Solar Thermal Panels – Flat bed or Evacuated Tubes

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?

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52 Responses to Solar Thermal Panels – Flat bed or Evacuated Tubes

  1. I think that it is inaccurate to describe fevacuated tubes as more complicated technology; high quality modern flat plate panels have complicated technology and Genersys produce an evacuated flat plat.
    Tubes are less relaible than panels becasue of tube failures. High quality panel failures are almost unknown, becasue the flat plates are designed to handle the heat stresses better.

    It is highly inaccuarte to describe flat plates as being with a panel of black painted metal; these types only work in very hot countries. Most flat plates sold in the Uk are selectively coated, not painted.

    Robert Kyriakides

  2. solarkent says:

    I agree that Robert is right about the risks of Tubes against flat plate, also the fact that modern flat plate panles are not just a sheet of copper with a bit of black paint, much more sophisticated that that!

    However to be fair to tubes, there are systems that avoid the cracking, in particular the failure where the copper meets the glass. I had a demo of solar tubes from solaruk (www.solaruk.com) where the evacuated tube is a totally seperate entity to the water circulation system, the tube fitting around the collector.

    This particular layout doesn’t seem to be that common but possibly resolves the real problems that Robert has identified.

    Certainly not a reason to go for tube wholesale, its just there are samples out there which might be appropriate.

  3. Gary Burk says:

    I’m now building a home in the Colorado hills at 7500 feet altitude. It gets lots of sun, cold winters, lots of snow, and has frequent high wind (50mph) and occasional very high wind and large hail (building code spec is for 136mph wind). I’m concerned about the use of evacuated tube collectors in regard to both snow and wind driven hail. I’d like to start out with solar DHW assist and the home’s in-floor radiant heating is designed for low-temp (110F) solar heated water – but this must come later for budgetary reasons. Do you have application experience in this type of mountain environment?

    Regards,
    Gary Burk

  4. Geoff says:

    There are clearly some confusions over the complication and reliability of vacuum tube systems and flat plate. Like all products, some are built better than others. Many modern vacuum tube collectors have eliminated the glass to metal seal that has failed on many tubes. Some collectors, like the LaZer2 from SolarUK.com come with a 25 year warranty. With flat plate collectors, the seal between the glass or perspex cover can leak. once water gets into the panel, algae grows and reduced the output of the system.

    • Nick says:

      Hi Geoff, had a look at the website, very useful , I’ve been looking at both flatplate and vacum tube and must admit that the tube option seemd the best way to go apart from price and reliability. The solaruk.net site along with teh solarkent site seem to indicate its almost down to personal preference – price seems about the same and witha 25 year warranty ! though must admit the tubes look to have the edge.

  5. Geoff says:

    Hi Nick, the thing about renewable products is that you are buying 25 years of energy in one go. This is why people should buy a good quality product and have it professionaly installed.

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  7. Bao Pilette says:

    Proper thanks are due for this awesome article. I’ve read id only a few months now and maybe they are always very informative. Thanks!

  8. Vishvesh Lakhani says:

    Can someone please tell me exactly how efficient both the technologies are.
    As of the moment im slightly confused if these have the same efficiency levels PV solar panels. Would really appreciate any kind of helpful information!
    Thanks

  9. SimonM says:

    Visvesh, don’t look so much at the efficiency of each, rather the long term cost (or benefit) of ownership. If one panel is 10% less efficient than another, but costs 1/2 and lasts just as long, then the cost benefit is far greater. So, consider:

    Price
    Nominal Kilowatts for that price
    For what period of the year will it displace all or some water heating.
    Quality – do a lot of web research
    How long between any services
    Cost of any service, what’s involved
    Expected lifespan
    Any guarantees (also how long has company been around)

    Realistically the efficiency is of marginal relevance apart from the effectiveness in extracting heat for a longer part of the year. But then your call is: more cheaper panels?

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