A comprehensive computer model of a packed bed thermal energy storage system originally developed for storage media employing either sensible heat storage (SHS) materials or phase-change material (PCM), was validated for the sensible heat storage media using a rather extensive set of data obtained with a custom-made experimental facility for high-temperature energy storage, The model is for high-temperature storage and incorporates several features including (a) allowance for media property variations with temperature, (b) provisions for arbitrary initial conditions and time-dependent varying fluid inlet temperature to be set, (c) formulation for axial thermal dispersion effects in the bed, (d) modeling for intraparticle transient conduction in the storage medium, (e) provision for energy storage (or accumulation) in the fluid medium, (f) modeling for the transient conduction in the containment vessel wall, (g) energy recovery in two modes, one with flow direction parallel with that in the storage mode (cocurrent) and the other,with flow in the opposite direction (countercurrent), and (h) computation of the first and second-law efficiencies. Parametric studies on the sensible heat storage system were carried out using the validated model to determine the effects of several of the design and operating parameters on the first and second-law efficiencies of the packed bed, Decisions on the thermodynamic optimum system design and operating parameters for the packed bed are based oil the second-law evaluations made.