This tutorial deals with the efficiency of the steam power process and explains the relevant context.
3. Efficiency
Generally speaking the efficiency is defined as quotient of output and input. The measure of efficiency is a non-dimensional number between 0 and 1 (respectively between 0 and 100%). Such a coefficient allows to measure and to compare the efficiency of processes. The higher the efficiency, the better is the transformation of energy fed into the process. Some examples for processes and related efficiencies:
- Gears: A gear transforms mechanical energy into mechanical energy. The goal is to change the rotating speed and the torque. Both input and output of a gear is mechanical energy. Some part of this mechanical energy is being lost due to friction and is transformed into heat. Efficieny: Up to 99 %.
- Petrol engine: The input here is fuel, which contains chemical energy. The output is mechanical energy to power the wheels. Due to friction and hot fumes, energy is being lost in the process and the output is consequently reduced. Efficiency: Up to 35 %.
- The first stem engines in the beginning of the 18th century: The input is chemical energy in coal, the output is mechanical energy, which could be used for indutrial purposes. Efficiency: < 1 %
Thermal energy conversion processes are bound to thermodynamic laws. The efficiency of processes within power plants can therefore not exceed a certain maximum. Technical implementation gives rise to further losses and an actual efficiency, which in general is well below the theoretical maximum. The theoretical maximum of such a process is called Carnot-Efficiency or Carnot-Factor. It denotes the efficiency of a Carnot Cycle and determines how much of the heat input can be transformed into work. The Carnot Cycle consists of four ideal changes of state, similar to the steam power process. Its efficiency can be calculated independently from working appliances or details of the process design. Two temperatures are the only factors that influence the efficiency:
- T1, temperature at which heat is fed into the process
- T2, temperature at which the heat is discharged from the process
The higher T1 and the lower T2, the higher is the Carnot efficiency. The lower boundary temperature T2 is in many cases the ambient temperature (in power plants that cool with water from the sea, heat can be discharged with a lower temperature than the ambient).
An efficiency of 100 % is a theoretical value, which can only be obtained, if temperature T2 reaches 0 Kelvin, -273.1 °C respectively or if T1 can be indefinitely high.
In a power plant the efficiency factor denotes the ratio of the “generated” energy and input of energy via fuels. The efficiency of steam power plants can reach up to 46 %. Nowadays, gas turbines reach efficiency factors of up to 35 %, combined-cycle gas turbines factors up to 62 %.
