Thermal Conductivity Enhancement of High-Temperature Thermal Energy Stores For Use with Solar Power Plants

Thermal energy storage is of critical importance in many engineering applications. The demand for CO2 reduction to curb global warming considerably increases the interest in utilizing renewable energy sources, especially solar energy. Due to the discrepancy between solar energy supply and energy demand, a thermal energy storage device has to be used. Thermal energy storage (TES) plays a vital role in solar energy applications in areas such as energy efficient buildings and solar power plants, and therefore, it has received significant attention. Thermal energy storage techniques can be classified as sensible heat storage and latent heat storage. Latent heat storage is particularly attractive, since it provides a high energy storage density and can store the energy as the latent heat of fusion at a constant temperature (phase change temperature of the corresponding PCMs). Although extensive investigations on low temperature latent heat storages mainly used for buildings have been conducted, very limited investigations on high temperature latent heat storages have been carried out for solar power plants. All the PCMs have a common problem of low thermal conductivity. The thermal conductivity is around 0.2 and 0.5 for paraffin and inorganic salts, respectively, which prolongs the charging and discharging period. Since metal foams have high thermal conductivities and high surface area density as well as good mechanical properties, they could be used as a metal skeleton embedded within PCMs to significantly enhance heat transfer both for low temperature and high temperature thermal storage systems reducing charge and discharge time. Most of PCMs undergo large change in volume (~10%) during melting. Volume contraction during solidification may not only reduce heat transfer area but also separate the PCM from the heat transfer surface, increasing the heat transfer resistance dramatically. But the presence of metal foams can quickly transfer heat through the structure surfaces embedded in the PCMs, and distribute the heat to the whole volume of PCMs from the heat transfer surface, thereby enhancing the heat transfer rate and reducing charge and discharge time. The energy storage mechanism in metal foams integrated with phase change materials (PCMs) is inherently complicated, since it involves solid/liquid phase change heat transfer, moving solid/liquid interface, porous metal foam microstructures, buoyancy induced natural convection etc.. Many underlying physical problems need to be understood, and all these warrant a detailed study for heat transfer enhancement of high temperature latent heat storages.The proposed research aims to experimentally and numerically study the feasibility of using metal foams to enhance the heat transfer capability of phase change materials (PCMs) in high temperature thermal energy storage systems used in solar power plants. The heat transfer enhancement caused by metal foam structures will be experimentally investigated. The effect of metal foam cell size and porosity on thermal energy storage will be examined. A numerical model will be developed to predict the complicated physical phenomena during the transient charging and discharging processes. Another major purpose of this collaborative research is to build the long-term concrete collaboration with one prestigious research group in China through the mutually interested research project. In this study, three inorganic PCMs: sodium nitrate (NaNO3), potassium nitrate, and an eutectic mixture of magnesium chloride, potassium chloride and sodium chloride (MgCl2/KCl/NaCl) will be employed as the latent heat storage materials. In this study, graphite foams will also be used for the experimental study, and the enhancing effect will be compared with the counterpart of metal foams (This work will be done by Xi'an Jiaotong University).