Characterization of a Compact Organic Rankine Cycle Prototype for Low-grade Transient Solar Energy Conversion

Abstract

Many people in rural regions of the worlddo not have access to electricity although they have abundant supplies of sunlight. An off-grid system to cheaply convert this solar energy into electricity could improve the lives of millions of families. Waste heat from high-concentration photovoltaic arrays and solar energy from evacuated tubecollectors are potential sources of abundant low-grade thermal energy. An Organic Rankine Cycle (ORC) is able to convert low-grade thermal energy into electricity at a relative low cost.While traditional ORCs rely on stable thermal energy inputs (i.e. geothermal energy), solar energy is not able to provide a constant supply of thermal energy. Understanding how transient solar fluctuations affect ORC performance is crucial to the development of high-performance, stable and stand-alone solar-driven ORC systems and further integration with heat storage units.In this work, an experimental ORC testbed was designed and fabricated for small-scale low-grade thermal energy utilization for transient characterization. Of particular interest in this work is the behavior of the ORC system as it approaches the operational limits. When the mass flow rate in the cycle is too high or the thermal energy input too low, the balance of the system is disturbed and strange oscillations are encountered. This work focuses on gaining a deep understanding of this abnormal behavior and itsinfluence on ORC performance. The physical phenomena that bring about these changes in an evaporator and their effect on ORC's moving components (i.e., diaphragm pump and scroll expander) are studied and characterized. A qualitative description of the phenomena is proposed and used to describe the changes within the cycle as a result of the imbalance. Once the individual processes in the oscillations are identified, quantitative analysis is carried out to better characterize the entire cycle. This characterization may be used to predict when the heat source conditions would lead to oscillations in ORCs and the magnitude of the reactions.These oscillations hinder the ORC performance and may even cause damage to the moving components under certain circumstances. Revealing the physical mechanismof oscillations helps attain advanceddesign and energy-efficient operation of ORCsunder transient thermal energy supplies, particularly solar. Cycle modifications such as heat storage units are also discussed to coping with transient solar energy inputs and night-time operation.