A Study On Multi-effect Bubbling Humidification-dehumidification Process Powered By Solar And Wind Energy

At present, the shortage of water resources is a global problem, which calls the common attention of many countries all over the world. Seawater desalination is an important solution to this problem.Widely used traditional desalination technologies such as low-temperature multi-effect distillation, multi-stage flash and reverse osmosis are only suitable for large-scale desalination projects. Besides, the use of conventional energy sources to drive these technologies has a negative impact on the environment. Solar desalination technology is freed from dependence on fossil fuels, however, with low efficiency, it covers a large area. Humidification-dehumidification desalination technology presents several advantages such as flexibility in capacity, moderate installation and operating costs, simplicity and possibility of using low temperature energy. It is suitable for small and scattered demand of fresh water in remote areas. And it serves as an important supplement to traditional desalination processes.A five-effect bubbling humidification-dehumidification desalination unit is design and constructed. Air serves as carrier gas, and transfers heat and mass with seawater by bubbling and passing through it. The air is heated and humidified in the humidifier and condensate in the dehumidifier. The latent heat of condensation is reused. The unit can be driven by electric (can be transformed from wind and solar energy) and solar energy. Operation parameters such as bubble velocity, temperature and height of saline water can be adjusted easily. Both single-effect and multi-effect experiments are conducted.The single-effect experiment shows that, the higher the temperature of saline water in the bottom humidifier is, the more fresh water will be obtained. There is a optimum bubble velocity for a certain heating power. The use of induced draft fan has a negative impact on the fresh water productivity. And the height of saline water in the bottom humidifier has slight influence on the fresh water productivity. The multi-effect experiment shows that decreasing the bubble velocity for the uppermost humidifier and increasing the bubble velocity for the bottom humidifier enhance the performance of the unit. The biggest fresh water productivity is3.156kg/h, and the corresponding biggest GOR is1.2.Based on the heat balance and the mass balance, a mathematical model of the unit is developed. Numerical simulation is conducted and the results agree with the experimental data. Several conclusions are obtained from the numerical investigation. For a given temperature difference between the uppermost and bottom humidifier, the higher the temperature of bottom humidifier is, the more fresh water will be got. For a given temperature of the bottom humidifier, the lower the temperature of the uppermost humidifier is, the bigger fresh water productivity will be. For given temperature of both the bottom and the uppermost humidifiers, the influence of temperature differences between other humidifiers on fresh water productivity is very slight. This provides basis for the setting of bubble velocities for humidifiers.Based on the experiment date and numerical investigations, optimization is conducted and solar and wind energy are used to drive the unit to get a fresh water productivity of28L/d under given conditions. Solar heating system, pump system and wind and photovoltaic hybrid power system are designed for the whole desalination system. It lays a foundation for practical application of multi-effect bubbling humidification-dehumidification technology.