Which is better - solar flat panels or evacuated tubes?

I am often asked about the difference between evacuated tubes and flat plate solar panels; people want to know which is better and there are some confusing websites out there, particularly those which extol the virtues of evacuated tubes, so I shall let you have my views on each type of product. You will have to bear in mind my own bias; after some considerable research I invested my own money in a flat plate factory for Genersys; I personally decided my preference and the reasons have not changed in the past eight years.

First I should explain about evacuated tubes. They are round long glass cylinders, which are evacuated and contain absorber fins. The fins are coated with a selective coating that reacts to light and converts it to heat, just like flat plate panels. Sometimes the fins are contained within a vacuum and other models simply put a vacuum shell, rather like the shell inside a thermos flask, around the fins. The fins attach to a copper heat pipe, sometimes containing a refrigerant gas, which conducts the heat to a manifold, to which a number of tubes are connected. The manifold contains a heat circuit which delivers the heat to where it is needed.

 

Most evacuated tubes are made in China where evacuated tubes comprise almost all the market. However, evacuated tubes were invented in Germany, by a team led by one of my friends, Dr Peter Schubert, who worked for Dornier, which was then taken over by Daimler Benz Aerospace, who sold the technology to a Chinese company as part of a deal which allowed Daimler Benz to sell trucks in China.

 

The evacuated tube system has claims to be high in efficiency because the vacuum prevents large heat losses. The design of evacuated tubes has remained relatively stable for the past ten or so years, with many new models coming on the market, but all operating on the same principles. There is no question that a vaccum is very good at limiting heat losses to the minium.

 

The “high efficiency” claim for evacuated tubes is somewhat misleading. If you had to pay for light then high efficiency would be desirable, but light was, when I last looked, free, so the marginal efficiency of evacuated tubes over flat plate panels makes no difference. If you were not able to store heat energy the extra marginal heat of evacuated tubes would be important, but you can and should store heat energy, because that is the most efficient way to operate a solar system; you collect heat energy in daylight hours in sufficient quantity to store it when you need it. Virtually every domestic solar system will spend a lot of the time “stagnating” because there is no useful demand for the heat energy.

 

One web site claims that evacuated tubes allowed water to be heated to higher temperature than flat plates; this claim is nonsense. The temperature to which any modern solar system may be heated is restricted by a number of factors including:-

 

1. The ability of the heat transfer fluid, typically glycol, not to vaporise. As soon as the fluid becomes gas it is useless for heat transference. Typically modern solar systems are pressurised, so the glycol can operate in excess of 120 degrees Celsius, but not above this figure.
2. All solar systems have to be regulated to prevent over heating; this is a large potential problem. Overheating is particularly a problem for evacuated tubes as it exposes weaknesses in the materials over time and this leads to a loss of vacuum.
3. The tube may be evacuated, but the manifold that the tubes feed into is definitely not evacuated; this is insulated and suffers heat loss. So, in some ways, evacuated tubes have a distinct disadvantage compared with flat plates; they are more prone the tubes over heating than a flat panel is, but they lose the heat anyway, through the manifold.
4. All modern solar systems, whether powered through evacuated tubes or flat plate will heat water to the same temperatures. Typically in Germany the solar panels heat water to 80 degrees Celsius in smaller stores and then the potable water in blended, to prevent scalding in the home. Around 92% of domestic solar systems in Germany use flat plates, not evacuated tubes. In the United Kingdom most solar systems are regulated to heat water up to 50 to 60 degrees Celsius, depending on consumer preference and life style.

 

However, even though it is a small matter, evacuated tubes are more “efficient” so if you have a very small roof area on which to fit your system, you might have to use evacuated tubes. Of course there is only a limited amount of energy is a given area of light – no form of solar panel can get more energy out of the light than exists in it, so if  you are restricted then you might need evacuated tubes with their smaller footprint. Generally (and it is difficult to give more than a rule of thumb) an array of evacuated tubes will probably provide around 10% more heat than an identically sized array of flat plate panels in identical conditions.

 

The trade off for the marginally higher “efficiency” of evacuated tubes is their unreliability. They do overheat and this leads to tube failures. If you think of an evacuated tube as a tungsten light bulb, you will understand that from time to time tubes “pop” and you need to get up on the roof to replace them. Often you will not be able to tell if a tube has lost its vacuum; in frosty weather the tube without a coating of ice is the one that has lost its vacuum.

 

Evacuated tubes have been around for thirty years; they are generally used in Germany (the developed world’s most sophisticated and largest thermal solar market) for specific projects, often industrial ones, where there is a regular maintenance on site. The Germans did use them extensively years ago, but found the reliability and durability of flat plates a better alternative, which is why the market share of tubes has declined and might, also, be a clue as to why Daimler Benz sold its solar thermal evacuated business to China.

 

Finally you can easily roof integrate flat plate panels, which you cannot do with tubes. If you want the extra heat form the same footprint then you can opt for the world’s only evacuated flat plate panel – by Genersys – which can be re-evacuated from within the home.

 

Whichever technology you opt for I should make one thing clear. Even if you have roofing skills and plumbing skills neither option is a “do it yourself” task. Solar systems must be fitted by trained installers, who have been trained to fit them.