Page 51 - Energize August 2021

TECHNICAL

Standby generators are not used on all required with any renewable resource. Because solar PV is generally the most stable source
systems, and systems can operate in an in the hybrid system, the degree to which wind can complement PV is a common criterion for
off-grid mode. complementarity. In designing a high reliability hybrid system three types of complementarities
The most common power sources need to be considered:
are photovoltaic (PV); wind turbines and
hydropower, combined with different Daily: The resources complement one another over a 24-hour cycle as shown in Figure 2. This
auxiliary fuel-based sources such as diesel is the most basic requirement for a hybrid system. It is normally found in solar/wind hybrids
generator sets, microturbines or fuel cells. where the wind power exhibits a high value at night compared to its daytime value. The daily
The control unit controls the generation, performance during bad weather needs to be included in any evaluation.
storage and dispatch of electricity, as
well as the connection to the grid where Seasonal: The resources complement one another over a seasonal cycle: where one is high
applicable. The unit can provide such in summer and low in winter, the other is low in summer and high in winter (see Figure 3).
services as maximum power point tracking, Seasonal complementarity is more effective when the daytime wind pattern is complementary
generation control and curtailment, charge to the solar pattern. South African weather does not exhibit this pattern, with aggregated
control and load management. In smaller wind and solar both high in summer and low in winter, although individual sites may exhibit a
systems the controller and inverter/grid different pattern.
connection form a single unit.
Storage systems generally consist of
battery banks, with other systems such
as flow batteries gaining in popularity.
Emerging storage technologies include
hydrogen and compressed gas (air or
carbon dioxide) energy storage. With
smaller systems, battery banks are the
technology of choice.
The inverter converts a DC input to an
AC output of the correct frequency and
voltage, and also controls the connection
to the grid and standby plant where
applicable. The controller/inverter may be
designed to serve two types of load, viz.,
primary and deferrable loads. The primary
load is the electrical load which must be
met at a specified time, such as IT, lighting,
etc.), while the deferrable load is the load
which need not be met within a specified
Figure 2: Daily complementarity (QTE energy)
time but could be deferred until later (e.g.,
water pumping). This facility is used in
combination with the controller to provide
load management.

Complementarity of renewable
resources
A basic requirement for a hybrid RE
system to be successful is that the energy
sources need to complement each other
or have a high degree of complementarity.
Resources are generally considered to be
complementary when the performance or
availability of one is high when the others
are low. Combining sources which are
supplementary rather than complementary
does not yield any benefit other than a
possible improvement in security of supply.

Complementarity could be a characteristic
of the natural resources or could be
manufactured, as in the case of storage.
Storage can be configured and controlled
to give any degree of complementarity Figure 3: Seasonal complementarity 5

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