A mathematical model for thermal analysis of a helical heat exchanger for long-term thermal energy storage in soil for use in arid zones was developed. The helical heat exchanger was modeled as a series of horizontal rings with a constant pitch distance between them. The model was solved by a finite difference method, using a microcomputer, and validated with experimental data obtained from field experiments. Based an the model, theoretical results of the following parametric studies are presented : thermal properties of the soil, cycle period, and height and pitch distance of the helical heat exchanger.