Energy Storage

Systems and Components

Alfred Rufer

• Editorial: CRC Press
• Año: 2017
• ISBN:
  9781351621922
• Edición: 1
• Categoría: Tecnología e ingeniería
• Categoría: Ingeniería eléctrica y telecomunicaciones

This book will provide the technical community with an overview of the development of new solutions and products that address key topics, including electric/hybrid vehicles, ultrafast battery charging, smart grids, renewable energy (e.g., solar and wind), peak shaving, and reduction of energy consumption. The needs for storage discussed are within the context of changes between the centralized power generation of today and the distributed utility of tomorrow, including the integration of renewable energy sources.

Throughout the book, methods for quantitative and qualitative comparison of energy storage means are presented through their energy capacity as well as through their power capability for different applications. The definitions and symbols for energy density and power density are given and relate to the volume and weight of a given system or component. A relatively underdeveloped concept that is crucial to this text is known as the theory of Ragone plots. This theory makes possible the evaluation of the real amount of energy that can possibly release out of a given system, with respect to the level of power dependency chosen for the discharge process.

From systems using electrochemical transformations, to classical battery energy storage elements and so-called flow batteries, to fuel cells and hydrogen storage, this book further investigates storage systems based on physical principles (e.g., gravitational potential forces, air compression, and rotational kinetic energy). This text also examines purely electrical systems such as superconductive magnets and capacitors. Another subject of analysis is the presentation of power electronic circuits and architectures that are needed for continuously controllable power flow to and from different storage means. For all systems described, the elementary principles of operation are given as well as the relationships for the quantified storage of energy. Finally, Energy Storage: Systems and Components contains multiple international case studies and a rich set of exercises that serve both students and practicing engineers.

1 Generalities on Energy Storage

1.1 HISTORY AND CONTEXT OF USE

Large facilities for electrical energy storage have been built in the second half of the twentieth century in the context of matching the variable power demand (daily cycles) with the installation of nuclear power plants, known for their mostly constant power production. Figure 1.1a shows the typical power profile of the weekly consumption, and Figure 1.1b illustrates one of the most important pump-storage facilities built in Luxembourg in this context. Many other facilities have been carried out with the same motivations, for example, the “Hongrin-Léman” facility in Switzerland, the Raccoon Mountain Pumped-Storage Plant west of Chattanooga, Tennessee, United States, or the Yagisawa Power Station in Japan.

From the end of the twentieth century, another trend emerges in the context of the development of renewable energy sources. From the classical centralized utility of today, there is a clear movement in the direction of distributed utility of tomorrow, together with the appearance of so-called smart grids (Figure 1.2).

Renewable energy sources are known by their variation in time or available power related to meteorology conditions. This is clearly a new motivation for the development and the realization of new energy storage systems. An additional reason can be found in the principle of decentralizing the energy production itself. This concerns the power matching between decentralized generators and their loads nearby that can generate significant and fast variations of the local power demand. Figure 1.3 illustrates the principle of “losing” the averaging effect of the power due to decentralization of production.

A general tendency toward an increasing use of energy storage can be observed.Two different aspects are considered:

1. First, the use of storage technology in order to solve the problem of availability of sources (day-to-night shift for photovoltaic plants as a first example, or the bridging of lack of production of fluctuating sources).

2. Second, the use of energy storage technology in order to assist some problematic consumers when the local generation cannot follow the strong and fast demand. In this context, two examples can illustrate the problem. First, in the case of a “microturbine” the fast increase of load must respect some minimum time constant (minutes, due to thermal constraints). Second, in the case of the use of “fuel cells,” the design of these systems for the maximum peak power can lead to unacceptable costs. The design for the “mean value” is more realistic, and the highest power demand can be taken out of a storage device.

FIGURE 1.1 Daily variations of the power demand: (a) profile of the demand over a week; (b) pumped-storage plant (Vianden, Luxembourg).

FIGURE 1.2 From centralized utility of today to distributed utility of tomorrow.

FIGURE 1.3 Power fluctuations (ΔP/P_n) in centralized and decentralized power generation systems.

1.2 GENERAL DEFINITIONS

1.2.1 Definitions of Energy

The most convenient way to define energy is to use its relationship to the integral of the exchanged power:

(1.1)

Another way is to consider the general definitions of a thermodynamic system and to combine them with the macroscopic forms of energy of the system. The sum of all forms of energy of a system is called the total energy.

The macroscopic energy of a system is related to its movement and to the external effects such as gravity, magnetism, or electricity.

The microscopic energy is related to the molecular activity of a system and is often called the internal energy.

A simple definition of the total energy is given by the sum of the internal energy U, the kinetic energy KE, and the potential energy PE, leading to the expression of Relation 1.2 [1]:

(1.2)

where

m is the mass

V is the velocity of the mass from a given reference point

g is the gravitational acceleration

z is the height of the mass center from the reference point

For a system with a rotating mass, the term for the kinetic energy becomes

(1.3)

where

J is the moment of inertia

ω is the angular velocity

Relation 1.2 is valid for closed systems. For so-called open systems, an additional term related to the flow of material must be introduced. This related energy is characterized through the mass flow rate:

(1.4)

where

is the volumetric flow rate

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