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Design Considerations for a Reliable Microgrid

Microgrid reliability

  

Reliable electric service is growing in importance to our lives and to the economy, but threats to reliable electricity are also proliferating.

The threats seem to be coming from an ever-growing list of sources: wildfires, extreme cold, record floods, hurricanes, extreme storms, and cyberattacks. Fortunately, innovation and technology are keeping pace with the threats. Increasingly, businesses, utilities, government agencies, hospitals, as well as universities and research organizations are installing microgrids to ensure that their electric power will continue to flow even if the surrounding grid goes down.

Green Mountain Power in Vermont recently announced plans to break ground this spring on a microgrid in Panton that will combine 4.9-MW of solar panels with a 1-MW/4-MWh battery storage system. The microgrid project is part of Green Mountain Power’s plan to create resiliency zones in towns that are affected by outages caused by severe weather.

On the other side of the country, Kaiser Permanente is building a microgrid at its hospital in Ontario that includes 2.2 MW of solar panels, a 1-MW fuel cell, and a 9-MWh battery.

One of the aims of the California hospital microgrid, as with the Vermont project, is to demonstrate that a microgrid can provide reliability without the need for fossil fuel generation.

In Chicago, Commonwealth Edison is building a microgrid in the city’s Bronzeville neighborhood that will include 5.5 MW of gas-fired generation, as well as 50 kW of solar panels and a 500-kW battery system. It is designed to provide low emission power if grid power suffers a disruption.

These projects represent just a small sample of the wide variety of microgrids being deployed today. And while they vary in many ways, including design, purpose and even size, they all share a common characteristic. Microgrids are designed to be self-sustaining with the ability to isolate or “island” from the surrounding grid during an emergency and return to synchronized grid operation after the emergency has passed. How any individual microgrid achieves that goal varies widely, however.

Microgrids come in a wide variety of configurations that allow them to be tailored to meet specific customer needs. One of the key defining characteristics of a microgrid’s design is the generation source it uses.

Fueling Reliability

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.

Batteries Included

In applications where emissions free generation is a primary concern, renewable energy microgrids fill the bill. Usually, those microgrids would be powered by solar panels or by wind turbines, or both, but if reliability is a top concern, they will invariably include battery storage so that power can be delivered seamlessly when the sun goes behind the clouds or the wind stops blowing.

Often microgrids powered by fossil fuel generators also include battery storage. In addition to adding redundancy, being able to store energy allows a microgrid to operate much more flexibly.

When energy storage is combined with a conventional generation source – diesel or gas-fired – a microgrid is able to arbitrage high-cost energy from the grid. If gas prices spike, a microgrid could switch to battery power until prices come more in line with the microgrid’s desired operating parameters. Energy storage is thus able to provide price arbitrage potential and cost savings that can be used to offset a microgrid’s capital and operating costs.

In addition to simple microgrid configurations such as a microgrid powered by a diesel generator or a microgrid composed of solar panels and battery storage, advanced microgrids now often combine elements of all three technologies. It is not uncommon to see microgrids with a secure form of generation, such as diesel, gas turbine or fuel cell generation, that also feature solar panels and battery storage. One of the hallmarks of a microgrid are the intelligent controls that allow the system to switch seamlessly between different forms of generation and to store and discharge energy to take advantage of optimal conditions.

A microgrid with solar panels, energy storage and a backup diesel generator could generate solar power, use excess solar energy to charge its battery array, discharge energy from the batteries as the sun fades, and turn to diesel power overnight or during emergencies, all under the direction of automated controls that could be optimized for cost savings, reliability and sustainability.

That is just one potential microgrid configuration. State-of-the-art technologies and a growing number of applications that can serve as examples make it possible for a microgrid to be scaled and designed to best meet individual customer needs.

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