Avoiding heat stress in cows is an important condition for animal productivity and the maintaining of animal health. For this, it is necessary to provide an optimal microclimate in cowsheds using systems of air cooling. The paper analyzes one of these systems—an air humidification−cooling system. The research was carried out in a semi-insulated box-type cowshed containing 244 places. The changes in temperature, relative humidity, and temperature humidity index (THI) were studied for the air coming from outside and for the air inside the cowshed. Considering the fact that the cows were in the cowshed most of the time (51.5%) under heat stress, the use of a cooling system is appropriate. It was established that a cooling system is capable of compensating for heat released by animals. It was determined that with an increase in air temperature the relative efficiency of a cooling system increases. An intensive constant air exchange provided using fan operation avoids an excessive growth of... [more]

The optimal management of multiple chiller systems calls for the solution of the so-called optimal chiller loading (OCL) problem. Due to the interplay of continuous and logical constraints, OCL is an NP-hard problem, so that a variety of heuristic algorithms have been proposed in the literature. Herein, an algorithm for its exact solution, named X-OCL, is developed under the assumption that the chillers’ power consumption curves are quadratic. The proposed method hinges on a decomposition of the solution space so that the overall OCL problem is decomposed to a set of equality constrained quadratic programming problems that can be solved in closed form. By applying the new X-OCL solver to well known case studies, we assess and compare the performances of several literature algorithms, highlighting also some errors in the published results. Moreover, X-OCL is used to design a greedy optimal chiller sequencing (OCS) solver, called X-OCS. The X-OCS is tested on two literature benchmarks an... [more]

The operating limits of oscillating heat pipes (OHP) are crucial for the optimal design of cooling systems. In particular, the dryout limit is a key factor in optimizing the functionality of an OHP. As shown in previous studies, experimental approaches to determine the dryout limit lead to contradictory results. This work proposes a compact theory to predict a dryout threshold that unifies the experimental and analytical data. The theory is based on the influence of vapor quality on the flow pattern. When the vapor quality exceeds a certain limit (x = 0.006), the flow pattern changes from slug flow to annular flow and the heat transfer decreases abruptly. The results indicate a uniform threshold value, which has been validated experimentally and by the literature. With that approach, it becomes possible to design an OHP with an optimized filling ratio and, hence, substantially improve its cooling abilities.

A review of the ongoing research on black start (BS) service integrated with offshore wind farms (OWFs) is presented in this paper. The overall goal is to firstly gain a better understanding of the BS capabilities required by modern power systems. Subsequently, the challenges faced by OWFs as novel BS service providers as well as an outlook on the ongoing research which may provide solutions to these are presented. OWFs have the potential to be a fast and environmentally friendly technology to provide BS services for power system restoration and, therefore, to ensure resiliency after blackouts. As a power electronic-based system, OWFs can be equipped with a self-starter in the system in order to perform BS. The self-start unit could be a synchronous generator (SG) or a power electronic unit such as a grid-forming (GFM) converter. Preliminary BS studies performed in PSCAD/EMTDC are presented in a simplified OWF system via an SG as the self-start unit. Consequently, technical challenges... [more]

This paper introduces an optimal sizing method for a catenary-free tram, in which both on-board energy storage systems and charging infrastructures are considered. To quantitatively analyze the trade-off between available charging time and economic operation, a daily cost function containing a whole life-time cost of energy storage and an expense of energy supplies is formulated for the optimal sizing problem. A mixed particle swarm optimization algorithm is utilized to find optimal solutions for three schemes: (1) ultracapacitors storage systems with fast-charging at each station; (2) battery storage systems with slow-charging at starting and final stations; (3) battery storage systems with fast-swapping at swapping station. A case study on an existing catenary-free tramline in China is applied to verify the effectiveness of the proposed method. Results show that a daily-cost reduction over 30% and a weight reduction over 40% can be achieved by scheme 2, and a cost saving of 34.23% an... [more]

One of the most important concerns in the planning and operation of an electric power generation system is the effective scheduling of all power generation facilities to meet growing power demand. Economic load dispatch (ELD) is a phenomenon where an optimal combination of power generating units is selected in such a way as to minimize the total fuel cost while satisfying the load demand, subject to operational constraints. Different numerical and metaheuristic optimization techniques have gained prominent importance and are widely used to solve the nonlinear problem. Although metaheuristic techniques have a good convergence rate than numerical techniques, however, their implementation seems difficult in the presence of nonlinear and dynamic parameters. This work is devoted to solving the ELD problem with the integration of variable energy resources using a modified directional bat algorithm (dBA). Then the proposed technique is validated via different realistic test cases consisting o... [more]

