Journal of Energy Engineering

The content of coalbed methane (CBM) is an important parameter that affects the performance of coal extraction and the safety of coal mines. The importance of this parameter is especially more pronounced when using the direct method to determine CBM content, in which the measuring accuracy of the lost gas affects the results. The loss is affected by the gas pressure and exposure time. In this article, the influence of gas pressure and exposure time on the estimation results and loss rate was investigated. Moreover, the influence of the sampling interval and observation time on the accuracy of the kinetic model in simulating the desorption process was analyzed. The obtained results demonstrated that the lost gas is positively associated with pressure and exposure time, and the lost gas rate is proportional to the exposure time. It was found that although the Barrer method outperforms the power function method, in terms of calculation accuracy, both methods have large errors. In terms of physical sense and model accuracy, the pore diffusion (IDM) model best fits the desorption kinetics of this coal. In the stable desorption stage, the desorption process is more consistent with Henry’s law. When the desorption changes violently, the desorption process is more consistent with Fick’s diffusion law. In order to ensure the consistency between the model and the experiment (95% confidence interval), the sampling interval of IDM model at 1 MPa should be more than 30 s.

Abstract

Days with greater amounts of sunshine often have higher cooling demands. This makes solar energy one of the best solutions to mitigate the use of fossil fuels in cooling systems. On the other hand, scientific studies on ejector technology have demonstrated promising improvements in terms of enhancing the efficiencies of cooling and refrigeration systems. This work involved field testing of a solar thermal plant combined with an ejector-compression system for space cooling applications in buildings. The thermal plant uses parabolic solar collectors that focus a large area of sunlight toward tubes where circulating oil captures the energy. This energy activates the ejector system, which produces a nominal 15-kW cooling effect. A solar ejector cooling system is integrated into the CanmetENERGY Research Centre’s building, covering part of its air conditioning load, and consequently decreasing the electrical consumption of the main building’s chiller. The system has operated with a coefficient of performance (COP) of up to 0.27 at this location. Design characteristics of the system are presented and the mode of operation and analysis of collected data are elaborated.

Abstract

A practical application for ejector-based cooling systems (ECS) used in combination with solar energy is in the design of solar-powered air conditioning systems for homes or commercial buildings. These systems use solar thermal power to generate heat, which is then used to power an ejector cooling cycle that uses a combination of evaporative cooling and mechanical compression to cool the air inside the building. This can significantly reduce the need for electricity from the grid and help to reduce greenhouse gas emissions. Another practical application for ejector cooling systems working with solar energy is in the field of solar thermal power generation. Solar thermal power plants use mirrors to focus the sun’s energy onto a fluid, which is then used to generate electricity. However, the high temperatures generated during this process can cause thermal stress on the system and reduce its efficiency. Ejector cooling systems can be used to cool the fluid and reduce thermal stress, increasing the efficiency of the power generation process. This can be done by using solar energy to power an ejector cooling cycle that uses a combination of evaporative cooling and mechanical compression to cool the fluid.

Abstract

The intermittent nature of solar energy poses a great challenge to the parabolic trough collector (PTC) technology. Although PTC technology is mature in application, its thermodynamic performance in transient processes is in need of a more comprehensive understanding. Here, a transient photothermo-hydraulic model of a parabolic-trough collector loop (600 m) was established with the finite volume method (FVM). A comprehensive comparative analysis for the transient photo-to-thermal characteristics (transient entropy generation and thermal-exergy efficiency) of PTC loop under a main weather disturbance [direct normal irradiance (DNI)], main control parameter (the mass flow rate of the heat transfer fluid $m_{\mathrm{HTF}}$), and geometry-dependent parameters (the collector width $a$ and the absorber tube diameter $d_{\mathrm{outer}}$) was carried out. The research shows that increasing DNI increases the entropy generation rate of each part of the absorber tube (up to 45.52%) and decreases the energy and exergy efficiency during the transient process, and vice versa. Transient entropy generation can be reduced by regulating the operating parameters (increase $m_{\mathrm{HTF}}$ rapidly) and changing the geometric parameters ($a$ or $d_{\mathrm{outer}}$) of PTC (up to 87.58%, 38.03%, and 32.92%, respectively). Moreover, the influencing mechanism of $m_{\mathrm{HTF}}$ ($a$ or $d_{\mathrm{outer}}$) on the transient entropy generation of the absorber tube is different from that in steady state. The transient thermodynamic response analysis is of great significance to the control and design optimization of PTC loop in the actual system.

Practical Applications

Thermodynamic analysis is very important for the performance optimization of parabolic trough collector (PTC) technology; entropy generation and exergy analysis can measure the irreversibility of the photothermo-hydraulic conversion process and can provide directions for its thermal performance breakthrough. Currently, the research on the thermodynamic performance of the PTC technology mostly adopts the overall steady-state analysis. However, simple steady-state analysis cannot reflect the full picture of the irreversibility of PTC in the process of photothermal conversion. For further analysis, more accurate transient response analysis is needed for the thermodynamic performance of PTC. The present study established a transient response analysis photothermo-thermodynamic model of PTC with finite volume method. The transient response of PTC thermodynamic parameters under different parameters was studied and analyzed. The thermodynamic performance influencing mechanism of PTC was analyzed. The thermodynamic performance comparison analysis information in the present study can guide the design optimization of collector and transient operational control of PTC loop in a more targeted manner. The present study is of great significance to the control and design optimization of PTC in actual system.

