SPI (Sustainable Process Index) is an approach to analyze the sustainability of a process is based on the calculation of an aggregate indicator proposed by Krotscheck and Narodoslawsky.
The SPI calculation is based on the mass and energy balances of the process. It is independent of the legal standards that can vary over time, making it particularly attractive. The aim of the SPI is to compare the mass and energy flows generated by human activities to natural material flows, on a global and local scale.
The total area to integrate a specific process in the ecosphere in a sustainable manner is then given by:
Where, AMP represents the area for the extraction of raw materials, AE denotes the area relative to the energy resource, AI denotes the area relative to facilities, AS denotes the area relative to staff, and AD denotes the area to discharge all waste and emissions.
Processes produce services or goods. The impact per unit of good or service is represented by a specific area atot:
Where, Np represents the number of goods or services produced by the process, such as the amount of kilowatt per hour produced by a specific energy system. The reference period is generally one year. Finally, we can link this specific area, for the production of a certain good or service, to the statistically available area per person to provide goods or services in a sustainable manner. The following ratio defines SPI as:
Where ain is the available surface relative to the annual supply of goods and energy per person. It is usually estimated by dividing the total area of a region by the annual number of its inhabitants.
Actually, the SPI indicates how much of the area, which is theoretically available per person to ensure their livelihood under sustainable conditions, is used for the production or the service in question: as the SPI (or atot) gets lower, the impact on the ecosphere to provide the good’s or service also becomes lower. A key point of the SPI assessment is the ability to specify and compare the different impacts of a technology.
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