Abstracts Physics

Solar Energy Storage in Molten Salt Shell Structures

by Nathan David Loyd

Molten salts (MS) in the 580°C range could be used to store excess energy from solar power stations and possibly from nuclear or coal. The energy can be stored up to a week in large containers at elevated temperature to generate eight hours of electricity to be used at night or during peak demand hours. This helps to reduce the fluctuation experienced at thermal solar power stations due to weather conditions. Our research supported by Office of Naval Research (ONR), presents a survey of salts to be used in molten salt technology and the design of large steel and hybrid molten salt storage shells. The physical characteristics of these salts such as density, melting temperature, viscosity, electric conductivity, surface tension, thermal capacity and cost are discussed. Cost is extremely important given the large volumes of salt required for energy storage at a commercial power station. Formulas are presented showing the amount of salt needed per required megawatts of stored energy depending on the type of salt. The estimated cost and the size of tanks required and the operating temperatures are presented. Recommendations are made regarding the most efficient type of molten salt to use. Commercial thermal solar power stations have been constructed in the US and overseas mainly in Spain for which molten salt is being considered. A field of flat mirrors together with collection towers are presently used in some designs and parabolic troughs used in others to produce electricity commercially. Two designs of tanks for the storage of excess energy from thermal solar power plants using molten salts (MS) at 580°C is presented. Energy can be stored up to a week in large containers to generate eight hours of electricity for use at night or to reduce weather related fluctuation at solar thermal energy plants. The research presented in this thesis shows detailed designs of cylindrical shells for the storage of high temperature molten salts. One storage shell consists of an inner stainless steel layer designed to resist corrosion and an external steel structural layer to contain the large pressures resulting from the molten salt with a steel bottom. The other storage shell consists of an inner stainless steel layer and an external reinforced concrete structural layer with a steel bottom. Both cylindrical tanks are 54 feet high and has an 80 foot diameter, with the salt level at a height of 42 feet. Given the heat of the molten salt and the size of the tank, designs include a flat shell cover supported on stainless steel columns and a semispherical utility access dome at the center. Considerations are made for the reduction of strength of steel at elevated temperatures. Layers of external insulation materials are used to reduce heat loss in the storage shells. Designs also present a 120 foot diameter posttensioned concrete foundation with 20 feet high steel side walls for the storage tank for the containment of molten salts in case of an accident. The tanks sit on a layer of sand to allow for thermal expansion. Advisors/Committee Members: Samaan G. Ladkany, William Culbreth, Nader Ghafoori, Ying Tian.