Embodied energy is defined as the sum of energy inputs (fuels/power, materials, human resources etc) that was used in the work to make any product, from the point of extraction and refining materials, bringing it to market, and disposal / re-purposing of it. Embodied energy is an accounting methodology which aims to find the sum total of the energy necessary for an entire product lifecycle. This lifecycle includes raw material extraction, transport,[1] manufacture, assembly, installation, disassembly, deconstruction and/or decomposition.
Different methodologies produce different understandings of the scale and scope of application and the type of energy embodied. Some methodologies account for the energy embodied in terms of the oil that supports economic processes.
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The history of constructing a system of accounts which records the energy flows through an environment can be traced back to the origins of accounting itself. As a distinct method, it is often associated with the physiocrat's "substance" theory of value [2], and later the agricultural energetics of Sergei Podolinsky, a Ukrainian physician [3], and the ecological energetics of V.V. Stanchinsky [4]
The main methods of embodied energy accounting as they are used today grew out of Wassily Leontief's input-output model and are called Input-Output Embodied Energy analysis. Leontief's input-output model was in turn an adaptation of the neo-classical theory of general equilibrium with application to "the empirical study of the quantitative interdependence between interrelated economic activities" [5]. According to Tennenbaum[6] Leontief's Input-Output method was adapted to embodied energy analysis by Hannon[7] to describe ecosystem energy flows. Hannon’s adaptation tabulated the total direct and indirect energy requirements (the energy intensity) for each output made by the system. The total amount of energies, direct and indirect, for the entire amount of production was called the embodied energy.
Embodied energy analysis is interested in what energy goes to supporting a consumer, and so all energy depreciation is assigned to the final demand of consumer. Different methodologies use different scales of data to calculate energy embodied in products and services of nature and human civilization. International consensus on the appropriateness of data scales and methodologies is pending. This difficulty can give a wide range in embodied energy values for any given material. In the absence of a comprehensive global embodied energy public dynamic database, embodied energy calculations may omit important data on, for example, the rural road/highway construction and maintenance needed to move a product, human marketing, advertising, catering services, non-human services and the like. Such omissions can be a source of significant methodological error in embodied energy estimations.[8] Without an estimation and declaration of the embodied energy error, it is difficult to calibrate the sustainability index, and so the value of any given material, process or service to environmental and human economic processes.
The SBTool, UK Code for Sustainable Homes and USA LEED are methods in which the embodied energy of a product or material is rated, along with other factors, to assess a building's environmental impact. Embodied energy is a concept for which scientists have not yet agreed absolute universal values because there are many variables to take into account, but most agree that products can be compared to each other to see which has more and which has less embodied energy. Comparative lists (for an example, see the University of Bath Embodied Energy & Carbon Material Inventory[9]) contain average absolute values, and explain the factors which have been taken into account when compiling the lists.
Typical embodied energy units used are MJ/kg (megajoules of energy needed to make a kilogram of product), tCO2 (tonnes of carbon dioxide created by the energy needed to make a kilogram of product). Converting MJ to tCO2 is not straightforward because different types of energy (oil, wind, solar, nuclear and so on) emit different amounts of carbon dioxide, so the actual amount of carbon dioxide emitted when a product is made will be dependent on the type of energy used in the manufacturing process. For example, the Australian Government[10] gives a global average of 0.098 tCO2 = 1 GJ. This is the same as 1 MJ = 0.098 kgCO2 = 98 gCO2 or 1 kgCO2 = 10.204 MJ.
In the 2000s drought conditions in Australia have generated interest in the application of embodied energy analysis methods to water. This has led to use of the concept of embodied water.
Some typical values of embodied energy in common materials are: [11]
| Material | Specific energy cost (MJ/kg) |
Density @ STP (g/cm3) |
Volumetric energy cost (GJ/m3) |
Principal inputs |
|---|---|---|---|---|
| Aluminium | 220 | 2.7 | 590 | bauxite |
| Asphalt concrete | 2.4 | 2.3 | 5.5 | gravel, bitumen |
| Brick | 2.88 | 1.69 | 5.17 | clay, shale |
| Concrete | 0.95 | 1.1 | 1.0 | rock, sand, Portland cement |
| Copper | 70 | 8.9 | 630 | chalcopyrite |
| Glass | 16 | 2.5 | 40 | sand |
| Plastic (LDPE) | 80 | 0.92 | 74 | petroleum |
| Steel | 35 | 7.8 | 270 | iron ore |
Treloar, et al. have estimated the embodied energy in an average automobile in Australia as 0.27 terajoules as one component in an overall analysis of the energy involved in road transportation.[12]