Plenary Lecture

Distributed Energy Cogeneration by means of Micro Gas Turbines: A Survey of Applications and Existing Commercial Systems and Outline of an Integrated Procedure for Design of Highly Efficient Prototypes

Professor Pietro Zunino
Department of Mechanical Energy, Management and Transportation Engineering
Università degli Studi di Genova
Italy
E-mail: pietro.zunino@unige.it

Abstract: Micro-gas turbines used in cogeneration power plants are a promising technical solution for distributed combined production of electricity and heat, especially due to their low emissions and fuel flexibility. A brief survey of the micro gas turbine applications and commercialized systems is presented in order to highlight the great potential of development as well as the factors limiting market grow and commercial success. The main part of the lecture is focused on the outline of an integrated procedure of design of highly efficient micro-gas –turbine system. Even though the global energy efficiency of micro-gas turbines is usually high, due to the use of waste heat for cogeneration, the electrical efficiency is lower than other competing technologies such as reciprocating internal combustion engines. As a consequence, an effort should be made to enhance its performance. This target can be obtained by increasing the turbine inlet temperature in order to improve Brayton cycle thermodynamic efficiency or by means of an integrated and optimized design of micro-turbine components (centrifugal compressor, radial inflow turbine, recuperator, combustor etc). Both these approaches are object of researches even though they have different impact on machine manufacturing process. The first approach requires the use of advanced materials for the hot gas path components, such as ceramic turbine impeller and casing and nickel based super-alloy recuperator, since cooling techniques, such as those used in larger heavy duty gas turbines, are hardly implementable with radial turbomachines. On the contrary, components redesign with advanced optimization techniques enables to keep the current technology for components manufacturing leading nevertheless to important performance enhancement. Summarizing, the key parameters of the gas turbine cycle determining efficiency are:
• Cycle pressure ratio;
• Turbine inlet temperature (TIT);
• Turbomachinery component efficiencies.
Turbomachinery efficiency should be always maximized. However for power lower than 1 MW and therefore for mass flow rate roughly lower than 10 kg/s, radial turbomachinery are used, instead, of axial ones, which, on the contrary, are used for larger mass flow rates. Centripetal turbines are more compact and less expensive than axial but cannot be cooled. Therefore, the maximum turbine inlet temperature depends only on the material and not on the cooling technology. Turbine inlet temperature is fixed and if ceramic material is not yet employed a TIT of 950 °C is the limit for nickel based super alloys. As a consequence, of the limited turbine inlet temperature, regeneration is necessary for reaching cycle efficiency level of the order of 30%, with pressure ratios compatible with single stage radial turbomachines. On the other hand, use of regeneration is made acceptable due to power, lower than 1 MW. Also for the recuperated Brayton cycle, efficiency depends on pressure ratio, turbine entry temperature, turbomachinery efficiencies and recuperator effectiveness. For the recuperated cycle, best cycle efficiencies are obtained at moderate cycle pressure ratios, of the order of 4 to 5. Pressure ratios as large as 6 are compatible for single stage centrifugal compressors and centripetal turbines. The research team of the Laboratory of Aerodynamics, Combustion and Turbomachinery of University of Genova has a long standing experience in the study, development and testing of gas turbine components for energy generation and propulsions. Since some years, the research team is active in the multidisciplinary optimized design (MDO) of the key components of micro gas turbines for energy cogeneration.

Brief Biography of the Speaker: Prof. Pietro Zunino received the M.Sc. degree in Mechanical Engineering with honors from University of Genova in 1976, and the Master after master degree in Turbomachinery from the Von Karman Institute with honors in 1979. He was design engineer of turbomachinery in the R&D department of Hydroart from 1977 to 1983. From 1983 to 2000, he was assistant professor and after associate professor. He is full professor of turbomachinery at the University of Genova since 2000, and visiting professor at the Saint Petersburg Polytechnic University since 2015. Prof. Pietro Zunino was director of the Fluid Machinery, Energy Systems and Transportation Department , University of Genova from 2005 to 2011 and since 2012 he is the director of the Department of Mechanical , Energy, Management and Transportation Engineering. Prof. Pietro Zunino is author of more than 150 scientific papers on aero-thermodynamics of turbomachinery for power generation and aeronautical propulsion. He has been session chairman of International Scientific Conferences, scientific reviewer for International Conferences and Scientific Journals, evaluator for the European Commission VI and VII Framework, Aeronautical and Space, scientific responsible of European Commission research projects on gas turbines, manager for industrial research contracts on turbomachinery. He is member of the International Scientific Committee of ISAIF Conferences since 2007. His present research activity includes rotor-stator aerodynamic interaction in turbomachines, development of high-lift profiles for aero-engine gas turbines, multidisciplinary micro gas turbine design.

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