Journal of Energy Engineering

In this study, we investigated uranium dioxide (${\mathrm{UO}}_2$) dissolution under geological repository conditions by applying a three-dimensional (3D) thermal-chemical reactive transport model. The transport of chemical species and thermal conduction in ${\mathrm{UO}}_2$ fuel pellets and chemical dissolutions of ${\mathrm{UO}}_2$ were considered. The mathematical and numerical formulations of the model are described in the paper. $\mathrm{Fluid}\text{-}{\mathrm{UO}}_2$ reactions were modeled to demonstrate the validity of modeling reaction processes. ${\mathrm{UO}}_2$ dissolution under low (25°C) and high temperatures (250°C) was simulated, taking into account the changes in aqueous uranium species with temperature. The predicted lifetime of one ${\mathrm{UO}}_2$ pellet is greatly dependent on the temperature. To illustrate the effect of uranium species on reaction rates, numerical studies were conducted at the same temperatures but with different reaction types and chemical species. It was found that reactions that produce ${\mathrm{UCl}}_4^0$ enhance the dissolution rates of ${\mathrm{UO}}_2$ by consuming the ${\mathrm{Cl}}^{-}$ in solutions. ${\mathrm{UO}}_2$ dissolution with varying pH values was also modeled. When pH increased to 6, the average dissolution rate of a ${\mathrm{UO}}_2$ fuel pellet was eight times slower than it was at $\mathrm{pH}=2$. Dissolution simulations were carried out on the images of fractured ${\mathrm{UO}}_2$ pellets. The impact of microfractures on ${\mathrm{UO}}_2$ dissolution was illustrated. The developed model is able to quantify ${\mathrm{UO}}_2$ dissolution behaviors and identify key parameters controlling the physiochemical processes involved. The model can be used as a predictive tool for applications such as spent ${\mathrm{UO}}_2$ fuel sequestration, contaminant transport, and geothermal resources development.

Abstract

In this study, the effect of the fin surface on the electrical performance of passively cooled photovoltaics (PV) modules equipped with flat (${\mathrm{PV}}_{\mathrm{f}}$) and tread (${\mathrm{PV}}_{\mathrm{t}}$) surface finned heat sinks was experimentally investigated. ${\mathrm{PV}}_{\mathrm{f}}$ and ${\mathrm{PV}}_{\mathrm{t}}$ modules were fabricated with heat sinks by filling phase change material (paraffin)/aluminum chip mixture between the PV module and heat sink. The mean ambient temperature, solar radiation, and solar power during the experiment were about 13°C, $864\mathrm{W}/{\mathrm{m}}^2$, and 247, respectively. The electrical power of the ${\mathrm{PV}}_{\mathrm{f}}$ and ${\mathrm{PV}}_{\mathrm{t}}$ modules was increased by 10.84% and 17.2%, respectively, while the average temperature was decreased by 11.78% and 16.5%, respectively, compared to the PV module. The ${\mathrm{PV}}_{\mathrm{t}}$ module provided 5.37% more effective cooling and a 5.74% higher electrical power than the ${\mathrm{PV}}_{\mathrm{f}}$ module. The electrical efficiency of the PV module was 11.35%, and the rate of increase was 1.21% and 1.95% for the ${\mathrm{PV}}_{\mathrm{f}}$ and ${\mathrm{PV}}_{\mathrm{t}}$ modules via cooling. Compared to the PV module, the electrical exergy of the ${\mathrm{PV}}_{\mathrm{f}}$ and ${\mathrm{PV}}_{\mathrm{t}}$ modules increased by 10.84% and 17.2%, respectively, at an average solar exergy of 230.61 W, while their thermal exergy decreased by 23.64% and 32.3%, respectively. Compared to the ${\mathrm{PV}}_{\mathrm{f}}$ module, the thermal exergy of the ${\mathrm{PV}}_{\mathrm{t}}$ module was reduced by 11.34%, and the electrical exergy was increased by 5.74%. The increase in exergy efficiency is 3.14% and 4.51% for ${\mathrm{PV}}_{\mathrm{f}}$ and ${\mathrm{PV}}_{\mathrm{t}}$ modules versus the PV module.

