Inverter-Driven Microgrids

Hot Springs, N.C., is a mountain community along the Appalachian Trail, north of Asheville. This small resort town is known in the region as a scenic riverside getaway. In March 2023, Hot Springs made news for employing a green microgrid. The town of about 600 people may help answer the question: How do we get utility-scale, green energy to rural communities across the United States?

The United States relies on a variety of energy sources to heat homes, depending on utility infrastructure and natural resources. An interactive Washington Post map breaks down the share of energy sources used by each community to heat its homes (https://wapo.st/3KrPqxV).

When you zoom all the way out, you notice that New England and Alaska still primarily use fuel oil. The Southeast and Northwest states consume most of the electric-resistance heating. Big cities and the Rocky Mountain states have a big share of natural gas distribution. Rural areas use a mix of propane and wood.

Not too many surprises here, but if you zoom in, you can find rural areas that seem to include an “anything-they-can-get” mix of energy sources from town to town. This brings us back to Hot Springs.

Islanded Microgrids

What do the communities surrounding Hot Springs currently use for home heat? To the South and West, 44 percent of homes use electricity for space heating. To the North, 29 percent of people heat their homes with wood. Another 30-plus minutes east, 40 percent of homes use fuel oil, making this one of the rare spots in the South where fuel oil is the most popular fuel.

The common thread in the extended neighborhood of Hot Springs is that people must get creative. The town might be a low priority for a new natural gas pipeline because of the mountain terrain and small population. Yet, it needed something to improve its power supply as its main, 10-mile-long power line was prone to outages. What options do residents have?

Microgrids, as defined by the U.S. Department of Energy (http://bit.ly/3ZTzeeE), are: “localized grids that can disconnect from the traditional grid to operate autonomously. Because they are able to operate while the main grid is down, microgrids can strengthen grid resilience and help mitigate grid disturbances as well as function as a grid resource for faster system response and recovery.”

If a tree falls onto a power line supplying a large network of homes, grid operators traditionally fire up a coal or natural gas plant to start the recovery process. However, Hot Springs is now doing the same start-up task with solar and batteries.

Instead of carving a new power line through the woods and over the Appalachians, Hot Springs opted to build a new solar photovoltaic microgrid array near town.

“杜克能源——镇上的效用——认为中方lding a second feeder line to fix the [grid consistency] problem but determined that a microgrid made more sense because, unlike a new line, the microgrid would not disrupt miles of scenic and environmentally sensitive terrain,” notes a T&D World article (http://bit.ly/3o9qBiI). “The North Carolina Utilities Commission agreed and approved the microgrid in 2019.”

The PV microgrid in Hot Springs is considered an islanded microgrid, meaning it does not need a kick-start from another power plant. The microgrid can support itself as if on an island. To date, this specific type of inverter-driven microgrid was never installed to serve a community of this scale. A few solar panels and batteries are typical residential-grade options. However, Duke Energy was able to build an inverter-based system to help power the area.

The specific technology used in Hot Springs is called an inverter-driven/inverter-based microgrid. This technology comes in handy because some fossil-fuel power plants have slow-moving wheels.

“Historically, electrical power has been predominantly generated by burning a fuel and creating steam, which then spins a turbine generator, which creates electricity,” the DOE’s Solar Energy Technologies Office explains (bit.ly/3MAZqb4). “The motion of these generators produces AC power as the device rotates, which also sets the frequency or the number of times the sine wave repeats.”

However, when you add too many devices to this grid, you are removing energy faster than it is coming out of the power plant, which slows AC frequency. “Because the turbines are massive spinning objects, they resist changes in the frequency just as all objects resist changes in their motion, a property known as inertia,” the DOE says.

Inverter-based generators can produce energy at any AC frequency. They are not spinning a single, big turbine that resists change. Grid operators can take the DC energy harvested from the sun through the PV panels, store it in batteries, and convert it to the AC voltage needed to smooth out or restart the power delivery to the area.

Renewables for Rural Communities

These PV microgrids can be helpful for rural communities in a variety of ways. They reduce the need to carve more power lines to rural communities to support grid maintenance. By reducing the number of long distribution lines, there are also reduced electrical transmission losses from the power plant to the homes.

As is the goal in some areas, there is the opportunity to minimize the carbon footprint of a city or region by building renewable infrastructure instead of more fossil-fuel plants. The ratepayers are less affected by fuel price fluctuations. Sunlight is free, whereas fossil fuels have a different market value each day and face supply disruptions across the globe.

Outside of Hot Springs, Duke plans to build this type of infrastructure in South Carolina, Florida, Indiana, Ohio and Kentucky.

“对我们会有其他的机会to do this in some of our coastal territories, some of our other mountainous territories and even potentially in downtown areas where they don’t want the noise,” explains Jason Handley, general manager of Duke’s Distributed Energy Group. “To be quite honest, generators make noise.”

Are we the big spinning turbines resisting change? Most of the community-level solutions proposed to deal with brownouts and blackouts involve building massive power plants and fixing thousands of miles of outdated grids to supply states or regions.

What if we reduce the number of new lines that are carved across the country to keep the massive fossil-fuel power plant turbines happy, as if the turbines are the customer? These islanded, inverter-driven stations could be a key piece of the grid resiliency and sustainability factors we are trying to balance to keep the lights on.