Abstract

This study employs machine learning techniques – random forest and extra trees – to predict the frictional pressure gradient during convective condensation in an inclined in-tube heat exchanger. The experimental data matrix (663) includes conditions for saturation temperatures of 30, 40, and 50oC, mass velocity 100-400 kgm-2s, quality 10-90%, and thirteen inclination angles between -90o and +90o for a smooth tube of an internal diameter of 8.38 mm. Based on statistical analysis, the extra trees outperforms the random forest. The average deviation (AD) and mean average deviation (MAD) are 2.88% and 6.72%, respectively, for random forest (RF) and 0.25% and 2.97%, respectively, for extra trees (ET).

Abstract

Nickel−cobalt oxide and carbon dots (Ni-Co oxide@C-dots) nanocomposite were prepared by hydrothermal synthesis followed by the pyrolysis technique. The nanocomposite exhibited a flower-like morphology with porous and interconnected monoliths. The nano-dimension carbon dot pockets throughout the composites provide an efficient resistance-free electron transfer pathway. Also, flower-petals type morphology at the surface attracts and enhances electrolyte-electrode interaction. Hence, Ni-Co oxide@C-dots act as a potential anode for a hybrid supercapacitor. As a negative electrode, a high-performance human hair-derived activated carbon is used. In asymmetric assembly, the device delivers a specific capacitance of 295 F g-1 at 1 A g-1. The device also exhibits an excellent energy density of 92 Wh kg-1 at a power density of 1000 W kg-1. The study suggests the synthesis and assembly of a high-performance energy storage device.

Abstract

This paper investigates the heat transfer and pressure drop characteristics of mango bark-CO2 nanofluid. The physical model comprises flow in a pipe under a constant heat flux of -10W/m2. The inclination angle varies between -90o and +90o, and volume fraction between 0% and 2.0% for Reynolds numbers of 50 and 100. The numerical code is validated using three research studies on heat transfer coefficient, Nusselt number, and pressure drop. Results show that the heat transfer coefficient and pressure increase with nanofluid volume concentration and Reynolds number. However, the heat transfer coefficient varies nonlinearly with the inclination angle and has a maximum occurring when the inclination angles are -30o and +60o while the pressure drop is relatively constant.

Abstract

Understanding the adsorption behaviors of CO2 and CH4 is of significance to exploit shale gas by CO2 injection. This paper simulates the adsorption of pure CO2, CH4 and their mixtures on kerogen for different pressures and compositions. The stability of adsorption, the affinity between gas and kerogen, and the efficiency of injecting CO2 to displace adsorbed CH4 are discussed. Results show that kerogen adsorbs CO2 more preferentially than CH4. The presence of CO2 causes a minimum and maximum in the adsorption isotherms of CH4 with increasing pressure. CO2 is adsorbed more stably than CH4 overall but adsorbed CO2 molecules are more heterogeneous in stability than CH4. The increasing pressure and CO2 mole fraction in free gas result in the decrease in adsorption selectivity of CO2 over CH4 under high pressure, while the trends are reversed under low pressure. Within the exploitable pressure range, the effects of injecting CO2 for displacing adsorbed CH4 are similar between different reservoir pressures. The adsorbed CH4 will be displaced more intensively in the early stage of CO2 injection than other stages. This research provides some recognitions for recovering shale gas by injecting CO2.

Abstract

Sub-seabed sequestration of CO2 is a potential method for reducing the atmospheric level of greenhouse gas. The geological exploitation in sub-seabed hydrate zone shows that free gas could be sealed beneath it, which provides a feasible idea for CO2 sub-seabed sequestration. In this study, the CO2 was injected into the water-containing porous media to simulate the CO2 sub-seabed sequestration process, and the magnetic resonance imaging (MRI) technique was employed to recognize the real-time reservoir state. The CO2 hydrate formed during the CO2 upward migration process and produced sealing effect for external fluids. In addition, it was found that at least 8 MPa of overpressure can exist stable beneath the CO2 hydrate cap, which ensures the security of CO2 marine sequestration and increases the CO2 sequestration depth. These results indicate that the hydrate cap provides a feasible and safe method for CO2 geological sequestration, and it is significant to promote the potential of sub-seabed CO2 sequestration.

