Efficiency Potential of Solar Thermochemical Reactor Concepts with Ecological and Economical Performance Analysis of Solar Fuel Production

The alternative fuel production pathway by solar thermochemical splitting of water and carbon dioxide into hydrogen and carbon monoxide by redox reactions of a metal oxide, and their subsequent conversion into liquid fuels by Fischer-Tropsch synthesis, is investigated. These fuels could provide a means to completely decarbonize the transport sector and thus to significantly reduce its climate impact. A generic model is developed for the description of solar thermochemical reactors including heat exchangers, where reduced elements of redox material move from the reduction to the oxidation chamber through a number of heat exchanger chambers, in which they transfer energy by radiation to the cold elements moving in counter-flow. In a first implementation of the model, infinitely fast thermal diffusion within the material is assumed and the influence on heat exchange of the wall separating the hot from the cold elements is neglected. A heat exchanger efficiency potential of over 80% is determined, where efficiency is increased towards higher reaction temperatures and reaches its maximum value in the reduction pressure range of 10-100 Pa due to a trade-off between additional energy requirements for vacuum pumping and enhanced fuel productivity. In a second implementation of the model, the effect of the separating wall is considered and heat diffusion in the porous redox material is solved. Heat exchanger efficiency is found to have a potential of about 70%, where heat diffusion in the redox material is identified to be a limiting factor for heat exchange. The system can be enhanced by increasing the porosity as well as optimizing material thickness and total residence time of the elements in the heat exchanger. Through an efficient design of the heat exchanger, efficiencies close to the optimal case of infinitely fast heat diffusion can be reached. The model is adapted for the description of heat exchange between two unmixed particle beds moving in opposite directions in a cylindrical enclosure. Heat exchanger efficiency is found to have a potential of close to 60% and to be limited by heat transfer within the particle beds which can be enhanced e.g. by optimizing the bed diameter, heat exchanger length, particle size, and the velocity of the beds. The analysis is complemented by an assessment of ecological and economic performance of a baseline case fuel production plant. The baseline assumptions are that the plant has an output of 1000 barrels per day (bpd) of jet fuel and 865 bpd of naphtha, uses water from seawater desalination and carbon dioxide by capture from the atmosphere, has a thermochemical efficiency of 20%, and uses heat and electricity provided by conversion of solar primary energy and combustion of the gaseous Fischer-Tropsch products. The energy conversion efficiency from incident sunlight to lower heating value of the produced fuels is determined to be 5.0%. A life cycle analysis shows greenhouse gas (GHG) emissions of 0.49 kgCO2-eq. per liter of jet fuel, which is a reduction of over 80% compared to conventional jet fuel. The main drivers of the GHG emissions are identified to be the origins of carbon dioxide and electricity, the combustion of gaseous Fischer-Tropsch products, and the construction of the solar concentration infrastructure. The water consumption is 7.4 liters per liter jet fuel for on-site processes and 40.2 liters for off-site processes, which is orders of magnitude lower than that of biofuels and about equal to that of fossil fuels. The area-specific productivity is 3.3 × 104 liters of jet fuel equivalents per hectare and year, which is lower than the best power-to-liquid pathways but about an order of magnitude higher than that of biofuels. An economic model based on the annuity method shows production costs of 2.23 € per liter of jet fuel for the baseline case. The economic drivers are the construction and operation of the solar concentration facility, the provision of electricity by an on-site concentrated solar power plant, carbon dioxide capture, and the lifetime of the fuel production plant. Thermochemical efficiency and solar irradiation have an important influence both on the GHG emissions and the plant economics through their determination of the required size of the solar concentration facility. It is found that jet fuel production with emissions significantly lower than conventional fuel requires a renewable source both for carbon dioxide and for electricity, such as carbon dioxide capture from the atmosphere and electricity generation from sunlight. Assuming favorable development of the involved process steps, production costs of 1.28 € per liter jet fuel at greenhouse gas emissions of 0.10 kgCO2-eq. per liter are estimated. The realization of high thermochemical efficiencies is crucial for the development of an economic fuel production pathway due to its direct influence on the required size of the solar concentration facility. The developed models can be used for the continued comprehensive analysis of the fuel production pathway including the large parameter space in the design of solar thermochemical reactors and thus provide a valuable tool for further research and design