Acknowledgement
The financial support of the Research Council of the KU Leuven
(OT-projectOT/03/35)is gratefully appreciated,as it allowed the
research on the Modave Sandstone. This work was also supported by the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evalution and planning (KETEP) through a grant that was funded by the Ministry of Trade,Industry & Energy, Republic of Korea