EME 807
Technologies for Sustainability Systems

3.5. Sustainability Index


3.5. Sustainability Index

Several quantitative metrics have been constructed by Brown and Ulgiati (1997), based on the emergy theory (see system diagram in Figure 3.5). The treatment below provides a good example of how environmental metrics can be blended with economic and social aspects and link them to the system sustainability in a broader sense. 

Emergy-based indices described in text below
Figure 3.5. Emergy-based indices, accounting for local renewable emergy inputs (R), local nonrenewable inputs (N), and purchased inputs from outside the system (F).
Credit: Brown, M.T. and Ulgiati, S., Ecological Engineering 9 (1997) 51–69

Figure 3.5 is a system diagram showing the energy flows and transformations within a generic locale (surrounded by the system boundary). The Economic Use box can be seen as a "transformer" of the available energy and resources into some Yield (Y), i.e., some product directly related to the function of this system. The inputs to the system are classified as renewable resources, non-renewable resources, local resources, and non-local (purchased) resources. In this model, it is presumed that system sustainability is favored by using renewable energy resources and local energy resources. The resources that are both renewable and local are denoted by R on this diagram. On the contrary, non-renewable local (N) and any non-local, i.e., purchased (F) resources are assumed to lower overall sustainability of the system. These assumptions set the basis for devising a few sustainability metrics in this study.

One of such metrics, which characterizes the environmental impact of an energy flow, is Environmental Loading Ratio (ELR):

ELR = (F + N) / R

From this relationship, we can see that the more non-renewable and outside resources are involved in the process, the higher the ERL index. An increase in renewable energy use in the denominator translates into a lower ELR value. As you can guess, lower ELR is beneficial for the environment.

Another index introduced here is Energy Yield Ratio (EYR):

EYR = Y / F

This metric characterizes the system's capability to exploit local resources (renewable or not). The more the system depends on imported resources or services (increasing F), the lower the EYR, and the higher the system's vulnerability.

Finally, the Sustainability Index (SI) combines both ELR and EYR as follows:


Obviously, for higher sustainability “score”, we are interested in having the highest EYR versus the lowest ELR. Within this approach, SI can be used as an aggregate measure to characterize the sustainability function of a given process, technology, or economy.

Please see further explanation of this method and example calculations of metrics in the reading material referenced below.

Reading Assignment:

Journal article: Brown, M.T., and Ulgiati, S., Ecological Engineering 9 (1997) 51-69.

This paper explains the calculation of environmental and sustainability indices based on the available energy flows. It illustrates the process of devising sustainability metrics and applying them to a number of technologies and products.

Please study this article. In this lesson activity, you will be asked to perform a simple calculation of the environmental metrics based on the approach described herein.

The article is available as PDF file in the Lesson 3 Module on Canvas or can be accessed through the databases of the PSU Library system.

Note that the above-described approach to assessing a system sustainability is just a single illustration of how sustainability metrics can be devised. The parameters chosen by the authors were specific to their objectives. Calculations they provide answer some of the questions, but may not answer other questions that different stakeholders may have. In that respect, setting the objectives for your assessment and stating clear definitions and assumptions is a very important step in any assessment study in order to make the results meaningful.