Thermosyphon vs. Active Circulation.
The most common image one has of a solar heater is of a roof tank with a flat plate collector positioned beneath it in what is known as a close-coupled configuration. This is the most common configuration. Known as thermosyphon, it relies on the fact that hot water rises.
Whilst this is the simplest of all configuration options, like many older designs, it has a number of disadvantages when compared to current technology.
Firstly the heat transfer rate is very slow. Secondly and more importantly it has no means to control top end temperatures.
For this reason many thermosyphon configurations are sized to prevent overheating and even boiling in summer conditions.
Being undersized to prevent overheating also prevents them from offering adequate performance in winter.
Conversely correctly sized systems having no control often present dangerously hot water temperatures.
Active circulation mechanisms however whilst being slightly more complicated are able to control temperature delivery and so can provide superb winter performance without the problem of overheating in summer or if the system is unused when the property is vacant for example during holidays, etc.
This is achieve by one of two methods, either diverting water flow to a heat dump to dissipate the excess energy on large scales systems or by allowing the collector to stagnate in the case of small systems.
Whilst it can be possible to locate a thermosyphon tank out of sight in a “separated” configuration, another benefit that active circulation brings is that the storage tank or geyser can be placed anywhere as opposed having to be above the collector.
In many cases active systems can employ the existing geyser as the primary storage tank rather than having to install a second or new one.
Although flat plate systems are more commonly used in close-coupled arrangements, certain manufacturers of evacuated tube systems also offer a close-coupled thermosyphon arrangements.
These systems however are equally as prone to overheating for the same reason having no means to turn off the energy delivery once the water is at the required temperature.
The result again is that such systems are under specified in order to prevent excessively high temperatures being reached in summer.
Not withstanding this these systems can still boil if left unused during holiday periods. This is of course is potentially dangerous as well as being wasteful of water, as ultimately the systems safety valve will discharge the entire volume of water to bring the temperature under control.
To borrow a statement from a Pirelli advertisement: – Power is nothing without control.
Conclusion: Active circulation provides control to achieve year round performance and protect against overheating.