Miyakojima Island is located about 300km to the southwest of Okinawa’s main island. Its size, 204.5km2, is slightly smaller than Osaka City. Its population is approximately 55,000, most of whom live in the Hirara urban district. Sugar cane fields spread across the flat landscape, and the island is crowded throughout the year with tourists who come to view the beautiful coastline and swim and dive in the sea endowed with coral reefs.
4MW solar panels combined with 4MW secondary cells
The island’s power demand peaks at about 50MW. Along with the core power supply, which is diesel engines with a total output of 59MW, gas turbine power generators with a total output of 15MW and wind power generators with a total output of 4.2MW are supporting the power demand of the island.
Adding to these power supplies, a mega-solar (large-scale solar) power plant with 4MW output started operation in October 2010 (Fig. 1). It was constructed using the "FY2009 subsidy for the project to verify stand-alone systems for new energy demonstration projects on remote islands" granted by the Ministry of Economy, Trade and Industry (METI).
The project was aimed at understanding what impacts the widespread introduction of solar power generation, whose output fluctuates widely, would have on the power transmission and distribution network (grid) and verifying the control function that benefits grid stabilization using secondary batteries. For this purpose, secondary batteries with a total output of 4.1MW were also installed.
Now the verification project has entered its final phase, having helped stabilize the power quality by smoothing the mega-solar power plant’s output using secondary batteries and generating power in accordance with the schedule.
The plant is located a 30-minute drive to the southeast of Miyako Airport. As you drive along with the steep coast on your right, you will see two wind turbines on your left and solar panels neatly arrayed along the road for about 1km from the foot of the wind turbines. That is the site of the verification project.
Since the place is frequently hit by typhoons, the solar panels have been tilted as low as 5 degrees (Fig. 2). The mounting system is reinforced in consideration of a reference wind velocity of 46m/s and a maximum instantaneous wind velocity of 73m/s. In addition, high-salt-resistant solar panels and mounting systems were deployed while the electric facilities including PV inverters and secondary batteries were all housed in the buildings (Fig. 3).
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