Renewable Integrated Microgrids

It is essential to ensure that the available renewable sources are utilized in the best possible manner. In order to maintain uninterrupted power supply, the authors propose that a pump hydro storage unit is used to manage the intermittencies associated with renewable energy sources…

Microgrids integrated with renewable energy sources are a promising step towards the accomplishment of goal of power to all. It is essential to ensure that the available renewable sources are utilized in the best possible manner. In order to maintain uninterrupted power supply, the authors propose that a pump hydro storage unit is used to manage the intermittencies associated with renewable energy sources.

The microgrid concept refers to a single controllable system designed to meet special requirements such as enhancing local reliability, supporting local voltages and reducing feeder losses. The accomplishment of the mission to provide power to one and all, makes microgrid an inevitable option. The mode of operation of a microgrid can broadly be categorized as grid connected and island mode. The former mode allows the microgrid to either cater to its entire load all by itself or export/import power to/from the main grid, whereas the latter supports the isolated grid operation without any power sharing with the main grid. As per Ministry of New and Renewable Energy (MNRE), India has a huge potential of electricity generation through renewables. It is thus realized that RES integration in microgrids could be a promising solution to tackle ever increasing energy demand. The nineties decade saw many power network companies changing their way of operation from vertically integrated units to open market systems. The major driving force behind deregulation was to encourage competition amongst utilities and marketers to reduce energy prices.

Out of the many energy pricing methods available, spot pricing or locational marginal pricing (LMP) is one of the most popular one . In LMP, determination at different buses with optimization of social benefit has been carried out where social benefit refers to the difference between the benefit of energy to society i.e. society’s willingness to pay for its demand and the cost of energy. Renewable energy based μg is aimed at minimizing lifecycle cost and highlights the uncertainties involved in storage devices in μg

Energy storage with newer battery technologies are descriptively outlined in with several types of storage technologies described. Despite all the merits, the battery storage suffers an obvious drawback of being bulky. Therefore, authors propose pump hydro storage, which is not only robust, but also cost effective as compared to battery storage. Keeping in view the reliability of supply, the authors encourages bidirectional power sharing between the microgrid and the main grid carried out in accordance with the locational marginal price (LMP) fixed by the main grid whereas the intermittencies in RES are managed by pump hydro storage unit installed within the microgrid.

Figure 1: Renewable Microgrid
(Source: Tatung Smart Energy-Micro Grid Solution, Taiwan)

Microgrid

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.( U.S. Department of Energy Microgrid Exchange Group).

Microgrids are electricity distribution systems containing loads and distributed energy resources, (such as distributed generators, storage devices, or controllable loads) that can be operated in a controlled, coordinated way either while connected to the main power network or while is landed (CIGRÉ C6.22 Working Group, Microgrid Evolution Roadmap).

Figure 2: Microgrid Architecture

TYPICAL MICROGRID REQUIREMENTS:

  1. Grid Connected Capabilities

– Optimization of Economic Operation

– Support Integration of Renewables

– Support for DER Market Participation

  1. Islanding Capabilities

– Emergency Islanding Support

– Managing Critical/Non critical Loads to Available Generation

– Island Operations with High Penetrations of Renewables

– Optimized Island Operation for Longevity

  1. Secure Operations

– Cyber Secure Communications Network

– Distributed and Resilient Architecture

A microgrid connects to the grid at a point of common coupling that maintains voltage at the same level as the main grid unless there is some sort of problem on the grid or other reason to disconnect. A switch can separate the microgrid from the main grid automatically or manually, and it then functions as an island. A microgrid not only provides backup for the grid in case of emergencies, but can also be used to cut costs, or connect to a local resource that is too small or unreliable for traditional grid use. A microgrid allows communities to be more energy independent and, in some cases, more environmentally friendly.

A microgrid could comprise a variety of generating sources, including both renewable and non-renewable sources. The dire need to cut down pollutant gas emissions has led to the development of distributed renewable energy generating sources. RES, such as photovoltaic or wind power generators are popularly being integrated in a microgrid to curtail greenhouse gas emissions. The performance of a renewable microgrid system is analyzed in or various situations considering both grid-connected and grid-isolated mode. The key issues suffered by any renewable mirogrid are due to the intermittencies in RES availability.

A broad layout showing the components of a renewable microgrid is outlined in Fig. 1 where varied, generating sources, including wind, solar PV are incorporated. The energy storage system smoothes out the irregularities in power supply due to renewable energy sources. Microgrid could either be connected to the main grid or work in isolated mode.

  1. MICROGRID ARCHITECTURE

A typical architecture of µg is shown in Fig. 2 comprising of dispatchable, non-dispatchable generating sources.

In order to ensure a systematic bilateral power exchange, Microgrid Operator can export or import power to/from the System Operator (controller of main grid) after assessing the excess or deficiency of power. Generating sources include both dispatchable and non-dispatchable units. Dispatchable sources have a controlled output, i.e. can be switched on/off as and when required, whereas the output from a  non-dispatchable unit is utilized as and when available, i.e. it is not controlled, renewable sources which includes solar, wind etc. are either completely put to use or are stored. This varied generation is the beauty of microgrid concept, i.e. hybrid generation is possible in the microgrid system setup. As can be seen in Fig.2, a cluster of consumers can be fed through this setup. Microgrid operator and system operator play a crucial role in the power exchange process. Microgrid operator controls the optimal operation of the microgrid, i.e. it assures the reliability of supply during all intervals of demand. The system operator is the controller of the main grid and coordinates the bilateral power exchange between the main grid and the microgrid.

Figure 3: Pump hydro storage system
(Source: Our world of energy)
  1. PUMP HYDRO STORAGE SYSTEM

Pumped hydroelectric energy storage (PHES), is a type of storage used by power systems for load balancing. It stores energy in the form of gravitational potential energy of water, which is pumped from a lower elevation reservoir to a higher elevation. The surplus off-peak electric power is used to run the pumps. During periods of high power demand, the stored water is released through turbines. Although the losses of the pumping process make the power plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of high demand.

A pumped hydro storage plant operates with an upper and a lower reservoir. In the generating mode, the water flows from upper to lower reservoir whereas during pumping mode, i.e. a period of low demand,  the generated power is used to pump water from the lower to the upper reservoir as can be seen in Fig.3. Though some losses are reported in this kind of storage system (approx. 15-30%), this however is still better than a battery storage system. Figure 3 illustrates the process of electricity generation and demand by such plants.

Pumped-storage hydroelectricity allows energy from intermittent sources (such as solar, wind) and other renewables, or excess electricity from continuous base-load sources (such as coal or nuclear) to be saved for periods of higher demand. The reservoirs used with pumped storage are quite small when compared to conventional hydroelectric dams of similar power capacity, and generating periods are often less than half a day.

Pumped storage is the largest-capacity form of grid energy storage available.The round-trip energy efficiency of PSH varies between 70%–80%. The main disadvantage of PSH is the specialist nature of the site required. It needs both geographical height and water availability. Thus the suitable sites are either in hilly or mountainous regions and potentially in the areas of outstanding natural beauty. Therefore, there are many social and ecological issues to overcome.

The idea behind a renewable microgrid is to accomplish the goal of power to all in an environment friendly manner. The microgrid self sustains its demand unless there is a shortage of power in which case, power is bought from the main grid. Likewise, power is sold when surplus amount is available with the microgrid.


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