Energy quality, emergy, and transformity: H.T. Odum's contributions to quantifying and understanding systems

We present in this article, a brief historical overview of the development of the concepts and theories of energy quality, and net energy that were the precursors to emergy. The concepts evolved over decades, beginning in the 1950s with Odum's work on tracing energy flows in ecosystems. During the 1970s, Odum's attention was drawn to larger scale systems that included the economies of humans and the concept of net energy. In the 1980s, Odum quantified energy quality and defined it as a "donor-based" evaluation technique. In the 1990s, energy quality was further refined and rigorous definitions for "emergy" and "transformity" were given. The units of emergy were defined as solar emjoules (abbreviated seJ) and the units of transformity were seJ/J. In addition, we provide some insights into the types of processes and systems that have been evaluated using emergy methods. (C) 2004 Elsevier B.V. All rights reserved.