A Second-Law Study on Packed Bed Energy Storage Systems Utilizing Phase-Change Materials

Abstract

Thermal modeling of packed bed, thermal energy storage systems has traditionally been limited to first-law considerations. The exceptions include a few second-law studies, noted in the Introduction, of sensible heat storage systems and the latent heat storage systems. The cited second-law studies treat the storage and removal processes essentially as “batch” heating and cooling. The approximation effectively ignores the significant temperature gradient, especially in the axial direction, in the storage medium over a substantial portion of both the storage and removal processes. The results presented in this paper are for a more comprehensive model of the packed bed storage system utilizing encapsulated phase-change materials. The fundamental equations for the system are similar to those of Schumann, except that a transient conduction equation is included for intraparticle conduction in each pellet. The equations are solved numerically, and the media temperatures obtained are used for the determination of the exergy (or availability) disposition in complete storage-removal cycles. One major conclusion of the study from both the first-law and second-law perspectives is that the principal advantage in the use of phase-change storage material is the enhanced storage capacity, compared with the same size of packed bed utilizing a sensible heat storage material. Thermodynamically, however, it does not appear that the system employing phase-change storage material will always, or necessarily, be superior to that using a sensible heat-storage material. The latter conclusion is reached only on the basis of the second-law evaluation.

Entropy generation analysis as a design tool—A review

“…Through a rather simplified black box model it is shown that such a temperature is the geometric mean between the HTF inlet temperature and the ambient temperature. Later, more refined black box models have been developed for the thermodynamic optimization of packed bed storage systems [59], shell-and-tube LHTES systems [60,61] and spherical capsules [62,63]. Second-law investigation of LHTES system is still a very active research area as illustrated in the comprehensive review by Jegadheeswaran [64].…”

Selection of heat storage materials for ammonia–water and lithium bromide solar-powered absorption heat pump systems

“…It has been established that sensible Downloaded by [Tulane University] at 11:41 04 October 2014 heat storage systems are more economical than latent heat storage systems only when heat is needed for short durations and in appreciable amounts. The works of Adebiyi (1991) have demonstrated that systems employing PCMs have the added advantages of enhanced second-law efficiency and lower investment in the storage material than systems employing sensible heat materials. The latter particularly revealed that the principal advantage of phase-change systems over sensible-heat systems is the enhanced storage capacity.…”

An overview on design methodologies for liquid–solid PCM storage systems

“…The benefit of a maximized exergy storage is a higher power output from the PCM storage. However, when the recovery process (additionally to the storage process) is taken into account, the optimum melting temperature for the full cycle is greater than that for the storage process alone [50][51][52].…”