Practical Applications

A small part of solar energy is converted into electricity, while a significant part is converted into heat and increases the temperature of the PV cells. An increase in the temperature of PV cells harms conversion efficiency and causes an increase in the share of converted heat and a decrease in electricity generation. The conversion efficiency can be improved by removing the accumulated heat of the module. In practice, there are various examples of passive and active cooling. Active cooling methods enhance heat transfer by forcing the fluid flow with a fan or pump to facilitate heat removal. Active cooling applications are heavy, expensive, and complicated. Natural heat convection can be maximized with heat spreaders or heat sinks during passive cooling. Passive cooling may not dissipate heat as effectively as active cooling. Passive cooling systems are cheap, lightweight, and uncomplicated, and they can become even more valuable as their thermal efficiency increases. This study addresses the effect of the heat sink fin surface on the cooling and electrical performance of flat and tread finned heat sinks coupled PV modules that are paraffin/aluminum chips blend backplane filled. 1st and 2nd law PV module efficiencies improved by about 1.21% and 1.95% and 3.14% and 4.51% with flat and tread finned heat sinks, respectively.

Cofiring of coal slime in existing utility boilers is a promising approach to the disposal of coal slime. However, reports about cofiring coal slime with anthracite in the down-fired utility boiler are still few. This work presents a comprehensive study on the combustion characteristics, slagging potential, and ${\mathrm{NO}}_{\mathrm{x}}$ emissions of cofiring coal slime and anthracite through lab-scale experiments, numerical simulations, and full-scale industrial measurements in a 600MW down-fired utility boiler. The lab-scale experiments show combustion interactions between anthracite and coal slime, which have both a promotive effect on the ignition of anthracite and inhibitive effect on the combustion and burnout of anthracite. Especially, the inhibitive effect is greater than the promotive effect when the blending ratio of coal slime is above 5%. The addition of coal slime can effectively improve the ash fusion temperature of blended samples. In addition, simulations and full-scale industrial measurements of anthracite cofiring with coal slime are performed in a down-fired utility boiler. The results show that as the blending ratio of coal slime increases, the carbon content in fly ash increases and the boiler efficiency and ${\mathrm{NO}}_x$ emissions decrease. A 5% ratio is validated to be a reasonable cofiring ratio of coal slime in the actual operation without no slagging phenomenon. These results provide more insights into the cofiring behavior of coal slime and anthracite and guide the application of blending coal slime in a down-fired boiler.

Abstract

Fractured horizontal wells and water-flooding development are effective means to stabilize oil production in low-permeability reservoirs; however, transient pressure behavior analysis based on a single-well testing model ignores the interference effect caused by water-injection wells. With advancements in horizontal drilling technology, multiple-fracture horizontal wells are widely used in tight reservoirs, and researchers have concluded that pressure drops in hydraulic fractures cannot be ignored. In this paper, we present a well testing model that considers the effects of finite-conductivity fractures and the interference of water-injection wells. The analytical solutions were derived based on the source function and superposition principle, and the characteristics of the transient pressure behavior at the bottom hole are analyzed and discussed. The typical pressure curve of our model showed that seven flow regimes exist: wellbore storage stage, transitional flow, first linear flow, fracture interference flow, second linear flow, pseudoradial flow, and interference flow. Furthermore, the results of the sensitivity analysis indicated that the flow regimes are greatly affected by the properties of the fractures and water-injection wells. The length of the fracture mainly affects the transitional flow, first linear flow, fracture interference flow, and second linear flow regimes, whereas the number of fractures predominantly affects the first two stages. The fracture spacing has a significant effect on the fracture interference flow and second linear flow regimes, and the water-injection rate mainly affects the interference flow regime. The well spacing determines when the interference effect occurs. These findings will support more accurate modeling of well production performance considering the impact of water-injection wells.

Practical Applications

Well test analysis is an effective measure in the process of reservoir development, and it is mainly used to monitor well production performance or estimate the physical parameters of the formation. Currently, multiple-fracture horizontal wells and water-flooding development are widely used in low-permeability reservoirs; however, a single-well test analysis method cannot reflect the reality of communication between production wells and adjacent wells. In this work, we present an interference well testing model that considers the effect of water-injection wells. The transient bottom-hole pressure response under different parameters was studied and analyzed, and the results of this study can be mainly applied in the arrangement of well groups in the development decision stage and adjustments of oil recovery plans in the middle production period. The cases investigated in this study provide several reliable references for engineers in well production management, and may also provide some guidance in further development decisions for oil fields applying multiple-fracture horizontal wells and water flooding.

