New techniques for quasi-isothermal volumetric machines and their use in thermodynamic cycles

This work is part of a project on a small plant for electric power generation using volumetric machines. The goal of the entire project is the realization of a small plant, of an electric power output from 1 to 3 kW, that must exploit low temperature thermal sources (up to 250 °C / 523 K). To compete with more affirmed technologies, it is necessary to limit the engines costs and maintenance costs. Small reciprocating machines usually require more maintenance than other machines, but they are relatively cheap and the maintenance itself does not require highly specialized manpower (at least not for ordinary maintenance). For this reason, such machines have been chosen for this project. An analysis on thermodynamic cycles at low maximum temperature has been performed and it is reported in paragraph 1.5. This analysis shows that the compression and the expansion must have low polytropic exponents, in order to achieve any work output from low temperature thermal sources. This work analyses in details the compressor and proposes a method to achieve a compression as close as possible to an isothermal: the use of a gas-liquid Micro Channel Heat Exchanger (MCHE) inside the compression chamber. So, this thesis starts with a theoretical analysis on reciprocating machine principles, focusing on non-adiabatic cases. The analysis includes some paragraphs on the reciprocating expanders (that are far less studied and known than the compressors) and on ideal cycles. The subsequent chapter (Chapter 2) focuses on the exchanger study. The exchanger was made in aluminium using the DMLS additive manufacturing technique. The preliminary calculations, design and experimental activities on the exchanger are all part of this work and are illustrated in chapter 2. Chapter 3 is about the experimental activities on the compressor. First, the compressor was characterised without modifications. Then the exchanger was added to the compressor and tested. The test bench was built up in the department laboratory and the software for the data acquisition is also part of this three years’ work. More details on these matters are shown in appendixes. Chapter 4 shows a one-dimensional calculation using Siemens Amesim software. The model has not been perfectly calibrated yet; however, it follows the experimental data well enough to support some speculations on the instantaneous heat transfer rate in the exchanger. Chapter 5 is about future activities. A new solution to overcome the limits highlighted in chapter 3 and 4 is illustrated.