What is Emergy?
Emergy is perhaps the most significant, little known or misunderstood quantity in the realm of science. It is a quantity that can be derived from the laws of thermodynamics, including the 1st and 2nd laws of thermodynamics and the maximum power principle, or the so-called 4th law (Lotka, 1922 a,b). However, it is not a physical quantity, per se. By this we mean that there is no place in the universe where you can make a physical measurement that will be equivalent to a quantity of emergy. In contrast, the available energy of an item is the potential energy within that item that can do work, such as that in a log from a tree that releases a measurable quantity of potential energy by burning the wood, degrading it into a quantity of heat. The available energy in the log can be measured in calories or joules, using the English or metric systems, respectively. Note that a calorie was originally defined as the heat required to raise the temperature of one gram of water one degree centigrade. Thus, the log contains a quantity of available energy that can be measured by the heat released when it is burned, regardless of the units used.
In comparison, emergy is measured by the available energy that has already been used-up, or degraded into heat in past transformations, which were required to create the available energy in the log mentioned above. Thus, emergy considers the available energy, for example in transpiration, used up in growing the tree to harvestable size in the forest and the available energy used in cutting down the tree and processing it into logs, which can then be burned to generate heat. Emergy is the record of the previously used-up available energy that is manifested in the properties of the smaller quantity of available energy in an item in the present. Therefore, emergy is expressed in the properties of an item that are acquired under evolutionary competition in the process of its creation (Campbell 2001). It measures the available energy that a system has invested over time to have a particular item as a part of its structure or a particular flow as a part of its function. Emergy allows the comparison of system products and flows on an equal basis, using the emergy unit, or solar emjoule, sej. Emergy is a measure of the overall benefits and costs to a system of the gain or loss of a particular storage or flow, but it does not mean that items of the same emergy can perform the same functions or are substitutable within the system.
From the above definition one can see that emergy can be used as a comprehensive and universal accounting quantity apart from its deeper significance as a thermodynamic quantity that demonstrates the maximization of quality–adjusted available energy flow or empower (emergy per unit time). Maximizing empower flow through a system defines the decision criterion in evolutionary competition between alternative designs, according to the maximum empower principle, which is Odum’s restatement of the 4th law of thermodynamics as proposed by Lotka.
Transformity and Its Importance
Emergy is an extensive variable, which sums the available energy (i.e., exergy) of one kind (e.g., solar equivalent joules) used in the past to make, produce , or create a storage or flow of available energy (i.e., joules) in the present. Transformity is the intensive variable that relates the past use of available energy in the production process to the available energy remaining in the item produced. Its units in the metric system are solar emjoules per joule (sej/J). Transformity is an important tool in emergy accounting, because it is used to relate all of the forms of energy in a series of energy transformations to one form of energy (usually solar joules). In a simple chain of energy transformations the energy decreases at each step up the chain, due to losses required by the 2nd law thermodynamics, but the emergy contributed at the base is of the chain is the same. As a result the transformity (sej/J) of the remaining available energy increases with each transformation. When chains of transformations of available energy are combined in a complex network of energy transformations, transformity measures the position of each kind of energy within the universal energy hierarchy (i.e., the network of interconnected energy flows). For this reason, transformity is a quantitative measure of the relative quality of everything produced within a system. The importance of transformity is that everyone seeks quality in their choices. People perceive quality, subjectively, and there are partial measures of quality such as, the degree of concentration, e.g., 24 carat gold, but transformity is the only universal objective measure of quality known to science. For the same or equivalent products, transformity is also a measure of the relative efficiency of production processes. Here the minimum transformity indicates the most efficient production process, because the maximum output (J of a product) is produced per unit of emergy input.