![Sample our Engineering & Technology journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5933/TOC-Subject-Sample-2021-Engineering-Tech.jpg"})
ABSTRACT
Deep coaxial borehole heat exchanger (DCBHE) is a promising kind of ground heat exchanger for geothermal exploitation. In this study, effects of stratified ground thermophysical properties (GTP) are considered, and then an improved semi-analytical heat transfer model is developed and validated to analyze the DCBHE performance. Then, influences of stratified GTP on the DCBHE performance are investigated. Finally, thermal response test (TRT) of DCBHE with stratified GTP is simulated by a numerical model, and by using the simulated TRT data, the improved model is combined with a parameter estimation method to identify the effective values of GTP, which are compared with actual values. The results show that stratified ground thermal conductivities have an important influence on the DCBHE performance, and that stratified volumetric heat capacities of ground have a small influence on the DCBHE performance. The DCBHE performances based on the effective values and actual values of GTP match well with each other for different cases and geothermal gradients, indicating that for stratified thermal conductivities or volumetric heat capacities of ground, the effective GTP are reliable and can be used for accurately predicting the DCBHE performance.
Nomenclature
| a | = | geothermal gradient, ℃ m−1 |
| C | = | volumetric heat capacity, J m−3 K−1 |
| G | = | G-function |
| h | = | convection heat transfer coefficient, W m−2 K−1 |
| L | = | length of internal pipe, m |
| Qout | = | heat output rate, W |
| q | = | heat flow from grout to annular fluid, W m−1 |
| Rae | = | thermal resistance between annular fluid and outside surface of external pipe, m K W−1 |
| Ria | = | thermal resistance between internal and annular fluids, m K W−1 |
| r | = | radial coordinate, m |
| T | = | temperature, ℃ |
| t | = | time, s |
| V | = | volumetric flow rate, m3 s−1 |
| z | = | axial coordinate, m |
| Greek letters | = | |
| λ | = | thermal conductivity, W m−1 K−1 |
| Subscripts | = | |
| 0 | = | initial condition |
| a | = | annular fluid |
| b | = | borehole wall |
| ei | = | inside surface of external pipe |
| eo | = | outside surface of external pipe |
| ep | = | external pipe |
| f | = | fluid |
| g | = | grout |
| i | = | internal fluid |
| ii | = | inside surface of internal pipe |
| in | = | inlet of DCBHE |
| io | = | outside surface of internal pipe |
| ip | = | internal pipe |
| j, n | = | j-th and n-th time nodes, respectively |
| out | = | outlet of DCBHE |
| s | = | ground |
| s,ave | = | weighted average value of stratified GTP |
| s,eff | = | effective value of GTP, which is estimated by TRT |
| sur | = | ground surface |
Acknowledgements
The authors would like to thank the Natural Science Foundation of Anhui Province (Grant No. 1808085QE178) for the financial support.
Disclosure statement
No potential conflict of interest was reported by the author(s).