Validating Sample Average Approximation Solutions with Negatively Dependent Batches

Sample-average approximations (SAA) are a practical means of finding approximate solutions of stochastic programming problems involving an extremely large (or infinite) number of scenarios. SAA can also be used to find estimates of a lower bound on the optimal objective value of the true problem which, when coupled with an upper bound, provides confidence intervals for the true optimal objective value and valuable information about the quality of the approximate solutions. Specifically, the lower bound can be estimated by solving multiple SAA problems (each obtained using a particular sampling method) and averaging the obtained objective values. State-of-the-art methods for lower-bound estimation generate batches of scenarios for the SAA problems independently. In this paper, we describe sampling methods that produce negatively dependent batches, thus reducing the variance of the sample-averaged lower bound estimator and increasing its usefulness in defining a confidence interval for the optimal objective value. We provide conditions under which the new sampling methods can reduce the variance of the lower bound estimator, and present computational results to verify that our scheme can reduce the variance significantly, by comparison with the traditional Latin hypercube approach

Association of Researchers in Construction Management

Design errors are claimed to account for 26% of the cost of defects, these in turn are stated to encompass 2-9% of production cost for building and constructions. Lack of knowledge and information has been identified as a major reasons for design errors. Recently Building Information Modelling (BIM) has been considered as a mean for reducing design errors. However, limited research has been conducted on the role of BIM as a means for transfer and sharing knowledge in order to reduce design errors. The aim of the paper is to analyse BIM's role of facilitating knowledge and expertise sharing in order to prevent design errors. The aim is achieved by analysing a case study of design errors in a construction project. By drawing on the concept of boundary object it is confirmed that BIM can serve a mean for preventing design errors by facilitating knowledge and expertise sharing, across discipline, time and space, and professional boundaries. Depending the kind of boundary knowledge and expertise should be shared across, different challenges emerge in organizing the knowledge and expertise sharin

Association of Researchers in Construction Management

Design errors are claimed to account for 26% of the cost of defects, these in turn are stated to encompass 2-9% of production cost for building and constructions. Lack of knowledge and information has been identified as a major reasons for design errors. Recently Building Information Modelling (BIM) has been considered as a mean for reducing design errors. However, limited research has been conducted on the role of BIM as a means for transfer and sharing knowledge in order to reduce design errors. The aim of the paper is to analyse BIM's role of facilitating knowledge and expertise sharing in order to prevent design errors. The aim is achieved by analysing a case study of design errors in a construction project. By drawing on the concept of boundary object it is confirmed that BIM can serve a mean for preventing design errors by facilitating knowledge and expertise sharing, across discipline, time and space, and professional boundaries. Depending the kind of boundary knowledge and expertise should be shared across, different challenges emerge in organizing the knowledge and expertise sharin

SOFC development program at Haldor Topsøe/Risø National Laboratory - progress presentation

The SOFC technology under development at Haldor Topsøe A/S and Risø National Laboratory is based on an integrated approach ranging from manufacturing of planar anode-supported cells and compact stacks to analysis of total systems. Today, the consortium of Haldor Topsøe A/S and Risø has an extended program to develop the SOFC technology all the way to a marketable product. The standard cells are thin and robust with dimensions of 12 x 12 cm2 and the cell stacks are based on internal manifolding. Production of cells in a pilot production plant is being up-scaled continuously. Stack and system modelling including cost optimisation analysis is used to develop 5 kW stack modules for operation in the temperature range 700-850°C. High volume power density stacks based on thin plate metallic interconnects have been tested formore than 10000 hours including thermal cycling with encouragingly small degradation. Stacks in the 1+ kW size classes have been tested in methane as well as CO rich gas. The SOFC program comprises development of next generation cells and multi stack modules for operation at lower temperature with increased durability and mechanical robustness. Development of cells with porous metallic support and new cathode materials is in progress in order to ensure long-term competitiveness

Fuktegenskaper hos cementbundet material med flygaskainblandning

Med cementbundna materials fuktegenskaper menas hur de binder och transporterar vatten. Känner man fuktegenskaper kan man räkna ut hur exempelvis en betong torkar. Mätning av fuktegenskaper för cementbundna material är något Avd. Byggnadsmaterial i Lund har stor erfarenhet av, särskilt för material med Portlandcement som bindemedel [1] [2]. På senare år har forskning vid avdelningen också visat hur fuktegenskaperna förändras när delar av Portlandcementet ersätts med slagg eller silika [3] [4]. Denna artikel är en kortfattad sammanfattning av resultaten från ett pågående doktorandprojekt inom vilket fuktegenskaperna undersöks för cementbundet material där delar av Portlandcementet ersatts med flygaska. Projektet kommer redovisas i sin helhet nästa år