Analytical Modelling on Simultaneous Phase Transitions in Low Temperature Evaporator for Organic Rankine Cycle Applications

A lab-scale organic Rankine cycle has equipped a Low Temperature Evaporator (LTE) for water recovery in addition to the High Temperature Evaporator (HTE) for heat recovery. The recovered water is reused as the make-up water line to save the fresh water consumption in the cooling tower [1, 2]. Water recovery efficiency was defined as the ratio of the water condensation rate from the flue gas side to the moisture flowrate at the flue gas inlet [3]. The LTE as cross flow heat exchanger is to recover water in condensate form from the combustion flue gas in the duct side while the recovered latent and sensible heats are transferred into the refrigerant R134a in the tube side. The LTE involves complicated phenomena since the condensation of water vapour in the flue gas duct side and the flow boiling of R134a in tube side were taken place simultaneously. Design of a LTE and its optimized operation depend on a knowledge and understanding of the heat and mass transfer occurred in the LTE. Analytical modelling would be essential to derive the critical parameters for design and operation to achieve the goal of the organic Rankine cycle. The objective of this research was to develop an analytical modelling for simulating the simultaneous phase transitions: 1) condensation of water vapour in the duct side and 2) flow boiling of the refrigerant R134a in the tube side. The control volume was confined to the LTE with two working fluids including the combustion flue gas in the duct side and R134a in the tube side. The work scope was to conduct derivations of the governing equations and numerical algorithm, program development, validations and verifications, and extensive case studies. The modelling was able to generate the spatial profiles of temperature and heat transfer coefficients of both sides, vapour quality of R134a, and condensation rate of water vapour in flue gas side, etc. The mean absolute deviation between the calculated and measured heat transfer coefficients was within 18 %. The calculated data including exit temperature of flue gas and R134a, and water recovery efficiency were in good agreement with the measured data within 15 %. The case studies with the developed software were conducted to examine the roles of sensible and latent heat transfer in flue gas side and boiling impact of R134a side with variations of design and operating parameters including heat transfer area, and inlet conditions of flue gas and R134a, etc. The performance was compared with the case of water coolant under same conditions. The results show that the water recovery efficiency was able to enhance from the current 50 wt% to 77 wt% as expanding its total heat transfer area up to 4 times than the baseline dimension. It was found that the ratio of mass flow rate of the coolant to flue gas was a strong function to improve the water recovery efficiency due to the higher heat transfer coefficients in R134a side induced from the flow boiling. The comparison case study predicted the water recovery efficiency of the R134a case to be increased up to 20 wt%p than the water coolant case under same condition.