The Evacuated Tube Solar Collector
An evacuated tube solar collector is an array of tubes.The vacuum tube solar is a second generation (more efficient) collector, the first generation was the flat plate solar collector. The vacuum tubes collector is the major component of the evacuated tube solar water heater.Because it is more expensive, use it only in cloudy and less sunny regions. It is used in countries like Japan and Germany and in the northern parts of the US, and also in canada
How are Solar Evacuated Tubes Built?
Each tube is built as co-centered rings. Going from the outer ring inside we find:
- Outer transparent glass tube
- Black absorbing material, attached to inner transparent glass tube
- The volume between the 2 glass tubes is evacuated (it explains the name I think…),so we can call it a vacuum tube solar collector
- Inside the inner tube there are two non-concentric pipes that flows anti freeze liquid (e.g. glycol). Greenhouse effect plays here the same role as it playes in the solar flat plate collector.
The Advantages of the Evacuated Tube Solar Water Heater
The advantages of evacuated tubes are:
- Graceful degradation: in case one tube is damaged, it is possible to replace only the damaged tube. Until the damaged solar tube is replaced, the other tubes still perform and the effect on the evacuated tube solar water heater performance is marginal.
- Scalability, you can start with a smaller amount of tubes and increase the number of evacuated tubes, if the manifold has enough spare slots "waiting" for the increased number of tubes.
- Solar evacuated tubes will perform and efficiently heat up water even in not-so-sunny regions where sun radiation is sparse. This fact disprove the claim that solar water heating can not work in the northern regions.
- Efficiency – more radiation is converted to useful heat because a circle exposes more angles to the sun, even when the sun is lower in the sky.
- Because the evacuated tube design is more efficient, the solar collector can be more compact in size
- The heat losses are minimized due to the vacuum (vacuum is an extremely good heat insulator); as a matter of fact, one can touch the tube surface and not get burned, even though the inside of the vacuum tube is extremely hot.
- Solar evacuated tubes are non-direct system, they use anti-freeze fluid (such as glycol) so they are not exposed to break downs due to freezing conditions.
Where Do You Need the Evacuated Tube Solar Collector?
The disadvantage of evacuated tube solar water heaters is the higher cost compared to the flat plate water heater system; however, the vacuum solar tubes collector is a very good solution for the colder, not so sunny states and countries. If you live in the warmer states or countries buy the flat plate collector and save the balance in price (or use it for a nice vacation…)
Vacuum tube solar collectors have typically 20 to 30 circular tubes, they are reliable and require a very low maintenance, the life span can be expected to be 10 years or more. If you live in the US, you are entitled to the 30% federal energy tax credit on the equipment and installation cost
Bellow is a schematic drawing of the solar evacuated tubes
Evacuated Tube Solar Collector Recommended Reading
Click to read on flat plate (flat panel) solar collectors: flat plate solar collector
Click to read on the greenhouse effect: the greenhouse effect
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