Design Considerations for a Reliable Microgrid

A microgrid built with a strong, and even sole, focus on reliability, one that is going to serve primarily as a source of backup power, is likely to feature a fossil fuel generator.

Generators that rely on fossil fuels, such as natural gas, gasoline or diesel fuel, generally have high reliability because they can operate regardless of the weather conditions that could reduce or halt the output of renewable energy sources. The power output of solar panels, for instance, can be reduced or even eliminated by cloud cover. A wind turbine’s electrical output is, of course, dependent on the wind blowing. Those factors are one of the key considerations in designing a microgrid that will primarily serve as a source of backup power.

The generator itself could be a diesel engine or it could be fired by natural gas or by gasoline. Among the considerations that factor into choosing between generator types are fuel availability, local emissions restrictions, available space, and even noise restrictions.

In some locations, there is limited pipeline infrastructure for the delivery of natural gas. That limit could tip the decision on microgrid design in favor of a reciprocating engine, such as a diesel engine or an internal combustion engine.

Diesel engines are very reliable and long lived, and diesel fuel can be stored on site, making the engine ready for service immediately during an emergency. One consideration in choosing a diesel engine, however, is the potential availability of fuel during an emergency should on-site storage be depleted. In that case, it is useful to have a reliable source of fuel delivery.

If natural gas is available, it is often seen as a good choice for reliability. Natural gas pipelines have a very low rate of failure or disruption, particularly relative to electric service because the overwhelming majority of gas pipelines are buried, so they are not subject to the same causes of disruption that can affect electric lines, such as wind or ice storms.

Fossil fuels are not the only choice for microgrids built for reliability. Fuel cells are also able to provide around-the-clock reliability, as long as they have a reliable fuel source. Fuel cells can be powered by either natural gas or they can be fueled directly with hydrogen. Hydrogen can be stored on site or even produced on site. Recently several utilities and companies have been investing in “green” hydrogen production, that is, hydrogen that is produced by electrolyzer equipment that uses clean sources of electricity, such as solar power or hydroelectric power.

Fuel cells are also a good choice for microgrids in settings where generation emissions are a concern because fuel cells have little or even no harmful emissions. Fuel cells are also quiet, so they are often used in dense urban settings or in hospital microgrids where noise from operations is a concern.

If local emissions restrictions are a concern, it is worth noting that although they do release emissions, gas turbines have roughly half the emission levels of a coal plant, so they can potentially reduce overall emissions compared with electric power taken from the grid. And modern diesel engine technology, those that comply with the Environmental Protection Agency’s latest Tier-4 emissions criteria, are also less polluting than many older forms of fossil fuel generation.