Computational fluid dynamics analysis of a thermocline thermal storage unit for solar thermal applications

World energy consumption increases every year therefore new power plants are necessary to meet the demand. Among many energy production systems, it is expected that share of renewable energy in total production is going to grow more than others due to environmental concerns and achievements at cost reduction. One of the renewable energy sources is sun and a solar power plant captures the sunlight and converts it into electricity. There are several ways to harvest the sun's energy. Photovoltaic systems use panels to directly convert sunlight into electricity whereas solar concentrating systems use mirrors to reflect and focus sunlight on heat collecting component of the system where heat is transferred to a fluid which is used to generate steam to turn the turbine and generator to generate electricity as in thermal or nuclear power plants. Concentrating solar systems has various configurations such as parabolic through, parabolic dish, and central tower. In parabolic through systems, there are reflector mirrors and receiver tubes which are located at focal axis of mirrors. Sun rays are reflected to the receiver tube and heat is transferred to fluid inside the tube. This fluid is then used with conventional steam generator to produce electricity. In parabolic dish systems, sun rays are reflected to the center of the dish where receiver is placed. The heat machine on the receiver that moves with the dish uses Stirling or Brighton cycle for power conversion. On the other hand, in central tower systems sun rays are reflected to the receiver at the top of the tower by mirrors around the tower. Conventional steam generator is used for electricity generation. The electricity form solar power plant is categorized as intermittent electricity due to the fact that it cannot be continuously available. As a result, fluctuating demand of electricity cannot be met therefore solar power plants are considered non-dispatchable. On the other hand, it is possible to constrain intermittency either with direct electricity storage for photovoltaic systems or with thermal energy storage for concentrating systems. In thermal energy storage, some of the heat form the receiver is stored in the storage unit for later use. Since storing thermal energy is cheaper than storing electricity itself, the focus is on development of thermal energy storage systems. Efficient and cost-effective storage is an important tool to increase the share of solar energy in the electricity market. There are two options available for thermal energy storage: two-tank storage and single-tank thermocline storage. In two tank storage, during charge cycle, fluid from cold tank passes through a heat exchanger if system is indirect or collector field if system is direct to hot tank and during discharge cycle fluid moves back from hot tank to cold tank after passing through steam generator. In single-tank thermocline storage, there is a filler material in the tank as energy storage medium. During charge cycle, cold fluid moves from the bottom of the tank towards the heat exchanger and returns to the tank from the top as hot fluid and during discharge cycle, hot fluid moves from the top of the tank towards heat exchanger and returns the tank from the bottom as cold fluid. Since part of fluid in the two-tank storage system is replaced with a filler material which is usually cheap, single-tank thermocline storage offers cost-effective energy storage. In the scope of this thesis, single-tank thermocline thermal energy storage discharge cycle analysis with computational fluid dynamics (CFD) was considered because the literature review showed that focus is generally on the properties of storage tanks and sensitivity analysis of storage tanks with correlation and formulation. Therefore, with the CFD analysis stream-lines and velocity and temperature distribution in the thermocline tank together with the effects of porosity, sphericity and type of fluid on discharge process were investigated. Seven different heat transfer fluids having various specific heat capacity values were considered. In order to see the effect of porosity in the tank, three porosity values were selected. The effect of the filler material geometry was included by using four different sphericity values. First of all, the geometry of the thermocline tank was determined with literature review. Then, the mathematical model to perform simulations were defined. The governing equations of continuity, time dependent momentum, and time dependent energy are included in openFOAM CFD code. Governing equations include not only effective conductivity and Forchheimer-Brinkman approximation but also Bouyssinisq approximation. The grid independence study which guarantees the independence of results from the mesh size was performed. Later, base input of the CFD code which includes the model of the reference thermocline tank from literature was validated by comparing simulation results with experimental data. Finally, simulations were performed with selected heat transfer fluids for different operating conditions i.e., porosity and sphericity and their effect on thermocline storage tank energy deposition and energy generation was discussed. The simulation results showed that when the fluid has high value of volumetric heat capacity (VHC), the initial energy stored in the tank increases. In addition, if VHC value of the fluid is lower than the value for the filler material, initial energy stored in the tank is mainly stored in the filler material. The percentage of the stored energy in the solid drops as low as 50% if the fluid has high VHC and tank has high porosity. Furthermore, for fluids with high values of VHC, when the porosity of the thermocline tank increases, the amount of energy remained in the tank during discharge cycle increases. On the other hand, fluids with low values of VHC, higher porosity value results in lower remaining energy in the tank. On the other hand, temperature profiles and streamlines of the 6 hours of discharge showed that for low values of porosity, higher sphericity value prevents mixing of hot and cold fluid and results in better discharge performance. It is clear from the simulations that discharge behaviour of the storage tank mainly depends on the relation between the VHC of fluid and solid filler material, high value of sphericity provides positive effect on discharge performance as long as porosity is low, and porosity value selection must include fluid and solid VHC values. Future studies can be done to increase the efficiency of the tank by improving stream-line pattern. It is known that when the stream-lines are uniform, mixing of hot and cold fluid reduces and efficiency of the tank increases. It is possible to reach this goal by making modifications inside the tank such as creating lanes for the flow. Another possible area of study can be usage of nano-fluids as heat transfer medium. These studies can be performed efficiently with computational fluid dynamics simulations which use the experimentally verified mathematical model developed in this study.