Thermal response test

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A thermal response test (TRT) is used to determine the thermal properties of the ground. There is no direct way to measure ground thermal conductivity and borehole thermal resistance. The TRT is vital for designing ground source heat pumps and seasonal thermal energy storage (STES) systems. A TRT is an indirect (in-situ) measurement method which is the simplest and most exact way to determine precise thermal properties (Gehlin 2002). [1] Thermal response tests were first suggested by Mogensen (1983) at an international conference in Stockholm. [2] Mogensen suggested a simple arrangement in which heat at constant power is injected into (or extracted from) a borehole while the borehole mean temperature is measured.

Contents

To determine the thermal properties of the intended site and utilize them in the design process, it is essential to conduct a thermal response test (TRT) before implementing a ground source heat system. Before the test, a borehole of equal depth to the planned boreholes is drilled. Inside the borehole, pipes (U-pipe or Co-axial) are installed and the gaps filled with grout. Various tests are then conducted to measure temperature properties, thermal conductivity, and thermal resistance of the borehole. The TRT incurs costs for the user, making it more practical to estimate the total borehole length using rule of thumb methods for small-scale ground source heat pump (GSHP) systems. However, for larger projects, it is strongly recommended to perform one or a few TRTs to ensure accurate results.

Equipment

The system consists of a borehole, pipe system, circulation pump, a chiller or heater with constant power rate, and continuous logging of the inlet and outlet temperatures of the heat carrier fluid. The equipment is normally contained within a single unit for ease of transport and efficient use. [3] The thermal response data (i.e. temperature development in the borehole at a certain energy injection/extraction) allows estimation of the effective thermal conductivity of the ground and the thermal resistance of the borehole.

Recommendations

In order to fulfil the TRT properly, the following recommendations should be considered (Gehlin, 2002, [4] Sanner et al., 2005 [5] and Kharseh [6] ):

Traditional response tests apply a constant heat flux to the pumped water, however the newer "constant temperature" method, which holds the outflow water at a constant temperature, has been shown to have many advantages, including shortening the test period and in improving the operating stability and test accuracy. [7]

Preparation

Before the test is started, the undisturbed ground temperature must be determined. This can be measured in various ways: e.g., by temperature loggings of the borehole or by measuring the temperature of the circulated water through the borehole without heating over 20–30 minutes. The mean fluid temperature corresponds to the undisturbed mean temperature along the borehole. The next step is to switch on the heater and the monitoring system. During the test, the heat transfer into the ground surrounding the borehole is essentially radial and relatively constant along the borehole.[ citation needed ]

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References

  1. ↑ "Simple search". epubl.ltu.se. Retrieved 2018-03-12.
  2. ↑ Mogensen P. 1983. Fluid to Duct Wall Heat Transfer in Duct System Heat Storage. Proc. Int. Conf. On Subsurface Heat Storage in Theory and Practice. Stockholm. Sweden, June 6–8, 1983. PP: 652-657.
  3. ↑ "Thermal Response Testing - Carbon Zero Consulting". Carbon Zero Consulting. Retrieved 2018-03-12.
  4. ↑ Gehlin S. 2002. Thermal Response Teast, Meathod Development and Evaluation. Doctoral Thesis 2002:39. Luleå University of Technology. Sweden.
  5. ↑ Sanner, Burkhard; Hellström, Göran; Spitler, Jeff; Gehlin, Signhild (April 2005). Thermal Response Test – Current Status and World-Wide Application (PDF). Proceedings World Geothermal Congress 2005. CiteSeerX   10.1.1.702.2864 .
  6. ↑ Kharseh, M. (2009). Reduction of Prime Energy Consumption in the Middle East by GSHP Systems. Department of Civil, Mining and Environmental Engineering. Luleå University of Technology Luleå .
  7. ↑ Yu, Xiaohui; Zhang, Yufeng; Deng, Na; Ma, Hongting; Dong, Shengming (2016). "Thermal response test for ground source heat pump based on constant temperature and heat-flux methods". Applied Thermal Engineering. 93: 678–682. doi:10.1016/j.applthermaleng.2015.10.007.
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