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Heiko Liebel, Norwegian University of Science and Technology (NTNU) (Norway)
Bjorn Frengstad, Geological Survey of Norway (NGU) (Norway)
Randi Kalskin Ramstad, Asplan Viak AS (Norway)
Bjorge Brattli, Norwegian University of Science and Technology (NTNU) (Norway)
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Geothermal energy is CO2-neutral and considered environmentally friendly. High-enthalpy geothermal energy is restricted to regions with high heat flow. Low enthalpy geothermal energy, however, can be obtained at any place with the help of a ground-source heat pump from the soil, rocks and from groundwater. It has a high potential for heating and cooling of buildings especially in temperate and polar zones. Systems using borehole heat exchangers have a Coefficient of Performance (COP, ratio of useful energy to electricity consumption of a heat pump at given operating conditions) higher than 3 so that electricity costs can be reduced by about 75 %.
Optimum efficiency of installations requires general knowledge on the heat flow and groundwater flow properties of the aquifer. Especially in fractured rock aquifers these properties are not well understood. Most parts of Scandinavia are built up by igneous and metamorphic rocks with very low permeability and porosity. Therefore stored groundwater and groundwater flow is observed almost exclusively in fractures.
A PhD project, started summer 2008, covers the simulation of groundwater flow and heat flow in fractured rock aquifers with commercial software packages like FEFLOW, HST3D or SUTRA. Thermal response tests with and without pumping of groundwater (to promote groundwater flow) will be carried out to quantify the contribution of the groundwater flow to heat flow. The calibration of the models will be based on tracer experiments including environmental (18O, 2H, 3H) and injected tracers.
Initial studies are to be carried out in a 70m deep single well at Lade (Sør-Trøndelag, Norway) in July 2008 comprising a thermal injection/withdrawal test to estimate fracture frequency in greenstone and trondhemite rocks and the energy storage potential in this aquifer. It will be the first test of its kind performed in a hardrock aquifer in Scandinavia. The main experiment and study area will be a well field in Holmedal (9 wells, Sogn og Fjordane, Norway), Bryn (5 wells, Brum, Norway) or the well field of EAB (Energiselskapet Asker og Brum) 3km southeast of Bryn (3 wells).
It is expected that both groundwater flow and heat flow in fractured rock aquifers follow similar rules as widely accepted for porous media. The influence of high conductivity via fractures will be quantified using numerical models and analytical calculations. Fracture frequency as an important information about the energy storage potential of an aquifer will be estimated with thermal injection and withdrawal tests.
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