Hydroelectric power generation

Japanese: 水力発電 - すいりょくはつでん

A method of converting the potential energy of water into kinetic energy and using that energy to generate electricity. Facilities that generate hydroelectric power are called hydroelectric power plants, and are generally named after the specific location where they are installed, and are referred to as "____ power plants." This method utilizes the difference in water level, which is achieved by either blocking river water with a dam or by building a long waterway along the main stream, to direct water from a high point through a pipeline (penstock), which drives a water turbine and generates electricity using a generator directly connected to the turbine. The history of hydroelectric power in Japan began in 1890 (Meiji 23) when electricity was generated for private use by the Ashio Mine (400 horsepower) and Shimotsuke Asa Spinning Mill (65 horsepower) in Tochigi Prefecture, and the first hydroelectric power supply for general use was in 1891, when the construction of a canal on Lake Biwa was started and the Keage Power Station (with a DC generator output of 80 kilowatts and a single-phase AC generator for 1,300 lamps = an AC generator that supplies 1,300 electric lamps with two electric wires) was built to transmit electricity for lighting and power in Kyoto City. The success of the Keage Power Station led to plans to transmit electricity over short distances in various locations. After that, the demand for electricity gradually increased, and hydroelectric power generation developed rapidly, mainly based on the three-phase AC generation method (a method of supplying electricity using three electric wires) that allows the voltage to be freely changed to facilitate long-distance transmission, and electricity generated at advantageous hydroelectric points in the mountains began to be transmitted to distant cities. A representative example of this was the Katsuragawa Electric Power Company's Komahashi Power Plant (Yamanashi), completed in 1907 (Meiji 40). At the time of completion, it had an output of 15,000 kilowatts, and the transmission voltage was boosted to 55,000 volts to transmit electricity to Tokyo, 75 kilometers away. This was followed by hydroelectric power plants developed on the Kinugawa River (Shimotaki-Tochigi), Kisogawa River (Yaotsu-Gifu), and Uji (Kyoto), and electricity was transmitted to Tokyo, Nagoya, and Osaka. In 1914 (Taisho 3), the abundant hydroelectric resources of Lake Inawashiro were developed, and Inawashiro Hydroelectric Power's Inawashiro No. 1 Power Plant was able to transmit 37,500 kilowatts of electricity at a transmission voltage of 110,000 volts to Tokyo, about 230 kilometers away, marking the start of full-scale long-distance transmission of hydroelectric power. In the 1920s, transmission of 154,000 volts was achieved, and the abundant hydroelectric power in the mountainous region of the Chubu region was developed one after another, allowing large-scale transmission of electricity to the urban areas of Tokyo and Osaka. Hydroelectric power development was active until World War II, but due to a shortage of materials and labor during and for several years after the war, development of hydroelectric power temporarily stagnated. In the 1950s, hydroelectric power generation was actively developed to meet the strong demand for electricity accompanying the postwar industrial recovery, and until 1961, hydroelectric power generation was the main source of power generation, with thermal power generation operating as a backup during dry periods. However, after that, the number of locations where hydroelectric power generation could be developed economically decreased, the demand for electricity increased significantly, and large-capacity, highly efficient thermal power plants and nuclear power plants were built, so the power generation style shifted to thermal power generation, with thermal and nuclear power plants providing the majority of the base demand for electricity. As a result, the development of hydroelectric power plants also shifted to large-capacity reservoir and pumped storage systems, which placed base load on new thermal and nuclear power plants and peak load on hydroelectric power plants. Especially since the period of rapid economic growth, the construction of large-capacity, high-head pumped storage power plants with individual unit capacities of 300,000 kilowatts, heads of 500 meters, and power plant capacities of 1,000,000 kilowatts has become mainstream.

