East Pyne Hall, Princeton University. Photo by Andreas Praefcke, Wikimedia Commons
By George Harvey
In 1996, Princeton University replaced its old coal-fired heating plants, which had been around since the 1920s, moving to natural gas as a fuel source. This was not a simple change, however, as it had a set of implications that went beyond merely switching furnaces.
The new plant was a single, cogenerating (cogen) central system that supplied both heat and electricity to about 180 buildings. It was built around a turbine that can be described as very much like the jet engines that are found in aircraft.
In a cogen system, the efficiencies of such engines can get to be very high, approaching 85%. The engine turns a generator, just as most gas turbine generators would, but the heat in the exhaust gas is captured to be used for the buildings.
The generator produces up to about 15 megawatts, which is only slightly less than the university typically uses. When demand is low on campus the turbine supplies all of the university’s power. At other times, some power is drawn from the grid. Sometimes, the university even supplies power to the grid.
In time, the cogen system had two very important alterations. One was a change to make it capable of running entirely independently of outside grid power. This turned it into what is called a “microgrid,” capable of standing alone. The second was the addition of renewable energy in the form of a solar photovoltaic system.
When Hurricane Sandy struck the mid-Atlantic area, shutting down the power grid, Princeton became an island of light and warmth, because its microgrid continued to function almost flawlessly. After an initial twenty minutes without power, the university was back to nearly full function. Though there were limits to the amount of power available, this did not represent a problem. On the other hand, the university was used by all sorts of people who were dealing with the difficulties of the storm. They came by to refresh themselves, warm up, and get together.
Since that time, the microgrid at Princeton has been an inspiration to people from all over the country. They have studied it, so they could understand better how to use microgrids in their own communities. Commercial operations, schools, hospitals, and emergency shelters have been installing such systems.
In a well-known example, Green Mountain Power installed a solar-powered microgrid at the high school in Rutland, Vermont, which serves as an emergency shelter. That system includes a number of other buildings, even including a few homes. It has both an extensive solar array and a large battery backup system.
Another interesting example that came into view recently belongs to Ameren, a large electric utility based in Saint Louis. Located in Champaign, Illinois, it is the only customer-connected microgrid that operates at the high voltages used by utilities, according to Ameren. It is powered by an array of several hundred solar panels and one wind turbine. The system has the latest battery technology for backup power with redundant backup from natural gas. It supplies power to 192 commercial and residential customers. Ameren is currently testing the Champaign microgrid, checking its ability to operate without any connection to the grid. Initial tests have gone well.
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