Journal of Energy Engineering

Two new hybrid systems incorporating photovoltaic, thermoelectric, and solar collector components were designed and characterized under natural sunlight conditions. Two types of photovoltaic cells were used: monocrystalline silicon and polycrystalline silicon connected on the hot side of the thermoelectric generator. The two types of solar collectors used were connected on the cold side of the thermoelectric generator. The experimental setup is presented in this paper. The behavior of each component related to irradiance, temperature, and wind speed was studied. The performances of the hybrid systems were determined and compared. The best performance was observed for the hybrid system with the monocrystalline photovoltaic panel. The maximum power of the photovoltaic cells increased by more than 11% when they were integrated in a hybrid system, due to a decrease in temperature. The photovoltaic cells’ temperature decreased by 6°C for the solar radiation of $440\mathrm{W}/{\mathrm{m}}^2$, to 19°C for $\mathrm{1,000}\mathrm{W}/{\mathrm{m}}^2$. The time stabilization was determined at 7–8 min for both hybrid systems in clear sky conditions. The software for the system control, data acquisition, and determination of the components’ parameters was developed using a graphical language program.

In northern China, wind curtailment and environmental pollution from the widespread use of traditional heating methods cause huge economic losses. This paper proposes an emerging heating technology using wind power to alleviate these problems. First, an overall scheduling framework for wind-powered heating systems is introduced. The optimal operation of wind power generation based on the periodic heating demands of residents in rural areas is then presented based on a mathematical problem formulated as a mixed-integer nonlinear program. Moreover, an innovative cross-period deployment is developed considering the intermittent and variable nature of wind generation. Through the application of a genetic algorithm, the thermal requirements of residents can be continuously and sufficiently supplied by clean energy under the premise of satisfying system operation constraints. Finally, the coordinated optimization of this research applied to wind-powered heating systems in western Inner Mongolia demonstrates the effectiveness of the proposed dispatching approaches, thereafter being repeatedly verified by extensive case analyses.

With the development of distributed energy systems (DES), energy storage has attracted much attention in the research world. Reversible solid oxide fuel cells (RSOFCs) have shown potential as a promising means of electrical storage with high efficiency, especially for multienergy distributed systems coupling electricity and gas. RSOFCs have both a generation mode and a storage mode with a high fuel flexibility and in order to implement the frequent switch of generation and storage, a detailed dynamic behavior is needed in both the single mode and transitional process. However, few studies have been conducted investigating the complex dynamic processes of RSOFCs. In this study, a one-dimensional model is established to study the basic dynamic processes of RSOFCs, especially those involved in mode switching. The model is based on rich SOFC models and is first validated with experimental data and then employed to study the response of the primary characteristics of the cell, including current density, temperature, power density, and mole fraction of the voltage step input. The calculation results show that both the electrochemical and heat transfer greatly influence the transient process, but on different time scales. The reaction dynamics is another primary factor that determines the gas mole fraction. Furthermore, especially regarding the complex mode switching, three different modes are proposed and investigated to study the different switch processes and ultimately to provide a basic guide to control the RSOFC stacks.

An existing Romanian hydro system (two reservoirs/peak hydropower plants), where the addition of a pumped storage plant with a weekly emptying/filling cycle was planned, was selected as a case study. For the main upstream reservoir/hydropower plant, an operational policy with a monthly time step was derived using a stochastic dynamic programming model. The optimal weekly scheduling problem of the whole system was solved by a genetic algorithm-based model. The monetary income maximization, by 2014 market energy prices for produced/consumed energy, was chosen as the objective function; any gain from ancillary services was ignored. An original methodology for coupling the monthly and weekly time-step results was used to obtain the annual performance estimation within the mean hydrological year conditions. At that level, the inclusion of the pumped storage plant would lead to a gain to nearly double the annual performance of the existing hydro system.

This study presents an investigation concerning the conversion of solar radiation into heat, cold, and electricity on the facades of virtual buildings located in four different cities that correspond to different climatic conditions. Two of the cities are in Eastern Europe, Romania, one city is situated in Western Europe, Spain, and one city is on the western coast of the United States. Four different solar active applications integrated in the facades were investigated: heating based on flat solar collectors, cooling based on a LiBr-${\mathrm{H}}_2\mathrm{O}$ absorption cycle, photovoltaic conversion of solar radiation in electricity, and conversion of heat into electricity based on an organic Rankine cycle. The facades most suitable for investments in the solar active applications were recommended and it was found that both location and the type of application were influencing such recommendations. For both locations in Romania the facade facing west was recommended for all types of applications. For the location in Spain, for both heating and cooling, the first recommendation is the facade facing east and the second recommendation is the facade facing south. Although for electrical applications, the first recommendation is the facade facing south, and the second recommendation is the facade facing east. For the location in the United States, for heating and cooling, the first recommendation is the facade facing west, and the second recommendation is the facade facing south. Although, for electrical applications, the first recommendation is the facade facing south, and the second recommendation is the facade facing west.

