Fossil fuels are the main source of power for all forms of transportation. The search for environmentally acceptable alternatives has gone in several directions. Today solar power produced by photovoltaic cells in panels such as those shown is becoming a viable alternative in pleas where there is plenty of sunlight much of the year. Technological advances are bringing down the cost, but a homeowner who wants to install enough solar panels to supply all of a home's electricity often must spend tens of thousands of dollars. Most automakers now sell hybrid vehicles that run on a combination of gasoline and electric power generated by batteries. There also has been much research aimed at developing all-electric vehicles and hydrogen fuel cells to power cars. So far, though, both are extremely expensive and impractical. Few electric-car "charging stations" exist, and the process for making hydrogen fuel cells generates high levels of greenhouse gases