The EcoCity concept presented here has been designed in Finland and improved through collaboration with local partners and stakeholders to adapt to varying contexts while trying to provide solutions for the improvement of human settlements around the world, particularly in the Global South. Supported by specific methodologies and effective facilitation processes and skills, also developed by VTT Technical Research Centre of Finland Ltd. (VTT), it provides a structured yet flexible framework for conducting the complex dialogue leading to ecocity implementation, the importance of which is typically overlooked. This article discusses the theoretical underpinnings of the concept in relation to the general ecocity debate, as well as its main historical influences linked to the Finnish urban development tradition. The process thus enabled is illustrated by a Libyan case study.

In modern aircraft designs, following the More Electrical Aircraft (MEA) philosophy, there is a growing need for new high-power converters. In this context, innovative solutions to provide high efficiency and power density are required. This paper proposes an unregulated LLC full-bridge operating at resonant frequency to obtain a constant gain at all loads. The first harmonic approximation (FHA) model is not accurate enough to estimate the voltage gain in converters with high parasitic resistance. A modified FHA model is proposed for voltage gain analysis, and time-based models are used to calculate the instantaneous current required for the ZVS transition analysis. A method using charge instead of current is proposed and used for this ZVS analysis. Using this method, an auxiliary circuit is proposed to achieve complete ZVS within the whole load range, avoiding a gapped transformer design and increasing the efficiency and power density. A 28 Vdc output voltage prototype, with 10 kW pea... [more]

This study explores the influence of hydrostatic pressure on the discharge along the oil-paper interface under AC voltage, especially for the normal operating condition and breakdown. In this paper, an experimental platform was set up to record the partial discharge (PD) parameters of the test sample under different hydrostatic pressures, while the applied AC voltage was increased to final flashover voltage step by step. Experimental results showed that higher hydrostatic pressure had different effects on PD under different voltages. Higher pressure decreased the PD energy and increased the flashover voltage. Furthermore, under higher hydrostatic pressure, discharge traces (white mark) were found on the surface of the samples after intense discharging on the oil-paper interfaces, indicating that the hydrostatic pressure can affect the gas generation and dissipation process underneath the surface of the pressboards. Finally, the mechanism of how hydrostatic pressure influences the PD, f... [more]

The influx of a switching impulse during DC steady-state operations causes severe electrical stress on the insulation of HVDC cables. Thus, the insulation should be designed to withstand a superimposed switching impulse. All major manufacturers of DC cables perform superimposed switching impulse breakdown tests for prequalification. However, an experimental approach to study space charge dynamics in dielectrics under a switching impulse superposed on DC voltage has not been reported yet. This is because, unlike the DC stress, it is not possible to study the charge dynamics experimentally under complex stresses, such as switching impulse superposition. Hence, in order to predict and investigate the breakdown characteristics, it is necessary to obtain accurate electric field distribution considering space charge dynamics using a numerical approach. Therefore, in this paper, a numerical study on the switching impulse superposition was carried out. The space charge dynamics and its distrib... [more]

The multi-degree-of-freedom high-precision positioning system (MHPS) is one of the key technologies in many advanced industrial applications. In this paper, a novel hyperbolic magnetic field voice coil actuator using a rhombus magnet array (HMF-VCA) for MHPS is proposed. Benefiting from the especially designed rhombus magnet array, the proposed HMF-VCA has the advantage of excellent force uniformity, which makes it suitable for multi-degree-of-freedom high-precision positioning applications. First, the basic structure and operation principles of the HMF-VCA are presented. Second, the six-degree-of-freedom force and torque characteristic of the HMF-VCA is studied by three-dimensional finite element analysis (3-D FEA). Third, the influence of structural parameters on force density and force uniformity is investigated, which is conducive to the design and optimization of the HMF-VCA. Finally, a prototype is constructed, and the comparison between the HMF-VCA and conventional VCAs proves t... [more]

This editorial summarizes the collection of papers in the Special Issue entitled Photovoltaic System Design and Performance, which was published in MDPI’s Energies journal. Papers on this topic were submitted in 2017 and 2018, and a total of 21 papers were published. Main topics included data analysis for optimal performance and fault analysis, causes for energy loss, and design and integration issues. The papers in this Special Issue demonstrate the importance of designing and properly monitoring photovoltaic systems in the field in order to ensure maintaining good performance.

