MODULE III Air-conditioning and Cooling technology Page 72

Solar energy source for a cooling system

The solar actuated cooling system consists of a cooling tank with an evaporator integrated into the lid, which contains water as a coolant. In addition there are one or more zeolite containers, a hand vacuum pump and a parabolic collector. In order to adequately dry the zeolite, a temperature of over 250 °C is necessary, making a concentrating system necessary. Flat collectors cannot reach such a high temperature. A solar boiler parabolic collector will be used for the described cooling system. Up until now, such collectors have generally been found mainly in developing countries, where they offered an alternative to wood burning or fuel consumption. In addition to the preparation of meals, the boiler is suitable for the sterilisation of medical instruments or for boiling water. Up to now more than 10,000 such collectors have been built. With a diameter of 1.4 m, it consists of highly polished, anodised aluminium sheets, and is thereby weather resistant. The mounting can rotate, and is set in a flat rolled steel frame on wheels, so that the entire collector can follow the "path of the sun" by rotating its frame and adjusting the angle of its mirrored surface. The capacity at an irradiation of 1000 W/m² amounts to ca. 800 Watts for the desorption. To dry the zeolite, the zeolite container filled with circa 4 kg zeolite is placed in the burning point of the parabolic mirror. At maximum temperatures of circa 350 °C the zeolite is regenerated after about 4 hours.


Adsorption cooling

For cooling purposes, a zeolite container filled with dry zeolite is connected to a flat evaporator filled with water. Next, the air is removed from the system with the hand powered vacuum pump. Following the connection of the zeolite container and accompanying the evacuation process, the water will begin to steam strongly, where there is sufficiently low pressure. As a result of the already described cooling process, the water in the evaporator will be cooled and subsequently freeze. Further water molecules evaporate from the resulting ice (sublimation), which results in the further cooling of the ice. Using a zeolite cartridge, the insulating box, with a usable volume of 55 l, can be cooled at an environmental temperature of 22 °C and maintained at a temperature level of between 0 °C and 6 °C for approximately three days. At the end of this time period, the evaporator must again be filled with water, a regenerated zeolite cartridge must be connected, and the air must again be pumped out of the system. In order to be able to continuously cool over a longer period of time, and to bridge over periods of poor weather, one must have a supply of several regenerated zeolite cartridges.

Fig. 57 Adsorption cooling box