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