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This study deals with the core design of a PM vernier machine considering modulation flux effects, and the comparative investigation on volume and performance characteristics of the vernier over conventional PM machines are addressed. To these ends, for a PM vernier machine in operation at the base-speed, the flux density equations for teeth and yokes considering the flux modulation effects are derived, where the air gap harmonic permeance function is used. Using the derived equations, a PM vernier motor with specified yoke flux densities is designed. To identify the predicted flux yoke densities, the flux distribution and iron losses in core parts are analyzed through time-step finite element (FE) simulations. Through Fourier series expansion of the air gap flux waves obtained by FE analysis at several specified times, the harmonic components constituting the flux waves are investigated and their speeds are also evaluated in numerical ways. Finally, to estimate the competitiveness of vernier machines versus conventional machines, the designed PM vernier motor is compared against two different conventional PM motors designed through the same design procedures in various aspects such as volume, torque capacity, efficiency, and power factor, in which, in particular, the core losses are included in efficiency calculation.

An optimal design method is proposed in this paper to improve the safety and price competitiveness of floating photovoltaic (PV) systems. From the standards for grounding by the International Electrotechnical Commission (IEC) 60364, the Electrical Equipment Technology Standards (EETS) are set up for the grounding resistance to be less than or equal to 10 Ω for high voltage (above 750 V DC) and extra high voltage (above 7000 V) systems. In order to satisfy this criterion, a parallel connection of grounding electrodes is essential in the system. Furthermore, inter-electrode interference should be considered to reflect the resistance increase due to the potential increase between electrodes. Therefore, in this study, the parallel grounding resistance according to the distance and number of electrodes, as well as the arrangement method were theoretically predicted and compared with the measured values. For the first time, the design of grounding electrodes has been applied to real floating PV systems and is expected to satisfy EETS.

In this paper, we present the results of an economic feasibility study and propose a system structure to test and maintain electrical stability. In addition, we present real operation results after constructing a remote microgrid on an island in South Korea. To perform the economic feasibility study, a commercial tool called HOMER was used. The developed remote microgrid consists of a 400 kW wind turbine (WT) generator, 314 kW photovoltaic (PV) generator, 500 kVA × 2 grid forming inverter, 3 MWh lithium ion battery, and an energy management system (EMS). The predicted renewable energy fraction was 91% and real operation result was 82%. The frequency maintaining rate of the diesel power plants was 57% but the remote microgrid was 100%. To improve the operating efficiency of the remote microgrid, we investigated the output range of a diesel generator.

This paper proposes a DC-bus signaling (DBS) method for autonomous power management in a DC microgrid, used to improve its reliability. Centralized power management systems require communication between the power sources and loads. However, the DBS method operates based on the common DC-bus voltage and does not require communication. Based on the DC-bus voltage band, the DC-bus voltage can be used to inform the status of the DC-bus in various scenarios. The DC microgrid operates independently to maintain the system stably in the DC-bus voltage band. The DC microgrid can be divided into a grid-connected mode and an islanding mode. This paper proposes a control strategy based on power management of various independent components in islanding mode. In addition, the autonomous control method for switching the converter’s operation between grid-connected mode and islanding mode is proposed. A DC microgrid test bed consisting of a grid-connected AC/DC converter, a bidirectional DC/DC converter, a renewable energy simulator, DC home appliances and a DC-bus protector is used to test the proposed control strategy. The proposed autonomous control strategy is experimentally verified using the DC microgrid test bed.

Recently, there have been frequent fluctuations in the wholesale prices of electricity following the increased penetration of renewable energy sources. Therefore, retailers face price risks caused by differences between wholesale prices and retail rates. As a hedging against price risk, retailers can utilize critical peak pricing (CPP) in a price-based program. This study proposes a novel multi-stage stochastic programming (MSSP) model for a retailer with self-generation photovoltaic facility to optimize both its bidding strategy and the CPP operation, in the face of several uncertainties. Using MSSP, decisions can be determined sequentially with realization of the uncertainties over time. Furthermore, to ensure a global optimum, a mixed integer non-linear programming is transformed into mixed integer linear programming through three linearization steps. In a numerical simulation, the effectiveness of the proposed MSSP model is compared with that of a mean-value deterministic model based on a rolling horizon method. We also investigate the optimal strategy of a retailer by changing various input parameters and perform a sensitivity analysis to assess the impacts of different uncertain parameters on the retailer’s profit. Finally, the effect of the energy storage system on the proposed optimization problem is investigated.

