Theoretical efficiency limits for energy conversion devices Jonathan M. Cullen * , Julian M. Allwood Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK article info Article history: Received 14 July 2009 Received in revised form 6 January 2010 Accepted 19 January 2010 Available online 5 March 2010 Keywords: Energy efficiency Sankey diagram Prioritisation Exergy analysis Conversion loss abstract Using energy more efficiently is a key strategy for reducing global carbon dioxide emissions. Due to limitations on time and resources, actions must be focused on the efficiency measures which will deliver the largest gains. Current surveys of energy efficiency measures assess only known technology options developed in response to current economic and technical drivers. However, this ignores opportunities to deliver long-term efficiency gains from yet to be discovered options. In response, this paper aims to calculate the absolute potential for reducing energy demand by improving efficiency, by finding the effi- ciency limits for individual conversion devices and overlaying these onto the global network of energy flow. The potential efficiency gains for each conversion device are found by contrasting current energy demand with theoretical minimum energy requirements. Further insight is gained by categorising conversion losses according to the underlying loss mechanisms. The result estimates the overall efficiency of global energy conversion to be only 11 per cent; global demand for energy could be reduced by almost 90 per cent if all energy conversion devices were operated at their theoretical maximum efficiency. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction: the efficient use of energy The reasons for using energy more efficiently are clear: to relieve pressure on scarce energy resources, to reduce energy costs by avoiding wastefulness, and perhaps most pressing, to reduce energy related carbon dioxide (CO 2 ) emissions which contribute to climate change. The well-known Kaya identity [1] expresses the generation of energy-based CO 2 emissions as the product of four drivers: pop- ulation, per capita wealth, energy intensity (energy per unit wealth) and carbon intensity (CO 2 per unit energy). The first two drivers are socio-economic and are difficult to limit in practice. The third and fourth drivers are technical options which require energy to be used more efficiently (which lowers energy intensity) and the de- carbonisation of energy supplies (which reduces carbon intensity). To date, emission reduction strategies have focused primarily on energy supply options: renewable energy technologies, nuclear power, carbon capture and storage (CCS) and fuel switching. Yet, the International Energy Agency (IEA) asserts that ‘energy efficiency improvements . represent the largest and least costly savings’ [[2], p. 4] available. There is further need to develop energy efficient technologies and understand the scope of efficiency measures to reduce CO 2 emissions. In the 1975 conference Efficient use of energy, Ford et al. [3] stated that the primary objective of any technical energy study is to define a target ‘standard of performance’ against which current demand for energy can be compared. Such a target may be chosen from several options, for example, current best practice, the extrapolation of an historical trend, or the projected gains from a specific design innovation. The difference between today's energy demand and this target provides a measure of the improvement potential, or possible energy savings from energy efficiency measures. Finding the global improvement potential from energy effi- ciency requires tracing the scale of energy flow through technical devices in the energy network, and assessing the efficiency gains available in each device. Equation (1) is used to calculate these potential savings from efficiency, in conversion devices: Potential for saving energy ¼ Scale of energy flow   Target efficiency  Current efficiency  (1) where the energy terms are measured in joules (J) and the effi- ciency terms in percentages (%). Assessing the scale of energy flow through the global energy network is the subject of a previous article by Cullen and Allwood [4]. Their research traces energy flow from fuel to final service, including the technical steps of fuel transformation, electricity generation, and end-use conversion, as shown in Fig. 1 . The results are presented visually in Sankey diagram form, permitting identi- fication of the technical areas with the largest energy flows. Particular attention is given to the technical components, rather * Corresponding author. Tel.: þ44 1223 760360; fax: þ44 1223 332662. E-mail address: jmc99@cam.ac.uk (J.M. Cullen). URL: http://www.lcmp.eng.cam.ac.uk Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.01.024 Energy 35 (2010) 2059e2069