(136c) Limiting Efficiencies for Solar Thermal Energy Conversion to Fuels

The conversion of solar energy into chemical energy in the form of a fuel derived from the raw materials of air, water and soil, is critical for energy storage and transportation in a future solar energy driven economy. A key metric of interest here is the fraction of incident sunlight recovered as fuel, or the process sun-to-fuel (STF) efficiency. The importance of the STF efficiency metric is seen in the context of the large magnitude of global primary energy consumption relative to the land resource available to harness solar energy.

In this presentation, we explore the STF efficiency limits of thermochemical processes producing carbon-based fuels and H₂ from sunlight starting with the raw materials of CO₂ and H₂O. Thermochemical STF processes absorbing the entire solar spectrum as heat at high temperatures (achieved through the use of concentrators) are expected to have higher STF efficiencies compared to photochemical processes relying on a specific wavelength band. Our analysis finds that the theoretical STF efficiency remains nearly unchanged whether using single or multiple blackbody absorbers for solar heat recovery. 

For feasible solar concentrations of 2000-8000, the theoretical maximum Sun-to-H₂ (STH₂)efficiency ranges from 72-79%, corresponding to operating temperatures of 1200-1600 K. The use of multiple reaction steps during the water-splitting process is shown to further lower the theoretical efficiency. The corresponding STF efficiency range for methane and methanol is 70-76% and 65-71% respectively.

To approach real processes, we discuss a “practical” STF and STH₂ efficiency limit, which considers the irreversible heat integration between the process streams, heat sources and sinks. A notable observation here is that the practical maximum efficiencies derived using irreversible heat integration are only 5-10% lower in value than the corresponding theoretical efficiencies.