Thermodynamic modeling of solarized microturbine for combined heat and power applications

Combined heat and power (CHP) plants utilize exhaust heat from thermal-based power generators to increase system efficiency beyond electrical efficiency alone. Many existing CHP systems use fossil-fueled generators to create electrical power for retail sale or on-site industrial or commercial uses. This study develops, validates, and exercises a quasi-steady state thermodynamic model of a 100 kWe/165 kWt rated microturbine that has been coupled with a concentrating solar power (CSP) tower to offset natural gas consumption. Exhaust heat is rejected at approximately 270 °C for CHP applications. Governing equations developed for eight components incorporate manufacturer data and empirical data to describe system-level operation with respect to intraday variation in the solar resource. Model validation at ISO conditions shows electric output of the simulated system is within 1.6% of the as-built system. Simulation results of the complete solarized system gave 31.5% electrical efficiency, 83.2% system efficiency, 99.5 kWe electrical power, and 163.5 kWt thermal power at nominal operating conditions for a DNI of 515 W/m2. The thermodynamic model is exercised under rated electrical load (base loading) and variable electrical load (load following) conditions with performance measured on 13 operating characteristics. Sensitivity analyses evaluate changes in performance with respect to operating variables (e.g., turbine inlet temperature) and environmental variables (e.g., elevation). Results show that a CSP plant with solarized microturbine can meet target performance specifications of a non-solarized microturbine (pure natural gas). Annual time series simulations completed for Phoenix, Arizona, USA indicate a solarized microturbine can reduce natural gas use by 26.0% and 28.4% when supplying rated power and variable power output, respectively. Annual operating time of the solarized microturbine at rated capacity included 59.8% fuel only, 12.4% hybrid, and 27.8% solar only modes for the selected study location.