The daily variation in air temperature is large compared with the temperature changes a short distance below the surface of the ground. In theory, a heat engine can be arranged to produce electricity from this temperature difference. In practice, the thermal efficiency of such a device will be low because of the small temperature differences involved. An energy harvesting device could produce sensor-scale amounts of electrical power by using a thermoelectric generator operating between the air and ground temperatures. This paper describes a proposed ground-air thermoelectric heat engine along with a procedure for the approximate optimal design of such a device. Simple design and performance equations based on thermal resistances of the thermoelectric module and coldand hot-side heat exchangers are derived and presented.