Use of Low/Mid-Temperature Solar Heat for Thermochemical Upgrading of Energy, Part I: Application to a Novel Chemically-Recuperated Gas-Turbine Power Generation (SOLRGT) System

Abstract

This paper is the first part of a study presenting the concept of indirect thermochemical upgrading of low/mid temperature solar heat, and demonstration of its integration into a high efficiency novel hybrid power generation system. The proposed system consists of an intercooled chemically recuperated gas turbine (SOLRGT) cycle, in which the solar thermal energy collected at about 220 °C is first transformed into the latent heat of vapor supplied to a reformer and then via the reforming reactions to the produced syngas chemical exergy. The produced syngas is burned to provide high temperature working fluid to a gas turbine. The solar-driven steam production helps to improve both the chemical and thermal recuperation in the system. Using well established technologies including steam reforming and low/mid temperature solar heat collection, the hybrid system exhibits promising performance: the net solar-to-electricity efficiency, based on the gross solar thermal energy incident on the collector, was predicted to be 25–30%, and up to 38% when the solar share is reduced. In comparison to a conventional CRGT system, 20% of fossil fuel saving is feasible with the solar thermal share of 22%, and the system overall efficiency reaches 51.2% to 53.6% when the solar thermal share is increased from 11 to 28.8%. The overall efficiency is about 5.6%-points higher than that of a comparable intercooled CRGT system without solar assist. Production of NOx is near zero, and the reduction of fossil fuel use results in a commensurate ∼20% reduction of CO2 emissions. Comparison of the fuel-based efficiencies of the SOLRGT and a conventional commercial Combined Cycle (CC) shows that the efficiency of SOLRGT becomes higher than that of CC when the solar thermal fraction Xsol is above ∼14%, and since the SOLRGT system thus uses up to 12% less fossil fuel than the CC (within the parameter range of this study), it commensurately reduces CO2 emissions and saves depletable fossil fuel. An economic analysis of SOLRGT shows that the generated electricity cost by the system is about 0.06 $/kWh, and the payback period about 10.7 years (including 2 years of construction). The second part of the study is a separate paper (Part II) describing an advancement of this system guided by the exergy analysis of SOLRGT.