Utilization Of Molten Nitrate Salt Nanomaterials For Heat Capacity Enhancement In Solar Power Applications

Abstract

Concentrated solar power (CSP) system use general thermodynamic cycle to produce electricity and thus the system efficiency is mainly determined by the working temperature of heat transfer fluid (HTF). Organic-based HTFs (e.g., mineral oil, ethylene glycol, etc.) were firstly used for this application. However, this has limited the working temperature of CSP around 300 °C since these organic material starts to decompose below 400 °C. Typical liquid salts are thermally stable to high temperatures (500~600 °C). Using these salts as HTF can significantly increase the working temperature and as a result the system efficiency can also be highly enhanced. For example increasing working temperature from 300 °C to 500 °C can simply increase Carnot efficiency from 48 % to 61 %. Moreover, these salts are eco-friendly and using them as HTF can reduce the potential environmental cost caused by the conventional HTF. These salts also have very low vapor pressure that can reduce the potential mechanical stress on the pipe / storage system caused by using the conventional HTF. Recently a binary liquid salt (NaNO3-KNO3; also termed as "solar salt") has been introduced and adapted in the most recent CSP plants. This solar salt is also used as thermal energy storage (TES) medium. Extra thermal energy collected in the daytime is stored in solar salt and kept in a TES for later use. When electricity demand peaks (e.g., evening time) solar salt in TES provide thermal energy to continue electricity production. One of the major challenges to use solar salt as HTF / TES is its high freezing point at 220 °C. This has the potential risk of crystallization in a pipe / storage system in a harsh condition (e.g., rainy season) and can result in high maintenance & operation costs for extra freezing protection system (e.g., insulation, auxiliary heater, etc.). Adding Ca(NO3)2 to solar salt can dramatically decrease the freezing point (down to 120 °C). However, this ternary salt mixture has relatively low thermo-physical properties. Doping this material with oxidized nanoparticles can improve these properties. Nanofluids are liquids doped with nanoparticles. They have been proposed for large enhanced thermo-physical properties. In this report, the low thermo-physical properties were highly enhanced by doping with nanoparticles (19 % increase by 1 % nanoparticle concentration by weight). The result of this study will be useful to develop advance HTF / TES material for CSP plants. This will also applicable for other high temperature HTF applications such as geothermal energy, nuclear energy, and other energy generation technologies using thermodynamic cycle.