1.3
biomasses, which is a secondary form of solar energy. The energy source was replenished
in the same characteristic time as the energy being consumed. In the pre-industrial era,
humankind was basically living on a secondary form of solar energy. However, also back
then the way we consumed energy was not fully sustainable. For example, deforestation
due to increasing population density was already playing a role at the end of the first
millennium.
Methods of energy conversion
Figure 1.4 shows different energy sources and the ways we utilize them. We see that
usually the chemical energy stored in fossil fuels is converted to usable forms of energy
via heat by burning, with an efficiency of about 90%. Using heat engines, thermal energy
can be converted into mechanical energy. Heat engines have a conversion efficiency of up
to 60%. Their efficiency is ultimately limited by the Carnot efficiency limit that we will
discuss in Chapter 10. The vast majority of the current cars and trucks works on this
principle. Mechanical energy can be converted into electricity using electric generators
with an efficiency of 90% or even higher. Most of the world’s electricity is generated
using turbogenerators that are connected to a steam turbine, where coal is the major
energy source. This process is explained in more detail in our discussion on solar thermal
electric power in Chapter 22. Along all the process steps of making electricity out of fossil
fuels, at least 50% of the initial available chemical energy is lost in the various conversion
steps.
Figure 1.4: The different energy carriers and how we utilise them (adapted from L Freris and D Infield, Renewable
Energy in Power Systems (copyright John Wiley & Sons Inc, Chichester, United Kingdom, 2008)) [12].
Chemical energy can be directly converted into electricity using a fuel cell. The most
common fuel used in fuel cell technology is hydrogen. Typical conversion efficiencies of
fuel cells are 60%. A regenerative fuel cell can operate in both directions and also convert
electrical energy into chemical energy. Such an operation is called electrolysis; typical
conversion efficiencies for hydrogen electrolysis of 50-80% have been reported. We will
discuss electrolysis in more detail in Chapter 23.
biomasses, which is a secondary form of solar energy. The energy source was replenished
in the same characteristic time as the energy being consumed. In the pre-industrial era,
humankind was basically living on a secondary form of solar energy. However, also back
then the way we consumed energy was not fully sustainable. For example, deforestation
due to increasing population density was already playing a role at the end of the first
millennium.
Methods of energy conversion
Figure 1.4 shows different energy sources and the ways we utilize them. We see that
usually the chemical energy stored in fossil fuels is converted to usable forms of energy
via heat by burning, with an efficiency of about 90%. Using heat engines, thermal energy
can be converted into mechanical energy. Heat engines have a conversion efficiency of up
to 60%. Their efficiency is ultimately limited by the Carnot efficiency limit that we will
discuss in Chapter 10. The vast majority of the current cars and trucks works on this
principle. Mechanical energy can be converted into electricity using electric generators
with an efficiency of 90% or even higher. Most of the world’s electricity is generated
using turbogenerators that are connected to a steam turbine, where coal is the major
energy source. This process is explained in more detail in our discussion on solar thermal
electric power in Chapter 22. Along all the process steps of making electricity out of fossil
fuels, at least 50% of the initial available chemical energy is lost in the various conversion
steps.
Figure 1.4: The different energy carriers and how we utilise them (adapted from L Freris and D Infield, Renewable
Energy in Power Systems (copyright John Wiley & Sons Inc, Chichester, United Kingdom, 2008)) [12].
Chemical energy can be directly converted into electricity using a fuel cell. The most
common fuel used in fuel cell technology is hydrogen. Typical conversion efficiencies of
fuel cells are 60%. A regenerative fuel cell can operate in both directions and also convert
electrical energy into chemical energy. Such an operation is called electrolysis; typical
conversion efficiencies for hydrogen electrolysis of 50-80% have been reported. We will
discuss electrolysis in more detail in Chapter 23.