Abstract

The present paper aims to analyse the thermal coupling of PEM fuel cells fed by hydrogen, stored in metal hydride tanks for maritime applications, considering the first Italian zero emissions ship with hydrogen PEM fuel cells propulsion (ZEUS), launched in 2022. A dynamic model is developed, and the integrated propulsion system is analysed in different operative conditions, considering the transitory that can be experimented during navigation (i.e., from harbour to cruise speed). The behaviour of the fuel cells in dynamic conditions is investigated, considering two different control systems, based on PID and Model Predictive Control approaches.

Abstract

Artificial intelligence is increasingly applied in electricity systems. This development results in opportunities and pressures for incorporating public values. Public governments can steer the implementation of AI to safeguard certain values. To understand what values might be pressing to cover, this paper maps the visions of various actors in the Dutch electricity system regarding which public values should be safeguarded. For this analysis, actors in different elements of the electricity system were interviewed, and a narrative analysis of grey material published by several of these actors was conducted. Multiple actors within the electricity system identified sustainability, reliability, affordability, equity and equality, and balances of power as important values to prioritize. This does not mean other values can be disregarded. At its core, identification and non-identification indicate which public values are present and absent in the current debate about the future Dutch electricity system.

Abstract

Currently Electric Vehicle batteries are considered to reach the End of Life once their State of Health reaches 70-80%. However, notions of circular economy suggest that the battery first-life should be extended as much as possible to reduce their environmental impact. Previous works have considered the range limitations of the drivers, arguing that the End of Life threshold is too conservative for many cases. However, to validate this statement, the increase of the internal resistance must be addressed. In this work a battery model is used to simulate the battery performance under different driving cycles synthesized from real data. Results show that the functional End of Life is forced by capacity, power or safety related issues. The dominant cause for reaching the End of Life depends highly on the battery size. The underperformance is in most cases found after 70-80% State of Health, validating that the first life of most batteries could be extended without affecting the performance of the vehicle.

Abstract

This paper proposed a heat-driven thermoacoustic heat pump system capable of utilizing medium/low-grade heat for heating supply in domestic applications. The system consists of one simple thermoacoustic energy-conversion unit with a direct-coupling configuration, which greatly reduces the system complexity compared to the traditional multi-stage thermoacoustic systems. The dimensional parameters of the system were optimized under nominal conditions, with heat-sink and heat-source temperatures of 55 °C and 7 °C. Simulation results show that the proposed system can obtain a heating capacity of 5.7 kW and an overall coefficient of performance of 1.40 at nominal conditions. Economic performance shows annual fuel energy savings and energy cost savings for individual systems are 20.3 MWh/year and 675 $/year. The environmental assessments show that the proposed system can displace CO2 emissions amounting to 3.72 tCO2/year. Exergy loss of each component was then given better to understand the energy conversion processes of the system. Moreover, system performance under different heat-source and heat-sink temperatures was studied.

Abstract

Heat transfer performance of molten salt (Hitec) flowing in various inclined tubes under uniform/non-uniform heat flux is numerically investigated. The effect of inclination angle (0o, 45o, 90o), inlet temperature (200 – 400 oC) and heat flux (50 – 350 kW/m2) on turbulent heat transfer features of molten salt are discussed, and the differences on heat transfer performance and wall temperature distribution between uniform and non-uniform heating are compared. The results show that increasing inlet temperature can lead to low heat transfer ability, but the tube inclination angle and heat flux have almost no effect on heat transfer features. The lateral side non-uniform heat flux also has little effect on heat transfer performance compared with uniform heat flux, but can cause large temperature gap up to 300 oC around the tube. Comparisons of simulation results with existing Nu-correlations show that Qiu-equation have better prediction accuracy for all researching cases with errors of ±8%, especially for high Reynolds numbers. The present results can be helpful for the design and operation of concentrating solar receivers.