Journal of Energy Engineering

The purpose of the study is to reveal changes of the hydrodynamics and sediment transport induced into the river channel by the operation of small hydropower plants (SHPs). A 36-km reach of the upper Prahova River in Romania has been chosen, along which 10 run-of-river developments are built. A 1D modeling software was used to compute hydrodynamic parameters over the selected reach. Two discharge scenarios were considered into the channel: (1) mean annual flow (for the natural regime, without SHPs), and (2) the minimum discharge (worst case scenario of SHP operation for the environment). Results of the second scenario show that ecologically relevant hydraulic parameters in the channel are approaching the thresholds of endangering the fish habitat. On average, depth and shear stress drop by 50%, mean velocity by 40%, and flow area by 75%. Comparison between computed nondimensional shear stress and its corresponding critical value indicates that minimum discharge can only transport finer particles than average from the bed surface, thus leading to potential filling of fish spawning areas with sand.

The conflict between energy utilization and environmental problems is deepening, urging society to promote the utilization of renewable energy in the future energy structure. In this paper, the processes of power-to-gas (PtG) are described in detail for improving the utilization and flexibility of renewable energy. The reversible solid oxide cell (RSOC) technology is considered as a promising route for PtG, especially power-to-methane (PtM). The working principle, electrochemical performance, and dynamic behaviors of RSOC and methane production characteristics are reviewed and manifested by summarizing the existing literature and the authors’ experimental data. From the data, the application of RSOC in the distributed energy system is specifically demonstrated. The basic structure and power management strategies of the distributed systems combining renewable energy and natural gas are simulated and optimized by trade-offs among system efficiency, power quality, and renewable power penetration as well as system cost. It is found that RSOC is able to better integrate the renewable power, heat, and gas in the distributed system to fully utilize various energy sources and achieve a more efficient PtM conversion.

By adding an injection system of n-butanol on the intake manifold, the four-cylinder direct injection (DI) diesel engine had been modified to operate in n-butanol/diesel compound combustion mode (BDCC). Effects of temperature, combustion, and emissions were experimentally investigated on this experimental engine. The temperature, combustion, and emissions characteristics were compared between the original neat diesel mode and the BDCC mode. The engine speeds were set to be 2,250, 2,800, and 3,350 rpm, and the engine loads were set to be 25, 50, 75, and 100%, respectively. The experimental results demonstrated that the intake temperature, the combustion temperature, and the exhaust temperature were decreased under BDCC mode, compared with that of the original neat diesel mode. The form of heat release rate was significantly changed using the BDCC mode, and the maximum combustion pressure of BDCC mode was higher than that of diesel mode. Under the low load, the ignition delay of BDCC mode was longer than that of original neat diesel mode, which caused the heat release was much more centralized and the peak of heat release was much higher. Under the high load, BDCC mode had two peak values of the heat release rate, the first small peak was formed by the small partial self-ignition of n-butanol, and the second peak was formed by the heat release of n-butanol and diesel compound combustion. The diesel engine operating with the BDCC mode could simultaneously reduce the ${\mathrm{NO}}_{\mathrm{x}}$ and soot emissions but increase the total hydrocarbons (THC) and carbon monoxide (CO) emissions compared with the original neat diesel engine.

Aimed at the sustainable growth of energy production from renewable sources, the accelerated development of reliable technologies for the future smart grid has become an immediate necessity. In addition to exploiting various other resources, an important measure is changing the conventional approach to consumer interaction with the grid, thereby pursuing the development of more flexible loads. In this context, the current paper presents a control solution for active loads (ALs) in order to enhance their controllability and interactivity with the grid. Providing dynamic frequency support in autonomous microgrids (MGs) is the main target application of the ALs. The proposed solution includes a novel approach to controlling the harmonics currently produced by the AL, which allows performing harmonic compensation in the presence of nonlinear loads or grid voltage distortions. To prove the concept, the paper includes an experimental analysis that highlights both the dynamic frequency support potential of the AL and its harmonic compensation capability.

With the growing need for alternative energy sources, research into energy-harvesting technologies has increased considerably in recent years. The particular case of energy harvesting on road pavements is a very recent area of research, with different technologies having been developed in recent years. However, none of them have presented high conversion efficiencies nor technical or economic viability. This paper deals with the technical study of existing road pavement energy–harvesting solutions using electromechanical systems to transmit energy from the device surface to an electrical generator and the development of a new mechanical system to implement on such devices. The main goal is to quantify the energy harvesting, transmission, and conversion efficiency of the new system and compare it with the existing systems. Conclusions about each system’s efficiency and the maximum theoretical efficiency of the proposed system are presented.

