Renewable energy refers to energy sources that are natural, continuous, and large. This includes solar energy, wind energy, water energy, geothermal energy, and certain biofuels. The downside to renewable sources of energy is that they generally require a substantial amount of infrastructure to extract appreciable power. For example, collecting solar energy requires expensive solar panels. Harvesting wind energy requires wind turbines. Getting energy from flowing water requires dams, and so on.
Today, renewable energy is poorly exploited. Most countries get less than 10% of their power from renewable energy sources. Fossil fuels are easier to come by, and they currently return more power per dollar invested in extracting them. Certain countries, like Iceland and Norway, get as much as 99% of their power from renewable energy sources, but this is because they are located conveniently in areas where there is abundant geothermal activity. For other countries to transition to renewable energy will require significant investments and starting costs.
Investments in renewable energy have been increasing since the environmental movement of the 1960s and 70s, and more recently due to renewed concerns about global warming and Peak Oil. Fossil fuels pollute the earth, are limited in nature, and are controlled by undemocratic states in politically volatile regions. Using our limited quantities of fossil fuel energy to invest in a renewable energy infrastructure seems like a wise collective decision for the human species to make.
Ultimately, the renewable energy source that will offer the greatest amount of power is solar energy. There are millions of venture capital and government dollars being invested in start-ups seeking to boost the efficiency and reduce the cost of solar cells, and progress is being made. Solar furnaces, which condense the sun's rays into a focus with mirrors, then use that heat to boil liquid and run turbines, are an even more efficient means of harnessing solar energy than conventional solar panels. The surfaces of the world's oceans are largely unused, and life in certain parts of the sea is very scarce, making them ideal venues for deploying floating solar panels that power the world's cities. Eventually, we will exploit the gargantuan volumes of space to set up solar panels and beam power to where it is needed.
In the meantime, we can extract appreciable quantities of energy from other renewable sources such as water and wind. For example, the recently completed Three Gorges Dam in China will produce 18 gigawatts of continuous power when all its generators are installed. This will supply about 3% of the national demand for electricity. The project cost roughly under a billion US dollars (USD).