22.1
22
Solar thermal energy
Solar thermal energy is an application of solar energy that is very different from
photovoltaics. In contrast to photovoltaics, where we used electrodynamics and solid state
physics to explain the underlying principles, solar thermal energy is mainly based on the
laws of thermodynamics. In this chapter we give a brief introduction to that field. After
introducing some basics in Section 22.1, we will discuss Solar thermal heating in Section
22.2 and concentrated solar (electric) power (CSP) in Section 22.3.
Solar thermal basics
We start this section with the definition of heat, sometimes called thermal energy. The
molecules of a body with a temperature different from 0 K exhibit a disordered movement.
The kinetic energy of this movement is called heat. The average of this kinetic energy is
related linearly to the temperature of the body.
1 Usually, we denote heat with the symbol
Q. As it is a form of energy, its unit is joule (J).
If two bodies with different temperatures are brought together, heat will flow from the
hotter to the cooler body and as a result the cooler body will be heated. Dependent on its
physical properties and temperature, this heat can be absorbed into the cooler body in two
forms, sensible heat and latent heat.
Sensible heat is the form that results in changes in temperature. It is given as
where Q is the amount of heat that is absorbed by the body, m is its mass, C p is its heat
capacity and (T 2 − T 1 ) is the temperature difference. On the other hand, if a body absorbs
or releases latent heat, the temperature stays constant but the phase changes. This happens
for example when ice is melting: when its temperature is equivalent to its melting point,
heat that is absorbed by the ice will not result in an increase in temperature but in
transformation into from the solid to the liquid phase, which is water. Mathematically, this
is expressed as
22
Solar thermal energy
Solar thermal energy is an application of solar energy that is very different from
photovoltaics. In contrast to photovoltaics, where we used electrodynamics and solid state
physics to explain the underlying principles, solar thermal energy is mainly based on the
laws of thermodynamics. In this chapter we give a brief introduction to that field. After
introducing some basics in Section 22.1, we will discuss Solar thermal heating in Section
22.2 and concentrated solar (electric) power (CSP) in Section 22.3.
Solar thermal basics
We start this section with the definition of heat, sometimes called thermal energy. The
molecules of a body with a temperature different from 0 K exhibit a disordered movement.
The kinetic energy of this movement is called heat. The average of this kinetic energy is
related linearly to the temperature of the body.
1 Usually, we denote heat with the symbol
Q. As it is a form of energy, its unit is joule (J).
If two bodies with different temperatures are brought together, heat will flow from the
hotter to the cooler body and as a result the cooler body will be heated. Dependent on its
physical properties and temperature, this heat can be absorbed into the cooler body in two
forms, sensible heat and latent heat.
Sensible heat is the form that results in changes in temperature. It is given as
where Q is the amount of heat that is absorbed by the body, m is its mass, C p is its heat
capacity and (T 2 − T 1 ) is the temperature difference. On the other hand, if a body absorbs
or releases latent heat, the temperature stays constant but the phase changes. This happens
for example when ice is melting: when its temperature is equivalent to its melting point,
heat that is absorbed by the ice will not result in an increase in temperature but in
transformation into from the solid to the liquid phase, which is water. Mathematically, this
is expressed as