At the end of a winding, tree-lined country road in western Finland, four young engineers believe they have a possible answer to one of green energy's biggest challenges. The challenge is how to provide a year-round, steady power supply from renewable energy during changing seasons and variable weather conditions. The answer nestling in Vatajankoski power plant, 270 km (168 miles) north-west of Finland's capital, Helsinki, is remarkably simple, abundant and cheap: sand.