ICELaND's largest energy firm is working on ambitious plans to supply Britain with power generated by volcanic heat.  Landsvirkjun, the stateowned giant that produces 75% of the island's electricity, is examining the feasibility of laying a 1,170km (730 mile) cable between Iceland and Scotland.  The cable would be the longest of its kind in the world, transporting up to six terawatt hours (TWh) of energy to Britain every year - enough to power 1.5m households.  The move comes as politicians are trying to create a European "supergrid". Better links would allow Europe to share renewable energy, meaning that wave and wind power generated in the North Sea could be used across the continent.  The EU wants to obtain 20% of its energy from green sources by 2020.  Landsvirkjun's proposed conduit between Iceland and Scotland would cost about $2.1 billion (?1.3 billion) to build. a further $2.4 billion would need to be spent on the construction of several geothermal plants and hydroelectric dams to generate the 6TWh of energy.  Geothermal plants exploit volcanic terrain by filtering water through hot rocks and then using the steam to drive electricity generators.  Iceland hit the headlines last year when the Eyjafjallajokull volcano erupted for the first time in almost 190 years, sending out a cloud of ash that disrupted flights across Europe for a week.  The proposed undersea cable to Scotland would produce annual revenues of about $400m, which would help Iceland repair its public finances after the collapse of its banking system in 2008.  Valdimar armann, an economist at Gamma, a Reykjavik asset manager, said the project would require foreig