Stoves and furnaces – Tool having fluid fuel burner – Curling iron
Patent
1987-08-21
1989-05-30
Jones, Larry
Stoves and furnaces
Tool having fluid fuel burner
Curling iron
126442, 126901, F24J 302
Patent
active
048340669
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an evacuated solar collector tube in which the vacuum is degradable under certain conditions to limit the stagnation temperature of the tube.
Collector tubes conventionally comprise an inner glass or metal tube, through which a heat exchange fluid is passed, and an outer glass tube enveloping at least a portion of the length of the inner tube. The space between the two tubes is evacuated and the outer surface of the inner tube is coated with a solar selective surface coating. The selective surface coating is chosen to provide for high absorption of solar radiation and low emittance of thermal radiation, so that the collection of incident solar radiation is maximised and the quantity of energy lost by infra-red radiation is minimised. A high level vacuum, in the order of 10.sup.-5 Torr surrounding the selective surface coating virtually eliminates heat loss from the surface by conduction and convection processes.
A selective surface coating which has been found to be particularly efficient comprises a base coating of copper and an outer iron-chromium-nickel-carbon cermet. The copper is sputtered onto the tube by a non-reactive process and the metal-carbide coating is deposited by sputtering the metal from a stainless steel electrode onto the tube surface in the presence of a reactive gas such as acetylene. The reactive sputtering process is controlled such that the resultant cermet is graded from a metal-rich coating at the interface with the copper to a carbon-rich coating at the outside.
The combination of an efficient selective surface coating and a high vacuum insulation results in high quality collectors which stagnate (i.e., reach an equilibrium condition where losses equal energy gain) at temperatures as high as 300.degree. C. in non-concentrated sunlight. Thus, the collector elements are particularly useful in thermal systems which are designed to operate at quite high temperatures. The collectors are also very suitable for use in systems which are intended to operate at relatively low temperatures, because the low losses and high collection efficiency permit high energy collection under various conditions, including when relatively heavy cloud cover exists.
However, in some low temperature systems, such as those which are used for producing domestic hot water, tubes which stagnate at very high temperatures (i.e., in the order of 300.degree. C.) can create significant problems. The systems must be designed to prevent the high temperature fluids from entering the system reservoir, where the heat may cause permanent damage to the reservoir lining. Moreover, the high temperature fluid may present a potential safety hazard to users of the system. Therefore, it is desirable under certain circumstances to limit the capability of collector tubes to reach very high temperatures, but any process which is employed to do so should not effect the high performance of the collectors at low temperatures.
One method of achieving high performance at low temperatures and of reducing the stagnation temperature has been suggested in a paper entitled "Thermal Conduction in Evacuated Concentric Tubular Solar Energy Collectors Degraded by Low Pressure Gas" by G. L. Harding and B. Window in Solar Energy Material 4 (1981) 421-434, North-Holland Publishing Company. In page 434 of this paper brief reference has been made to the feasibility of using a gas in the evacuated space of a collector tube to limit the stagnation temperature.
The present invention extends this basic concept by proposing the employment of a gas which exhibits hydrophobic characteristics, which adsorbs onto the selective surface coating at low temperatures (e.g., at temperatures less than about 150.degree. C.) and which desorbs into the otherwise evacuated space at higher temperatures.
Thus, the present invention provides a collector element for use in a solar energy collector system and which comprises an inner (glass or metal) tube into which a heat exchange fluid may be directed, an outer glass tube enveloping at least a port
REFERENCES:
patent: 4142509 (1979-03-01), Hermann et al.
patent: 4186724 (1980-02-01), Nelson
patent: 4282857 (1981-08-01), Pei
Bourke Brendan
Collins Richard
Pailthorpe Bernard
Jones Larry
Rheem Australia Ltd.
The University of Sydney
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