Google’s new $700 million data centers in Taiwan will make ice at night, when electricity is significantly cheaper, and use it to cool the buildings during the day, reports Rich Miller at Data Center Knowledge. It’s called thermal storage, and it’s basically a battery, but for air conditioning.

Energy storage is a big deal. It allows companies to save money by banking up energy when it’s cheaper — and it also spreads out the pressure on the electrical grid, because large companies can shift their extensive energy needs to lower-demand times.

A typical energy storage system, notes Katie Fehrenbacher at GigaOm, is literally a giant ice-maker that sits next to a conventional a/c system. The unit freezes 450 gallons of water overnight, creating a “battery” of cold that can be used in place of a conventional a/c compressor for at least 6 hours on the following day.

Google is installing these systems in order to save itself money on electricity in Taiwan, where it’s quite expensive. But given the existence of companies like EnerNoc, which pay companies to free up the electricity grid when others need the juice, it’s not hard to imagine a future in which units like Google’s pay for themselves.

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