and Bumthang (Chhumey). Bhutan's first mini-hydroelectric facility was built in 1967 in Thimphu; it operated until 1988. Until the 1970s, Bhutan constructed many other small hydroelectric plants. During the 1970s, Bhutan and India began to partner on larger projects aimed at electrifying larger regions of Bhutan and addressing transnational energy needs. Alternative energy In the face of climate change and growing energy demands, Bhutan has sought additional energy security through developing its alternative energy sources. Solar Energy As of 2015 there are approximately 4,600 solar power systems operating in Bhutan, with 2,750 on-grid systems and 1,848 off-grid systems. The development potential is estimated at around 12,000 megawatts. Solar energy in Bhutan has received direct investment from domestic and international sources. In 2010, Asian Development Bank provided a grant of over USD21 million for electrification of rural homes, aiming to provide power both on-grid and off-grid. The Bhutan Power Corporation provided solar electrification training for villagers from rural eastern areas of Bumthang, Lhuentse, Mongar, Pemagatshel, Samdrup Jongkhar, Sarpang, and Wangdue Phodrang Districts Solar powered lighting is also available to many nomads living within protected areas of Bhutan. Biomass and biogas In order to shift household dependence on firewood, Bhutan began re-exploring biogas development from cow dung. This included a five-year trial program in Chukha, Samtse, Sarpang, and Tsirang Districts from 2011 to 2015. Bhutan had previously explored generating biogas in an identical fashion in the 1980s, but the program was abandoned after failures in training of masons and users, after-sales service, and site follow-up. Wind energy The theoretical development potential for wind power in Bhutan is an estimated 761 megawatts. Potential is highest at Wangdue Phodrang at 141.7 megawatts and Chukha at 91.8 megawatts. In 2010, pilot wind turbine programs were implemented to investigate the feasibility of using wind energy to alleviate hydropower drops during the dry winter seasons. It is located at Wangdue Phodrang in the western part of Bhutan. Bhutan launched its first wind turbines in 2016 in Rubesa gewog in Wangdue Phodrang. It consists of two wind turbines with an estimated production capacity of 600 kilowatts. History Since the late twentieth century, hydroelectric power has been a very important aspect of Bhutan's economic development as a low-cost energy source supporting more capital-intensive industries, such as forestry, mining, and cement and calcium carbide production. Bhutan's steep mountains, deep gorges, and fast-flowing rivers create abundant hydroelectric potential, which the government