Thermo-Economic Analysis of an Organic Rankine Cycle Powered Cascaded Vapor Compression-Absorption Refrigeration System

Abstract

Application of refrigeration is present in countless sectors ranging from food and drink industry to air-conditioning and also plays a major role for healthcare, energy, manufacturing and the environment. Refrigeration sector (including air conditioning) is one of the major consumers of electrical energy across the world which accounts 17% of the global electricity consumption. Moreover, the worldwide global warming by refrigeration sector accounts 80% from depletion of fossil fuel and 20% from direct leaks of refrigerants. Food and Agricultural Organization predicts that food production should increase worldwide by 70% by 2050, so that refrigeration sector has a vigorous role to play. Researchers and engineers are tirelessly working for inventing newer energy efficient low carbon technologies to provide alternatives. Efficient utilization of available energy sources propelled researchers to design integrated technologies. Integration of thermal systems improves the energy efficiency, decreases the carbon emissions and economizes the usage of fossil fuels. In the present research work, an integrated system, which includes the organic Rankine cycle (ORC) and cascaded vapor compression–absorption refrigeration system (CVCARS) is proposed. The cascade refrigeration system combines the advantages of the conventional standalone vapor compression refrigeration system (VCRS) and vapor absorption refrigeration system (VARS), and provides efficient low temperature cooling. Integration of ORC avails the usage of low temperature energy sources and works as a cogeneration unit which fulfils the electricity and heat energy requirement of VCRS and VARS respectively of CVCARS. The proposed ORC integrated CVCARS is assessed as an alternative refrigeration technology considering detailed energy, exergy and economic analysis. The proposed system powered by waste heat is independent of the grid supply compared to 19.15 kWe requirement of standalone VCRS to meet the equivalent cooling demand (30.7 kW). The energetic and rational efficiencies of the proposed system, with n-pentane as an ORC working fluid, are calculated as 79.02% and 46.7%, respectively. Considering manufacturer’s cost data, the economic analysis reveals that the simple payback period and breakeven point for the proposed integrated system are 6.8 years and 5.5 years, respectively. The net present value and internal rate of return are calculated INR 16,24,652 and 0.14, respectively. Based on the condition of equality of the annualized cost of viii the standalone system and proposed system, a methodology for selection between these two configurations, called selection diagram, is also presented which offers quick suggestion about the optimal configuration at the initial design stage. Higher investment cost compared to the standalone cooling system is one of the practical limitations for the proposed system. Thus after requisite results of thermodynamic analysis, the economic model of the proposed system is developed based on the size and cost. The system size and annualized cost are optimized to make the system potentially attractive for the industrial sector. The simple payback period and break-even point are calculated (for the base case) as 5.26 years and 4.22 years, respectively. The net present value and internal rate of return are calculated INR 28,46,170 and 0.19, respectively. Optimization results reveal that the annualized cost of the present system is decreased about 12%. Moreover, the simple payback period and break-even point are reduced to 4.50 years and 3.48 years, respectively. The economic factor such as higher value of electricity price and the lower value of discount rate are favorable for the selection of the proposed system compared to the standalone vapor compression refrigeration system. Integration of renewable energy sources with the proposed ORC integrated CVCARS makes it suitable for the decentralized applications. In the present work, hybrid solar-biomass source is integrated with the proposed ORC integrated CVCARS for availing the benefits of clean and efficient low temperature cooling and heating with zero dependency on fossil fuels. The solar fraction and break-even point (BEP), considering paraboloid dish, n-pentane organic fluid, straw type biomass, and Jodhpur location, are calculated as 0.254 and 7.71 years, respectively. In addition, the net present value and the internal rate of return are calculated as INR 10,03,660 and 0.11. Due to lower annual efficiency, the solar fraction for the linear Fresnel reflector (LFR) based system is 0.179; however, the lower cost of LFR field and lower cost of energy generation from biomass leads to the lower BEP (7.43 years). Thermo-economic performance of the system is also affected by the ORC working fluid and the calculated breakeven values are 7.85 years for toluene and 8.16 years for R245fa. In comparison with the solarbiomass powered system, the fully biomass powered system requires 39% lower capital cost and 30% lower BEP.

A micro-scale indigenous organic Rankine cycle power system test rig has been developed to justify the assumptions related to the thermo-economic parameters used in the analysis of ORC powered CVCARS. Due to unavailability of the local ORC expander manufacturer a microscale ORC test rig is developed using scroll compressor converted expander. The experimental results show the good match with the assumptions taken in the analysis of ORC powered CVCARS. The isentropic efficiency of the expander and thermal efficiency of the cycle are in a range of 60-73% and 2.5-4.8%. The calculated value of the levelized cost of energy for the micro scale ORC based system is 5.31 INR/kWh, which is 37% lower compared to the supply cost of unit electricity for industrial sector in India. The proposed system, with a gross capacity of about 1.03 kWe, reduces the emissions of combined NOx and HC, CO, and PM of about 28.1 kg/y, 24.6 kg/y, and 1.4 kg/y, respectively.

Keywords: Cascade refrigeration system; Co-generation; Concentrated solar power; Exergy; Hybrid system Optimization; Organic Rankine cycle; Selection diagram; Biomass