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Development of a soil thermal conductivity map of Bavaria for design of shallow geothermal applications

Toshihiko Momose1, Marcellus Schluze1, Bernhard Wagner1
1 Bayerisches Landesamt für Umwelt

P 8.1 in Geothermie und Geocooling

Introduction

    Soil thermal conductivity (λ) is a primary property determining the heat extraction potential of ground heat exchangers (GHE). In the context of the EU co-financed project “Information Offensive Geothermal Energy (IOGI)” Bavarian Environment Agency – Geological Survey creates a soil map of the λ values in Bavaria in the scale of 1:25 000, to provide an overview of the spatial efficiency for horizontal GHE. The model for the λ estimates will allow the prediction of heat extraction potential for near surface geothermal applications and thus is a major milestone of the project.

Materials and methods

   Twenty-one Bavarian soils with a large variety of soil textures were selected for the λ measurements. For each soil, seven samples with different water content were prepared at a constant dry bulk density and their λ data were measured by the heat-probe method. Additionally, the 21 soils were tested for physical properties, such as grain size distribution and mineral composition, required for model development.

Results and discussion

    The λ values increase with the saturation degree (S) for each soil. We find that the increase in λ over log S values can be divided into two categories at a particular S value (Sp). In the range of S < Sp, the λ values keep almost the same to that at dry conditions (λdry). For S > Sp, the λ values are linearly correlated with log S. The correlation coefficient exceeds 0.9 for all tested soils.

   The above findings provide the basis to develop the model for the λ of Bavarian soils:  

For 0 < S < Sp, λ = λdry.

For Sp < S < 1, λ = λdry + (λsatλdry) * (1 – log S / log Sp),

where λsat is the λ value at saturated conditions. Using the physical properties, the multiple regression analysis derives the equations for λdry, λsat and Sp:  

λdry = 0.30 * rd – 0.21,

λsat = (7.7Vq * 2(1-Vq))Vs * 0.6(1-Vs), and

Sp = 0.10 * mclay * rd + 0.08,

where rd is the bulk density, Vq is the volumetric quartz content expressed by “0.14 * msand – 0.67 * mclay + 0.57”, Vs is the volumetric solid content expressed by “rd/2.67” and mclay is the clay content.

    Figure 1 shows the measured and calculated λ values for different soil textures. The statistical analysis reveals that, on the average for all tested soils, the λ estimates correspond to the λ measurements within the RMSE of 0.11 Wm-1K-1.

Conclusion

   We have developed a new model that successfully fits to the λ measurements of 21 selected Bavarian soils. As a future work, further model validation based on a larger dataset will be performed. The validated model will be applied for predicting the λ values of soil units of the soil survey map of Bavaria 1:25.000 (ÜBK25) thus enabling it to be a management tool for optimized site specific layout of horizontal GHE. Data will be made available for users in the internet (www.bis.bayern.de).

 


Letzte Änderung 23.10.2013