Effect of an alternative operating strategy for gas turbine on a combined cooling heating and power system

Abstract

The off-design performance of a combined cooling heating and power (CCHP) system is of great importance and is affected by the system configuration and operating strategy. This paper proposes an alternative operating strategy called flue gas reinjecting (FGR) for a CCHP system consisting of a small-scale gas turbine, a double-effect absorption chiller and a hot water exchanger. The FGR operating strategy elevates the compressor inlet temperature by reinjecting a portion of the flue gas into the ambient air, which seems to contradict the well-proven concept of gas turbine inlet air cooling. In contrast, improving inlet air temperature cannot obviously influence the off-design performance of the gas turbine, and more high-temperature flue gas is produced in the gas turbine, which leads to more cooling and heating output in the absorption chiller and hot water exchanger, respectively. To extend the power output range (100–20%), reducing the turbine inlet temperature (TIT) operating strategy is introduced as an auxiliary means. The off-design performance of the CCHP system is investigated under the combined FGR and TIT operating strategy and independent TIT operating strategy. It is found that improved total efficiency and energy saving performance of the CCHP system are presented with the combined operating strategy at partial load. At a load rate of 50% (876kW), the fuel energy saving ratio (FESR) increases from 20.52% to 22.96% as the temperature of the inlet air is increased from 25 to 45°C using the combined operation strategy. The exhaust gas reinjection reduces the excess air resulting in inhibition of the formation of NOX in the combustor. Moreover, using the proposed operating strategy contributes to promoting more effective utilization of low-temperature heat and the direct blending of two fluids reduces heat and pressure losses emanating from the heat exchanger. This study may provide a new operating strategy for small-scale gas turbines to improve the off-design performance of CCHP systems.