Page 55 - Energize August 2021

TECHNICAL

up to have enough capacity to serve the
load. The case presented above considers
a significant amount of solar penetration.
In order to make the system more
resilient and stable to the intermittent nature
of solar fluctuations, a battery storage
system is added to the system. There are
various roles a battery storage system
can play, including PV output smoothing,
frequency control and load shifting.
Further, the battery storage system serves
as a spinning reserve replacement when
switching off engines is required, to cover
unforeseen events or to provide sufficient
time to start gensets. Consequently, its
capacity would need to be added to the
blue dotted line shown in Figure 2.
Figure 3: Some of the results from the case study with solar capacity of (a) 10 MW; (b) 20 MW; (c) 30 MW; (d) 50 MW
The battery’s main function is to
smooth the solar output and keep
engine operation and ramping within Case study analysis
operational requirements. The steep spikes A case study was carried out on the above-mentioned off-grid hybrid system to assess the
(represented in light yellow) show an electricity cost reduction potential when hybridising existing HFO genset based power. The
exemplary fluctuation of solar power due study was conducted for a fictious mining site in Burkina Faso with a total load demand of 30
to a small cloud cover, which is absolved MW. This load demand was originally served by seven HFO gensets of 5 MW each.
by discharging the battery. The genset The sizes of the solar and battery systems were varied and the renewable energy (RE-
operation therefore only sees the smoothed share) penetration and levelised cost of electricity (LCOE) produced by the overall system
output of the hybrid system represented by including HFO cost of electricity, were accessed as the key performance indicators. It was
the red line. assumed that any realistic size of solar and the battery systems could be integrated with the
From Figure 1 it can be seen that the existing gensets and all technical requirements and possible constraints can be managed.
battery and the solar plant are a combined Figure 3 shows some of the example cases which were considered during the case study.
system and any power produced from the The size of the solar system is increased as the percentage of load, from 30% to over 150%
solar plant is smoothed through the battery in these cases. At higher solar penetrations, the remaining HFO gensets are not sufficient to
storage system by a hybrid controller. This stabilise the grid, as too many gensets are switched off. Hence batteries must act as a grid-
hybrid controller decides if the battery forming component to the system with sufficient power and capacity to ensure reliable and
shall be charged or discharged or if and stable operation.
when genset capacity needs to be added Further, the HFO fuel system needs to be kept in hot conditions for genset operation.
or switched off. Another key element of a The required heat is usually provided through gensets under operation. In scenarios where all
hybrid system is solar power forecasting. gensets are switched off, an electrical heater system is powered by the solar-battery hybrid
The above-mentioned hybrid controller plant. At very high solar penetration and thus RE-share as seen in Figure 3(d), the battery
requires a forecasted solar power to also provides load shifting services in addition to the smoothing and grid stability. Excess
efficiently determine a suitable generator solar energy is used to charge the battery which can be discharged during the evening hours.
dispatch schedule and identify any loss While assessing the economics of the system, various assumptions must be made. Table 1
of solar power production sufficiently in summarises the technical and financial assumptions which were considered during this study.
advance to switch on gensets. In the case
of HFO based gensets, this is vital since
it can take 10 to 30 minutes to start an Parameter Value Units
engine. Without solar power forecasting, Total load demand 30 MW
the decision whether a genset can be
Number of HFO gensets 7 –
switched off or would need to be online
Derated capacity of each genset 5 MW
in 15 minutes cannot be made. The
Lifetime of the mine 10 Years
forecasting enables this decision and helps
to maximise fuel saving through efficient Mine availability 93 %
genset operation. HFO fuel price 0,64 US$/litre
There are different types of forecasts OPEX cost escalation 2,5 %
available depending on the required
Annual solar irradiation 2155 kWh/m2
forecast duration. For up to 30 minutes, a
Specific yield for single axis tracker system 2095 kWh/kWp/year
sky imager is required, whereas satellite-
Solar system CAPEX 720 US$/kWp
based forecasting combined with digital
weather prediction can efficiently forecast Battery CAPEX (1C rated) 440 US$/kWh
from 30 minutes to several days ahead. Table 1: Main assumptions considered during the case study

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