Efficient Computing

Step 2: Efficient Data Centers

Reducing the environmental footprint of our data centers starts with reducing their electricity consumption. Several years ago we surveyed existing approaches and traditional data center designs and realized they could not meet our goals. A typical data center facility spends almost half of its energy consumption on the systems powering and cooling the computers inside, and not on the computers themselves. In other words, they use twice as much power than they would in a perfectly efficient scenario. In a 2007 report the U.S. Environmental Protection Agency (EPA) confirmed this sad state and estimated the average energy overhead of current data centers at 96%. Back in 2004 we decided to move much of our infrastructure into new data centers built for efficiency and cost, and our engineers set out to find ways to eliminate wasted power. Our goal was simply to build the world's most efficient data centers. Here's how we did it:

A data center is a fancy name for a warehouse-like building that contains thousands of servers. Because these servers consume a lot of electricity, there is a great deal of electrical equipment required to bring the power to the servers, including equipment to provide power in the case of a utility power failure. Typically 10-20% of the incoming power is lost in this machinery. We first focused on streamlining this equipment with a goal of eliminating most of the waste.

But we still had to address all of the power spent on cooling. All the electricity that goes into a data center building ultimately turns into heat, and thus there are fans, pumps, and air conditioning equipment to remove all that heat. In many data centers, cooling alone is responsible for a 30-70% overhead in energy usage. Fortunately, there is a simple way to remove all of that heat - let water evaporate.

Evaporation is a powerful tool. It maintains our body temperature even when outside temperatures exceed our normal 98.6°F. How? Energy is required to change water into vapor; this energy is heat that is removed from the immediate surroundings, causing a cooling effect. Our data centers make extensive use of this evaporative process with cooling towers. Below is a simple representation of a cooling tower. Here's how it works:

The goal of using evaporative cooling is to minimize the time chillers need to run. A chiller works basically the same way as an air-conditioner, except that it could be operating in liquid or air. When chillers are required to run, they can consume many times more power than the rest of the cooling system combined. With cooling towers, our data centers spend most of their time running in a mode called "free cooling". This means the chillers are off. Free cooling isn't technically "free," but it is really inexpensive and really efficient.

Optimizing every part of the system took a lot of work, but it really paid off. We reduced the energy-weighted average overhead across all Google-built data centers to 19% versus the average of 96% reported by the EPA. In other words, compared to standard data centers we've reduced the overhead by more than fourfold. To our knowledge, no other large-scale production data center has ever operated as efficiently. In fact, one of our data centers is running at an even lower overhead of only 15%, a sixfold improvement in efficiency. (For those interested, read the technical details of our measurements.) Operating data centers at this level of efficiency has a significant impact on both our operating expense and our environmental footprint.

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