Abstract
The aim of the study is to investigate the influence of geometry on the thermal capacity and stratifications of a water pit heat storage for solar district heating. A TRNSYS component model for a truncated cone water pit was developed based on the coordinate transformation method and validated by experimental results from the water pit heat storage in Huangdicheng in 2018. The thermal performance of 26 water pits with different heights and side wall slopes was calculated for 10 consecutive years. It takes four to six years for the water pit to reach steady-state operation. The operation data from the tenth year was selected to evaluate the thermal performance of each configuration. The results show that because of the thermal insulation on top of the water pit, the height to diameter ratio of a water pit with minimum annual heat loss was always smaller than 1.0. The annual storage efficiency of a water pit increases with side wall slope due to the reduced side wall area. There is an almost linear increase in the thermal stratification number of a water pit with height. With an increase in the height, thermal stratification in water pits with a steeper slope increased more gradually than water pits with a lower slope. The findings in this paper are relevant for the design optimization of water pits as seasonal thermal energy storages.
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Abbreviations
- A :
-
total surface area (m2)
- c :
-
specific heat capacity (J/(kg·K))
- d :
-
bury depth (m)
- D :
-
diameter (m)
- Depth:
-
calculation depth (m)
- E :
-
energy (J)
- Gr :
-
Grashof number
- h :
-
heat transfer coefficient (W/(m2·K))
- H :
-
height (m)
- I g :
-
global irradiation (W/m2)
- ṁ :
-
flow rate (kg/s)
- n :
-
number
- Pr :
-
Prandtl number
- r :
-
radial direction
- R 1 :
-
radius of the top wall (m)
- R 2 :
-
radius of the bottom wall (m)
- Radius:
-
calculation radius (m)
- Ra :
-
Rayleigh number
- Rt :
-
thermal resistant ((m2·K)/W)
- Str :
-
Stratification number
- T :
-
temperature (K)
- u :
-
wind velocity (m/s)
- V :
-
volume (m3)
- y :
-
height direction
- α :
-
thermal diffusivity (m2/s)
- β :
-
thermal expansion coefficient (K−1)
- γ :
-
increase factor
- δ :
-
thickness (m)
- Δ :
-
difference (m)
- η :
-
efficiency (%)
- θ :
-
side wall slope
- λ :
-
thermal conductivity (W/(m·K))
- ν :
-
kinematic viscosity (m2/s)
- ξ :
-
horizontal direction in the new coordinates
- ρ :
-
density (kg/m3)
- τ :
-
time (s)
- ϕ :
-
absorption factor
- ϕ :
-
vertical direction in the new coordinates
- − :
-
average
- 0:
-
initial
- a:
-
air
- bot:
-
bottom
- bw:
-
water back from end user
- ch:
-
charging
- con:
-
concrete
- dc:
-
discharging
- G:
-
down side node
- end:
-
end of the experiment
- env:
-
environment
- exp:
-
experiment
- i, j, k :
-
grid number
- ini:
-
start of the test
- insu:
-
insulation
- L:
-
left side node
- nup:
-
upper opening node
- nlow:
-
lower opening node
- num:
-
numerical
- p:
-
profile
- R:
-
right side node
- s:
-
soil
- U:
-
up side node
- w:
-
water
- CFD:
-
computational fluid dynamics
- FVM:
-
finite volume method
- FEM:
-
finite element method
- FDM:
-
finite difference method
- KPI:
-
key performance indicator
- STES:
-
seasonal thermal energy storage
- WPTES:
-
water pit thermal energy storage
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Acknowledgements
The authors thank the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA21050200), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013N070) and the State Grid Corporation Science and Technology Project “Research on Comprehensive Development and Utilization Technology of Renewable Energy in Multi-format Ecological Development Zone” for funding this project.
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Bai, Y., Yang, M., Fan, J. et al. Influence of geometry on the thermal performance of water pit seasonal heat storages for solar district heating. Build. Simul. 14, 579–599 (2021). https://doi.org/10.1007/s12273-020-0671-9
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DOI: https://doi.org/10.1007/s12273-020-0671-9