Journal of Energy Engineering

The initial ignition characteristic of fuel spray in a diesel engine is of great significance in the cold start problem. In this paper, the constant volume combustion vessel was applied to simulate the practical diesel engine condition near the end of the compression stroke and investigate the autoignition threshold temperature and other ignition characteristics for the fuel spray such as ignition delay time and initial ignition location. First, the influence of the ambient temperature and the ambient density in the vessel on ignition delay time, initial ignition location, and fuel mixture autoignition threshold temperature was researched. Then, the influence of fuel injection pressure and the plate surface temperature on ignition delay time and initial ignition location were also investigated in this paper. The results have shown that the autoignition threshold temperature was approximately 803 K for the fuel spray in the combustion vessel, which simulated the practical diesel engine condition and the ignition delay time for both free jet and impinging jet increasing with the ambient temperature and the ambient density decreasing, respectively. The experimental map has also been developed to show the zone where initial ignition would occur with the different ambient temperatures and the ambient densities for the diesel fuel; the change of the injection pressure has no obvious influence on ignition delay time, but it reduces ignition success rate. The ignition delay time becomes longer as the plate surface temperature reduces and the variation rate of ignition delay time differs greatly in different temperature zones.

Power transmission expansion planning has been frequently solved through economical and reliability perspectives, without focusing on the environmental implications of the incorporation of transmission lines. In this paper, the authors study the environmental effects on global and local pollutant emissions derived from the incorporation of some specific new transmission circuits in both new and existing corridors and their interrelationship with power system costs and system reliability variations. For this purpose, the authors develop a methodology that quantifies the indirect impact on pollutant emissions due to variations in power plants’ generation when adding a line circuit to a hydrothermal power system. The methodology also allows the analysis of the effect of $\mathrm{N}–1$ security criterion over the pollutant emissions displacement, as well as the effect of changes in demand, the hydrological scenarios, and the failure cost. The authors illustrate the methodology using a 203-node simplified version of the main Chilean network. Results suggest that pollutant emissions show an important relation with the network’s structure and the system’s reliability level. Furthermore, it is shown that there may be some unpredicted distributed environmental implications of adding a line circuit into a hydrothermal system.

The microwave-assisted liquefaction of Ulva prolifera under atmospheric pressure conditions was investigated for an Fe species–modified HY catalyst. The characterization of Fe/HY confirmed that the Fe species existed in the form of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ and that the framework of HY remained intact. It was also found that the doped ${\mathrm{Fe}}_2{\mathrm{O}}_3$ distinctly enhanced the total acid sites of the catalyst and thus increased the bio-oil yield from 40.6 to 52.6%. The GC-MS results demonstrated that the application of the ${\mathrm{Fe}}_2{\mathrm{O}}_3/\mathrm{HY}$ solid catalysts facilitated the production of long-chain compounds (octadecanoic acid methyl ester and hexadecyl ester), thus leading to a distinct decrease in the oxygen content of the bio-fuel. Therefore, the obtained ${\mathrm{Fe}}_2{\mathrm{O}}_3$-modified HY exhibits the potential for bio-fuel production from Ulva prolifera.

Integrated transmission and distribution (T&D) analysis is a new research area in power systems that has gained traction in recent years to study the impacts of rapidly growing penetration levels of distributed generation, particularly solar photovoltaics (PV), on power systems. This paper demonstrates the advantages of studying the impact of PV on power system dynamics using three phase, integrated T&D models instead of the positive-sequence transmission-only models. The three phase integrated T&D models include detailed models of substations that connect the transmission and distribution systems. It is demonstrated that by more accurately simulating disturbances as they would occur in the field, and by preserving the geographic diversity of PV generators, dynamic simulation of integrated T&D models avoids overestimation or underestimation of the impact of PV on power systems.

A new mathematical model is presented to investigate the random combustion of a combustible mixture of micro-organic dust particles and air. For this purpose, a model is developed assuming a flame structure composed of three zones: a broad preheat zone, a reaction zone, and a postflame zone. The random modeling of the combustion process is brought about by adding a source term in the energy equation indicating the random states of particle volatilization in the preheat zone. The effect of thermal resistance on the combustion characteristics, such as flame temperature and burning velocity, is studied using a nonzero Biot number in the model. Moreover, it is observed that particle size considerably impacts the flame propagation through organic dust particles. As a result, it is concluded that the increase in Biot number leads to a decrease in the burning velocity as well as flame temperature. The novelty of this research is the improved understanding of the random distribution of the combustion phenomenon, which leads to more realistic and reasonable predictions of the combustion physics compared to the previously published analytical and experimental results.

Using solar energy for space heating is a promising technology to offset buildings’ energy consumption and reduce environmental pollution. Solar air heating system is free of freezing and inexpensive owing to its inherent simplicity in many circumstances. In this paper, the performance of a straight-through evacuated tube (ST tube) which opens on both ends for space heating, is studied. A thermal model for a single ST tube installed on a building façade for solar space heating using hot air was developed and an experimental measurement was conducted. The results predicted by the thermal model and experimental measurement agreed well, which confirms the validity of the thermal model. The performance and some aspects for practical application of the ST tubes for solar space heating are studied and discussed. The daily average efficiency of the ST tube was approximately 55% under the experimental conditions. The radiative thermal resistance from the absorber tube to the cover tube was almost equal to the total resistance, and the emissivity of coating on the outside surface of absorber tube had great effect on total heat loss. The cost and useful heat gains for a single room in a building in Beijing was studied. The payback period of this solar space heating system is approximately 8 years. The performance of solar collectors, including multiple ST tubes, will be studied in the next research.

