Evaluation of Electrical Characteristics of Energy Storage Devices

...

Figure 1: Operation of the Flywheel Storage system

The Battery Electronic Storage System (BES)

The second storage category uses the Battery technology and is referred to as the Battery Electronic Storage system or the BES system. The BES system, which is the most commonly used electronic storage system, is made up of a group of electrochemical cells. The cells are interconnected in either parallel or series and are meant to produce a certain amount of desired voltages after a series of chemical reactions. Over the years, many different types of electrochemical batteries have been developed including the Lead acid batteries, valve regulated batteries, lithium ion batteries, nickel cadmium batteries, among many others. However, these batteries carry several similar characteristics despite their differences. For lithium ion batteries, self-discharge rate is low, rated at about 5% maximum discharge rate. As a result, they have a longer lifetime than flywheel storage systems reaching at a minimum of 1500 cycles. (Lou et al., 2015). Different rechargeable batteries have different efficiency, self-discharge and cycle rates. The table below describes the characteristics of some of the major batteries used in storage technology[pic 5]

The most widely used rechargeable batteries are the lead-acid batteries. The cells are made up of PbO2 cathodes and Pb anodes. The electrolyte is sulfuric acid.

Super-capacitors

The third category is the category that makes use of the capacitors and super-capacitor as the electrical storage systems. Devices that rely on the super-capacitor technology have operations that resemble traditional capacitors. However, super-capacitors have higher capacities and discharge larger currents that traditional capacitors. The design and structure of a supercapacitor allow it to have the characteristics of traditional capacitors as well as rechargeable batteries. Super-capacitors are made up of two conductor electrodes, an electrolyte as well as a membrane separator that is porous in nature. Super-capacitors store energy in the form of static charge on the surfaces of the electrodes and the electrolyte. The surfaces are greatly increased because of the great porosity exhibited by the electrodes. Super-capacitors are generally of great use in areas with weak networks to compensate for the voltage drops that are faced in such areas. Super-capacitors have a high average efficiency of 90%achieved after a complete charging and discharging inclusive cycle. This indicates that only a loss of 190% is realized during the process of discharging (Lou et al., 2015). Super-capacitors have many features that make them superior to many other methods of energy storage. However, their main and most important features are their long cycling time of more than 1× 105 cycles as well as their high-efficiency rates per cycle. As discussed earlier, efficiency here refers to the ratio between the energy released and the energy stored. However, super-capacitors are only suitable for short-term energy storage and not for large scale and long-term uses (Triprathi and Amrita, 2013).

It is important to note that research on the use of super-capacitors has become more extensive. Research, in this case, has broadened to include materials that can be used to increase storage such as the use of carbon materials such as those used in rechargeable batteries.

Superconducting Magnetic Energy Storage (SMES)

Another category is the SMES known as the Superconducting Magnetic Energy Storage. The SMES devices are typically made up of three major components; a superconducting coil unit, a power conditioning subsystem, and a refrigeration and vacuum subsystem. SMES devices store energy in the form of magnetic fields that are created by the superconducting coil as a result of the direct current flowing through it. The superconducting coils used in these storage devices are usually of two different types. The first type is the low temperature superconducting coils (LTS) that work at 5K and the second is the high temperature superconducting coils (HTS) that operate at 70K (Lou et al., 2015). With the use of SMES technology, very small response time is achieved with high power peaks. The system also provides high efficiency as there is virtually no energy lost by the superconducting coil. The only loss realized by the system occurs in the place of the converter during discharge. The SMES system uses a device for signal conditioning. The device allows for a connection between the DC bus of the coil and the AC bus for charging and discharging processes. LTS technology is better developed and hence more commercially available when compared to the HTS technology. However, just like with super-capacitors, research is still ongoing (Lou et al., 2015). The figure below represents a diagrammatic representation of the SMES system[pic 6]

Figure 2: SMES System

There are many other forms of storage technologies such as the air compressors and thermal systems. All these different types of technology have the same basic guiding principles. Electric energy is converted for storage and then reconverted back to electricity when its use is required. Additionally, the different energy storage devices can be categorized as either direct or indirect storage devices as illustrated in the figure below[pic 7]

Figure 3: Storage technologies classification

[pic 8]

Graph 1

The graph 1 above indicates the re amount of energy released by the storage devices in comparison to energy that is taken in by the storage devices. The energy released slowly increases, in direct proportion to what is taken in until a point where it becomes constant even when there is more electrical energy that remains from when the device was discharged. The storage device is said to have reached a peak point. Similarly, we see that the effective capacity and power of the storage device continues to decrease with time as the device discharges energy. At first, the rate of discharge is fast and continues, however, the device then recovers and the rate of discharge reduces completely and comes to a stop even with increase in frequency. This behavior is common in rechargeable batteries. Super capacitors also act the same way, this characteristic is what allows them to become useful in their application in alternative energy storage devices such as electric generators

[pic 9]

Graph 2

Unlike graph

...