Solar Energy: There are two main forms of solar energy, heat and photovoltaic. Heat energy is obvious on a hot desert day. Photovoltaic energy is more subtle. Both forms are clean, but do require a lot of land for the installations. Scientific American calculated a couple years ago that by 2050 all of the power needs of the US could be met by some 250,000 square miles of solar installations...that's an area 500 miles by 500 miles by the way.
Solar reflective concentrators have also been used for years to successfully generate heat, which is then converted to electrical power. Solar power should be neat the top of any list of renewable resources. It is truly renewable until the sun runs down that is.
Biomass: Wood chips, waste agricultural materials, and other plants are burned like coal and natural gas.
Biofuels: Biofuels like ethanol and biodiesel are distilled and extracted from plants high in sugars and starches such as corn, sugar cane, and switchgrass to name a few. Brazil has long used ethanol from sugarcane to run a large fraction of its autos. Ethanol is the main biofuel today, though there pros and cons.
One promising technology is algae oil. The process does work, but it is area and water intensive, and still too expensive. Algae oil has the benefit of actually using up carbon dioxide. Of course if you burn algae fuel in cars....it goes right back into the air.
Geothermal Energy: In areas of the world near tectonic plate boundaries or perhaps mantle hot spots, there is a lot of natural heat energy. This energy is often released as steam which can be used to spin turbines. Water can also be injected into wells drilled into the hot rock formations. The result is again usable steam. The geothermal process releases a lot of ionic type salts from the brine solutions, but is otherwise not too hard on the environment.
Geothermal energy makes the list of renewable resources, though it is not infinite. In specific locations, geothermal heat will decline with use and not renew for a long time, if ever.
Hydro Power: One of the oldest worldwide energy sources on the list of renewable resources is running water. Water falling from a higher to a lower elevation has a lot of energy. Hydro power usually requires some sort of dam, which tends to be hard on fish and other river living creatures.
Peat: Peat sometimes makes the list of renewable resources for Political Reasons. Peat is basically low grade coal. It takes a Long time to renew. And practically is not renewable, but it goes to show the power of money.
Wood Pellets - Packed with Energy
and A $7,000,000,000 market
Wood Pellets: One form of wood produce is wood pellets. Wood pellets are made from compressed wood chips or sawdust. Pellets are packed tight with energy, and burn cleanly compared to other wood products, produced with a 10% moisture content providing a high combustion efficiency. The European market for pellets is growing rapidly, expanding to a value of some seven billion USD.
Tidal and Wave Energy: The tides are relentless, and so is their potential for power output. The power potential is off the charts. However, the locations of productive tides to where it is needed are not so many. Wave energy is not so great, but more accessible. Still, there is a lot of energy waiting to be tapped especially for areas near strong tides and currents. There are many different mechanical devices to transform tide and wave power to electric power.
Wind Energy: Wind turbines also have a long history of energy production. They are popping up all over the place. It appears that Germany is now leading the pack in power production from wind turbines.
Following below is a list of renewable resources. Renewable energies have been used for hundreds of years. Research is ongoing and changing almost daily.