Browsing by Subject "Efficiency"

Now showing 1 - 20 of 62

Open Access
Accuracy and efficiency considerations in the solution of extremely large electromagnetics problems
(IEEE, 2011) Gürel, Levent; Ergül, Özgür
This study considers fast and accurate solutions of extremely large electromagnetics problems. Surface formulations of large-scale objects lead to dense matrix equations involving millions of unknowns. Thanks to recent developments in parallel algorithms and high-performance computers, these problems can easily be solved with unprecedented levels of accuracy and detail. For example, using a parallel implementation of the multilevel fast multipole algorithm (MLFMA), we are able to solve electromagnetics problems discretized with hundreds of millions of unknowns. Unfortunately, as the problem size grows, it becomes difficult to assess the accuracy and efficiency of the solutions, especially when comparing different implementations. This paper presents our efforts to solve extremely large electromagnetics problems with an emphasis on accuracy and efficiency. We present a list of benchmark problems, which can be used to compare different implementations for large-scale problems. © 2011 IEEE.
Open Access
Capacity analysis of a PMR system with DAB downlink
(IEEE, 2003) Şengül, Ersin; Can, B.; Akar, Nail; İder, Yusuf Ziya; Köymen, Hayrettin
Several trunked private mobile radio (PMR) systems have been designed over the last decade, most of which have symmetric downlink and uplink channel capacities. These systems may not be spectrally efficient in case of group or broadcast-based voice and data calls, a common feature of PMR systems. We propose a new asymmetric PMR system comprising a wideband OFDM-based downlink and a narrowband uplink, which not only achieves a better spectral efficiency but also can support high bit rate multimedia applications. The system is shown to have high trunking efficiency since all users are assumed to use the pool of channels available in the wideband downlink. In this paper, we study the performance and capacity of a private mobile radio system using a digital audio broadcasting (DAB) downlink. In particular, we study the efficiency of such a system for voice calls using voice activity detection and statistical multiplexing. Moreover, we show that, the efficiency of the system can significantly increase, if the incoming calls, which can not find an available channel, are allowed to wait a certain amount of time before occupying a channel.
Open Access
A charge inverter for III-nitride light-emitting diodes
(American Institute of Physics Inc., 2016) Zhang Z.-H.; Zhang, Y.; Bi, W.; Geng, C.; Xu S.; Demir, Hilmi Volkan; Sun, X. W.
In this work, we propose a charge inverter that substantially increases the hole injection efficiency for InGaN/GaN light-emitting diodes (LEDs). The charge inverter consists of a metal/electrode, an insulator, and a semiconductor, making an Electrode-Insulator-Semiconductor (EIS) structure, which is formed by depositing an extremely thin SiO2 insulator layer on the p+-GaN surface of a LED structure before growing the p-electrode. When the LED is forward-biased, a weak inversion layer can be obtained at the interface between the p+-GaN and SiO2 insulator. The weak inversion region can shorten the carrier tunnel distance. Meanwhile, the smaller dielectric constant of the thin SiO2 layer increases the local electric field within the tunnel region, and this is effective in promoting the hole transport from the p-electrode into the p+-GaN layer. Due to the improved hole injection, the external quantum efficiency is increased by 20% at 20 mA for the 350 × 350 μm2 LED chip. Thus, the proposed EIS holds great promise for high efficiency LEDs.
Open Access
The comparative performance of the public enterprise sector in Turkey: a Malmquist productivity index approach
(Academic Press, 1997) Zaim, O.; Taskin, F.
The public enterprise sector in Turkey has grown appreciably since the 1950's and has made a marked impact on aggregate production, employment, and saving. However, since the early 1980's, public enterprises have been accused of absorbing the government's financial resources and are being held responsible for Turkey's large external debt. The purpose of this study is to compare the performance of the public sector with that of the private sector for the various subsectors of manufacturing in Turkey. The Malmquist productivity index, constructed using nonparametric linear programming methods, is employed for the relevant comparisons. (C) 1997 Academic Press.
Open Access
Comparative study of optically activated nanocomposites with photocatalytic TiO2 and ZnO nanoparticles for massive environmental decontamination
(S P I E - International Society for Optical Engineering, 2007) Tek, S.; Mutlugun, E.; Soganci, I. M.; Perkgoz, N. K.; Yucel, D.; Celiker, G.; Demir, Hilmi Volkan
Nanocomposites that incorporate TiO2 and ZnO nanoparticles separately in three-dimensional solgel matrices through full chemical integration are prepared to perform highly efficient photocatalytic activities for applications of environmental decontamination. Spectral responses of photocatalytic TiO2 and ZnO nanoparticles exposed to UV activation for self-cleaning process were obtained as also their optical relative spectral efficiency curves from 270 to 370 nm in the UV regime. Our investigations of the optimal conditions to increase their spectral photocatalytic efficiencies resulted in remarkably high levels of optical recovery and efficiency.
