Study On Thermal Energy Storage Performance Of Solar Thermal Power Thermocline Single Tank

As an important way of renewable energy utilization, the rapid development of solar thermal power is of great significance, especially under the background which traditional fossil fuels are running out. However, the intermittence essence of solar energy makes it a puzzle for the steady output of solar thermal power and its connection to the power grid. Although the currently utilized dual tank thermal energy storage(TES) strategy or natural gas backup system could help make the solar power plant operate steadily, their high costs make it a hinder for its competition with traditional coal fired power plants. Thermocline single tank thermal energy storage is a relatively cheap method for TES, if it can be properly hybridized and integrated with solar aided coal-fired power plants, to form a complementary power generation mode, then the cost of power generation could be expected to be decreased.This thesis starts from the modeling of thermocline single tanks, with methods like mathematical analysis, experimental investigation, numerical simulation and system level simulation, makes a comprehensive investigation of thermal energy storage characteristics of solar thermal thermocline single tank. The main contents and conclusions include:Firstly, based on more accurate method of CFD numerical simulation, the temperature profile and heat transfer fluid outlet temperature of three typical TES structure, including plate, cylinder and tube, is investigated. It is found that the occurrence of thermocline phenomenon varies from structure to structure, whereas the plate, with its higher effective heat transfer coefficient and higher heat exchange area, has the best TES performance among the three. The results are compared with the proposed 1D solid-fluid TES transient model for all three structures respectively. The 1D model uses an extended lumped capacitance method to account for the internal heat conduction resistance in a solid structure(for Biot number ranges from 0.1 to 100), where an effective heat transfer coefficient is used to replace the former heat transfer coefficient. The results show that two methods agree well with each other, therefore the 1D model could be used to analysis solid-fluid TES process.Secondly, the idea of sand-based and sand/oil solid-fluid thermal storage media is proposed and experimentally validated with indirect contact experimental setup. The feasibility of sand at various grain sizes as filler material for solar sensible heat thermal energy storage(TES) is investigated. The results show that voidage is the determining factor for sand’s TES performance. With the lowest voidage, coarse sand demonstrates the best performance among the four types while filter sand is the worst. For materials like sand whose basic thermal properties only vary a bit, the changing of charge temperature can hardly affect the TES performance, so the results obtained in lower temperature could be easily extended to higher temperature conditions. When the flow rate increases, the energy stored in the tank is also increased, yet much weaker than linear correlation. The use of sand soaked with Xceltherm 600 synthetic oil can enhance TES efficiency, besides, the radical temperature profile becomes uniform and the heat conduction is accelerated. It is found that the TES efficiency of sand-oil mixture increases by 18.5% compared to that of pure sand. In the future, when the mixture idea becomes more mature, industrial waste oil and edible oil may be used to replace the synthetic oil and further lower the TES cost.Furthermore, with the introduction of three coefficient including goodness factor, cost factor and comprehensive efficiency coefficient, the evaluation standard system is established for thermocline single tank TES system, so that the TES performance of single tank and dual tank could be compared. Referring to the evaluation standard, the numerical analysis of three typical solid-fluid configurations:flat plate, cylinder, and tube are analyzed. It can be observed that the void fraction ε varying within the range of 0.25-0.33 seems to be of little impact on the storage performance of the TES system. While larger ε tends to show better characteristics among all three cases, the use of heat transfer fluid (HTF) is also increased. While smaller ε may lead to weaker performance, the use of HTF may be reduced. Therefore, when the HTF is much expensive than the TES materials, smaller ε may be more economic. Three factors, mass flow rate, TES unit’s height and unit’s cross section areas, can affect the TES performance. Within a proper operation range, lower mass flow rate, higher unit’s height and smaller unit’s cross section areas are recommended. In addition, when the deviation extent of goodness factor is properly adjusted, the TES performance of a single tank could be enhanced while its cost could be reduced, endowing single tank a potentiality to overcome dual tank with oil-salt heat exchanger.Finally, thermocline single tank system is integrated into a solar aided coal-fired power plant for the first time. A system level simulation is conducted for a solar aided power plant with/without TES system, dual tank and single tank TES under the typical weather conditions of Beijing. Four dates, including equinox and solstice, are chosen, and factors like coal consumption rate, solar power generation, and solar capacity are analyzed. It is found that when replacing the first stage of higher pressure reheater under the fuel saving mode, the best performance could be reached. The introduction of TES system could significantly lower coal consumption rate, increase the power generation and solar capacity. During Autumn sunny days, the daily coal consumption rate of power generation could be reduced from 290.62g·kWh-1 by more than g·kWh-1. However, when the weather is cloudy, single tank TES system is not recommended due to its lower efficiency.