*2.1. Large Hydropower*

Large hydropower in developed (20th century) and developing countries (e.g., China, Brazil, and South Africa) in the early 21st century has been very large, with this generation system providing the main energy source in those countries where topography, hydrology and climatology allow hydropower recovery.

Although China began to develop its hydropower strategic plan in the 1950s, it has quickly overtaken other countries. Its development started with the Liujiaxia dam, which was completed in 1974 and has an installed capacity of 1250 MW. In 2012, China finished the Three Gorges Dam, which has a total installed capacity of over 20,000 MW. A project is currently being developed in the Yarlon Tsagpo Canyon with an installed capacity of over 40,000 MW, double the installed power in the Three Gorges Dam [8]. China has far exceeded the milestone achieved by the Hoover Dam (Colorado River, United States) in 1936, where 2000 MW were installed; or the 12,000 MW installed in the Itaipu Dam (Parana River, Brazil and Paraguay) in 1966, now with 14,000 MW. In Spain, up to 2013, the installed capacity is approximately 18,000 MW, and the hydropower installations do not exceed 400 MW on average.

Ansar [22] made an inventory of the 245 largest hydropower sites in the world (1934–2007). Among them, 72 are located in East Asia, 50 in Latin America, 40 in North America, 29 in Africa, 29 in Europe and 25 in South Asia. Of these, 97 dams are producing electricity, 89 are multipurpose (including hydropower) and 59 are devoted to irrigation and other uses. These plants are occasionally reversible to take advantage of the available volume of water, adjusting the electric energy injected to the grid according to the energy demand. Rehman et al. [10] established that the worldwide installed capacity of reversible plants is 104 GW (presently, the total installed capacity of hydropower is 1000 GW), of which 22.2 GW are installed in North America, 44 GW in Europe (5.3 GW in Spain), 33 GW in Asia, and the remainder in Africa and Russia. These authors refer to efficiencies between 70% and 80%.

Regarding environmental performance indicators of hydropower solutions, these plants have a positive impact on global climate change [23], based on the carbon footprint, which is the parameter used to determine the environmental impact, which has taken on special importance since the 1990s. This parameter is defined as the sum of the greenhouse gases emitted by an organization, event or product, expressed in terms of CO2 equivalent units (CO2-e) [24]. According to Zhang and Xu [25], the influence of the carbon footprint depends on many factors, most importantly the construction and maintenance costs (because these represent more than 60% in earthen dams and 50% in concrete dams) [9]. The range of emissions for these systems is between 2 and 240 gCO2-e/kWh [9,26], with the carbon footprint in hydropower plants being smaller than that in coal plants. These non-renewable energy plants have emissions above 890 gCO2-e/kWh [26–28]. Considering these emissions, hydropower plants saved 3.3 billion tons of CO2 emissions in 2014 and will help reduce emissions by over 120 billion tons between 2015 and 2050 [13] compared to coal plants.

Regarding the economic aspects of large hydropower, the investment ratio (€/kW) decreases as the installed capacity increases, reaching values from 2170 to 470 €/kW for power ranges between 200 and 1400 MW [29,30]. Civil works represent between 70% and 80% of the total investment, and the remaining costs are devoted to electro-mechanics and hydraulic equipment [10].
