Cloud computing requires massive data centers, and the rows upon rows of computers burn lots of electricity, generate massive amounts of heat--and then require yet more electricity to power cooling systems. (In fact, more electricity goes into the cooling than the actual computing.) Obviously this is a problem; according to The Times, "Data centers such as those run by Google already use around 1 percent of the world's energy, and their demand for power is rising fast with the trend to outsource computing."
Helsinki, Finland may have the answer. Next month the city will see the launch of a massive data center buried in a WWII-era bomb shelter under one of their biggest tourist attractions, Uspenski Cathedral. Clever technological planning means the heat from the data center will be siphoned off and delivered to Helsinki residences, reducing the cooling bill, and attendant electricity cost, by some $561,000 per year. Read all about it here.