1. Keage Power Plant: Japan’s First Hydroelectric Plant for Public Use
The Keage Power Plant in Kyoto uses water from the Lake Biwa Canal and began operation in 1891. According to the Agency for Cultural Affairs’ description of Japan Heritage, it is considered Japan’s first hydroelectric power plant designed to supply electricity to the general public [6]. The electricity was used for streetlights and factory power in Kyoto, and later for the country’s first commercial electric railway, the Kyoto Electric Railway [6]. The force of falling water became the town’s lights and trains.
It is introduced that the plant currently in operation is the third generation, built in 1936 [7]. Because there are differences in the output figures for the initial equipment depending on the source, specific numbers are not listed here.
2. How a Water Wheel and Generator Work Together
The flow of hydroelectric power is surprisingly simple. Water enters from an intake, passes through a waterway to turn a water wheel, and a generator directly connected to the wheel rotates to create electricity [2]. It is a transfer of energy: falling water force → water wheel rotation → generator rotation → electricity.
The rotation speed of water wheels varies by model. According to power company materials, it is said to be between 120 and 1,200 rotations per minute [1]. This works out to 2 to 20 rotations per second. The conversion efficiency, which shows how much energy can be turned into electricity, is described in the same power company materials as about 80% for hydroelectric power, compared to 30 to 40% for thermal power. Hydroelectric power is said to be about twice as efficient as thermal power [1]. However, these are explanatory values from power companies and change depending on the model and conditions.
3. How Water Height and Flow Rate Decide Power Output
The output of a power plant is determined by the multiplication of the height the water falls (effective head) and the amount of water flowing per second (flow rate). The more water there is, and the greater the drop, the greater the electrical energy produced [2].
As a formula, Output (kW) = 9.8 × Flow Rate (cubic meters per second) × Effective Head (m) × Efficiency of the water wheel and generator [3]. The number 9.8 represents the strength of gravity. Because you multiply by efficiency, the actual output is smaller than this result. Let’s try a calculation example. If the flow rate is 1 cubic meter per second, the head is 100 m, and the combined efficiency is 0.8, then 9.8 × 1 × 100 × 0.8 = 784, which is about 780 kW. This is just an example applied to the formula, not figures from a real power plant. From this formula, we can see that if either the head or the flow rate doubles, the output also doubles.
4. Three Types of Water Wheels and When Each Is Used
Water wheels are chosen based on the head and flow rate [2]. Based on explanations from Chubu Electric Power, let’s look at three representative types [4].
For power plants with a large head, the Pelton wheel is suitable. It is a water wheel that spins by directing the force of water ejected from a nozzle onto cup-shaped blades. For medium heads, the Francis wheel is used. It can be used in a wide range of heads, roughly 10 to 300 m, and it is said that about 70% of Japan’s hydroelectric power plants use this wheel. The applicable head range varies by source, with some explanations stating 40 to 600 m. For power plants with a relatively low head and high flow, propeller wheels and Kaplan wheels, which have adjustable blade angles, are used. It is also introduced that cross-flow wheels are mainly used in small hydroelectric plants under 1,000 kW. To get the same output, if the head is low, you need a design that passes a large amount of water.
5. Pumped Storage: How a Generator Can Also Work as a Motor
Types of hydroelectric power plants include run-of-river, regulating reservoir, dam, and pumped storage. Dam types are large capacity (1 million kW class) and are said to be for times when electricity use is high [3]. Pumped storage has two reservoirs: an upper reservoir and a lower reservoir. During the day, water is dropped from the upper to the lower reservoir to generate power. At night, water is pumped from the lower to the upper reservoir [5]. It is explained as playing the role of a battery that stores electricity in the "form of water."
What is interesting is that the generator works as a motor when pumping water. At night, it receives electricity and moves as a motor; during the day, it is turned by the water wheel to generate power. It is also said that once generation starts, it reaches maximum output in a few minutes [5]. However, some electricity is lost when pumping water. How much returns after the round trip changes depending on the equipment and conditions.
7. An Experiment: Does Drop Height Change the Force of Water?
Does the feeling on your hand change depending on the height from which you drop water? Try dropping water from a paper cup slowly onto your hand from 10 cm above and from arm’s length in a place where getting wet is okay. You can check for yourself with your own hands whether you feel a stronger force when it is higher.
If you want to look at the materials, there are the following three: Chubu Electric Power’s explanation of water wheels, where you can compare the shapes of the three wheels with diagrams [4]. J-Power’s explanation of pumped storage, where you can understand the mechanism of the upper and lower reservoirs [5]. The Agency for Cultural Affairs’ Japan Heritage page, where you can read the history of the Keage Power Plant [6].