Open Access
Comparative study of thin film n-i-p a-Si: H solar cells to investigate the effect of absorber layer thickness on the plasmonic enhancement using gold nanoparticles
(Elsevier Ltd, 2015) Islam, K.; Chowdhury F.I.; Okyay, Ali Kemal; Nayfeh, A.
In this paper, the effect of gold nanoparticles on n-i-p a-Si:H solar cells with different intrinsic layer (i-layer) thicknesses has been studied. 100nm and 500nm i-layer based n-i-p a-Si:H solar cells were fabricated and colloidal gold (Au) nanoparticles dispersed in water-based solution were spin-coated on the top surface of the solar cells. The Au nanoparticles are of spherical shape and have 100nm diameter. Electrical and quantum efficiency measurements were carried out and the results show an increase in short-circuit current density (Jsc), efficiency and external quantum efficiency (EQE) with the incorporation of the nanoparticles on both cells. Jsc increases from 5.91mA/cm2 to 6.5mA/cm2 (~10% relative increase) and efficiency increases from 3.38% to 3.97% (~17.5% relative increase) for the 100nm i-layer solar cell after plasmonic enhancement whereas Jsc increases from 9.34mA/cm2 to 10.1mA/cm2 (~7.5% relative increase) and efficiency increases from 4.27% to 4.99% (~16.9% relative increase) for the 500nm i-layer cell. The results show that plasmonic enhancement is more effective in 100nm than 500nm i-layer thickness for a-Si:H solar cells. Moreover, the results are discussed in terms of light absorption and electron hole pair generation. © 2015 Elsevier Ltd.
Open Access
Contests over joint production on networks
(Wiley, 2020) Doğan, Serhat; Keskin, K.; Sağlam, Çağrı
We consider a network of heterogeneous agents where each edge represents a two‐player contest between the respective nodes. In these bilateral contests, agents compete over an endogenous prize jointly produced using their own contest efforts. We provide a necessary and sufficient condition for the existence of Nash equilibrium and characterize the equilibrium total effort for every agent. Our model has insightful results regarding the network type, that is, depending on whether the network is bipartite or nonbipartite. Finally, considering the sum of all expected utilities as an efficiency notion, we investigate the optimal network structure.
Open Access
Coupling and power transfer efficiency enhancement of modular and array of planar coils using in-plane ring-shaped inner ferrites for inductive heating applications
(American Institute of Physics Inc., 2017) Kilic V.T.; Unal, E.; Demir, Hilmi Volkan
We propose and demonstrate a highly effective method of enhancing coupling and power transfer efficiency in inductive heating systems composed of planar coils. The proposed method is based on locating ring-shaped ferrites in the inner side of the coils in the same plane. Measurement results of simple inductive heating systems constructed with either a single or a pair of conventional circular coils show that, with the in-plane inner ferrites, the total dissipated power of the system is increased by over 65%. Also, with three-dimensional full electromagnetic solutions, it is found that power transfer efficiency of the system is increased up to 92% with the inner ferrite placement. The proposed method is promising to be used for efficiency enhancement in inductive heating applications, especially in all-surface induction hobs.
Open Access
Delegation in a duopolistic differentiated goods market with Bertrand competition
(1995) Yıldırım, Hüseyin
The impact of delegation in a firm has been observed by many modern authors. Vickers(1985), Fershtman and Judd(1987), Sklivas(1987) considered the problem as part of positive economic theory whereas Koray and Sertel(1989) treated it as a regulation problem. We examine a similar problem for a duopolistic dilTerentiated good market with Bertrand competition and lengthen the delegation chain to 5 managers. Our findings show that the firms’ profits are monotonically increasing, i.e. there is a positive incentive to redelegate for each firm. Our natural conjecture is that, in the limit, firms reach collusion non-cooperatively.
Open Access
Delegation in a duopolistic differentiated goods market with Cournot competition
(1995) Ünver, Mustafa Utku
We consider the impact of delegation in a Cournotic duopoly with differentiated goods upon the firms’ profit maximization behavior. In an oligopoly, delegation in each firm can be modeled through a specific non-cooperative game. Delegation games in a differentiated goods market with affine demand are studied within the Cournot competition concept where redelegation is permitted in a symmetric duopoly. The following results are demonstrated: The maximand delegated by each primary delegator, i.e. owner of each firm, converges in monotonically decreasing fashion to the true profit function in the absence of delegation costs, and total industry output at the Cournot equilibrium converges in monotonically increasing fashion to some output level. Welfare changes due to redelegation are also considered.
