most of our energy resources, such as coal, petroleum, and natural gas, are non-renewable. if the..................of these energy sources continues at the....................., we can foresee their being used up. ......................this reason, scientists are taking a closer look at alternate................: nuclear, geothermal, hydroelectric, wind and solar. .................. solar energy is used in many places to ................ water, the technology for its ................. as a source of electricity is still in its infancy. scientists are now looking ................. photovoltaics as a growing solar technology that will be .................. valuable sources of power in the future. However, there are still problems to ..................... before photovoltaic properties can be widely used to generate power.