Journal of Energy Engineering

This article reviews the main development of advanced components and control algorithms in engine thermal management systems (TMS). First, the limitations in traditional mechanical coolant pumps, radiator fans, and thermostats are discussed, such as waste of power and poor temperature control. To solve these problems, replacing the executors of TMS with electric components and intelligent control schemes is possible. The development status and characteristics of electric fans, pumps, thermostats, cooling systems, and an advanced cooling concept and cooling medium are summarized, all of which contribute to the improved efficiency of TMS. The reliability of the system is directly affected by the control strategies; therefore, the research progress of different control algorithms, including proportional-integral-derivative (PID), Smith predictor, model predictive control (MPC), active disturbance rejection control (ADRC), Lyapunov, fuzzy control algorithms, are reviewed. In addition, the current methods of building zero-/one-/three-dimensional simulation object models used for control algorithm development are then summarized. Finally, the coordinated development of electric components and control strategies are concluded to have great significance in improving the fuel economy and safety of internal combustion engines.

The ever-increasing penetration of intermittent renewable energy generation from wind farms (WFs) and photovoltaic (PV) plants has significant impacts on the stability of the power system concerned. This paper examined the effect of the power oscillation damping controller (PODC), consisting of a power system stabilizer (PSS) and a unified power flow controller (UPFC), on the stability of a power system with doubly fed induction generators (DFIGs) based WFs and PV plants. The small-signal stability of a power system with intermittent renewable energy generation was enhanced by coordinately adjusting the parameters of damping controllers of the PSS and UPFC-PODC, as formulated as a multiobjective optimization and solved by the steepest descent algorithm (SDA). The well-coordinated application can suppress significantly the low-frequency oscillation (LFO). In addition, an index based on eigenvalues and damping ratios is proposed to measure the damping change of the power system. A four-machine, two-area sample power system was employed to test the proposed method through several designed scenarios, including changing the tie-line transmission power, increasing wind power and PV outputs, and changing WF and PV plant access points. The performance of the proposed method was demonstrated through small-disturbance eigenvalue analysis and short-circuit fault dynamic time-domain simulation. Simulation results showed that simultaneous optimization of parameters among different controllers effectively suppresses local and interarea oscillations.

In order to study the structural characteristics and self-similar propagation of a syngas/air mixture turbulent expanding premixed flame, experimental studies under various equivalent ratios (0.6, 0.8, and 1.0) and hydrogen fractions (20%, 40%, and 60%) were carried out. The effects of the hydrogen fractions and equivalence ratios on the local radius and pulsation radius of the flame front were studied, and perturbations at different flame positions were quantitatively analyzed. The correlation between flame structure and initial flame structure during flame propagation was analyzed, and the self-similarity of syngas/air mixture turbulent premixed flames at different hydrogen fractions and equivalence ratios was also analyzed. The results showed that the fluctuation of the ratio of the pulsation radius to the area equivalence radius at the flame front notably increased with increasing hydrogen fraction, and the correlation coefficient of the flame front decreased at the same radius. Although the equivalence ratio increased, the standard deviation of the pulsation radius decreased at the same radius, and the degree of uniformity of the flame front decreased as well. The accelerated propagation process of the flame had self-similar characteristics. The Reynolds number of the fit index of the dimensionless propagation speed decreased from 0.45 to 0.30 with increasing hydrogen fraction. When the equivalence ratio increased, the fit index decreased from 0.41 to 0.31.

The aim of this study was to prove that an optimization method combining genetic algorithms with a direct-forcing immersed boundary method as a distinguished numerical method could improve the performance of vertical-axis wind turbine blades. The proposed direct-force immersed boundary (DFIB) flow solver was tested using the benchmark laminar flow problems (lid-driven flow, flow over a stationary cylinder, and vortex-induced vibration of the circular cylinder). Two cases were analyzed, one of a stationary airfoil and another of a rotating airfoil in a vertical-axis wind turbine. The analysis was carried out using two-dimensional flow simulations in a laminar flow regime. A NACA 0012 airfoil was used as the original airfoil cross section of the vertical-axis wind turbine. The proposed method successfully simulated the moving blade in the flow field, and the results revealed that the optimized wind turbine blade designed using the proposed method had an efficiency improvement of 5.61% compared to the original airfoil.

Large two-stroke marine diesel engines have special injector geometries which differ substantially from the configurations used in most other diesel engine applications. In particular, injector orifices are distributed in a highly nonsymmetric arrangement, thus affecting the spray characteristics. In this study, a new approach was implemented in computational fluid dynamics (CFD) simulations of these sprays, for nonevaporating conditions, for one noneccentric and two eccentric atomizers. The approach consisted of modeling spray primary breakup using large eddy simulation (LES), and modeling spray secondary breakup by means of Reynolds-Averaged Navier–Stokes (RANS) equations in conjunction with validated spray models. LES results were processed using a new droplet identification method, yielding probability density functions (PDFs), which were used as input in the secondary breakup simulations. Spray morphology was characterized in terms of penetration length, deflection angles, and spray cone angles. Results were in good agreement with those of recent experiments in a large spray combustion chamber, verifying strong deviation from an axisymmetric spray structure; they outperformed CFD results using a conventional (RANS-only) method. Overall, the present approach was shown to be effective for CFD studies of asymmetric sprays in large marine engines.

