1. https://www.eia.gov/energyexplained/renewable-sources
2. https://ourworldindata.org/renewable-energy
3. https://www.utilitydive.com/news/renewable-energy-prices-keep-falling-when-do-they-bottom-out/555822/
4. https://www.eia.gov/energyexplained/hydropower/
5. Bay JA, Golomb DA, Energy and the environment, 1st Edition, Oxford University Press, New York, 2002