Renewable energy comes from resources that are naturally replenished, such as sunlight, wind, rain, tides, waves and geothermal heat. Some key advantages of renewable energy sources are that they are available freely in nature, produce little to no pollution, and are inexhaustible. However, renewable energy is generally available in a dilute form, the cost of harnessing it is often high, availability depends on unpredictable natural phenomena, and transporting the energy can be difficult.