Page 62 - Energize August 2022

TECHNICAL

were reduced further, but plant was retained to cater for inclement Solar PV
weather (which does happen). The final addition for very large Solar PV is the general choice of renewable energy in off-grid
installations has been megawatt-sized wind turbines. hybrids, mainly because of the small modular size, allowing fairly
close matching of load and generation. The challenge with solar
Mine energy needs PV is not the size of the system, but the variation in output with
Hybrid power systems produce electricity, but the energy weather conditions. The pattern of solar radiation will depend
requirements of a mine extend beyond electricity. In addition to on the conditions at the site and may include several days of low
mine plant operation such as mine hoists, compressed air, pumps, solar radiation. Any RE system has to cater for the worst-case
ventilation, crushers and ore processing, there is a need for energy situation.
for motive power and heating. Figure 2 shows typical power needs. Solar PV is a periodic, intermittent source with a varying peak
Most hybrid systems currently used in mines consist of a value. A typical solar PV system consists of an array that collects
combination of ICE generators, solar PV installations and battery the full day’s energy requirement and a BESS that stores energy
electricity storage systems (BESS). A few large mines use wind for use outside sunlight hours. The challenge here is not only
generation as well. Motive power and heating usually use liquid to provide sufficient storage for a full day’s requirement but to
fuel as well. provide for days when solar output is below normal.
Under normal conditions the BESS will need to store 12 to
18 h of energy. However, under inclement weather conditions,
the PV output can drop in excess of 50%. Figure 3 shows the
aggregated utility PV generation for systems connected to the SA
grid for a ten-day period where the output is lower than normal.
(Individual sites are likely to vary to a greater extent).
In an off-grid system without ICEs the BESS will be required
to provide the deficit. In this example a total of approximately
2,5 times the daily requirement will have to be provided by the
BESS. If the BESS normally stores 50% of the daily requirement,
Figure 2: Mining energy needs 1 this means an increase in size of 500%. If the BESS normally stores
75% of the daily requirement, this will still require an increase in
Emission free hybrids (100% renewable) size of 330%. Depending on the weather pattern for most of the
The aim of many mining companies is to achieve emission free year the excess storage will not be used. In addition, the PV array
mining, which means zero use of fossil fuels. This implies that the will have to be oversized to provide excess energy to charge the
total energy requirements of the mine should be met by renewable excess storage. This all increases the cost of the installation and
energy sources. Emission-free means either 100% renewables or may make a PV/BESS system economically unviable.
using a renewable fuel for the ICE plant. To supply a mine with
100% renewable energy, the energy system should be able to meet
the mine’s total energy demands, including all forms of electricity,
motive and heating requirements. The inability to meet the
mine’s total energy demand is a potential obstacle that prevents
successful implementation of standalone renewable energy
systems in mining industries. 1
Any power system requirement can be met if enough money is
thrown at it. The challenge here is to provide a system that will meet
the energy demand at all times at an affordable cost. Most systems
today use some form of ICE generation together with renewables.
Although renewable energy is increasing as a component
of hybrid systems, the barrier to greater use is stochastic and Figure 3: Record of the aggregated utility PV generation (Max. 2000 MW)
unpredictable weather behaviour. Both solar and wind power for systems connected to the SA grid for the period (Eskom)
are affected by weather which varies depending on the location.
Because of this intermittent nature of renewables, a single Wind generation
renewable energy source tends to be problematic in terms The attraction of wind is that supply is continuous throughout
of energy yield and operational cost, and it is necessary to the day although it can be highly variable. Wind turbines produce
complement renewables with an additional source such as ICEs or energy at night, and where this is a primary supply, the amount
BESS. The limitations of renewable energy and the dependence of storage needed is less than the equivalent sized solar. The
of systems on the vagaries of weather ensure that ICE electricity problem with wind as a component of a hybrid system is twofold:
generation will remain a major component of off-grid hybrid power • Modularity (granularity): Wind turbines come in two ranges -
systems for many years to come. small (kW) and very large (MW) size.

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