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 focus on sun-to-H2 (STH2) production because of the importance of H2 as a secondary energy carrier in a renewable energy future. We begin with exploring the theoretical STH2 efficiency limits for thermochemical water-splitting processes over a range of solar irradiation concentration ratios (C) and water-splitting reaction temperatures. Thermochemical STH2 processes absorbing the entire solar spectrum as heat at high temperatures (using concentrators) are expected to have higher STH2 efficiencies compared to photochemical processes relying on a specific wavelength band. For example, at C=8000, the theoretical STH2 efficiency for thermal water-splitting is found to be 79.2% corresponding to reaction temperatures greater than 1200 K.We extend the analysis to derive achievable STH2 efficiencies, which account for realistic process inefficiencies such as those arising from positive heat and mass transfer gradients. For a given C, there exists an optimal range of water-splitting reaction temperatures beyond which one sees a large drop in the achievable STH2 efficiency. We find that thermal water-splitting processes operating within the reaction temperature range of 1200-2000 K can approach STH2 efficiencies close to 42-49% (C=8000). These suggested high STH2 efficiency process designs require efficient process heat integration as well as the development of high temperature membrane reactors and catalysts for surpassing equilibrium limitations.

Limiting efficiencies for solar thermal energy conversion to fuels

245th National Meeting (2013)

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