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Solar–wind hybrid renewable energy system 223

the system components, a high number of variables and parameters that need to be
considered for the optimum design and also because of the fact that the optimum
configuration and optimum control strategy of the system are interdependent. This
complexity makes the HRES more difficult to be designed and analysed [10].
Proper sizing of HRES is another important factor that needs to be considered;
oversized or undersized system might lead to increased establishment cost or inability
to supply the load demand. Intense research is being undertaken to design and improve
the performance, reliability and prediction accuracy of the output power of HRES to
make them more acceptable [14].
As mentioned earlier, in the case of grid connected HRES, if the power gener-
ated by the RES is less than the load demand, the excess power requirement can be
supplied by the gird. In case the power generated by the RES is more than the load
demand the excess energy can be sent to the grid. The standalone HRES does not
have these advantages. Hence, before designing any standalone HRES system for any
application, there are certain facts that one should consider. These are:

• It is important to make sure that the targeted location is suitable for setting up a RES. For
example, if someone is considering to design some standalone RES comprising of solar
panels and storage, they need to make sure that the average solar irradiation of that location
is reasonably good. Otherwise, the solar panels would not be able to work efficiently or
generate the expected amount of energy. Similarly, to consider wind generators for certain
location it is important to ensure that the average wind speed of that location is good enough
for the wind generator to start generating power.
• When several renewable energy sources are available at the prospective region, the first step
in planning some RES is the selection of appropriate RES technology or technologies which
can improve the cost effectiveness and reliability of the system.
• Generally, the lifetime of such systems are around 25 years. While designing some stand-
alone RES one must consider the increasing load demand in modern life. So, a load forecast-
ing of coming 25 years is very important for the designed system to work efficiently for its
entire lifespan.
• In comparison with energy systems with only one type of source, the HRES has increased
complexity. Due to this reason, the optimum design of a hybrid system becomes complicated
through uncertain renewable energy supplies and load demand, non-linear characteristics of
the components, a high number of variables and parameters that must be considered for the
optimum design [10]. Another fact is that the optimum configuration and optimum control
strategy of the system are interdependent. This complexity makes the hybrid systems more
difficult to be designed and analysed. To efficiently and economically utilize the RES, op-
timum sizing is necessary. This is extremely important to avoid inability to meet the load
demand and increased establishment cost.
• While designing these HRES, one must consider the fact that the dynamic interaction be-
tween load demand and the HRES can lead to critical stability and power quality problems
[14]. Hence, the power flow management in HRES is very important to ensure the continu-
ous energy flow between system components. This is also required to increase the operating
life of the HRES and to ensure the quality of energy flow.
Several studies have been conducted by the researchers around the world to
improve the performance and efficiency of the HRES. Researchers have developed
different methods of the feasibility study, load forecasting, system modelling, optimally