Smart Grids and Micro-Grids. Umashankar Subramaniam
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energy gap can be moved by using ESS (Energy Storage Systems) to meet the peak demand and level the load curve. The other way for levelling the load curve is to instruct the consumers on energy efficiency, demand response programs, cost of the microgrid operation and energy utilization and management using ESS. This approach favours to utilities in generation expenses, line loss reduction, and volt support [1].
2. Power Quality Support: The ESS can support the microgrid or main grid for power quality improvements such as low-voltage ride through (LVRT) and voltage regulation. The capability of generators to stay operative under voltage sag conditions without getting disconnected from the grid is known as low-voltage ride-through (LVRT). The implementation of ESS can substantially improve the low-voltage ride-through of the generator. Moreover, The ESS can also be used to improve the voltage regulation because of its rapid voltage support.
3. Smart Grid: Smart grids are turning into a noticeable option for efficient energy transmission and distribution from multiple power supplies. ESS in a smart grid ensures the reliability of energy supplied to the consumers. It also includes upcoming prosumers such as smart houses and electric vehicles as major contributors of grids to avoid network over-loads by regulating and controlling the inconsistent power [2].
4. Electric Vehicles: Plugin and hybrid Electric vehicles are becoming prevalent and attracting recent research. Recent research in battery management system (BMS), drives and battery are promoting electric vehicles (EV) at a fast pace and with drop in cost of ESS. The ESS of the EV (Electric Vehicles) can also feedback the power to the grid under appropriate planning and scheduling. More than one type of batteries can also be coupled together in the form of hybrid ESS to deliver the required energy [3].
5. Smart Homes: ESS is an integral part of energy sustainable buildings and smart homes. An associated smart energy management system can efficiently manage energy storage and power utilization. The accurate data of consumption and subsequent emissions can also be provided. Smart homes efficiently use the energy with lesser emission.
6. Uninterruptible Power Supply (UPS): UPS is the significant application of ESSs in microgrids, particularly for the islanded microgrids. Renewable energy sources, such as solar photovoltaic and wind turbines, may unexpectedly stop producing power because of clouds, nightfall or lack of wind. However, turning on the backup generators may take few seconds [4]. To avoid the chances of outages in the microgrid during this period the ESS in the microgrid rides through the power shortage.
2.3 Available ESS (Energy Storage Systems) Technologies
The energy storage system has been used in power systems for a very long time. The energy storage technology also enables high intervention and incorporation of variable renewable energy sources for grid application. It has a critical role in improving the working abilities of the grid. ESS can be categorised as mechanical, electro-chemical, electrical and thermal systems based on its characteristics, as shown in Table 2.1.
Table 2.1 Classification of ESS [5].
| Mechanical | • Pumped Hydro Storage (PHS)• Compressed Air Energy Storage (CAES)• Flywheel Energy Storage Systems (FESS |
| Electro-Chemical | • Hydrogen Storage• Battery Energy Storage Systems (BESS) |
| Electrical | • Super-conducting Magnet Energy Storage (SMES)• Electric Double Layer Capacitors or Super-capacitors (SCES) |
| Thermal | • Molten Salt Storage• Adiabatic CAES |
2.3.1 Type of ESS (Energy Storage Systems)
This subsection discussed different types of ESS (Energy Storage Systems) technology available that can be utilized in the microgrid. The categorised ESS technology along with its corresponding advantages and disadvantages is enumerated [6, 7].
1. Mechanical storage: Various technologies that falls under this category are:a. Pumped Hydro Storage (PHS): Pumped Hydro Storage has been extensively implemented for a very long period and is considered as a developed technology of energy storage system for power grid applications. PHS projects store water in the reservoir/pond placed at higher altitudes in times of energy availability with aim to save the energy and then electrical energy can be produced by transforming the penitential energy to electrical energy during the release of stored water through turbines. Significant benefits of pumped hydro system are:Huge power and energy ratingProlonged lifespanHigh efficiencyLess discharge lossesThe geographical dependence, large site area and long gestation periods are the major obstructions in this type of energy storage system. Several modifications have also been suggested and established for PHS.b. Compressed Air Energy Storage (CAES): CAES is a strategy to store energy during low energy demand in by means of compressed air in underground air chambers and utilise the same to meet higher demand. There are two methods of CAES to store the air: A diabatic and Diabatic. Adiabatic storage has much higher efficiency because it preserves the heat from compression and re-uses this when the air is expanded to generate the power. Adiabatic storage continues to store the energy produced by compression and returns in to the air as it is expanded to generate power. Whereas in diabetic storage, the heat generated during compression is released to the atmosphere through heat intercoolers. The advantages of CAES are:Huge power and energy ratingLong lifespanMinor discharge losses.Similar to PHS, prime complication in CAES is geographical dependence, extensive investments and commonly appropriate for only grid-level applications.c. Flywheel energy storage System (FESS): Like PHS, a prime complication in FESS can supply instantaneous support of active power for the microgrid. It contains a disk with a certain amount of mass charged by accelerating it to very high speed and storing energy by keeping it rotated at high speed when there is excess electricity. The flywheel is decelerated to generate power during the demand period. The disk of flywheel ESS is placed in the rotor’s perpendicular position to avoid the effect of gravity. Flywheel energy storage loses some energy due to friction; hence, minimizing friction can enhance its efficiency. The friction can be reduced by making a vacuum environment for the flywheel to spin in, ensuring no air friction, or having a permanent magnet or electromagnetic bearing to make the rotor float. It has several qualities, such as high-power density, high conversion efficiency, short response time, low maintenance costs, no greenhouse emission, no toxic by-product, and long-life span. During the last several decades, FESU has been used as an uninterruptible power supply in short-duration power inconsistency. Flywheels are also used in transportation and space applications for power delivery and, significantly, to stabilize or drive satellites (gyroscopic effect) [8]. The drawbacks of flywheels are a small capacity and high-power loss, ranging from 3% to 20% per hour. CAES is geographically dependent, extensive investments and commonly appropriate for only grid-level applications.
2. Electrical storage: Several electrical energy storage technologies have been developed and a few of them are briefly explained as follows:a. Superconducting Magnetic Energy Storage (SMES): under this technology, a large quantity of energy from the grid is stored within the magnetic field of a superconducting coil and discharged within a fraction of a cycle to reinstate a abrupt loss or dip in power. The energy is stored in the magnetic field established by direct current flow in a cryogenically refrigerated superconducting coil below its critical temperature. The stored energy can be supplied to the electrical network by discharging the coil using an inverter/ rectifier to transform DC power back to AC power. The inverter/rectifier incurs about 2–3% energy loss in each transformation. The advantages of the SC are:Instantaneous power transferabilitySuitable for small-scale storage systems.Several modifications of the strategic injection of surges of power in the grid through SMES have been proposed.b. Supercapacitor: A supercapacitor (SC), also called an ultra-capacitor, is developed by placing an electrolyte solution between two conductors.