Journal of Energy Engineering

The horizontal ground heat exchanger has been extensively employed to collect geothermal energy from mine backfill bodies. A novel method for avoiding thermal interference using the thermal influencing radius coupling pipe spacing was investigated. Thermal interference between adjacent heat-transfer pipes can be neglected when $D_{40°\mathrm{C}}=2R_{40°\mathrm{C}}$ is used as the pipe spacing. The comprehensive impacts of heat-transfer fluid and backfill body on the heat-transfer efficiency and thermal influencing radius of the horizontal single pipe were investigated. The growth ratio of heat-transfer capacity can be as high as 227.3% when thermal conductivity increases from $0.5\mathrm{kJ}/(\mathrm{kg}·°\mathrm{C})$ to $2\mathrm{kJ}/(\mathrm{kg}·°\mathrm{C})$. The heat-transfer capacity increases by 4% for every 1°C reduction in inlet temperature. The thermal influencing radius is significantly influenced by specific heat capacity and thermal conductivity. This study can provide theoretical recommendations for the design and improvement of heat-transfer pipes in the backfill body.

Clean heating transformation and photovoltaic (PV) promotion gradually have become the keys to realize dual carbon strategy goals. Based on this, the combined application of building-integrated PV (BIPV) systems and air-source heat pump (ASHP) systems has received widespread attention. However, the current combined system still presents problems such as the mismatching in time between PV output and heating load. Therefore, in this paper, a novel BIPV and ASHP coupling system is proposed, and the operation parameters were optimized using low-carbon community as an example. A conventional PV and heat pump (HP) combined system, ASHP heating, and coal-fired heating were employed as the baseline and compared with the novel system. The results show that the optimized system can improve the PV power local accommodation rate by 43% compared with a conventional PV and ASHP coupling system. In terms of economic and environmental benefits, the operating cost of the optimized system is 50% of the cost of the ASHP system. Compared with coal-fired heating system, the annual air pollutant and carbon emissions can be reduced by 92% and 57%, respectively. The system also can improve the stability of power grid operation. The demand for power capacity can be improved by 62% and the annual load balance of the power grid can be improved by 29.5%.

Water huff-n-puff is considered to be an available method to exploit unconventional oil reservoirs, which can significantly promote the reservoir pressure and facilitate water–oil exchange under the imbibition effect. To explore the production characteristics and well performance during water huff-n-puff in the Huanjiang oil field that is a tight oil reservoir, field tests were carried out and production characteristics and well performances were investigated. Moreover, reservoir simulation works were conducted to study the various factors that influence the water huff-n-puff performance. Furthermore, sensitivity analysis for various factors was carried out with the tornado diagram method and the importance of different influencing factors was ranked quantitatively. The results demonstrate that water huff-n-puff has great significance to effectively replenish reservoir pressure as well as promote oil production. Field tests demonstrate that the average liquid rate as well as the oil rate of the horizontal wells increased by 5.94 and $1.18{\mathrm{m}}^3/\mathrm{day}$, respectively. Considering the different oil production characteristics, the oil production process can be classified into three different stages. The levels importance of different influencing factors were ranked as rock wettability, water huff-n-puff cycles, water injection volume, hydraulic fracturing sections, well position, and soaking time.

In order to clarify the relationship between the conventional diffusion models and the time-dependent diffusion model during gas desorption, a porous diffusion model during gas desorption in coal was established by modeling the pore structure according to the porous diffusion theory and fractal theory. Gas diffusion processes were experimentally measured using a series of coal samples with different ranks and types under different gas equilibrium pressures. And the established theoretical model was furthermore verified by experimental data. The theoretical data derived from the established porous diffusion model agreed well with the experimental data with correlation coefficients greater than 99.0%. The attenuation of diffusion coefficient during gas desorption was essentially attributed to the gradual decrease of the gas diffusion amount for large pores during unsteady diffusion of gas in porous media. The effective diffusion coefficient presented a first fast and then slow decreasing trend with the prolonged diffusion time. The diffusion coefficient increased with the increase of gas equilibrium pressure as well as the metamorphic grade and fragmentation degree of coal. The pore connectivity enhanced as the surface area proportion of larger pores increased, leading to an increase in the proportion of the gas diffusion quantity in larger pores and the diffusion coefficient and meanwhile a decrease in the fractal dimension. This study will provide theoretical support for in-depth understanding of the gas diffusion mechanism in gas-containing coal.

