10 Module Deployment and Energy Rating
251
10.2.1 Effect of Module Orientation
The first and foremost effect that limits the energy generation of PV modules in
the field is their orientation with respect to the position of the Sun. In fact, fixed
installations receive light mostly from tilted angles that limit the total incident power
per square meter. As a result, all over a single day the power input to the module
grows from zero to the maximum irradiance at solar noon,
1 and then decreases again
to zero.
In Chap. 2, the in-plane global irradiance G(t), and its components (the in-plane
direct irradiance B(t), the in-plane diffuse irradiance D(t), as well as the in-plane
albedo R(t)) have already been defined. Let us ignore, for the moment, the diffuse
component and the albedo and define the orientation of a PV module, whether in the
field or on the top of a roof, as shown in Fig. 10.1a, where:
• the module tilt or slope β is the inclination
2 between the surface of the module
and the horizontal plane;
• the module azimuth ϕ is the direction towards which the module is tilted: the
usual convention in the PV community is to refer to as 0° azimuth the installation
facing South, with positive azimuths referring to west-facing orientations.
From the definition of β it follows that a module with β = 0° tilt lies horizontally
on the ground (Fig. 10.1b) and another module with β = 90° tilt is a vertically
installed module, like on a building façade or on solar fences (Fig. 10.1c).
Let us now refer, for simplicity, to a module installed at the Equator at β = 0° tilt,
in clear-sky conditions
3 and at one of the equinoxes (i.e. when the Equator is aligned
to the sun rays and the sun rays at solar noon are perfectly perpendicular to the 0°
tilted module, see Fig. 10.2). The total flux of power absorbed by the PV module
(the so-called in-plane global irradiance G(t)) in such conditions varies with time
according to the relation
G(t) = Scosθ (t),
(10.1)
where S is the power of the sunrays per unit area, and θ(t) is the angle of incidence
and is defined as the angle between the sunrays and a line perpendicular to the surface
of the PV module (and, of course, this angle varies with the position of the Sun).
The presence of the cosine of θ (t) in (10.1) (also referred to as the “cosine effect”)
accounts for the fact that the PV module absorbs no irradiance when θ = 90
◦ , i.e.
at sunrise and sunset and cosθ (t) = 0. On the other hand, G(t) is maximum at
1 Solar noon denotes the time at which the sun reaches the highest point above the horizon in the
precise geographical position where we are standing. It can be detected on a sundial and it is
generally different from the local noon, which is the time we see on our smartphones and is set
conventionally for each Time Zone, serving as a reference for the local time.
2 β is the symbol for both the temperature coefficient for V oc and the tilt of the module.
3 Clear-sky condition is generally defined as the absence of visible clouds in the sky dome.
251
10.2.1 Effect of Module Orientation
The first and foremost effect that limits the energy generation of PV modules in
the field is their orientation with respect to the position of the Sun. In fact, fixed
installations receive light mostly from tilted angles that limit the total incident power
per square meter. As a result, all over a single day the power input to the module
grows from zero to the maximum irradiance at solar noon,
1 and then decreases again
to zero.
In Chap. 2, the in-plane global irradiance G(t), and its components (the in-plane
direct irradiance B(t), the in-plane diffuse irradiance D(t), as well as the in-plane
albedo R(t)) have already been defined. Let us ignore, for the moment, the diffuse
component and the albedo and define the orientation of a PV module, whether in the
field or on the top of a roof, as shown in Fig. 10.1a, where:
• the module tilt or slope β is the inclination
2 between the surface of the module
and the horizontal plane;
• the module azimuth ϕ is the direction towards which the module is tilted: the
usual convention in the PV community is to refer to as 0° azimuth the installation
facing South, with positive azimuths referring to west-facing orientations.
From the definition of β it follows that a module with β = 0° tilt lies horizontally
on the ground (Fig. 10.1b) and another module with β = 90° tilt is a vertically
installed module, like on a building façade or on solar fences (Fig. 10.1c).
Let us now refer, for simplicity, to a module installed at the Equator at β = 0° tilt,
in clear-sky conditions
3 and at one of the equinoxes (i.e. when the Equator is aligned
to the sun rays and the sun rays at solar noon are perfectly perpendicular to the 0°
tilted module, see Fig. 10.2). The total flux of power absorbed by the PV module
(the so-called in-plane global irradiance G(t)) in such conditions varies with time
according to the relation
G(t) = Scosθ (t),
(10.1)
where S is the power of the sunrays per unit area, and θ(t) is the angle of incidence
and is defined as the angle between the sunrays and a line perpendicular to the surface
of the PV module (and, of course, this angle varies with the position of the Sun).
The presence of the cosine of θ (t) in (10.1) (also referred to as the “cosine effect”)
accounts for the fact that the PV module absorbs no irradiance when θ = 90
◦ , i.e.
at sunrise and sunset and cosθ (t) = 0. On the other hand, G(t) is maximum at
1 Solar noon denotes the time at which the sun reaches the highest point above the horizon in the
precise geographical position where we are standing. It can be detected on a sundial and it is
generally different from the local noon, which is the time we see on our smartphones and is set
conventionally for each Time Zone, serving as a reference for the local time.
2 β is the symbol for both the temperature coefficient for V oc and the tilt of the module.
3 Clear-sky condition is generally defined as the absence of visible clouds in the sky dome.