Energy prices have recently risen due to the increased demand for energy by consumers, businesses and developing countries. Renewable energy sources (RES) and distributed generation are practical substitutes for the conventional expensive ways of producing energy. However, problems arise when providing energy over an extended period of time as RES are periodic in nature. Electricity demand may be lower while RES are producing electricity, but these sources may not be able to meet peak demand. Furthermore, there is a problem with reliability due to the fact that RES supply varies monthly, seasonally and annually due to weather conditions.

Energy storage systems (ESSs) can be deployed to enhance power system quality and overcome the limitations of RES. The flywheel is one such type of mechanical ESS that can either be connected to the grid or operated off grid. It stores energy by converting electrical energy into mechanical energy, specifically rotational kinetic energy. The energy that feeds into the flywheel ESS (FESS) is typically drawn from an electrical source, which may or may not be connected to the grid. As the flywheel stores energy, its speed increases, and as it discharges, it slows down. An energy conversion mechanism is responsible for converting these two forms of energy, which drives the rotating flywheel.

FESS characteristics

A flywheel's many appealing features include its minimal environmental effect, longer lifespan, high efficiency, high power density, high cycle life, high capacity for charging and discharging, and rapid reaction. The speed at which a flywheel rotates can be used to measure the state of charge of a material without impacting its temperature or lifespan. However, the increased self-discharge rate compared to other storage technologies is the main drawback of FESS. While other ESS are subject to regulation by depth of discharge, flywheels can quickly charge and discharge themselves.

FESS can function effectively on shallow discharges. Since flywheels do not require a prolonged charging and discharging cycle, they are expected to have a lifetime of over 20 years and hundreds of thousands of cycles, in addition to robust performance. A flywheel's efficiency ranges from 90% to 95%, allowing it to rapidly transfer a great deal of power. Because it uses non-hazardous ingredients, it has always been an environmentally benign technology that produces no pollutants when it operates. Energy and power ratings can be fine-tuned separately according to their respective uses.

The size and speed of the rotor determine the energy rating of the FESS, while the size of the grid and related power electronic components determine the power rating. A high-power FESS can be used for shorter periods of time, but a longer-duration FESS can be built with few changes to the rotor material and design. Power density is five to 10 times higher in flywheels than in batteries. In certain applications, like electric vehicles, flywheels might reduce the need for batteries due to their longer lifespan and smaller bulk.

Challenges and solutions

Flywheel material

The use of composite materials in flywheels, specifically carbon fiber, has reduced production scale while nearly tripling energy density compared to steel flywheels. Future designs of steel-based flywheels are anticipated to have rotors that are both safe and stress-free, allowing them to function at a higher speed compared to composite ones. Since steel is readily available and processing routes are well-established, it offers the benefit of low-cost manufacturing. As a result, the price of a flywheel can be cut even further. Furthermore, steels are more easily recycled than batteries; nevertheless, flywheels will not require recycling due to their extended lifespan.

Losses in FESS

The charge holding capability of most flywheels on the market today is compromised by their high loss rates. Power converter-based losses include conduction and switching, mechanical losses include friction, bearing and drag, and electrical losses include hysteresis, eddy current and copper. Most applications can employ FESS with current technologies if losses can be reduced by 10% to 20%. Using a lightweight ball-bearing system, hermetically sealing the system and engineering the flywheel to reduce electromagnetic drag are all potential future solutions that could lead to this reduction. The total loss of the system can be kept within budget by implementing these suggestions.

Flywheel rotor strength

Research around materials science should be conducted on how to enhance the robustness of highly stressed rotors spinning at high speeds. The self-discharge rate can be reduced through the implementation of active rotor magnetic bearings, which should be an area of primary emphasis.

FESS hybridization

Due to its limited storage capacity, the flywheel is best suited for use with lower-range grids. Therefore, additional research into combining FESS as a farming system with other suitable methods is required.

Yarn-integrated FESS structures

Researchers should investigate employing carbon nanotube yarns impregnated with superconductors because (a) they have a higher specific energy, (b) they have a higher energy density, (c) they are smaller and (d) they have a lower bearing cost for housing.

Addition of advanced machines to FESS

In the future, high-speed machines like induction machines, permanent magnet synchronous motors and bearing-free machines can be used to increase speed. A multilevel power electronic inverter that is well-suited to operate multiphase equipment is another appealing alternative. One great choice is an active neutral point clamped multilevel inverter. To further decrease costs and losses, it will be necessary to promote the use of power electronic interfaces based on metal-oxide-semiconductor transistors in the future.

Conclusion

Flywheel ESS is a solid option for managing supplementary benefits, pushing for the integration of renewable energy production and providing a bright future in commercialization. There are a lot of advantages to FESS over other ESS units: they are modular, recyclable, fast to respond, have a huge peak power, last a long time, are very efficient, are environmentally friendly and have a high energy density.