Optimization of solar-coal hybridization for low solar augmentation

Abstract

This work presents a process to determine the preliminary optimal configuration of a concentrating solar power-coal hybrid power plant with low solar augmentation, and is demonstrated on a regenerative steam Rankine cycle coal power plant in Castle Dale, UT, USA (average DNI of 542 W m−2). A representative plant model is developed and validated against published data for a coal power plant. The simplifications that lead to the representative model from a coal power plant model include combining multiple feedwater heaters, combining turbines, and using a mass-average calculation for extraction steam properties. Comparing net power generation and boiler heating estimates from the representative model to the benchmark power plant, the representative model predictions are accurate to within ±2.5% of the accepted value. Methods for quantifying solar resource based on geography and simulating a concentrating solar power field arrangement are provided and the solar contribution to electrical power output is estimated using an exergy balance. A financial model is also included to estimate the solar marginal levelized cost of electricity and payback time using a cash-flow analysis. A multi-objective optimization routine is then employed to determine the optimal configuration using the models described in this study.It is shown that a solar augmentation of >3% of boiler heating is required for a hybrid design to be considered thermodynamically feasible. However, as the augment fraction is increased, the financial benefit from fuel savings is insufficient, without a carbon tax, to offset the higher capital cost. Optimization results, constrained to a maximum solar field size of 20 ac, are also provided assuming a common carbon tax value (16 USD sh.tn.−1). The resulting optimal design for the Castle Dale Plant with a carbon tax and no premium indicates the use of parabolic trough collector technology at an augment fraction of k=9% to bypass feedwater heater 6. The resulting marginal solar levelized cost of electricity is 9.5 x 10-4 USD kWh−1 with an estimated payback time of 25.2 years. With a green energy premium price of 0.018 USD kWh−1 and no carbon tax, the payback time reduces to 3.9 years. This process can be applied to a subcritical Rankine cycle coal power plant for which operating data and meteorological data are available to evaluate preliminary hybridization feasibility.