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Öğe Development and evaluation of transfer functions for a variable speed compressor(2012) Ekren O.; Şahin S.; Işler Y.In this study, transfer functions of electronic expansion valve and variable speed scroll compressor in a chiller system are developed based on an experimental analysis. Dynamic behaviours of water temperature at chiller output and refrigerant superheat at evaporator outlet have either the first- or the second-order linear models, respectively. Different from the existing modelling studies in literature novel models are presented for variable speed refrigeration system with scroll compressor and liquid type evaporator. The scroll compressor used in the current study is not produced for variable speed operation however it is operated with some limitations in the range of 30 to 50 Hz through an inverter. Similar restrictions are presented for the electronic expansion valve as 10 per cent to 35 per cent opening range because of operational limitations. To validate transfer functions of the variable speed scroll compressor and electronic expansion valve, mean square error values for all transfer functions are calculated. The mean square error values vary between 0.0126 and 0.3116. In addition, all transfer functions are designed and tested using corresponding proportional-integral-derivative controllers in order to see their applicability. The results indicate that energy savings are a consequence of operating the chiller at smaller frequencies than the basic 50 Hz while partial load occurred. © Authors 2011.Öğe Energetic and exergetic performance evaluation of an AC and a solar powered DC compressor(2013) Ekren O.; Çelik S.This study represents experimental performance analyses of an alternative current (AC) and a direct current (DC) refrigeration compressors implemented in a 79 liter refrigerator. Experiments were carried out at continuously running (ON) and periodically running (ON/OFF) operation modes. Data was analyzed and a comparison in terms of cooling capacity, power input, coefficient of performance (COP), Carnot COP, and exergy efficiency was conducted. The comparison showed that DC compressors can be much more efficient than AC compressors in refrigeration units. © Springer-Verlag Berlin Heidelberg 2013.Öğe Evaluation of fuel production technologies by using renewable energy for smart cities(Elsevier, 2022) Canbaz C.H.; Ekren O.; Ekren B.Y.; Kumar V.Fuels that are environment-friendly resources play a vital role in the development of economies. Each fuel resource has intensive, however, inadequate specifications in terms of security, sustainability, capacity to compete with other sources. Although oil and gas resources lead the way of fuel production due to their low cost, alternative sources such as wind and solar renewable energy and nuclear energy have also gained popularity forcing the technological development of fuel production technologies. Hydrogen is a source for the production of energy by using any energy resource. In this chapter, we investigate the latest fuel production technologies providing energy outputs from renewable energy resources for their usage in smart cities. We study the role of renewable resources under the current dynamics of the global energy market situation as well as the effects of fuel production technologies to increase the efficiency and sustainability for the future of smart cities. This review includes a broad spectrum that spans from the technical energy resources literature with very comprehensive research by using SCOPUS database to nontechnical but renowned resources including journals and other publications including raw data as well as forecasts and opinions that respected experts have performed. Specifically, solar and wind production technologies, as well as future forecasts compared with the conventional fuel resources, are reviewed comprehensively. By that, we aim to shed a light on the current status of fuel production technologies in renewable energy resources and their usage in smart cities. © 2022 Elsevier Inc. All rights reserved.Öğe Experimental design and regression analysis for performance of a chiller system(2011) Ekren O.; Ekren B.Y.In this paper, we perform experimental design and regression analysis for performance of a chiller system. First, we complete design of experiment (DOE) on response so that determine the main and the interaction effects of the pre-defined factors. Second, we omit the insignificant factors from the analysis and, develop regression functions by considering the significant factors (inputs). Four factors are considered in DOE, these are: the water temperature, water flow rate, electronic expansion valve (EEV) opening percentage and compressor speed. Eight responses (outputs) are considered in DOE. They are the coefficient of performance (COP), capacity of evaporator, capacity of condenser, power consumption of the compressor, temperature of condensing, temperature of evaporation, superheating, and sub-cooling. The DOE results are analyzed, and regression functions are developed by MINITAB - a statistical software - at a 95% confidence level. ©2008 TIBTD Printed in Turkey.