Observations on the Decay of a Thermocline in a Rock Bed With No Net Fluid Flow

The transient thermal response of a rock bed with no net fluid flow is examined following all-day charging under clear sky conditions. The experimental system consists of 1.86 m² (20 ft²) of flat-plate solar collectors using air as the working fluid, a flow control system, and a 0.357 m³ (12.6 ft³) rock bed for thermal energy storage. A thermocline is established in the bed during charging due to the time-varying nature of the collector outlet temperature. Experimental measurements of the temperature distribution in the bed for a 13-hour stagnation period allow a preliminary estimate of the loss of available energy in the storage medium. The net loss in thermodynamic availability is 30 percent. Since the temperatures in the upper regions of the bed are lower than those in the central regions at the end of charging under clear sky conditions, the possibility of natural convection motion of the fluid in the bed exists. An “apparent” local thermal diffusivity is calculated and from comparison with stagnant bed values indicates that natural convection motion may occur in the upper regions of the bed.