Universal expressions of solar thermochemical efficiency

The calculation of efficiency of solar thermochemical processes has become a field of dispute for the relevant scientific community. Numerous expressions are available in the literature but most of them are ambiguous, conflicting or even overlapping. The lack of a unanimously acceptable approach is caused mainly by the form of the proposed equations, many of which are unclear and open to interpretation. This article proposes a set of simple, yet concrete expressions that can fully describe the solar thermochemical processes starting from the material, to reactor and finally the system level. According to the scale, each expression incorporates the key factors that determine the performance of the technology and all parameters involved per case are explained in a clear manner. Furthermore, they can be readily used to assess any thermochemical process without any limitation, thereby contributing towards the standardization of STCs benchmarking.