Interesting article on German geothermal, thanks.  Notice they hope it'll be competitive with oil and gas in 20 yrs, if the oil/gas prices rise fast enough.
 
A liquid would be a better choice for storing solar energy if you want high temps.  John Hait's "Passive Annual Heat Stoage" works well for low temp storage.  I've built 2 and have consulted on several.  Great way to passively heat/cool a house, not so good for electric generation.
 
Raising the mass temp will reduce the required size, but it's not a simple thing to do.  And as you increase your delta T, losses become pronounced.  Dirt's a good medium mostly for its cost, not efficiency.  Hait's primary contribution was measuring heat movement through dry ground at the rate of 20 feet (6 m)/half year and combining that with a simple umbrella for storage.  Simple, effective. 
 
Another good way to store solar energy is biomass.  As Jean Pain demonstrated, relatively uncomplicated to convert to electric generation.  That's my direction, whether composted or from woodgas.  KISS works for me.