AbstractsPhysics

Nowadays, great effort is made by the research community to develop efficient energy conversion engines. The motivation for this effort derives from the growing demand for electricity, the exhaustion of fossil fuels reserves and the environmental pollution caused by the exhaust emissions from engines. A technology that seems to play an important role in the development of energy sector is the microturbines. The microturbines have many advantages over the conventional reciprocating engines, which make them suitable for distributed generation applications, smart grids, hybrid and cogeneration systems. In this thesis, applications of microturbines are studied, both numerically and experimentally. The numerical works include the study of hybrid SOFC-GT (Solid Oxide Fuel Cell – Gas Turbine) systems. These systems combine a microturbine with a SOFC generator that operates at elevated pressure conditions. The fuel reacts electrochemically within the fuel cell stack (which substitutes the combustion chamber of the microturbine) and produces electric power and heat. The heat is utilized from the microturbine to produce additional electric power. The hybrid SOFC-GT systems have high efficiency and are considered as an alternative to conventional electric plants.For the modeling of the hybrid systems a general purposes software (AspenPlus) was used. Starting from a method published in the literature for the modeling of a SOFC generator, changes were made in order to develop an improved model. Moreover, simulation models for the part-load performance of turbomachines were developed for the first time using this general software. In the present thesis, hybrid systems were examined which are based on commercially available microturbines and a successful SOFC generator, which have been tested in various research programs. Models were developed and validated against experimental data from the available bibliography. The models were used to study the behavior of hybrid systems under full and part-load operating conditions. Also, the influence of important performance parameters, such as the SOFC stack temperature, was investigated. The results showed that these systems can achieve high efficiency values with a proper choice of the design parameters as well as of the size of the microturbine. The previous experience was used to develop a method for the optimization of the compressor and turbine components in hybrid SOFC-GT systems. The method consists of two steps. In the first step a parametric study is conducted in order to assess the operating range of the hybrid system and, in the second, the compressor and turbine geometric parameters are calculated using in-house turbomachinery design codes and an optimization procedure. Based on the new turbomachines the hybrid system has shown a clear efficiency advantage over the whole operating range. The developed optimization tools allow the study of further design options, such as a separate compressor and turbine selection or a different compressor type.The experimental activities…