Page 261 - Hybrid-Renewable Energy Systems in Microgrids

238 Hybrid-Renewable Energy Systems in Microgrids

4.2.6 Hybrid methods

The optimal sizing of HRES techniques that combine two or more different methods
can be called as hybrid methods. This technique possibly is the best to solve multi-
objective design problems. There are two common methods available in the literature
for multi-objective problems. The first one is to merge the individual objective func-
tions into a single compound function and the second approach involves the determi-
nation of an entire Pareto optimal solution set [99].
In their work, Arnette and Zobel presented a multi-objective linear programming
(MOLP) model to determine the optimal combination of renewable energy sources
and existing fossil fuel facilities on a regional basis [100]. Another study [88] rep-
resents a methodology to optimally size the wind turbine generators, PV panels and
batteries to build an HRES that are optimal in terms of multiple criteria including cost,
reliability and emissions. Dufo-Lopez et al. suggested a triple multi-objective design
method of isolated hybrid systems minimizing, concurrently, the total cost throughout
the useful life of the installation, pollutant emissions and unmet load [98].

4.2.7 Computing tools
Simulation programs/ Software are the most common tools for evaluating the per-
formance of the HRES. Using the computer simulation, the optimum sizes of the
components of an HRES can be determined by comparing the performance and
energy production cost for different system configurations. Several software tools
are available for this purpose, such as HOMER, HYBRID2, iHOGA, HYBRIDS,
RETSCREEN etc.
One of the most popular tools for optimally designing the system components
of HRES is HOMER (The Hybrid Optimization Model for Electric Renewables),
which is designed by National Renewable Energy Laboratory. HOMER is very fre-
quently used for optimisation of hybrid renewable energy systems, both off grid and
grid connected [101]. This software can be performing multiple analyses and can be
helpful to address a wide range of design questions. It can be used to design most
cost-effective systems, for optimum sizing, economic analysis. It also can be used
to analyse the sensitivity of the system under changing conditions like, load varia-
tion, battery price etc. HOMER finds the combination of components with least cost
to meet specific load demand. The principal tasks are simulation, optimisation and
sensitivity analysis.

4.2.7.1 Simulation
HOMER simulates the operation of a system by making energy balance calculations
for each of the 8,760 hours in a year. HOMER compares the electric and thermal load
of every hour to the energy that the system can supply in that hour. HOMER also
decides for each hour whether to charge or discharge the batteries for systems. Should
the system meet the loads for the entire year, HOMER estimates the lifecycle cost of
the system, accounting for the capital, replacement, operation and maintenance and
interest costs.