Current earth coil design methods are based on line source theory. Unfortunately, these methods cannot account for a number of factors - including moisture movement and drying, shrinkage, surface effects, etc. - that can have a dramatic effect on earth coil performance. Eventually, computer models of earth coils that include these factors should be developed. Until they are, small scale experimental methods offer a way to quickly and inexpensively determine how well an earth coil will perform in a particular soil environment . Small scale tests can also provide valuable information to aid in computer model development which cannot be obtained from field data.

The analytical basis for small scale testing is developed in this paper. Several proven and potential applications are also presented. The operation of thermal probes that are currently used to determine underground electric cable capacity is discussed. A probe such as this can be used to simulate an earth coil during the cooling season, and, with some modification, be used to simulate the annual cycle undergone by a reversible heat pump earth coil. The operation of a laboratory simulator is also described . This simulator can account for most of the factors listed earlier and also isolate the effect of each factor . The annual cycle undergone by a heat pump can be simulated in 2.3 days. The ability to isolate effects and complete a test in a short time makes the laboratory simulator an extremely useful tool for computer model development and experimental optimization studies.

Units: I-P