Optimal power flow (OPF) is critical for maintaining the secure and economic operation of power systems. In this paper, a fundamental analysis of the OPF problem is provided. As a common nonlinear programming (NLP) problem for mathematicians, the reason why the OPF problem is challenging for power engineers is analyzed. It is shown that the nonlinearity of power flow equations is the main factor that makes the OPF problem hard to solve. Ideas for handling the power flow equations are summarized. Existing OPF methods are further divided into three categories based on how the power flow equations are handled: OPF methods with the strict alternate current (AC) network model, OPF methods based on convex relaxation, and OPF methods with linearized network models. A comprehensive review of the state-of-the-art OPF algorithms in each category is presented. The features, advantages, and disadvantages of different categories are analyzed. Additionally, the prevalent industrial practices related to OPF calculations are presented. Extensions of the OPF problem, including security-constrained OPF (SCOPF), OPF with discrete variables, and OPF with uncertainties, are discussed.

The present study investigates the feasibility of using winglet as a novel approach to reduce aero-acoustic noise of wind turbine blades. For this purpose, the National Renewable Energy Laboratory (NREL) Phase VI wind turbine is used as the baseline case. The flow field is simulated by computational fluid dynamics and SST $k\text{-}\omega$ turbulence model. Further, the Ffowcs Williams and Hawkings (FW-H) acoustic analogy is implemented for noise prediction. The aero-acoustic simulations are performed for different wind speeds and distances from the base of the tower. The results indicated that the aerodynamic sound pressure level (SPL) is intensified in all frequencies by increasing the wind speed; however, its production rate is decreased in high wind speeds. Moreover, the SPL at the downstream of the tower is higher than the upstream due to the disturbances emerged in the flow field. Furthermore, the SPL for the baseline and the modified blade shows a little difference in low frequencies; however, in high-frequencies, differences become noticeable. Introducing winglet at the blade tip is effective in high frequencies and can lead to a significant decrease in noise emission especially near the tower, but it has no perceptible effect on noise emission in low frequencies.

This paper presents a solution to improve the dynamic performance of the frequency-control process in autonomous microgrids by means of photovoltaic (PV) power plants. An energy storage system was integrated in the PV power plant primarily to enhance the power-control capability of the PV system. Because the the focus of this paper is on how the PV-storage system can provide dynamic frequency support in the microgrid, a frequency controller was added to the control scheme of the voltage source converter (VSC) that interfaced the PV power plant with the microgrid. The PV response to frequency deviations was adapted from the existing Romanian grid code and, according to the system’s characteristics, a modified power-frequency curve was proposed. The system being studied was examined through both simulations and experiments, and the results showed that, when the storage system was enabled, the PV power plant improved the microgrid’s dynamic frequency response.

This paper describes the implementation and comparison of two current-control strategies for a three-phase voltage source inverter (VSI) system grid connected via a LCL filter and used in distributed power generation systems (DPGS) applications. The performance of a proportional integrator (PI) current controller in the dq-synchronous reference frame and a proportional resonant (PR) controller in the $\alpha \beta$ stationary reference frame are evaluated and compared. A voltage source (playing the role of a generic renewable energy source) is connected to the grid using a two-level inverter in order to verify the performance of the proposed approach. Both methods are implemented and tested at the laboratory-level using a dSPACE setup. The comparative study is performed to demonstrate the stability and effectiveness of the control algorithms across a wide range of operating conditions. The experimental results confirm that the proposed control achieves good current tracking, thus satisfying the grid-connection harmonics standards.

With the goal of providing basic thermal data for the thermal management of a power-generation cabin at Dome A in the Antarctic, a dynamic model of annual temperature variations in the power-generation cabin is developed by using a lumped-parameter method. The dynamic temperature variations and thermal responses of a power-generation cabin system throughout the year are investigated. The effects of ventilation rate and intermittent operation on the indoor air temperature are examined and analyzed. The results indicate that the air inlet and exhaust fan should be configured in the power-generation cabin to control the indoor air temperature at an acceptable level. The total ventilation rate for the power-generation cabin should between $G=0.68$ and $1.38{\mathrm{m}}^3/\min$. Under intermittent operation, the indoor air temperature can easily be lower than 0°C and may even reach $-40°\mathrm{C}$ in the cold season. The thermal response times during the start-up of the diesel generator unit are 0.7 h, 1.0 h, 15 days, and 50 days for the indoor air, cabin shell, upper oil layer, and lower oil layer, respectively. If the diesel generator unit fails, the oil temperature will decrease to $-40°\mathrm{C}$ after 65 days in the cold season and 110 days in the warm season.

A preliminary study on the increased thermal capacity in thermal energy storage concrete solids with inclusions of phase change materials is presented here. Particularly, the change in the thermal behavior of such composites, by varying the inclusions percentage, is evaluated. An ad hoc hygrothermal finite-element code has been developed, able to evaluate the nonlinear concrete behavior during the phase changes of phase change materials (PCM), subjected to a load history of the test plant designed by ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development). Transient thermal analyses have been conducted, assuming a homogeneous distribution of phase change materials within the cementitious matrix. Hygro-thermo-mechanical models have been developed to evaluate the heat storage capacity of the composite material, and its change in mechanical strength has been analytically and numerically investigated, both at environmental temperature and during heating. The adopted code takes into account a homogenized composite, having a uniform distribution of PCM within the cementitious matrix and characterized by homogenized hygrothermal properties.