A novel water desalination system was designed and manufactured at the Institute of Sustainable Energy Laboratories at Nottingham University. This system employs the concept of humidification and dehumidification based on the pyschrometric energy process using a special membrane to convert the saline water into fresh water. Figure 27.1 provides schematic diagrams of the desalination process and system components. This system includes the water desalination unit, evacuated solar collector, storage tank, circulation pumps, air fan and auxiliary components. The desalination unit consists of the humidification chamber, which contains an evaporation core (E-core) where evaporation takes place, and the dehumidification chamber, which contains an evaporation-condensation core (E/C-Core) where both evaporation and condensation takes place for energy recycling and water production respectively. The key innovation is the re-use of the psychrometric energy created by the condensing of the moisture in the carrier gas: a little thermal energy is supplied to the humidification and dehumidification process. The HDD system could be one – stage, or multiple stages. Figure 27.4 presents a schematic diagram of a one-stage solar HDD process. The Evaporation Core (E-Core) inside the humidifier is made from packing materials of cellular paper with a specific area of 450 m2/m3 volume while the evaporation condensation (E/C) core contains a special membrane of a plastic heat exchanger with two channels; the evaporation channel has a hydrophilic surface and the condensation channel has a hydrophobic surface. This makes the condensation and evaporation process in the E/C core very efficient.
The desalination unit is connected to a 120 L storage tank through the humidifier with a circulation pump and flow meter regulator to adjust the mass flow rate of hot water. This storage tank is fully insulated with foam insulation materials to reduce the heat losses and to keep the system running during the night utilizing the heat stored from solar energy during the day. The water inside the storage tank is heated by a helical copper tubular heat exchanger fixed inside the storage tank as shown
Fig. 27.2 Evacuated solar collector manifolds and heat tubes assembly
in Fig. 27.1; the outer diameter of copper pipe is 22 mm and the total length of heat exchanger is 5.73 m with 6 turns. The inlet and outlet of heat exchanger are connected, respectively, to the outlet and inlet of the manifold at the top of evacuated solar collector so that these form a closed loop and an electrical pump circulates the water in the loop.
The copper manifold header pipe of the collector is a long horizontal cylinder with a volume of approximately 0.45 L. The header pipe also contains 20 small cylindrical heat pipe housing ports, as shown in Fig. 27.2 . The axis of each housing port is perpendicular to the flow direction in the header pipe. In the solar collector, the head of each evacuated tube heat pipe is inserted into a separate housing port and the heat from the heater pipes is transferred to the flow inside the header pipe through the walls of the housing ports. The thermal contact between the heads of the heat pipes and the housing ports is provided by using a special metallic glue compound. An expansion vessel is also incorporated into system in order to prevent the possibility of system damage due to an increase in pressure. The vessel has two halves: one half connects directly to the water system while the second, separated by a special diaphragm, contains air. As pressure rises and the volume increases, the diaphragm is displaced. In addition, the fluid pressure in the solar collector manifold is monitored by a pressure gauge—Bar 100XKPA, 0.60 psi—as illustrated in Fig. 27.2.
A solar sunlight simulator of the application of artificial radiation in the form of comprising of an array of 30 halogen floodlights significantly extends the range of insolation values in the experiment. The floodlights are evenly spaced on a frame installed above and in parallel to the evacuated tubes, as shown in Fig. 27.3. The array is divided into three groups and is connected to the grid via a 3-phase transformer, which enables the level of the radiation flux to be gradually regulated. The maximum electrical power consumed by each floodlight is 400 W. A pyronometer with sensitivity of 17.99 * 10-6 Volts/W/m2 measures the radiation flux at 20 different locations on the surface of the evacuated tubes and in the spaces between them. The results were averaged as presented in Fig. 27.6. The test rig is also equipped with a metered cylinder tank, circulation pumps and a set of K-type thermocouples to measure the temperature of the water circulating in the heated circuit and the temperature of the saline water at several points in the system. An anemometer device measures air flow velocity and water flow meters measure the flow of the saline hot
Fig. 27.3 Solar simulator with desalination rig
water, the cooling water and the fluid inside the solar collector manifold. In addition, humidity sensors measured the relative humidity at various locations in the rig.