Browsing by Author "Nadar, Emre"
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Item Open Access The benefits of state aggregation with extreme-point weighting for assemble-to-order systems(Institute for Operations Research and the Management Sciences (INFORMS), 2018) Nadar, Emre; Akçay, A.; Akan, M.; Scheller Wolf, A.We provide a new method for solving a very general model of an assemble-toorder system: multiple products, multiple components that may be demanded in different quantities by different products, batch production, random lead times, and lost sales, modeled as a Markov decision process under the discounted cost criterion. A control policy specifies when a batch of components should be produced and whether an arriving demand for each product should be satisfied. As optimal solutions for our model are computationally intractable for even moderately sized systems, we approximate the optimal cost function by reformulating it on an aggregate state space and restricting each aggregate state to be represented by its extreme original states. Our aggregation drastically reduces the value iteration computational burden. We derive an upper bound on the distance between aggregate and optimal solutions. This guarantees that the value iteration algorithm for the original problem initialized with the aggregate solution converges to the optimal solution. We also establish the optimality of a lattice-dependent base-stock and rationing policy in the aggregate problem when certain product and component characteristics are incorporated into the aggregation/disaggregation schemes. This enables us to further alleviate the value iteration computational burden in the aggregate problem by eliminating suboptimal actions. Leveraging all of our results, we can solve the aggregate problem for systems of up to 22 components, with an average distance of 11.09% from the optimal cost in systems of up to 4 components (for which we could solve the original problem to optimality).Item Open Access Cirosu düşük perakende noktalarına Unilever ürünlerinin dağıtımı için karma sistem tasarımı(TMMOB Makina Mühendisleri Odası, 2007) Körpeoğlu, Ersin; Nadar, Emre; Bozkaya, Efe Burak; Sever, Derya; Karaşan, Oya EkinUnilever’in doğrudan ulaşamadığı düşük potansiyelli ama sayıca fazla perakendecilere Unilever ürünlerini ulaştırmayı amaçlayan bu projede, öncelikle bu noktalara hizmet vermeyi engelleyen etkenler belirlenmiştir. Başta yüksek ulaşım maliyetleri olmak üzere sistemdeki bu etkenleri ortadan kaldırmak amacıyla, firmanın istekleri doğrultusunda bir karma dağıtım sistemi tasarlanmıştır. Sıcak satış, telefonla satış ve depodan satış bölümlerinden oluşan bu sistem; doğrusal olmayan tamsayılı programlama tekniğiyle modellenmiş ve tanımlanan ek kısıtlar ile doğrusal hale getirilmiştir. Problemin boyutunun büyük olması nedeniyle CPLEX ile doğrudan çözülemeyen modelimiz için buluşsal bir algoritma geliştirilmiştir. Buluşsal algoritmanın başarısını ölçmek amacıyla model, kesin olarak çözülebildiği 37 tane koşturum örneği için buluşsal algoritma ile kıyaslanmış; buluşsal algoritmanın en iyi değerinin modelin en iyi değerinden ortalama %14.6 saptığı, 37 koşturumun 14’ünde algoritmanın en iyi hedef değerini verdiği gözlemlenmiştir. Modelleme sürecinde rassal olmadığı kabul edilen parametrelerin benzetim çalışması aracılığıyla rassal olduğu durumun ele alınmasıyla, modelin belirsizlik içeren durumlarda da işlevselliğini koruduğu gösterilmiştir. Veritabanı ve müşteri kartı tasarımı gibi yan uygulamalarla da desteklenen proje, firma yönetimi tarafından pilot uygulamaya alınmıştır. Sonuç olarak firmaya karlı ve esnek bir dağıtım kanalı oluşturulmuş ve düşük potansiyelli perakende noktalarına düşük maliyetle ulaşım sağlanmıştır.Item Embargo Deviations from commitments: Markov decision process formulations for the role of energy storage(Elsevier BV, 2022-11-11) Karakoyun, Ece Çiğdem; Avci, H.; Kocaman, Ayşe Selin; Nadar, EmreWe study the energy commitment, generation, and storage problem for a wind power producer who can own and operate a battery for different purposes. We consider two main problem settings: In the first setting, the producer may choose to deviate from her commitments based on the latest available information, using the battery to support such deviations. In the second setting, the producer is required to fulfill her commitments, using the battery as a back-up source. We also consider the special cases of these settings with no battery. In these settings, the producer decides how much energy to commit to purchasing or selling, how much energy to generate in the wind power plant, and how much energy to charge into or discharge from the battery. We formulate the producer’s decision-making process as a Markov decision process (MDP) by taking into account uncertainties in the electricity price and wind speed in a market setting where the price can be negative. We analytically compare the total profits of the two main settings. We then conduct data-calibrated numerical experiments to examine the effects of system components, imbalance pricing parameters, negative prices, and wind availability on the system operations and profits. Using the battery to intentionally deviate from commitments rather than to minimize such deviations improves the total profit by 16.5%, while this change in the role of the battery increases the total imbalance by 16 times, on average, in our experiments.Item Embargo Diffusion control in closed-loop supply chains: Successive product generations(Elsevier BV, 2023-12-10) Bayrak, B.; Guray, B.; Uzunlar, N.; Nadar, EmreWe consider a durable-good producer who