The operating principle of indirect collection systems involves the implementation of two separate subsystems: a renewable energy collection system (solar collector, PV, wind turbine, etc.) and a plant for transforming the collected energy to freshwater. Some examples employing renewable energy to power desalination plants are presented in this section; a more extensive review is presented in Section 8.5. The plant subsystem is based on one of the following two operating principles:
• Phase-change processes, for which either multi-stage flash (MSF), multiple effect boiling (MEB), or vapor compression (VC) are used.
• Membrane processes, for which reverse osmosis (RO) or electrodialysis (ED) are applied.
The operating principle of phase change processes entails reusing the latent heat of evaporation to pre-heat the feed while at the same time condensing steam to produce freshwater. The energy requirements of these systems are traditionally defined in terms of units of distillate produced per unit mass (kg or lb) of steam or per 2326 kJ (1000 Btu) heat input, which corresponds to the latent heat of vaporization at 73°C. This dimensional ratio in kg/2326 kJ or lb/1000 Btu is known as the performance ratio, PR (El-Sayed and Silver, 1980). The operating principle of membrane processes leads to the direct production of electricity from solar or wind energy, which is used to drive the plant. Energy consumption is usually expressed in kWhe/m3 (Kalogirou, 1997b).
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.