The contradiction between supply and demand of energy leads to more and more attention on the large-scale energy storage technology; Compressed Air Energy Storage (CAES) technology is a new energy storage technology that is widely concerned in the world. The research of coupled heat transfer and seepage in fractured surrounding rocks is the necessary basis to evaluate the operation safety and effectiveness of CAES. Current studies point to the possibility of cracking in concrete liner seals, but the thermodynamic processes and leakage characteristics of compressed air in the presence of cracking and the heat transfer characteristics of seepage have not been addressed and reported. In order to investigate the leakage, the gas seepage and heat transfer law in fractured rock when the hard rock CAES gas reservoir seal cracks, the COMSOL fracture Darcy module, and the non-Darcy Forchheimer model are used as the constitutive seepage. The global ODE is used to calculate the thermodynamic process of compressed air in gas storage with coupled seepage and heat transfer process. The pressure and temperature of compressed air are obtained as the unsteady boundary of the seepage heat transfer model. A program for calculating the seepage and heat transfer characteristics of fractured surrounding rock in the CAES gas reservoir is established. On this basis, with the proposed Suichang CAES cavern as the background, the seepage and heat transfer characteristics of the fractured surrounding rock of the gas storage are studied. The results showed that when there are fewer cracks in the lining and surrounding rock of the air reservoir, the air pressure decreases due to a small amount of air leakage after 30 operation cycles, and the leakage rate of each cycle is 0.7% of the gas storage capacity, but it still meets the engineering requirements. If the plant is operating under these conditions, the charging rate will need to be increased by 1.2 kg/s per cycle charging stage. In the discharging and power generation phase, the high-pressure air that previously percolated into the rock mass cracks could flow back into the air storage through the lining cracks. Therefore, it is incorrect and unreliable to consider the gas which flows out from the inner surface of the lining as unusable. When the lining crack width is less than 0.3 mm, the seepage flow is Darcy flow and the non-Darcy effect can be ignored; when the lining crack width is greater than 0.5 mm, the non-Darcy effect of seepage cannot be ignored. The gas velocity in the surrounding rock fracture medium is on the order of 0.01 m/s with an influence range of over 100 m, and the gas velocity in the pore medium is on the order of 10-6 m/s with an influence range of 50 m. The findings of this study contribute to a better understanding of the interaction between the thermodynamic properties of compressed air and the seepage heat transfer process in compressed air storage underground reservoirs, as well as the gas leakage process in the event of liner seal cracking.
The spatiotemporal distribution characteristics of water and nitrogen in the soil profile are essential influencing factors that determine the development of crop root systems. The purpose of this study was to clarify the inter-row and inter-tree variability in soil moisture in the apple root zone, and to determine the effective root diameter ranges of apple trees that influence water and nitrogen absorption. The method used was a 2-year border irrigation experiment carried out in a traditional apple orchard in Zuncun, Shanxi Province, China. Dynamic variations in the soil moisture between trees within the row (perpendicular to the direction of border irrigation) and between rows (along the direction of border irrigation) were continuously measured from 2015 to 2016, and a specific soil profile was excavated to analyze the distribution characteristics of soil water, nitrogen, and roots with different diameters. The results showed obvious variations in soil moisture in the surface soil of 0–30 cm, and the soil moisture content between rows was 5% higher than that between trees within the row. The root length density in the soil between trees within the row was 33.5% higher than that in the soil between rows. Bivariate correlation analysis showed that the correlation between the root system and nitrogen and water was ranked from highest to lowest: total nitrogen (0.741) > nitrate nitrogen (−0.36) > soil moisture (−0.273). The correlation coefficient between trees within the row was higher than that between rows. Lower soil moisture between trees within the row resulted in increased root biomass and more active uptake activity between trees within the row. There were different significant correlations between the specific root diameter and the contents of soil water and nitrogen, showing that the 1.5 mm diameter roots correlated with the water content, whereas the 2.0 mm diameter roots correlated with the nitrogen content. The findings of this study provide a deeper understanding of the absorption mechanism of crop roots for soil water and nitrogen.
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