Global energy efficiency

Engineering fundamental of energy efficiency

Advances in energy efficiency are essential if carbon emissions are to be reduced. However, the current rational for identifying and ranking efficiency options appears to lack any clear or consistent basis. In response, this research creates a framework for assessing energy efficiency options by considering the scale of opportunity and the technical potential for improvement. The result is a fundamental analysis of the technical limits to global energy efficiency, which can be used to direct research priorities and energy policy in the area of efficiency.

There are good reasons to improve energy efficiency: scarcity of energy resources, avoiding wastefulness, reduced expenditure on energy, and perhaps most pressing the threat of climate change. Previous attempts to assess energy efficiency options-for example, Lovins’ Factor Four (1) and Solcolow’s stabilisation wedges (2) – provide useful technical examples, but make no attempt to rank the options they describe. At best, efficiency options are prioritised by their marginal cost of reducing emissions, for example in the abatement curves produced by the International Energy Agency (3). However, these curves consider only known technologies in the current economic market, instead of comparing options based on the long-term physical potential for improvement.

Therefore there is a need to identify and rank efficiency options using a physical basis that is independent of drivers in today’s market. This research responds to this need by asking two key questions:

The scale of opportunity is found by tracing the global flow of energy through society from fuel to service. The complex flow of energy is shown below, beginning with primary energy sources on the left, flowing through various energy transformation steps, and finishing with final energy services on the right. The analysis focuses on the technical components of each energy chain, making an important distinction between conversion devices (e.g. engines, furnaces, and light bulbs) and passive energy systems (e.g. vehicles, building and factories). The diagram shows which technical devices transform the most energy, and therefore where sizable efficiency gains are likely to be available.


Figure 1: From fuel to service: tracing the global flow of energy through society

Figure 1: From fuel to service: tracing the global flow of energy through society

Each energy chain contains multiple conversion steps, and energy efficiency of each step can be calculated. For this analysis, we firstly calculate the theoretical limit of efficiency in each conversion device. A second-law approach is employed which considers both the quantity and quality of energy use (or energy’s ability to perform work) which allows the conversion devices to be compared with their theoretical ideal. Thus an upper limit for the improvement potential can be calculated.

The resulting map provides a graphical measure of the efficiency of each conversion step: fuel transformation, electricity generation and distribution, end-use device conversion. What is apparent is that current energy efficiency policy, which focuses on light bulbs, standby losses and aircraft engines, is misguided. Instead, the focus should be on improving engines and burners, in which the largest potential gains are found. Energy losses from each conversion step are collated into useful engineering loss mechanisms (the bottom right-hand corner) providing a useful breakdown to focus research priorities.


Figure 2: Energy conversion devices: a global map of efficiency

Figure 2: Energy conversion devices: a global map of efficiency

It is unlikely that real conversion devices will ever approach the theoretical efficiency limits because of thermodynamic and engineering constraints. Therefore, the current research attempts to define a practical improvement potential by creating an engineering model of each conversion device and exploring the possible ranges of key variables that affect energy use. Practical models are also created for passive energy systems, for which no theoretical efficiency can be calculated. These estimate the energy savings from technical improvements such as the insulation of a building to conserve heat, or the streamlining a vehicle to conserve motion.

1. E. von Weizacker, A. B. Lovins, and L. H. Lovins. Factor four: doubling wealth, halving resource use. Earthscan, London, 1998.
2. S. Pacala and R. Socolow. Stabilization wedges: solving the climate problem for the next 50 years with current technologies. Science, 305(5686):968-972, 2004.
3. OECD/IEA. Energy Technology Perspectives: scenarios and strategies to 2050. International Energy Agency, Paris, 2006.


A list of publications on our work to date on global energy efficiency can be found here