This study proposes to use gas-fired boilers as peak shaving heat sources in heating substations due to their capability to increase the reliability, flexibility and heat capacity without the need to change the district heating network (DHN). However, the design and operational requirements with different connection modes for this kind of DH system are still not clear. This paper presents a systematic study on this kind of DH system, analyzes the connection modes of series and parallel connections between the gas-fired boilers and the heat exchangers. For each connection mode, we figured out the thermal balances and obtained the design and operational parameters including the supply temperatures of the heat exchangers, gas-fired boilers and their variations under different network temperature levels and the base load ratios (β). Under the series connection mode, the design supply temperature of the heat exchangers has no relation with the design peak shaving flow ratio (ω′); it decreas... [more]

Helium is the second most abundant element in the Universe after hydrogen. Considerable resources of helium-3 isotope (He3) are located mostly outside the Earth. He3 is very important for science and industry, especially for airport neutron detectors, lung tomography and helium dilution refrigerators. Besides, global warming is forcing the industry and governments to search for alternative energy sources, and He3 has the potential to be used as fuel in future nuclear fusion power plants. Unfortunately, the price of gaseous He3 has recently increased from $200 per liter to over $2750. The expected further increase in price and demands led us to present an analysis of the economic profitability for He3 separation process, which utilizes the properties of superfluid helium. This paper shows the arguments supporting the idea that extraction from natural sources is the only economically viable way of obtaining He3 isotope nowadays. The method could be relatively easily implemented into the... [more]

Interleaved zero-voltage switching boost converters have been known for over 20 years. However, the influence of parasitic capacitance of transistors has not been described. In this paper the converter is analyzed and the equation for voltage ratio is derived for any number of converter phases. First a general description of the topology, including main voltage and current waveforms, is provided. Subsequently the converter is analyzed and conditions for soft switching are derived. Next the analysis results are compared to measurements of an experimental converter setup. Discrepancies are identified and the influence of parasitic capacitance is analyzed. By considering the parasitic capacitance the difference between experimental and analytical results of voltage ratio is reduced from 0.2 to less than 0.05.

The regional allocation of carbon emission quotas is of great significance to realize the carbon emission target. Basing on the combination of the multi-index method and the improved equal-proportion distribution method, and fully considering the differences in economic factors, population factors, energy factors, technological factors among cities, China’s 2030 carbon intensity reduction target was allocated. The results indicate that: (1) Under the target constraint of 60% reduction in CO2 emissions per unit of Gross Domestic Product (GDP) (carbon intensity) in 2030 compared to 2005, the carbon intensity target reduction rate (CITRR) of 285 Chinese cities is between 17.65% and 141.14%, with an average reduction rate of 51.52%; (2) the CITRR of cities presents significant spatial positive correlation, and the Global Moran I correlation index is 0.38; and (3) the distribution trend of CITRR is the same as the general trend of economic development of China, showing a basic trend of grad... [more]

Bus networks in many capital cities are long distances and partially overlapping with each other. As a result, waiting time is high and energy consumption efficiency is poor. Bus operators in many countries tried to reform their bus routes to reduce waiting time and fuel costs by reducing overlapping bus routes. However, most of the reformed bus routes were complicated, which caused discomfort to passengers to use the bus service. To overcome this problem, this study proposed a new bus reformed model called the Partial Bus Routes Reduction in Urban Fringe Model (PBRU) which was a simple and passenger-friendly route operation. It resulted in 14% of total inbound and 16% of total outbound passengers receiving the benefit of waiting time reduction. Most passengers wait twice at the resident bus-stop and transfer point. As a result, the overall waiting time increased by 0.72−3.75 min. The reduction of fuel consumption was consistent with increasing load factors and dependent on the time pe... [more]

This paper gives a comprehensive approach to the emulation of photovoltaic (PV) plants made of different module technologies as well as varying peak power through the advanced fast PV power emulation technique. Even though PVs are recognized as a technology for CO2 emissions mitigation, the proposed emulation technique provides the opportunity to replicate PV plant operation without a carbon footprint because of its working principle. The process of PV power plant emulation consists of several stages which are described in detail. An algorithm for determining PV power plant configuration based on the technical characteristics of the PV emulation system equipment is developed and presented, as well as an algorithm for preparing data on the current−voltage (i−v) characteristics used as input data into programmable sources that mimic the power plant PV array. A case study of a single day operation of PV power plants made of two different topologies and technologies was carried out with th... [more]

Solar photovoltaic (PV) power generation has strong intermittency and volatility due to its high dependence on solar radiation and other meteorological factors. Therefore, the negative impact of grid-connected PV on power systems has become one of the constraints in the development of large scale PV systems. Accurate forecasting of solar power generation and flexible planning and operational measures are of great significance to ensure safe, stable, and economical operation of a system with high penetration of solar generation at transmission and distribution levels. In this paper, studies on the following aspects are reviewed: (1) this paper comprehensively expounds the research on forecasting techniques of PV power generation output. (2) In view of the new challenge brought by the integration of high proportion solar generation to the frequency stability of power grid, this paper analyzes the mechanisms of influence between them and introduces the current technical route of PV power... [more]

