Rotor Over-Speed Analysis of a Hybrid Solar Gas Turbine

Abstract

As the demand for environmentally friendly power production is increasing, the Hybrid Solar Gas Turbine (HSGT) power plant has the potential to provide a reliable electricity production with low carbon foot print by combining the solar heat and the fossil fuel. Compared to the conventional gas turbine system, the HSGT system has an additional heat source and larger volume due to the solar receiver and the ducting system required for guiding the air to and from the solar receiver. The additional heat source and larger volume increase the thermal capacitance which adds complexity to the operational behavior of the gas turbine. This paper presents an investigation of the transient behavior of a hybrid solar gas turbine. The OP16 gas turbine, rated at 1.85MWe, is integrated into a solar thermal power plant as part of an ongoing R&D project. To analyze the shaft over-speed of the system, a model is created using the commercial cycle analysis software GSP. The model is validated against the measured data for a non-solar mode operation as it is similar to the conventional gas turbine operation on pure fossil fuel. A rotor over-speed analysis of the OP16-HSGT system during load-shed shows that the rotor over-speed exceeds the acceptable limit due to the thermal capacitance and volumetric effects. An increase in shaft inertia, or re-direction of the air flow from hybrid-solar mode to non-solar mode by using control valves, or the use of blow-off valve to vent out the solar heat can be taken as safety measures to reduce rotor over-speed. It was found that the control valve should act within 0.3s to maintain acceptable rotor over-speed. As a control valve with such characteristics is scarcely available, a concept where the OP16-HSGT system uses a blow-off valve in addition to the control valve was developed. The blow-off valve reduces the rotor over-speed from 111% to 103% by venting out the thermal capacitance from the OP16-HSGT system and by that one can achieve safe operation during a load-shed or an emergency shutdown.