• Energy Research

AbstractThe recent rise in atmospheric methane (CH4) concentrations accelerates climate change and offsets mitigation efforts. Although wetlands are the largest natural CH4 source, estimates of global wetland CH4 emissions vary widely among approaches taken by bottom‐up (BU) process‐based biogeochemical models and top‐down (TD) atmospheric inversion methods. Here, we integrate in situ measurements, multi‐model ensembles, and a machine learning upscaling product into the International Land Model Benchmarking system to examine the relationship between wetland CH4 emission estimates and model performance. We find that using better‐performing models identified by observational constraints reduces the spread of wetland CH4 emission estimates by 62% and 39% for BU‐ and TD‐based approaches, respectively. However, global BU and TD CH4 emission estimate discrepancies increased by about 15% (from 31 to 36 TgCH4 year−1) when the top 20% models were used, although we consider this result moderately uncertain given the unevenly distributed global observations. Our analyses demonstrate that model performance ranking is subject to benchmark selection due to large inter‐site variability, highlighting the importance of expanding coverage of benchmark sites to diverse environmental conditions. We encourage future development of wetland CH4 models to move beyond static benchmarking and focus on evaluating site‐specific and ecosystem‐specific variabilities inferred from observations.

The development and initial application of a mechanistic model (TOUGHREACT‐N) designed to characterize soil nitrogen (N) cycling and losses are described. The model couples advective and diffusive nutrient transport, multiple microbial biomass dynamics, and equilibrium and kinetic chemical reactions. TOUGHREACT‐N was calibrated and tested against field measurements to assess pathways of N loss as either gas emission or solute leachate following fertilization and irrigation in a Central Valley, California, agricultural field as functions of fertilizer application rate and depth, and irrigation water volume. Our results, relative to the period before plants emerge, show that an increase in fertilizer rate produced a nonlinear response in terms of N losses. An increase of irrigation volume produced NO2− and NO3− leaching, whereas an increase in fertilization depth mainly increased leaching of all N solutes. In addition, nitrifying bacteria largely increased in mass with increasing fertilizer rate. Increases in water application caused nitrifiers and denitrifiers to decrease and increase their mass, respectively, while nitrifiers and denitrifiers reversed their spatial stratification when fertilizer was applied below 15 cm depth. Coupling aqueous advection and diffusion, and gaseous diffusion with biological processes, closely captured actual conditions and, in the system explored here, significantly clarified interpretation of field measurements.

Abstract Unique reservoir performance was observed in the Shenhua 100,000 t/year Carbon Capture and Storage (SHCCS) Demonstration Project. Suggested by the geological pre-assessments, hydraulic fracturing and a multi-layer injection procedure were employed to improve the injectivity and reduce the risk of an overpressure. However, in-situ data showed the total injection rate increased after the injection started, while the injection initiation pressure decreased with only a minor pressure build-up development. Additionally, the injectivity of the uppermost injection layer, which was not fractured, grew considerably over the years, making this layer potentially able to meet the target rate by itself. To clarify this unforeseen observation, the reservoir performance was investigated through numerical simulations and comparison against the 2.5-year historical data. The simulation results indicated that permeability heterogeneity of the injection layers might explain the observed reservoir performance. High CO 2 injectivity in the uppermost injection layer could be attributed to its overall permeability being higher than that of other layers, and the considerable injectivity increase over the years could have been caused by the substantial permeability increase along the principal direction of CO 2 migration in this layer. The injectivity improvement caused by hydraulic fracturing was significant in the early time of injection, but it dramatically reduced afterwards. The intermittent injection procedure could effectively reduce the pressure build-up in the reservoir and helped to maintain the injection at the target rate. Based on these assessments, the cumulative injected CO 2 mass could reach 300,000 t in December 2015, but the yearly average injection rate would drop slightly. The predicted cumulative mass could be underestimated because the higher injectivity in 2014 was not accounted for in the calibration, and because the model size could have affected the reservoir performance, as shown by the sensitivity analysis. This research indicated that permeability heterogeneity and the injection procedure could significantly affect the reservoir performance, and should be given consideration in the performance assessment.

Widely distributed aquifers have been proposed as effective storage reservoirs for compressed air energy storage (CAES). This aims to overcome the limitations of geological conditions for conventional utility-scale CAES, which has to date used caverns as the storage reservoirs. As a promising technology, compressed air energy storage in aquifers (CAESA) has received increasing attention as a potential method to deal with the intermittent nature of solar or wind energy sources. This article presents a selective review of theoretical and numerical modeling studies as well as field tests, along with efficiency and economic analyses, to assess the feasibility of the emerging technology. Although some field tests suggest that a large bubble could be created in aquifers to sustain the working cycles at target rates, challenges remain before the technology can be recommended for wide deployment. The geological critical safety factors affecting the gas bubble development and sustainability of operation cycles include the geological structure, aquifer depth, and hydrodynamic and mechanical properties, such as porosity, permeability, compressibility, and mineral composition. Moreover, the injection/withdrawal well configurations and oxidation reactions caused by the oxygen in compressed air should also be considered. The failed attempt of renewable energy combined with CAESA in Iowa is described and the lessons learned are summarized. Combining CAESA with thermal storage, using CO2 as cushion gas, horizontal wells or hydraulic fracturing, and man-made boundaries are proposed to improve CAESA efficiency but need further study for future applications.

An experimental study was conducted to test the hypothesis that the biomass growing after an increase in available nutrient in an aquatic ecosystem affects the flocculation dynamics of suspended particulate matter (SPM). The experiment was carried out in a settling column equipped with a turbulence generating system, a water quality monitoring system, and an automated μPIV system to acquire micro photographs of SPM. Three SPM types were tested combinatorially at five turbulence shear rates, three nutrient concentrations, and three mineral concentrations. Analyses of experimental data showed that nutrient availability together with the presence of biomass increased the SPM size by about 60% at low shear as compared to nutrient- and biomass-free conditions; a lower increase was observed at higher shears. In contrast, only 2% lower fractal (capacity) dimension and nearly invariant settling velocity were observed than in nutrient- and biomass-free conditions. Likewise, SPM size and capacity dimension were found to be insensitive to the SPM concentration. Although limited to nearly homogeneous mineral mixes (kaolinite), these experimental findings not only reject the hypothesis that SPM in natural waters can be dealt with as purely mineral systems in all instances, but also anticipate that SPM dynamics in natural waters increasingly exposed to the threat of anthropogenic nutrient discharge would lead to an increased advective flow of adsorbed chemicals and organic carbon.