The water-wet sandstone reservoirs in the Liushagang Formation of the Beibuwan Basin has tremendous exploration potential for oil reserves in the South China Sea. However, owing to the weak correlations between petrophysical parameters and oil productivity, the production performance and seepage capacity showed significant differences in various reservoirs. The production performance of the field was evaluated by characterizing the mobility of two-phase flow through the concept of classical global mobility. However, global mobility still does not consider differential displacement pressure (DDP) and movable oil saturation (MOS). Therefore, a new index based on the Li–Horne model was established to simultaneously evaluate the seepage capacity of water-wet sandstone reservoirs, accounting for DDP and MOS. The detailed work stemming from the model included selecting water-wet sandstones with different lithologies and comparing the displacement effects of different flooding systems, oil viscosities, injection water salinities, pressure conditions, and MOS in multiple sets of parallel core samples. Finally, the new index was validated. A strong correlation was found between the new index and oil productivity per-unit differential pressure through displacement analysis of rock/oil/water, rock/oil/gas, and rock/oil/polymer systems on several core samples. The developed index for evaluating seepage capacity is suitable for different sandstones, flooding systems, oil viscosities, injection water salinities, pressure conditions, and productivity test data, thus providing insight into the production performance and seepage capacity parameters of water-wet sandstone reservoirs.

Monitoring wind turbines is essential for their safe operation on wind farms. However, the majority of data-driven monitoring strategies do not consider expert knowledge. Consequently, they cannot simultaneously monitor statistical and physical characteristics and have poor monitoring results. This study proposes a knowledge-aided adaptive parallel monitoring strategy to monitor the process by evaluating the physical and statistical characteristics using two submodels. First, we propose a novel knowledge-aided monitoring statistic to characterize energy conversion efficiency, thus monitoring both the conversion efficiency using one of the submodels and the physical performance of the wind turbine. Subsequently, the process characteristics covering both steady and varying states can be monitored with another submodel using two monitoring statistics, which can accurately detect unusual behaviors from a statistical perspective. Generally, we can use three statistics to monitor the process from two perspectives. With the physical and statistical characteristics captured, we propose a novel adaptive monitoring strategy to adjust the model performance and accurately detect fault conditions. Real-world experiments demonstrate the effectiveness of the proposed method. Among the four monitoring methods, the monitoring strategy aided by knowledge showed the highest detection accuracy.

Productivity prediction of perforation completion is key for well optimizing completion methods and reservoir engineering research. Productivity prediction models using an additional pressure drop to represent skin factor are limited. In this study, a steady-state productivity prediction model coupled with reservoir seepage and fluid flow in the wellbore was developed. The impact of different perforation parameters on well production was studied. Considering perforation depth, perforation density, perforation diameter, phase angle, depth of contaminated zone, and pollution degree of the contaminated zone, a semi-analytical skin calculation model under different reservoir conditions and completion methods with the least-squares method combining the consideration of perforation depth, perforation density, perforation diameter, phase angle, depth of contaminated zone, and pollution degree of the contaminated zone was developed. Perforation parameters were optimized. A 3D directional well unsteady productivity prediction model was established using the finite-volume method considering the influence of conventional perforation completion parameters, gravel packing perforation completion parameters, natural fractures, reservoir heterogeneity, reservoir scale, skin factor, fluid gravity, fluid compressibility, and rock compressibility. A new method for optimizing perforation parameters and characterizing skin factors was developed, providing a theoretical basis and simulation means for deeper understanding of the development law of directional wells.

Steam-assisted-gravity-drainage (SAGD) is a widely employed method to enhance heavy oil production and efficiency, and the key to ensuring its steady operation is the maintenance of a steam chamber. Heat conduction and convection of steam to connate water are essential factors in the heat transfer process. In this paper, we developed a novel analytical model to investigate the heat transfer process on the boundary of the steam chamber, driven by the pressure difference between the injected steam and the original reservoir. Both conduction and convection are considered in this new model, and the calculated results are in good agreement with the simulated data by using CMG-STARS. After model validation, the effects of physical properties and operating conditions (relative permeability, initial water saturation, and steam temperature) on the heat transfer process are investigated. The results reveal that connate water saturation plays a large role than the steam temperature in conductive heat flux; low saturation leads to more efficient crude oil exploitation. This work provides new insights into the recovery mechanisms of a SAGD process.

Physical similarity simulation is one of the best methods for improving coal bed methane extraction efficiency, and the development of reasonable materials is important for improving physical similarity simulation testing results. This study aimed to investigate the mechanical properties of coal-like materials in a solid–gas coupled physical simulation test experiment. A new type of material with sand as aggregate, cement gypsum as cement, and silica gel as adsorbent was developed by an orthogonal design method. ANOVA and theoretical analysis were used to investigate the sensitivity and effects of different content ratios on the volumetric weight, uniaxial compressive strength, and elastic modulus of coal-like materials. The results of the investigation showed that the order of volumetric weight sensitivity ranging from large to small was the contents of silica gel, gypsum, and cement. The order of the sensitivity of uniaxial compressive strength and elastic modulus ranging from large to small was the ratio of the contents of silica gel, cement, and gypsum. Furthermore, the volumetric weight, uniaxial compressive strength, and elastic modulus of coal-like materials increased linearly and exponentially with the increase of cement and gypsum contents, respectively, whereas they decreased exponentially with the increase of silica content. Based on the results and analysis, a comprehensive model for the characteristic parameters of coal-like materials was established, and the prediction error was controllable to within 25%.