German energy group Uniper SE (<a href="https://www.boerse-frankfurt.de/equity/uniper-se-na-o-n-1" target="_blank">ETR:UN0</a>) said on Thursday that it will invest around EUR 250 million (USD 268.2m) to re-commission its Happurg pumped-storage hydropower plant (HPP) in Bavaria and bring it back online in 2028.