Thus, hydroelectric power generation is playing an important role in peak shifting and power storage, using surplus electricity from thermal and nuclear power plants at night to pump water from the lower reservoir to the upper reservoir and generate electricity during the daytime when the load is heavy. In addition, since it is easy to control the output of hydroelectric power, it also plays a role in load frequency control (LFC), which controls the power output according to the load and monitors the frequency of the grid. As a new hydroelectric power generation technology, pumped-storage power generation uses a generator turbine rotated in reverse to create a pump, but since the rotation speed at which it is most efficient is different, variable-speed generation systems that change the rotation speed of the generator motor have been adopted since around 1990 (Heisei 2) at power plants such as Okawachi (Hyogo), Shiobara (Tochigi), and Okukiyotsu (Niigata). In addition, the world's first seawater pumped-storage power plant (output 30,000 kilowatts, head 136 meters) that uses seawater (lower reservoir) was built in northern Okinawa in 1999 and began operation. Currently, large-capacity pumped-storage hydroelectric power plants, which were built after the period of high economic growth, are combined with large-capacity thermal and nuclear power plants to improve overall economic efficiency. As of the end of fiscal 1999, Japan's hydroelectric power facilities had a capacity of 44.33 million kilowatts, accounting for 19.8% of the total power generation facilities, and in terms of generated electric power (actual generation), hydroelectric power was 89.3 billion kilowatt-hours, accounting for 9.7% of the total electric power (thermal power generation was approximately 55%, and nuclear power generation was 34%). Since the Great East Japan Earthquake in March 2011, this ratio has changed significantly due to the decline of nuclear power generation.

[Tsutomu Michigami and Ryuichi Shimada]

format

Hydroelectric power generation can be divided into four types: canal type, dam type, dam and canal type, and pumped storage type.

(1) Channel type: A method in which flowing water is taken from a single point in a river, a long, gentle waterway is constructed, and electricity is generated by utilizing the gradient of the river to obtain the difference in head.

(2) Dam type: A method of generating electricity by building a dam on a river and taking advantage of the difference in head between the dam and the downstream.

(3) Dam and waterway type: A method of generating electricity by combining the dam and waterway types to obtain power from the difference in head.

(4) Pumped-storage type: An upper reservoir is built at an elevation higher than the power plant site using an artificial dam or natural lake, and a lower reservoir is built at a lower elevation. Water from the lower reservoir is pumped into the upper reservoir by a hydroelectric turbine using surplus electricity generated by thermal power generation late at night, and then released into the lower reservoir during the daytime peak load periods to generate electricity.

Hydroelectric power generation can be classified into three types based on how it uses river flow:

(1) Run-of-river type: This type generates electricity by utilizing the natural flow of a river. Also known as the run-of-river type, this type includes the canal type. With this type, the generated power varies with changes in the flow rate.

(2) Regulating pond type: A regulating pond is provided on the river, and the flow rate during the nighttime when the load is light is stored in the pond, and electricity is generated during the daytime peak hours. Power generation is carried out on a daily cycle. The storage capacity of the regulating pond is smaller than that of a reservoir.

(3) Reservoir type: A large reservoir is built and river flow is adjusted based on a seasonal power generation cycle, storing water during the off-peak periods of spring and autumn and generating electricity during the peak periods of summer and winter.

[Tsutomu Michigami and Ryuichi Shimada]

Waterway power plant

Let's take a look at the main equipment of conduit power plants, which have been built since ancient times and are commonplace.

The intake is a small dam that blocks the main stream of the river and takes in water for power generation. This water is sent to a settling pond. To prevent the sediment contained in the water from flowing directly into the facilities after the waterway, the water is first directed to a large pond where the sediment is deposited.

The water that leaves the settling basin enters a waterway. Waterways include open channels, tunnels, and aqueducts. There are two types of tunnels: those with air in the upper part, and those whose entire cross section is filled with water. The former are called non-pressure tunnels, and the latter are called pressure tunnels.

At the joint between the waterway and the penstock, a water tank is required to store the amount of water used at the power plant for 2 to 3 minutes. This tank may be equipped with a water tank called a surge tank (pressure adjusting tank) to reduce the increase in pressure inside the penstock caused by water splashing when the water supply is suddenly stopped at the power plant. The water coming out of the tank enters the penstock installed on the inclined surface, and from its end enters the turbine through the main valve.

Water turbines used for hydroelectric power generation are roughly divided into impulse and reaction types, the former being the Pelton type used in high head areas, the latter being the Francis type for medium head areas, the mixed flow type for medium to low head areas, and the propeller type for low head areas. Water turbines can also be installed with either a vertical or horizontal axis, with most large-capacity hydroelectric power plants being vertical. In general, the output of a water turbine is proportional to the product of the head at that point and the flow rate.

