Hello, welcome to unit four, Discovering cleaner energy sources to power the world. Earlier in the course, we discussed how fossil fuels release greenhouse gases into the atmosphere. And that has caused the earth to get warmer at a very fast and dangerous pace. Scientists, are therefore, trying to discover new types of energy that won't have such a harmful effect on the environment. In this unit, we will look at some examples of those new cleaner energy sources. In this video, we're going to talk about some new technologies. We'll start by looking at the different types of renewable energy. Then we'll look at some of their advantages and challenges of each. Finally, we'll introduce some advances in technology that may help increase the use of renewable energy in the future. Let's start by explaining what renewable energy is. Renewable energy is energy from a source that is not depleted when used. Depleted means used up or emptied. Some of the most common examples are wind, water or solar power. For example, if you get your energy from a windmill, you don't have to worry that someday you'll use up all the wind the same way that fossil fuels will someday be used up or depleted. The main types of renewable energy sources are biomass energy, hydropower, geothermal energy, wind energy and solar energy. Biomass energy is energy made from non-fossilized plant materials. In other words, plants that have not been turned into fossil fuels. Wood and wood waste are common forms of biomass energy. In fact, a couple hundred years ago, before the use of coal and oil became so important, wood supplied 90% of the energy needed in the United States. Another example of biomass is ethanol, which is fuel made of corn. For the most part, sources for biomass energy similar to fossil fuels have to be burned to release the energy. But there are other forms of renewable energy that are not burnt and do not release any greenhouse gases into the atmosphere. Let's look at some types of clean renewable energy. Hydropower is energy created by moving water. Geothermal energy is energy taken from the hot interior of the Earth. Wind energy is power from the wind, and solar energy is power created by capturing the radiation of the Sun. These types of energies are different, because they are clean sources of renewable energy,. This means, they do not pollute our atmosphere with greenhouse gases. You might be asking yourself, why don't we use renewable energy from clean sources that don't cause pollution? One challenge is these natural sources are not always available,. There may be cloudy days, or days with low winds that can decrease the power from solar or wind energy. Another challenge, in the past creating these types of energy has been more expensive than fossil fuels, so some countries that are developing quickly have chosen to use cheaper and easier methods to get fossil fuels to power their growth. But many scientists are working hard to create new ways to help increase the use of these clean renewable sources of energy. For example, they are discovering ways to store the extra energy collected on extra sunny days, so it can be used later on cloudy days. All of this research is paying off. In the United States, the amount of electricity produced by renewable energy increased from 13% in 2012 to 22% in 2013. It's predicted by 2020, 26% of the electricity in the US will be produced by renewable sources, and this increase is happening all over the world. One great example is Denmark. In 2015, Denmark produced over 40% of its power by wind energy. In this video, we looked at a few examples of renewable energy. We learned about some of the challenges renewable energy faces, and introduced some advances in technology that might help make it more available in the future. In the rest of this unit, we will go into more detail about a few specific, new clean energy sources. We'll start, in the next video, by looking at wind, sun, and water energy sources.