Abstract
The number of heating and cooling system using vertical borehole heat exchangers is increasingly rapidly in Sweden. Currently, there are over 600,000 installations, mostly using U-tube ground collectors. In recent years, several new ground collectors with innovative designs and configurations have been launched. They claim to have superior performance and competitive advantages over the conventional collectors.
This project was aimed at assessing the performance and effectiveness of various types of ground collectors through full-scale experimental testing. The tested types of ground collectors included single U-tube, double U-tube configured in parallel, double U-tube configured in series, coaxial and U-tube with internally rifled pipes.
The ground collectors were tested one after another in the same borehole heat exchanger over a period of two and a half years. The testing was performed in both heat-injection and heat-extraction modes with flows ranging from near-laminar to fully turbulent. A custom-built thermal response test rig was developed to perform tests of this type. To ensure similar geothermal conditions for all tests, the ground was rested long enough to allow full thermal recovery before each test.
Among all tested ground collectors, the coaxial collector had the most consistent thermal and hydraulic performance over the tested range of thermal and pumping power conditions. However, the first cost and the time and labour expense for installing the coaxial collector were substantially higher than any other type of ground collectors. A project-specific cost-benefit analysis was deemed necessary to determine if the thermal and pump energy saving from the coaxial and other new collectors are large enough to overweigh their higher first costs and installation difficulties.
Among other collectors, the U-tube collector with internally rifled pipes had inferior thermal and hydraulic performance than the ordinary U-tube collector. Double U-tube collector, configured in series, had by far the worst hydraulic performance among all tested ground collectors.
The results from this project have been presented in six technical reports, three book chapters, and eleven journal and conference proceeding papers. Some more publications are in the pipeline and will be published in 2019.
This project was aimed at assessing the performance and effectiveness of various types of ground collectors through full-scale experimental testing. The tested types of ground collectors included single U-tube, double U-tube configured in parallel, double U-tube configured in series, coaxial and U-tube with internally rifled pipes.
The ground collectors were tested one after another in the same borehole heat exchanger over a period of two and a half years. The testing was performed in both heat-injection and heat-extraction modes with flows ranging from near-laminar to fully turbulent. A custom-built thermal response test rig was developed to perform tests of this type. To ensure similar geothermal conditions for all tests, the ground was rested long enough to allow full thermal recovery before each test.
Among all tested ground collectors, the coaxial collector had the most consistent thermal and hydraulic performance over the tested range of thermal and pumping power conditions. However, the first cost and the time and labour expense for installing the coaxial collector were substantially higher than any other type of ground collectors. A project-specific cost-benefit analysis was deemed necessary to determine if the thermal and pump energy saving from the coaxial and other new collectors are large enough to overweigh their higher first costs and installation difficulties.
Among other collectors, the U-tube collector with internally rifled pipes had inferior thermal and hydraulic performance than the ordinary U-tube collector. Double U-tube collector, configured in series, had by far the worst hydraulic performance among all tested ground collectors.
The results from this project have been presented in six technical reports, three book chapters, and eleven journal and conference proceeding papers. Some more publications are in the pipeline and will be published in 2019.
| Original language | English |
|---|---|
| Place of Publication | Gothenburg |
| Publisher | Chalmers University of Technology |
| Number of pages | 27 |
| Publication status | Published - 2018 Dec |
| Externally published | Yes |
Subject classification (UKÄ)
- Energy Systems
- Energy Engineering
Free keywords
- borehole heat exchanger
- ground collector
- coaxial
- U-tube
- thermal resistance
- pressure drops
- comparative performance
- heat transfer
- loop design
- ground source heat pump