Mass, Energy, Efficiency in the US Economy
Abstract
This paper summarizes energy (exergy) flows for the US from 1900 through 1998. It then considers the various processes for converting crude exergy into "useful work", as the term is understood by engineers and physicists. There are five types of work, namely muscle work by humans or animals, mechanical work by stationary or mobile heat engines (prime movers) and heat, either at high temperatures (for metallurgical or chemical processes) or at low temperatures for space heating, water heating, etc. The ratio of output work to input exergy is the thermodynamic efficiency of the conversion process. Efficiencies vary considerably from process to process, and over time. In general, primary conversion efficiencies have increased dramatically during the 20th century. While electric power may be regarded as (almost) pure work, it is convenient to define "secondary work" as the work done by electricity, such as electric light, electromotive power, electric furnaces, electrochemistry and electronics. Surprisingly, the efficiency of secondary work has barely increased during the century, because high efficiency uses have declined in terms of market share, while low efficiency uses have increased share. In conclusion, it is argued that overall exergy efficiency constitutes a good measure of technological change and may prove to be an important explanatory factor for economic growth.