How Green Is Your Microgrid

By Rachel Kaufman

Lessons learned from Superstorm Sandy and Princeton University.


Superstorm Sandy in 2012 knocked out power to more than six million people in the Northeast, but in some small parts of the region, the lights stayed on. Princeton University, especially, received kudos (and a visit from President Obama) for keeping the main campus’s 180 buildings, including mission-critical research equipment, running after the storm, with a downtime of only a minute or so.

Students, freed from having to worry about heat or light, began mobilizing to help the surrounding city, where families were still in the dark, traffic lights failed, and most of the city remained devastated for days.

The school also provided hot meals for 150 first responders and invited locals to warm up, charge their phones, and use the school’s wi-fi.

The university did it with its microgrid, a hundred-year-old idea that is regaining popularity. More cities, institutions, and developers are turning to microgrids not just as a way to save money, but to become more resilient in the face of changing climate and to be greener.


Princeton’s microgrid supplies not only electricity but heating and cooling through a cogeneration plant (shown below right). The campus also employs solar arrays to capture energy. Diagram and photos: Princeton University

A microgrid is a group of buildings connected to locally generated power and to the main energy grid. Some of the advantages to a microgrid are obvious: If you can generate your own electricity locally, then it doesn’t matter if a tree falls on the power line five miles away. Some are less intuitive but just as important: Princeton’s microgrid, which supplies not just electricity but heating and cooling through its cogeneration system, helped cut its CO2 emissions to 106,764 metric tons in 2012, down from 131,377 five years prior, despite adding more than a half million square feet of building space in the same period.

Princeton’s cogeneration plant uses a gas-powered jet engine to spin a turbine to create power. But two thirds of the fuel going into the engine, instead of producing thrust, is simply wasted as excess heat. By capturing that heat and using it to warm buildings (or power air conditioning compressors) through the school’s district energy system, the school increases its efficiency to 80 percent.

Of course, Princeton’s district heating system is over a hundred years old—so the school has been generating its own steam for a century. Switching to a cogeneration model, then, was an easier sell. And from there, turning the whole enterprise into a true microgrid—one that can disconnect from the grid if necessary—was only an incremental cost more. “It costs more to install [district energy systems],” says Ted Borer, energy plant manager at Princeton, “but the lifecycle cost is much lower because the energy savings pay you back and more. If you’re looking to solve a problem, these things don’t have a payback in your election cycle.”

But it has had a payback in savings, resiliency, and good public relations. “Mayors are competing with the next town to attract that blue chip employer, like a Google or a pharma company,” says Rob Thornton, president and CEO of IDEA, the International District Energy Association. “They’re not looking for the lowest price power, they need reliable power. They want cleaner energy. They want it to be resilient. That all points to district energy [and] microgrids. So after Sandy—we’ve been around 105 years. We’re now getting calls from mayors, planners, city leaders, saying, ‘I want what Princeton has.’” The plant has built up a reputation, as well.

“When I saw the news that a 100-year-storm was about to slam Princeton while I visited, I was immediately grateful that I would be on campus,” a former student wrote to Borer a few days after the storm. “I was pretty confident that it would be one of the most reliable places for power in the whole region.”

Because Princeton has a near-constant need for heat or chilled water, its cogeneration plant runs all the time. What changes is the amount of power it buys from the local utility (PSEG) and the amount of energy it gets from a 5MW photovoltaic array. During the day, when rates and demand are both high, its power comes from the gas turbine and the sun, and it buys a small amount from PSEG. At night, when power is inexpensive, it lowers the turbine’s output and buys cheap power. “And typically, in New Jersey, I’m buying a whole lot of nuclear power and not a lot of fossil fuels [at that time of night],” Borer says.

Microgrids can be more efficient than tapping into the macrogrid, even if they don’t accompany a district heating and cooling system, because of “line loss,” which causes anywhere from 3 to 7 percent of energy vanishing when transmitted over long distances. But, said IDEA’s Thornton, the real green really is in the synergy between district energy and electrical generation. Any part of a city that’s being redeveloped all at once, rather than piecemeal, is a candidate for installing a district energy system: Boston’s 1,000-acre “Innovation District” is installing a district energy system. In Vancouver, the Olympic Village that was built for the 2010 games included one that reduces the neighborhood’s emissions by half.

Other prime candidates for district energy (and by extension a microgrid): sports arenas, says Thornton. In Phoenix, Arizona, Chase Field has 8,000 tons of air-conditioning capacity “to serve that arena for 82 home games a year,” he says. “The other 283 days, that capacity is idle, which is a waste of capital.” So instead, it makes use of a district chilled water plant that serves 30 buildings in downtown Phoenix when there are no games on.

However, despite the green qualifications of both microgrids and district energy, the reason they’re getting attention now is for their resiliency. Superstorm Sandy was a wakeup call for many cities. Climate change means that extreme storms, floods, tornadoes, and blizzards are becoming more commonplace. And it’s only getting worse. Miami’s building codes require multifamily buildings to have generator capabilities, and have done so since Hurricane Andrew in 1992; how long before other major cities, maybe those that never thought they were vulnerable to storms, require the same—or a more stringent microgrid requirement?

Resiliency is important even without the threat of major storms. Sandy was the most high-profile time Princeton’s microgrid stayed on when the power went off elsewhere, but Borer says that the local utility, PSEG, has a dozen very brief outages a year, during which Princeton’s generator seamlessly takes over. (The school also improved reliability by adding a second set of wires from the utility to its substations.) “It could be 20 seconds up to 10 minutes,” Borer says. “But if you’re running some really fancy experiment, or even if you’re stuck in an elevator, 10 minutes matters.”