Novel Integration of Organic Rankine Cycle (ORC) with Vapor Compression Refrigeration Cycle (VCC) for Recovery of Low-Grade Waste Heat for Electricity Production


Student thesis: Doctoral Thesis

View graph of relations


Related Research Unit(s)


Awarding Institution
Award date25 Nov 2019


The refrigeration and air-conditioning consume a major portion of total energy in hot and humid regions. As a result, a significant amount of low temperature waste heat is rejected in the atmosphere causing a net increase in the carbon footprints. In this research, a methodology for recovering the low temperature low grade waste heat has been analyzed by developing a novel integrated vapor compression cycle and the organic Rankine cycle (i-VCC-ORC) that recovers this waste heat from air-conditioning system and utilizes it into the organic Rankine cycle for electricity production. The research also aims to improve the thermodynamic response of the integrated system as compared with the standalone system by analyzing the coefficient of performance, ORC thermal efficiency, net work output and exergy efficiency as performance indicators. In order to achieve this goal, a system integration feasibility based on computational analyses is developed and thermodynamically analyzed in detail. The waste heat from the vapor compression cycle is recovered by the organic Rankine cycle for electricity production. The main focus of the research is to choose the suitable working fluid pair for the integrated vapor compression cycle and the organic Rankine cycle. The waste heat temperature from the vapor compression cycle ranges from 50-89°C whereas the cooling capacity (heat input) of the vapor compression cycle is taken as 35 kW and 100 kW for system analyses. Two different heat recovery techniques, namely full condensing and desuperheating, using different integrated cycle configurations are applied to assess the performance of the system. A novel concept of recovering waste heat has been introduced in which no external heat source drives the organic Rankine cycle. Instead, the integrated system uses a shared heat exchanger which is a single unit that acts as a condenser of the vapor compression cycle and evaporator of the organic Rankine cycle simultaneously. Based on the computational analyses, the overall COP of the system improves and the heat recovery efficiency (thermal efficiency) of the system reaches >3% by full condensing method and 3-7.50% by desuperheating method using various cycle configurations. In addition to this, the exergetic response of the system using full condensing method is better as compared with the desuperheating approach. Thermo-economically, the desuperheating method (water-water cooled and air-air cooled systems) shows a shorter payback period for the selected refrigerant pairs (7.56 years for R407C-R141b and 8.56 years for R410A-R141b) as compared with refrigerant pair selected for full-condensing method (14.54 years for R600a-R123).

In the later part of the research study, the integrated cycle is improvised introducing a sub-cooler in the vapor compression cycle. The ORC cycle includes a single evaporator and dual evaporator configuration. However, the desuperheating method is applied to both improvised versions of the integrated cycle to record the thermodynamic response. Another prospect of the research is the use of zeotropic mixtures for waste heat recovery from the vapor compression cycle. The use of zeotropic mixture in the integrated system (VCC integrated single and dual evaporator ORC systems) has some concrete advantages over pure fluids. The irreversibilities associated to non-isothermal heat addition will be reduced by using zeotropic mixtures. The zeotropic mixtures have got a special characteristic property of temperature glide at constant pressure which ensure the better thermal matching and reduce the exergy loss during heat transfer process. This results in achieving the higher conversion efficiency (ORC thermal efficiency) as compared with the pure refrigerants. Secondly the desired properties can be achieved by changing composition of the mixture. The zeotropic mixtures of R245fa/R365mfc and R123/R236ea are used in the single evaporator ORC cycle whereas R245fa/R365mfc is used in the dual evaporator cycle to compare the thermodynamic improvement of the system. The single objective optimization using genetic algorithm is applied to maximize the net work output by optimizing the evaporator temperatures and mole fraction of the 1st component. The integrated systems using zeotropic mixtures perform better as compared with their pure components both energetically and exergetically. However, the vapor compression cycle integrated dual evaporator ORC using mixtures perform better as compared with the vapor compression cycle integrated single evaporator ORC using mixtures.