When the Power Goes Out, Microgrids Keep Electricity Flowing

When the Power Goes Out, Microgrids Keep Electricity Flowing

To make sure the lights stay on, some institutions are creating their own power grids.

Microgrids, small-scale power networks that can operate independently of the main grid, are generating new interest for two main reasons: the spread of alternative power sources like solar, which make local networks more feasible, and the power-snapping impact of severe storms like Hurricane Sandy and the derecho windstorms earlier in 2012, which both knocked out electricity to millions.

"The need for resiliency has led to the microgrid movement," says Scott Clavenna, chief executive of Greentech Media, a market-research firm in Boston. The state of Connecticut this year has begun rolling out a number of microgrids for emergency use, and the U.S. Department of Energy is making $7 million available for microgrid design.

Microgrids offer distributed power generation, or power that's created outside of the utility grid. In some ways, they update the idea of district energy, which predates the grid and originated with buildings clustered together and sharing a source of heat. The savings derived from centrally heating a campus don't translate as neatly for electricity, but microgrids reduce the risk of losing power in a storm and can use multiple power sourcesthough usually only two, unlike the hypothetical example in the accompanying graphic.
Power Sources

District energy: Its heating source can be used to produce electricity as a byproduct, in what is called cogeneration. For example, a turbine might serve mainly to produce heat, but the spinning of its blades can also create electricity. District cooling networks, which draw on various forms of cold-water storage, are often built alongside district heat networks. Princeton University does all three. It heats and cools 180 buildings from three central plants, instead of having to put equipment in all its buildings.

District heating and power typically uses underground pipes and wires, which are far less likely to be affected by storms than utility lines on poles. Campuses that use district energy can turn themselves into "islands" that function off the grid in times of emergency.

Microturbines: Microgrids often use smaller-scale versions of the turbines used in utility power plants. Usually powered by natural gas, these are more expensive but also more efficient than diesel-powered generators. Besides being less costly, natural gas is also generally transported in pipelines, so supply is likely to be reliable even in bad storms.

Solar: As prices of solar arrays and storage have dropped, solar power has become an increasingly popular choice for microgrids. Konterra, a planned sustainable community being built between Baltimore and Washington, has a 402-kilowatt solar array. Battery storage is essential, because the sun doesn't always shine and power grids need consistent flows of energy. Solar is also a direct-current source and generally has to be converted into alternating current, requiring specialised control systems.

Fuel cells: They convert chemical energy to electricity and can be designed to avoid greenhouse-gas emission. The major energy source for fuel cells is hydrogen, though they can also run on natural gas. Fuel cells are more efficient than other sources of energy. But they generally require the storage or transport of hydrogen, and they are currently more expensive than other forms of energy production. Fuel cells are also direct-current systems and usually need to be converted to alternating current.

How Microgrids Work

In normal times, microgrids usually complement the main power grid, connecting to it via a point of common coupling, or PCC. The PCC maintains even voltage levels between the main grid and the microgrid to prevent problems, including the possibility of explosions, caused by uneven loads. A circuit breaker sits between the PCC and the microgrid's main control system, which is used to separate the two if the main grid fails.

In a crisis

A storm or other disaster causes a power outage on the main grid. A circuit breaker is used to separate the microgrid from the main grid, making the microgrid an island of power.
The microgrid's control systems balances power needs with capacity. It may tap into alternate and backup systems. Storage batteries ensure a smooth flow of power from these systems to the microgrid.
Circuit breakers between the control system and various buildings on the microgrid are automatically or manually flipped, to take nonessential buildings offline.
Depending on fuel supplies, a microgrid can operate indefinitely, delivering power and in some cases heating or cooling.
When the main grid is restored, the microgrid operator prepares to reconnect to the grid, shutting down excess capacity and preparing to restore power to buildings that have been offline.

Source: The Wall Street Journal

SMART GRID Bulletin March 2017


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