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Thermal performance and evaluation of a novel stratified and mixed flexible transformation solar heat storage unit

  • Research Article
  • Building Systems and Components
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Abstract

It is necessary to satisfy the flexible requirements of solar heat storage systems to provide efficient heating and constant-temperature domestic hot water at different periods. A novel heat storage tank with both stratified and mixing functions is proposed, which can realize the integration of stable stratification and rapid mixing modes. In this research, a three-dimensional heat transfer model of the heat storage tank with stratified and mixed dual modes was established, and a thermal performance test system for the tank was built in the State Key Laboratory of Green Building in Western China. Moreover, a new evaluation index representing the mixing speed is proposed. The stratification effect and mixing characteristics of the tank were studied under different comprehensive conditions. The results show that the exergy efficiency of the tank with a stratified pipe can be increased by 10%–15% compared to that of a conventional tank. Additionally, the recommended optimal flow rate range for well-stratified tanks is 4–6 L/min. The mixing nozzle of the tank reduces the mixing reaction coefficient by 0.27 and significantly reduces the mixing time. This study provides critical guidance to meet the flexible thermal needs of users and implement high-performance applications using the stratified and mixing modes of heat storage tanks.

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Abbreviations

C P :

specific heat capacity of water (J/(kg·K))

D :

diameter of stratified inlet (mm)

g :

gravitational acceleration (m/s2)

MIX:

mixing number representing the stratification effect of the tank

M :

moment of energy, or weighted energy by height (J·m)

M i :

energy moment of the i-th layer of the heat storage tank (J·m)

M str :

energy moment of a perfectly stratified tank (J·m)

M mix :

energy moment of a completely mixed tank (J·m)

M exp :

energy moment of an experimental tank (J·m)

m i :

water mass of the i-th layer of the tank

N :

number of nodes of the heat storage tank

ΔP :

pressure difference between the inside and outside of the stratified inlet (Pa)

Q 1 :

actual energy stored in the heat storage tank (J)

Q 2 :

ideal energy stored in the heat storage tank (J)

T :

water temperature of heat storage tank (K)

T 0 :

initial temperature of heat storage tank (K)

T n :

water temperature inside a stratified inlet (K)

T w :

water temperature outside a stratified inlet (K)

T inlet :

inlet temperature of the heat storage tank (K)

T outlet :

outlet temperature of the heat storage tank (K)

T average,inlet :

average inlet temperature of the heat storage tank during the charging process (K)

T i :

water temperature of the i-th layer (K)

T 1,i :

water temperature of the i-th layer at state “1” (K)

t start :

time when the experiment begins (s)

t stop :

time when the experiment ends (s)

Vs :

heat storage volume (m3)

V i :

volume of water in the i-th layer (m3)

v n :

flow rate of water inside a stratified inlet (m/s)

v w :

flow rate of water outside a stratified inlet (m/s)

\(\dot v\) :

inlet flow rate of water storage tank (m3/s)

X sim,i :

simulated value in the i-th layer

X exp,i :

measured value in the i-th layer

Y i :

vertical distance from the tank bottom to the middle of the i-th layer (m)

Z :

height between the stratified inlet and the tank bottom (m)

δ :

relative error

η :

thermal storage efficiency

η str :

stratification efficiency

η ξ :

exergy efficiency

ξ 1-0,exp :

change in useful energy of an experimental tank from state “1” to initial state “0” (J)

ξ 1-0,str :

change in useful energy of a fully stratified tank from state “1” to initial state “0” (J)

ξ mix :

mixing reaction coefficient

ρ :

density of water (kg/m3)

ρ i :

density of water at temperature Ti (kg/m3)

ρ n :

density of water inside a stratified inlet (kg/m3)

ρ w :

density of water outside a stratified inlet (kg/m3)

τ :

current time since the experiment started (s)

τ* :

dimensionless time

τ mix :

end time of the mixed experiment, complete mixing state (s)

τ str :

start time of the mixed experiment, stable stratification time (s)

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52078408, No. 51908442), the State Key Program of National Natural Science Foundation of China (No. U20A20311), the Science Foundation for Outstanding Youth of Shaanxi Province (No. 2020JC-43).

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Authors and Affiliations

  1. State Key Laboratory of Green Building in Western China, Xi’an University of Architecture and Technology, Xi’an, 710055, China

    Dengjia Wang, Yingying Wang & Yanfeng Liu

  2. School of Building Services Science and Engineering, Xi’an University of Architecture and Technology, Xi’an, 710055, China

    Dengjia Wang, Hui Liu, Yingying Wang, Kaili Liu, Yanfeng Liu & Meng Gao

  3. Department of Civil Engineering, Technical University of Denmark, Brovej 118Kgs., Lyngby, DK2800, Denmark

    Jianhua Fan

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  1. Dengjia Wang
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  2. Hui Liu
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  3. Yingying Wang
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  4. Kaili Liu
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  5. Yanfeng Liu
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  7. Jianhua Fan
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Correspondence to Dengjia Wang.

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Wang, D., Liu, H., Wang, Y. et al. Thermal performance and evaluation of a novel stratified and mixed flexible transformation solar heat storage unit. Build. Simul. 16, 1881–1895 (2023). https://doi.org/10.1007/s12273-022-0930-z

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  • DOI: https://doi.org/10.1007/s12273-022-0930-z

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