is not coping well with solar stagnation on sunny summer days. The solar circuit has boiled and exploded a pressure vessel, leaking solar antifreeze onto the attic floor, down the cavity walls and affecting the mains wiring of the house. What can I do?
A – This solar panel stagnation problem can be solved in several ways. Before suggesting a possible new design of solar water heating system, I have to say first that although I have seen many, I have never installed a solar thermal system like yours, since this overtemperature challenge can be completely got rid of by improved system design.
To expand on Solartwin’s improvement in solar thermal system design, before returning to your solar panel problems, we install heat-export-capable systems instead. These solar heating systems do not use antifreeze, nor do they have a heat exchanger on the way into the hot water store, although there may be one on the way out in the case of thermal stores – these account for about half of our solar installations. Instead our solar panel is connected across the top and the bottom of the store. In addition, a minimum water volume to solar panel size ratio is followed, which is about 35 litres of stored solar hot water per sqm of solar panel (for our specific type of matt black, double glazed long wave IR emissive solar panel). If you ignore this ratio, the solar heating system may boil in summer holidays during sunny heat waves.
For conventional single glazed selective solar collectors (panels), which I suppose yours will be, the minimum volume per sqm, will be much higher, perhaps about double, so this simple approach may well be impractical for the solar panels which you use (even if other issues such as solar panel corrosion protection can be addressed). So I will not be suggesting heat export design using your existing solar thermal panels in my list of solutions to your high temperature solar problem below.
In a Solartwin solar water heating system, when it gets hottest, typically in early afternoon, when the cylinder top exceeds 65C, (you can define this temperature in 5 degree intervals on the solar controller) the solar controller goes into heat export mode. This heat export mode means that our solar pump just keeps on pumping (irrespective of whether the solar panel gains or loses heat) until either the light has faded (It is a PV powered pump) or until the top of the store is less than 65C again. If it is still not below 65C in the morning, then heat export by the solar panel continues, once the sun rises. The solar water heating system’s controller only reverts to normal differential (ie heat import) mode when the water store’s top is less than 65C again.
This preventative approach to control of a solar water heating system’s potential overheating works very well because the sun does not come on and off suddenly (in a square wave), as a light switch operates a domestic light bulb. Instead, the intensity of sunlight landing on the panel is a gradual affair. The sun brightens, peaks, and then dims again gradually during the day (as an approximate sine wave). So at low solar radiation (light) levels, in morning and evening there is usually some time available when the panel is cooler than the stored water and it can therefore be used to cool the water down.
I have mathematically modelled this approach to controlling solar overheating (in response to our clever way of working being misrepresented to government by some thuggush competitors). The model fits very closely with real time field data produced by independent energy consultants. The solar test results shows that during an extended sunny summer heat wave, with no hot water drawn off from a 120 litre hot water cylinder at any time, the maximum water temperature at the top of the cylinder was 87C. This occurred in early afternoon. This was the worst case in all of the tests – it simply did not boil. There is a youtube video about this here – move the playhead to 3 minutes and 30 seconds..
I have heard from solar thermal panel installers in Ireland that such overheating failures are happening frequently, because a poorly thought out solar panel state subsidy scheme pays out “per square metre of solar panel”. This perverse solar subsidy is incentivising the installation of large numbers of square metres of solar panels coupled with what can be an undersized system infrastructure. It seems that, like some Irish solar thermal installations, your solar water heating system has boiled and perhaps burst its pressure vessel, in your case, leaking antifreeze into the attic floor, down the cavity walls and affecting the mains wiring of the house.
You are now worried about taking a summer holiday having been told that you need to “use up more solar hot water” to prevent your poorly designed solar system from boiling and bursting its pressure vessel again. I think that even if your system is meant to boil its antifreeze on occasion, it should never be bursting like this. Any solar water heating system should always be completely self-restting after any solar boiling / stagnation event. I think that you have a valid case to request that this avoidable defect in your solar be rectified at no cost to you. Administratively, options for enforcing this rectification include the REAL code, via a trade association quality procedure, via Trading Standards or, as a last resort, via a court.
Technically, your solar stagnation / overheating problem is almost certainly solveable and it might be addressed in several ways:
1/ Installing a much larger pressure vessel and integrity checking that the solar plumbing of your system never allows the solar collectors to refill once they have boiled dry. However regular stagnation degrades antifreeze and so this may need to be replaced fairly frequently as a consequence.
2/ Increasing the size of your solar hot water store is, perhaps, an option. But you may not have space – and there may be some basic besign problems still to be addressed in point 1.
3/ Reducing the number of solar collectors is another option. Drawbacks include less hot water.
4/ Position your solar panels steeper. Don’t collect the summer excess. I particularly like the idea of flattening out your solar thermal system’s annual performance to make it less peaky in summer. Roofs are sometimes too shallow in pitch for solar heating panels, if you take a year-round approach. Might it be possible to try repositioning one or more of your solar collectors, to be much steeper, for example at 70 degree to 90 degrees (vertical) affixed onto a sunny S facing wall. This will usualy reduce the summer overheat while allowing them to catch as much low angle winter sun as possible. This positional approach is attractive in theory, but not always in practice. If your panels end up on 2 different orientations, the plumbing and controls can get complex – so it is best to steepen them all, keeping them as southerly facing as possible.
5/ Fit a Solartwin. Replacing the whole lot with an inherently simpler and more stable zero carbon Solartwin solar water heating system, one which uses heat export, is, of course, my favourite option – but of then it would be, since I helped to develop Solartwin’s award winning solar water heating technology!
I hope this is useful. Regards, Barry.
PS – A few extra thoughts, in case they are of interest – as an explanation of our “design it out in the first place” approach to solar thermal overtemperature control.
1/ The table of theoretically possible overtemperature control options (at 1 min 35 sec) in this video has an overview of the theory of the different approaches to overtemperarture control.
2 / On the issue on operating carbon budget and annual (including winter) performance, I would suggest looking at the DTI side by side solar heating systems testing report and its eight appendices. Their monthly energy graphs – showing power consumption – are interesting. Some years ago I made an explanatory video on it: our most popular solar video to date.
A few other solar technology subtleties which may interest you:
- The solar collector is double glazed, so it has a lower base efficiency but also a lower decline in performance with temperature. This helps winter performance slightly.
- The solar collector is not selective but matt black. This facilitates long wave infrared heat loss, particularly towards low radiant-temperature clear skies. (Sunny days tend to have clear skies.) This means heat is lost fastest when it needs to be lost.
- PV variable speed pumping is more precise than on-off solar pumping. Plus the 20% or so carbon clawback is avoided.
- The solar controller’s default “implement heat export” setting is a cylinder-top temperature of over 65C.
- Regarding solar hot water storage. Having no heat-in heat exchanger means that the store can be stratified by 20C or more. A cool base means more pump-on time and hence better performance. Stratification is less pronounced in indirect solar cylinders. You cannot effectively use heat export in indirect cylinder unless either the heat exchanger extents to the top or there is a destratification pump in place.
- Our narrow microbore pipes mean less distribution heat loss, (both dead leg and surface related).