Evacuated Tubes are sold largely on the basis that they are “more efficient”; however the reference to high efficiency is specific to only one stage within the multi-stage heat transference process and not a reference to the over all system efficiency.
New and inexperienced installers often favour the evacuated tube collector thinking that if it is more efficient it must be better. However as they gain experience they learn (as we did) that the high efficiency is often more of a hindrance than a help due to over heating on bright days.
The reason for this is that if one stage of the heat transfer process is more efficient than the next, it will generate heat quicker than it can dissipate it, which leads to over heating. When any solar system over heats it goes into what’s known as a state of stagnation, which essentially means that the solar heat transfer fluid within the collector has turned to steam, the pump cannot pump steam, so the system shuts down until it cools down allowing the steam to condense back to a liquid.
The problem is caused by the insulation properties of the vacuum within the tube. The sole purpose of the vacuum is to insulate the collector surface and to keep the heat in, this is
fine under normal circumstances; however when the system has over heated and gone into stagnation the heat needs to escape in order for the collector to cool down and for the
system to start working again. The vacuum keeps the excess heat in the collector which leads to a prolonged state of stagnation, possibly way into the evening (just like a thermos flask keeping coffee warm for hours).
A Flat Plate Collector by comparison does not have the vacuum insulation, so as and
when it goes into stagnation it can allow the excess heat to radiate out, bringing the temperature down, condensing the steam and allowing the pump to switch on and produce more useful energy.
What this ultimately means is that on a nice bright day an evacuated tube system will
heat up quicker than it can dissipate the heat and then spend the day shut down full of
steam producing no usable energy whilst the flat plate system continues on working, harvesting considerably more energy that day. In this example the Flat Plate Collector’s
over all system efficiency is far greater than that of the highly efficient Evacuated Tube Collector that has over heated and been shut down all day.