Advanced Review Integrated solar thermochemical cycles for energy storage and fuel production J¨ org Petrasch ∗ and James Klausner Integrated solar thermochemical cycles comprise a range of promising novel process technologies that use concentrated solar energy to drive endothermic chemical reactions at elevated temperatures. The most promising application is the production of carbon-neutral fuels, particularly via single or multistep water and CO 2 splitting or via the solar thermochemical upgrading of carbonaceous fuels such as biomass, waste, or oil residues. Furthermore, intermediate storage of solar energy in reversible reactions, the so-called solar thermochemical heat pipes, shows great promise to replace latent heat storage for concentrating solar power generation. Potential niche applications are material processing and mate- rial testing. Widespread deployment of solar thermochemical cycles hinges on the development of several key technologies: (i) reaction systems and catalysts able to endure tens of thousands of conversion cycles without significant degradation, (ii) reactors and heat recuperation systems that fully exploit the theoretical potential of solar thermochemical cycles, (iii) industrial-scale reactor technologies, and (iv) process control technologies that address the inherently transient nature of solar power. C  2012 John Wiley & Sons, Ltd. How to cite this article: WIREs Energy Environ 2012. doi: 10.1002/wene.11 INTRODUCTION S olar energy is by far the most abundant source of renewable energy. 1 Because solar energy is an intermittent power source, and the most suitable lo- cations for solar power collection are desert regions, away from population centers, storage is critical for the eventual large-scale deployment. Among the nu- merous storage solutions that are currently being pur- sued, thermochemical storage of concentrated solar energy as a fuel has several advantages: chemical en- ergy carriers have a high-energy density, they are sta- ble, and can be stored indefinitely; a complete infras- tructure for hydrocarbon fuel transport, storage, and conversion already exists. Integrated solar thermochemical cycles feature highly complex coupled, transient multiphysics phe- nomena, including radiation, conduction, and con- vection heat transfer; fluid dynamics; and high- temperature heterogeneous chemical kinetics. The ∗ Correspondence to: petrasch@ufl.edu Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, USA DOI: 10.1002/wene.11 fundamental principle of solar thermochemical cycles is schematically depicted in Figure 1. Solar radiation is concentrated by concentrating optics (CO), typi- cally consisting of one or several highly reflective, sun- tracking mirrors. Radiation is then converted to heat, which in turn is used to drive a high-temperature en- dothermic reaction. Conversion to heat and chemical reaction is usually collocated in a solar thermochemi- cal reactor (SCR). Reaction products are referred to as solar fuels. They can be stored indefinitely and trans- ported over large distances. Substances emitted dur- ing the consumption of solar fuels are either recycled or released to the atmosphere. The latter scenario is only feasible if the products of the solar thermochem- ical reaction are innocuous, natural constituents of the atmosphere, such as water or CO 2 . FUNDAMENTALS Radiative Transfer and Concentrating Optics The maximum normal solar irradiance, I, is approx- imately 1000 W/m 2 at sea level. If sunlight is to be used to drive thermochemical processes, it must be Volume 00, July/August 2012 1 c  2012 John Wiley & Sons, Ltd.