Assessment of Direct Steam Generation Technologies for Solar Thermal Augmented Steam Cycle Applications

Abstract

A conceptual design study was performed to quantitatively evaluate a range of solar augmented steam cycle design options for natural gas combined-cycle (NGCC) and pulverized coal power plants. Solar augmented steam cycle options were modeled for two direct steam generation (DSG) solar technologies: 500°C line-focus (parabolic trough or linear Fresnel reflector) and 565°C central receiver. Fossil-fuel power plant designs included NGCC with duct firing and subcritical pulverized coal. All of the conceptual designs utilized steam generated by a solar field to offset some of the fossil fuel required to generate power. The performance of multiple integration options was evaluated using thermodynamic models to provide a quantitative comparison of plant retrofit design options. Based on the available steam conditions for the DSG technologies and the results of past modeling studies, solar steam was assumed to be integrated with the main steam supply at the exit of the heat recovery steam generator superheater for the NGCC plant. For the pulverized coal plant, solar steam was integrated into the main steam header after the superheater. Steam integration at these high temperature points in the Rankine cycle is the most thermodynamically optimal design. For both plant types, feedwater was extracted from the boiler feedwater pump discharge. Using the thermodynamic analysis results, the solar augment potential across nearly half of the U.S. states was estimated for existing coal and gas plants that met certain age, solar resource, land availability and other criteria. The gigawatt (GW) deployment potential for DSG solar technologies was compared to the equivalent potentials for previously analyzed oil-based parabolic trough and molten salt central receiver technologies [1]. Solar use efficiency and plant heat rate improvement were the primary metrics used to compare the relative performance of the solar technology options. The deployment potentials were significant, particularly for the technologies with the highest solar steam temperatures. If line-focus DSG technologies can successfully achieve temperatures approaching 500°C (932°F), this study indicates that the solar augment potential is nearly double the capacity estimates for lower temperature troughs. Over 25 GW of solar augment capacity was estimated in the U.S. Compared to nominally 200 MW of worldwide solar augment capacity in operation today, the study indicated a considerable opportunity for solar augment applications to play a role in the total energy mix.