Emergy Sustainability and Its Indicators

Unlike classical energy and economic analyses that only consider items that can be quantified in energy or money terms, thus omitting most free inputs from the environment, emergy analysis is a thermodynamic methodology which considers both the economic and environmental aspects of a system by converting all inputs, flows, and outputs to the common denominator of solar energy, the basic energy behind all the processes of the biosphere.

This is a primary factor because, although the market only considers monetary value, the economy is also based on quantities from the environment, which must be considered and assigned a value, if resources are to be exploited sustainably in the long period.

Emergy analysis is useful to check applications of Herman Daly's first rule of sustainable development, the so-called sustainable yield principle, that states that resources should be exploited at a rate compatible with their replacement by nature. It can be used to define guidelines for consumption of resources compatible with their formation times.

Emergy can be regarded as the solar energy necessary to sustain a system; the greater the total emergy flow necessary for obtaining a product, the greater the consumption of solar energy necessary for its re-formation once it has been used, and thus the greater the past and the present environmental cost to maintain it.

The intensive use of the services and products of an ecosystem can degrade its structures and functions, decreasing the capacity of the ecosystem to self-organize efficiently. In order to facilitate the measurement of a system's sustainability, some emergy indicators were introduced.

To explain them a simple model of a system is shown in Figure 4. Emergy flows to the system are divided into the main categories as given in the following:

• local renewable resources (R);

• local nonrenewable resources (N);

• feedback (F): purchased resources and services from outside system; and

• the yield (Y): the output of the system ('virtual' in the case of territory).

These flows can be combined to obtain a set of indicators; here are most common ones.

1. Emergy yield ratio (EYR) is the ratio of the output of a system (Y) to the feedbacks from outside (F). Therefore, it is the ratio of total emergy to the nonrenewable, economic inputs. Considering that the total emergy is the sum of all local and external emergy inputs, the higher the ratio, the higher is the relative contribution of the local sources of emergy to the system. This index therefore shows the ability of a system to use the available local resources. Generally, EYR values less than 5 are typical of secondary energy sources and primary materials (e.g., cement and still), whereas values greater than 5 are shown by primary energy sources. In the case of processes that give products with EYR values less than 2, the processes

Plan Chantournage Gratuit
Figure 4 Energy system diagram of a generic system.

are not considered an energy source but rather a consumer or a transformation process:

2. The environmental loading ratio (ELR) is the ratio of renewable to nonrenewable emergy use inside a system. It is a measure of the renewability of a production system or a system state. The higher the ratio, the lower the system's sustainability. A high ratio suggests a high technological level of emergy use and/or a high level of environmental stress (either local or global). It sounds an alarm-bell of nonequilibrium which could become irreversible for a state or a production system:

3. Emergy money ratio is the ratio of emergy use in a country (or a region) to its gross national product. It measures how much emergy is associated with the economic wealth of a state. The emergy money ratio provides a link between emergy evaluation and economics. The use of emergy with traditional economic evaluation methods can provide additional information to guide human activities, as in planning land use or designing production processes. This tool has a twofold function: (1) it provides information about systems, for example, comparison of national economies with different emergy flows and GDP, or evaluation of the trend of an economic system in time; (2) it can be used as a conversion factor for further emergy evaluations when monetary parameters must be converted into emergy and vice versa:

where U is the total emergy used in a territorial system (U — R + N + F).

4. The emergy investment ratio (EIR) is the ratio of feedback to local resources (renewable and otherwise). It measures how much a system depends on the outside rather than on local resources, and how much a system or process uses invested emergy in comparison with alternatives:

5. Emergy per person is the ratio of emergy used in a country divided to population. It measures how much emergy is available per person. This indicator suggests a measure of the standard of living in a country in terms of current availability of resources and goods, though it does not give any indication about the future availability of resources. Two different meanings are associated with emergy per person: in the case of a low ELR (low use of nonrenewable inputs), a high emergy per person ratio indicates resource availability, whereas when ELR is high (high use of nonrenewable inputs), emergy per person represents consumption and the system needs to invest in order to decrease emergy use and enhance renewable uses:

Emergy per person (sej/pers.) = U/person where U is the total emergy used in a territorial system (U = R + N + F).

6. Empower density is the ratio of emergy flow into a system to area of the system. It measures the concentration of emergy in space and time. This indicator is a measure of spatial concentration of emergy flow within a process or system. It can be used to highlight areas under environmental pressure with respect to more natural areas. A high empower density can be found in processes in which emergy use is large with respect to the available area. This suggests that land is a limiting factor for the future economic growth. The emergy density can be used to plan environmental policy. Areas with high emergy densities have a concentration of emergy use and call for different policies than natural or agricultural areas. For example, cities and industrial districts should invest in increasing efficiency of energy use, transport and so on, whereas agriculture areas need more attention to soil management and fertilizer use:

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