THEORETICAL AND EXPERIMENTAL ANALYSES OF MICRO ORC SYSTEMS FOR LOW GRADE HEAT SOURCES

The growing cost for energy production and distribution as well as problems related to environmental pollution have motivated the increasing interest in the research of alternative solutions and, in particular, innovative technologies capable of compromising efficient use of energy, production cost, optimization and guaranteeing environmental sustainability. One of the promising technologies suitable for waste heat recovery and low grade heat is the Organic Rankine Cycle (ORC). ORC’s offer some advantages such as: reliability, safety, noise, emissions and flexibility. The work presented in this thesis is aimed at studying and developing systems capable converting thermal energy that can be low grade heat sources like solar systems or waste heat recovery from industrial wastes or wastes from internal combustion engine (ICE) into electrical and/or mechanical energy, based on the ORC technology in the power range of 1-10 kWe. The work consists essentially of two parts. The first part includes literature overview, state of the arts on ORC systems in general and the implementation of a process simulation model of micro ORC system and its integration in a solar thermal plant. The second part includes laboratory work on both ORC test bench and prototype, and the field tests on a combined solar ORC facility. As for the first part, the literature work overviews the major fluids commonly employed in ORC applications in general and the micro systems up to a 100 kWe in particular, the overview extends to the main components installed in a simple ORC like the expander types, feed pump types and finally the heat exchangers suitable for heat recovery and low grade heat systems. The overview ends with a general description of the various potential uses of this technology, with particular focus on the kWe size applications. The thermodynamic cycle of the ORC has been studied by creating a simulation model for the considered working fluid, R245fa, but capable of running with different working fluids using the EES software. Considering system integration, a co-simulator has been created using the Trnsys-EES platforms to study the possibility of coupling an ORC to a solar collector system. Trnsys is suitable for the modeling of the solar system and is largely used by professionals in assessing the performance of thermal and electrical energy systems. In the second part the experimental activity is presented. The laboratory is equipped with an ORC test bench used to investigate the performance of the expander and a separate test bench used to characterize the feed pump both available at the ENESYSLAB of the University of Trieste-Italy. During the laboratory experiments, the expander isentropic efficiency measured is above 60%. Considering integration of the ORC system analyses have been performed on the ORC prototype coupled to a solar field at the University of Florence Italy. The developed models have been used to study and predict some cycle performance parameters, in particular, the model developed for the combined solar ORC offers a fast comprehensive analysis of the system and allows to easily predict and assess the performance of a solar powered ORC and its system behavior at different operating conditions, in particular, the model show efficiencies above 4% for the combined solar ORC system. An economic analysis concludes the work where the cost of realizing a solar-ORC is presented, in the analyses, it has been shown that the payback time of the considered plant is 10 years with reference to the Italian plan of incentives as a plan of investment. This analyses confirms the numerous advantages obtainable from this type of technology and highlights improvements that could be made to achieve better performance hence the feasibility of the system if the installation cost is reduced.