In this study, the spray and atomization characteristics of neat biodiesel, ethanol-biodiesel blends (BDEs), and neat diesel in a high-pressure common-rail injection system were investigated experimentally and analytically. Schlieren-type high-speed photography was utilized to capture the experimental images. The findings indicated that spray penetration and peak tip velocity decreased and the spray cone angle increased with increase in the ratio of ethanol in the blends. Two predictive models were used to generate data to compare with measurements of spray penetration and peak tip velocity. The empirical formula of equivalence ratio, Ohnesorge number, and Sauter mean diameter (SMD) were calculated to evaluate atomization levels.

This paper first proposes the concept of using subcritical water as a thermal energy storage (TES) material. Subcritical water has a maximum TES temperature of 330°C and an internal energy storage density as high as $977.75\mathrm{MJ}/{\mathrm{m}}^3$, which is very suitable for industrial waste heat recovery. This paper investigates the TES characteristics of subcritical water in the heat preservation process. An experimental apparatus was set up, and experiments at different initial temperatures were performed. The experiments show that subcritical water has greater TES capacity than normal water because of its high section energy density. The water tank can be divided into three regions according to the section energy density. The section energy density rapidly increases in the lower head region, slowly increases in the transition region and remains stable in the isothermal region. The section energy density is distributed according to the tank shape and temperature distribution. When the initial temperature increases, the changing rate of the section energy density along the axial direction rapidly increases in the lower head region, slowly increases in the transition region and does not change in the isothermal region. The section exergy density has similar characteristics to the section energy density.

In this paper, a molten carbonate fuel cell (MCFC)–heat engine hybrid system is investigated in terms of the environment and sustainability. The MCFC is a premier power generator, and the high-temperature waste heat supplied by the fuel cell is used as a heat source at an irreversible heat engine. This irreversible heat engine is the secondary power generator of the system. The thermo-environmental function and exergetic sustainability index are applied to design an environmentally friendly MCFC–heat engine hybrid system. Results are obtained numerically, and an ecologically friendly operation range is attempted to determine the considered system. According to calculations, the maximum power density of the hybrid system is $\mathrm{4,242.62}{\mathrm{Wm}}^{-2}$, and the maximum thermo-environmental function density and the exergetic sustainable index are equal to $0.0309{\mathrm{Wmpts}}^{-1}{\mathrm{m}}^2$ and 107.36, respectively. Finally, it is recommended that current density should be chosen between current density at the maximum exergetic sustainable index and current density at the maximum thermo-environmental function for a less environmentally harmful system.

A numerical analysis of the effect of spark plug geometry on the cyclic combustion variability and fuel consumption is presented. Cycle simulations are performed at different operating conditions of a spark-ignition engine including changes of engine speed and load. After calibration of the model constants for an average cycle using the experimental results of the considered engine, the cyclic combustion variability over 1,500 cycles with the application of a classic J-gap spark plug is simulated. With the same values of calibration constants and perturbations of parameters that cause the cyclic combustion variability, cycle simulations are performed with a fine central (iridium) electrode spark plug. The cycle simulation results for the evaluation of combustion stability and fuel consumption are compared with the previous simulation results achieved with the classic spark plug geometry. The application of the iridium spark plug electrode improves the combustion stability over the analyzed operating range of the engine, with a maximum improvement of 13.5% at part load and low engine speed. A reduction of fuel consumption of approximately 1.25% is achieved at part load conditions. The simulation results confirm that the application of the iridium spark plug geometry has potential to improve engine stability and fuel economy.

