The Benefit of Dividing an Indirect Thermal Storage into Two Compartments: Discharge Experiments


Experiments are presented to demonstrate the benefits of dividing an indirect thermal storage into two compartments. The transient discharge experiments were conducted in an undivided and equally divided 126 l rectangular storage vessel, which has a height to depth aspect ratio of 9.3:1 and is inclined at 30° to the horizontal. A 240-tube copper heat exchanger with a total surface area of 2.38 m2 was immersed in the storage fluid. For the divided storage, the heat exchanger flow path was in series through the two compartments. Water flow rate through the heat exchanger was varied from 0.05 to 0.15 kg/s to demonstrate the effect of varying the number of transfer units (NTU) from 2.2 to 7 on the relative performance of undivided and divided storage vessels. Reported measurements include transient storage temperature distribution, heat exchanger outlet temperature, delivered energy, and exergy of the divided and undivided storage. The divided storage provides higher energy delivery rates and higher heat exchanger outlet temperatures during most of the discharge. The magnitude of these benefits depends on NTU and the extent of discharge. For a flow rate of 0.05 kg/s, corresponding to a nominal NTU of 7, the divided storage delivers a maximum of 11% more energy than the undivided storage when 100 l of hot water or 55% of the stored energy has been delivered. For a flow rate of 0.15 kg/s, corresponding to a nominal NTU of 2.5, the divided storage delivers a maximum of 5% more energy at the same level of discharge. Data agree with first and second law analyses of a storage system comprised of two tanks in series.


Mechanical and Aerospace Engineering


United States. Department of Energy
National Renewable Energy Laboratory (U.S.)
University of Minnesota
University of Missouri--Rolla

Keywords and Phrases

Solar Hot Water; Thermal Storage; Heat Exchanger; Natural Convection

International Standard Serial Number (ISSN)


Document Type

Article - Journal

Document Version


File Type





© 2006 Elsevier, All rights reserved.

Publication Date

01 Jan 2006