Condensation irrigation simulations of heat and mass transfer

Sammanfattning: There is a growing water demand in the world. Along with the deterioration of existing water supplies, the escalating world population leads to the assumption that two out of three people will lack sufficient freshwater by the year 2025. As competition for freshwater increases, water of lower quality, for example saline or waste water, is often used in irrigation at the risk of seriously degrading the farmland. Desalination is the only possible way to produce more freshwater. Each day, 23„ª106 m3 of freshwater is produced from seawater by Reverse Osmosis, Multi Stage Flash and Multi Effect distillation. Most of these plants are driven by fossil fuels and only 0.02% by renewable energy. A sustainable development requires simple inexpensive desalination systems driven by renewable energies. Condensation irrigation (CI) is a newly developed idea that meets these requirements. CI is a combined system for solar desalination and irrigation and/or drinking water production. Solar stills are used for humidifying ambient air flowing over the saline water surface in the still. This warm, humid air is then led into an underground pipe system where it is cooled and vapour precipitates as freshwater on the pipe walls. If drainage pipes are used the water and some of humid air percolate through the pipe perforations and irrigates and aerates the ground. Drinking water can be collected at the pipe endings when using non-perforated pipes. The CI system has attracted attention from several North African countries. Pilot plants are now in operation in Tunisia and Algeria where LTU is collaborating with the Tunisian Institute for Research on Rural Engineering, Water and Forestry and the University of Tlemcen in Algeria. LTU is also collaborating with Al Fatah University and the International Energy Foundation in Tripoli, Libya. Ongoing work aims at developing a design tool and monitoring program for the CI system to be used in the design and operation of a demonstration plant in Libya. Mass and heat transfer in the soil aroundthe buried pipes has been modelled in Matlab to evaluate the theoretical potential for these types of systems and to gain understanding of the mechanisms governing their productivity. It was concluded that CI could be used for both irrigation and drinking water production at relatively low operational costs. For a presumed reference system, the mean water production rate in the drinking water system was 1.8 kg per meter of pipe and day. When using drainage pipes for subsurface irrigation, this number increased to 3.1 kg/m/d, corresponding to 2.3 mm/d of supplied irrigation water. The main parameters affecting the water production efficiency were inlet air temperature and humidity, and the pipes were in both systems recommended to be placed at shallow depths. However, since the numerical models disregard from solar radiation and crops, somewhat different conclusions on how the pipe configuration affect the irrigation yield is expected when these factors are included in succeeding models. When irrigation is intended, the pipe spacing must be determined with respect to the soil and local climate as well as the type of crops to be cultivated. Future work on the CI system will include validation of numerical simulations, studies on how solar irradiation and vegetation affect the system, and the construction of a full-scale pilot plant in Libya.

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