In this paper, the effect of oil injection flow rate on the performance of a variable speed twin-screw compressor was investigated experimentally. Comprehensive experiments were conducted at different compressor rotational speeds and oil injection flow rates. As the rotational speed increased from 1000 to 3000 rpm, the compressor volumetric and adiabatic efficiencies increased while the specific power consumption decreased. However, the oil injection flow rate showed different influences on the compressor performance. As the oil injection flow rate increased from 27 to 50 L/min, the adiabatic and volumetric efficiencies increased while the compressor discharge temperature and specific power decreased. Further increases in the oil injection flow rate had little effect on compressor performance at flow rates above 50 L/min. The analysis indicated that the oil injection flow rate was limited by the discharge temperature at low compressor rotational speeds and was a trade-off among two eff... [more]

The corona onset characteristic of bundle conductors is an important limiting factor for the design of UHV AC power lines in high-altitude areas. An experimental study on the corona characteristics of 8 × LGJ630, 6 × LGJ720, 8 × LGJ720 and 10 × LGJ720 bundle conductors commonly used in UHV power lines under dry and wet conductor conditions, as well as artificial moderate and heavy rain conditions, was conducted in Ping’an County, Xining City (elevation 2200 m). By using the tangent line method, the corona onset voltages and onset electric field of four types of conductors at high altitudes are obtained for the first time. In addition, the calculation model of corona onset voltage considering the outer strands’ effect on the electric field and the geometric factor in the corona cage in high altitude areas is established. The comparison of the calculation results and experimental results under dry conditions verifies the model’s correctness. Based on the results, an optimal selection sch... [more]

Massive Multiple-Input Multiple-Output (MIMO) alludes to the theory of having a large number of transmitter chains at the base station, which in turn provides the higher spectral and energy efficiency with reduced radiated power and greater simplicity in the signal processing. In this paper, we have improved the energy efficiency of Massive MIMO by considering the effects of nonlinear amplifiers in each transmitter branch. We have designed the system by calculating the optimal number of transmitters and receivers with the optimal transmitted power and their corresponding spectral efficiency in terms of energy efficient prospective of Massive MIMO under both the perfect and imperfect channel conditions at different power consumption and area of coverage. We have evaluated the impacts of nonlinear amplifiers by calculating the energy efficiency at different efficiencies and distortion losses of nonlinear power amplifiers. In order to solve the optimization problem of energy efficiency, w... [more]

The high dv/dt and di/dt outputs from power devices in a high-low voltage DC-DC converter on electric vehicles (EVs) can always introduce the unwanted conducted electromagnetic interference (EMI) emissions. A conducted EMI prediction and mitigation strategy that is based on transfer function for the high-low voltage DC-DC converter in EVs are proposed. A complete test for the DC-DC converter is conducted to obtain the conducted EMI from DC power cables in the frequency band of 150 kHz-108 MHz. The equivalent circuit with high-frequency parasitic parameters of the DC-DC converter is built`1 based on the measurement results to acquire the characteristics of the conducted EMI of the DC power cables. The common mode (CM) and differential mode (DM) propagation coupling paths are determined, and the corresponding transfer functions of the DM interference and CM interference are established. The simulation results of the conducted EMI can be obtained by software Matlab and Computer Simulation... [more]

This paper describes the introduction of three-dimension (3-D) blade designs into a 5-stage axial compressor with multi-stage computational fluid dynamic (CFD) methods. Prior to a redesign, a validation study is conducted for the overall performance and flow details based on full-scale test data, proving that the multi-stage CFD applied is a relatively reliable tool for the analysis of the follow-up redesign. Furthermore, at the near stall point, the aerodynamic analysis demonstrates that significant separation exists in the last stator, leading to the aerodynamic redesign, which is the focus of the last stator. Multi-stage CFD methods are applied throughout the three-dimensional redesign process for the last stator to explore their aerodynamic improvement potential. An unconventional asymmetric bow configuration incorporated with leading edge re-camber and re-solidity is employed to reduce the high loss region dominated by the mainstream. The final redesigned version produces a 13% in... [more]

The intimidating surge in the procurement of Distributed Energy Resources (DER) has increased the number of prosumers, creating a new possibility of local energy trading across the community. This project aims to formulate the peer-to-peer energy (P2P) sharing model to encourage the DERs to share surplus energy among the consumers. An effective pricing method is developed based on the supply-demand ratio (SDR) with the importance of self-optimization, which allows the prosumers to maximize their energy sharing and profits. To implement this pricing method, a simplified dynamic matchmaking algorithm has been deployed to introduce the Outstanding Prosumer to interact with existing consumers to increase the efficiency and profitability of the trade network. Consumers also benefit from this model, as they can pick the most economical energy supplier instead of relying on the utility grid. The prosumer with high excess energy and the consumer with the highest energy demand will be prioritiz... [more]

