Microgrids are used to efficiently distribute locally produced power to local consumers of power, as well as energy storage providers. Interest in microgrids is increasing as households and local organizations increasingly install their own power generators, such as photovoltaics and wind turbines. This makes a great deal of economic sense for such communities as generation only makes up a fraction of the cost of power supplied by conventional grids. Most of the cost to the consumer is actually transmission, distribution, grid services and the markups charged by the various systems operators. For example, solar power and onshore wind energy currently have a levelized cost of energy (LCOE) of around $50/MWh, but residential users in the U.S. pay $133/MWh. As battery electric vehicles become increasingly popular, residential users can use these to provide energy storage within the microgrid, further enhancing its ability to match local supply with local consumption.
Grid Connections, Islanding and Autonomous Microgrids
Most applications of microgrids still connect to the conventional grid, allowing power to be bought in or sold out of the microgrid as required. If a major disruption cuts off the conventional grid, they can still function to some extent, in a mode known as islanding. Additional storage within a microgrid can increase this type of resilience. Some microgrids, designed for remote locations, have no connection to a wider grid; these are normally referred to as isolated, stand-alone or autonomous microgrids. However, even for very remote locations, some connection to a larger grid is usually beneficial, such as with the microgrids on Scotland’s Orkney Islands.
Early Adopters of Microgrid Technology
A recent report in IEEE Spectrum describes how a microgrid of 27 households in Basalt Vista, Colo. has reduced its utility bills by 85 percent. This has in part been achieved by using smart devices that maximize energy efficiency. Each home is equipped with an 8kW solar panel mounted on its roof, along with a lithium iron phosphate battery. All space heating and cooling, water heating and appliances are also electrical.
Power produced within the microgrid is consumed within the microgrid, with the economic advantages described above. The community is also able to function in islanding mode, even relying entirely on battery backup for up to two days. This is an important capability in a location where there is a risk of wildfires cutting off supply from the conventional grid. In many ways, the Basalt Vista demonstration is very similar to the Nice Microgrid, which has been operating in Southern France for several years.
Autonomous Energy Grids
What sets the Basalt Vista microgrid apart is that it has been designed following the National Renewable Energy Laboratory’s “Autonomous Energy Grids” (AEG). The “autonomous” in AEG doesn’t signify a microgrid that operates in isolation from the wider-scale electrical grid, as the term “autonomous microgrid” would do. Instead, an AEG performs load leveling autonomously, with large numbers of smart devices, variable renewable sources and energy storage being controlled intelligently.
“The future grid will be much more distributed and too complex to control with today’s techniques and technologies. We need a path to get there—to reach the potential of all these new technologies integrating into the power system,” stated Benjamin Kroposki, director of NREL’s Power Systems Engineering Center.
An AEG includes sensors and distributed controllers that work in an integrated way to match supply and demand in real time. This means controlling both consuming devices and generators, in contrast to today’s system, which controls the supply to suit demand. Controlling the consuming devices in this way is sometimes referred to as demand side response (DSR). Another key innovation of AEG is that it combines aspects of both optimization and control theory in an integrated way. Optimization typically finds solutions assuming nominal conditions, while control algorithms stabilize systems in response to real-world feedback. The AEG algorithms perform optimization in real time, using concepts from control theory to ensure power quality.
The ultimate vision is that entire national grids will be made up of multiple layers of AEG operating within each other. At the lowest level, these could be considered microgrids. These individual microgrids would be connected together at the power distribution scale using the AEG principles and the distribution grids would themselves be connected to a transmission scale AEG.
“It all started with NODES [Network Optimized Distributed Energy Systems] … NODES covers just one cell—one bounded community. Then [Benjamin Kroposki] had this idea of having cells that communicate with each other to form a hierarchical system that could cover the entire grid. That’s how it went to the multicell perspective,” said Andrey Bernstein, AEG technical lead.
DC-Microgrids
Another approach to microgrids is the DC-microgrid. The use of alternating current (AC) power is largely a legacy of older technology. Most modern devices require low voltage direct current (DC) power and are therefore fitted with transformers and rectifiers, or some other form of power converter. Examples of devices that require DC power include LED lighting, virtually all electronic devices, electric motors in domestic appliances, and battery chargers. Electrical resistance devices, such as incandescent bulbs, cookers and convection heaters, can run equally well on DC or AC power.
Fitting large numbers of devices with their own power converters adds significantly to the cost of the devices while also reducing their energy efficiency. Traditionally, generators were designed to produce AC power because this meant that transformers could be used to step up the voltage for efficient transmission over large distances, and then step it down again for local consumption. However, modern power semiconductors enable DC-to-DC conversion at very high efficiencies. High-voltage direct current (HVDC) is now the preferred method of transmitting very large quantities of electrical power over large distances—so-called power superhighways, which can form intercontinental connections. The much lower voltage DC grids required for local distribution are, therefore, easily implemented and produce significant energy and device cost savings. The issue, however, is compatibility. Since most devices are already produced with power converters to accept a main’s AC input, special devices are required to operate within a DC-microgrid.
The Future of Microgrids
The extent to which microgrids are adopted will depend on how the energy transition takes place. Some commentators predict a community lead transition in which households embrace local renewable energy generators, such as solar panels, as well as energy conservation and storage. In such a scenario, microgrids will likely play a major role in making these consumer investments more cost-effective and energy-efficient. However, there are also more centralized scenarios in which most renewable power is produced in large offshore windfarms and nuclear power stations, with little reliance on energy efficiency to achieve carbon neutrality and hydrogen enabling patterns of consumption more similar to today’s fossil-fuel-based energy system. In a more centralized scenario, the electrical grid is likely to look much more like the grid of today.