In this paper, the interaction between the energy storage (ES) power distribution and system efficiency enhancement is researched based on the energy stored quasi-Z-source inverter. The corresponding current counteraction, stress reduction, power loss profile, and efficiency enhancement around the embedded energy storage units are studied in details. Firstly, the current counteraction effect on the device current is presented with the embedded ES source. The corresponding reduction in the device current stress is revealed. Then, the detailed device power loss expressions with current redistribution in the impedance network are explored mathematically. A quasi-inverted-trapezoidal power loss profile is found with the embedded source power distribution. To further increase the overall system efficiency, an optimal energy efficiency tracking strategy is proposed for the ES-qZSI (energy-stored quasi-Z-source inverter) based on the power distribution control. Both the simulation and the exp... [more]

The increasing installation numbers of ventilation units in residential buildings are driven by legal objectives to improve their energy efficiency. The dimensioning of a ventilation system for nearly zero energy buildings is usually based on the air flow rate desired by the clients or requested by technical regulations. However, this does not necessarily lead to a system actually able to renew the air volume of the living space effectively. In recent years decentralised systems with an alternating operation mode and fairly good energy efficiencies entered the market and following question was raised: “Does this operation mode allow an efficient air renewal?” This question can be answered experimentally by performing a tracer gas analysis. In the presented study, a total of 15 preliminary tests are carried out in a climatic chamber representing a single room equipped with two push-pull devices. The tests include summer, winter and isothermal supply air conditions since this parameter v... [more]

This paper proposes a current sensor for wide bandgap (WBG) devices. The current of a power semiconductor device is mainly measured using a Rogowski coil, an active current transformer, and a coaxial shunt resistor. The Rogowski coil and active current transformer are not suitable for the current measurement of surface mount WBG devices due to their operating principle. The coaxial shunt resistor causes parasitic inductance. Since WBG devices are more sensitive to parasitic inductance than silicon devices, parasitic inductance may affect circuit operation. To overcome these problems, this paper proposes a printed circuit board (PCB)-embedded spiral pattern pick-up coil current measurement for WBG devices. The proposed pick-up coil has high mutual inductance compared to the conventional pick-up coil, so the measurement sensitivity is high. In addition, there is no need for additional processing outside the PCB. Experimental results using a double pulse tester circuit are provided to ver... [more]

One of the main issues that affect the development of high-voltage direct-current (HVDC) cable insulation is the accumulation of space charge. The load operation of an HVDC cable leads to the formation of a radially distributed temperature gradient (TG) across the insulation. In this study, the space charge accumulation in a cross-linked polyethylene (XLPE) cable is measured under a DC electric field and TG using the pulsed electro-acoustic (PEA) method, and the effect of the TG on the space charge behavior is investigated. In addition, the bipolar charge transport (BCT) model and the conductivity model based on an improved cylindrical geometry are used to simulate the charge behavior in the HVDC XLPE cable under TG, and the experimental and simulated results are compared. The results show that the higher temperature of the cable conductor promotes the accumulation of homocharge near the side of high temperature. Additionally, with the increase of the TG, not only does more heterocharg... [more]

The suspension electromagnet (SEM) is one of the most critical components of the low-speed maglev train to achieve the stable suspension and non-contact operation with the track. Therefore, it is valuable for optimizing the maglev system to study the characteristics of the SEM. This paper proposes a novel analytic method to calculate the equivalent stray capacitance of the SEM efficiently. It considers the capacitances inside the winding (turn to turn and layer to layer) and between the winding and core. Firstly, utilizing the compensation analytic method (CAM) calculates the static capacitances to reduce the calculation error, and the results of the CAM have significant improvement comparing with that of the traditional analytic method (TAM). Secondly, the analytical formula of the core floating potential is derived based on the partial capacitance theory, which has a 2.1% relative error to the finite element method (FEM). Finally, the experiment results of the SEM of a test rig prove... [more]

The purpose of this article is to present the key differences between the Polish Guaranteed DSR (demand-side response) Scheme operating from 2017 to 2020 and the Polish Capacity Market DSR Scheme operating from 2021, and their impacts on the decision to participate in the DSR scheme. The present study attempts to compare the rules of the old and new schemes and their impacts on the participation decision depending on the industry of the potential scheme participant. The study was conducted on a group of 50 randomly selected companies. A structured interview was used to collect information, and a chi-quadrant independence test and a Wilcoxon−Mann−Whitney test were used in the analysis of the collected data. The study was conducted in selected enterprises located in Poland. The results of the study indicate the elements that are similar to the literature on the subject, and key aspects arising from the unique characteristics of the Polish market and the industry of the potential particip... [more]

