Evaluating the potential of phase-change induced volumetric expansion in thermal energy storage media for passive solar tracking in high-temperature solar energy systems

As traditional fossil energy sources are continuously diminishing, the demand for optimising output from renewable energy sources is gaining particular importance. Among these, solar energy is certainly one of the most prominent technology and it is widely used in a variety of applications, either concerning electricity and heat production. Nonetheless, the global efficiency of solar systems still has to be largely improved, reducing at the same time generation costs, in order to make solar an even more relevant source of clean energy. In modern photovoltaic, concentrated photovoltaic as well as concentrated solar power plants, the net output can be increased through solar tracking solutions aiming at the optimal positioning of the solar panels/mirrors on a daily and seasonal basis. This typically requires electromechanical motors, which are designed to align the incident solar radiation with the optical axis, thus enhancing the overall energy conversion efficiency but draining at the same time up to 1–2% of the theoretically achievable net power output. Furthermore, to increase energy dispatchability concentrated solar power plants usually incorporates thermal energy storage units, which can be of the sensible-heat or latent-heat storage type. The latter imply phase transition of the storage material which, in turn, can generate up to 20% volumetric expansion for a solid-to-liquid transition. Although generally assumed as an undesired side effect, such expansion can represent an opportunity to extract mechanical work and thus increase the overall efficiency of the solar system. The main objective of this study is to provide an initial quantitative assessment of the passive tracking potential related to the phase-change induced expansion of thermal storage media in concentrated solar power plants. To this aim, a solar-integrated waste-to-heat steam power plant, rated at 15 MWe, has been taken as a reference and a coupled finite-difference/finite-volume numerical model of the latent-heat thermal energy storage unit of the plant has been developed. The model takes input data from the power plant operating conditions and is able to retrieve time-resolved temperature and volumetric density changes of the thermal storage media. Results from the numerical model shows that passive solar tracking is achievable for a fraction of the heliostat field that ranges from 10% to 100%, depending on the season and operating pressure of the tracking system. In terms of electrical power savings, this is up to 2% of the net power output of the reference plant, thus representing a promising basis for further investigations on the applicability of the proposed novel integrated passive solar tracking concept.