During China’s critical transition toward smart grids, great attention has been devoted to demand response programs and energy storage technologies. Vanadium redox flow batteries (VRBs) are regarded as the most promising storage technology owing to their flexible energy and power capacity configurations. However, the lack of a deep understanding of the economic viability and VRB flow-rate control strategies remains a major barrier to the expansion of VRBs. To investigate the economic viability of VRB-based load shifting under the time-of-use tariffs in China, this paper (1) optimizes VRBs’ energy and power capacity to minimize the total cost (electricity bills plus levelized storage cost) for a university campus in Hangzhou, China; (2) proposes a dynamic VRB (dis)charge and electrolyte flow rate control strategy by considering both economics and system efficiency of VRB-based systems; and (3) evaluates the cost-effectiveness of VRBs for load shifting under various storage lifetimes. Simulation results show that the proposed strategy could increase the VRB system efficiency by approximately 3%, limit the electrolyte temperature increment within 3.08 K, and be cost-effective in the near future.

The energy used in the built environment represents approximately 40% of the total energy consumed by the community, with significantly higher values in rural communities. Out of this, a significant share is used for heating, cooling, and domestic hot water (DHW) production. Therefore, identifying renewables-based thermal energy production systems represents an essential step in the transition toward the status of sustainable communities; however, their wide spread has to be supported by efficiency and a high social and economic acceptance. The use of renewable energies in collective households is currently facing certain technological and financial barriers; therefore, acceptable and affordable implementation needs to be stepwise designed by (partially) involving the already existing infrastructure. Technical and economic feasibility is supported by accurate design that considers the local variability of the renewables’ potential and consumption. In a first step, renewables-based DHW production can be considered because this is relatively constant throughout the year, and solar–thermal systems represent good candidates because there already exists a high degree of acceptance at the community level for this type of system. This paper presents a novel methodology for the design of solar–thermal systems implemented in rather small communities, considering the actual energy consumption for DHW production in collective households (based on the local solar energy potential; the methodology targets the identification and use or extension of energy production and distribution systems). A case study (Taberei District in Odorheiu Secuiesc City, Harghita County, Romania) is detailed, allowing technical analysis and feasibility assessment and also considering affordability and sustainability.

As the fastest growing type of renewable generation, wind power integration has been widely studied to address both the environmental and the energy concerns. The intermittent and stochastic features of wind power, however, cause remarkable uncertainty in operations, resulting in high complexity in system state analysis. It is desirable to evaluate the system conditions as precisely and efficiently as possible. To handle this problem, this paper proposes a novel probabilistic load flow approach by combining randomized quasi–Monte Carlo (RQMC) sampling with Johnson transformation to achieve satisfying accuracy within a low time consumption. For efficient and sufficient sampling, the low discrepancy sequence is scrambled in a fully random strategy, forming the RQMC sampling approach. Furthermore, considering the distribution characteristics and correlation features of wind energy, the Johnson translation system is introduced. Tests on the wind-integrated IEEE 118-bus system and the French high-voltage transmission network show that the proposed approach is able to achieve satisfactory accuracy and efficiency. Different wind profile models, including Weibull distribution and the historical measurements–based probability density functions, can be precisely handled.

This study conducts experimental and numerical investigations on the thermal storage characteristics of the vertical cylindrical water tank in the heat preservation process. A new experimental rig is established under the laminar condition, and the numerical model is developed for the detailed simulation of flow and heat transfer within the water tank. The numerical results agree well with the experimental data. According to the characteristics of flow and heat transfer, the space in the water tank can be divided into the vertical boundary layer, the bottom static region, the middle circulation region, and the top eddy region. Heat dissipation has decisive effect on the flow and temperature distribution pattern in the water tank. Heat transfer increases sharply at the corner region of the tank wall, but it keeps stable at the rest part of the walls. The thermal storage efficiency is positively proportional to the thermal conductivity and inversely proportional to the insulation layer thickness, while the specific heat capacity and the density have little influence on the thermal storage efficiency.

Flue gas treatment systems have been widely installed in power plants to recover the remaining energy from the exhaust flue gas and to address pollutant emissions by heating the cleaned flue gas. In this study, three modes of flue gas treatment are compared by economic analysis in which the capital cost of the added equipment is taken into account. In addition, the influence of the flue gas treatment system on the operational pressure of the condenser is also analyzed. The results show that flue gas treatment systems can heat the cleaned flue gas effectively. Treatment system with feedwater-gas and steam-gas heaters can obtain remarkable economic benefit and have good adaptability to different operational parameters. In addition, the capital cost of the added heat exchangers is acceptable. Under 100% turbine heat acceptance conditions, the relative variation ratio of the thermal efficiency could be increased by 0.33%, whereas it can be decreased by 0.18% when the outlet temperature of the cleaned flue gas increases by 8°C.