Open Access
Design and analysis of mechanisms for decentralized joint replenishment
(Elsevier B.V., 2017) Güler, K.; Körpeoğlu, E.; Şen, A.
We consider jointly replenishing multiple firms that operate under an EOQ like environment in a decentralized, non-cooperative setting. Each firm's demand rate and inventory holding cost rate are private information. We are interested in finding a mechanism that would determine the joint replenishment frequency and allocate the joint ordering costs to these firms based on their reported stand-alone replenishment frequencies (if they were to order independently). We first provide an impossibility result showing that there is no direct mechanism that simultaneously achieves efficiency, incentive compatibility, individual rationality and budget-balance. We then propose a general, two-parameter mechanism in which one parameter is used to determine the joint replenishment frequency, another is used to allocate the order costs based on firms’ reports. We show that efficiency cannot be achieved in this two-parameter mechanism unless the parameter governing the cost allocation is zero. When the two parameters are same (a single parameter mechanism), we find the equilibrium share levels and corresponding total cost. We finally investigate the effect of this parameter on equilibrium behavior. We show that properly adjusting this parameter leads to mechanisms that are better than other mechanisms suggested earlier in the literature in terms of fairness and efficiency. © 2016 Elsevier B.V.
Open Access
Dynamic power supply design for varying envelope signals using envelope tracking method
(2012) Özel, Muhlis Kenan
In modern communication systems, demand for higher data-rate is consistently growing. Higher data-rate within a limited bandwidth tends to require more amplitude modulation (AM) to increase number of symbols per second. Amplitude modulated carriers should be amplified using linear power amplifiers (PAs). Although, linear PAs have good performance in terms of linearity, they are efficient only when they transmit at maximum power. However, modern communication signals have high peak-to-average-power ratio (PAPR), therefore probability of a PA transmitting at maximum power is low. As a conclusion, efficiencies are degraded to alarmingly low values and the problem translates to heat and shorter battery life issues. In this thesis, we investigate how dynamic power supplies (DPS) perform for non-constant envelope RF signals. We have designed a DPS and used it to test efficiency enhancement of two PAs compared to the same PAs operating with constant supply. Our aim was to obtain efficiency increase compared to constant supply case without introducing extra non-linearity. We have obtained satisfying results for both amplifiers.
Open Access
Effect of discrete batch WIP transfer on the efficiency of production lines
(Taylor & Francis, 1993) Erel, E.
In this paper, the effect of discrete batch transfer of WIP between workstations on the efficiency of asynchronous production lines is analysed via a simulation model. The processing times are assumed to be random variables distributed according to specific distribution functions. The WIP transfer design problem involves determining the number of containers to allocate to each buffer location and the container capacity. Interesting and valuable information for practitioners has been obtained. It is found that loss in capacity occurs in the first few stations. Another finding is that an important portion of the lost capacity can be regained by allocating two containers to each buffer location, and if it is impossible to assign two containers to each location, then no single-container location should be adjacent to another single-container location.
Open Access
The effects of privatization on efficiency: how does privatization work?
(Pergamon Press, 2006) Okten, C.; Arin, K. P.
Uncovering the effects of privatization is difficult, because privatization of a particular firm usually is not an accident. This paper tests the effects of privatization on productive and allocative (market) efficiency using a rich panel data set of 22 privatized cement plants from Turkey in the 1983-99 period. Since, all public cement firms were privatized and we have preand post-privatization data for all, we are able to avoid the problem of endogeneity associated with sample selection. Our analysis goes beyond just examining the privatization effects and explores how privatization really works. Changes in objectives of the firm (ownership effect) and changes in market structure (environment effect) may both be responsible for privatization outcomes. We find that ownership effects are sufficient to achieve improvements in labor productivity. Our results on allocative efficiency, however, are dependent on changes in the competitive environment. While all plants seem to improve labor productivity through works force reductions, plants privatized to foreign buyers also increase their capital and investment significantly. (c) 2006 Elsevier Ltd. All rights reserved.
Open Access
Efficiency and stability of probabilistic assignments in marriage problems
(Academic Press, 2016) Doğan, B.; Yıldız, K.
We study marriage problems where two groups of agents, men and women, match each other and probabilistic assignments are possible. When only ordinal preferences are observable, stochastic dominance efficiency (sd-efficiency) is commonly used. First, we provide a characterization of sd-efficient allocations in terms of a property of an order relation defined on the set of man-woman pairs. Then, using this characterization, we constructively prove that for each probabilistic assignment that is sd-efficient for some ordinal preferences, there is a von Neumann-Morgenstern utility profile consistent with the ordinal preferences for which the assignment is Pareto efficient. Second, we show that when the preferences are strict, for each ordinal preference profile and each ex-post stable probabilistic assignment, there is a von Neumann-Morgenstern utility profile, consistent with the ordinal preferences, for which the assignment belongs to the core of the associated transferable utility game. © 2015 Elsevier Inc.
