Page 112 - Advances in Renewable Energies and Power Technologies

4. Cost of Intermittence and Benefit of Forecasting 85

As a result, the overall relevant horizons of forecast should range from 5 min to
several days [30]. It is apparent that the time step of predicted data (daily or hourly
energy, 10-min or 20-min energy, etc.) varies depending on objectives and on fore-
casting horizon [25].

4. COST OF INTERMITTENCE AND BENEFIT OF FORECASTING
As seen before, deleting or reducing the “unpredictable” variations of ISRES re-
quires the use of energy storages and back-up energy production means able to
compensate immediately the power variations. As a consequence, backup generators
must often remain switched-on to maintain the production/consumption balance
promptly. Besides, PVand wind energy systems should be switched off, for example,
when their electrical production exceeds a certain percentage of the global produc-
tion (ISRES power curtailment). Such difficulties entail an additional production
cost compared with conventional production. The accurate prediction of ISRES
intermittence would represent a cost-effective access to these energy resources.
As the cost of the intermittence of solar and wind resources is quite important
[33,34], a reliable forecasting of these intermittences should not only allow to
manage more efficiently the overall electrical system but also significantly reduce
the negative cost impact of these ISRES on the electrical network; at last, the
cost-effectiveness of PV and wind energy systems is increased.
Evaluation and forecasting of ISRES power appears to be an essential tool for
renewable energy power plant developers to identify the location where ISRES
could be installed and to decrease the use of conventional electricity production
means as much as possible while optimizing the profitability of ISRES.

4.1 COST OF INTERMITTENCY
There are multiple reasons why the ISRES power variations are very troublesome:
because the purchase electricity contracts are set up in advance, because back-up
generators must be stopped or switched depending on the ISRES production varia-
tions, because some of them have to be operating even without producing to
compensate instantaneously the short production variations; all of them contribute
to the cost linked to variability.
Besides, ISRES production remains uncertain until the last moment, and as elec-
tricity trading takes place the day before delivery, the deviations between forecasted
and actual production have to be balanced on short notice, which is costly as well
[35]. Electrical systems need to incorporate additional flexibilities (new operational
practices, storage, demand-side flexibility, flexible generators, etc.) to adapt them-
selves to the constraints because of the variability of renewables.
The integration costs are evocated as “an increase in power system operating
costs” [36], as “the additional cost of accommodating wind and solar” [37],as
“the extra investment and operational cost of the non-ISRES part of the power