Operation Of Thermal Energy Storage Systems Research Articles

Large supercooling of Al[sbnd]Si alloy in microcapsule composed of Al2O3 shell

Al–Si-based microencapsulated phase change materials (MEPCMs) with a eutectic temperature of 577 °C are promising thermal energy storage (TES) materials owing to their large latent heat. Various preparation methods for Al–Si-based MEPCMs with Al2O3 shells have been reported; interestingly, Al–Si-based MEPCMs have large supercooling that does not depend on the preparation method. Supercooling is an important property of MEPCMs in the engineering aspects of designing a TES system. However, limited studies have reported the mechanism of supercooling in MEPCM. Moreover, the large supercooling of AlSi alloys in microcapsules compared with bulk materials is an interesting phenomenon in the field of physical chemistry. This study investigates the supercooling phenomenon in eutectic, hypoeutectic, and hypereutectic AlSi alloy MEPCMs, that is, AlSi alloys in a microcapsule comprising an Al2O3 shell. For the MEPCM with a hypoeutectic composition, supercooling is not observed because the primary Al acts as a nucleation site. For MEPCMs of eutectic and hypereutectic compositions, supercooling is observed at 25–37 °C. When the MEPCMs are heated to 1000 °C and then cooled, the supercooling degree increases significantly. In MEPCMs with a hypoeutectic composition, the precipitation of primary Al crystals does not occur under these conditions. Such findings contribute to the optimization of the design and operation of TES systems.

Performance evaluation of thermal energy storage systems

We have developed two case studies to review and analyze energy performance of thermal energy storage (TES) systems in commercial buildings. Our case studies considered two partial ice storage systems in Northern California. For each case, we compiled historical data on design, installation, and operation of the TES system; and also metered data. This information was further enhanced by data obtained through interviews with the building owners and operators. The performance and historical data of the TES systems and their components were grouped into issues related to design, installation, operation, and maintenance of the systems. Our analysis indicated that (1) almost all problems related to the operation of TES and non-TES systems could be traced back to the design of the system, and (2) the identified problems were not unique to the TES systems. Judging from the problems related to non-TES components identified in these two case studies, it is reasonable to conclude that conventional systems have as many problems as TES systems. After the initial trouble-shooting period, in a typical summer week for Case 1, the TES system saved 640, 780, and 660 on-peak kW compared to the single chiller, split chiller, and two-chiller systems, respectively, at a cost of additional energy use of 20.5, 37.6, 47.2 MWh per week. For Case 2, the diurnal energy performance of the conventional single chiller system and the TES system were comparable; however, the TES system used less power than the conventional system. Once the performances of the TES with split and two chiller conventional systems were compared, the energy penalties increased substantially and the peak power savings diminished.