Eficiencia Exergética

Energy 35 (2010) 2059e2069

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Energy
journal homepage: www.elsevier.com/locate/energy

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

abstract

Article history:
Received 14 July 2009
Received inrevised form
6 January 2010
Accepted 19 January 2010
Available online 5 March 2010

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
developedin 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 efficiency limits for individual conversion devices and overlaying these onto the global network ofenergy
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 bereduced by almost 90
per cent if all energy conversion devices were operated at their theoretical maximum efficiency.
Ó 2010 Elsevier Ltd. All rights reserved.

Keywords:
Energy efficiency
Sankey diagram
Prioritisation
Exergy analysis
Conversion loss

1. Introduction: the efficient use of energy
The reasons for using energy more efficiently are clear: to relieve
pressure on scarce energyresources, to reduce energy costs by
avoiding wastefulness, and perhaps most pressing, to reduce energy
related carbon dioxide (CO2) emissions which contribute to climate
change. The well-known Kaya identity [1] expresses the generation
of energy-based CO2 emissions as the product of four drivers: population, per capita wealth, energy intensity (energy per unit wealth)
and carbon intensity (CO2per 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 decarbonisation 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 CO2 emissions.

*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
0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.energy.2010.01.024

In the 1975 conference Efficient use of energy, Ford et al. [3] stated
that the primary objective of any technical energy study is todefine
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...