Numerical simulations were carried out to assess the technical and economic feasibility of a solar water desalination system that has a novel hybrid Concentrating Photovoltaic Thermal (CPVT) collector coupled with a Vacuum Membrane Distillation (VMD) process. A special characteristic of this CPVT is its triangular receiver with PV cells facing the reflecting surface. This type of receiver has the advantage of generating more electricity with less PV surface area and great potential to be used to hybridize conventional parabolic thermal collectors. TRNSYS was employed to analyze the annual performance of the CPVT-VMD system evaluating parameters such as solar fraction, specific permeate production and specific energy production for different coastal cities. In the dynamic simulations, local annual weather data and specific information about the characteristics and operating conditions of a real CPVT collector and a VMD module were considered. From the parametric analysis the optimal sur... [more]

As the demand for electricity continues to grow in Saudi Arabia, finding ways to increase power generation becomes increasingly important. However, conventional power generation methods such as burning fossil fuels contribute significantly to environmental pollution and harm human health through the emissions of greenhouse gases. One potential solution to this problem is the use of solar energy, which has the advantage of being abundant in Saudi Arabia due to its location in the sun belt. When compared to conventional power generation methods, solar energy is a viable alternative, particularly when the indirect costs of fossil fuels, such as harm to the environment and human health, are considered. Using photovoltaic cells to convert sunlight into electrical energy is a key method for producing clean energy. Despite the initial cost of investing in solar energy infrastructure, it is ultimately less expensive than electricity derived from fossil fuels. In recognition of the potential of... [more]

The thermal environment quality of office buildings has an important role because thermal comfort is directly related to human productivity. Thermal comfort conditions are influenced by climate, location, and the built environment; hence, comfort standards are required to assist building designers in creating a comfortable indoor environment for building occupants. In this context, the present study analyzes the adaptive thermal comfort studies conducted in office buildings from various countries. A large number of research articles selected from the Scopus database were considered for this study. Based on the analysis, outdoor climatic conditions have a greater influence on indoor thermal conditions in naturally ventilated than in air-conditioned office buildings. The temperature required for comfort is as low as 17.6 °C and as high as 31.2 °C in naturally ventilated buildings. An adaptive comfort equation for naturally ventilated and air-conditioned office buildings has also been pro... [more]

South African universities boast a remarkable solar photovoltaic (PV) resource as a primary renewable energy component. Due to high peak demand tariffs and inherent prominent heating and cooling loads, fast and granular demand response programs are well established within typical campus grids, with electrical networks adapted towards hosting centralized PV plants and emergency diesel generation. With unreliable utility supply and aging infrastructure comes a natural landscape and niche application for campus microgrids (MG) in South Africa. One such case, the University of the Free State’s QwaQwa satellite campus in the Phuthaditjhaba district, is no exception to this, as it has sufficient solar PV generation, but it also has an unreliable utility component. This paper investigates a possible MG for the UFS QwaQwa campus with an emphasis on Hybrid PV-Diesel dispatch strategies, specifically, to ensure uptime during the loss of grid supply and decrease fuel usage. The proposed centraliz... [more]

This article presents an analysis and methodology for designing resonant inverters with energy dosing for induction heating applications. These power topologies are characterized by the fact that the power consumption of the DC power source does not depend on the magnitude and changes of the load but is a function of the operating frequency, the value of the resonant capacitor and the DC supply voltage. Based on a description of the electromagnetic processes in the power circuit, analytical dependencies have been determined that describe the behavior of the studied power electronic devices. The expressions for the current of the AC circuit in the various stages of the converter’s operation are obtained, and on this basis an engineering methodology for design and prototyping is presented. The proposed methodology is verified through two specific numerical examples, simulation and experimental studies. In this way, the possibilities of these power electronic devices for self-adaptation t... [more]

The effective exploitation of renewable energy and the recovery of waste heat are two crucial strategies in achieving carbon neutrality. As an efficient and reliable heat−to−power conversion technology, the organic Rankine cycle (ORC) has been recognized and accepted by academia and industry for use in solar energy, geothermal energy, biomass energy, and waste heat applications. However, there remain unsolved technical challenges related to the design and operation of the components and system. As the exergy destruction and investment cost of heat exchangers exert significant influence on the performance of ORC, investigations on the performance improvement of heat exchangers are of great significance. The aim of this paper was to provide a review on the performance improvement of ORC in relation to heat transfer enhancement, heat exchanger design optimization, and cycle construction based on a novel heat exchanger. The performance of ORC using different types of heat exchangers was di... [more]

All the energetic management and controlling strategies in ambient air conditioning systems (ACS) are aimed to match design load to current needs. This might be achieved by determining a rational value of design thermal load without overestimation that can minimize its deviation from the actual values. The application of variable refrigerant flow (VRF) systems with speed-regulated compressors (SRC) is considered as the most advanced trend in building air conditioning due to the ability of SRCs to cover changeable heat loads without lowering their efficiency. The level of load regulation by SRC is evaluated as the ratio of the load range, regulated by SCR, to the overall design load range. With this, the range of actual changeable loads is usually supposed to be covered by SRC entirely while keeping the rest, unregulated, and load range unchangeable. However, to confirm this, the rest load range behind the regulated one should be investigated to estimate the efficiency of SRC operation.... [more]

