BASIC MECHANISMS

Sun-heating by Several Means

In the FPC model, which is the simplest and the most common type of solar thermal device, the system works basically on a flat, blackened absorber plate inside the collector. When sunbeams are allowed to strike this plate, heat gets trapped inside the collector. This heats up the water in the copper tubes that run through the plate, causing the water to circulate through the system by natural conduction or convection. The heated water then gets transported to a storage tank placed above the collector, under well insulated conditions, for further channelling to the user point.
In the ETC model, the system works on two concentric tubes made of tough borosilicate glass. The outer tube is transparent and allows sunrays to pass through it with minimal reflection. The outer surface of the inner tube is selectively coated with an ultra-efficient absorber (A1-N/A1) for maximal solar spectrum absorption and minimal heat loss. The tops of the two tubes are fused tight and all gases in the space between the two glass layers are pumped out while exposing the tube to high temperature. The resulting vacuum acts as an excellent insulator just like in a glass-lined thermos flask.
Vacuum is a valuable factor in the performance of solar heaters because once the solar tube absorbs the radiation from the sun and converts it to heat, vacuum ensures that this heat is not lost. In fact, this technique gets so good that the inner space of the tube would be 150oC (304oF) when the outer side is just cold to touch! This obviously shows that solar tube water heaters perform fine even in winter and rainy seasons.
In order to maintain the vacuum between the two glass layers, a barium getter is used (just as in TV tubes). During the manufacture of the solar tube, this getter is exposed to high temperatures so as to allow the bottom of the tube to be coated with a pure layer of barium. This barium layer actively absorbs CO, CO2, N2, O2. H2O, H3 and other gases released from the solar tube during storage and operation, thus enabling perfect retention of the vacuum.
In the Thermosiphon model, hot water is generated by natural, atmospheric-pressure-based circulation and no external pressure is used. But this principle can work only up to a particular tank capacity and therefore, for larger capacity systems, hydropneumatic principle involving forced, supernormal circulation is used.
In the FTC model, water is supplied at a fixed, pre-specified temperature under any climatic condition, whereas in the DTC model, water is supplied at the maximum temperature accessible on a given day’s solar radiation, and the temperature can be manually optimized on cloudy days.
In the Heat Exchanger model, only soft water (like mineral water, distilled water, glycol or some other heat-transfer fluid) in minute quantity is fed into the system so that it gets heated first and then exchanges its heat with the hard water to be heated. The soft water comes back again & again by natural circulation and the hard water is prevented from entering the system directly so that it doesn’t cause scaling and eventual damages to the system’s structure. In a similar way, in subzero temperatures, a special variety of thermic fluid and anti-freeze agents are used to make sure that the water doesn’t freeze, the pipe doesn’t burst and the efficiency or the life of the system doesn’t suffer from frost-induced effects.