The solar collector is the engine of any solar water heater. Solar vacuum tubes have always been the most efficient solar power production systems but were more expensive than other flat panel system. However the growing demand of solar energy and modern manufacturing techniques has driven down the cost such that vacuum tube technology provides the greatest return on investment versus any other solar collector system.
The principle behind solar vacuum tubes is simple. A solar vacuum tube works similar in design to a coffee thermos. It consists of a two layers of glass with a vacuum in between the layers. The outer layer of the solar tube is Borosilicate glass which is very low in iron and allows 98% of light energy to pass through. The 2nd inner layer has very special coatings applied to it.
The SunRain solar vacuum tubes Northern Lights supplies use a patented 3-Layer process that results in a coating that can absorb more of the sun's energy while being able to withstand temperatures in excess of 300 degrees (575 F) without deteriorating. This coating is what makes these solar heat collectors superior to any other collector on them market. The special selective coating change the short wave solar radiation into long wave heat radiation and is almost 94% efficient meaning only 6% of the suns energy is lost!
The absorbed solar radiation is then transferred to a heat transfer fluid with in the tube (liquid inside the thermos), This liquid quickly heats up and rises to the top. In an open vacuum tube system the potable water is heated directly with in the vacuum tube. Thermosiphon causes the hot water to rise and be replaced by colder water. The hot water is the collected in the top collection chamber and pumped though the system.
Heat Pipe Technology
Is the most efficient heat transfer technology available for solar system on the market today. Instead of water flowing in the center of the vacuum tubes, a hollow copper tube is inserted through the length of the tube. This special tube contains a small amount of special liquid that acts as heat transfer medium. The hollow chamber is made under vacuum conditions which cause the fluid to vaporize at lower temperatures (30 C or 86 F) as a result of the low pressure. The vapor rises to a condenser bulb (heat dissipater) where it is cooled back to a liquid. Copper heat pipes are used because they can absorb and transfer heat very efficiently with virtually no energy loss. This technology allows the energy to be transferred indirectly to the hot water system. It is used extensively in systems that require a heat exchanger such as close loop system. Any hot water system being designed in freezing conditions will need to use heat pipes as antifreeze must be used and the therefore the water needs to be separated from the heating system via a solar heat exchanger. Heat pipe systems are more efficient than open vacuum tubes but also come at a significantly higher price with the added copper materials.
Vacuum Tube Heat Pipe Construction/1800-30G
Collector Header Construction - TZ58/1800-30G
Physics of Open Vacuum Tubes & Heat Pipe Vacuum Tubes
These heat tubes utilize an open vacuum tube. Water actually fills the inner vacuum tube and is heated internally by the sun (just like liquid in a thermos). Warm water is lighter than cold water and rises to the top of the tube where it fills the collector. The heavier cold water sinks to the bottom of the vacuum tube to replace the heated water and to be reheated by the sun. This natural phenomena is know as thermo-siphoning. The advantage of this type of heat system is that there is no costly copper piping involved therefor these open tubes are the most cost effective. The disadvantage is that they can only be used in an open loop system that is controlled by the atmospheric pressure. Applications such as heating swimming pools are ideal with this type of solar water heater because water flows directly into the tubes. These type of solar collectors should only be used in applications above freezing. They can withstand small amounts of pressure but should not be used in pressure system found in most closed loop applications such as domestic hot water and space heating.
These high performance heat tubes extend the possibilities of solar water heat. The principle is similar to that ofopen solar vacuumtubes (above) except that water does not physically enter into the vacuum tube. Instead, specially designed hollow copper heat pipes are inserted into the vacuum tube. Copper being and excellent thermo-conductor, conducts heats efficiently. The inside of the heat pipe contains pure waterresiding within in a vacuum. This vacuum means the water will boil at less than 30 degrees. Hotvapor rapidly rises to the top into a condenser tube. These condenser bulbs will transfer the heat to the manifold (heat sink) in contact with it. The condenser bulbs are designed to reach temperatures of 200 Degrees Celsius or more. High temperature translate to higher efficiencies Using multiple tubes, the condenser bulbs are inserted into a copper heat transfer manifold. These manifolds can withstand up to116 PSI of pressure. The water in a heat pipe system never actually enters the vacuum tube. This means it can be used in systems involving colder climates. If a vacuum tube ever broke, the system would continue to function. The Heat Pipe Solar Tubes can be used for multiple applications such as hot water tanks, radiators and in floor heating. The copper collector only holds a small amount of liquid and can be used with any heat exchange using glycol mixture to prevent freezing.