This module applies basic laws of thermodynamics to the study of the efficiency at which heat can be converted to other useful forms of energy, including heat at low temperatures. The module is divided into four major sections. Section I treats energy conversion efficiency and defines the concept of first-law efficiency, the most widely used indicator of efficient energy use. Past, present, and future trends in first-law efficiency for a variety of energy conversion devices are presented. Section II introduces the concept of available work and defines second-law efficiency (which is based on the available work concept). Second-law efficiency provides another, possibly superior, means of determining the potential for reduction in energy use due to improvement in energy conversion processes. Several examples of second-law efficiency are worked out for single source/single output devices. In addition, second-law efficiency is determined for a variety of tasks. In section III several energy conversion systems are analyzed from the standpoint of both first-/second-law efficiency. Section IV provides Fortran computer programs which apply concepts developed in the module to the analysis of combined energy systems and their potential for reduced fuel. Bibliography, an audio-visual materials list, and a list of prerequisite mathematics/physics skills needed are included. (JN)