This paper describes measurements made on a prototype ground-source heat engine to quantify the performance and operating characteristics of the device. The ground-source heat engine generated very small amounts of electric power from the daily fluctuations in the air-ground temperature difference. The long-term average electricity generation was relatively steady and was largely independent of daily weather conditions. It was estimated that significant improvement in operating power could be achieved by using optimally matched device components instead of the off-the-shelf components used for the prototype. The relationship between total temperature difference and temperature drop through components of the device was found to depend on the time of day. This dependence was partly attributable to the effects of direct solar insolation. Power production from the device was nearly continuous but the bulk of the electrical energy was produced between the hours of noon and four p.m.
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