Abstract
Globally there are several viable sources of renewable, low-temperature heat (below 130 °C), particularly solar energy, geothermal energy, and energy generated from industrial wastes. Increased exploitation of these low-temperature options has the definite potential of reducing fossil fuel consumption with its attendant very harmful greenhouse gas emissions. Researchers have universally identified the organic Rankine cycle (ORC) as a practicable and suitable system to generate electrical power from renewable sources based on its beneficial usage of volatile organic fluids as working fluids (WFs). In recent times, researchers have also shown a preference towards deployment of zeotropic mixtures as ORC WFs because of their capacity to improve thermodynamic performance of ORC systems, a feat enabled through the greater matching of the temperature profiles of the WF and the heat source/sink. This paper demonstrates the thermodynamic, economic and sustainability feasibility, and the notable advantages of using zeotropic mixtures as WFs through a simulation study of an ORC system. The study examines first the thermodynamic performance of ORC systems using zeotropic mixtures to generate electricity powered by a low-temperature solar heat source for building applications. A thermodynamic model is developed with a solar-driven ORC system both with and excluding a regenerator. Twelve zeotropic mixtures with varying compositions are evaluated and compared to identify the best combinations of mixtures that can yield high performance and high efficiency in their system cycles. The study also examines the effects of the volume flow ratio, and evaporation and condensation temperature glides on the ORC’s thermodynamic performance. Following a detailed analysis of each mixture, R245fa/propane and butane/propane are selected for parametric study to investigate the influence of operating parameters on the system’s efficiency and sustainability index. For zeotropic mixtures, results disclosed that there is an optimal composition range within which binary mixtures are inclined to perform more efficiently than the component pure fluids. In addition, a substantial enhancement in cycle efficiency can be obtained by a regenerative ORC, with cycle efficiency ranging between 3.1–9.8% and 8.6–17.4% for ORC both without and with regeneration, respectively. Results also revealed that exploiting zeotropic mixtures could enlarge the limitation experienced in selecting WFs for low-temperature solar ORCs. Moreover, a detailed economic with a sensitivity analysis of the solar ORC system was performed to evaluate the cost of the electricity and other economic criteria. The outcome of this investigation should be useful in the thermo-economic feasibility assessments of solar-driven ORC systems using working fluid mixtures to find the optimum operating range for maximum performance and minimum cost.
Highlights
Climate change remains a critical and topical issue affecting every aspect of the natural environment.The climate crisis is the biggest single threat to all of humanity, with devastating consequences that have the potential to reverse decades of progress and to thrust people into poverty
Twelve zeotropic working fluids (WFs) mixtures with various compositions of R245fa/R152a, R245fa/propane, R245fa/hexane, R245fa/heptane, butane/propane, butane/hexane, butane/heptane, isobutane/propane, isobutane/hexane, pentane/hexane, and isopentane/hexane were evaluated and compared through baseline, parametric, and optimisation performance studies to identify the best combinations of mixtures for yielding both high system cycle efficiencies and power outputs at optimum conditions, taking into account effects of the volume ratio, and the evaporation and condensation temperature glides of the zeotropic mixtures on the organic Rankine cycle (ORC) thermodynamic performance
This study considers the second law efficiency of the ORC system that is being evaluated at the dead-state temperature of 20 ◦ C
Summary
Climate change remains a critical and topical issue affecting every aspect of the natural environment. The present study attempts to demonstrate the thermo-economic feasibility, sustainability, and potential advantages of using zeotropic mixtures as WFs in an ORC system to generate heat and power driven by low-temperature solar heat sources for building applications. Twelve zeotropic WF mixtures with various compositions of R245fa/R152a, R245fa/propane, R245fa/hexane, R245fa/heptane, butane/propane, butane/hexane, butane/heptane, isobutane/propane, isobutane/hexane, pentane/hexane, and isopentane/hexane were evaluated and compared through baseline, parametric, and optimisation performance studies to identify the best combinations of mixtures for yielding both high system cycle efficiencies and power outputs at optimum conditions, taking into account effects of the volume ratio, and the evaporation and condensation temperature glides of the zeotropic mixtures on the ORC thermodynamic performance. The results of this analysis offer new understanding into the thermo-economic and sustainability performance of solar-driven regenerative ORC systems using sustainable working fluid mixtures for building applications and should prove useful in the feasibility assessments to find the optimum operating range for maximum performance and minimum cost
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