In this study, the direct current (DC) electric field effect on the diffusion-swirling flame dynamics and flame temperature profiles was investigated experimentally with the aim of establishing the feasibility of electric control of combustion in controlling the heat transfer rate and ${\mathrm{NO}}_{\mathrm{x}}$ formation. It was observed that with an increase in voltage and change in polarity, the flame configuration, such as shape, size, and length, changes as a result of drastic change in temperature profile for the fuel lean case along the combustor length. Although air flow was turbulent, generating an irregularly dynamic flame, the applied voltage and polarity made the temperature profile more uniform throughout the combustor. Consequently, it was observed that the applied electric field played a vital role in manipulating the flame significantly in terms of flame shape, size, temperature distribution, and flame behavior. Hence by varying the applied voltage from 0 to 2 kV with positive and negative polarities, a field-enhanced reduction in flame temperature was obtained due to increased convective heat transfer to the cooling water, which finally led to lowering thermal ${\mathrm{NO}}_{\mathrm{x}}$ formation during the fuel combustion. In the current experimental case the maximum ${\mathrm{NO}}_{\mathrm{x}}$ reduction was approximately 30%. A clear difference in sooting behavior was also observed depending on the polarity. While the soot particles accumulated on the injector base away from the positive electrode, they stuck to the negative electrode.

The supercritical carbon dioxide (${\mathrm{sCO}}_2$) Brayton cycle has been the focus of much research in recent years because of its high efficiency and compactness. One of the key issues is the heat rejection of about half heat addition to the ${\mathrm{sCO}}_2$ cycle, which needs a strong and reliable cooling system. In this paper, indirect and direct cooling systems using a 20-m natural draft dry cooling tower (NDDCT) were proposed and investigated. One-dimensional models for these two cooling systems were selected based on the experimental data of the cooling tower. The effects of the ambient temperature on the heat rejection rate and ${\mathrm{sCO}}_2$ outlet temperature for indirect and direct cooling systems were investigated, respectively. The results show that the optimal values of the water mass flow rate can be found in the indirect cooling system, and several optimal values are obtained under different ambient temperatures. Under the same operation temperature, the overall cooling performance of the direct cooling system is better than the indirect system, especially under low ambient temperature conditions. Under 15°C ambient temperature, the exergy efficiency of the direct cooling system is greater than that of the indirect cooling system ($>200\%$). The results of this work illustrate that the direct cooling system is more appropriate for the Gatton cooling tower and also provide some references for the dry cooling system design for small-scale ${\mathrm{sCO}}_2$ cycles in the future.

This work reports on the testing of a 100-W intermediate-temperature solid oxide fuel cell stack (IT-SOFC). The stack was tested both in feeding conditions with pure hydrogen and with syngas. In the latter case, different types of feeding gas mixtures were considered that differ from each other in the composition of their gaseous substances and the quantity of CO in the mixtures. For cases of stack feeding with mixtures containing carbon monoxide, the influence of the reformer temperature on the output obtained was also evaluated. The stack operation was tested by changing some operating conditions such as the temperature of the SOFC stack and the fuel use factor. The polarization curves of the stack were obtained in order to vary the operating conditions. Furthermore, mapping of the stack performances in terms of electric efficiency and power produced was carried out.

As the wind turbine structure flexibility is increased with its larger power capacity, the control system plays a more important role in the effective wind energy generation. This paper presents a robust model reference adaptive control (RMRAC) scheme for turbine speed regulation in the high wind speed region. The existing model reference control (MRC) is designed with respect to the linearized turbine dynamics at the selected operating point, and the adaptive law is introduced that is capable of adjusting the MRC parameters continuously to handle the varying system dynamics. In addition to model reference adaptive control (MRAC), disturbance accommodating control (DAC) is introduced to explicitly cancel out wind disturbances. The authors employ a robust modification mechanism, dead zone with dynamic normalization, to further improve the stability and robustness of the RMRAC. The proposed RMRAC scheme is verified using the high-fidelity turbine simulator FAST, which models an actual utility-scale wind turbine, CART3, located at the National Wind Technology Center (NWTC) in the United States. The proposed scheme is fully evaluated and compared to the baseline linear controls under various wind conditions generated by TurbSim. Under RMRAC, simulation results indicate more accurate generator speed and rotor power regulation performance, as well as enhanced robustness of the closed-loop system.

