| Thermal energy storage technology could solve the problems of the discontinuity and instability in the energy utilization process, therefore it has been applied in many industries, such as power, steel and aerospace. Water is a kind of good heat storage material and water heat-storage technology has become one of the most widely used heat storage technologies. However, based on the atmospheric conditions, the current water heat-storage technology can only be applied in low-temperature heat storage field. This paper aims to expand the water heat-storage technology to the high-temperature scope, that is, water is in the board sense of subcritical state. Using the experimental and numerical methods, this paper deeply investigated the thermal energy storage performance of the subcritical water in the heat preservation process.First of all, a subcritical-water heat storage experimental platform with maximum heat storage temperature of 280℃ and highest pressure of lOMPa has been established. Heat preservation processes with different initial temperatures have been carried out. The results show that thermal stratification is observed in the liquid zone of water tank. The temperature in the vapor zone is uniform, slightly lower than the liquid zone. The dimensionless temperature at the same height decreases slightly with the initial temperature increases. With the increased initial temperature, the initial thermal energy amount, the heat dissipation rate, the thermal energy storage efficiency, the initial thermal exergy amount, the thermal exergy loss rate and the thermal exergy storage efficiency rise.A mathematical model under laminar natural convection is established and its numerical solution method is studied. Compared with the experimental results, the mathematical model is proved to be valid. The evolutions of the flow and temperature fields in the water tank during heat preservation process are analyzed. The results show that the heat preservation process can be divided into the unsteady stage and the quasi-steady stage. The unsteady stage can also be divided into the initial stage and the development stage. The initial stage is controlled by the step temperature difference in the initial conditions. At the initial stage, the thermal stratifications are gradually formed in the liquid zone and the vapor zone, and the temperature in the wall zone decreases rapidly. The development stage is mainly affected by the uneven temperature distribution between the different zones in the water tank. At the development stage, the thermal stratification degree in the liquid zone gradually strengthened and the flow becomes weaker. The vapor zone is mainly occupied by vortex instead of thermal stratification, with uniform temperature in the zone. In the wall zone, the temperature in the middle and upper parts rises again, and the heat transfer in the axial direction can’t be ignored.In the thermal insulation layer, the temperature rises firstly at the inner surface, expanding towards the outside surface. The quasi-steady state is mainly affected by the temperature difference between the water tank and the environment. In the quasi-steady state, the liquid zone is divided into the top vortex area, the central circulation area and the bottom stagnation area. There are two main vortices in the vapor zone. The bigger clockwise vortex occupies most of central region, while the smaller counter-clockwise vortex exists in the lower right corner. The wall zone is characteristic by the higher temperature in the middle and upper parts and the lower temperature at the two ends. There is a large temperature gradient near the liquid-vapor interface. In the thermal insulation layer, temperature of the inner surface is higher than temperature of the outer surface. The isothermal line is parallel to the surface.On the basis of the numerical results, the influences of the system parameters on the thermal energy storage performance in the heat preservation process are investigated. The results show that the higher initial temperature can help to increase the initial thermal energy amount, the initial thermal exergy amount and the thermal exergy storage efficiency. The thermal energy storage efficiency first increases and then decreases with the increased initial temperature. The initial thermal energy amount and the initial thermal exergy amount increase with the aspect ratio, the specific heat and density of the wall material, the thickness of the wall increase, or with the environment temperature decreases. The thermal energy storage efficiency and the thermal exergy storage efficiency increase with the aspect ratio, the specific heat and density of the wall material, the conductivity of the thermal insulation layer and the convection heat transfer coefficient decrease, or with the thickness of the wall zone and the thickness of the thermal insulation layer increase. Other parameters have no obvious effect on the thermal energy storage performance.A large-scale subcritical-water heat storage experiment platform has been established, with maximum thermal energy storage amount of 5GJ. A series of experiments under different operation conditions have been carried out. The experimental results show that, when the initial temperature is 150℃ and the liquid height is 3.5m, the water tank has the maximum initial thermal energy and exergy amounts, with 4.12GJ and 682.7MJ separately. After 8 hours and 24 hours, the thermal energy storage efficiencies are 97.95% and 93.81% separately, and the thermal exergy storage efficiencies are 96.17% and 88.67% separately. With the initial temperature increases, the initial thermal energy amount, the heat dissipation rate, the initial thermal exergy amount and the thermal exergy loss rate increase gradually, while the thermal energy and exergy storage efficiencies decrease. With the liquid height increases, all indices for the thermal energy storage performance increase. If the thermal energy stored in this system is used for thermal utilization, the initial operation condition with the low initial temperature and the high liquid level should be selected. If the stored thermal energy is used for power utilization, the initial condition with the high initial temperature and the low liquid level can store more thermal exergy, with lower efficiency; while the initial condition with the low initial temperature and the high liquid level can store less thermal exergy, with higher efficiency. |
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