Jun Xia's research while affiliated with Wuhan University and other places

Compared with individual heatwaves or storm events, the compound extreme precipitations preconditioned by heatwaves (CHEPs) usually amplify their adverse repercussions on both ecosystems and society. However, little is known about the physical mechanisms of generations, especially considering precipitation types triggered by various factors and synoptic patterns. By classifying extreme precipitations based on duration, we conduct an event-based analysis and comprehensively assess CHEPs using the machine learning-constrained framework and binning scaling methods over China. We find the fraction of CHEPs to total extreme short-duration/long-duration precipitations (ESDPs/ELDPs) has substantially increased by 18%/15% from 1979 to 2021, when using dry-bulb temperature to identify heatwaves. More notably, the hotspots of CHEPs are generally consistent with those of ESDPs. The ESDPs play a dominant role in shaping CHEPs episodes, which are governed by enhancing atmospheric instability due to preconditioned heatwaves. The horizontal moisture advection and transient vertical dynamic motion of moisture, which are paramount to LDPs, is not significantly enhanced by the overheating atmosphere, leading to a small fraction of LDPs to CHEPs. In addition, the intensity of ESDPs tends to increase with air temperature at higher rates than that of ELDPs. As short-duration storms may trigger severe flash floods, ample attention should be paid to the escalating risks of CHEPs under climate change.

Underground H2 storage (UHS), i.e., injecting H2 into subsurface geological formation and its withdrawal when needed, is identified as a promising solution for large-scale and long-term storage of H2. In this study, molecular dynamics (MD) simulation was performed at a typical temperature 320 K with pressure up to 60 MPa to predict H2 transport properties and H2–H2O–rock interfacial properties, which are compared with those of CO2 and CH4. The MD results show that the CH4 profiles of property variations with pressure lie between those of H2 and CO2 and more comparable to CO2. The interaction of H2 with H2O/silica is much weaker than that of CH4 and CO2. It is found that the effect of H2 pressure on altering the water contact angle and interfacial tension is negligible under all conditions. Unlike the multi-adsorption layers of the confined CO2 and CH4, there is only one adsorption layer of H2 confined by silica nano-slit. The planar diffusion of H2 in the confined system is slower than that in the bulk system at pressures lower than 20 MPa. The data and findings of this study will be useful for modeling the multiphase flow dynamics of UHS on reservoir scale, optimizing UHS operation, and assessing the performance of a cushion gas, e.g., CO2 or CH4.

Underground H2 storage (UHS) is a promising technology to achieve large-scale, long-term H2 storage. Using CO2 as a cushion gas to maintain the pressure of the reservoir and withdraw stored H2 in the saline aquifer simultaneously enables the implementation of UHS and underground CO2 storage (UCS). The difference in the molecular properties of CO2 and H2 leads to distinct interfacial behavior when in contact with the brine and rock, thereby affecting the flow patterns and trapping mechanisms of gases in geological formations. Accurate prediction of the interfacial properties of CO2, H2, and the mixtures when interacting with brine and rock is crucial to minimizing the uncertainties in UHS and UCS projects. In this study, molecular dynamics (MD) simulations are performed to predict the interfacial tension, surface excess, bubble evolution, and contact angle of CO2, H2, and the mixtures at 10 MPa and 300–400 K. The MD results show that the interaction of CO2 with H2O and hydrophilic silica is considerably stronger than that of H2. The interfacial tension reduces linearly with the temperature in H2-dominated mixture systems, and the surface adsorption of H2 can diminish in a CO2-dominated system or at high-temperature conditions. The hydrophilic silica is more CO2-wet than H2-wet, and the attached CO2 bubble is more easily disconnected. Ions and the temperature play different roles in the contact angle.

