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.
Bibliographical noteFunding Information:
This work was supported by the National Renewable Energy Laboratory, the US Department of Energy, the University of Minnesota Initiative for Renewable Energy and the Environment and the University of Missouri-Rolla. The heat exchanger was donated by Heat Craft, Inc.
- Heat exchanger
- Natural convection
- Solar hot water
- Thermal storage