Introduction

This chapter covers the concepts of energy, electrical work, power and efficiency. It is important to start by making a clear distinction between energy and work, as these two quantities are closely related and even share the same unit, but describe different things.

The different variables are clearly distinguished from one another using the example of a pumped storage power station (1). A pumped storage power plant serves to convert electrical energy into potential energy and vice versa. As soon as the electrical energy demand of consumers is lower than the available supply, the electric motor of the pumped storage power plant pumps water from a lower to a higher level. In this process, the electric machine is operated as a (pump) motor. Via the magnetic field generated by an electric current in the motor winding, this produces a rotary motion which is used to pump the water into the upper reservoir. The electric motor thus converts electrical energy (the current flowing through the motor winding) into mechanical work (the water is pumped into the upper reservoir). Once the water has reached the upper reservoir, it has a higher potential than before in the lower reservoir. The water in the upper basin therefore stores potential energy and, in the case of a loss-free pump driven by a loss-free electric motor, this is exactly the same amount as was supplied to the electrical system as electrical energy. This is because the energy within a closed system must remain constant. In physics, this fundamental property is known as energy conservation.

As soon as the demand for electrical energy exceeds the supply, the water from the upper reservoir is discharged via a turbine. The turbine converts the previously stored potential energy of the water in the upper reservoir into mechanical work. This work performs a rotary motion in the electric machine. Since the machine now functions as a generator, it converts the mechanical work performed on it into electrical energy. During this process, a sluice valve in the upper reservoir can be used to reduce the amount of water flowing through the turbine within a certain period of time. This means that less potential energy is converted into mechanical work and thus electrical energy per unit of time. The work performed per unit of time is referred to as power. The power of a system therefore indicates how much work can be performed per unit of time. It is an important parameter for electrical systems. Within the electrical machine, part of the energy supplied is converted into heat in the line resistance, which is released into the environment. Furthermore, friction losses also occur in the mechanical elements (pump, turbine) and on the walls of the pipes, which is ultimately also released into the environment as heat. Consequently, the amount of electrical energy supplied to the system is always greater than the amount of electrical energy that can be extracted from the system. The ratio of the energy extracted to the energy supplied is described as efficiency.

The terms energy, power, electrical work and efficiency are explained in more detail below. For a better understanding, the basics from the previous chapter on charge, electrical potential and electrical current are assumed here.