Energy 31 (2006) 3041–3061 CO 2 -emissions reduction potential and costs of a decentralized energy system for providing electricity, cooling and heating in an office-building in Tokyo Ce´line Weber à , Michihisa Koyama, Steven Kraines Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 25 August 2003 Abstract Decentralized energy systems are thought to have great potential for supplying electricity, cooling, and heating to buildings. A decentralized system combining a solid oxide fuel cell (SOFC) with an absorption chiller-heater (ACH) is proposed. The CO 2 -emissions and costs of using different configurations of this SOFC-based system to provide an office building in Tokyo with electricity, cooling and heating are calculated by using an SOFC-model and an absorption-chiller model together with data for cooling and heating loads measured at an office building in downtown Tokyo. The results are compared with the CO 2 -emissions and costs of a conventional system that obtains the base electricity requirements as well as electricity for an electric chiller–heater system from the central power grid. The fully decentralized SOFC-based energy system could result in a potential CO 2 reduction of over 30% at an estimated cost increase of about 70% compared to the conventional system. r 2005 Elsevier Ltd. All rights reserved. Keywords: Decentralized energy system; SOFC; Absorption-chiller; CO 2 -emissions; Costs 1. Introduction This study focuses on the analysis of decentralized energy systems, systems of energy technology devices that provide electricity to single or small groups of buildings, but not to entire cities or urban regions. In decentralized energy systems, instead of taking the electricity from the grid, each building or group of buildings has its own independent energy generation system. We believe that decentralized energy systems could be important for meeting future energy requirements in Japan for the following reasons:  the predicted increase in energy demand in Japan, especially for computers and air-conditioning in buildings [1],  the flexibility of a decentralized system to meet the specific load patterns of a given building or a group of buildings in the most efficient manner, ARTICLE IN PRESS www.elsevier.com/locate/energy 0360-5442/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.12.003 à Corresponding author. Tel.: +41 21 693 35 13; fax: +41 21 693 35 02. E-mail address: celine.weber@epfl.ch (C. Weber).