Off-design heating/power flexibility for steam injected gas turbine based CCHP considering variable geometry operation

Owing to the fluctuating energy demand from consumer side, combined cooling, heating and power (CCHP) system operates at off-design condition most of the time. Moreover, the heating/power surplus is a common problem for the conventional CCHP system. In order to improve the precision of performance prediction and partly relieve the heating surplus, a steam injected gas turbine (STIG) based CCHP system considering off-design condition is proposed. The system consists of a steam injected gas turbine, double-pressure heat recovery steam generator (HRSG), heat exchanger, absorption chiller and auxiliary boiler. Two different operation strategies including turbine inlet temperature (TIT) strategy and variable guide vanes (VGVs) strategy, are illustrated to access the part-load performance of gas turbine and CCHP system. The performance indicators, like primary energy saving rate (PESR), primary energy rate (PER) are set up to evaluate the thermodynamic performance of the CCHP system. A simple gas turbine (SGT) based CCHP is chosen as the comparative object and two energy demand cases are selected to validate the advancement. The results show that VGVs strategy does not improve the gas turbine efficiency significantly but bring more exhaust heat into HRSG. VGVs strategy advances the heating to power ratio, PESR and PER around the whole part load condition, i.e., PESR is elevated from 0.3254 to 0.3743 by VGVs at the half of gas turbine load without injected steam. Injecting steam is a more effective way to improve gas turbine efficiency rather than using VGVs strategy while implementing same gas turbine load. Steam injection does not always enhance the PESR, but the PER keeps decreasing as injected steam flow increases around the whole load. STIG based CCHP system with VGVs operation strategy gets the best performance among these studied approaches, and the maximum enhancement of PESR reaches 0.3171 in the proposed energy demand cases.