Results of a study to examine the operating characteristics of a 100 kWh thermal energy storage (TES) system suitable for solar thermo electric applications is described. The system chosen consisted of a pebble bed as the primary storage medium and oil as the heat transfer cum storage medium. The operating temperatures considered were between 230 and 250°C with a 20 deg C swing. A full‐size unit consisting of a steel tank of volume 10 m3 with 50 mm pebbles, suitable instrumentation and facility for heating the oil was built. The important operating variables and characteristics of the system studied included the transient behaviour of the bed, namely the thermal wave front characteristics. Results of the theoretical analysis of the transient bed behaviour were compared with the experimental data on the wave front propogation characteristics and the comparisons are discussed. The uniformity of flow distribution is also examined. Copyright © 1990 John Wiley & Sons, Ltd