Abstract

Capacity Expansion Models (CEMs) are widely used in the academic literature to understand the needs and dynamics of highly renewable energy systems. Due to computational constraints, it is common to aggregate time-series data such as hourly power output from Variable Renewable Energy Sources (VRES) using clustering algorithms. However, there is evidence that the presence of wind data leads to increased clustering errors and biased investment decisions. With the above motivation, we combine two approaches from the literature and compare them against the state-of-the-art approach. For a small number of clusters, the proposed approach recovers 95% of the original variance and correlation. This leads to more robust investment decisions. However, we stress the increased computational burden involved.

Abstract

Hydrogen combustion reactions produce nitrogen oxides as a byproduct. They can be reduced by exploiting the catalytic combustion of hydrogen in a monolith. However, some nitrogen oxides can still be produced in the gas-phase of the catalytic combustor. The present work numerically investigates the production of nitrogen oxides as a by-product of the combustion of a lean airhydrogen mixture in a catalytic monolith with an equivalence ratio lower than 0.3. The analysis is carried out with a 2D dynamic numerical model implemented in MATLAB. The model solves mass and energy balances in
a domain describing a single channel of the monolith. The model involves a detailed reaction mechanism for the gas-phase combustion, including the subsets that model the production of nitrogen oxides. As a result, the model indicates that the catalytic combustor does not produce nitrogen oxides with an inlet hydrogen fraction lower than 9%vol. Furthermore, the maximum value of nitrogen oxide at the outlet of the channel is lower than 0.4 ppm, obtained with the highest hydrogen fraction simulated in this work (12%vol inlet hydrogen fraction).

Abstract

Absorption thermal storage technology has emerged as a viable option for both long and short term energy storage applications. Aqueous sodium hydroxide (NaOH) water pair has shown potential for high storage density and long-term storage. The present study experimentally investigates the impact of various design and operating conditions on the absorption performance of an aqueous sodium hydroxide storage system. The heat and mass transfer characteristics are further analysed and compared for two finned heat exchangers with various aspect ratios along with their energy and exergy performance characteristics. It has been observed that the solution depth is more critical to the system
performance over the heat exchanger surface area, with a storage density reduction of up to 41.2% for the heat exchanger with higher fin height. Higher exergy efficiency is also observed for the heat exchanger with lower fin height owing to its better absorption characteristics. The performance of NaOH-H2O based heat exchangers is finally compared with that of the LiBrH2O pair over a microchannel membrane heat exchanger design from the literature. It is seen that the LiBr-H2O pair offers a lower storage density by around 55% compared to the NaOH-H2O pair. The findings of the present study suggest significant potential for the NaOHH2O pair and can assist in identifying better heat exchanger designs for thermal storage applications.

Abstract

The calcium looping (CAL) process is a cost-effective method for post-combustion CO2 capture, using CaO as a sorbent in the carbonator and CaCO3 for CO2 regeneration in the calciner. A pilot-scale cold model dual fluidized bed (DFB) reactor system was constructed at the Waste-to-Energy Research Facility (WTERF) at Nanyang Technological University (NTU). This system includes a bubbling fluidized bed (BFB) as a carbonator to capture CO2 from WTERF’s flue gas, and a circulating fluidized bed (CFB) as a calciner to regenerate CO2 for storage. The hydrodynamics of the pilot-scale system
were investigated through a series of cold model experiments. The effects of various operating conditions—such as gas velocities in loop seals, the carbonator and calciner, and solid inventories—on pressure distribution and solid circulation rates were studied. The preliminary objective of these cold model studies was to assess the feasibility of the design for
developing a hot model capable of continuously capturing CO2 from real flue gas. The results indicated that changes in gas velocities caused variations in the pressure profile and axial solid holdup within the system. Gas velocities in the calciner significantly influenced the total solid circulation rate. Stable operation was maintained throughout the system, as determined by the pressure drop between the reactors. These studies contribute to the development of an efficient hot model for carbon capture and lay the groundwork for scaling up the calcium looping process for industrial applications.

Abstract

Vertical borehole heat exchangers are the main method for utilizing shallow geothermal energy and are more suitable for small to medium-sized cities and town buildings with both heating and cooling demands. The imbalance of heating and cooling loads is a common issue during the operation of borehole heat exchanger systems, therefore, an appropriate arrangement of borehole heat exchanger arrays is crucial for alleviating thermal accumulation of the system. This paper constructs a heat transfer model for borehole heat exchanger arrays and selects Harbin, Tianjin, and Guangzhou as typical cities to discuss the differences in heating and cooling load ratios. Based on the analytical solution of the ground temperature field under dynamic loads, the study investigates the impact on the ground temperature field of sequentially arranged ground heat exchangers within square and rectangular areas. The results show that for a group of 16 borehole heat exchangers arranged in a 45 m × 45 m square configuration, after 10 years of system operation, the ground temperature in Guangzhou, which has a higher cooling demand, increased by 15.45°C; in Harbin, which has a higher heating demand, the ground temperature decreased by 18.55°C; and in Tianjin, where the heating and cooling loads are more balanced, the ground temperature fluctuated by about 2.06°C, showing relatively minor changes. When the borehole heat exchanger group is arranged in a rectangular configuration, the ground temperature in Guangzhou increased by 12.50°C, in Harbin, the ground temperature decreased by 15.45°C, and in Tianjin, the ground temperature changed by 1.55°C, which is significantly less than the changes observed with the square configuration. Therefore, the rectangular arrangement of ground heat exchangers is superior to the square arrangement. The research results can provide theoretical guidance for the arrangement of borehole heat exchangers in practical engineering applications.

