Thermal characterization of Belgian subsoil for optimising geothermal application
In Belgium, 75% of the installed ground source heat pump are vertical loops. After the ThermoMap project the next essential step is therefore to assess the potential for a greater depth. However no fundamental research has yet been conducted on geological characterisation of the Belgian subsoil to improve the performance of these systems. There are wide variations possible with regard to the effective heat transfer between the transport medium and the rocks in the underground, which is crucial to optimise performances and reduce the costs. Nowadays, large differences in design of vertical closed loop systems are observed for a comparable geological situation. This potentially harmful situation to the breaktrough of geothermal energy in Belgium has two reasons, a lack of knowledge on the site-specific thermal characteristics and an insufficient analysis of the thermal energy need profile of the building, resulting in large uncertainties on the design. Following the experience of Geological Survey of Belgium , also based on its existing rock and borehole collections, the current project will focus on the subsoil aspects, while other initiatives like Smart Geotherm conducted by CSTC/BBRI (Luc François) and Geotherwal (Robert Charlier ,ULG) concentrates on the building part and the heat exchanger performances.
Objectives
The project aims to assess the shallow geothermal potential in Belgium through analysis of thermal properties of rocks from the surface to a depth of 150 m, which covers the standard depth for a vertical loop system currently installed in Belgium.
The project will focus on the thermal properties (thermal conductivity and capacity) and the mineralogical composition of about 400 rock samples corresponding to 25-30 different lithologies. Thermal properties of rocks depend on water content, porosity, dominant mineral phase and fabric anisotropy. Thermal conductivity is also a function of temperature, pressure, saturation and saturant. The sample selection will be conducted in order to be representative of the various lithologies composing the Belgian subsoil, taking into account their mineralogical composition and petrological texture, along with degree of alteration and fracturation. Thermal parameter measurements will be performed for both saturated and unsaturated conditions. Special attention will be given to lithologies with wide-ranging heat conductivity values to assess the influence of porosity and/or minor mineralogical constituents on the heat transfer. Emphasis will be placed on areas with the highest geothermal demand, e.g. the Sambre & Meuse valley and large cities of Flanders . Lithology samples will primarily be selected from the borehole collection at RBINS-GSB managed by Royal Belgian Institute of Natural Sciences (RBINS) Collection Service. If necessary fresh samples will be also obtained from outcrops or new boreholes to assure the geographical and geological representativeness of the measurements. These will be integrated into the existing rock samples collection of RBINS-GSB.
The characterization of the thermal properties of the Belgian subsoil forms an essential step towards the establisment of the shallow geothermal potential. The construction of such potential maps will be an additional innovative approach (not financed yet), since only two similar projects are currently conducted in Europe: e.g. in Germany, shallow geothermal potential maps are in production for the southern Molasse Basin and in North Rhine-Westphalia (www.geotis.de); in southern Italy, the VIGOR project is under progress (www.vigor-geotermia.it).
Measurement equipment
The thermal conductivity and diffusivity of the rock samples are performed with a TCS (Thermal Conductivity Scanning) at the RBINS-GSB laboratory. For unconsolidated rock and sediment samples a transient method will be applied (TEKAO4 and/or KD2Pro ).
Mineralogical and petrological analyses of the samples will be conducted with the analytical equipment of the Mineralogical and Petrological laboratory at the RBINS-GSB. The ratio of the different mineralogical phases will be measured with the new Panalytical X-ray Diffraction equipment, while the EDS (Energy-Dispersive X-ray Spectroscopy) and EBSD (Electron BackScattered Diffraction) modules will be applied in order to evaluate the chemical and micro-textural content.
