Titles are made to be taken away, I say (only once:right now), and whichever building was once the largest solar energy office is no longer because a bigger dog is in town now. Well, in China.
Specifically, located in Dezhou, the largest solar energy office building is 75,000 square meter. It's so big, in fact, that it's not just an office building.
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Interior Cooling Canals
Solar Power Towers
Solar Jigsaw Buildings