Öğe Experimental investigation of cooling performance of milk cooling tank with the vapor compressed and solar absorption (LiCl-H2O) cooling system [Süt sogutma tankinin buhar sikiştirmali ve güneş enerjili absorpsiyonlu (LiCl-H2O) sogutma sistemleriyle sogutma performansinin deneysel incelenmesi](Gazi Universitesi Muhendislik-Mimarlik, 2016) Demir V.; Günhan T.; Şencan Şahin A.; Ekren O.; Bilgen H.; Erek A.In this study, utilization of a solar energy powered absorption cooling system has been investigated instead of conventional cooling systems for milk cooling purpose as an alternative method. In this study, a novel designed milk tank (1000 liter tank capacity and 2BII performance class) with rollbond type heat exchanger around the wall and conventional heat exchanger bottom of the tank for vapor compression cooling system was used. Performance investigation of the tank has been realized while the first and last milking experiments according to International and National Standards for the milk storage tank. Experimental studies have been realized for the hybrid combination of the conventional (vapor compression) cooling and the LiCl-H2O absorption cooling systems with an internal energy storage ability. According to experiments, the conventional and absorption cooling method were cooled down the milk at the first milking period in the reasonable time interval. On the other hand, while the conventional method was being completed in the reasonable time interval, hybrid method was not able to complete in accepted time limit. Utilization of the hybrid combination of the LiCl-H2O absorption and conventional cooling system showed 18.01 Wh/L specific energy consumption, but conventional cooling system showed 21.06 Wh/L specific energy consumption at the same cooling amount. In Turkey, utilization of solar and geothermal energy powered absorption cooling systems to increase energy savings in different cooling application can be significant method since Turkey has advantages about renewable energy sources such as solar and geothermal energy.Öğe Heat exchanger applications in wastewater source heat pumps for buildings: A key review(Elsevier Ltd, 2015) Culha O.; Gunerhan H.; Biyik E.; Ekren O.; Hepbasli A.Abstract Wastewater heat recovery applications are becoming widespread in energy saving applications. A sustainable and low emissions operation in air conditioning and heating processes is achieved by harvesting the otherwise wasted energy in wastewater through specially designed heat exchangers, lying at the core of heat pumps. This combined system is called wastewater source heat pump. In this study, a review of wastewater heat exchangers in wastewater source heat pump applications is presented, and wastewater heat exchangers are classified in detail based on multiple features, including utilization and construction methodology. Also, the potential of wastewater, types of wastewater source heat pumps, and their applications are briefly discussed. © 2015 Elsevier B.V.Öğe Impact of the mixing theories on the performance of ejector expansion refrigeration cycles for environmentally-friendly refrigerants [Impact des théories de mélange sur les performances des cycles frigorifiques à détente par éjecteur pour des frigorigènes respectueux de l'environnement](Elsevier Ltd, 2019) Atmaca A.U.; Erek A.; Ekren O.Ejector expansion refrigeration cycle was investigated theoretically to compare the constant pressure mixing and constant area mixing ejector theories making use of thermodynamic models. Performance improvement was presented according to optimum and constant value pressure drop assumptions in order to simulate the design and off-design conditions, respectively. The agreement between the results with respect to both pressure drop assumptions is better at higher condenser and lower evaporator temperatures for each ejector mixing theory. Optimum area ratio (?opt) is less for constant area mixing approach since the ratio of the secondary flow velocity to the primary flow is higher for this ejector theory at each operation point. Although a group of low-global-warming-potential refrigerants were compared in terms of the performance and expansion losses, deep investigations were presented for R1234yf in main and sometimes R1234ze(E). Finally, when compared to other ejector sections, the mixing section efficiency