optimizes its sales decisions for two successive product generations with refurbishing and recycling potential. Customer arrivals follow a multi-generation diffusion process that takes into account the word-of-mouth feedback spread within each customer population of successive generations as well as the substitution effect among these generations. We investigate whether the producer can profit from partially satisfying the new-generation demand to slow down the product diffusion and improve the refurbishing and recycling volumes in the long run. We derive conditions for optimality of this partial-fulfillment policy. In fast-clockspeed industries, if the producer enters the refurbishing market for both generations, the partial-fulfillment policy is optimal if (i) the profit margin ratio of the early-generation product to the new-generation product is high enough, (ii) the profit margin ratio of the refurbished item to the new item is large enough for the new-generation product, and (iii) the fraction of customers willing to buy the refurbished item is only modestly large for each generation. If the producer uses the recycled content obtained from early-generation returns in new-generation production, the partial-fulfillment policy is optimal if (i) the number of early-generation end-of-life returns and the amount of recyclable material from each such return are large and (ii) the number of customers initially attracted by the early-generation product is high. We also characterize the critical time period beyond which initiating the partial-fulfillment policy provides no improvement in the refurbishing and recycling volumes for the new-generation product.Item Open Access Eco-design of eco-labels with coarse grades(Elsevier, 2020-02-03) Nadar, Emre; Ertürk, M. S.We consider a third-party eco-label certifier that aims to reduce the total environmental impact of production on non-target resources and the total consumption rate of target resources. With these objectives the certifier determines the environmental standards for coarse grades of certification. Eco-labeled production involves a fixed investment cost, a larger marginal production cost, and a restricted access to supply, while eco-labeled products command a price premium. In response to a given grading scheme, we analytically characterize the subgame perfect equilibria for a two-stage game of environmental quality investment and quantity competition in a duopolistic setting. We then conduct numerical experiments to gain insights into whether and when multi-grade eco-labels may be more useful than single-grade eco-labels by providing better environmental protection in the dual objectives of the certifier. While multiple grades of an eco-label may appear in a market with asymmetric firms, our results indicate that multiple grades can indeed be useful under additional conditions: (i) when the investment costs are asymmetric, (ii) when the investment and production costs are asymmetric, either if the firm with the cost advantage has sufficiently low production cost under mild supply restrictions or if the production costs are very different, or (iii) when the supply bases are only moderately asymmetric and sufficiently large. Our findings uncover the role the cost and supply characteristics of the market play in eco-design of eco-labels, calling into question the common practice of offering single-grade eco-labels.Item Open Access New-product diffusion in closed-loop supply chains(INFORMS, 2021) Nadar, Emre; Kaya, B. E.; Güler, K.Problem definition: We study the sales planning problem of a producer who sells new and remanufactured versions of a durable good over a finite life cycle. We investigate whether slowing down product diffusion by choosing to partially satisfy demand might be profitable for the producer. Academic/practical relevance: We provide new insights into sales management in closed-loop supply chains by uncovering the role key market characteristics play in profitability of partial demand fulfillment as well as its optimal timing and magnitude. Methodology: We develop a dynamic model in which demand arrives as a slightly modified Bass diffusion process, and end-of-use products required for remanufacturing are constrained by earlier sales. Results: The optimal sales plan involves partial demand fulfillment when the product diffusion rate is high, the profit margin from remanufacturing is large, and the remanufactured item is in limited demand. Partial demand fulfillment extends to earlier stages of the life cycle as the diffusion rate grows, the demand for remanufactured items shrinks, or the number of consumers who return their end-of-use items increases. It is profitable to backlog more customers when the word-of-mouth effect dominates the diffusion process or when the demand for remanufactured items is lower. Finally, the benefit of delaying product diffusion tends to increase with diffusion rate. Managerial implications: Our findings suggest that deliberately backlogging some customers may be an effective lever (in the absence of flexibility to dynamically adjust prices) for durable-good producers in fast-clockspeed industries to improve their total profits from the jointly optimized sales of new and remanufactured items.Item Open Access Operational benefit of transforming cascade hydropower stations into pumped hydro energy storage systems(Elsevier, 2022-07) Toufani, Parinaz; Nadar, Emre; Kocaman, Ayse SelinThis study evaluates the potential benefit of retrofitting existing conventional cascade hydropower stations (CCHSs) with reversible turbines so as to operate them as pumped hydro energy