Identifying power curtailment is essential for system reliability, especially in hydro–photovoltaic (PV) hybrid energy systems (HPESs) exposed to future climate variability. However, the relatively low temporal resolution of climate projections challenges the effectiveness of current numerical simulation methods for evaluating the curtailment rate of HPESs. This study derives an analytical method based on daily hydropower and PV power rather than the commonly used hourly data in numerical methods, to reduce reliance on the temporal resolution of climate inputs. A short-term operation model is constructed to verify the effectiveness of the derived power curtailment function. A long-term operation model incorporating the analytical power curtailment function is then established to analyze variations in future curtailment rates using climate information projections. Results of a case study in the Yalong River Basin validate the accuracy of the derived PV curtailment function. Compared with the reference period, future hydropower and PV power will increase in most climate scenarios with substantial variances, whereas the PV curtailment rate exhibits an overall increase in the near future (+2.49%) and a larger increase in the far future (+5.53%). The most noticeable increases in future PV curtailment rates occur in June, July, and October (+3.15%, +5.47%, and +3.11%). The proposed analytical method provides valuable insights for the risk evaluation of HPESs by coupling the short-term power curtailment rate with mid- and long-term climate variations.

Complementary operation with hydropower can facilitate the integration of intermittent wind and photovoltaic (PV) power by the regulation ability of reservoirs and the flexibility of hydro units. However, changes in operating patterns of hydropower stations in hydro-wind-PV complementary energy systems (HWPESs) induce potential impacts on hydropower efficiency, which has been seldom studied. To quantify the changes in hydropower efficiency in HWPESs, the complementary and separate short-term operation models are constructed to identify the changes in hydropower efficiency. Then, an accurate calculation method for power generation of HWPESs in the mid- and long-term operation is proposed by considering the short-term hydropower efficiency changes and power curtailment patterns. The JP-I HWPES in the Yalong River Basin is selected as a case study. Results indicate that the average daily hydropower efficiency under complementary operation decreases from 9.04 to 9.02 compared with separate operation, which results in a 3.73 % (1.36 billion kWh) decrease in the total hydropower generation. The proposed method can reduce calculation errors of hydropower and system total power generation by 99.59 million kWh (from 0.65 % to 0.09 %) and 126.78 million kWh (from 0.60 % to 0.03 %) in the validation period, respectively. From the hydropower perspective, this study quantified the changes in short-term hydropower efficiency, which can provide a practical tool for the accurate mid- and long-term operation of HWPESs.

The determination of critical management areas for nitrogen (N) and phosphorus (P) losses in large-scale basins is critical to reduce costs and improve efficiency. In this study, the spatial and temporal characteristics of the N and P losses in the Jialing River from 2000 to 2019 were calculated based on the Soil and Water Assessment Tool (SWAT) model. The trends were analyzed using the Theil-Sen median analysis and Mann-Kendall test. The Getis-Ord Gi* was used to determine significant coldspot and hotspot regions to identify critical regions and priorities for regional management. The ranges of the annual average unit load losses for N and P in the Jialing River were 1.21-54.53 kg ha-1 and 0.05-1.35 kg ha-1, respectively. The interannual variations in both N and P losses showed decreasing trends, with change rates of 0.327 and 0.003 kg ha-1·a-1 and change magnitudes of 50.96% and 41.05%, respectively. N and P losses were highest in the summer and lowest in the winter. The coldspot regions for N loss were clustered northwest of the upstream Jialing River and north of Fujiang River. The coldspot regions for P loss were clustered in the central, western, and northern areas of the upstream Jialing River. The above regions were found to be not critical for management. The hotspot regions for N loss were clustered in the south of the upstream Jialing River, the central-western and southern areas of the Fujiang River, and the central area of the Qujiang River. The hotspot regions for P loss were clustered in the south-central area of the upstream Jialing River, the southern and northern areas of the middle and downstream Jialing River, the western and southern areas of the Fujiang River, and the southern area of the Qujiang River. The above regions were found to be critical for management. There was a significant difference between the high load area for N and the hotspot regions, while the high load region for P was consistent with the hotspot regions. The coldspot and hotspot regions for N would change locally in spring and winter, and the coldspot and hotspot regions for P would change locally in summer and winter, respectively. Therefore, managers should make specific adjustments in critical regions for different pollutants according to seasonal characteristics when developing management programs.