For consistent, long-term efficient use of flat plate solar collectors drain back is a necessity to overcome the issues resulting from stagnation.
Stagnation is the term applied to a solar system that has shut down as the preheat vessel is up to temperature. Even in a commercial environment, there will be times of low demand. In an office building, for example, outside of working hours while the sun is still up, or a hotel in the middle of the day, between the times people check out and check-in. During stagnation, all solar thermal systems without protection are prone to overheating of the solar fluid (glycol) which can boil, losing its antifreeze properties and become corrosive, attacking panels and components. To avoid overheating the solar fluid its manufacturers recommend preventing exposure to excessive temperatures over long periods, especially during stagnation periods. Under permanent stagnation conditions, the solar fluid will solidify. This is because glycol is essentially a sugar-derived fluid, and like sugar, when it is boiled it transforms into a black sticky treacle-like tar which kills the system’s efficiency and can even clog the system rendering it inoperable. This irreversible and costly fluid damage can take place within a matter of just two to three weeks and is hard to get out of the system, even with special cleaners. In particularly bad cases it can even affect some of the ancillaries, such as the pump, which must then also be replaced.
In addition, when fluid boils under stagnant conditions, a cubic inch becomes a cubic foot during the phase change, so there is a pressure issue to also consider. This is especially the case in the commercial built environment where multiple array systems are commonplace. It’s critical to ensure fluid is not trapped in the system where it just boils and boils, causing pressure buildup. This overpressure will trigger the relief valve to open and discharge the fluid. Studies have previously suggested as many as 30% of sealed systems discharge due to overpressure and it’s a number, through our wide experience of solar thermal system installations, that we would agree with.
The only thing that is essential in a solar thermal system then is that you avoid stagnation as it creates too many problems and overcomplicates maintenance. Even under a best-case scenario, if system stagnation occurs and only damages the solar fluid it’s still going to need to be changed every two years.
Systems will need to be specified with large more costly and difficult-to-locate tanks or fewer collectors to limit stagnation, but commercial buildings will have a limitation on cylinder size. This curtails heating energy which can be provisioned by the collectors because there’s not usually room for a whole day’s demand to be stored. That is counterproductive when high demand requires more power out of the system.
So, to avoid stagnation the options are to either use a heat dump or drain back. A heat dump is exactly what it sounds like, and one popular way of doing it is with a fan coil unit (FCU). It is positioned on the roof and dumps heat when the cylinders are unable to accommodate any more heat from the solar collectors. It is an approach which is suitable for both evacuated tubes and flat plates, but it does require parasitic electricity, so we ‘lose’ that free system energy, which is currently one of the premier reasons for adopting a solar thermal system to deliver commercial grade water heating. The other issue with heat dumps is that they are reliant on working parts, which can obviously fail, and you will only find out about that when the relief valve discharges.
Drain Back, The Answer To Stagnation
Drain back is a system, as the name implies, that drains the fluid out of the collectors when not in use because the cylinder is up to temperature or there is no solar energy available. It’s an intrinsically safe way of avoiding stagnation without requiring additional operating electricity. It Prevents boiling, overpressure, and fluid degradation but, it can only be used with flat plate collectors.
The solar thermal system now has a collector, cylinder, and pump, to which is added the intermediate drain back vessel which is partially filled. The system still works in the same way with the temperature in the collector and the cylinder monitored. When there is usable energy, the pump starts, pushing solar fluid up into the collector and short circuits in the drain back vessel to deliver the thermal energy to the cylinder. It will continue to run like that if there is usable energy, and the cylinder is not up to temperature.
When the cylinder is up to temperature or there is no longer usable energy, then the pump will stop. With a connected fluid at two levels and an air bubble in between gravity takes over draining the fluid back out of the collector, through the pump and up into the drain back vessel coming to rest at the starting position with no fluid in the collector. The collector, now empty of fluid, can continue to get hotter and hotter, but it does not affect the system.
The drain back vessels are tubular and hang on the bottom of the collectors. Typical installations will use a ratio of one drain back vessel for every two to four collectors (depending on if the collector plates are installed in a horizontal or vertical orientation). With drain back vessels employed, the system can also leverage more collectors for faster cylinder heat-up and increased provision of heat when demand peaks. With an air gap allowing for the expansion, and no need to make up for the expansion of steam the system also does not require an expansion vessel.
However, all pipework is required to tip back to the cylinder as the system relies on gravity. From an installation perspective, you cannot create traps in the pipework, such as exiting the collector and then running the pipe up over a parapet wall. Once below the roof, it becomes less of a problem because that pipework will always be flooded. Anything above the roof needs to have a continuous fall to allow the system to drain back. This is why we strongly advise that installers working with flat plate collectors and drain back are trained by Adveco. In our experience, things can quickly go wrong and become very costly with delayed assessments or even aborted commissioning visits.
For robust commercial solar thermal application design overheating must be dealt with. With multiple arrays collecting from an uncontrollable heat source drain back systems provide the simplest necessary management to protect against overheating. Drain back ensures systems always stay safe in times of low use, countering issues found in stagnation periods, most notably protecting against overheat. As a result, glycol fluid used with drain back will typically last five times longer (and in some cases more). With drain back in place the expectation for a system fluid change goes from every two years to every 10 years.
With reduced maintenance and fluid costs added to the offset costs of energy being generated and no parasitic electrical energy demands, solar thermal with drain back represents one of the most proven and cost-effective renewables available to commercial buildings. And, with continuing high grid electricity costs, commercial organisations seeking to transition to low-carbon electric water heating are finding capital investments in solar thermal applications deliver far quicker returns on investment than ever before.
Learn more about Adveco solar thermal applications, flat plate collectors and drain back. Or download the free solar thermal handbook.
