Solar heating
Stopping stagnation Pull the plug on solar thermal bugbears
Stagnation and overheating are side effects of solar thermal systems, reducing solar efficiency, shortening component life and burdening the owners or tenants of commercial buildings with unnecessarily expensive repairs. Hugh Jones outlines the measures needed to guard against stagnation and introduces a new technology which automatically prevents it
WHEN DISCUSSING solar ther- mal, there’s one word none of us like to hear – stagnation. One dictionary defines stagna- tion as “the state of being still or not moving, like a sitting puddle of water where stagnation attracts mosquitoes.” Not a pretty picture, is it? And although you’re unlikely to encounter these conditions when solar thermal collectors suf- fer stagnation, you might well encounter steam, changes in fluid colour, leaks, strange smells, diminished system performance, and worse.
Experienced solar thermal installers will recognise these symptoms. Sometimes it’s possible to fix the problem, sometimes it will be necessary to replace mal- functioning collectors and present the client with an unwelcome bill. But the problems associated with overheating and stagnation in solar thermal are entirely prevent- able. And the good news is that a new technical solution, just brought to the UK market, ends the problem.
So let’s briefly remind ourselves how the problem of stagnation arises, check what measures need to be taken to guard against stag- nation with conventional solar thermal systems, then take a look at the innovative new solar ther- mal flat panel that has been creat- ed to prevent these stagnation issues from occurring, through the use of the highest quality materials and innovative technology.
Problems and risks
Stagnation occurs when the solar/ buffer cylinder reaches its set tem- perature and cannot accept any more heat from the solar collec- tors. This can happen on especial- ly sunny days, or when there has been a prolonged period of low hot water demand. In such cir- cumstances the solar pump shuts down and the system switches off. Circulation within the system is halted and for some time the gly- col fluid continues to heat.
31 | May 2016 | HVR Viessmann’s
Vitosol 200-FM flate plate collector (right) and
Vitosol 200-T vacuum tube collector (left)
New technical solution While there are ways to limit the effects of solar thermal stagnation, it would be better if systems could be prevented from collecting any more heat than they can cope with. That’s why, after years of development in partnership with Nancy University in France, Viessmann has patented an auto- mated temperature shutdown technology for thermal panels. Called ThermProtect, it protects the collector with an absorber coating based on the principle of “intelligent layers”.
Ultimately at stagnation, it’s possi- ble for the glycol water mix to reach over 200°C in flat plate col- lectors and up to 300°C in vacuum tubes. As temperature rises the fluid can become hot enough to ‘flash’ to steam.
This process is damaging. It accelerates component wear and tear, sealant breakdown and solar antifreeze decay. And when propylene glycol degrades, its increased acidity puts system com- ponents at greater risk of corro- sion, ultimately leading to expen- sive parts replacements. There’s also the threat of a more sudden and disruptive problem: if antifreeze properties are sufficient- ly degraded, fluids in the system might freeze solid in winter, burst- ing pipes and fittings.
Preventative measures The simplest way to guard against stagnation is to correctly match, or even under-size, the solar collec- tor area. This, however, is not being done often enough. Architects, specifiers and installers are focused on achieving solar fraction goals, optimistic local planning requirements and meet- ing worse-case scenario hot water demands; as such there can be a tendency to oversize the number of solar collectors. A solar thermal system should never be sized for more than the building can use or store. For a commercial building,
the system may typically cover around 70 per cent of the load in summer, and in winter the solar contribution might be as little as zero to five per cent. Over the year this averages at around 30 to 35 per cent.
Another essential preventative measure is to look in detail at the system that’s being specified. Some solar thermal systems, such as Viessmann’s Vitosol 200-T vac- uum tube heat pipe, contain a separate process medium that transfers heat to the glycol through the heat exchanger, allowing the system to empty the collector manifold of glycol as the fluid flashes to steam. Glycol is forced away from the damaging heat in the collector, returning only when temperatures have dropped and the steam has condensed.
System commissioning and maintenance are also critical. In commissioning, removal of air from the system will improve performance and reduce the risk of premature degradation of the solar glycol. Carefully designed pipework and positioning of air vents and separators will help. In maintenance, fluid testing, system flushing and the addition of stag- nation coolers can help manage stagnation once the solar thermal system is operational – but only if the system is correctly specified to start with.
The coating adjusts automatical- ly, through temperature-depend- ent changes in its crystal struc- tures, to changes in sunlight and heat absorption. It limits further energy collection when the panel reaches a certain temperature. Above an absorber temperature of 75°C the coating emits a much greater proportion of the incoming solar radiation and in effect shuts down. If the temperature of the collector falls below 75°C, the structure of the coating returns to its original state and 95 per cent of the incoming solar energy is absorbed and converted into heat. Because automatic changes in the coating’s structure can be made an unlimited number of times, this process ensures that the problems associated with stagnation are avoided entirely. The solar ther- mal system still stagnates, but this innovation ensures that the stag- nation occurs at a low tempera- ture, below the level associated with glycol water flashing to steam and degrading.
This technology was introduced to the UK this spring on the Viessmann Vitosol 200-FM flat- plate collector, which also sets a new standard for solar system effi- ciency and reliability. So the next time someone mentions stagna- tion, you can advise them that there’s no need for it - the prob- lem is solved. • Hugh Jones is Viessmann’s technical manager
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