The most efficient modern solar water heating systems utilize the latest evacuated tube and heat-pipe technology to make them extremely efficient. They can convert 60% of the solar energy that falls on them into useful heat energy. They can operate in temperatures as cold as -60°C enabling them to contribute to water heating throughout the UK winter. The solar collector is connected to a closed loop which circulates an antifreeze mixture which transfers the heat to the water tank. Hence there is no risk of frost damage and they are durable, and resistant to hail. They are so efficient that they are capable of heating water to boiling in summer (only where required for commercial applications).
Our solar collectors are comprised of evacuated vacuum tubes. They absorb solar energy, converting it directly into heat for use in water heating. Each tube is constructed like a Thermos™ flask, so the heat energy goes in but doesn't come out. Solar tubes have been used for years in Germany, Canada, Sweden, Norway, China and the UK. Each solar tube consists of two glass tubes made from extremely strong borosilicate glass (same as Pyrex™). The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating (Al-N/Al) which features excellent solar radiation absorption and minimal emission/reflection properties. The ends of the tubes are fused together with the space between them being evacuated. The insulation properties of the vacuum are so good that while the inside of the tube may be 175°C, the outer tube is cold to touch. This means that solar tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss. Once in position the tubes are very strong and can withstand hailstones. These systems are CE, ISO9001 and Solar Keymark certified.
How Heat Pipes Work
Inside each evacuated tube is a Heat pipe. This is an extremely efficient conductor of heat (in one direction only). The heat is transferred up to the bulb of the heat pipe, which fits snugly into the manifold, through which the working fluid flows. As the working fluid (an antifreeze mixture) circulates through the top (horizontal) pipe it is heated by the concentrated heat conducted up from each of the vacuum tube/heat pipes. Thermal grease applied to the heat pipe bulb before inserting into the manifold ensures excellent thermal transfer to the manifold. They are very simple and hence very reliable. Heat pipes might seem like a new concept, but you are probably using them every day and don't even know it. Laptop computers often use small heat pipes to conduct heat away from the CPU, and air-conditioning systems commonly use heat pipes for heat conduction.
The heat pipe is hollow with the space inside evacuated, much the same as the solar tube. In this case insulation is not the goal, but rather to alter the state of the liquid inside. Inside the heat pipe is a small quantity of purified water and some special additives. At sea level water boils at 100°C, but if you climb to the top of a mountain the boiling temperature will be less than 100°C. This is due to the reduced atmospheric pressure at altitude.
Based on this principle of water boiling at a lower temperature with decreased air pressure, by evacuating the heat pipe, the same result is achieved.
The heat pipes used in our solar collectors have a boiling point of only 25°C. So when the heat pipe is heated above 25°C the water vaporizes. This vapour rapidly rises to the top of the heat pipe transferring heat. As the heat is lost at the condenser (top), the vapour condenses to form a liquid (water) and returns to the bottom of the heat pipe to once again repeat the process.
At room temperature the water forms a small ball, much like mercury does when poured out on a flat surface at room temperature. When the heat pipe is shaken, the ball of water can be heard rattling inside. Although it is just water, it sounds like a piece of metal rattling inside.
This explanation makes heat pipes sound very simple - a hollow copper pipe with a little bit of water inside, and the air sucked out! Correct, but in order to achieve this result more than 20 manufacturing procedures are required and with strict quality control.
Material quality and cleaning is extremely important to the creation of a good quality heat pipe. If there are any impurities inside the heat pipe it will affect the performance.
The copper used is oxygen-free copper, thus ensuring excellent life span and performance.
Each heat pipe is tested for heat transfer performance and exposed to 250°C temperatures prior to being approved for use. Further Heat pipe info.