storage (PHES) systems. We examine the energy generation and storage problem for a CCHS with two connected reservoirs that can be transformed into a PHES system in a market setting where the electricity price can be negative. We formulate this problem as a stochastic dynamic program (SDP) under uncertainty in the streamflow rate and electricity price. We analytically derive an upper bound on the profit improvement that can be obtained from the PHES transformation. We conduct numerical experiments with data-calibrated time series models and observe that the PHES system provides a greater benefit under more limited streamflow conditions or more frequently observed negative prices.Item Embargo Optimization of pumped hydro energy storage systems under uncertainty: A review(Elsevier, 2023-12-20) Toufani, P.; Karakoyun, E. Ç.; Nadar, Emre; Fasso, O. B.; Kocaman, Ayşe SelinThis paper provides an overview of the research dealing with optimization of pumped hydro energy storage (PHES) systems under uncertainty. This overview can potentially stimulate the scientific community’s interest and facilitate future research on this topic. We review the literature from various perspectives, including the optimization problem type, objective function, physical characteristics of the PHES facility, paradigm used to capture uncertainty, and solution method adopted. We then identify several research gaps and future research directions for energy researchers. This review highlights the need for developing optimization models such as Markov decision processes that can represent uncertainties in renewable energy sources and electricity markets more accurately, constructing multi-objective models that consider not only economic but also environmental impacts, investigating underrepresented solar-PHES systems and PHES sizing problems, addressing nonlinear characteristics of PHES facilities, and optimizing bidding strategies in sequential or coordinated electricity markets.Item Open Access Retail location competition under carbon penalty(Elsevier, 2018) Dilek, H.; Karaer, Ö.; Nadar, EmreWe study the retail location problem in a competitive linear market in which two retailers simultaneously choose their locations. Both retailers procure identical products from a common supplier and each consumer purchases from the closest retailer. Each retailer incurs transportation costs for inventory replenishment from the warehouse and consumer travels to the store. We consider two carbon tax schemes imposed on retailers: for supply-chain-related transportation and for consumer-related transportation. Our analysis indicates that intense competition between retailers leads to a “minimal differentiation” equilibrium, which substantially increases the total system emissions. According to our numerical experiments with realistic parameter values, carbon tax on supply-chain-related transportation does not affect retail location decisions. Carbon tax on consumer transportation, however, may effectively induce the retailers to approach the middle of their respective markets, reducing the total system emissions. Our analysis also indicates that a low carbon price, relative to market profitability, only reduces the total system profit without any effect on emissions. Our findings suggest that the central policymaker avoid a uniform carbon price across different sources of emission and sectors with different characteristics.Item Open Access Short-term assessment of pumped hydro energy storage configurations: Up, down, or closed?(Elsevier Ltd, 2022-11-09) Toufani, Parinaz; Nadar, Emre; Kocaman, Ayşe SelinWe study the energy generation and storage problem for various types of two-reservoir pumped hydro energy storage facilities: open-loop facilities with the upper or lower reservoir fed by a natural inflow and closed-loop facilities. We formulate this problem as a stochastic dynamic program under uncertainty in the streamflow rate and electricity price. We include the streamflow rate and electricity price as exogenous state variables in our formulation. We compare the short-term total cash flows obtained by running different pumped hydro energy storage configurations in a market setting where the electricity price can be negative. We first derive theoretical bounds on the revenue gains and losses from switching from one configuration to another. We then conduct numerical experiments by employing time-series models to formulate the evolution of our exogenous state variables. We consider three distinct seasons with different streamflow rates, different negative price occurrence frequencies, and different reservoir capacities. Our results show that: (1) The open-loop facility with the upstream flow can yield cash flows that are up to four times as large as those of the closed-loop facility; (2) The cash flow from operating a large closed-loop facility can be achieved by operating an open-loop facility with 10 times smaller reservoirs; and (3) The open-loop facility with the downstream flow can be more advantageous than the open-loop facility with the upstream flow (with an improvement of more than 10% in the cash flow) if the negative electricity price occurs more than 30% of the time.Item Open Access Structural results for average‐cost inventory models with Markov‐modulated demand and partial information(Wiley-Blackwell, 2020) Avcı, Harun; Gökbayrak, Kağan; Nadar, EmreWe consider a discrete‐time infinite‐horizon inventory system with non‐stationary demand, full backlogging, and deterministic replenishment lead time. Demand arrives according to a probability distribution conditional on the state of the world