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Ernst Worrell


Understanding energy use is crucial if society is to make a transformation to more sustainable production and consumption patterns, since energy use patterns are key factors in climate change, (air) pollution, and the depletion of nonrenewable resources. Measuring the sustainability of energy use has many aspects that are closely interrelated (e.g., production, conversion, supply, price, efficiency), and their relationships change over time, moving and removing the boundaries of the energy system and its sustainability. This chapter focuses on two boundaries and the implications these have for measuring sustainability: the supply side, which results from changing sources and reserves, and the demand side, due to changes in use and the impact on supply side development. The economy is a basket of activities or, better, energy services. An energy service is the service provided by the energy-using device. Ultimately, the economy is not interested in using energy, but rather in the provision of energy services, and this at the lowest cost to society. Changes in the demand of energy services have altered the way society supplies energy, and will do so in the future, making a sustainable provision of energy services key to a future sustainable energy system. While energy demand is a key issue to determine sustainability of the energy system, reliable data is still lacking with which to measure the sustainability of demand. Understanding of (future) energy demand and end-use technology is thus still limited.


Understanding energy use, conversion, and supply is a key issue in the transformation of society to more sustainable production and consumption patterns. Not only is energy strongly connected to emissions to the environment (e.g., climate change, air pollution, solid waste, thermal pollution from cooling water), it is also strongly connected to the depletion of natural resources (e.g., fossil fuels, uranium, freshwater) and to public health (e.g., air and indoor pollution). The analysis of life cycle assessment (LCAs) of a broad range of products has demonstrated that (fossil) energy use is the primary determining factor in those LCAs for almost all impact areas, except toxics (Huijbregts et al. 2006). Reserves of oil and natural gas have already been depleted in some areas of the world, where economies like Indonesia and the United States have become net importers of oil and natural gas within a few decades. North Sea oil and gas supplies are rapidly decreasing. Whereas air pollution (e.g., ozone, particulate matter) affects the health and quality of life for millions of people around the world, in developing countries indoor air pollution (due to the use of traditional fuels and heating practices) leads to serious health effects for millions of (mainly) women. The principal challenges for the global energy system are twofold: (a) the world is running out of a sink for carbon dioxide (CO2), i.e., atmosphere, to use fossil fuels (climate change), and (b) the world is running out of low-cost energy sources.

Climate change and the policies to mitigate the impact will seriously impact the energy system and direct the transformation toward a more sustainable system over the next decades. While the scientifi c understanding of climate change is still growing, the IPCC Fourth Assessment Report provides an in-depth review of the trends and necessary changes needed to avoid serious impacts of climate change.

Currently, the world is also facing drastic price changes of energy resources. Despite climate change policy, demand for fossil fuels has increased rapidly over the past period. Reserve (recoverable) estimates have proven difficult and are subject to many uncertainties, estimates, and assumptions. While fossil fuel reserves are finite, there are probably still large reserves available, especially if nonconventional sources are included. However, exploiting these reserves is limited by access (e.g., North Pole, deep waters) and necessary recover technologies (e.g., tar sands). Most certainly, this spells the end to the era of low-cost fossil fuels.

Measuring the sustainability of energy use has many aspects that are closely interrelated: production, conversion, supply, price, efficiency. These relationships change over time, moving and removing the boundaries of the energy system and its sustainability. In this chapter, I focus on two boundaries and the implications they have for measuring sustainability:

• Supply: changing reserves and sources.

• Demand: changing uses and impacts on supply.

I will first discuss the ways that energy is measured, and how energy use has changed over time. This is followed by a discussion how moving boundaries affect the way we look at energy supply and demand, and how we evaluate methods to assess the sustainability of energy use.

