For solar thermal power to be cost effective as a baseload generation paradigm, the cost and efficiency of every subsystem must be improved. To reduce the cost of thermal storage and increase the working temperature of the subsystems for greater efficiency, appropriate storage materials must be identified for study. Unfortunately, there is an enormous search space of salt mixtures that are suitable candidates for thermal storage and must be analyzed for cost effectiveness to determine which candidates are the best. To lessen the experimental burden, this article combines previously validated theories for estimating the properties of salt mixtures with an analytic design optimization method to better estimate the true cost of each candidate. Five hundred and sixty three binary, ternary, and quaternary mixtures are analyzed. Five mixtures are identified as the prime candidates for a cascaded latent heat thermal energy storage system for a supercritical CO2 Brayton cycle generator. This system is cheaper than the Sunshot Initiative cost targets, bringing the solar thermal paradigm into cost competition with fossil fuels.

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reflect and concentrate sunlight onto a central receiver system that is situated at the top of a tower. This system is able to achieve higher operating temperatures at the receiver and much larger solar concentration ratios compared to parabolic trough systems, [2] resulting in higher thermodynamic efficiencies. [3] Several CSP tower plants are currently in successful operation, however they suffer from the same drawbacks as many renewable energy technologies in that their energy generation is intermittent. This is a major hindrance if renewable energy technologies are to compete with coal-fired power plants in providing baseload power to the grid. A solution to this for CSP plants is the use of thermal energy storage (TES) systems, in order to store thermal energy captured from sunlight for future use (during overcast weather conditions, or at night). [4] There are three types of TES systems. The first uses sensible heat (sensible heat thermal energy storage, SHTES), where a substance with a high heat capacity stores the thermal energy for future use. Typically, current operating plants (both parabolic and tower configurations) store thermal energy with molten solar salt-a combination of potassium nitrate and sodium nitrate. Energy is stored in or extracted from the solar salt by raising or lowering its temperature in response to the energy demand. These solar salts are limited by their maximum operating temperature of 585 °C and their energy storage capacity is determined by the heat capacity of the material. [5] The second TES system, latent heat thermal energy storage (LHTES), uses phase change materials (PCMs) that extract and store energy in the form of latent heat as the material transitions through a phase change (e.g., from solid to liquid). The energy of transformation is large, and therefore the storage material is able to charge and discharge significantly larger amounts of energy than sensible heat systems. This in turn improves the efficiency of the storage system, and occupies a smaller volume. Many PCMs can operate at much higher temperatures than sensible heat materials, which is a desirable property since the latest generation of CSP towers is capable of reaching temperatures in excess of 550 °C. [6,7] Additionally, the heat transfer process of extracting energy from PCMs is a relatively isothermal process, which translates to better temperature control and management of the entire heat transfer system that leads to higher efficiencies. [3,8] With current concerns about the environmental impact of greenhouse gas emissions, reducing our reliance on fossil fuels has become an ever-growing necessity. A thermal energy storage system that utilizes phase change materials (PCMs) in the form of molten salts, coupled with a concentrating solar power tower plant, is proposed as an effective means of achieving highly efficient and cost competitive power generation on par with traditional fossil fuel-based power. In this study, a set of five selection criteria are applied to a wide range of salt mixtures to...

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