THERMODYNAMIC AND DESIGN CONSIDERATION OF A MULTISTAGE AXIAL ORC TURBINE FOR COMBINED APPLICATION WITH A 2 MW CLASS GAS TURBINE FOR DEZENTRALIZED AND INDUSTRIAL USAGE

asme-orc2015 Tracking Number 80

The continuous growth of the part of renewable energy resources within the future mixture of energy supply leads to a trend of concepts for decentralized and flexible power generation. The raising portion of solar and wind energy, as an example, requires intelligent decentralized and flexible solutions to ensure a stable grid and a sustainable power generation. A significant role within those future decentralized and flexible power generation concepts might be taken over by small to medium sized gas turbines. Gas turbines can be operated within a large range of load and within a small reaction time of the system. Further, the choice of fuel, burned within the gas turbine, is flexible (e.g. hydrogen or hydrogen-natural gas mixtures). Nevertheless, the efficiency of a simple gas turbine cycle, depending on its size, varies from 25% to 30%. To increase the cycle efficiencies, the gas turbine cycle itself can be upgraded by implementation of a compressor interstage cooling and/or a recuperator, as examples. Those applications are cost intensive and technically not easy to handle in many applications. Another possibility to increase the cycle efficiency is the combined operation with bottoming cycles. Usually a water-steam cycle is applied as bottoming cycle, which uses the waste heat within the exhaust gas of the gas turbine. In small to mid-sized gas turbines the temperature and heat amount within the exhaust gas are often not sufficient to operate a water steam cycle efficiently. Further, in industrial applications, a part of the heat, within the exhaust gas, is often used in secondary processes which lower the total amount of heat, which can be transferred to a bottoming cycle. An alternative to water-steam bottoming cycles can be given by organic Rankine cycles (ORC) based on organic fluids. An advantage of organic fluids is the characteristic of evaporating at lower temperatures and lower heat amounts and thus, the usability in a Rankine cycle even at low heat source temperatures. This paper discusses an ORC process design for a combined application with a simple cycle gas turbine in the 2 MW class. The thermodynamic cycle configuration is shown and it will be pointed out that the cycle efficiency (simple GT) of 26.3% can be increased to more than 36% by application of a bottoming ORC cycle (simple GT+ORC). A key component within the ORC cycle is the turbine. This paper shows the results of an extended aerodynamic ORC axial turbine pre-design based on the thermodynamic cycle considerations. Within the design study the real gas properties of the organic fluid are taken into account. Based on the outcome of the pre-design a 3D aerodynamic axial turbine design is investigated. By application of CFD simulations the turbine design has been optimized and the ORC power output could be increased from 598kW (pre-design) to 657 kW, which lead to a combined cycle efficiency of more than 36%.