Various problems often arise in high-rise buildings during the winter months due to the stack effect. In this study, the high-rise building of interest, located in South Korea, was experiencing constant loud noises in the winter due to the stack effect. Thus, we created a noise level reduction plan by creating a method for pressurizing the high-rise zones of the building according to outdoor conditions. To discover the appropriate pressurization operating modes, we applied a two-year commissioning process to the 50-story building of interest. The 1st- and 47th-floor elevator halls were identified to have the highest noise levels of all other floors. Prior to applying the reduction plan, the maximum noise level on the first floor with the HVAC system turned off was 85 dB(A) and with the HVAC system turned on it was 70 dB(A). Both values exceeded the criteria of 57 dB(A) for a lobby space of a commercial building. In the case of the 47th floor, the maximum noise level with the HVAC system turned off was 58.7 dB(A) and with the HVAC system turned off was 56.0 dB(A), despite the latter having increased airtightness performance and applying preliminary pressurization (i.e., HVAC operation mode 2). These values exceeded the criteria of 48 dB(A) for an elevator hall in a commercial building. Following this initial data, we determined to pressurize the high/mid-rise zones of the building according to the outdoor air temperature and wind velocity conditions, which we categorized into four types (i.e., HVAC operation mode 4). To this effect, the first-floor elevator hall’s maximum noise level was 56.6 dB(A), meeting the criteria, and the 47th-floor elevator hall’s maximum noise level was 49.5 dB(A), still exceeding the criteria but by an insignificant amount. Although the HVAC pressurization operation we utilized resulted in favorable results for the target building A, it may not be as effective in other new high-rise buildings, creating changes to the indoor air environment or to the energy costs in maintaining a building. However, for the purposes of resolving the stack effect, we believe that the commissioning process we took to optimize the HVAC operation that is presented here can be applied to other new and existing high-rise commercial buildings.

Heated glass can be applied to improve windows’ condensation resistance and indoor thermal comfort in buildings. Although this applied technology has advantages, there are still some concerns in practical applications, such as additional energy consumption and control issues. This study evaluates the effectiveness of a heated window heating (HWH) system in terms of thermal comfort and heating energy performance (HEP). The simulation-based analysis is performed to evaluate the effectiveness of the HWH using a residential building model and to compare it with radiant floor heating (RFH) and hybrid heating (HH) systems (i.e., combined HWH and RFH). This study also investigates the peak and cumulative heating loads using HWH systems with various scenarios of control methods and setpoint temperature. The predicted mean vote (PMV) is used as an indoor thermal comfort index. The ratio of cumulative thermal comfort time to the entire heating period is calculated. The results show that HWH and HH can reduce the heating load by up to 65.60% and 50.95%, respectively, compared to RFH. In addition, the times of thermal comfort can be increased by 12.55% and 6.98% with HWH and HH, respectively. However, considering the social practices of South Korea, HH is more suitable than HWH. Further investigations for HH show that a surface setpoint of 26 °C is proper, considering both heating demands and thermal comfort. In addition, the setpoint temperature should be determined considering HEP and the thermal comfort for HWH, and the optimal setpoint temperature was suggested under specific conditions.

This paper presents the reliability evaluation for a microgrid composed of conventional generators (CGs), a photovoltaic (PV) system, and a small hydropower plant (HPP). The PV system has an intermittent output and the HPP output is limited depending on several constraints. Therefore, their reliability evaluation should be performed according to the CGs. This paper proposes a methodology to evaluate the reliability of the microgrid based on Monte Carlo simulations. To this end, a simple operational strategy for the HPP and reliability models for each component of the grid are introduced. By the operational strategy consisting of two stages, the HPP would be operated to generate power whenever water stored in the reservoir exceeds a certain threshold (called volume reference) or to discharge extra water when it is beyond the maximum volume of the reservoir. To demonstrate the proposed methodology, a case study is performed according to various volume references, and the results are obtained in terms of several reliability indices. Further, the appropriate volume reference is determined considering a trade-off relationship between the reliability and economics aspects.