What are the leading geothermal power generation operator companies in Japan? Which companies are actively involved in developing and operating geothermal power plants? What are some key players like Kyushu Electric Power, Mitsubishi Gas Chemical, J-POWER, Idemitsu Kosan, and INPEX and how do they contribute to geothermal energy? How do joint ventures and partnerships influence geothermal projects in Japan? What challenges and opportunities exist for geothermal power operators in Japan’s renewable energy sector?