Numerical And Experimental Study On Heat And Cold Energy Storage Using Supercritical Air

Compressed Air Energy Storage (CAES) is a way to store energy in the form of compressed air on a large scale, it has many advantages including great energy storage capacity, low cost and high efficiency. However the energy storage density of compressed air is low and large underground caverns would be required to serve as the pressurized air container. Simultaneously, CAES still uses fossil fuel before expansion. These drawbacks have restricted the widespread application of CAES. An energy storage system using supercritical air is a new type of CAES and can solve the main technical problems faced by it. Low-cost off-peak electricity is used to pressurize air to supercritical state, and then the supercritical air cools down and is liquefied. During peak time, the liquid air is pumped into a gasifier, after a further heating, the supercritical air expands through a turbine to generate power. During the cycle of energy storage, the compression heat and cold energy is recycled by the heat exchanger/storage device to increase the efficiency. Heat and cold regenerative heat exchangers are the key components and play decisive roles in the performance of the energy storage system using supercritical air. Combined with the characteristics of energy storage system using supercritical air, we discuss and compare the advantages, disadvantages and applications of the various types of heat and cold regenerative heat exchanger, and find that the rock bed is the best option for the heat and cold energy storage using supercritical air. We design and build the experimental system of heat and cold energy storage. Using this experimental method, supplemented by theoretical analysis and numerical simulation, we study the flow and heat transfer characteristics as well as cycle characteristics in supercritical air-rock packed bed. The main tasks and conclusions of this paper are as follows:By theoretical analysis, we establish a mathematical model related with the real experimental system and case, and validated with experimental data. The proposed packed bed model is then used to study the heat and cold storage process using supercritical air. The changes of temperature distribution in packed bed with time are presented, and give us a guidance and help for the experimental research.By dividing the packed bed into the central core region and the near wall region, and using some reasonable simplification, we develop a one-dimensional and two-phase model. The change of supercritical air and rock surface temperature with time are experimentally determined to solve the energy conservation equations of the model, and the heat transfer coefficient between flow and particle is obtained. The results show that the air pressure has very little effect on the heat transfer coefficient within the experimental range of Reynolds numbers (60<Re <125), but can significantly increase the effective thermal conductivity coefficient in the packed bed containing stagnant supercritical air. The reason is that the main modes of heat transfer in the bed with stationary air include the natural convection and air mixing of different temperature, and they are greatly affected by the density of air. The experimental data shows that the temperature decrease in the near wall region of the6.6MPa packed bed is about1times greater than the atmospheric one. At the same Reynolds number in the packed bed, the entrance effect of the heat transfer coefficient between supercritical air and rock particles is less pronounced than the one between atmospheric air and rock particles, but they seems to approach the same value with increasing the distance from the inlet of the packed bed.The typical operating parameters of heat storage cycle using supercritical air has been selected based on the simulation results. Energy and exergy analysis in the processes of heat storage, backup and heat recovery have been presented, the results show that about70%of the stored heat in the packed bed is preserved in rock particles, and the rest is absorbed by stainless cylindrical tank. Half of the total heat loss occurs in the backup service period, and the main sources of exergy losses are caused by temperature difference between rock and steel in the heat storage process. The effect of air temperature, pressure and flow rate on the heat storage performance is studied. The results show that the energy and exergy efficiencies strongly depend on the properties of the insulating layer and the average temperature of the stainless cylindrical tank during the heat storage cycle, higher storage tank temperature result in higher heat loss from the packed bed, however if the growth rate of stored heat is greater than its loss rate, the energy and exergy efficiencies will increase. It is found that the air pressure has limited impact on the energy efficiency, but influences the exergy efficiency greatly at high Reynolds numbers, due to the pressure loss increases as the square of flow rate. If the mass flow rate is the same, the higher the air pressure, the lower the air velocity in the packed bed, and the pressure loss through the packed bed will be significantly reduced which in returns increase the exergy efficiency substantially. The air flow rate has significant influence on the energy and exergy efficiencies. By increasing the air flow rate, the air velocity within the packed bed increases, and the heat transfer coefficient increases as well thus making heat storage and recovery time shortened, and finally reduce the amount of heat loss and increase the energy and exergy efficiencies until reaching a stable value. So in the heat storage rock bed using supercritical air, we can obtain a high heat transfer coefficient by increasing the air mass flow while maintaining a very low pressure loss.Cold storage experiments with liquid nitrogen as the heat transfer fluid are carried out, the flow and heat transfer patterns are changed compared to the heat storage, the heat transfer between liquid nitrogen and the packed bed is dominated by conduction, and the amount of it crucially depends on the volume of the rock and tank cooled down by liquid nitrogen. The typical operating parameters of cold storage cycle under supercritical pressure have been selected based on the simulation results. Energy and exergy analysis in the processes of cold storage, backup and cold recovery have been presented. The results show that the thermal efficiency of the cold storage cycle is approximately15percentage points lower than that of the heat storage cycle, this is because the insulation material performs better job at higher temperatures than at cryogenic temperature. The exergy efficiency is almost half of the value in the heat storage cycle, and this can be attributed to the much larger temperature difference across the thermocline and more thermal energy loss in cold storage cycle. The effects of operating pressure and flow rates on the cold storage performance are studied, the change of vaporization ratio of liquid nitrogen with the flow rate is discussed and its mechanism is studied. The results show that when the flow rate is constant, the vaporization ratio of the liquid nitrogen changes little as the pressure increases, but it shows an approximate linear increase with liquid nitrogen flow rate, indicating the rise of the level of the liquid nitrogen in the packed bed as increasing the liquid nitrogen flow rate, the volume of the rock and tank cooled down by liquid nitrogen is also increased, and the vaporization ratio increases. By analysing the cold loss in the energy storage process, we conclude that the amount of cold energy stored is determined by the total weight of the entire storage tank packed with rock, and the cold energy loss is reduced by shortening the time of cold storage process.