The Okinawa Yanbaru Seawater Pumped Storage Power Station (沖縄やんばる海水揚水発電所, Okinawa Yanbaru Kaisui Yōsui Hatsudensho) was an experimental hydroelectric power station located in Kunigami, Okinawa, Japan and operated by the Electric Power Development Company. It was the world’s first pumped-storage facility to use seawater for storing energy. Its maximum output was 30 MW. Construction of the plant started in 1987 and was completed in 1999  at a cost of ¥3.2 billion. It was dismantled in 2016.
|Opening date||May 16, 1999|
|Operator(s)||Electric Power Development Company|
|Total capacity||564,000 m3 (457 acre⋅ft)|
|Installed capacity||30 MW (40,000 hp)|
The power station was a pure pumped-storage facility, using the Pacific Ocean as its lower reservoir, with an effective drop of 136 m and maximum flow of 26 m³/s. Its pipelines and pump turbine were installed underground. Its maximum output was approximately 2.1% of the maximum power demand in the Okinawa Island recorded on August 3, 2009.
The upper reservoir, artificially excavated, was approximately 600 metres (2,000 ft) away from the shoreline and approximately 150 metres (490 ft) above sea level. It had an octagonal planar shape with a maximum width of 252 metres (827 ft). Its maximum depth was 25 metres (82 ft) and its effective storage capacity was 564,000 cubic metres (457 acre⋅ft). The entire inner surface of the reservoir was covered with an impermeable liner to prevent seawater from leaking and damaging the surrounding vegetation.
Fiber-reinforced plastic tubes were adopted for the penstock and the tailrace instead of steel tubes in order to avoid seawater corrosion and adhesion of barnacles. The pump turbine was partially made of stainless steel resistant to seawater.
The power station was a pilot plant funded by the Agency for Natural Resources and Energy and constructed by the Electric Power Development Company. A five-year verification operation was conducted beginning on May 16, 1999. The Japan Society of Civil Engineers presented the company an Outstanding Civil Engineering Achievement Award on May 26, 2000 for its construction of the plant.
The operator could not put the power station into practical use because the demand for electric power in Okinawa had not grown as predicted, and the plant was not profitable as a business. The power plant was dismantled in July 2016.
- 沖縄やんばる海水揚水発電所の実証試験運転開始について [Verification operation started at Okinawa Yanbaru Seawater Pumped Storage Power Station]. News Release (in Japanese). Electric Power Development Company. 17 March 1999.
- Shibuya, Yōko; Ishimura, Yōsuke (March 2010). 学生が行く今月の土木日本一: 世界初の海水揚水発電所: 沖縄やんばる海水揚水発電所 [This month’s civil engineering number one in Japan visited by students: World’s first seawater pumped-storage power station: Okinawa Yanbaru Seawater Pumped Storage Power Station] (PDF). JSCE Magazine (in Japanese). 95 (3): 34–35. ISSN 0021-468X.
- "Japanese pumped storage embraces the ocean waves - International Water Power". www.waterpowermagazine.com. Retrieved 2019-10-06.
- 最大電力の記録を更新 (今年2回目) [Maximum power demand again hits record high] (PDF). Press Release (in Japanese). Okinawa Electric Power Company. 4 August 2009.
- Fujihara, Tetsuo; Imano, Haruo; Oshima, Katsuhiro (October 1998). "Development of pump turbine for seawater pumped-storage power plant" (PDF). Hitachi Review. 47 (5): 199–202. ISSN 0018-277X.
- Ōmoto, Kazuhiro (October 1994). 現場を訪ねて: 沖縄海水揚水建設所: 自然にやさしく [Visiting construction site: Okinawa Yanbaru Seawater Pumped Storage Power Station: Nature-friendly]. Soil Mechanics and Foundation Engineering (in Japanese). 42 (10): 47–50. ISSN 0041-3798.
- 「沖縄やんばる海水揚水発電所の建設」の土木学会技術賞の受賞について [JSCE Outstanding Civil Engineering Achievement Award for construction of Okinawa Yanbaru Seawater Pumped Storage Power Station]. News Release (in Japanese). Electric Power Development Company. 26 May 2000.
- "Experimental power plant in Kunigami dismantled". Japanupdate. 29 July 2016.
- Okinawa Yanbaru Seawater Pumped Storage Power Station (Official site, in Japanese)
- "Seawater Pumped-Storage Power Plant". Japan Commission on Large Dams. Archived from the original on 2002-07-08.
- "沖縄やんばる海水揚水発電所". Panoramio. 2008-12-31.
- "The Okinawa Yanbaru Seawater Pumped Storage Power Plant". flickr. 2008-11-11.