Long-term predictions of the energy consumption of a building can be a reference in formulating energy conservation policies to achieve carbon neutrality. However, existing research on the prediction of energy consumption of urban buildings mainly adopts top-down or bottom-up single models that do not consider the coupling effect of macro and micro factors. Therefore, a coupled top-down and bottom-up prediction model has been proposed in this paper. The applicability of the proposed method was investigated based on residential buildings in Beijing, China. First, based on a top-down methodology, the energy consumption data of residential buildings in Beijing were investigated using an urban statistical yearbook, and the energy consumption of residential buildings under different energy-saving policies was analyzed. Second, micro factors, such as envelope parameters, personnel behavior, air conditioning, and electrical usage of typical residential buildings, were investigated. Subsequently, a simulation model of residential building energy consumption was constructed according to the survey data. The actual and simulated values for 2017 were $0.264\mathrm{GJ}/{\mathrm{m}}^2$ and $0.252\mathrm{GJ}/{\mathrm{m}}^2$, respectively. Finally, pessimistic and optimistic scenarios are proposed using the Human Impact, Population, Affluence, Technology (IPAT) model. The energy consumption of residential buildings in Beijing for the next decade was predicted under different scenarios by adopting the gray prediction method and multiple regression analysis, which was verified using the back-propagation neural network algorithm. In the baseline scenario, the projected 2021 heating energy consumption value was $13.4\mathrm{kgce}/{\mathrm{m}}^2$ with a relative error of 6.52%.

With wind and other variable renewable energy (VRE) generation gradually becoming the main power source, it is difficult to ensure the flexible and secure operation of the system by completely relying on conventional sources to balance the uncertainty and volatility of VRE. The increase of VRE penetration leads to the shortage of operation reserves, and wind power is allowed to participate in reserve services. However, the output of wind power is uncertain, and how to ensure the reliability and sustainability of wind power reserves is still an open question. A robust energy and reserve dispatch method considering the flexibility of wind power is proposed to solve this problem. To effectively measure the output and reserve margin of wind power under uncertain scenarios, (1) the grid connection coefficient of wind turbine generators (WTGs) is defined, and the uncertainty set considering active wind power curtailment is established; and (2) based on two kinds of WTG control methods, the reserve model of wind farms is established. The model links the reserve capacity between the base scenario and uncertain scenarios through the reserve coefficient, which ensures that the wind power reserve determined in the base scenario is still deliverable in any uncertain scenario. The robust scheduling model determines the optimal dispatching and reserve plan of conventional units and wind farms by balancing the system operation economy and the schedulable reserve of wind power. The column-and-constraint generation algorithm is used to solve the model. The effectiveness of the proposed model is verified by the illustrative results of the IEEE normal system.

Lower Silurian Longmaxi and Upper Ordovician Wufeng shales are gas-producing formations. These formations have ultralow porosity and permeability in the southeastern Sichuan Basin and have normal formation pressures with pressure coefficients of less than 1.2. Hydraulic fracturing has been proven as the best development strategy to produce gas. But choosing hydraulic fracture treatment parameters becomes challenging due to strong reservoir heterogeneity, significant horizontal stress contrast and high in situ stress in this region. We employed the pseudo-three-dimensional (P3D) model to study fracturing fluid types, injection rates, and proppant sizes to optimize the fracturing design of these shale formations. First, this model was solved in the simulator by the finite element method (FEM) to obtain the fracture height, length, and width. Then the results were validated by 3D Tip dominated model, and the perkins-kern-nordgren (PKN) and khristianovic-geertsma-deklerk (KGD) analytical models, which are popular and most used in designing hydraulic fractures. It was found that as the volumetric injection rate and gel loading in the fracturing fluid rise, so do the generated fracture length and width. Furthermore, the formations’ stress contrast affected the shape of the fracture, the interval with lower stress had a wider fracture compared to the interval with higher stress. Also, the higher stress in the layers above and below the shale formations contained the fracture height, which favored the growth of fracture in the shale formations. Lastly, it is suggested that a fluid with gel loading of 60 ppgt, proppant with 12/20 mesh size and an injection rate of ${6.36\mathrm{m}}^3/\min$ be used in these shale formations with normal reservoir pressure. These parameters’ combinations created the most extended propped fracture length of 264.8 m, and the average width was 1.06 cm.

Efficient water management in the gas diffusion layer (GDL) facilitates the efficient and stable operation of proton exchange membrane fuel cells (PEMFCs). GDL perforation is an effective method for fuel cell water management. The stochastic reconstruction of multiperforated GDL structure with different perforation diameters, perforation spacings, and array modes was established to simulate the transport process of water in multiperforated GDLs. The multiple relaxation time lattice Boltzmann method (MRT LBM) was used to simulate the water transport process in perforated GDLs. The result showed that the diameter of the perforation greatly influenced the water transport process; as the perforation diameter increased, the height of the water breakthrough rose, and water saturation significantly increased. When the perforation spacing was extremely small, the impact area of perforation spacing overlapped, and the area of the water breakthrough–prone region decreased, thus reducing water transportation efficiency. The array method had little effect on water saturation and water breakthrough height. These findings show that array parameters can influence the behavior of the water transport process in perforated GDLs and have significant implications for the application of perforation methods in GDL water management.