9 Solar Module Technology
223
that may require larger currents for faster battery charging, modules can contain
paralleled strings. This has the advantage of reducing shading losses. However, the
resulting higher currents will lead to higher resistive losses.
Thin-film solar modules, irrespective of what technology, are in general characterized by a higher number of cells connected in series, and by smaller cell active
areas. They are, thus, characterized by higher voltages and lower currents.
9.1.3 Cell-to-Module Losses
Just as in the case for solar cells, the performance of modules is characterized by
the power P max and efficiency η measured at Standard Test Conditions (STC =
1000 W/m
2 , AM1.5, 25 °C). On a module level, we can distinguish between totalarea efficiency η tot and aperture-area efficiency
1
η apa . The former is determined using
the total area (including the edges and also the frame, if present); the latter using only
the surface of the active-area of the cells (which corresponds to a smaller surface).
Understandably, η tot < η apa . When comparing the efficiency of different technologies
and/or products we need to make sure that we are referring to the same parameter.
In general, η tot tends to be a more reliable parameter for a direct comparison.
When assembling solar cells in a module, the overall efficiency of a module (i.e.
both η tot and η apa ) is generally lower than the average efficiency η cell of the cells
used.
The difference in the efficiency (or power output) between cells and modules is
described by a parameter called the “Cell-to-Module (CTM) ratio”, which accounts
for the so-called CTM losses.
For example, a conventional c-Si module (with a power of 293 W p and an area:
~1.63 m
2
= 1.65 m × 0.992 m), made of 60 cells (each 156 mm × 156 mm) with an
average efficiency η cell of 21%, would have a typical total-area efficiency η tot of 19%
with a CTM ratio (calculated for the efficiency) of ~90%. Typical CTM values lay
in the range of 85–95%, even if novel concepts (see Sect. 9.2.3) can lead to higher
CTM ratios.
2
The difference in efficiency between cells and modules depends on several factors, mainly: geometrical factors (e.g. module margins, cell spacing, etc.), optical
factors (e.g. glass/air interface and front cover reflections, encapsulant absorption,
etc.) and electrical losses (increased series resistance due to ribbons, interconnects,
1 For c-Si aperture-area or active-cell area are sometimes used interchangeably. For thin films, the
active-cell area is generally smaller than the aperture-area, which includes the portion of the cell
covered by grid fingers and interconnects.
2 It should be noted that CTM ratios may be determined using either power or efficiency values
for the module and the cells. These ratios do not correspond, because when efficiency values are
used, the cell and module areas are factored as well in the computation. CTM ratios are higher if
calculated using power values, rather than efficiency values.
223
that may require larger currents for faster battery charging, modules can contain
paralleled strings. This has the advantage of reducing shading losses. However, the
resulting higher currents will lead to higher resistive losses.
Thin-film solar modules, irrespective of what technology, are in general characterized by a higher number of cells connected in series, and by smaller cell active
areas. They are, thus, characterized by higher voltages and lower currents.
9.1.3 Cell-to-Module Losses
Just as in the case for solar cells, the performance of modules is characterized by
the power P max and efficiency η measured at Standard Test Conditions (STC =
1000 W/m
2 , AM1.5, 25 °C). On a module level, we can distinguish between totalarea efficiency η tot and aperture-area efficiency
1
η apa . The former is determined using
the total area (including the edges and also the frame, if present); the latter using only
the surface of the active-area of the cells (which corresponds to a smaller surface).
Understandably, η tot < η apa . When comparing the efficiency of different technologies
and/or products we need to make sure that we are referring to the same parameter.
In general, η tot tends to be a more reliable parameter for a direct comparison.
When assembling solar cells in a module, the overall efficiency of a module (i.e.
both η tot and η apa ) is generally lower than the average efficiency η cell of the cells
used.
The difference in the efficiency (or power output) between cells and modules is
described by a parameter called the “Cell-to-Module (CTM) ratio”, which accounts
for the so-called CTM losses.
For example, a conventional c-Si module (with a power of 293 W p and an area:
~1.63 m
2
= 1.65 m × 0.992 m), made of 60 cells (each 156 mm × 156 mm) with an
average efficiency η cell of 21%, would have a typical total-area efficiency η tot of 19%
with a CTM ratio (calculated for the efficiency) of ~90%. Typical CTM values lay
in the range of 85–95%, even if novel concepts (see Sect. 9.2.3) can lead to higher
CTM ratios.
2
The difference in efficiency between cells and modules depends on several factors, mainly: geometrical factors (e.g. module margins, cell spacing, etc.), optical
factors (e.g. glass/air interface and front cover reflections, encapsulant absorption,
etc.) and electrical losses (increased series resistance due to ribbons, interconnects,
1 For c-Si aperture-area or active-cell area are sometimes used interchangeably. For thin films, the
active-cell area is generally smaller than the aperture-area, which includes the portion of the cell
covered by grid fingers and interconnects.
2 It should be noted that CTM ratios may be determined using either power or efficiency values
for the module and the cells. These ratios do not correspond, because when efficiency values are
used, the cell and module areas are factored as well in the computation. CTM ratios are higher if
calculated using power values, rather than efficiency values.