Measuring Energy

Energy analysis started back in the 1970s and has provided various tools to measure energy use. As such, it laid the basis for LCA. Energy is measured in various ways using different system boundaries. Applying thermodynamic laws, energy is expressed using the first law (enthalpy) and the second law (entropy, exergy). Entropy expresses the quality of the energy (i.e., the amount of work for a given unit of energy), whereas exergy is used to estimate the overall efficiency with which society uses its energy sources. Using exergy as a measure, Ayres (1989) found that the overall efficiency of the U.S. economy is only 2.5%. Enthalpy is the commonly used term in energy analysis, and expresses the amount of heat released for a given amount of fuel. The energy content can be given in lower heating value (LHV), as is commonly used in international statistics, and higher heating value (HHV), which is predominantly used in North America. The difference is that HHV accounts for the energy content of condensed water formed during the combustion of the fuel.

Energy is converted in the energy supply chain at various times, starting with extraction, conversion, transport, and end-use conversion, before it is finally used to provide the energy service. Each time, losses are generated. In analysis and statistics, two ways are commonly used to measure energy: final energy use is the amount of energy used in a given step or end-use, while primary energy accounts for the losses during earlier conversions (e.g., the fuels needed to generate a unit of electricity). In the calculation of primary energy, arbitrary rules are sometimes used in statistics to assign the primary energy content of electricity generated by nuclear power (33%) and selected forms of renewable power (100%). Note that not all conversions may be contained in primary energy fi gures as system boundaries may vary. The gross-energy requirement (GER) includes all energy used in the whole supply chain for a given service or product (i.e., including extraction, conversion, and transport), expressed as total fuel demand (for GERs of various industrial materials, see Worrell et al. 1994).

Moving Boundaries in Energy Supply

Global energy use has risen dramatically over the past century. Globally, the use of fossil fuels has increased from 14 GJ/capita in 1900 to about 60 GJ/ capita in 2000, while population more than quadrupled in the same period. By 2000, the use of fossil fuels was estimated at 320 EJ (1018 joule) and biomass energy use at 35 EJ/year.1 In 1900, the use of fossil fuels and biomass were each estimated at 22 EJ (Smil 2003:6).

This demonstrates that not only has energy use changed rapidly, so has the composition of the fuel mix. Due to the rise of fossil fuel use in the nineteenth century, fossil fuels surpassed the use of biomass already in 1900. The use of fossil fuels started with coal, followed by oil, and finally natural gas. The major nonfossil primary energy sources are hydropower and nuclear energy (since

The so-called noncommercial use of biomass for energy purposes (mainly in developing countries) is very hard to establish. Currently, only rough estimates exist; actual biomass use may be higher than these figures suggest.

1960). In Figure 18.1, Gautier (this volume) illustrates the varying composition of the global demand for commercial primary energy sources.

The composition of the final demand of energy (e.g., at the final consumption) is also changing, with electricity demand growing due to increased demand for electricity services and cost reductions in power generation over time. Today, electricity generation has increased to almost 20,000 TWh (from 12,000 TWh in 1965) and is currently growing more rapidly than fuel use. The large increase of the (secondary) energy carrier electricity enabled a diversification of energy supply sources. Despite the dominance of fossil fuels, at no time in human history have we seen a more diverse mix of energy supply sources/fuels.

The reserves of fossil fuels are by definition finite. With increased consumption, reserves are being slowly depleted. However, reserve estimates are an inexact science (see also Gautier, this volume), driven by large uncertainties, estimates, and assumptions. Despite increasing oil consumption, proven oil reserves have increased over the past thirty years (see Figure 21.1).

One way to quantify the sustainability of consumption of nonrenewable fuels may be to measure the speed with which we extract the fuel from the resource base. This is often expressed as the reserve to production (R/P) ratio (i.e., the number of years of reserves at current consumption levels). Globally, coal has the largest R/P ratio of 130 in 2007, while the R/P ratio for conventional oil is probably around 50 (BP 2008). The R/P ratio is strongly dependent on current consumption levels as well as estimated reserves. While consumption is likely to grow for most of these fuels, when left unabated, the reserve base estimate is surrounded by large uncertainties.

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