Most generators that are directly connected to water turbines to generate electricity are three-phase AC synchronous generators. The frequency is 50 Hz in eastern Japan and 60 Hz in western Japan, and the generator capacity varies from several hundred kilowatts to several hundred thousand kilowatts. The generator voltage is 3,000 to 11,000 volts for small and medium capacity generators, and 13,000 to 16,000 volts for large capacity generators. The number of generators installed at a power plant is determined according to the power plant's output, but most are around 2 to 5, which are electrically connected in parallel to operate.

The three-phase AC voltage generated by the generator is converted to a high voltage of 60,000 to 500,000 volts by a transformer on the power plant premises to improve transmission efficiency, and then sent to the transmission line.

According to the load curve of demand, the power required for hydroelectric power plants is highest during the day and drops sharply at night, and cannot be covered by conduit-type power plants that simply use the natural flow of rivers. This requires balancing ponds and reservoirs, and the development of large-capacity thermal and nuclear power plants has further increased this demand, leading to the emergence of many pumped-storage power plants that overcome this.

[Ryuichi Shimada]

Dam-type power plant (dam waterway type power plant)

This is a method of generating electricity by constructing a large dam in the main stream of a river to hold back water and using the water level that occurs between the upstream and downstream of the dam. This method requires a suitable site for dam construction, and the dam is built in a place where mountains are close to both banks of the river. Not only must the topography be suitable, but it must also be easy to transport the enormous amount of materials needed to build the dam and power station. Furthermore, since the power station is built on the side of the base of the dam or inside the dam, the iron pipe that guides the water is short and a large drop in head cannot be expected. Therefore, if a larger drop is required, the dam and waterway type is used, in which a waterway is laid to create a drop like the waterway type. The water stored upstream of the dam serves as a regulating reservoir and reservoir. Most of the large-capacity hydroelectric power stations built after the 1960s that use fire as the main source of power generation and water as the secondary source of power generation are rockfill dams and arch dams. In the case of a dam type dam, there is no need for a waterway or other facilities for transporting water as there is in a canal type dam, but because it blocks the main stream, consideration must be given to securing water for irrigation and installing a fishway to allow fish to pass through the dam site.

[Tsutomu Michigami and Ryuichi Shimada]