This paper reports the development of a high load-carrying capacity piezoelectric generator (PG) to harvest the great amount of compressed air energy wasted by the moving cylinder in a pneumatic system. The PG is designed to convert the pressure energy induced by compressed air into electrical energy. The coupling model of the piezoelectric patch is established under pressure motivation. The relationship between the output voltage and the piezoelectric patch deformation has been analyzed, and the output voltage is calculated through the simulation model. The simulation result demonstrates that the output voltage is $72.00\mathrm{V}$ when the load resistance is $\mathrm{1,000}\mathrm{k}\mathrm{\Omega }$. The prototype generator consists of a chamber and a piezoelectric patch, which is sealed at the bottom. The prototype is installed in a real pneumatic system to test energy conversion performance, and it is driven by the compressed air that can be influenced by the moving piston. The energy exchange capacity of the proposed PG is analyzed by testing the output voltage and power generated under real working conditions. The test results show that the output voltage is increased when the moving distance decreases. The maximal output voltage and power are $72.80\mathrm{V}$ and 7.77 mW, respectively. The analysis of the testing results shows that the energy exchange capacity of the PG is highly relevant to the piston moving distance.

A three-dimensional nonisothermal two-phase flow model is developed to analyze the effects of cathode gas diffusion layer (GDL) porosity variations on the current density distribution and liquid water saturation in a proton exchange membrane fuel cell (PEMFC). Water phase changes among the vapor, liquid, and dissolved phases are considered in porous zones, whereas water transport in the channel is simplified by considering convection only. To investigate the current distribution quantitatively, the standard variance of current density over the cathode catalyst layer (CL) is applied. Aimed at improving the uniformity of current distribution in a cathode CL as well as further decreasing the liquid water in the cathode GDL, the optimization of graded porosity in a cathode GDL is conducted by parametric design in two directions, where one is the flow direction and the other is the thickness direction. Results indicate that compared with uniform porosity and porosity variations along a single direction, optimized porosity variations along two directions could achieve at most 18.3% improvement in current density distribution and a nearly 5% decrease in liquid water saturation in a high-current-density situation (operated at voltage 0.2 V). Thus, by applying porosity variations along two directions, fuel cell performance is improved and cell usable time is extended, which provides guidance for future material design on GDLs.

The transient model of a hybrid solar collector/nocturnal radiator with thermal energy storage for solar water heating and its nocturnal radiative cooling was deployed in simulating space comfort cooling in Owerri in southeast Nigeria. The physical system comprises a spectrally selective absorber with 12 copper tubes spaced 0.1 m apart, an area of $1.704{\mathrm{m}}^2$, a thickness of $1.03\times {10}^{-3}\mathrm{m}$, a 20-μm-thick polyethylene film 20 mm from the absorber surface, and two 300-L hot- and cold-water storage tanks installed in a $142.6\text{-}{\mathrm{m}}^3$-envelope room. Water was pumped at a flow rate of $8.33\times {10}^{-2}\mathrm{kg}/\mathrm{s}$ from the two tanks through the system for daytime heating and nighttime cooling, respectively. The following day, cooled water from the cold storage tank was utilized to provide comfort cooling within the envelope room. The temperature histories of the different components that make up the system, namely, the hybrid SCONOR, cold storage tank, and building indoor (room) temperatures, as well as the energy absorption and losses from the hybrid plate, were studied. The results obtained show that maximum energy absorption occurred around the bond region, whereas the lowest energy absorption was along the collector fin width. Also, after sunset, the SCONOR witnessed continuous losses until it reached the ambient state for nocturnal radiation to commence. Water cooled to a minimum of 18°C during nocturnal radiative cooling phase, while the cooled space temperature fluctuated between 26.6°C and 26.8°C. The indoor room temperature depression varied over 0.1°C–5.7°C from ambient temperature. Ambient temperature ranged from 25.6°C to 32.5°C during the day. Parametric analyses showed that an aluminum substrate showed the best promise in terms of the thermal performance of the integrated energy system.

Hybrid utilization of solar and geothermal energy is an attractive option to solve the global energy crisis as well as environmental issues. A solar-assisted ground source absorption heat pump (SGSAHP) system is proposed to provide a solution to energy shortage, especially in remote regions without reliable electricity supply. The SGSAHP system requires little electricity input and is able to maximize the use of renewable energy and minimize the peak demand to the power system. The system exploits solar and geothermal energy, which can improve the coefficient of performance (COP) of the system and make it operate with little electricity input. SGSAHP can run under both heating mode and cooling mode. In this paper, a SGSAHP mathematical model is developed and simulation study is conducted including parameter analysis, economic analysis, and system optimization. The results show that there exists an optimal value of the generator temperature to reach the maximum COP, while higher condenser temperature and evaporator temperature have negative and positive influence on system performance, respectively. The optimized thermodynamic and economic performance is obtained. The exergy analysis shows that the major exergy losses are contributed by solar collector and heat exchanger.

To analyze the combustion and exhaust characteristics of internal combustion engines, it is important to analyze combustion phases, such as ignition delay, premixed combustion period, and diffusion combustion period, by defining the start and end of combustion. This paper proposes a method for detecting and calculating combustion phases based on the difference between combustion and motoring pressures. The time at which 10% of the maximum pressure difference is observed can be considered as ignition timing, while that at 95%–99% as the end timing of combustion. Ignition delay and combustion duration calculated using the proposed method were almost identical to those calculated based on cumulative heat release. In addition, both methods exhibited a similar tendency with respect to the injection timing. A comparison of the results obtained via the two detection methods demonstrates that the error in the ignition delay was found to be considerably small, while that in the combustion duration reached 0.57°. Furthermore, a comparison of the proposed method and the complicated conventional method for detecting combustion phases based on cumulative heat release confirmed that the combustion phase can be detected via a simple calculation using the difference between the combustion and motoring pressures.