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Research Papers

Horizontal Inlets of Water Storage Tanks With Low Disturbance of Stratification

[+] Author and Article Information
Corsin Gwerder, Lukas Lötscher, Jason Podhradsky, Matthias Kaufmann, Andreas Huggenberger, Igor Mojic

Institute for Solar Technology SPF,
University of Applied Sciences (HSR),
Rapperswil CH-8640, Switzerland

Simon Boller, Boris Meier

Institute for Energy Technology IET,
University of Applied Sciences (HSR),
Rapperswil CH-8640, Switzerland

Michel Y. Haller

Institute for Solar Technology SPF,
University of Applied Sciences (HSR),
Rapperswil CH-8640, Switzerland
e-mail: michel.haller@spf.ch

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received May 22, 2015; final manuscript received July 1, 2016; published online August 15, 2016. Assoc. Editor: Jorge E. Gonzalez.

J. Sol. Energy Eng 138(5), 051011 (Aug 15, 2016) (9 pages) Paper No: SOL-15-1154; doi: 10.1115/1.4034228 History: Received May 22, 2015; Revised July 01, 2016

Solar combi-storages are used in many countries for storing solar heat for space heating and domestic hot water (DHW) in one device. When a combi-storage is used in combination with a heat pump, the temperature stratification efficiency of the storage is a decisive factor for the overall efficiency and thus, for the consumed end-energy of the system. In particular, fluid that is entering the storage with a high velocity may cause considerable mixing, thus, destroying stratification and leading to poor system performance. This work presents computational fluid dynamics (CFD) simulations of direct horizontal inlets at midheight of a typical solar combi-storage of about 800 L volume. Different inlet diffusor designs were simulated, and laboratory measurements were used to validate CFD experiments. For the given tank geometry, mass flow rates, and inlet position, it is found for a fluid inlet temperature of 30 °C that fluid velocities should be below 0.1 m/s and Reynolds numbers below 3000–5000 at the outlet of the diffusor in order to avoid the disturbance of a hotter 50 °C zone above the inlet. Furthermore, the fluid path within the diffusor must exceed a minimum length that corresponds to three to four times the hydraulic diameter of the diffusor.

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Figures

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Fig. 1

Simplified scheme of a combi-storage in combination with a solar thermal system and a heat pump

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Fig. 2

The analyzed vertical cylinder combi-storage in the initial state

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Fig. 3

Geometries of inlet diffusors and connecting pipes before the inlet

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Fig. 4

Hydraulic scheme for the laboratory measurements

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Fig. 5

Results of the mesh-study, with measured points at 1200 s of the experiment

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Fig. 6

Root mean square errors of temperatures and simulation time for different mesh sizes

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Fig. 7

Comparison between full and simplified simulation

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Fig. 8

Temperature profiles for a 1 in. horizontal inlet in simulation and experiment for different mass flow rates and different turbulence models used for the CFD simulations, full simulation without simplifications

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Fig. 9

Temperature profiles for an inlet with a diffusor plate in simulation and experiment for mass flow rates of 0.125 kg/s, 0.25 kg/s, and 0.5 kg/s. Simulated with the SST-SAS model.

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Fig. 10

Selected temperature profiles after 1 h of simulation with mass flow rates of 0.25 kg/s (top) and 0.5 kg/s (bottom row) for the inlet at midheight of the storage

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Fig. 11

Temperature profiles for various diffusor geometries and mass flow rates colored by the Reynolds number for the flow in the diffusor or inlet

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Fig. 12

The same temperature profiles as in Fig. 10, colored by the velocity of the flow at the entrance

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Fig. 13

Influence of the length of the mitigation zone on the effectiveness of the diffusor

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Fig. 14

Displacement of the thermocline for different hydraulic diameters and mass flow rates

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