Microgrids vs. the Macrogrid: The Applications of Microgrids in Today’s Power Systems

Learn more about the argument for microgrids.

An even in well-served areas, microgrids make it easier to incorporate renewable energy and to help their operators to save money. Here's a look at why microgrids may be important to the future of grid power.

 

What Is a Microgrid?

As reported by the Lawrence-Berkeley Lab, the U.S. Department of Energy Microgrid Exchange Group characterizes microgrids in this manner:

"A microgrid is a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode."

Or, putting it differently, a microgrid is a system of energy sources, energy consumers, and energy storage. This system can operate completely independently from the traditional centralized power grid (macrogrid) in “island” mode. Alternatively, in the grid-connected mode, it can be a source of power to the macrogrid or it can draw power from the macrogrid as conditions demand.

 

Microgrid Applications

The classical case is a microgrid whose main energy source is renewable power.

When the sun shines and the wind blows, the microgrid may generate more power than its users need—and even more than it can store—so it transfers power to the macrogrid. On cloudy days when the wind is calm, it can’t generate sufficient power and may have even run out of stored energy, so the macro grid is called on to transfer power to our microgrid.

 

A Microgrid that connects to the Macrogrid when conditions mandate. Image used courtesy of Berkeley Lab.

 

Berkeley Labs and the US Department of Energy (DOE) actively promote microgrids. They maintain a webpage entitled Examples of Microgrids that lists and provides links state-of-the-art examples of projects accomplished or still under construction.

There are many motivations for the establishment of microgrids, and three of the most common are:

• Providing power if the macrogrid fails
• Power for remote areas
• Saving money

When the Macrogrid Fails: Puerto Rico

Puerto Rico after Hurricane Maria is a famous example of when a macrogrid has failed. Maria left the island residents with no electricity at all for extended periods of time. This left many American citizens in this US commonwealth in situations where they couldn't rely on power, including keeping insulin refrigerated and useable.

At present, the Puerto Rican utility authority, PREPA, is restructuring the island's power grid—likely by establishing multiple renewable-energy microgrids. The plan involves dividing the island into eight inter-connectable, regional minigrids, each capable of functioning on its own if needed. The minigrids, themselves, will be divided into microgrids, also able to go it alone.

An integrated resource plan (IRP), submitted by Siemens, places heavy emphasis on solar energy, liquefied natural gas (LNG), and energy storage capability. An important hallmark is that each microgrid component will be able to function on its own no matter what happens to the utility as a whole.

Additionally, because Puerto Rico is an island, a ship-based LNG terminal on the island is highly desirable. Many different scenarios were presented, but all emphasized decentralization and the capability to segregate the system into eight minigrids, subdivided into microgrids.

Power for Remote Areas: Huatacondo

The University of Chile has developed a microgrid for the town of Huatacondo. Too far off the beaten track for any established macrogrid, the town’s only previous source of energy was a 150 kW diesel generator, which only operated 10 hours each day.

 

A wind turbine in Huatacondo, Chile. Image from Berkeley Lab

 

The new microgrid includes a 22 kW tracking solar photovoltaic (PV) system, a 3 kW wind turbine, and a 170 kWh storage battery system.

There is also an energy management system. Part of its job is to be aware of the weather forecast to estimate the amounts of energy that the PV system and the wind turbine can contribute. It will also point the PV cells at the correct angle to absorb the most sunlight. It will also strive to minimize use of the diesel to save on fuel costs as much as possible, while making sure basic energy needs are met.

Saving Money: Combined Heat and Power (CHP)

The generation of electricity often involves the generation of heat. Safely venting this heat can be expensive. It is also wasteful, as this heat can be used to provide hot water and to heat building, and a well-designed microgrid will do just that.

The formal term of such an arrangement is combined heat and power (CHP). It can be taken a step further in a process called trigeneration, which adds a device called an absorption chiller, AKA an absorption refrigerator. On hot days, the excess heat from a system is routed to the absorption refrigerator and used to provide cooling as an air conditioner equivalent.

The absolute beauty of this concept is that the excess heat inexorably generated in the process of providing electricity was previously a problem to be dealt with. Now, it is an asset to be profitably exploited.

Going Forward

Microgrids are growing in popularity so quickly that they were seen as a threat to utilities that could siphon away their business and revenue. Southern Company is an American gas and electric utility holding company that is taking a different approach—they are buying into the trend.

Southern Company has formed a strategic alliance with Advanced Microgrid Solutions to jointly develop and deploy behind-the-meter distributed energy resources across the United States.

 

Microgrids can be used for large utilities. Image used courtesy of the University of Cordoba

 

The parallels between smart buildings are microgrids are obvious. Both take advantage of solar energy, and neither let any heat energy to go to waste. The goal may or may not be independence from the macrogrid but rather always relying on the macrogrid as little as possible.

In almost all cases, whether the microgrid is grid-connected or island-mode, a major goal and accomplishment is to use as little fossil fuel as possible while still serving the energy needs of a connected community.