Research On The Desalination Technology And System Based On The Solar Heat Method

As freshwater resources being depleted, seawater desalination has become one of the most important means to solve freshwater shortage. Conventional desalination is an energy-intensive industry, and what is more serious is that the depletion of fossil resources is forthcoming at the same time. The conventional desalination not only exacerbate the situation but also produce much Nitrogen Oxides and Carbon Dioxide. So it's significant for sustainable development of our society if water crisis can be solved with the clean energy such as solar energy.Aiming at different geographical conditions and support requirements, two different solar desalination devices had been designed in this research, one is multi-stage intensify condensation solar desalination device, and the other is falling film evaporation solar desalination device. The operation principle, test of the steady state, the actual operating situation and the analysis of the performance simulation about the desalinations had been researched in detail, and the economical efficiency of two systems had been analyzed.Heat-transfer mathematical model of the multi-stage intensify condensation solar desalination device had been established. The heat-transfer equations between the different stages had been obtained and the performance coefficient of the device was calculated.The test about the transient and steady state of the three-stage intensify condensation solar desalination device under the actual weather was accomplished, the variation principle of operating temperature, freshwater production rate, cumulative production of freshwater during all the stages were obtained. The experiment results show that: the device basically reached a stable rate of freshwater production after running 3 hours; the higher the operating temperature was, the higher the freshwater production rate and performance coefficient were, and when it's operating temperature was above 70℃, the device could reach a performance coefficient of above 1. The experimental results in the actual weather conditions show that: when the amount of solar radiation was 19~20 MJ/( m2?d), water depth of 2cm, 3cm and 4cm, the fresh water production was respectively 7.29kg/d, 6.56kg/d and 5.12kg/d, and the device performance coefficient was respectively of 1.12, 0.98 and 0.78; and the variation trend of the freshwater production rate was according to the variation trend of the solar intensity, but there were varying degrees of time lag. The more shallow the depth of the seawater was, the higher freshwater production rate, cumulative production of freshwater and performance coefficient were obtained.The theoretic calculation about the operating performance of the intensify condensation solar desalination device were accomplished, the influence of such factors to the device performance, including diego disc series, energy matching factor, all levels of disk water depth, diego disc surface evaporation and distance of condensation surface and other factors, were theoretically predicted, and the best match and the device coefficient of performance under different water depths were analyzed and calculated. Contrastive analysis of the three-stage system operation performance suggested that measured values were primarily similar to theoretical values although the former was slightly lower, and the theoretical values results were basically reasonable and referential.Based on theory of film evaporation and film condensation, the falling film evaporation device drived by the solar energy and wind energy was designed. The heat and mass transfer of the device was analyzed, the mass conservation equation of the salt, energy balance equation, heat transfer equation and the calculation model for the freshwater production were set up.According to the analytical testing on the main influential factors of solar falling film evaporation desalination device, the suitable operating conditions were obtained: the actual operating temperature range of the host of the desalination device was 70℃to 80℃, the suitable temperature range of cooling water was 15℃to 25℃; the appropriate flux range of heating water and cooling water was 2.5 m3/h to 3.5 m3/h; the suitable operating pressure range of the host was 15kPa to 20kPa; the device had high performance coefficient, and the performance coefficient value was about 2.0 when the operating temperature was 70℃to 75℃. The best match operating parameters of the falling film evaporation solar heating system and the optimization parameters of the whole system were obtained after the research on device performance optimization. The actual dynamic operation test of solar falling film evaporation desalination device had been completed, and the operating situations of the desalination host, the solar collectors and the automatic control systems had been tested, and basic operating situation of the device had been tested in typical months during the past two years. Experimental results show that: water production rate of the plant was higher when solar radiation was 20 MJ/( m2?d), the system had an average freshwater production up to 10~15L per square meter of collector area, and the quality of freshwater was up to the standards(GB5749-2006).Economical efficiency of two types of solar desalination system had been analyzed by discounted present value method. Mathematical models of a falling film evaporation solar desalination system was set up, the actual return on investment and freshwater production cost of two types of devices was calculated. The freshwater production cost of the falling film evaporation solar desalination system was 11.45 RMB Yuan/ton, while the cost of the multi-stage intensify condensation solar desalination device was 10.9 RMB Yuan/ton. Both types of solar thermal desalination installations have a good economic value.