Waterway hydroelectric power generation
©Takashi Aoki

Waterway hydroelectric power generation

Dam-type hydroelectric power generation
©Takashi Aoki

Dam-type hydroelectric power generation

Dam and waterway hydroelectric power generation
©Takashi Aoki

Dam and waterway hydroelectric power generation

Pumped storage hydroelectric power generation
©Takashi Aoki

Pumped storage hydroelectric power generation

How pumped storage hydroelectric power generation works
©Takashi Aoki

How pumped storage hydroelectric power generation works

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

水の位置エネルギーを運動エネルギーに変え、そのエネルギーを利用して発電する方式。水力発電を行う設備を水力発電所といい、一般に設備が設置されている場所の固有名称がつけられて、○○発電所と称している。この方式は、河川の水をダムでせき止めるか、または本流に沿って長い水路をつくるかして得られる水位の高低差を利用し、高所から水を管路（水圧管）で導き、水車を駆動し、水車に直結した発電機で発電する。日本の水力発電の歴史は、1890年（明治23）に栃木県の足尾鉱山（400馬力）および下野麻(しもつけあさ)紡績（65馬力）の自家用として発電したのが最初で、一般供給用は、1891年に琵琶湖(びわこ)の疎水工事をおこし、蹴上(けあげ)発電所（直流発電機出力80キロワットと1300灯用単相交流発電機＝電線2本で電灯1300個に供給する交流発電機）をつくり京都市の電灯・電力用に送電したのが最初である。蹴上発電所の成功により、各地で至近距離に送電する方式が企てられた。その後、電力需要もしだいに増加し、電圧を自由に変えて遠距離送電を容易にする三相交流発電方式（電線3本で供給する方式）を主体に水力発電が急激に発展し、山間の有利な水力地点で発生した電力を遠隔地の都市に送電するようになった。その代表的なものとして1907年（明治40）に完成したのが桂川(かつらがわ)電力の駒橋(こまはし)発電所（山梨）で、完成当時の出力は1万5000キロワット、送電電圧を5万5000ボルトに昇圧して75キロメートル離れた東京まで送電した。これに続いて鬼怒川(きぬがわ)（下滝(しもたき)―栃木）、木曽川(きそがわ)（八百津(やおつ)―岐阜）、宇治（京都）などで水力発電が開発され、東京、名古屋、大阪に送電された。1914年（大正3）に猪苗代湖(いなわしろこ)の豊富な水力資源が開発され、猪苗代水力電気の猪苗代第一発電所から出力3万7500キロワットの電気を送電電圧11万ボルトで約230キロメートル離れた東京に送電できるようになって、水力発電の本格的長距離送電が開始された。1920年代には15万4000ボルト送電が行われるようになり、中部山岳地帯の豊富な水力が続々と開発され、東京および大阪の都市部に大規模に送電されるようになった。第二次世界大戦までは水力の開発が盛んであったが、大戦中と戦後の数年間は資材と労力の不足のため、水力発電の開発は一時停滞した。1950年代に入って戦後産業の復興に伴う旺盛(おうせい)な電力需要に対処するため、水力発電の積極的な開発が進められ、1961年（昭和36）までは水力発電を主体とし、火力発電を渇水期の補給用として稼動する水主火従（水力が中心で、火力がそれに従うの意味）の発電形態が電源開発の中心であった。しかし、その後は水力発電を経済的に開発できる地点の減少、電力需要の大幅な増加に加えて、大容量の高効率火力発電所や原子力発電所が建設されたため、電力需要のベースの大部分を火力や原子力発電が分担する火主水従（火力が中心で、水力がそれに従う）の発電形態に移行した。これに伴って水力発電の開発も、新鋭の火力・原子力発電にベース負荷をもたせ、ピーク負荷を水力発電に負荷させる大容量の貯水池式、揚水式のものが多くなった。とくに高度経済成長期以降は単機容量30万キロワット級、落差500メートル級で発電所容量100万キロワット級の大容量・高落差の揚水発電所の建設が主流となってきた。

　こうして水力発電は、深夜の火力・原子力発電の余剰電力を活用し下池の水を上池に揚水させ、昼間の重負荷時に発電する、ピークシフト、一種の電力貯蔵の役割が重要になっている。また水力は出力の増減制御が容易なので負荷にあわせて発電出力を制御して、系統の周波数を監視する負荷周波数制御（LFC：Load Frequency Control）の役割もある。新しい水力発電技術として、揚水発電は発電水車をそのまま逆回転させて揚水ポンプにするが、その最高効率の回転スピードが違うことから、発電電動機の回転数を変えて運転する可変速発電システムが、世界に先駆けて1990年（平成2）ごろから大河内(おおかわち)（兵庫）、塩原（栃木）、奥清津（新潟）などの発電所に採用されている。また、海水（下部池）を利用した世界初の海水揚水発電所（出力3万キロワット、落差136メートル）が1999年沖縄北部に建設され運転を開始した。現在は、高度成長期以降に建設された大容量揚水式水力発電と、大容量の火力・原子力発電とを組み合わせ総合的な経済性を高める運用が行われている。日本の水力発電設備は、1999年度末時点で4433万キロワットで全発電設備の19.8％となっており、発電電力量（発電実績）では、水力発電は893億キロワットアワーで全電力量の9.7％であった（火力発電が約55％、原子力発電が34％）。2011年（平成23）3月の東日本大震災以降、原子力発電の後退によってこの比率は大きく変わっている。

［道上　勉・嶋田隆一］

形式

水力発電の形式は、水路式、ダム式、ダム水路式、揚水式の四つに分けられる。

(1)水路式　河川の一地点で流水を取水し、緩やかな長い水路をつくって、その河川の勾配(こうばい)を利用し落差を得て発電する方式。

(2)ダム式　河川にダムをつくり、下流との間に落差を得て発電する方式。

(3)ダム水路式　ダム式と水路式を混合して落差を得て発電する方式。

(4)揚水式　発電所地点より高い所に人工のダムや天然の湖沼を利用した上部池を、また低い場所に下部池をつくり、深夜の火力、主として原子力発電の余剰電力により下部池の水を、発電用水車をポンプ運転して上部池に揚水し、昼間のピーク負荷時に下部池に落水して発電する方式。

　また、水力発電を河川流量の使用方法の見方から分類すると、次の三つになる。

(1)流れ込み式　河川の自然の流量をそのまま利用して発電するもので、別名自流式ともよばれ、水路式がこれに該当する。この方式は流量の変化によって発電電力が変化する。

