About Vacuum Tube Collectors
Solar water heater vacuum tubes are the new technology in solar thermal applications. They are inexpensive, easy to install and individual tubes can be replaced if one of them breaks. Best of all, the vacuum allows for efficient capture of heat even in the coldest climates.
How Heat Transfer Works
There are 3 forms of heat transfer.
When two objects come into contact, heat transfers between the two surfaces. The heat transfer rate is based on the type of materials and contact surface area. copper is without question, the fastest heat conducting material. These solar water heaters use copper heat pipes to rapidly transmit heat from the tubes to the manifold. The speed of the heat transfer is aided by use of evaporation and condensation of an antifreeze fluid which is inside of the heat pipe. After heat has transferred to the manifold, a working fluid circulates through the manifold to collect the heat from the copper piping inside of the manifold.
The vacuum tubes are very well insulated from heat loss by use of vacuum. They are comprised of 2 tubes, an inner and an outer where a vacuum is applied in between them. While they do not have a full vacuum, they are very close to it with a pressure rating of only 5 x 10-3 Pa. Conduction requires matter to transfer through. In a pure vacuum, absolutely no conductive heat transfer can occur. In between the tubes there is very little air, so conduction of heat through it to the ambient air outside of the tube is negligible to the efficiency of the solar water heater system.
Convection occurs when a fluid flows across a material or another fluid. Convective heat transfer can be much more rapid than conduction. Convective heat transfer can only occur through matter. It cannot occur through a vacuum. Unlike in flat panel collectors, when wind blows across the solar vacuum tubes, there is no heat lost to the ambient through convection.
Radiation occurs all the time with any object. Heat can travel through a void space through radiation. Radiation, however, transfers at a very low rate compared to conduction and convection. While the tubes do lose some of the heat through radiation, the amount of heat loss is negligible to the solar system since radiation transfers at such a slow rate.
Construction of Duda Solar Vacuum Tubes
Solar vacuum tubes comprise of two tubes. The outer tube is a clear 58mm OD borosilicate glass and allows for sun to pass right through. The inner tube is made of a tri-metal coated glass. This is also known as a "Three Target" material which is comprised of Cu/SS-ALN(H)/SS-ALN(L)/ALN. The OD of this tube is 47mm, and it has a dark coating which absorbs the sunlight and converts the sunlight into thermal energy (heat). The inner part of the tube has a shiny coating which helps influence the heat to dissipate towards the center of the tube.
Between the outer and inner tube, there is a sealed vacuum which is applied with a vacuum pump by the end of the tube and then sealed off by melting the glass. This vacuum greatly reduces the amount of heat able to escape the tube, thus trapping it inside of the inner tube. Conduction and convection through the tubes is minimal since there is very little air inside of the tubes (there is no way to achieve a full vacuum) and so heat can only escape by radiating out of the tubes. Radiation has a very slow rate of transfer compared to the other two forms of heat transfer and so most of the heat remains trapped inside the inner tube, even in high operating temperatures.
Once the heat is trapped inside the inner tube, it then travels through the air inside towards the middle where the copper heat pipe is. There are aluminum fins which surround the copper heat pipe. These fins help aid in rapid heat transfer to the copper heat pipe.
Once the heat reaches the heat pipe, it then transfers up the heat pipe and into the copper header pipe which is inside of the manifold. The heat pipe rests inside of a port which fully encloses the condenser (top) of the heat pipe. This part of the heat pipe is called the condenser because the heat pipe is hollow and filled with a fluid which evaporates at the bottom of the heat pipe, travels up to the top and then condenses inside of the condenser, dripping back down to the bottom of the heat pipe to collect more heat.
The working fluid in the active system passes through the inside of the header pipe, collecting heat as it passes through. The header pipe is insulated by high temperature rock wool which has a high density and great R-value for resisting the heat loss. This insulation is enclosed by an aluminum casing and sealed away from the ambient air. This enclosure is called the manifold.