Page 63 - Energize August 2022

TECHNICAL

• Variation: Wind also suffers from long periods of low generation, This results in curtailment on normal days which increases
and unless wind on site-speed studies show that a consistent the cost of generation. Oversizing will also result from seasonal
average could be maintained, should be approached with caution. variations, as meeting the demand in the low generation season
Figure 4 shows the aggregated output of the wind farms connected will result in overproduction in the high generation season. Surplus
to the Eskom grid. On day 21 the total wind output dropped to 1% electricity cannot be used for any other function, so curtailment
of the max output for a period of 1hr, and remained below 5% for a results, which increases the unit cost of RE. Design requires a balance
period of 16 h, and below 10% for a period of 24 h. between the cost of storage and the cost of oversized generation.
All of these factors make the use of BESS to cater for long
periods of low RE output economically inviable and make non-
renewable fuel-driven generating sources an essential component
in most hybrid power systems today.

Figure 4: Aggregated grid wind power (Eskom)

A wind-powered system would also require additional storage
to cater for low days. If the maximum output of the system is M Figure 5: Effect of oversizing of RE plant by 50%
MW and the average value 30% of maximum, the stored energy
required to make up this deficit would be 0, 2 I *24 = 4,8 M MWh. The fuel cell (FC) hybrid system
Nonetheless there are systems under construction that use The hydrogen energy storage system (HESS), consisting of
wind power as a component the viability of wind/BESS depends electrolyser/storage system/fuel cell (E/FC), is set to become a
on site conditions. A common argument is that wind and solar game changer in the field of hybrid power systems. Hydrogen
complement each other (high/low) but although some sites show storage systems can help mitigate the problem with overgeneration
this, many others exhibit days when both wind and solar are low. and storage and facilitate the application of a standalone
The additional stored energy has to come from somewhere and the renewable energy system. The possibility of producing “green”
solar and wind arrays have to be oversized to cater for this. hydrogen using renewable energy from PV systems and the
development of fuel cell technologies has opened a new possibility
Storage for hybrid power systems, that of the PV/E/FC hybrid. FCs are
Although several cases of successful implementations have been available in multi-megawatt capability and produce heat as well as
reported in mining literature, clean energy systems have generally electricity. Hydrogen FC emission consists of water, a non-polluting
not been able to provide the power for off-grid mining to date. substance. The HESS could become the alternative to batteries as
Storage has become a prime component in hybrid power systems the bulk energy storage system. Figure 6 shows a possible layout of
for mining, and the high cost and short lifetime of BESS storage a PV/E/FC system.
systems are among the main restrictive factors in achieving 100%
power from RE systems. Usually, BESS represents a sizable portion
of a standalone system’s overall cost and needs to be replaced
throughout the mine’s lifetime1.
The additional storage capacity required to cater for low
RE days will only be used for a small portion of the year, thus
increasing the cost of the stored energy. Keeping batteries at a
constant charge (or discharge) level for long periods is unhealthy
and will affect the performance of the battery.

Curtailment and oversizing
A common approach to the problem of periods of low RE production
is to oversize the renewable source so that the generation on bad
days is increased, reducing the storage required (Figure 5). Figure 6: Layout of a PV/electrolyser/fuel cell hybrid system

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