Energy infrastructure in large, multi-site organisations such as municipal authorities, is often heterogeneous in terms of factors such as age and complexity of the technology deployed. Responsibility for day-to-day operation and maintenance of this infrastructure is typically dispersed across large numbers of individuals and impacts on even larger numbers of building users. Yet, the diverse population of stakeholders with an interest in the operation and development of this dynamic infrastructure typically have little or no visibility of energy and water usage. This paper explores the integration of utility metering data into urban management processes via the deployment of an accessible “smart meter” monitoring system. The system is deployed in three public authorities and the impact of the system is investigated based on the triangulation of evidence from semi-structured interviews and case studies. The research is framed from three perspectives: the bottom-up micro-level (individua... [more]

In central Italy, there are extensive European black pine (Pinus nigra Arn.) plantations which range from 30 to 60 years of age and where no thinning operations have been made. The main purpose of this study was to provide a comparative analysis of two cable yarder technologies (Maxwald, mobile pulley carriage and Savall, semi-automatic carriage), in terms of fuelwood production and cost, from the first thinning of a 33 year old plantation in slope areas of these plantations. The results showed that fuelwood production was cost-effective in both systems (Savall by 15.1 and Maxwald by 14.8 € m−3), although the productivity of the Savall system was higher than the Maxwald system (6.1 vs. 5.7 m3 h−1). The respect amounts of productivity have the potential to increase by 27% for the Savall yarder and 25% for the Maxwald yarder upon condition that the delay times are reduced to minimum level by proper training of workers, by a better organization, and planning of operations. The total effec... [more]

The aim of this work is to analyze and stabilize the power system when connecting an energy storage system (ESS) to replace the traditional power reserve of a power plant. Thus, it is necessary to validate and simulate the power facility protection system using a relay coordination approach. The input feasibility of the generator for the frequency regulation (FR) of the operational ESS is also validated through detailed analysis studies including power flow, short circuit and relay coordination analysis. The case scenarios for ESS installation are categorized based on its operation mode and location in the power system. These studies are carried out on the power system at the peak load condition specified for both grids. With the electrical transient analyzer program (ETAP), an analysis is performed to study the implementation of the ESS in a large, integrated power system to determine which location best fits the installation of ESS considering the load flow, short circuit and relay c... [more]

A comparative evaluation of economic efficiency was performed for native polyculture microalgae oil production in an oxidation ditch (OD) process wastewater treatment plant (WWTP). A cost function was developed for the process. The operational cost per 1 m3 of wastewater (w.w.) was 1.34 $/m3-w.w. in the existing scenario, 1.29 $/m3-w.w. in algal scenario A (no cost for CO2 and waste heat) and 1.36 $/m3-w.w. in algal scenario B (no cost for CO2). The conditions were set as follows: hydraulic retention time (HRT): 4 days, microalgal productivity: 0.148 g/L and daily treatment volume: 81.6 m3-w.w./d. The cost differences were related to the increase in polymer flocculants for algae separation (+0.23 $/m3-w.w), carbon credits from CO2 absorption (−0.01 $/m3-w.w), the sales of biocrude (−0.04 $/m3-w.w) and sludge disposal (−0.18 $/m3-w.w). Hence, the introduction of the algae scenario was the same cost-effective as the existing scenario. Microalgae oil production in an OD process WWTP can s... [more]

Cable lines are one of the basic components of power systems. Medium and high voltage cables mainly comprise a metallic sheath, which is concentric to the main core conductor. There are several operating schemes of such cable lines, which differ in the place of earthing of sheaths and the possible use of the sheaths and/or conductors crossing. The sheaths cross-bonding is typically done in two places of one cable line section, and it allows to reduce power losses. Nevertheless, the use of incomplete sheaths crossing—only in one place on cable route may have economic justification. The paper presents an incomplete sheaths cross-bonding analysis of an existing medium voltage cable line. The results obtained by the mathematical model are validated by measurements taken on 30 October 2019 on an existing cable line. Measurements recorded on a real object for various systems of crossing sheaths are presented. The influence of incorrect sheaths crossing on the measured quantities was shown. I... [more]

Attenuation of solar radiation between the receiver and the heliostat field in concentrated solar power (CSP) tower plants can reduce the overall system performance significantly. The attenuation varies strongly with time and the average attenuation at different sites might also vary strongly from each other. If no site specific attenuation data is available, the optimal plant design cannot be determined and rough estimations of the attenuation effect are required leading to high uncertainties of yield analysis calculations. The attenuation is caused mainly by water vapor content and aerosol particles in the lower atmospheric layer above ground. Although several on-site measurement systems have been developed during recent years, attenuation data sets are usually not available to be included during the plant project development. An Atmospheric Attenuation (AATTENUATION) model to derive the atmospheric transmittance between a heliostat and receiver on the basis of common direct normal i... [more]