Photovoltaics in Buildings A Design Guide - DTI Home Page

34
Figure 4.3
Unit electricity cost
Figure 4.3 shows a graph of the notional unit cost of electricity.
The unit cost of all PV electricity produced falls with increasing array size
because of economies of scale. The cost of the electricity produced and
used on site initially falls (Zone A) because of economies of scale but as the
array size gets larger it is likely that all the electricity cannot be used on site
and so more will be exported. Since this electricity is not bought back at
cost, the unit price of electricity used on site rises (Zone B).
In practice, the form of the curve and the point of ‘minimum’ cost will depend
on the specific PV installation. An approach based on sizing for the ‘minimum’
cost, Cmin, could be labelled the IMBY strategy, ie ‘in my backyard’, and
represents that of a client adopting a ‘rational’ approach to their PV
installation. More altruistic clients who wished to reduce CO 2 emissions for
society as a whole could opt for larger arrays.
A start to a solution is to examine the load pattern. If we consider a building
with a somewhat higher base power demand in winter than in summer (as is
likely to be the case in naturally ventilated offices), then sizing the array to
meet the summer base load will tend to ensure that the winter output (when
the array output is less) will also be used on site. This is an array size which
will export little energy and so is likely to be in Zone A.