Low grade heat conversion into power using small scale organic rankine cycles

Abstract

The present thesis regards the study of small scale organic Rankine cycles (ORCs) considered in the two philosophies that embodies its applications. Major power cycles, including vapor and gas cycles were recalled with emphasis on advanced Rankine cycles and its derivates that are Kalina and Uehara cycles. Organic Rankine cycles as presented are well adapted for low and medium temperature heat resources below 400 ºC. The applications intensely investigated are: solar electricity, solar desalination, biomass combined heat and power plants (CHP), geothermal and energy recycling from thermal devices and processes. Suitable Working fluids for a solar ORC driven by heat source temperature below 100 ºC are selected using predefined criteria such as higher fluid densities, maximum cycle efficiency, safety and environmental data, and moderate critical temperature. R134a, R600, R152a emerged as good fluids among 20 potential candidates. The ORC configurations for the same level of temperature are also subject to an investigation. Exergy topology analysis was applied to suitably choose the optimal configuration based on additional parameters: exergy efficiency, degree of thermodynamic perfection and coefficient of influence. The simple Rankine cycle proved to be more adequate and potentially cost effective in comparison with modified Rankine cycles with internal heat exchangers or with integrated feedliquid heaters. Experimental investigations carried out on a small ORC designed for exhaust gas heat recovery confirmed the technical feasibility of the technology. The test bench was made up of several heat exchangers, two diaphragm pumps and an open-drive oil-free scroll expander. Cycle and expander performances were evaluated and compared for three fluids: R123, R245fa and HFE7000. The maximum power output delivered is about 2 kW, the cycle efficiency is less than 8% and the global efficiency does not exceed 5%. The oil free open-drive expander yields a maximum efficiency of about 70%. The final section deals with the economic evaluation and optimization of small scale ORCs in heat recovery application. The heat source considered is hot air at about 180 °C with a mass flow rate of 0.21 kg/s. The optimized cycle system is based on the experimental investigations and models derived. It produces 2 kW power and yields 8% thermal efficiency. Using appropriate mathematical formula the system was scaled-up to 50 kW. The study concludes that the organic Rankine cycle is a promising technology for small-scale waste heat recovery applications. For illustration, the levelized electricity cost (LEC) is about 13.27 c€/kWh for very small systems (2 kW) and decreases down to 7 c€/kWh for a 50 kWe system. This value could be significantly lower, below 5 c€/kWh for medium and large size systems. By means of a thermo-economic model the mismatch between the optimal technical point and the minimum specific cost was demonstrated. The analysis of the mismatch allowed concluding that economic optimization instead of thermodynamic optimization or a compromise between them is recommended when seeking for profitable environmental solutions.