Abstract

The intelligent water control sleeve can measure water cut and flow rate in real-time, remotely control the sleeve opening, and adjust the flow rate from different formation segments into the wellbore. To enhance the water control efficiency of the intelligent sliding sleeve, a multi-segment well reservoir model was established based on Well A01H in the M oilfield of the Bohai Sea. Reservoir numerical simulations were conducted for the water control process during the water-free period, low and medium water cut period, and high water cut period to study the adjustment strategies for the intelligent sliding sleeve valve opening. The results indicate that during the water-free period, targeting the liquid production profile, the duration of water-free oil production can be extended by reducing the sleeve valve opening in high production sections and increasing the opening in low production sections. During the low to medium water cut periods, with water cut as the control target, the rate of water cut increase can be reduced by decreasing the sleeve valve opening in high water cut sections and increasing the opening in low water cut sections. In the high water cut period, the focus shifts to maximizing daily oil production. Cumulative oil production can be increased by further extracting liquid production within the permissible range of production pressure difference. This study proposes control objectives and methods for each stage of intelligent sliding sleeve adjustment in horizontal wells, providing theoretical and practical guidance for water control development in CNOOC oilfields.

Abstract

Maintaining fracture conductivity in acid-fractured carbonate reservoir presents a significant challenge as the fractures tend to close due to closure pressure. A viable approach to prevent the decline of conductivity is closed-fracture acidizing (CFA). In this study, we introduce a field-scale numerical model to simulate the acid-etching pattern in CFA and its effect on the conductivity of acid-fractured fractures. The accuracy of the CFA model is validated through experiments under identical acid-etched fracture morphology. The simulation results indicate that the morphology of closed fractures determines the acid-etching patterns. When the mean fracture aperture is small (≤2 mm), roughness is high (SD>0.05), and the dimensionless correlation length is extensive (≥0.05), acid etching becomes non-uniform, forming grooves and channels. In this case, the live acid reaches farther, and the conductivity remains high under closure stress (improved 8 to 33 times compared to before acidizing). Conversely, the acid uniformly etches the fracture surface, the acid treatment distance is short, and the conductivity rapidly decreases, making the acidizing performance negligible. In short, acid tends to flow into areas with the least resistance, and ultimately affecting acid-etching patterns and conductivity.

Abstract

In this paper, the effect of near-wall distance on flow-induced vibration and heat transfer characteristics of a circular cylinder is numerically investigated. The amplitude ratio, frequency ratio, near-wake structure, and temperature distribution are analyzed. The results show that, the amplitude ratio increases with rising reduced velocity when U*≤5, and further increasing the reduced velocity leads to a transition of the cylinder’s behavior from the upper branch to the lower branch. When H=1D, the cylinder obstructs the temperature boundary layer development, resulting in reduced heat transfer, accompanied by the highest average temperature (Tmean) and the lowest Nusselt number (Numean); conversely, as the near-wall distance increases to H≥1.5D, along with a rise in wall height, heat transfer enhances, yielding higher Nusselt numbers.

Abstract

The vibration and heat transfer characteristics of two tandem cylinders with two degrees of freedom at different reduced velocities(U*) and spacing ratios were numerically investigated. The amplitude, trajectory, time averaged Nusselt number, vortex shedding, and temperature fields were analyzed. Results show that the dimensionless amplitudes of the two cylinders exhibited similar variation trends with increasing U*, first increasing and then decreasing. The trajectories of both cylinders formed 8-shaped patterns. The time averaged Nusselt number (NuA) of cylinder 1 was higher than that of cylinder 2, and both increased with increasing U*. The vortices shed from cylinder 1 covered the surface of cylinder 2, creating high temperature gradients and enhanced heat transfer on the surfaces of both cylinders.

Abstract

Reduced-oxygen air flooding can serve as an effective alternative for water injection in the development of low-permeability reservoirs. During the development process, there is a phenomenon of increased viscosity due to the mixture of crude oil oxidation and water. It is essential to clarify the impact of viscosity increase after crude oil-water mixture during low-temperature oxidation on oil recovery. This paper investigated the impact of water content on the viscosity increase of oil-water mixed fluids during air oxidation through static oxidation experiments. The results indicate that heavy components generated after crude oil oxidation can interact with water to form highly viscous fluids. As the water content increases from 0% to 70%, the viscosity of the mixed fluids initially increases and then decreases. It reaches its maximum value of 32.5 cp at a water content of 30%. The long core displacement experiment studied the impact of transitioning from water flooding to air flooding and nitrogen flooding on crude oil recovery under optimal viscosity conditions. Two sets of experiments, water flooding and gas flooding, were conducted for comparison. The results indicate that the highest oil recovery was achieved when transitioning from water flooding to air flooding. This suggests that the heavy components generated during crude oil oxidation in the air flooding process, along with the highly viscous fluid formed by interaction with water, can block high-permeability channels, exerting a localized profile modification effect. This leads to changes in gas flow channels and expands the effective gas sweep range. This leads to changes in gas flow channels and expands the effective gas sweep range.