|
Residential Wind Power
Solar Thermal Collectors
WINDerFUEL |
Operating principles of double wall vacuum tube with selective surface coating The key component of the solar collector is the double wall vacuum tube. It is made by two concentric transparent borosilicate glass tubes (Pyrex) able to resist impact from hail up to 25mm in diameter. The inner glass tube is coated with a selective coating of Al-N/Al, which absorbs and converts the maximum of solar radiation & infrared light into heat and reduces emissivity. A barium getter is used at bottom of the inner evacuated tube. This barium layer actively absorbs all CO, CO2, N2, O2, H2O The "tube" is one continuous piece with no metal embedded which makes the vacuum tube have longer service life time. A heat-pipe is a sealed tube containing a small quantity of a volatile liquid with no air or other "permanent" gas present. When it is placed vertically and the lower end is heated, liquid will evaporate and the vapour so formed will travel to the cooler parts of the pipe where it will condense and give up its latent heat to vaporisation. The condensate will then run back to the heated end where it can re-evaporate. This is illustrated above, right. Because the heat transfer within the pipe comes from boiling liquid and condensing vapour, both of which processes have inherently very high heat transfer coefficients, and because the amount of material which has to move from one end of the pipe to the other is small, the effective thermal conductivity of the heat-pipe is very large. To illustrate the magnitude of these quantities imagine that the heat-pipe is transmitting one kilowatt using refrigerant as the working fluid. The mass flow would be just under 0.5 g/s. At a temperature of 100 C in a 20 mm diameter pipe this would correspond to a vapour velocity of about 2.5 m/s. In more sophisticated versions, the heat pipe contains a capillary wick to assist the return of the liquid from the condenser end to the evaporator end. Such pipes will work without the aid of gravity, for example in spacecraft. However, for terrestrial applications it is far cheaper and simpler two-phase thermosiphon, as the gravity return heat-pipe is usually known, is adequate. The maximum operating temperature of a heat pipe is the critical temperature of the used heat transfer medium. Since no evaporation/condensation above the critical temperature is possible, the thermodynamic cycle interrupts when the temperature of the evaporator exceeds the critical temperature. The main useful characteristics
of the two-phase thermosiphon are:
These characteristics make
heat-pipes useful wherever a large amount of heat needs to be conducted
through a small cross-section. They have been used in cooling space-craft
components, in cooling plastics-forming dies, for the construction of air-to-air
heat exchangers for industrial and domestic energy recovery, and in cooling
electronic components mounted in confined spaces. Or, as the first heat
exchanger in a solar thermal collector for domestic applications
|
|
|