Renewable energy is energy from a resource that is replaceable by existing flows of energy, such as sunshine, wind, water, biological processes and geothermal heat flows. These energy resources might be used directly or indirectly as forms of energy. The contemporary interest in renewable energy is linked to the exhaustion of fossil fuels and environmental challenges created by the use of fossil fuels and nuclear energy. However, the burning of biomasses for heat and light have been exhausting natural resources for centuries, but the oil crisis and the green house effect have been strong motivations for the expansion of renewable energy. Some countries may see renewable energy as strategically more secure, especially if this means they do not need to rely on foreign energy supplies.