Energy-saving policies for temperature-controlled production systems with state-dependent setup times and costs

There are numerous practical examples of production systems with servers that require heating in order to process jobs. Such production systems may realize considerable energy savings by temporarily switching off the heater and building up a queue of jobs to be processed later, at the expense of extra queueing costs. In this paper, we optimize this trade-off between energy and queueing costs. We model the production system as an M/G/1 queue with a temperature-controlled server that can only process jobs if a minimum production temperature is satisfied. The time and energy required to heat a server depend on its current temperature, hence the setup times and setup costs for starting production are state dependent. We derive the optimal policy structure for a fluid queue approximation, called a wait-heat-clear policy. Building upon these insights, for the M/G/1 queue we derive exact and approximate costs for various intuitive types of wait-heat-clear policies. Numerical results indicate that the optimal wait-heat-clear policy yields average cost savings of over 40% compared to always keeping the server at the minimum production temperature. Furthermore, an encouraging result for practice is that simple heuristics, depending on the queue length only, have near-optimal performance

A share-first-plan-second policy for efficient cooperation in a multi-modal transportation corridor

Cooperation in transportation networks has been a cornerstone of policies towards more sustainable transportation, aiming to improve the modal split and increase the fill rates of transportation resources. Effective cooperation between transportation firms requires some form of joint planning, which is often challenging to implement from an IT perspective and difficult to sustain due to the reliance on advanced planning software. In this paper, we present a simple but effective policy for cooperative transportation that does not require a complex joint optimization of operations. In this share-first-plan-second policy, cooperating firms first develop a cyclic schedule for a fleet of shared transportation resources and then assign their shipments to the transportation resources in real time. The policy performs nearly as well as a jointly optimized planning of operations while not requiring advanced IT systems and planning software. Finally, the share-first-plan-second policy exhibits robustness against deviations from planned transport operations, enhancing its practical applicability

Assessing quality of care for the dying from the bereaved relatives’ perspective: using pre-testing survey methods across seven countries to develop an international outcome measure

Background: The provision of care for dying cancer patients varies on a global basis. In order to improve care, we need to be able to evaluate the current level of care. One method of assessment is to use the views from the bereaved relatives. Aim: The aim of this study is to translate and pre-test the ‘Care Of the Dying Evaluation’ (CODETM) questionnaire across seven participating countries prior to conducting an evaluation of current quality of care. Design: The three stages were as follows: (1) translation of CODE in keeping with standardised international principles; (2) pre-testing using patient and public involvement and cognitive interviews with bereaved relatives; and (3) utilising a modified nominal group technique to establish a common, core international version of CODE. Setting/participants: Hospital settings: for each country, at least five patient and public involvement representatives, selected by purposive sampling, fed back on CODETM questionnaire; and at least five bereaved relatives to cancer patients undertook cognitive interviews. Feedback was collated and categorised into themes relating to clarity, recall, sensitivity and response options. Structured consensus meeting held to determine content of international CODE (i-CODE) questionnaire. Results: In total, 48 patient and public involvement representatives and 35 bereaved relatives contributed to the pre-testing stages. No specific question item was recommended for exclusion from CODETM. Revisions to the demographic section were needed to be culturally appropriate. Conclusion: Patient and public involvement and bereaved relatives’ perceptions helped enhance the face and content validity of i-CODE. A common, core international questionnaire is now developed with key questions relating to quality of care for the dying

