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Baseline 2030 Baseline 2050 ACT Map 2050 BLUE Map 2050

Figure 22.6 Transport energy use in the Baseline, ACT, and BLUE scenarios (IEA 2008g). Reprinted with permission.

Measurement Framework

Before exemplifying this approach, we derive general equations for measuring sustainability. Sustainability specifies a relation between the "opportunities" available to current generations and those that are passed on to future generations. If we take energy resources, broadly defined, to be synonymous with energy opportunities, then each generation must pass on to the next an equal or greater endowment of energy resources (or the ability to provide energy services) in order for the energy system to be sustainable. This is a strong energy sustainability requirement, since it does not admit that other factors may be substituted for energy to produce an equivalent level of well-being. We will return to this issue shortly.

By "broadly defined," we do not mean energy resources measured simply in joules, but rather energy resources measured by their ability to be transformed into energy services that contribute to human well-being. This concept of sustainable energy cannot be reduced to a single equation. Nonetheless, equations are an invaluable tool for representing relationships between variables that can be measured. In that spirit, we seek to define the energy sustainability relationship between generations in mathematical form. To do this, it is useful to work at a high level of generality and abstraction, while bearing in mind that to be useful the equation must be applicable to specific, real energy resource estimates.

The difficulty in the parameterization is to define a basket or set of energy services to describe human well-being. The definition of human well-being will vary across cultures and time. Still, in economics, attempts have been undertaken to define a set of human activities and services to enable a comparison of "human well-being" between countries, the so-called purchasing power parity. Although by definition incomplete, the adaptation of a similar approach to determine and defi ne a basket of energy services could provide an indicator for our exercise and proposed metric for the sustainability of the energy system.

Energy resources can be found in the form of stocks of nonrenewable resources that may be consumed over time (e.g., such as oil, coal, uranium, or natural gas) or in the form offlows of renewable energy resources (e.g., solar insolation, wind velocity, or mass of available biomass). Let the total quantity of energy resources from stocks at time t, measured in joules, be Q. There are many forms of energy resource stocks which must be treated individually. However, for the sake of simplicity, we assume that all forms of energy resource stocks can be measured in joules. Let et be the energy intensity of the conversion of energy resource stocks into energy services in time t, with units of joules per unit of energy service. The total amount of energy services available in the form of stocks is Qt !ef Let the annual flow of energy in joules per year from all renewable sources be qt and assume—although the conversion efficiency for renewables is in many cases much lower than for fossil fuels (e.g., solar to electricity is only 5-20%, geothermal 10-25% vs. gas or coal plans which range from 30-55%)—that renewable energy has the same conversion to service efficiency as energy stocks, et. It is important to note that neither Qt nor qt represent all the energy potentially available but rather those portions that are technically feasible and economically practical to produce given existing technological, economic, environmental, and social conditions.

The total stock of nonrenewable energy is QJe, but what is the stock of renewable energy? We know that the total flow of renewable energy handed forward to future generations is qt /et per year, but how much nonrenewable energy is available each year? With these definitions, stocks and flows cannot be combined to obtain total energy resources; one is expressed in joules, the other in joules per year. One solution to this dilemma—converting fossil energy resources into a flow—can be deduced from the definition of sustainability. Let the use of fossil energy per year be g, then Nt = Qt/gt is a measure of the number of years of fossil resources available relative to current use. Sustainability implies that the current generation should not leave the next generation with less energy relative to current use than it inherited. Finally, since the total needs of future generations may be expected to grow with population, P, it seems necessary that the endowment of energy resources should be expressed on a per capita basis.

The current per capita endowment of energy resources expressed as an annual flow of energy services is:

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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