|Fig. 1: Gezhouba Hydroelectric Power Plant, China's largest plant on the Yangtze. (Source: Wikimedia Commons.|
Harvesting energy from the flow of rivers and streams using hydroelectric technology may be one of the cleanest sources of renewable energy and can be widely scaled.  China is the most polluted country in the world and has embraced this innovative method of energy production. The Yangtze River is one of the largest streams of water all throughout China and has been utilized to supplement Central and Eastern China with electricity using a hydroelectric power plant. 
The potential for harvesting energy from the flow of a river seems limitless as long as there is no drastic widespread drought throughout China. It is also the most efficient technique in producing energy, as it can capture up to 90% of the available energy from the river into electricity. The development of a hydroelectric plant on the Yangtze River has reduced the adverse gas produced from the emissions of industrial and commercial institutions, as the hydroelectric method leaves no carbon footprint. However, in order to construct a hydroelectric power plant, it is vital to first build a dam which may cause a submergence of the surrounding land and cause an environmental concern. 
The power plant operates starting from the dam, which in turn collects water, controlling the flow of the river or stream. As a result, water levels in the dam are higher than the downstream levels. As the water flows downward due to gravity from the difference in water levels, a generator captures the kinetic energy created from the falling water flow, creating electricity.
The variability of the electricity produced by the Gezhouba Hydroelectric Power Plant (See Fig.1), the largest power plant on the Yangtze River, depends on three main factors. On average, the plant produces an annual output of 15.7 billion kWh. However, this is dependent upon the total amount of water flow, the distribution of water flow, and the water level difference within the dam. The relationship between water flow and electrical production can be quantified by the equation 
|P||=||-4.7 × 10-6 Q2 + 0.2398 Q||Q ≤ 18,000||=||-0.039 Q + 3498.33||Q > 18,000|
where Q is the water flow in m3/s and P is the power produced in Megawatts. If water flow is between 16,400 m3/s and 20,000 m3/s, then the plant will function at full capacity. However, if the water flow increases above 20,000 m3/s, then sluice gates will release the excess water, increasing the water levels downstream, which will ultimately decrease electric generation since there would be very little difference in water levels. Anything over 65,000 m3/s, the generator will not function as water levels are not significant enough to produce power. 
Due to the efficiency and theoretically infinite supply of water flow, many hydroelectric power plants will continue to be implemented and developed throughout China. However, the factors of rainfall and drought in China determine how much energy it produces annually. The total amount of water flow per year is of less concern than how well the river water flow is distributed throughout the year, as having too much or too little water flow during a period reduces the optimality of the hydroelectric generator.  Despite the variability of the temporal distribution of water flow, the Gezhouba Hydroelectric Power Plant has already revealed its effectiveness in powering Central and Eastern China, and many more plants are now being installed and utilized. However, China has already shown signs that their hydroelectric resources may be exhausted, as their hydroelectric production is slowing down. According to the BP Statistical Review of World Energy 2017, in 2016, China had 263.1 million tonnes oil equivalent with a growth rate of only 4%. The total is 3053 million tonnes oil equivalent, so the power produced in 2016 only produced 8.63% of the total power produced. The average growth rate from 2005 to 2015 is 10.9, revealing that 2016 had been significantly lower than the average, signaling that China has already exploited their maximum potential from hydroelectric generation. 
© Jesse Kuet. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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