Inventory Control for Multi-location Rental Systems

Verhuurartikelen vervullen vraag naar producten die duur zijn in aanschaf en vaak alleen nodig zijn voor een beperkte periode. Voorbeelden van dit soort producten zijn auto’s, films, boeken, gereedschap, gelegenheidskleding en juwelen. Complexe voorraadproblemen komen voor bij verhuurbedrijven, vooral wanneer sprake is van meerdere vestigingslocaties. Bedrijven moeten bijvoorbeeld beslissen hoeveel artikelen toe te wijzen aan iedere locatie en hoe om te gaan met aanvragen voor artikelen die op dat moment aan andere klanten zijn verhuurd. Bij dergelijke aanvragen is het gebruikelijk om artikelen te verschepen vanuit andere locaties of vanuit een opslagdepot. Omdat de handelingskosten voor transporteren hoog kunnen zijn, is het belangrijk om de verschepingen goed te coördineren en tevens het proces van herpositioneren van artikelen over de locaties goed te regelen. Dit proefschrift presenteert nieuwe methoden voor voorraadbeheer en het organiseren van logistieke processen in verhuursystemen met meerdere locaties die onderling verhuurartikelen kunnen verschepen in reactie op de vraag. Voor verschillende veelvoorkomende types verhuursystemen optimaliseren we beslissingen voor voorraadniveaus, verschepingen en herpositionering. Aan de hand van wiskundige analyses leiden we theoretische eigenschappen van optimaal beleid af. Aan de hand van numerieke methoden bepalen we dit optimale beleid voor verhuursystemen met een gelimiteerd aantal locaties. Dit geeft belangrijke inzichten in effectieve strategieën voor het verschepen en herpositioneren van voorraad in verhuursystemen met meerdere locaties. Met deze inzichten ontwerpen we heuristieken die eenvoudig te implementeren zijn en die significante verbeteringen geven ten opzichte van beslissingsregels uit de praktijk

Redistributing Stock in Library Systems with a Depot

Public library organizations often utilize depots for carrying out shipments to libraries in case of stock-outs and for storing low demand rental items at low cost. Similar systems may be employed by rental companies for other rental products such as tools, DVDs, and jewelry. Since shipments deplete the depot’s inventory, stock must be taken back from the libraries in order to deal with future shipment requests. These shipment and take-back operations are carried out periodically, e.g. daily or weekly. This work focuses on optimizing the decisions for shipments and take-backs. We model the system by means of a Markov decision process and investigate its optimal policy for various problem instances. For the take-back decision, we distinguish between so-called threshold, reactive, and preventive take-backs. We use the insights from the MDP to develop a three-phase take-back heuristic. In experiments, our heuristic performs within 1% on average from the optimal solution. For settings with a large number of libraries, it is shown that an acceptable performance can be achieved by setting a base-stock level at the depot and taking back sufficient stock from the libraries to achieve this level

Optimizing stock levels for rental systems with a support warehouse and partial backordering

Various rental systems, such as public libraries and tool rental companies, have a support warehouse for storing rental products at low cost and carrying out expedited shipments in response to stock-outs at local warehouses. In case of stock-outs, rental systems commonly employ partial backordering, i.e., a limited number of demands is backordered while additional demand is lost. We include partial backordering in our models and demonstrate its relevance. We fully characterize costs and optimal base stock policies in cases with one support and one local warehouse and provide upper bounds for base stock levels in the general system. By enumerating base stock policies and evaluating exact costs numerically, we determine optimal base stock policies for small systems with up to six local warehouses. For larger systems we develop a greedy algorithm based on approximate cost evaluations. The resulting base stock policies have costs within 0.2% from optimality on average in experiments. Among other things, experiments indicate that existing methods with no backordering may lead to poor solutions with partial backordering and that partial backordering is an effective means to reduce lost demand even with limited partial backorder levels. (C) 2017 Elsevier B.V. All rights reserved

Transshipment of cross-channel returned products

Companies increasingly employ dual-channeling strategies with online and o_ine channels to reach customers. The combination of high return rates in e-commerce and the possibility for customers to return products ordered online at any offline store may result in unbalanced inventories. Transshipments can be used to deal with these unbalanced inventories. In this paper we study dynamic policies for transshipment of products that are returned cross-channel from online to offline stores. At the end of each period in a finite sales season, cross-channel returned products can be transshipped back to the online store or kept on-hand at the offline store. Optimal transshipment policies are obtained using a Markov decision process. We introduce a well-performing heuristic based on the expected costs during the sales season, with a maximum deviation of 1:59% from the optimal costs in experiments. Furthermore, we show that in all instances our heuristic outperforms static policies in which products are either always or never shipped back to the online store. We observe that dynamic transshipment policies are more effective than static policies in dealing with imbalances in the initial stock. Dynamic transshipment of cross-channel returns seems to open up possibilities for more effective demand fulfillment of dual-channel companies