Study On Integration And Performance Of Combined Cooling,Desalination And Power System Driven By Low Grade Thermal Energy

The shortage of energy and fresh water are the two greatest challenges that plague the economic and social development of our country.China is rich in low-grade thermal energy,and the South China Sea has huge reserves of ocean thermal energy,which has important utilization value.However,owing to the limitations of low efficiency and high fixed costs,ocean thermal energy conversion(OTEC)has not been commercialized.Meanwhile,desalination has been proved an effective solution to the problem of freshwater shortage.However,as an energy-intensive industry,desalination based on conventional energy not only requires high cost of energy,but also exacerbates greenhouse gases emissions.Therefore,actively carrying out research on the comprehensive and efficient utilization of low-grade thermal energy such as ocean thermal energy,solar energy,and industrial waste heat is of great significance for achieving the national energy-saving and emission-reduction strategic goals and the sustainable development of energy and freshwater resources.Firstly,considering the actual demands of remote islands at low latitudes,a combined cooling,desalination and power(CCDP)system by coupling organic Rankine cycle(ORC),ejector refrigeration cycle(ERC)and multi-effect distillation(MED)desalination is proposed based on the principle of energy cascade utilization.The performance of the CCDP system is investigated from the perspectives of the first and second law of thermodynamics and exergoeconomic properties.The results indicate that the highest PESR and exergy efficiency of the proposed system are up to 33.72% and 29.33%,respectively.The improvement of both thermodynamic and exergoeconomic performances can be achieved at a higher generation pressure and a larger distillate production.Furthermore,a multi-objective optimization is conducted in terms of exergy efficiency and total cost rate as objective functions.The Pareto optimal solutions(POS)for five working fluids are determined based on a fast and elitist non-dominated sorting genetic algorithm(NSGA-II)and decision-making technique.According to the results of POS,R601 has the best performance in thermodynamic and economy.Secondly,considering the temperature slip in the phase transition process,an innovative ammonia-water based combined cooling,desalination and power system is proposed,consisting of Kalina cycle-based ocean thermal energy conversion,ERC and spray flash evaporation(SFE)desalination unit.An ejector is introduced for coupling the Kalina cycle and ERC,which creates a larger pressure difference across the turbine.Besides,a multi-stage SFE desalination system is integrated into the proposed system for making full use of seawater resources and providing fresh water output.Mathematical model is established for simulating steady operation of the proposed system,and parametric analysis is performed to determine the effect of influencing factors on the system thermodynamic and exergoeconomic performances.Results show that by comparison with the stand-alone Kalina cycle and the ORC based multi-generation system,the novel CCDP system is more advantageous with regard to the exergy efficiency and net power output.Effective and exergy efficiencies of the Kalina based CCDP system are 6.77% and 37.08%,respectively.The improvement of both effective efficiency and exergy efficiency can be accomplished by reducing the condensation pressure and raising the basic ammonia concentration.Finally,study on flash evaporation desalination as a novel technology is conducted,owing to its advantages of high heat and mass transfer rate and fit for low-temperature operation.To perform an in-depth analysis,a detailed thermodynamic model based on droplet analysis has been presented.Based on the simulation results,the mechanism of heat and mass transfer of seawater droplets sprayed into a low-pressure environment is revealed,and the relationship between flash characteristics and the traveled distance is explored.Besides,the effects of initial parameters and injection direction on evaporation rate and spray flash speed are analyzed.Furthermore,a detailed mathematical model of multi-stage flash evaporation desalination system is established based on droplet evaporation model.The effects of different thermodynamic parameters,including top brine temperature and total number of stages,on the thermodynamic and economic performance are investigated.Results show that higher numbers of operating stages is benefit to enable higher freshwater output and lower specific energy consumption,and the minimum unit cost of fresh water is 3.28?/t when the operating stage is 4.