Collectors used in modern domestic solar water heating systems

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Solar thermal collectors capture and retain heat from the sun and transfer this heat to a liquid. Two important physical principles govern the technology of solar thermal collectors:
  • Any hot object ultimately returns to thermal equilibrium with its environment, due to heat loss from the hot object. The processes that result in this heat loss are conduction, convection and radiation[17]. The efficiency of a solar thermal collector is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heat energy that can be retained for a predefined period of time). Within the context of a solar collector, convection and radiation are the most important sources of heat loss. Thermal insulation is used to slow down heat loss from a hot object to its environment. This is actually a direct manifestation of the Second law of thermodynamics but we may term this the ‘equilibrium effect’.
  • Heat is lost more rapidly if the temperature difference between a hot object and its environment is larger. Heat loss is predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Conduction, convection as well as radiation occur more rapidly over large thermal gradients[18]. We may term this the ‘delta-t effect’.
The most simple approach to solar heating of water is to simply mount a metal tank filled with water in a sunny place. The heat from the sun would then heat the metal tank and the water inside. Indeed, this was how the very first SWH systems worked more than a century ago[3]. However, this setup would be inefficient due to an oversight of the equilibrium effect, above: once when the tank and water has started to be heated, the heat gained would be lost back into the environment, ultimately until the water in the tank would assume the ambient temperature. The challenge is therefore to limit the heat loss from the tank, thus delaying the time until thermal equilibrium is reached.
ICS or batch collectors overcome the above problem by putting the water tank in a box that limits the loss of heat from the tank back into the environment[19][20]. This is achieved by encasing the water tank in a glass-topped box that allows heat from the sun to reach the water tank[21]. However, the other walls of the box are thermally insulated, reducing convection as well as radiation to the environment[22]. In addition, the box can also have a reflective surface on the inside. This reflects heat lost from the tank back towards the tank. In a simple way one could consider an ICS solar water heater as a water tank that has been enclosed in a type of ‘oven’ that retains heat from the sun as well as heat of the water in the tank. Using a box does not eliminate heat loss from the tank to the environment, but it largely reduces this loss. There are many variations on this basic design, with some ICS collectors comprising several smaller water containers and even including evacuated glass tube technology[19]. This because ICS collectors have a characteristic that strongly limits the efficiency of the collector: a small surface-to-volume ratio[23]. Since the amount of heat that a tank can absorb from the sun is largely dependent on the surface of the tank directly exposed to the sun, it follows that a small surface would limit the degree to which the water can be heated by the sun. Cylindrical objects such as the tank in an ICS collector inherently have a small surface-to-volume ratio and most modern collectors attempt to increase this ratio for efficient warming of the water in the tank.

Flat plate and evacuated tube collectors side-by-side.
Flat plate collectors is an extension of the basic idea to place a collector in an ‘oven’-like box[19]. Here, a pipe is connected to the water tank and the water is circulated through this pipe and back into the tank. The water tank is now outside the collector that only contains the pipes. Since the surface-to-volume ratio increases sharply as the diameter of a pipe decreases, most flat-plate collectors have pipes less than 1 cm in diameter. The efficiency of the heating process is therefore sharply increased. The design of a flat-plate collector therefore typically takes the shape of a flat box with a robust glass top oriented towards the sun, enclosing a network of piping. In many flat-plate collectors the metal surface of the pipe is increased with flat metal flanges or even a large, flat metal plate to which the pipes are connected[24]. Since the water in a flat-plate collector usually reaches temperatures much higher than that of an ICS, the problem of radiation of heat back to the environment is very important, even though a box-like ‘oven’ is used. This is because the ‘delta-t effect’ is becoming important. Formed collectorsare a degenerate modification of a flat-plate collector in that the piping of the collector is not enclosed in a box-like ‘oven’. Consequently these types of collectors are much less efficient for domestic water heating. However, since water colder than the ambient temperature is heated, these collectors are efficient for that specific purpose[25].
Evacuated tube collectors is a way in which heat loss to the environment[19], inherent in flat plates, has been reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes[26]. Since two flat sheets of glass are normally not strong enough to withstand a vacuum, the vacuum is rather created between two concentric tubes. Typically, the water piping in an evacuated tube collector is therefore surrounded by two concentric tubes of borosilicate glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent[27].
Flat plate collectors are generally more efficient than evacuated tube collectors in full sunshine conditions. However, the energy output of flat plate collectors drop off rapidly in cloudy or cool conditions compared to the output of evacuated tube collectors that decrease less rapidly[19]. In-depth discussion of different solar collector types and their respective applications and performance, also those used in industrial applications, can be found in the Wikipedia article on Solar thermal collectors.
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