Energy Storage Device Fundamentals and Technology

Himanshu Priyadarshi, Ashish Shrivastava, and Kulwant Singh

Manipal University Jaipur

Introduction

Futuristic planning is an important aspect for any society that wants to prosper progressively. Energy security is very crucial for any civilization. Substantial reserves of energy are necessary to keep the wheel of civilization pacing, otherwise it might come to a screeching halt. The economies of many societies are abjectly dependent on fossil fuels, and this dependence has caused colossal loss of lives. The increased awareness about the dangers of fossil fuels has forced the intellectual community to think in non-conventional ways, as sticking too much to the convention of fossil fuels has not done any good either to the planet or its people.

TABLE 7.1

Predictability of Solar Photovoltaic Energy

Factors

Specifications

Predictability

Geographic location

Latitude, meridional, and diurnal changes, elevation with respect to the mean sea level - affects the vertical air column, and hence the attenuation

Deterministic

Collector plate disposition

Collector tilt angle, orientation with respect to horizontal

Deterministic

Time of day

Hour angle - represents temporal variation

Deterministic

Time of year

Declination - model seasonal effect on insolation

Deterministic

Atmospheric conditions

Ozone layer status, dust, humidity, and cloud cover

Stochastic

Mother Nature provides abundant energy resources which are much more than that can be ever utilized or even that is being currently harnessed. Hence, it stands to reason that a strong support system be developed for the harvesting of natural energy resources such as solar photovoltaic energy, wind energy, and so on. An important challenge in the harvesting of energy resources such as wind and solar is to support these sources of energy with predictability. The availability pattern of these resources is highly stochastic, i.e., it varies with climatic conditions. A typical case in point is detailed in Table 7.1.

The last item in the table showrn above is the atmospheric conditions, which include several important sub-factors, and hence solar and wind energy get very much affected by these atmospheric conditions. Sometimes there is surplus generation and sometimes there may be deficit; the elements in nature balance each other in terms of cause and effect. This provides a clue as to how the variegatedness of the energy procurement quanta can be managed by making use of energy storage devices.

Storage of electrical energy harnessed from renewable sources of energy will provide consistency of availability of electrical power. Energy storage is a necessity, and not an option. Right from the discovery of electrical pow'er, batteries have been at the forefront of energy dispensation. At the inception of batteries, their utilization was limited to being used only once, and this limitation in terms of lifetime and non-reusability arrested the growth of this sector. The progression of time unveiled the “reusability” capability of batteries and their corollary devices such as supercapacitors and pseudo-capacitors. This energy storage sector has attracted a lot of investment from advanced nations because they could see the absolute requirement of energy storage devices as the essential support system for the renewable energy sector. Various energy storage technologies are being utilized all over the world, with electrochemical energy storage devices being particularly useful due to their commendable energy storage and power transaction capabilities.

Generic Energy Storage Device Concepts

The foresighted investors who can envision the worth of the energy storage market are pumping money for the manufacturing of energy storage devices which are being engineered with variegated nomenclatures such as batteries, supercapacitors, ultracapacitors, pseudo-capacitors, fuel cells, etc. The expected outcome of this section is not to present a differentiational discussion of these various devices meant for energy storage. As we go through this section, we intend to present a unified outlook for electrochemical energy storage. The rationale behind prioritizing a unified approach is to develop sound fundamentals, which can then be utilized in the design of devices with niche applications and incumbent features. This synergistic view will also protect the reader from the misconceptions due to the jargon of devices that are being proposed in multitude from the research community. Simplistic understanding of a generic electrochemical energy storage device with regard to the way it exchanges energy with external loading circuits can be initiated with the crude analogy of gravitational potential energy storage and exchange with skateboard, illustrated as follows. An energy storage device is a bidirectional energy exchange device, and the same device must be capable of accumulating as well as dispensing electrical energy as per the situation. Hence, repetitiveness in reusability with longevity is an important consideration for the design of electrochemical energy storage devices. The skateboard may go upward as well as downward in the concavity of the gravitational potential well many times if the mechanical design of the skateboard remains unaffected from its locomotion with negligible wear and tear. Similarly, if the structure and material composition of an electrochemical energy storage device remain preserved in the course of multiple energy exchanges, the energy storage device will have a long lifetime and consistency in performance.

