A Thermodynamic and Cost Analysis of Solar Syngas From the Zn/ZnO Cycle

Abstract

We present a thermodynamic and cost analysis of synthesis gas (syngas) production by the Zn/ZnO solar thermochemical fuel production cycle. A mass, energy, and entropy balance over each step of the Zn/ZnO syngas production cycle is presented. The production of CO and H2 is considered simultaneously across the range of possible stoichiometric combinations, and the effects of irreversibilities due to both recombination in the quenching process following dissociation of ZnO and incomplete conversion in the fuel production step are explored. In the cost analysis, continuous functions for each cost component are presented, allowing estimated costs of syngas fuel produced at plants between 50 and 500 MWth. For a solar concentration ratio of 10,000, a dissociation temperature of 2300 K, and a CO fraction in the syngas of 1/3, the maximum cycle efficiency is 39% for an ideal case in which there is no recombination in the quencher, complete conversion in the oxidizer, and maximum heat recovery. In a 100 MWth plant, the cost to produce syngas would be $0.025/MJ for this ideal case. The effect of heat recuperation, recombination in the quencher, and incomplete conversion on efficiency and cost are explored. The effects of plant size and feedstock costs on the cost of solar syngas are also explored. The results underscore the importance improving quencher and oxidizer processes to reduce costs. However, even assuming the ideal case, the predicted cost of solar syngas is 5.5 times more expensive than natural gas on an energy basis. The process will therefore require incentive policies that support early implementation in order to become economically competitive.