has the most critical effect on the optimum performance improvement ratio of R1234yf with reference to constant pressure mixing assumption. © 2018 Elsevier Ltd and IIRÖğe Investigation of the liquid-vapor separator efficiency on the performance of the ejector used as an expansion device in the vapor-compression refrigeration cycle(American Society of Mechanical Engineers (ASME), 2020) Atmaca A.U.; Erek A.; Ekren O.Ejector expansion refrigeration cycle with reference to the constant pressure mixing theory is investigated to display the effects of the liquid-vapor separator efficiency on the performance, entrainment ratio, and area ratio at various operation conditions. Reversible ejector assumption is used for the highest theoretical performance limit, whereas efficiency of the liquid-vapor separator and all ejector components is added to the model to calculate more realistic performance improvement potentials. R1234yf and R1234ze(E) having low global warming potential values are used in the analyses. Zero-dimensional thermodynamic models are constructed applying the conservation equations between the inlets and outlets of the refrigeration cycle and ejector components. Percentage performance decrease is higher when the mixing section and the separator efficiency is added to the model at higher condenser temperatures compared with the lower evaporator temperatures according to the investigated operation ranges. Vapor and liquid separation efficiency affects not only the performance but also the design of the ejector although it is an external component since it has influence on the area ratio and entrainment ratio. Finally, the difference between the percentage performance improvement of the reversible ejector cycle and the realistic ejector cycle including the separator and ejector components efficiencies is as high as 35% at the highest investigated condenser temperature for R1234yf. Copyright © 2019 by ASME.Öğe A novel spherical packed bed application on decentralized heat recovery ventilation units(EDP Sciences, 2019) Genc A.M.; Karadeniz Z.H.; Ekren O.; Toksoy M.Decentralized heat recovery ventilation (HRV) systems are assumed as simple solutions to obtain a healthy and comfortable indoor environment. A wall or window mounted compact version of decentralized HRV systems (mono unit) are used for small scale, mostly residential applications. A fan and a heat exchanger are the critical components of this compact system. The flow capacity of these units are down to 10 m3/h, where efficiencies over 90% are commonly declared by the manufacturers. On the other hand, spherical packed beds (SPD) are widely used in the heat transfer applications such as; chemical reactors, grain driers, nuclear reactors, thermal storage in buildings and in solar thermal power plants, due to operational convenience. These systems are operated under steady flow conditions, unlike decentralized HRV systems which are designed for cyclic operation. In this study, heat recovery performance of a spherical packed bed heat exchanger for a decentralized HRV system is investigated. A one dimensional mathematical model for a SPD is obtained and an in-house computer code is developed to solve the transient heat transfer inside the packed bed under cyclic operation conditions. Well known convenient correlations were used for pressure drop calculations. A number of bed and sphere diameters were studied in a wide range. Various flow time and number of cycles were studied for the hot and cold flow to understand the SPD performance for HRV applications. This novel application also has the potential for regenerative heat recovery systems. © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0Öğe Numerical analysis of a near-room-temperature magnetic cooling system [Analyse numérique d'un système de froid magnétique proche de la température ambiante](Elsevier Ltd, 2017) Ezan M.A.; Ekren O.; Metin C.; Yilanci A.; Biyik E.; Kara S.M.In this study, for a near-room-temperature magnetic cooling system, a decoupled multi-physics numerical approach (Magnetism, Fluid Flow, and Heat Transfer) is developed using a commercial CFD solver, ANSYS-FLUENT, as a design tool. User defined functions are incorporated into the software in order to take into account the magnetocaloric effect. Magnetic flux density is assumed to be linear during the magnetization and demagnetization processes. Furthermore, the minimum and maximum magnetic flux densities (Bmin and Bmax) are defined as 0.27 and 0.98, respectively. Two different sets of analyses are conducted by assuming an insulated cold heat exchanger (CHEX) and by defining an artificial cooling load in the CHEX. As a validation case, experimental work from the literature is reproduced numerically, and the results show that the