Solar Thermal Panels or Solar Water Heaters come in two main types… Flat bed panels and evacuated tubes. So which are better?
Well first of all, the theory behind the two systems is much the same. See this post for more. Solar collectors (which are essentially just thin pieces of metal) absorb energy from the sun, and transform this energy into heat. This heat is then transferred to the houses hot water cylinder where it provides useful hot water.
The main difference between two technologies refers to the way the metal collectors are configured.
Flat Bed Panels are a simpler technology. A thin panel of metal, usually painted black, is positioned just below a pane of glass. The reason for the glass is to prevent heat escaping into the air, and so maximise the amount of heat transferred into the hot water cylinder.
The benefits of the system
Flat bed plates are relatively cheap
When the sun is shining directly at the panels, they have the maximum plate surface area exposed to the sun, and so achieve the maximum possible
They look very inoffensive
The drawbacks of the system
At times when they are not receiving direct sun, they’re not as effective evacuated tubes
If the panel is damaged, it will have a very significant effect on efficiency of the system
Evacuated Tubes are a more complicated technology. Thin strips of twisted metal are positioned in the centre of a vacuum filled tube. The vacuum tube means conduction losses are reduced, while the glass acts to reduce convection losses. The idea behind the twist on the metal is that it means part of the panel is at the correct angle to receive the maximum energy from the sun, and therefore providing energy to the cylinder at lower sunlight times of the year.
The benefits of the system
Provide a significant amount of energy to the cylinder at all times of year
If one of the tubes is damaged, it will have little effect on efficiency of the others. Also it is easy to change one tube if necessary
The drawbacks of the system
More expensive than flat bed systems
They are less effective during times of direct sunlight. Does it matter?
They are more remarkable looking than flatbed panels. A bad thing?