NetApp cogeneration project cools corporate data center for free - but ROI a moving target

NetApp built a cogeneration system that provides both power and cooling to its data center, but volatile energy prices have greatly changed the economics of the system since it was first installed four years ago. The lesson: combine heat and power (or CHP) systems may be green, but the payback can be unpredictable.

For most people, the term "cogeneration" conjures up images of power plants that capture the low-grade waste heat from the generator's "tailpipe" and use it as a source of space heating or water heating for buildings. Network Appliance uses its cogeneration plant to generate cool air for its data center.

The system, profiled in this case study published today, uses a technology called an adsorption chiller to do that.

Thermodynamically, cogeneration is very efficient, since waste heat is reused, and the cost of generating power is partially offset by electricity savings that come from turning off the chillers and using "free" cooling from the absorption units. While some cogeneration systems use biomass, methane from landfills, or even wood chips to fuel generators, most commercial buildings use systems that rely on gas turbines. NetApp's generator uses a gas-powered internal combusion engine.

The problem lies in the fact that you still have to generate the power, and the cost of that power varies with the cost of the fuel used. The other variable in the cost savings equation is the cost of utility power, which varies over time as well as by season and time of day. Both numbers can vary significantly during the lifespan of a cogeneration system, changing the ROI.

In NetApp's case, its cost of fuel tripled from the original estimates, greatly extending the payback period for the project. Furthermore, variations in utility rates also restricted the times when the system would be more economical to run. Today NetApp only turns on the CHP system during peak hours in the summer when electricity rates are at their highest.

Even if NetApp could run the system economically at other times, however, the savings wouldn't be as great. That's because NetApp already uses an air-side economizer, an air conditioning system design that takes advantage of outside air temperatures on cool days to help cool the data center. (Few data centers use such systems today, even though they're economical and proven technology, according to PG&E.) The systems are cheaper and easier to run than a CHP system and, depending on the climate, can provide some or all of the cooling power needed for a data center. As it stands now, the times when NetApp runs the CHP system - during the hottest days or summer - are the times when the air-side economizers won't work. While PG&E isn't opposed to cogeneration, a spokesperson's recommendation is to investigate using economizers first.

Copyright © 2007 IDG Communications, Inc.

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