The repetitiveness in reusability with longevity feature is a very crucial engineering consideration because the generic energy storage design technology must be adopted at the global level in this era of increased competitiveness and ambitious business expansion mentality of the investors in technology. Hence, it becomes imperative for us to understand how the energy storage design considerations play an important role in meeting this criterion. To this end, we must understand the most fundamental repetitive unit of the energy storage cell design. A cell is the most fundamental unit of an energy storage system.

In general, the cell acts as a voltage source, of course except solar cells, which are a current source. Strictly speaking, solar cells cannot be termed an energy storage device, as current flow is a kinetic parameter, whereas energy storage devices require a potential parameter. The potential parameter in the electrical domain is the voltage or electromotive force, and for an energy storage cell, the following parameters are very important.

Cell Voltage

It is the potential difference available across the electrical current outlet terminals of a cell. It depends on the active materials used inside the energy storage cell device.

Depending upon the electrochemical material constitution of the energy storage cell, the cell voltage varies.

If we contemplate the chronological evolution of energy storage cells, it gives us the clue of contemporary dominance of lithium ion cells in the market of energy storage. The Daniel cell opened the avenues for utilization of the electrochemical domain for energy storage, and the discovery of lead acid batteries further consolidated the prospects of electrochemical energy storage by increasing the cell voltage and the amount of energy that can be stored. The discovery of nickel metal hydride cells proved to be a milestone in the market of energy storage as it opened up the gateway to the technological landscape for intercalated electrode structures. The phenomenon of intercalation has proved to be very instrumental in the growth of lithium-based energy storage.

In an electrochemical energy storage device, the terminal potential difference depends on the electrochemistry. The difference in the electrode potentials of the positive and negative electrode determines the cell voltage to a considerable extent.

Generally, the elements placed at the left hand side of the periodic table are good candidates for being the active material in positive electrodes, as such elements act as good reducing agents. The elements placed near the right hand side of the periodic table are more suitable for being the active material in negative electrodes, as such materials are good oxidizing agents. However, this tuning between the left and right column elements of the periodic table cannot be done arbitrarily, as the electrolyte must be able to withstand the potential difference across it. An increase in the potential difference may be done by connecting energy storage cells in series; however, stable holistic electrochemistry and the safety cannot be compromised.

Energy Capacity of a Cell

The energy capacity of an energy storage cell is defined as the amount of energy the unit is able to deliver at the nominal cell voltage defined in the foregoing section. Generally, an energy storage device’s capacity is commercially proclaimed in terms of ampere-hours. The term ampere-hours gives an index of its charge bearing capacity, and charge multiplied by the potential difference gives the energy capacity.

Cell Charging Rate or C-Rate

The time rate at which the energy capacity of an energy storage cell is being built-up or being discharged is called the cell charging rate or C-rate.

Energy Density and Specific Energy

The energy stored in an energy storage device per unit volume is termed energy density; whereas the energy stored per unit mass is known as specific energy. Thus for a given volume, the device with greater energy density can store more energy; whereas for a fixed energy rating, the device with greater energy density will be more compact. Similarly for a given mass, the device with more specific energy will store more energy; for a fixed energy capacity, the device with higher specific energy will be lighter.

Power Density and Specific Power

The cost of an energy storage device has an energy capacity component and a power component. While the premium on the energy storage is dependent on watt-hour, the premium on the power it is capable of transacting is dependent on the number of watts. In this light, the size and weight of an energy storage device have to be understood with respect to power transaction. The ability to discharge/accept power to a load/from a source per unit volume is known as the power density of an energy storage device, and when this ability is specified per unit mass, it is called the specific power.

 
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