Role Of Energy Storage In A RE Powered Microgrid Solution

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Opinion piece by DIBIN CHANDRAN – Senior Engineer, Power & Renewables

Microgrids, Renewable energy (RE) and energy storage technologies (EST) are highly promising and frequently discussed topics in the energy community for the last decade. Incensed cybersecurity threats and frequent natural disasters that pose a risk to the electric distribution system have made RE based microgrid solutions a desirable infrastructure for reliable electricity distribution.

Microgrids allow electricity users to safely and reliably disconnect from the utility grid connection and independently serve on-site electric loads. Mass deployment of distributed renewable energy (RE) into the grid enables local generation and utilization of electricity. This disconnected state is commonly referred to as “islanding” because it’s effectively a small powered system that serves its requirements without transferring power in or out of the island. 

The microgrid connects to the power grid through the point of common coupling (PCC). Given the increased microgrid installations, distribution systems pose significant changes in characteristics compared with the present distribution system. Therefore, suitable control strategies must be adopted to manage these differences and improve overall efficiency. The following figure shows the typical microgrid structure, where PV panels provide energy and EST balances the demand for and supply of energy.

Typical microgrid structure

Many vital considerations exist for the energy storage system in microgrids. Efficient management of EST, power electronic interfaces, charging and discharging, conversion mechanism of power, reliability, and protection from dangers are the significant issues for the development of the energy storage system in microgrid applications. The following figure describes the impact of an energy storage system on a power system network.

Load demand profile with Energy storage system (Source : ‘‘An overview of energy storage and its importance in Indian renewable energy sector: Part II—Energy storage applications, benefits and market potential,’’ J. Energy Storage,  Oct. 2017.)

Since power generation from the RE generators is fluctuating,  it won’t be easy to depend on the RE generation (e.g. Solar PV) completely during the shift from the utility grid to the local generation during islanding operation. If no backup generation exists or low power generation cases, the microgrid would only be sustained with EST until the storage capacity is exhausted.

Typically aggregated and distributed energy storage are the two basic configurations of EST for microgrid applications, as shown in the following figure.

Typical ESS configurations (Source: ‘‘Advances and trends of energy storage technology in microgrid,’’ Int. J. Elect. Power Energy Syst., 2013)

The amount of power flow from distributed energy resources  to the PCC bus remains constant for the aggregated system. Moreover, the total capacity of this ESS can be applied to assuage power flow fluctuations. If the capacity of an energy storage system increases, the cost also increases. Manufacturing and controlling large-scale energy storage are complex. Thus, small-scale and distributed energy storage systems are utilized to attain reliable and adequate power regulation. Energy storage devices in distributed storage configurations are directly connected to specific distributive sources with numerous interfaces. However, controlling power flow is the main challenge faced by the distributed system. Following are some of the applications of energy storage systems in power system:

Applications of energy storage systems in power system (Source: ‘Energy storage systems in modern grids—Matrix of technologies and applications,’ J. Energy Storage, May 2016)

Modern storage systems are unique in that they are very fast responding resources that can generate and absorb power and, in some cases, regulate real and reactive power quality in an electric distribution system. These capabilities allow storage to serve various roles within a microgrid for instances where customers need uninterrupted islanding, have no on-site generation, or need to supplement the on-site generation that exists in their distribution system.

The effectiveness of islanded operation depends on coordination and high-speed control action – on the scale of milliseconds. The high power and fast response capabilities of battery storage systems (e.g. lithium-ion) will provide effectively instant power to the microgrid for a limited duration of time to bridge an outage period. 

Considering the characteristic of distributed RE, energy storage technologies are an alternative solution for the potential utilization of renewable energy in microgrid applications. Continuous research and optimizations are performed in the energy industry to develop ESTs and their utilizations in microgrids to manage the decent power balance by storing energy during off-peak hours with reduced cost. Therefore, perfection in EST modelling with optimization characteristics is the key feature of next-generation EST.

By DIBIN CHANDRAN – Senior Engineer, Power & Renewables

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