This paper presents a model for a transmission expansion planning (TEP) problem in which both active and reactive power as well as voltage magnitude of buses are considered through linearized alternating current (AC) load-flow constraints. The proposed approach uses the special ordered set of Type 2 (SOS2) to obtain the optimal global solution of the approximated linear model of TEP, which is indeed a mixed-integer linear programming (MILP) problem. This linear binary model can be effectively solved by existing off-the-shelf solvers using the branch and bound algorithm. The solution obtained is guaranteed to be globally optimal, whereas most mixed-integer nonlinear programming (MINLP) solvers could not guarantee an obtainable global optimal solution for nonconvex problems. The accuracy level of the solutions for the approximated linearized model can be easily controlled by adjusting specific parameters to suitable values. Results obtained through a simulation study show the effectiveness and applicability of the linear model presented. As numerous simulation studies show, the proposed methodology is reliable and robust.

This paper presents a novel method to enhance the low-voltage ride-through (LVRT) capability of a doubly fed induction generator (DFIG) equipped with an energy storage system (ESS) to provide voltage support during grid faults. Specifically, the transient DC-link voltage is regulated by controlling the ESS, which is connected to the DC-link capacitor of the DFIG. This is actually equivalent to absorbing the power imbalance between the captured wind power and the power delivered to the grid by the DFIG during the fault so that the DC-link voltage and the rotor current dynamics can be greatly improved. Moreover, both the rotor speed and the pitch angle are also controlled to initially react to the low-voltage transient, which includes increasing rotor speed to store the unbalanced energy as kinetic energy and augmenting the pitch angle to reduce the captured wind power. Thus, because of less unbalanced power flowing into the rotor, the DFIG dynamics can be further improved and the required ESS capacity can be saved. As the grid-side converter (GSC) is not necessarily required to manage the transient DC-link voltage, it is manipulated to devote its entire capacity to injecting reactive power into the grid for voltage support during a fault. Comparisons with the conventional control strategy are obtained by simulating a single machine-infinite bus power system. It is verified that the proposed method can not only ensure the LVRT capability of DFIG by saving the ESS capacity, but also significantly improve the terminal voltage dynamics during the grid fault.

Exergy analysis, thermoeconomics, and combined pinch and exergy analyses are useful methods for improving design and performance of processes such as thermal power plants. However, these methods are usually applied separately. In this paper, the methods are applied simultaneously to the 423-MW Neka combined-cycle power plant and the 315-MW Ramin steam power plant to evaluate and compare the performance of the systems and their components under different load conditions. To perform these analyses, a computer simulation and analysis program is developed. The simulator can predict the cycle behavior for different operating conditions with relative errors of less than 1.5%. The models are refined using performance test data from these plants. The system information is displayed graphically to visualize the performance of the systems for different conditions by applying combined pinch-exergy analysis. To better illustrate the plant performance and benefits of knowing the exergy destructions, the exergy destruction level (EDL) and the exergy cost destruction level (ECDL) are proposed and applied. Correspondingly, a new graphical representation is developed to illustrate the performance of each component based on exergoeconomic analysis, providing enhanced combined pinch-exergy and EDL/ECDL representations.

A survey of 1,244 households in three representative Chinese provinces was undertaken to assess energy consumption and cost for rural and urban residents. After accessing a considerable amount of information on energy consumption and cost, significant independent variables were selected by stepwise regression to analyze the main influencing factors of household energy consumption, and the Kruskal-Wallis test was used to research regional differences of energy application in six study regions. The results showed that energy consumption profiles in urban areas were similar, with no significant differences in main energies and cost between three study cities, whereas energy consumption profiles in rural areas were quite different. Different urban-rural energy divides were evident in different provinces. Wealthy areas tended to have a smaller urban-rural energy divide, whereas impoverished areas tended to have larger energy divide. Urban and wealthy rural residents tended to use commercial energies, which resulted in urban resident energy cost being significantly more than a rural resident in the same province, and particularly so in an impoverished province. These outcomes illustrate the different energy application profiles in representative areas and will help to develop appropriate energy policies.

This paper presents the technical and economic interaction with the grid of an offshore wind power plant development in Romania, on the coastal area of the Black Sea. Offshore wind power plants are currently very expensive and they can hardly compete with other power plants, even onshore wind power plants. In the context of capital costs decreasing over time, of possible European Union incentivizing schemes, and of requirements resulted from the twenty-first Conference of Parties (COP 21), this paper analyzes the economics of offshore developments in Romania considering the current status of the national grid. The results of the study provide guidance on project economics and development, and also on the need for transmission grid development. The results indicate that an investment starting within a few years on the Black Sea coastal area has economic potential for a large offshore wind power plant with additional costs for grid strengthening limited to 10% of the main investment. No strengthening investments may be required if the plant capacity is limited to 264 MW.