Influences Of Regenerative Scheme On Performances Of A Power/Cold Cogeneration Kalina Cycle System

The Kalina cycle system and the power/cold cogeneration system developed on the basis of Kalina cycle system are deeply analyzed in this paper by combining theoretical analysis with numerical simulation.Specific contents include:The triple-pressure Kalina cycle system suitable for medium and high temperature heat sources was simulated by using programming software EES（Engineering Equation Solver）,and some parameters affecting the cycle performance were analyzed and optimized.The analysis results show that the triple-pressure Kalina cycle system achieves a good match with the heat source by optimizing the working medium concentration and other parameters,thus showing excellent thermal performance.The superheat in the boiler was optimized to reach the peak point of the power recovery efficiency for better performance of the cycle under the given condition of heat source temperature.The overall cycle efficiency increases as the temperature of the heat source rises.However,the waste heat recovery ratio of the triple-pressure Kalina cycle system decreases with the increase of the heat source temperature,that is to say,the exhaust temperature of the waste heat discharged from the boiler is also higher when heat source inlet temperature is higher.Thus the Kalina cycle can be further improved by adding the refrigeration sub-cycle to reclaiming further the exhaust heat.This paper chooses to improve and analyze the parallel power/cold co-supply cycle（PPR-KC）after generating and comparing many power/cold co-supply cycles.Firstly,a heat exchange surface was added in the first heat exchanger R1 of the original cycle,and a dilute solution from the outlet of the second heat exchanger R2 was introduced to participate in the heating of the basic solution,so as to alleviate the imbalance of heat recovery in the regulation of different work/cooling ratios probably.The diluted solution from the generator is transferred through the regenerator R2 and R1 in turn to fully exchange heat,so that the heat contained in the diluted solution at the outlet of the generator can be more fully recovered and utilized when the shunt is relatively small,that is,the cooling capacity is relatively large,so as to make up for the shortage of turbine exhaust heat due to the reduction of working fluid,and expand the regulating range of the shunt ratio.Secondly,change the practice of setting the minimum end difference in the boiler to the minimum limit value（20K）in previous studies,that is,take the utilization of the heat source by the dynamic sub-cycle as the primary consideration.According to the characteristics of distributed energy to ensure the cooling capacity as the primary task and the power generation can be adjusted by the power grid,the solution evaporation process in the boiler is no longer set as the minimum end difference,but the end difference at the solution bubble point in the generator of the refrigeration loop is always set as the minimum end difference limit value,in order to expand the adjustable range of the diversion ratio in this paper.The mathematical model for the above cycle is established and analyzed based on the first and second laws of thermodynamics.The results show that,the cycle efficiency first increases and then decreases with the increase of superheat.The cycle efficiency decreases gradually with the decrease of the refrigeration shunt ratio or the increase of the temperature difference of boiler bubble pointΔt_n when the optimal superheat corresponding to the highest efficiency is taken.So when changing on the curve ofΔt_n=20 K and the connection curve of the highest efficiency point at the maximum refrigeration split ratio under different boiler bubble point,the cycle is always at the optimal performance,and the parameters of the cycle are the best working conditions.It means when the refrigeration split f_ref is relatively small,it should be operated according toΔt_n=20 K,and when f_ref is required to further increase,Δt_n should be appropriately increased to reduce the heat recovery of the power sub-cycle.The temperature of the waste heat at the outlet of the boiler（the inlet of the generator）increases,so the available heat in the generator increases,which will provide more working fluids to output the refrigerant and cooling capacity,and the cold-work ratio will increase accordingly.But the power recovery efficiency of the cycle has decreased.Therefore,when the cooling capacity required by the user is large,the boiler bubble point end difference can be appropriately improved to meet the cooling capacity requirements of the user at the expense of the overall cycle efficiency.At the same time,a three-way valve was added to adjust the access of refrigerant in the circulation system in order to meet the demand of distributed energy supply system for air-conditioning cooling capacity in summer,so as to realize the switch between air-conditioning temperature area and ice temperature area.The cycle can be used to obtain the amount of cold in the ice temperature zone when the refrigerant goes to the low pressure absorber,and the amount of cold in the air conditioning temperature zone when it goes to the medium pressure absorber.The ice making mode can be used as a means of storing energy.In addition,a comparative analysis of the situation where the flow direction of dilute solution to medium-pressure absorber（Plan A）and low-pressure absorber（Plan B）of the air-conditioning temperature zone is carried out.Due to the limitation of heat exchanger end difference and flue gas outlet temperature,the increase of working concentration gradually reduces the range of split ratio,and the refrigeration temperature also increases accordingly when the end difference at boiler bubble point is always 20K.At the same time,the adjustable range of working solution concentration is limited due to the constraints of heat source discharge temperature and exhaust gas dryness of turbine.Therefore,the circulating cooling temperature is lower than-17℃when the refrigerant leads to the low-pressure absorber and the working solution concentration is within the adjustable range.And the refrigeration temperature is below 10℃when the refrigerant goes to the medium pressure absorber.The variation trend of cyclic variables and performance indexes of plan A and B is similar,but the adjustable range of parameters and the optimal performance are different under the same parameters.The xw variable range of the two schemes is about 0.46～0.54 when the refrigeration temperature requirement is 0～10℃,and the power recovery efficiency of plan B is slightly higher than that of plan A in this range.