The combined solar-photovoltaic electricity and solar-themal heat application in power to chemicals process schemes: consideration of optimal solar photovoltaic surface area fractions

Abstract

Power-to-chemicals processes hold tremendous potential to enable the widespread adoption of renewable sources of energy and to open many avenues to the green hydrogen economy. These processes will play a critical role in reducing mankind reliance on fossil fuels derived energy, thereby helping to abate the harmful emissions concomitant to the use of fossil fuels. Improving energy efficiency and process economics is key to unlocking the potential of power to-chemicals processes. This study examines the energy efficiency, cost reduction and environmental benefits, if any, of using the combination of solar photovoltaic and solar thermal heat in power-to-chemicals processes. The power-to-hydrogen, syngas, ammonia, methane, methanol, propane, ethylene, ethanol, and propanol are selected for the analyses. Thermodynamic analysis using the first and second law of thermodynamics together with the g-h graphical analysis is used to determine the performance limits of processes as well as to set theoretical targets. The theoretical targets of interest are the work storage efficiency and overall system work efficiency. Aspen plus® commercial software is then used to model processes to establish how far the theoretical targets can be met and to also determine the cost of processes. The analyses revealed that there will always be work efficiency losses when the processes are supplied with solar-photovoltaic electricity alone. The work efficiency is improved when the solar-thermal heat is used to supplement the solar-PV electricity, with maximum work efficiencies achieved at specific solar-photovoltaic area fractions. The analysis further showed the total energy cost of the processes to increase with the solar-PV area fraction, while the area and cost of electrolysis stack decrease with the increasing solar-PV area fraction. The observed trade-off in energy and electrolysis cost suggests that there is an optimal solar-PV area fraction (fop) between the minimum (fmin) and f=100% where the cost is minimised. Aspen simulation showed the overall system work efficiencies of 51.7, 52.7 and 54.3 % for power-to-ammonia, methanol, and methane, respectively. The corresponding Levelized costs of production are 543, 533 and 1158 USD/tonne.