current methodology is fairly accurate. Moreover, parametric analyses are conducted to investigate the effect of the velocity of heat transfer fluid (HTF) and types of HTF on the performance of the magnetic cooling system. Also, the performance metrics of the magnetic cooling system are investigated with regards to the temperature span of the magnetic cooling unit, and the cooling load. It is concluded that reducing the cycle duration ensures reaching lower temperature values. Similarly, reducing the velocity of the HTF allows reducing the outlet temperature of the HTF. In the current system, the highest temperature spans are obtained numerically as around 6?K, 5.2?K and 4.1?K for the cycle durations of 4.2?s, 6.2?s and 8.2?s, respectively. © 2016 Elsevier Ltd and IIRÖğe Numerical analysis of magnetic field and heat transfer of a reciprocating magnetocaloric regenerator using a halbach magnet array(American Society of Mechanical Engineers (ASME), 2020) Akiş T.; Hamad A.; Ezan M.A.; Yanik E.; Yilanci A.; Çelik S.; Ekren O.In this study, a numerical model of a reciprocating magnetocaloric regenerator using a Halbach magnet array is developed in ANSYS-FLUENT software. The model consists of three components, namely, (i) the Halbach magnet array, (ii) the magnetocaloric material (MCM), and (iii) the heat transfer fluid. A two-dimensional (2D) domain is studied due to the axisymmetric geometry of the physical model. A pressure difference is defined between the inlet and outlet sections of the fluid domain to maintain a reciprocating fluid flow. In the proposed computational scheme, a segregated approach is followed to consider the spatial distribution of the magnetic field in the thermal analyses. Therefore, a 2D magnetic field within the MCM is computed using an analytical approach at first, and its results are integrated into ANSYS-FLUENT with a user-defined function (UDF). Hydrodynamic and heat transfer characteristics of the proposed regenerator model are evaluated under various Reynolds numbers and cycle durations. Moreover, the temperature drop at the cold side of the regenerator is represented in terms of the pressure difference, flow duration, and the diameter of Gadolinium (Gd) as the MCM. For the current geometrical configurations, it is observed that the magnetic field varies from 0.4 T to 1 T within Gd. The highest temperature spans are measured as 8.4 K, 7.5 K, and 7.2 K numerically for the cycle durations of 1.2 s, 2.2 s, and 4.2 s, respectively. Copyright © 2020 by ASME.Öğe Preliminary design of the two-phase ejector under constant area mixing assumption for 5 kW experimental system(EDP Sciences, 2019) Atmaca A.U.; Erek A.; Ekren O.Ejector expansion refrigeration cycle is the modification of the vapour compression refrigeration cycle with the implementation of a two-phase ejector and a vapour-liquid separator to improve the cycle performance. In this study, main geometrical parameters of an ejector, i.e. diameters of the motive nozzle throat, motive nozzle outlet, suction nozzle outlet, and constant area mixing section are calculated in order to provide the preliminary design aspects at various operation conditions. The thermodynamic model of the ejector is established with reference to constant-area mixing assumption. The equations are solved in Matlab®. The environmentally-friendly refrigerants, R1234yf and R1234ze(E) from the hydrofluoroolefins (HFOs) and R134a which is about to be phased out by the F-gas Regulation are used in the analyses. When compared to the previous literature findings, the current research aims to compare the dimensions of a two-phase ejector to be used in an experimental system having 5 kW cooling capacity for these three refrigerants. © 2019 The Authors, published by EDP Sciences.Öğe Size optimization of a PV/wind hybrid energy conversion system with battery storage using simulated annealing(Elsevier Ltd, 2010) Ekren O.; Ekren B.Y.In this paper, we perform Simulated Annealing (SA) algorithm for optimizing size of a PV/wind integrated hybrid energy system with battery storage. The proposed methodology is a heuristic approach which uses a stochastic gradient search for the global optimization. In the study, the objective function is the minimization of the hybrid energy system total cost. And the decision variables are PV size, wind turbine rotor swept area and the battery capacity. The optimum result obtained by SA algorithm is compared with our former study's result. Consequently, it is come up with that the SA algorithm gives better result than the Response Surface Methodology (RSM). The case study is realized for a campus area in Turkey. © 2009 Elsevier Ltd.