Bench Talk

Alcatraz is home to the largest microgrid (powered by a 305kW solar-array charging battery bank) in the US. (Source: National Park Service)

Microgrids hold the key to smart grid technology. These localized, compact distribution networks promise to answer a triple conundrum facing utility engineers: that is, how to build reliable, resilient, (and above all) clean electricity systems. The vision reveals a patchwork of interconnected microgrids coordinated by smart automation. Such microgrids would allow engineers to build an electricity distribution network, which reduces reliance on giant, centralized (and polluting) power plants by tapping into a nationwide distribution of small energy-harvesting renewable energy (RE) sources. These RE sources would not only provide power to the local area but also smooth power variations in other parts of the grid by exporting excess capacity.

Unfortunately, the hype and the reality are somewhat different. Today in the US, thousands of uncoordinated microgrids that use polluting fossil-fuel power sources (such as diesel generators, which only kick in when the main grid is down) make up the microgrid majority. There is a dearth of RE, and it seems that the microgrid vision has stalled. But there’s hope; modern communication technology, advanced distribution automation equipment, and increasingly efficient wind and photovoltaic (PV) solar sources are coming together to reboot the clean power dream.

Bringing Resilience to Electricity Generation

The National Renewable Energy Laboratory (NREL), which is a US-based energy research facility, says that a microgrid enables local power generation assets (including traditional generators, RE, and electricity storage) to service a locality even when the larger grid experiences interruptions or to service remote areas when there is no connection to the larger grid. Apart from remote examples, a microgrid normally operates connected to and synchronously with the “macrogrid,” but it can also disconnect to function autonomously and in isolation as an “electrical island.”

Resilience is assured, because failure of one microgrid has no impact on neighboring networks. Each can be rapidly isolated from faulty networks while continuing to supply power (from localized generation) to its own sector. Contrast this with today’s centralized model: Failure of a giant power station impacts a wide area and often causes other plants on the network to trip out (due to macrogrid interconnectedness), thereby plunging whole cities and even regions into darkness.

Moreover, localized networks powered by RE promise to have a dramatic impact on carbon emissions by reducing reliance on fossil-fuel generation.

With multiple semi-autonomous microgrids operating in parallel, the resulting smart grid would transform vulnerable networks—where a single point of failure can cause a cascade of shutdowns affecting millions of people—into ones that are more efficient, secure, and robust by offering redundancy and eliminating reliance on a chain with many potential weak links.

“Nanogrids” not Microgrids

There are around 1900 electricity networks described as microgrids in the US alone, according to analyst Wood Mackenzie. That sounds promising, but many of these grids fall well short of addressing the triple conundrum facing utility engineers. One problem is that these installations include many microgrids that are actually closer to “nanogrids” (systems that are often little more than diesel generators, like those in a hospital basement serving as a standby when the city’s power goes out or like those in industrial premises taking advantage of a gas-fired CHP (combined heat and power) unit in an attempt to contain energy costs). Such designs typically fail to meet the NREL definition of a microgrid since they are incapable of feeding excess capacity back into the national system.

Worse yet, because many of those 1900 microgrids rely on fossil-fuel generation, they do nothing to reduce carbon emissions. The vision of a wind farm coupled with battery storage, powering a genteel suburb, is far from reality in the US and limited to a handful of examples in environmentally-attuned Europe.

Little will change until there’s a fundamental shift in the way national utility systems are designed. Today, these systems typically comprise a massive network built around a limited number of very large power plants; this network would need to transform into a distributed mesh, served by many (comparatively) tiny electricity generators and a good proportion of RE, to meet the vision. Currently, particularly in the US, momentum towards this transformation is slow—but there is cause for optimism.

Realizing the Microgrid Dream

Stone Edge Farm in Sonoma, California, demonstrates how a microgrid should be built. First combining conventional power generation from a gas turbine with solar power, the farm then stores this energy in eight different types of battery, which ultimately powers the site’s irrigation system. The farm’s disparate power sources are managed by a “distributed optimizer” (a custom device that manages the power feeds, or in and out power switches, from different sources). 

Stone Edge Farm’s microgrid received an unplanned test during the late 2017 Californian wildfires. The farm was evacuated, as flames came within 8 km of the property. Through remote access, the facility was switched to “island mode” for ten days of autonomous operation, until it was safe for people to return.

Alcatraz Island is home to another microgrid—claimed to be the largest in the US—powered by a 305kW solar array. The solar panels connect to a battery bank and power inverters that help supply energy to the island, instead of Alcatraz solely relying on diesel generators. The microgrid has reduced the island’s fuel consumption by 45 percent since its 2012 installation.

The Sonoma and Alcatraz microgrids both demonstrate what’s possible when leveraging modern RE technology and distribution automation equipment. By aggregating together distributed, small-scale resources that are linked by IEDs (Intelligent Electronic Devices), a microgrid can connect safely to the grid while maintaining power quality—and that’s no mean feat.

Such microgrids make things much easier for utility operators. Communicating with hundreds of discrete generators is daunting, but microgrids can gather small resources together into manageable units: thereby, supplying energy to a wider smart grid when power is in high demand, storing energy when it’s not needed for later release, smoothing out frequency and voltage fluctuations on the wider grid, and operating remotely for the benefit of the local community when things go wrong elsewhere.