| The use of sun energy by flat, liquid solar collectors The solar collectors use more than a half of the total sun radiation that reaches the earth. Taking the mean value of insolation (according to the standard) on the level of 1000 kWh/m2, the collector gains up to 600 kWh/m2. The construction and materials used in production of collectors are the result of dozen years of research and improvements since the collectors were used on a mass scale. The interest in solar collectors as a devices of a common use has began in seventies of the past century after the fuel crisis caused by OPEC . In Europe , the devices that use solar energy are applied successfully since than and have more and more share in an energy balance of the european countries. Especially in Spain, Italy and Greece but also in the scandinavian countries. In Germany, which have a similar heliocentric conditions to polish, the Federal Government implements intensively the One milion roofs covered with collectors program by a simple system of direct subvencions to every square meter of colector.  The most important element of the flat, liquid solar collector is the absorber - the element, which gains sun rays. The absorber is a copperplate covered with a selective material. On the properties of selective layer of copperplate depend the absorption coefficient and the efficiency of the whole collector. So far as a selective material was used the oxide of chrome called the black chrome.
But nowadays the most popular material for absorber is a tytanium cover. The process of sputtering ensures the excellent mechanical properties and durability. Also important are the very good absorbtion (α) and emission (ε) coefficients.
The next crucial part of the collector is the collector glass. Usually it's made of hardened glass with low content of iron oxide Fe2O3 (<0,05 %) in order to assure a good light transmission. To lower the heat loses the collector is isolated with mineral wool under the absorber and on the sides of collector. The box of collector is usually made of aluminium boxes or aluminium profiles. The vacuum tube collectors  The vacuum collectors is an advanced, top product of the solar technic. It is up to 30 % more efficient than a flat collector with the same area of absorbtion, especially in spring, autumn and winter. It issues from the ability of vacuum collector to absorb the dispersed radiation of sun and from radically restricted loses of heat thank to the vacuum in the collector tubes. The collector tube consist of two parts which are connected at their tops and ends. Between these two parts is a vacuum.
Tha absorption layer in vacuum collectors in most cases has a form of narrow strip with the copper pipe soldered to it. This strip with the copper pipe goes and along the vacuum tube. Some of the manufacutres apply the method of covernig the outer surface of inner tube with tha absorbtion layer.
The vacuum tubes are connected together in mainfold bar, which also is isolated thermally.  The tubes can be twisted in the optimal direction to the angle of sun rays incidence and thanks to that ability the vacuum collectors can be mounted on facades of buildings or horizontally on flat roofs. 
The next advantage of vacuum collectors is possibility of easy exchange of damaged tubes without necessity of closing the whole solar installation.
The next advantage of vacuum collectors is possibility of easy exchange of damaged tubes without necessity of closing the whole solar installation. High efficiency of vacuum collectors enables supporting of central heating during an early spring and autumn (only with low-temperature floor and wall heating) . |