that undergoes Markovian transitions over time. But the actual state of the world can only be imperfectly estimated based on past demand data. We model the inventory replenishment problem for this system as a Markov decision process (MDP) with an uncountable state space consisting of both the inventory position and the most recent belief, a conditional probability mass function, about the actual state of the world. Assuming that the state of the world evolves as an ergodic Markov chain, using the vanishing discount method along with a coupling argument, we prove the existence of an optimal average cost that is independent of the initial system state. For our linear cost structure, we also establish the average‐cost optimality of a belief‐dependent base‐stock policy. We then discretize the uncountable belief space into a regular grid and observe that the average cost under our discretization converges to the optimal average cost as the number of grid points grows large. Finally, we conduct numerical experiments to evaluate the use of a myopic belief‐dependent base‐stock policy as a heuristic for our MDP with the uncountable state space. On a test bed of 108 instances, the average cost obtained from the myopic policy deviates by no more than a few percent from the best lower bound on the optimal average cost obtained from our discretization.Item Open Access Sustainable transportation system design(Springer, 2020) Utku, İ. Y.; Çolak, M.; Özmısır, D.; Boz, A.; Aydoğdu, T.; Didiş, M. C.; Nadar, EmreAs reducing the carbon footprint became one of the topmost concerns of the firms, Company X Turkey has a goal of transforming all of its operations in environmentally sustainable manner. Therefore, they specified their main goal as reducing yearly carbon emission levels of the company by five percent calculated in key performance indicator. Although there are several causes of increased levels of carbon emission, since the control capability of the company is limited in other fields, this study focuses specifically on developing a strategy for reducing the carbon emission generated due to Company X’s transportation system in Turkey. The aim of this study is to create a well-designed transportation network through the detection of CO2 emission causes. To lower route-based emission levels, more utilized use of cross-dock locations and alternative fuel usage is recommended, while routing is provided by integer programming. Improvement suggestions including fleet aerodynamics, tire pressure, optimal speed, and acceleration for fleets are constructed in a separate branch to decrease fleet-based carbon emissions in the system.Item Open Access Technical note—optimal procurement in remanufacturing systems with uncertain used-item condition(INFORMS Inst.for Operations Res.and the Management Sciences, 2023-05-08) Nadar, Emre; Akan, Mustafa; Debo, Laurens; Scheller-Wolf, AlanWe consider a single-product remanufacture-to-order system with multiple uncertain quality levels for used items, random procurement lead times, and lost sales. The quality level of a used item is revealed only after it is acquired and inspected; the remanufacturing cost is lower for a higher-quality item. We model this system as a Markov decision process and seek an optimal policy that specifies when a used item should be procured, whether an arriving demand for the remanufactured product should be satisfied, and which available item should be remanufactured to meet this demand. We characterize the optimal procurement policy as following a new type of strategy: state-dependent noncongestive acquisition. This strategy makes decisions, taking into account the system congestion level measured as the number of available items and their quality levels. We also show that it is always optimal to meet the demand with the highest-quality item among the available ones. We conclude with extensions of our model to limited cases when the used-item condition is known a priori (for two quality levels) and remanufacture-to-stock systems in which the standard push strategy is optimal in the remanufacturing stage. © 2023 INFORMS.Item Open Access Tüketici davranışlarının zamanla değiştiği kapalı devre tedarik zincirlerinde satış planı eniyilemesi(Gazi Üniversitesi , Fen Bilimleri Enstitüsü, 2019) Nadar, EmreBu çalışmada, yeni ve yeniden imal edilen olmak üzere iki farklı şekilde satılabilen ürünlerin satış planını eniyileme problemi ele alınmaktadır. Ürün talep eğrisi, literatürde yaygın olarak kullanılan ve gerçek verilerle birçok dayanıklı ürün için geçerliliği onaylanmış Bass yayılım sürecini takip etmektedir. Üretici belirli dönemlerde belirli taleplerin karşılanmasını geciktirerek yayılım sürecini yavaşlatabilmektedir. Satılan ürünlerin bir kısmı belirli bir süre kullanıldıktan sonra üreticiye geri dönerek yeniden imalata hazır hale gelmektedir. Yayılım sürecinin yavaşlatılarak yeniden imal edilen ürün talebinin kullanılmış ürün arzıyla daha etkin bir şekilde eşleştirilmesi üretici açısından daha kazançlı olabilmektedir. Müşterinin ürünün markasına (veya üreticisine) bağlılığı ve yeniden imal edilen ürün talebi zamanla değişebilmektedir. Bu satış planı eniyileme problemini makul ölçekli örneklerde çözebilen bir dinamik programlama algoritması önerilmektedir. Sayısal çalışmalarda bu algoritma kullanılarak en iyi satış politikalarının yayılım süreci ve tüketici davranışı parametrelerine göre değişimi incelenmiştir. Müşterinin markaya bağlılığının veya yeniden imal edilen ürün talebinin zamanla azalmasının, yeniden imalattan daha fazla gelir elde etmek için zamanında karşılanan toplam talep sayısını azalttığı gözlenmiştir.