Wind
Uneven heating of the Earth's surface causes the wind to blow. Wind energy has historically been used to push sailing ships great distances across the oceans, pump water, grind grain, or saw wood. Wind power is the conversion of wind energy into more useful forms, usually electricity using wind turbines.
Europe is at the forefront in utilizing wind energy. The American Wind Energy Association advocates for this power source in North America.
Wind turbines can capture the solar energy stored in wind and convert it into electricity or use the mechanical power directly. Wind turbines convert the kinetic energy of the wind into other forms of energy. Large, modern wind turbines operate together in wind farms to produce electricity for utilities. Small turbines are used by homeowners and remote villages to help meet energy needs. Wind power will become more efficient as new turbine designs reduce the costs, making wind turbines economically viable in more places.
The effectiveness of wind power is closely related to the local micro-climate. This effectiveness is related to the number of days per year the wind blows, the average wind velocity and the steadiness of the wind speed. Certain places, such as the high plains of North America, are more desirable as wind power generating locations than others.
Geothermal
Geothermal power makes use of heat from the interior of the Earth. This is the same energy which rises naturally to the Earth's surface in the form of hot springs, geysers, and volcanoes. Geothermal systems are located in areas where the Earth's crust is relatively thin. By drilling into the ground and inserting pipes, hot water or steam can be brought to the surface. In some applications, this heat is used directly to heat homes or provide process heat for businesses. In other areas, the steam is used to drive a turbine to generate electricity. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the Earth's surface, and down even deeper to the extremely high temperatures of molten rock called magma.
This video presents the process by which geothermal power is harnessed in California just north of San Francisco Bay.
Biomass
In a process called "photosynthesis", plants capture sunlight and transform it into chemical energy. This energy may then be converted into electricity, heat, or liquid fuels using a number of different energy conversion processes. The organic resources that are used to produce energy using these processes are collectively called "biomass".
The term "biomass" means any plant derived organic matter available on a renewable basis which can be converted to energy. It includes dedicated energy crops and trees, agricultural food and feed crops, agricultural crop wastes and residues, wood wastes and residues, 
aquatic plants, animal wastes, municipal wastes, and other waste materials. It is possible to convert municipal waste, manure or agricultural products into valuable fuels for transportation, industry, and even residential use.
Biofuels made from plant matter - sugarcane, corn or soybeans, for instance - include ethanol and biodiesel, with ethanol currently accounting for more than 90 percent of global biofuel production. PBS had an excellent program focused on biofuels on February 8, 2008.
The combustion of biofuels results in far lower emissions of several pollutants, including carbon monoxide, hydrocarbons, sulfur dioxide, and particulate matter, than burning petroleum fuels would. Unlike fossil fuels—which contain carbon stored for millennia beneath Earth’s surface and which release enormous amounts of greenhouse gases when burned—biofuels have the potential to be “carbon-neutral” over their life cycles. Large-scale and widespread demand for biofuels could offer new markets for farm and forest products as well as new jobs and industries in rural areas. On the negative side, it could increase exploitative pressures on our remaining natural areas.
One interesting alternative source of biomass is algae, if the price of production can be reduced. Algae grows much faster and in less space than conventional energy crops. An acre of corn can produce 20 gallons of oil per year. One acre of algae can produce 15,000 gallons.
An algae farm could be located almost anywhere. It would not require converting cropland or forests to energy production. It could use sea water and could consume pollutants from sewage and power plants.
Solar
An immense amount of energy from the sun strikes the surface of the earth every day. This energy may be captured and used in the form of heat in "solar thermal" applications, or it may be converted directly into electricity to power electrical devices using photovoltaic cells.
Today, solar collectors can gather solar thermal energy in almost any climate to provide a reliable, low-cost source of energy for many applications including hot water, residential heating. In recent years, utilities have begun to use solar thermal energy to generate electricity by boiling water and using the steam to drive a turbine which generates electrical power. In 2008, Pacific Gas & Electric contracted for as much as 900 megawatts from solar thermal power plants in the Mohave Desert.
Photovoltaic cells convert sunlight directly into electricity and are made of semiconductors such as crystalline silicon or various thin-film materials. Photovoltaics can provide tiny amounts of power for watches, large amounts for the electric grid, and everything in between.
As can be seen in the image, solar energy generation is not limited to the earth's surface. Solar power generation can be most efficient in the vacuum of space. Scientists have even envisioned solar power satellites transmitting energy to the earth.
This is not science fiction. In 2009, Pacific Gas & Electric announced it had contracted for 200 megawatts from space by 2016.
Hydrogen
Hydrogen is a clean energy carrier (like electricity) that can be made by use of diverse resources such as renewable energy (e.g. solar, wind, geothermal), nuclear energy, and fossil energy (combined with carbon capture/sequestration). Hydrogen in the long-term could simultaneously reduce dependence on oil and emissions of greenhouse gases and criteria pollutants. Hydrogen is a clean fuel in the sense that its byproduct is clean water. Nevertheless, there are problems with hydrogen.
Hydrogen production by today's methods is costly and can result in as many greenhouse gases as fossil fuels. A production method that would avoid the creation of greenhouse gases is hydrogen dissociation from water using sunlight. Three technologies are available: solar cells which are efficient but expensive, microorganisms which are inexpensive but produce little hydrogen, and photocatalysis using semiconductors which has shown potential using nanotubes. The search for a successful hydrogen production method that does not worsen global warming continues.
Given that the only byproduct of burning hydrogen is water from which hydrogen can be dissociated, it is renewable.