The inductor−inductor−capacitor (LLC) resonant converter is a suitable topology for wide output voltage and load range applications with limited circuit parameters. One of the most significant design boundaries of an LLC resonant converter in high-voltage applications is the parasitic capacitance effect of the main circuit components, particularly the transformer and junction capacitances of the secondary rectifier network. Parasitic capacitance effects are much higher in high-voltage applications than in low-voltage applications. Therefore, the use of an LLC resonant converter is limited to high-voltage applications. This study proposes to reduce the capacitive effects of high-voltage transformers and rectification networks with a multi-winding transformer with an integrated rectifier design and to use it in high-voltage applications with the advantages of the LLC resonant converter. For the proposed prototype, comparative experimental measurements were conducted using a conventional... [more]

Axial-flow pumps, in addition to providing high anti-cavitation properties, must have high anti-erosion properties to ensure the required lifetime of the pump. Erosion damage of surfaces occurs when the net positive suction head (NPSH) significantly exceeds its critical value. The object of the study in this article is the axial-flow pump with a specific speed of 600 in two alternatives: № 1 and № 2. By analysis of the flow in the impeller blade systems, the ratio value between the NPSH, which ensures the absence of erosion, and the NPSH3, at which pump operational failure occurs, was determined. Impeller variant № 1 did not provide the required ratio. Impeller variant № 2 had higher cavitation qualities, and the required ratio was achieved for it. Energy, cavitation, and erosion characteristics of the axial-flow pump with impeller № 2 in rotational frequency n = 2000 rpm were investigated. Easily breakable paint coatings were used for the accelerated study of cavitation erosion. The e... [more]

The hydraulic system of a powered roof support performs two functions. The first function is to control the powered roof support in the extraction wall. The second function is to protect against adverse overloads resulting from rock mass pressing directly on the powered roof support. This damaging phenomenon is prevented by the protection of the powered roof support, with a safety valve built into the hydraulic system or directly into the prop. However, the third function proposed by the authors based on the research results is to minimize leaks. These leaks usually develop in the props or in the hydraulic system. The authors propose implementing changes to the hydraulic system for this purpose. The change consists of replacing the existing support block with a double block with charging. Tests were carried out in real conditions, that is, a mining wall. Tests in the mining wall were carried out on the powered roof support’s leaking prop. As a result of charging, the actual load capaci... [more]

Greenhouses extend growing seasons in upper latitudes to provide fresh, healthy food. Costs associated with carbon-emission-intensive natural gas heating, however, limit greenhouse applications and scaling. One approach to reducing greenhouse heating costs is electrification by using waste heat from cryptocurrency miners. To probe this potential, a new quasi-steady state thermal model is developed to simulate the thermal interaction between a greenhouse and the environment, thereby estimating the heating and cooling demands of the greenhouse. A cryptocurrency mining system was experimentally evaluated for heating potential. Using these experimental values, the new thermal model was applied to the waste heat of the three cryptocurrency mining systems (1, 50, and 408 miners) for optimally sized greenhouses in six locations in Canada and the U.S.: Alberta, Ontario, Quebec, California, Texas, and New York. A comprehensive parametric study was then used to analyze the effect of various para... [more]

This work presents a design for uninterruptible power supply inverters using Pareto front optimization for improved cost and efficiency. Three PWM modulation techniques applied to the full-bridge inverter are analyzed. As a result, the best MOSFET design solution in terms of the cost and efficiency of the inverter is evaluated based on a database with 47 power MOSFETs. Using the Pareto front, the optimal and sub-optimal solutions are compared, considering the three modulation techniques and the characteristics of MOSFETs manufactured for different voltage levels. Thermal and electrical measurements are used to validate the models.

A surface-mounted permanent magnet synchronous motor (SPMSM) is an electric motor with a simple magnetic circuit design, fast responsiveness, linear torque−current characteristics, speed−voltage characteristics, and constant operating speed. SPMSMs use only magnetic torque; however, interior PMSMs (IPMSMs) have high power densities because they can use reluctance torque. In addition, when flux-weakening control is used, the operating range is wide compared with the SPMSM. This study presents a comparative analysis of the characteristics of SPMSM and bar-type IPMSM. Characteristic analyses are performed by setting the same stator shape, rated speed, number of turns, winding specifications, voltage limit, and magnet usage in a pole/slot combination of six poles and 27 slots. Next, we compare the no-load back electromotive force, cogging torque, and loss characteristics, and perform a characteristic analysis of each model while satisfying the design specifications. No-load and load tests... [more]