Microgrids are small networks that rely on nearby solar, wind or other renewable sources to provide reliable green power. They can also be powered by fossil fuels if circumstances require it. Microgrids can either operate independently of the main grid, known as “island mode,” or they can be connected to the main grid at the same voltage, bringing in power from the main grid at peak times and sending excess power back to the main grid at low times. When the power grid is switched over, its availability is not affected. If battery storage is added, the microgrid can be isolated and operate independently in “island mode” when the broader utility network fails. Microgrids have gained momentum in recent years, due in part to the potential to increase the efficiency of critical facilities. These facilities include wastewater treatment plants, drinking water systems, and health care facilities.

“Microgrid” is a concept compared to the traditional “big grid”. It uses advanced control technology and power electronics devices to connect distributed energy with the load and energy storage devices to form a miniature complete grid. This “miniature” power grid is a complete power system from power generation, transmission and transformation to the end user. It can not only form a fully functional local performance source network, but also “run alone” in a way that does not interfere with the transmission and distribution system. It can also be connected to the municipal power grid through a common connection point: when the power supply function of the micro grid is insufficient, the large power grid can supplement the shortage, and when the power generation is large, the excess power can be fed back to the large power grid. When necessary, the two modes can be switched between, which fully maintains the safe and stable operation of microgrid and large grid.

Composition of island mode

Distributed renewable energy technologies

Whether in offshore islands, remote areas, or in densely populated buildings, communities, or factories, people will increasingly see such distributed energy applications. For example, a microgrid of distributed photovoltaics, wind power and diesel generators could meet all the energy needs of fishermen on remote islands. For example, CCHP and distributed renewable energy technologies are integrated into urban community microgrid systems to provide residents and businesses with locally produced and cost-effective electricity, hot water and cooling services. Much of this is due to microgrid technology, which is no longer limited to the centralized power supply model of the municipal grid. It also allows remote islands, rural areas and special requirements to be built outside the scope of the municipal grid. Distributed energy supply systems can be set up close to the user side according to their individual needs.

As an integrator of a variety of distributed energy sources, “microgrid” technology has broad development space and application scenarios. In a complete microgrid system, distributed energy is the main body of power supply, and different types of energy can cooperate and complement each other. On the power side, the system monitors and controls the power load. In terms of the control system, the microgrid needs internal scheduling and external communication to achieve a high degree of autonomy. Cold, heat and electric storage make microgrids secure and flexible. Depending on whether the microgrid is connected to the large grid or not, the microgrid can be divided into off-grid type and grid-connected type. The application scenarios of off-grid microgrid include solving electricity problems in islands and remote areas, while grid-connected microgrid adds a guarantee for the safety of energy supply for users, and networked operation can also improve the economy of the entire system.

Diesel generators in a microgrid

Great distances separate our remote communities from their neighbors – and the energy systems many of us take for granted when we flip a switch. Because remote communities are not connected to either natural gas infrastructure or the grid, they must generate their own energy by burning diesel (derived from oil) to heat their homes and buildings, and to power their small microgrids. Like any power system, reliable operation of a microgrid requires the exact match of supply and demand for electricity, every minute of every day. To achieve this balance, microgrids rely on only a small amount of energy — usually a single diesel engine. By contrast, larger grids are more resilient and better able to manage sudden surges in demand or unexpected power losses.

Maintaining 24-hour system reliability is a top priority for all power system operators. Continuous (or non-intermittent) energy sources such as diesel and hydropower provide energy around the clock. In contrast, intermittent energy sources such as wind and solar provide variable output, complicating grid operations for microgrids in many ways. Still, there are many successful examples of renewable energy being integrated into remote communities to create hybrid microgrids – and the list continues to grow as projects prove they reduce operating costs, carbon pollution and reliance on imported diesel fuel.

Using diesel generators as a backup power source can be a good option for microgrids that are backed by renewable energy. Whether you are looking to improve the reliability of your microgrid or improve the level of service, diesel generators can be an effective solution. Diesel engines have a proven technology and can offer a high level of reliability. They can also provide a backup source for power in microgrids. In addition to providing backup power, diesel engines can be joined together to create a load balancing system that can synchronize power.

Renewable energy permeates microgrids

The amount of energy from renewable sources in a hybrid microgrid is known as the penetration level. Because the energy resources available at any one time are extremely diverse, large grid systems with many different energy sources are able to handle large amounts of intermittent renewable energy (80-100 percent). By contrast, the current technical limit for intermittent renewable energy penetration in remote microgrids without storage is about 20 to 30 percent.

Hybrid microgrids have only a small amount of energy available. Because intermittent renewable energy is not produced consistently, diesel generators must increase or decrease their power output depending on the amount of renewable energy available at any given time. (Remember: Supply must always match demand exactly.) Older diesel generators tend to operate very inefficiently at lower speeds, so frequent up-and-down ramps shorten their service life and undercut the cost savings of operating microgrids using renewable energy.

The Solution

Fortunately, newer technologies are available to mitigate these problems. One promising technology to consider when replacing older diesel equipment is the variable speed generator (VSG). VSGS produce electricity efficiently at a much lower rate than conventional generators, which means they consume less diesel when supplementing with renewable energy. In addition, VSG allows for higher renewable energy penetration at maximum levels.

Another technology that complements intermittent renewable energy and is familiar to most people is battery storage. When used in conjunction with wind or solar power generation, batteries can help smooth out hourly or daily variations in energy production, keeping electricity flowing when the sun isn’t shining or the wind isn’t blowing. However, batteries cannot store enough power to cope with longer seasonal supply changes – for example, during the long Arctic winter when there is little sunlight.

The role of more renewable energy in microgrids is also important. When energy demand fluctuates throughout the day, making matching supply and demand more challenging — as people cook, run appliances and use electric heaters. Reducing overall demand through energy efficiency and making demand more consistent throughout the day through smart grid technology can help make microgrids easier to operate.