Page 106 - Applied Photovoltaics

Using current manufacturing technology, typical crystalline silicon modules have
EPTs ranging from 2 to 8 years, while thin film modules have EPTs of 1 to 3 years
(Kato, 2000; Alsema, 2000). These are expected to be below 2 years and 1 year,
respectively, as newer technologies are installed (Alsema et al., 1998). However, the
EPT gives no indication that the life of a PV module (L pv ) is likely to be 20—40
years, so that the PV module will generate significantly more energy over its lifetime
than was needed in its manufacture. The term energy yield ratio (EYR) (Pick, 2002),
defined as

E gen L pv
EYR (5.5)
E
input
is used to reflect this. An EYR greater than one indicates the PV module (or system)
is a net energy producer. For a module with an EPT of 4 years and a life of 20 years,
EYR would be 5, indicating that the module would generate five times the energy
used in its manufacture over its lifetime.

EXE RCI S ES
5.1 A hypothetical solar module consists of 40 series-connected identical solar
cells, each giving an open circuit voltage of 0.61 V and a short circuit current
of 3 A under bright sunshine. The module is short-circuited under bright
sunshine and one cell is partly shaded. Assuming that the cells have an
ideality factor of unity, and neglecting temperature effects, find the power
dissipated in the shaded cell as a function of the fractional shading of the cell.

5.2 (a) Briefly discuss features of a silicon solar cell that affect its spectral
response.

(b) When do you need to consider spectral response differences between
cells and why?
5.3 (a) Explain how localised ‘hot spots’ can occur in a partially shaded cell
connected into a large photovoltaic array.
(b) Explain the steps that can be taken to prevent damage arising from
such ‘hot spots’.
5.4 (a) A nominal 12V photovoltaic module contains 36 identical solar cells,
each with a short circuit current of 3.0 A, and a fill factor and open
circuit voltage typical of those found with commercial solar cells.
Draw and label as appropriate the expected current-voltage
characteristic of such a module at 25°C.
(b) By mistake, the manufacturer connects one cell in the wrong way
(reverse polarity). On the previous diagram, superimpose the
corresponding current-voltage characteristic and indicate how it was
determined.

(c) What effect will result from shading the wrongly-connected cell and
why?

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