Evaluation of Electrical Characteristics of Energy Storage Devices

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Evaluation of Electrical Characteristics of Energy Storage Devices

Introduction

For many years, energy storage has relied on many forms of technology for it various applications. For instance, energy storage systems have been used in dam and diversions of river courses to create energy in the form of mechanical energy. These facilities rely on hydraulic technology to create energy. Modern systems used in rivers now convert energy into electrical energy. Electricity has two major uses that make it suitable for use in storage. The first characteristic is that it is used up at the same time as it is generated. This means that an imbalance between supply and discharge can lead to low quality of energy. The second characteristic of electricity is that it is often consumed in places that are far from where it is generated (Brunet, 2011)

Electrical energy storage devices, therefore, must contain the necessary characteristics to deal with the characteristics of electricity. Additionally, electrical energy storage technologies do not use electricity in its primary form. Rather, they convert it into another form that can be stored. Examples of such forms of energy include; active, electrochemical, electrostatic and potential energy forms. When the energy is required, it is reconverted back to electrical format after which it can now be used. A conclusion can be drawn from this information about electrical energy storage devices. Energy storage devices must have the ability to both act as energy consumer during charging, and as energy producers during discharging. This is perhaps the most important characteristic of electrical energy storage devices. This paper seeks to summarize the extensive topic regarding the characteristics of electrical storage systems. It discusses different technologies associated with electrical energy storage, therefore drawing useful characteristics from the various technologies (Brunet, 2011)

Parameters for Defining Storage Technologies

Storage devices and technologies rely on different parameters to define them. The first is capacity which refers to the amount of energy in a storage device after the charging phase is completed. It refers to the extensive amount of energy that a device can hold, Wst, which is higher than the actual amount of energy that can be used up when the device is in operation, Wut. The second parameter used during definition is the energy available. The energy available is determined by using the system of generators or motors that are used during the process of energy reconversion back to electricity. It is expressed as an average value indicated as a percentage of the ratio between the energy released and the total amount of energy that can be stored. Thirdly, discharge time, expressed by the expression 1 below, is also used as a defining parameter.

[pic 1]

Where: τ(s) is the Discharge time, in s, Wst is the Total energy stored, in Wh and Pmax is the Maximum or peak power, in W

Additionally, efficiency is also used for definition of storage devices. It refers to the ratio between the energy released by a storage device and the maximum amount of energy that the device can store. An efficient storage device releases a high amount of energy, close to what it stores. The expression for efficiency is illustrated below

[pic 2]

Where:  η refers to the Efficiency of storage technology and Wut the Useful or recoverable energy for a given point of operation, in Wh

A fifth parameter referred to as durability is also relied upon. Durability as it relates to storage devices refers to the number of times a storage device can release energy. It is calculated in the maximum number of complete charge and discharge cycles the device undergoes.

The last parameter is the autonomy parameter. It refers to the maximum time a system can release energy. It is expressed in the expression below where:

[pic 3]

a is the Autonomy, in s and Pdt is the Maximum power discharge, in W (Brunet, 2011)

Electrical Characteristics of Energy Storage Devices

Electrical energy storage technologies are divided into different classifications, each with their own characteristics. The relevant characteristics of each device are discussed in this review.

Flywheel

The first classification of storage technologies is the mechanical system, commonly seen in the flywheel devices. As seen in the flywheel store energy in the form of kinetic energy. The standard flywheel device is comprised of a flywheel, a group of bearings, a reversible electrical motor or generator, a power electronic unit, and a vacuum chamber. In a flywheel system, the amount of energy stored is largely dependent on the movement of the flywheel (Lou et al., 2015). This means that the rotating speed and inertia of the wheel are the major contributing factors to the efficiency of the flywheel energy storage system. Flywheel; energy storage systems are categories in two different groups. The first is low-speed systems that rely on steel as the make-up component of the flywheel.  These systems also rely on traditional designs which means that the wheels have large diameters accounting for their low speeds. These systems rotate at a speed well below 6 × 103 rpm. The second category is the high-speed flywheel energy storage systems. These systems use advanced composite rotors that are made up of materials such as graphite or carbon fiber. These composite materials are effective because they provide high specific energy. Low-speed flywheel storage systems are designed for short-term uses while Flywheel systems of higher speeds can operate for longer periods of time allowing them to be usable in vehicles and power plants. The flywheel storage systems have high discharge and charge rates for a numerous number of cycles. This is their main advantage. The only limitation of the flywheel systems is the lifetime, often estimated at around twenty years. Flywheel systems have an energy efficiency of about 90% at their rated power. Flywheel storage systems are not reliable for long term storage operations. As such, they work best as reliable standby power sources. Fly wheel systems have high discharge rate. They have a minimum rate of 20% of their total capacity per hour (Lou et al., 2015). As a result, flywheels have declining energy efficiency when cycling does not occur in a continuous manner. This characteristic of flywheel systems is what makes them highly efficient when interpreted with the plants used for the production of renewable energy. Flywheel storage systems are used to give out power where insufficient power is being provided from the grids. They do this by storing power when it is being produced in excess. Additionally, flywheel storage systems may be used to condition power and to provide power in case of total loss that may be caused by failure of the grid system.  Flywheel storage systems may be used as backup power sources but only for short periods of times. (Lou, Wang, Dooner and Clark, 2015).  The general operation of the flywheel is illustrated in the figure below        [pic 4]