(2)調整池式　河川に調整池をもち、夜間の軽負荷時に流れ込む流量をこの池に貯水しておき、昼間のピーク時に発電する。発電の運用は1日間のサイクルで行われる。貯水池に比べて調整池の貯水容量は小さい。

(3)貯水池式　大きな貯水池をつくり、季節的な発電サイクルにより河川流量の調整を行い、春・秋のオフピーク期に貯水し、夏・冬のピーク期に発電する。

［道上　勉・嶋田隆一］

水路式発電所

古くからつくられ、身近にある水路式発電所のおもな設備を取り上げてみる。

　取水口は小さいダムで河川の本流の水をせき止め、発電のための用水として取り入れる所。この水が沈砂池に送られる。送られた水に含まれる土砂がそのまま水路以降の施設に流れ込むのを防ぐためにいったん広い池に導き、土砂を池に沈める。

　沈砂池から出た水は水路に入る。水路には、開渠(かいきょ)、トンネル、水路橋などがある。トンネルには、上部に空気が入っているものと、断面全体が水に満たされているものの2種類があり、前者を無圧トンネル、後者を圧力トンネルとよんでいる。

　水路と水圧鉄管の継ぎ目に、発電所で使用する水の2～3分間に相当する量を蓄える水槽が必要であるとされている。この水槽には、発電所で急に水を止めた場合の水の跳ね返りによる水圧鉄管内の圧力上昇を緩和する水槽を設ける場合があり、それをサージタンク（調圧タンク）とよんでいる。水槽から出た水は傾斜面に据え付けられた水圧鉄管に入り、その終端から主弁を経て水車に入る。

　水力発電として使用される水車は大別して衝動型と反動型があり、前者には高落差領域で使用されるペルトン型、後者には中落差領域のフランシス型、中・低落差領域の斜流型、低落差領域のプロペラ型がある。また、水車の据付け方法として縦軸と横軸とがあり、大容量の水力発電所は大部分が縦軸である。一般に、水車の出力はその地点の落差と流量の積に比例する。

　水車に直結し電気を発生する発電機は、ほとんどが三相交流同期発電機である。周波数は、東日本では50ヘルツ、西日本では60ヘルツ、発電機容量は数百キロワットから数十万キロワットと種々ある。また発電機電圧は中・小容量のもので3000～1万1000ボルト、大容量のもので1万3000～1万6000ボルトとなっている。一つの発電所に据え付けられる発電機の台数は発電所の出力に応じて決められるが、多くは2～5台程度で、これらは電気的に並列されて運転する。

　発電機により発電された三相交流電圧は送電効率を高めるため発電所構内の変圧器により6万～50万ボルトの高電圧にして送電線に送られる。

　水力発電所に要求される発電力は、需要の負荷曲線から昼間はもっとも大きく、深夜は激減することとなり、河川の自然流量をそのまま取り入れる水路式発電ではカバーできない。このため、調整池や貯水池が必要となり、さらに大容量の火力・原子力発電の開発により、いっそうこの要求が高まり、これを克服した揚水式が多く出現している。

［嶋田隆一］

ダム式発電所（ダム水路式発電所）

河川の本流に大きいダムを建設して水をせき止め、ダムの上・下流の間にできる水位を利用して発電する方式である。この方式では、ダム建設に適した地点が要求され、川の両岸に山が迫っている所を選んでダムを築く。また地形が適当であるばかりでなく、ダムや発電所建設の膨大な資材の搬入にも便利でなければならない。さらに発電所はダムの裾(すそ)の横側、またはダムの内部などに設けるので、水を導く鉄管は短く、大きな落差を望めない。そこで落差をさらに大きくしたい場合は、水路を引いて水路式のように落差をつくるダム水路式が採用される。ダムの上流に蓄えられる水は、そのまま調整池・貯水池の役目を果たすこととなる。1960年代以降につくられた、火主水従の発電形態の大容量の水力発電所は、大部分がロックフィルダムとアーチダムである。ダム式の場合は、水路式のように水路、その他の水の運搬に関する設備は必要ないが、本流をせき止めるので、灌漑(かんがい)用水の確保、魚がダム地点を通過できるようにする魚道(ぎょどう)の設置などの配慮が必要である。

［道上　勉・嶋田隆一］