Open Access
Efficiency of the Turkish stock exchange with respect to monetary variables: a cointegration analysis
(Elsevier BV, 1996) Muradoglu, Y. G.; Metin, K.
In this study, we test the semistrong form of the efficient market hypothesis in Turkey by using the recently developed techniques in time series econometrics, namely unit roots and cointegration. The long run relationship between stock prices and inflation is investigated by assuming the possible existence of a proxy effect. Conclusions are made as to the efficiency of the Turkish Stock Exchange and its possible implications for investors. To our knowledge, this is among the pioneering studies conducted in an emerging market that uses an updated econometric methodology to allow for an analysis of long run steady state properties together with short run dynamics.
Open Access
An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays
(AIP, 2013) Liu, S. W.; Wang, J. X.; Divayana, Y.; Dev, K.; Tan S.T.; Demir, Hilmi Volkan; Sun, X. W.
We report phosphorescent organic light-emitting diodes with a substantially improved light outcoupling efficiency and a wider angular distribution through applying a layer of zinc oxide periodic nanopillar arrays by pattern replication in non-wetting templates technique. The devices exhibited the peak emission intensity at an emission angle of 40° compared to 0° for reference device using bare ITO-glass. The best device showed a peak luminance efficiency of 95.5 ± 1.5 cd/A at 0° emission (external quantum efficiency - EQE of 38.5 ± 0.1%, power efficiency of 127 ± 1 lm/W), compared to that of the reference device, which has a peak luminance efficiency of 68.0 ± 1.4 cd/A (EQE of 22.0 ± 0.1%, power efficiency of 72 ± 1 lm/W). © 2013 American Institute of Physics.
Open Access
Efficient ownership patterns: three examples
(1994) Sever, Murat
This stud\^ provides three exanpdes in whicli llie ownership structure of productive assets affects efhciency oF the economic outcome. We show how the incompleteness of contracts and the s])ecificit y of investments cause inefficient l^ehaviours and reductions in the efficient level of relation-specific investment because of the individuals' o])])ortunisti(' behaviour. We show the importance of ownershi]) on l^ehaviours of agents and their investment decisions l)y affecting the distribution of residual rights over assets and so the distribution of tlie sur])lus from investments. VVV' observe the effects of monitoring on the l)ehaviour of individuals in dilfeient types of ownership patterns.
Open Access
Enhanced Performance of Nanowire-Based All-TiO2 Solar Cells using Subnanometer-Thick Atomic Layer Deposited ZnO Embedded Layer
(Pergamon Press, 2015) Ghobadi, A.; Yavuz, H. I.; Ulusoy, T. G.; Icli, K. C.; Ozenbas, M.; Okyay, Ali Kemal
In this paper, the effect of angstrom-thick atomic layer deposited (ALD) ZnO embedded layer on photovoltaic (PV) performance of Nanowire-Based All-TiO2 solar cells has been systematically investigated. Our results indicate that by varying the thickness of ZnO layer the efficiency of the solar cell can be significantly changed. It is shown that the efficiency has its maximum for optimal thickness of 1 ALD cycle in which this ultrathin ZnO layer improves device performance through passivation of surface traps without hampering injection efficiency of photogenerated electrons. The mechanisms contributing to this unprecedented change in PV performance of the cell have been scrutinized and discussed.
Open Access
Enhancement of polycrystalline silicon solar cells efficiency using indium nitride particles
(Institute of Physics Publishing Ltd., 2015) Alkis, S.; Chowdhury, F. I.; Alevli, M.; Dietz, N.; Yalızay, B.; Aktürk, S.; Nayfeh, A.; Okyay, Ali Kemal
In this work, we present a hybrid indium nitride particle/polycrystalline silicon solar cell based on 230 nm size indium nitride particles (InN-Ps) obtained through laser ablation. The solar cell performance measurements indicate that there is an absolute 1.5% increase (Δη) in the overall solar cell efficiency due to the presence of InN-Ps. Within the spectral range 300-1100 nm, improvements of up to 8.26% are observed in the external quantum efficiency (EQE) and increases of up to 8.75% are observed in the internal quantum efficiency (IQE) values of the corresponding solar cell. The enhancement in power performance is due to the down-shifting properties of the InN-Ps. The electrical measurements are supplemented by TEM, Raman, UV/VIS and PL spectroscopy of the InN-Ps. © 2015 IOP Publishing Ltd.