Photovoltaics in Buildings A Design Guide - DTI Home Page

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Figure 2.7
Typical module constructions
a. Glass/EVA/Tedlar/Polyester/Tedlar
b. Glass/Resin/Glass
Figure 2.8
TFS using amorphous silicon
The cells, which are of low voltage, are joined in series to form a module of
a higher, more useful voltage. The modules (Figure 2.7) are constructed like a
sandwich (and sometimes referred to as laminates) and have a backing sheet
and a cover of low-iron glass which protects the front surface of the material
while maintaining a high transmissivity. A structural frame is used in a number
of designs to protect the glass.
The backing sheet need not, however, be opaque. At the Doxford Solar
Office (Figure 1.2), the PV cells are encapsulated between two layers of glass
with transparent spacing between cells (Figure 2.7(b)); thus light passes
through the transparent areas. This produces an effect inside the building
which in the architect’s words is like “sunlight filtered through trees”.
Thin film silicon (TFS) PVs using amorphous silicon are manufactured by a
vapour deposition process. Between the p and n layers is the i (for intrinsic)
layer. Overall, thicknesses are much less than with crystalline technologies,
hence the name. Typically, the cells are laminated into glass (Figure 2.8) but
modules can also be made flexible by using plastics (Figure 2.9) or metal.
Modules electrically connected together in series (Figure A.4) are often
referred to as a string and a group of connected strings as an array. An
array is also a generic term for any grouping of modules connected in series
and/or parallel. Power from the array (Figure 2.10) goes to a Power
Conditioning Unit (PCU). PCU is a general term for the device (or devices)
which converts the electrical output from the PV array into a suitable form for
the building. Most commonly, the PCU has a principal component, an inverter
(which converts DC to alternating current, AC) and associated control and
protection equipment. PCU and inverter are sometimes loosely used
interchangeably. The AC output from the PCU goes to a distribution board in
the building or to the grid if supply exceeds demand.
Figure 2.9
TFS module with metal
backing sheet and
plastic cover