Page 184 - Applied Photovoltaics

Figure 9.5. Loss-of-load-probability (LOLP) as a function of days of storage
(©1987 IEEE Jones & Chapman).

From Fig. 9.5, it can be seen that an array sized to meet the load, in conjunction with
one day’s battery storage, gives an average availability (1 – LOLP) of approximately
94%. This value should be taken as the upper bound on the fraction of the energy in a
hybrid system that can be economically displaced by the photovoltaic array. In
reality, to make use of all the photovoltaic-generated power in summer, the array size
needs to be a little less than that needed to supply the energy requirements of the load.
The implications of this on photovoltaic array sizing and tilt angle are that:
x An increased tilt (i.e. greater than the latitude) gives more uniform all-year-
round generation, but at the expense of total annual output.
x It is good policy to select the array size and tilt angle together, aiming for the
photovoltaic generation on a clear summer day to be equal to the load.
x The array generation should not be too much below the load or else the diesel
will be required semi-continuously to supply the difference. (Not everyone
agrees with this aim.)
x As a guide, do not consider a hybrid system unless the inclusion of
photovoltaics offsets at least 30% of the estimated diesel’s operation and
maintenance costs (Jones & Chapman, 1987). (Refer to the hybrid indicator
of Chapter 7.)
x The system design should provide for infrequent use of the diesel, but for
relatively long periods of time on each occasion used, at close to full load.
x It is preferable for the array not to meet the load fully so as to optimise the
system cost and ensure the generator will be operated for a reasonable length
of time every two weeks.
x The system voltage should be selected in accordance with the inverter
characteristics and requirements. In general, this involves limiting the DC
current to 100 A maximum.

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