There are a number of advantages to tidal energy. Because the force behind tidal energy comes from the pull of the moon, it is an inexhaustible energy source. As long as the moon continues to orbit the earth, there will be energy in the tides. This relationship to the moon also makes tidal energy a predictable energy source. Other forms of renewable energy, such as wind and solar energy, are dependent on random weather patterns. But tidal energy is based on the rise and fall of tides, which is more uniform and reliable. It is a clean energy source because, unlike the burning of fossil fuels, it does not release greenhouse gases or other pollutants into the air. It is also a cheap energy source. After the initial investment is paid off, the cost of generated electricity is very low. Tidal energy has a high energy density, meaning that the tides store a larger amount of energy than most other forms of renewable energy, such as the wind.