They have a material that supplies electrons and a power source that lifts the energy level of those electrons.

VOA: special.2010.09.01

The plant is a major source of food energy and a major food security crop.

VOA: special.2010.06.08

If properly managed, clearing and replanting trees in a 30-year cycle creates both a renewable source of energy and maximum rubber production.

VOA: standard.2009.09.13

The shutdown is bad news for the recession-hit country partly because it will lose a source of cheap electricity and be forced to import more expensive energy.

VOA: standard.2010.01.01