The results of experimental studies of the cocombustion of coal–water fuel with fuel oil are presented. According to the results of experimental studies, the efficiency of joint ignition of stationary single droplets of coal–water fuel and fuel oil was substantiated. A decrease in the ignition delay times of fuel droplets was found in comparison with the ignition of only coal–water fuel droplets under identical conditions. A significant decrease in the ignition delay times of coal–water fuel droplets is typical for droplets with sizes of 2 and 3 mm in the entire studied temperature range of the oxidizer (from 873 to 1,273 K) and distances (from 2 to 4 mm) between drops of coal–water fuel and fuel oil. It has been experimentally established that the reduction of ignition delay times is 58 and 70%. For droplets of coal–water fuel with a characteristic size of 1 mm, the changes in ignition delay times are ambiguous. The joint combustion of coal–water fuel and fuel oil affects the ignition of droplets of coal–water fuel, and the characteristic size of which is more than 1 mm. With this method of burning coal–water fuel and fuel oil, the latter acts as a catalyst when igniting coal–water fuel, thereby ensuring a stable combustion of such fuel. The combustion of two-component coal–water fuel together with fuel oil can avoid the problems typical of such fuels: reduce the requirements for spraying devices, solve the problem of low reactivity, increase the completeness of combustion of this fuel, and fully use it in the production of thermal and electrical energy.

Abstract

A 2D numerical model has been created to simulate the gas flow in a porous hydrogen (${\mathrm{H}}_2$) and air-fed solid oxide fuel cell (SOFC) and analyze the generation of entropy involved via the major key contributing factors. On this basis, flow, thermal, and mass transfers have been numerically handled with a validated lattice Boltzmann method (LBM), including flow channels that have an impact on the entropy generation assessment. Flow, thermal, and mass paths have been simulated throughout the SOFC. It turned out that the ohmic losses are largely predominant compared with those of the other factors (irreversibilities due to fluid friction, heat transfer, mass/chemical transfer, and activation). In addition, under equal current density, the partially obstructed anode channel exhibits lower entropy generation compared with the free (unobstructed) anode channel, thereby indicating higher heat and mass transfer performance. These findings indicate the modeling efficiency considered and the potential of the LBM approach to address the processes involved in a porous ${\mathrm{H}}_2$ SOFC.

Practical Applications

SOFCs directly convert chemical energy into electrical energy, with high efficiency, strong reliability, and low emissions. SOFCs have become attractive for automotive and aerospace industries due to their energy flexibility. The design of their channels directly affects heat and mass transfer capability and their output performance. For a better view of the thermal performance of any thermal system, aspects such as pressure drop and entropy generation must be considered in addition to heat transfer factors. Entropy generation analysis is one of the most used techniques to refine the SOFC design and investigate their performance. This can be achieved by modifying the anode channel. One option is to partially obstruct the channel with differently shaped obstacles. Further, the numerical simulations can provide a solid reference point for future CFD models and are relevant to thermal dynamics in these devices and chemical-to-electrical energy conversion industries.

Abstract

Phase change materials (PCMs) have attracted greater attention in battery thermal management systems (BTMS) applications due to their compact structure and excellent thermal storage performance. This work developed a BTMS model based on composite phase change material (CPCM) for a cylindrical lithium-ion battery pack. The initial state of the CPCM was set as a liquid phase, and the insulating performance of the BTMS was investigated numerically, considering the CPCM solidifying from liquid phase to solid phase in a low-temperature environment. The numerical model was built, and calculations were performed using Ansys Fluent 19.2. The research results showed that a CPCM effectively can extend the time that the battery pack remains above 0°C by releasing a large amount of latent heat during the solidification process in a low-temperature environment. As the convective heat transfer coefficient increased, the insulation effect provided by CPCM to lithium-ion battery packs decreased, exacerbating the temperature inhomogeneity inside the low-temperature thermal management system. Increasing the thermal conductivity of the CPCM ensured that the maximum temperature difference of the battery pack during operation remained within a safe range. Finally, the simulation results indicated that adding different thicknesses of insulation materials at the periphery of the low-temperature thermal management system effectively can improve the insulation effect. However, it also is necessary to select the appropriate thickness of insulation materials in combination with the actual demand.

Practical Applications

This paper establishes a model based on CPCM for the low-temperature thermal management system of cylindrical lithium-ion batteries. The thermal insulation and temperature homogenization performance of the CPCM-based BTMS were analyzed under various conditions, including different ambient temperatures, convective heat transfer coefficients, and thermophysical properties of materials, as well as the addition of insulation layers, considering time-dependent variations. Based on this analysis, the physical requirements of CPCM and the package structure for BTMS under different operating conditions were determined. The simulation results demonstrate that the liquid-phase CPCM solidifies and releases the stored heat through latent heat to warm and insulate the battery when the discharging process is stopped at lower temperatures. This effectively extends the battery’s holding time in the high-temperature domain and above 0°C. As the external convective heat transfer coefficient increases, the effectiveness of CPCM in insulating the battery decreases, exacerbating the temperature nonuniformity within the low-temperature thermal management system. Increasing the thermal conductivity of the CPCM ensures that the maximum temperature difference during battery operation remains within safe limits. In addition, the results indicate that the insulation effect can be improved significantly by adding an appropriate thickness of insulation material to the periphery of the thermal management system, based on specific requirements. This study provides a valuable reference for the design and optimization of thermal management systems for batteries operating in a wide temperature range.