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Evacuated tubes: make sure you don’t get burnt

Posted by Gabriel Wondrausch on 4 February 2009 at 8:34 am

Evacuated tube collectors are the Formula One of the solar thermal world. They are more efficient than other types of collector as they are almost perfectly insulated, and heat cannot pass through a vacuum. As the tubes are cylindrical, they are always perpendicular to the sun reducing the amount of energy that is reflected before it hits the absorber. They are capable of achieving much higher temperatures than other solar collectors as the vacuum ensures that no heat is ever lost.

Some evacuated tubes get as hot as 298˚C while the outside of the tube is still cool to touch. The vacuum in the evacuated tubes also has the advantage of allowing the tubes to function regardless of how hot or cold it is outside. They will start working as soon as there is radiation. This means that even on a freezing cold winter’s day as long as there is sunlight, your solar system will still be providing you with energy. Evacuated tubes do not rely on direct sunlight in the way that photovoltaic panels do. They will still work in low light conditions.

Unfortunately a large number of the evacuated tube collectors sold originate in China and are of very poor quality. Quite often the components are shipped from China, put together in Europe and sold as European. Generally speaking they are made with lower grade raw materials and are not manufactured to a high quality and as a result are not as efficient as evacuated tube collectors should be. Admittedly, they are an awful lot cheaper than their European manufactured counterparts, but you don’t really want to skimp with something that is going to be attached to your roof and potentially reach temperatures in excess of 200˚C.

At SunGift Solar we only use high quality evacuated tube collectors. In fact, our most popular evacuated collectors are manufactured here in the UK and have the added advantage of working particularly well in our often cloudy climate.

So let’s say you’ve done your homework and you have found a good collector. The next essential point to consider is that evacuated tube collectors can be too good at what they do. Unless your system is designed correctly, the fluid in the tubes can reach extreme temperatures, causing it to vaporize and stagnate, in turn reducing the efficiency of the collector. This can be prevented by simply ensuring that you have a storage tank big enough to remove the heat.

It is also a good idea to add a thermostatic mixing valve to the cylinder. This allows the temperature of the water in the cylinder to be heated to 85˚C, enabling the solar to work for longer and harvest more energy. The thermostatic mixing valve then blends the hot water with cold water to your desired temperature before going to the hot water outlets.

A lot of the repairs that we carry out on evacuated tube systems are the result of poor installation. This is why, to get the most out of your evacuated tubes, it is important that the solar system as a whole is designed correctly. A correctly designed and installed evacuated tube system will provide you with many years of trouble-free hot water.

If you are thinking about going to go for evacuated tubes, it is well worth doing your homework. Personally, I am confident that they are the best type of solar collector on the market at the moment. We wouldn't fit them otherwise.

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

If you have a question about anything in the above blog, please ask it in the comments section below.

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2 comments - read them below or add one

Northstar Energy

Northstar EnergyComment left on: 3 April 2009 at 12:31 pm

Keith, That's an excellent point. The difference in the lifespan of a solar collector can generally be assessed by the guarantees given. For example, we offer Genersys solar panels which have a 20 year unconditional guarantee - which covers even any loss of performance - compare this to the average 12 months guarantee on an evacuated tube system.. Just a point about the performance; The difference in performance between an evacuated system and a 'flat-panel' system depends on the outside temperature. We are based in the South of England and the mild temperatures we experience mean that, for a given area of roof-space, a good flat-panel gives a higher yearly output than an evacuated tube system due to their very large absorber area. This can be easily demonstrated using the Governments Standard Assessment Procedure (SAP 2005) Higher performance, better looking and longer lasting – you made the right decision in choosing flat-plate solar collectors.

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keith poulter

keith poulterComment left on: 7 February 2009 at 8:43 pm

When we bought our Roth flat plate solar panels, the person selling them to us said that the performance was about the same between flat plate and evacuated tubes, but as there were many fewer joints involved in the manufacture of flat plate panels, they needed less maintenance. Yet you say that an evacuated tube system will provide many years of trouble-free hot water. Is this a fair point, or a bit irrelevant?

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