Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Determination of the thermal characteristics of a saturated sand layer

The thermal characteristics (borehole resistance, thermal conductivity) of an underground type are strongly influenced by local circumstances such as homogeneity, sand/silt/clay content, water saturation, and groundwater level. Within this project the characteristics of a homogeneous water saturated sand layer are measured in-situ by logging temperatures and flows while supplying heat to a vertical heat exchanger.

This so called TRT-test (Thermal Response Test) also provides information on the borehole refilling material as three tests on three different boreholes with three different fillings were performed. In the past, the thermal characteristics of the underground were estimated based on experiences and lab tests. This led to a wide range with a recommended value, e.g. water-saturated sand has a conductivity between 1,6 and 5,2 with 2,5 (W/(mK)) as a recommendation.

It's clear that significant differences can occur between the best and worst circumstances and that in one case the value is overestimated while another case leads to an underestimation. Lab tests can indeed contribute to a better estimation but an in-situ measurement takes in account all influencing parameters. In water saturated sand layers, little experience with TRT-tests was gained in the past. It was also a surprise that borehole filling with drilled out sand provided the best results compared to a filling with a sorted sand mixture. These conclusions gave better insight in creating optimal borehole storage design.

The thermal characteristics are not only of importance in huge borehole energy storage projects. Ground coupled heat pump systems also make use of vertical heat exchangers. There is a significant growth in the yearly installed heat pump systems all over the world. Installation designers do not know the performance of the source side of a heat pump system very well. In the past, this led to installation problems or even failures that had a negative impact on the heat pump market growth. The accurate determination of thermal conductivity and borehole resistance leads to an optimal system design that brings on two main advantages: a reliable and efficient system operation and high cost-effectiveness.

Designers of underground thermal energy storage and large geothermal heat pump systems can benefit from a better insight on the thermal characteristics of the underground. Nowadays, they rely on values from literature and they neglect the importance of the borehole resistance. Thermal enhanced grouting can be developed in order to reduce this resistance value. This can contribute to a higher system power capacity, in many cases the major bottleneck in the design of an energy storage or heat pump system. At residential heat pump systems, drilling companies often advise on the heat source system. A careful preparation by examining the thermal characteristics is also for them of great importance in order to guarantee long-term system reliability.

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