Answer:
There is a lot but here are some!
Wind: Wind power is growing rapidly, and is becoming a well-recognized renewable energy resource. Using wind power to turn turbines that generate electricity can provide a cheap source of energy. Building and maintaining equipment could provide thousands of jobs and cost-efficient and clean electricity. Wind farms, however, are not particularly popular. They can impact local environment and wildlife, and even provide noise pollution. Additionally, many people feel that the equipment used obstructs scenic views.
Solar: This is perhaps the most recognized renewable energy source. Energy from the sun is captured using cells made from special materials (silicon is quite popular right now) and then converted into electricity. The biggest factor in solar cell production is cost. However, with technological advancements solar is becoming more cost efficient, and high efficiency solar cells are being developed. This is important, since high efficiency cells are hard to come by. New materials are providing solar cells that are easier to transport and install. Flexible solar cells can be used for residential use, and building solar arrays is becoming popular. One of the main factors in efficiency is the fact that solar panels only generate electricity during daylight hours, and can be hampered by cloudy conditions or pollution. Some sort of storage is needed in order to make full use of solar power.
Water: We have been studying water-based renewable energy for quite some time. Hydroelectricity has been a source of energy for years. However, even though the energy production process does not put off pollution, there are other environmental concerns associated with the damming of rivers and ecological impacts stemming from this practice. But hydroelectric power remains one of the more cost-efficient means of generating renewable energy.
Geothermal: Geothermal energy is extracted from the natural processes of the earth. A great deal of heat is created below Earth's surface, and efforts are being made to extract and use this power. While the ancient Romans knew about and used geothermal heating, now Earth's processes are being used to generate electricity -- going beyond space heating. Geothermal power does not put off greenhouse gases (although some harmful gases from deep in the earth would be released -- and need to be contained), and it is reliable. However, it can only be used in areas where there is tectonic activity.
Nuclear: Perhaps the most controversial form of renewable energy is nuclear energy. Electricity is produced from the energy released by nuclear reactions. While fission (splitting) is the main source used today, interest continues in developing cold fusion. Currently, though, power plants generating power using nuclear fission are among the safest plants. They also generate power without emitting pollution. In Europe, France benefits greatly as its nuclear energy produces the cheapest electricity (according to 60 Minutes).
