Solar Thermal systems and Heat Dumps

Chris Flaherty is Genersys’s technical expert who has travelled the world supervising and advising on the installation of Genersys thermal solar panels. As a result he has seen many alternative ways of heating water and many different types of panels. With the Renewable Heat Incentive gaining momentum there will be installers who are not familiar with some of the techniques and intricacies of thermal solar (or solar water heating) and may well have gaps in the knowledge which the qualification courses they have been on have not covered. We will aim to fill in some of the gaps and one of the issues is heat dumps.

In many countries, such as the USA, heat dumps are a familiar feature of solar water heating systems. Solar panels get very hot and because the energy is drawn on demand and there is a sizing ratio between the surface area of solar panels and the amount of hot water you can store, when panels overheat many brands of panels need to have somewhere to dump the neat, otherwise the system will fail. When you understand that Genersys panels are designed not to get hotter than 170 degrees Celsius, you will have some insight into the heat that these panels can generate.

Many panels have to have a heat dump fitted to them. I have seen systems in the United States where the heat dump comprised a radiator fitted at the back of the installation, often on a back roof, in order to dissipate dangerously hot heat. That constitutes a heat dump. Other places dump surplus heat into a nearby watercourse of pond – not a practice that I would recommend.

Genersys panels do not need a heat dump, even if they are situated in the middle of the Sahara Desert (as some of them are). I now set out Chris’ paper on heat dumps in thermal solar systems.

He covers the different types of solar thermal panels (also known as collectors) and explains the technical advantages of not needing a heat dump.

Genersys panels do not require a Heat Dump

1.  Collectors that cook out

 These are collector types like some panel makes or all tube systems with only two connections. This means a few fewer connections but the disadvantage is that once the panels start to stagnate (reach very high temperatures) all of the glycol needs to steam up to get transported out of the collector. This is a process akin to cooking the glycol. It means very high stress on the glycol and the membrane of the expansion vessel because of the large amount of steam which reaches all the way down to the vessel. On larger system you would always need to install a pre-expansion vessel to protect the membrane of the main expansion vessel.

2.  Collectors that steam out

These are collector types like Genersys 1000-10 with 4 connections. This type of collector will push the glycol out of the absorber as soon as just a little steam develops and pushes it to the expansion vessel. The glycol will then be protected inside the expansion vessel. As soon as the panel cools down and the steam starts to condensate the glycol gets pushed back into the panels, this design increases the longevity of the glycol and the entire solar system, without the need for expensive heat dumps.

 The expansion vessel should have at least the pressure related to the height of the system + 0.5 bar. Though the system should have a minimum pressure of 1.5 bar on the entrance of the pump. This pressure is important for the pump to work properly.

 That is the technical position. In effect if you do not overheat the panels and have a Genersys solar system your system will:-

  • Not need a heat dump, which is a cost saving.
  • The glycol (which is the heat exchange fluid inside the panel) will last longer and keep its qualities better
  • There will be no need to take overheating precautions, like covering the panels
  • The pressure vessel and pumps will last longer

Some two connector systems like vacuum tubes are not fitted with heat dumps as a matter of course and this often causes overheating problems and an excessive wear on the solar panels (which can cause loss of vacuum) and excessive wear and tear on the system.

You should only have to fit a solar system once, and once fitted you will not want to have to pay to have someone go to the roof to carry out repairs on the panels or replace them.  That is why fitting a high quality system, like Genersys, is important.

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