Öğe Sizing of autonomous wind/solar hybrid energy conversion systems for URLA, Turkey(2009) Özerdem B.; Ekren O.In this paper, an optimum sizing procedure of autonomous hybrid (wind + solar) energy system is presented which can be used to satisfy the requirements of given load distribution. The main purpose of this study is to find out an appropriate wind-photovoltaic hybrid energy system to satisfy electricity consumption of GSM (Global System for Mobile communication) base station at Izmir Institute of Technology Campus Area, Urla, Izmir, Turkey. To do this, monthly average daily solar radiation and wind speed data are collected. The monthly average wind speeds are measured at 10 m height during 3 years period on Izmir Institute of Technology Campus Area. The monthly average wind speed values are obtained between 5.7 and 7.7 m/s, on the other hand, the monthly average daily value of solar radiations on horizontal surfaces are ranged from 1.4 to 6.9kWh/m2 at Izmir Institute of Technology Campus Area. The hybrid system considered in the present analysis consists of one 5kW nominal power wind energy conversion systems (WECS), 21.82 m2 of photovoltaic (PV) panels (18 mono crystal PV panels each having 75 W power output) together with a battery storage system.Öğe Solar powered mechanical ventilation: A case study(EDP Sciences, 2019) Birtürk A.; Ekren O.; Aktakka S.; Özel Ö.; Toksoy M.In this study, a solar powered mechanical ventilation unit has investigated and tested in terms of efficiency and performance. Test unit can be divided into two parts, the first one is ventilation unit with 370 m3/h max airflow rate and max 167 W fan power provides fresh air for a residency and recovers heat from the climatized exhaust air. Total area is 70 m2 for the residency and total occupant is four. The second part of the test system is solar energy power system with two 325 W polycrystalline photovoltaic panels, an inverter and two batteries. The mechanical ventilation unit has energized by a solar photovoltaic system; if the solar energy is not available then ventilation unit has connected to the national electricity grid. This is an alternative option to consumers to use electricity by the grid in case the PV system does not produce enough energy because of the usage or the technical problems or the weather conditions based on the seasons. On the other hand, in some cities, number of photovoltaic panels rolled up upper number according to solar energy potential and therefore resulted excess electricity has assumed to sell to the national grid. According to the results, the test system is able to operate at maximum ventilation necessity and power consumption without grid connection in Izmir. Furthermore, we have compared Izmir and Romania in accordance with feasibility for the same mechanical ventilation system at max flow rate and required ventilation rate is determined depends on daily usage scenario of the room. © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0Öğe Thermodynamic performance of the transcritical refrigeration cycle with ejector expansion for R744, R170, and R41(Turk Isı Bilimi ve Teknigi Dernegi, 2018) Atmaca A.U.; Erek A.; Ekren O.; Çoban M.T.For more than a decade, there is a great demand for finding environmentally-friendly refrigerants obeying the global warming potential value restrictions of the tough environmental legislation. Among the candidate working fluids, R744 (carbon dioxide or CO2), R170 (ethane), and R41 (fluoromethane) are selected to be investigated parametrically in this paper. Performance comparison is made for these three working fluids individually in both transcritical (supercritical) refrigeration cycle and modification of this cycle with ejector expansion. As the first step, the effects of the gas cooler outlet temperature, evaporator temperature, and evaporator outlet superheat temperature difference on the overall performance and percentage expansion losses are investigated within a specific gas cooler pressure range. Evaporator outlet superheat temperature difference is found to be the least effective parameter on the performance; hence, secondly, the transcritical ejector expansion refrigeration cycle is analyzed considering only evaporator temperature and gas cooler outlet temperature based on the same gas cooler pressure ranges. Thermodynamic models are constructed in Matlab® and the ejector equations for the ejector expansion refrigeration cycle are established with reference to constant pressure mixing assumption. Comparisons of the performance, percentage expansion losses, and performance improvement potential through the implementation of the ejector instead of the expansion valve among these three refrigerants having low critical temperatures represent the main objective of the paper in order to make contributions to the previous researches in the literature. ©2018 TIBTD Printed in Turkey.
