The growing consumption of fossil energy resources increases greenhouse gas emissions (GHG). To meet the industry sector's target of reducing GHG emissions by 49% in 2030, relative to the levels in 1990, there is a need for energy storage. Energy storage offers a potential solution to enhance the utilization of renewable energy, by addressing the intermittent nature of these sources. This study includes a conceptual design for three Thermal Energy Storage (TES) materials, steel slag, phase change salt, and molten salt, able to generate steam in the industry (4-20 bar). Moreover, this study attempts to contribute to faster commercialization of the phase change materials, by aiming to contribute to a thermal conductivity improvement method. The thermal conductivity limits the discharging and charging ranges nowadays.
The concept designs are based on a fluctuating steam demand pattern provided by Arcadis. Two storing modes are extracted, a stand-alone thermal energy source which can meet a 12-hour steam demand and an energy source intended for peak shaving. Based on literature, the storage configurations, with the associated key parameters are extracted, for each material, to attain an estimation of the dimensions and losses. Due to the inflexibility of literature on phase change salt, a semi-empirical model is developed. With this model, the temperature of the heat transfer fluid (HTF) temperature at the outlet of the storage can be calculated based on various input parameters. The input parameters include the type of phase change salt, type of HTF, mass flow rate of the HTF, and the dimensions of the storage. The HTF outlet temperature in combination with the mass flow determines the discharge power. Based on the output of the TES, an economically optimized design, of a shell and tube steam generator, using AspenEDR software, is conducted.
The results for the stand-alone case showed almost similar volumes for the steel slag and phase change salt. The molten salt storage solution was larger based on the requirement of two tanks, a hot and a cold one. Furthermore, phase change salts resulted in the highest total stored energy, which is a measure of the inability to completely discharge the phase change storage in combination with a maximum temperature drop of the HTF outlet temperature. An increase in the thermal conductivity of the phase change salt can reduce this problem. Another advantage is the possibility of using any HTF for charging and discharging the phase change salt. Due to diffusion inside the TES upon partly charging and discharging, steel slag and phase change salt seemed not suitable for peak shaving.
Next, the focus is on increasing the thermal conductivity of the phase change salt. A readily available, stainless steel, wire mesh implemented in the phase change salt, at the shell side of a shell and tube storage, already yields a 10% increase in overall discharging performance. A graphite coating, applied on the mesh, could enhance the corrosion resistance, thereby enabling the the use of a higher conductive steel, such as carbon steel. Furthermore, could a graphite coating increase the overall thermal performance, by generating highly conductive pathways parallel to the mesh. A suitable graphite coating is obtained by exploring graphite coatings in various application areas. Together with the requirement of this coating, the first choices are made regarding the components of the coating. In addition, various experimental results are conducted to determine a potential coating composition and application method.
Results showed that a graphite coating consisting of graphite, Polyvinylidene Fluoride (PVDF) and N-Methylpyrrolidone (NMP), applied with the use of a dip coater, followed by drying, could be attached to the carbon steel with a thickness of 2 mm in four layers. The influence of the graphite coating is estimated with the use of a resistance model.
Further research should focus on quantitatively determining the influence of the graphite coating application in combination with the evaluation of the corrosion resistance of the coating.