A Study Of Thermal Vapor Compressor And Hybrid Renewable Energy System For MEE Seawater Desalination Systems

Thermal vapor compressor(TVC), a key auxiliary unit for thermal vapor compression aided multi-effect evaporation(MEE-TVC) seawater desalination systems, its design and operation parameters are important influencing factors of the MEE-TVC system performance. With a given design structure, the vapor flow state inside TVC and the suction location of TVC have significant influences on the TVC and the MEE-TVC system performance. But there are relatively little studies of these two problems, especially the in-depth systematic theoretical and experimental studies.The influences of the flow state and characteristics on the performance of the TVC and that of MEE desalination system performance have not been clearly and fully established, which in turn limits the improvement and enhancement of MEE-TVC system performance. Application of renewable energy, especially HRES(Hybrid Renewable Energy System) including two or more kinds of renewable energies in MEE seawater desalination systems are receiving more and more attention with the increased concern about of environmental pollution and energy shortage. The main work conducted in this paper are as follows:A new method which could improve the vapor flow characteristics and increase TVC entrainment ratio by entrained vapor preheating using of external heat sources is proposed. On this basis, the vapor preheating based TVC and MEE-TVC seawater desalination system are presented. Thermodynamic analysis of TVC performance is conducted. The influences of entrained vapor superheat on TVC theoretical entrainment ratio is calculated and analyzed. The results show that the entrained vapor superheat has little effect on the TVC theoretical entrainment ratio and the effect could almost be neglected, and a TVC reversible entrainment ratio efficiency is defined to describe the TVC themodynamic completeness. If the increase in the TVC reversible entrainment ratio efficiency resulted from the entrained vapor preheating could offset the decrease in the TVC theoretical entrainment ratio, then the TVC entrainment ratio could be increased. With this consideration, the vapor preheating based TVC and MEE-TVC seawater desalination system are proposed, the process is given, and its characteristics and the possible applications are stated.A MEE-TVC seawater desalination experimental system is set up, which uses solar heating system and electric boiler steam as preheating sources. The influence of entrained vapor preheating on MEE-TVC system performance is examined by experimental study. In order to verify the effectiveness of the new method, a four-effect-parallel-flow MEE-TVC experimental system with design productivity of 120kg/h is set up. The experimental system is composed of falling film evaporator system, vacuum tube solar heating system, electric boiler system and data acquisition system. Making use of this experimental system, MEE-TVC system performance experiments under four different superheats of 0℃、5℃、10℃、15℃ are carried out to examine the influence of entrained vapor preheating on TVC entrainment ratio, motive steam consumption, system performance ratio and mass flow rate of cooling water, which used hot water from solar heating system and steam from electric boiler as the preheating heat source. The experimental results show that the TVC entrainment ratio significantly increases with the increase of entrained vapor superheat. TVC entrainment ratio of the entrained vapor superheat 15 ℃ is increased by 31.2% over that of the entrained vapor superheat 0℃. Motive steam consumption and cooling water mass flow rate decrease with the increase of entrained vapor superheat, and these experimental results prove the effectiveness of the foregoing new method.Theoretical and experimental study of influence of suction location on TVC and MEE-TVC system performance are conducted. A more complete mathematic model considering seawater physical property and thermodynamic losses in evaporators and pipes is established to carry out theoretical calculation and analysis of influence of suction location on TVC and MEE-TVC system performance. In order to confirm the reliability of mathematic model and examine the influence of suction location on TVC entrainment ratio, performance ratio and specific heat transfer area of the MEE-TVC system, the experiments are conducted making use of above MEE-TVC experimental system with TVC suction located in different positions. The theoretical and experimental study results both show that the suction location has a very important effect on both the TVC and the MEE-TVC performance. With the suction location shifted to the higher position of the MEE effects and the decrease of the compression ratio, the entrainment ratio increases singularly, the performance ratio increases first and then decreases, the specific heat transfer area decreases first and then increases. There are an optimal TVC suction location and a compression ratio that make performance ratio getting its maximum value and specific heat transfer area getting its minimum value.A novel MEE-TVC-MVC system driven by solar and wind energy HRES(Hybrid Renewable Energy System) is proposed. Put forward A novel MEE-TVC-MVC seawater desalination system which conduct series connection of mechanical vapor compressor(MVC) and thermal vapor compressor(TVC) behind of the last evaporator so as to share pressure increase burden of entrained vapor is presented. Solar and wind energy HRES is chosen as driven energy of the novel MEE-TVC-MVC system in order to reduce the use of fossil fuel and environmental pollution. Mathematic model is established to conduct technical and economical analysis of the influence of wind power proportion, MVC compression ratio, the heating steam temperature, the effect number and fresh water production on water production cost. The analysis results show that the novel MEE-TVC-MVC system proposed in this paper has a much smaller water production cost and a better economy comparing with that of MVC driven by wind power. There is a certain gap between water production cost of novel MEE-TVC-MVC system and that of MEE-TVC system driven by conventional fossil fuel. But the gap would decrease with the development of renewable energy technologies and the novel MEE-TVC-MVC system could get almost the same water production cost of the MEE system driven by conventional fossil fuel.