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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Storage & grid integration' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Fragiskos Mouzakis Center for Renewable Energy Sources and Savings (CRES), Greece

(1) Center for Renewable Energy Sources and Savings (CRES), Pikermi, Attiki, Greece (2) Aalborg University, Aalborg, Denmark (3) Aalborg University, Aalborg, Denmark

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Optimal sizing and operation of stand-alone hybrid power systems: case study of Agios Efstratios


Most of the inhabited Greek islands are not connected to the central electricity grid and their energy needs
are satisfied by autonomous diesel power stations.
The operation of such stations arises a number of economic and environmental issues.
The replacement of conventional stations with hybrid ones that combine renewable energy technologies with energy
storage systems will provide a promising clean energy generation alternative and lessen the dependence on fuel prices.
The aim of this study is the optimal sizing and operation of such a hybrid power system in the Greek island
Agios Efstratios.


The hybrid power system needs to be designed in a way that satisfies a number of technical,
economic and policy requirements. Renewable energy penetration along with system stability and cost,
are the main optimization parameters.
Analytical steady-state and dynamic models of wind, solar, diesel and energy storage (ES) units
are developed in HOMER and DIgSILENT Power Factory. Simulations are carried out based
on existing network parameters and measured time series of power demand, wind speed and solar irradiation.
Simulation results indicate the optimal unit configuration, capacity and location.

Main body of abstract

The study begins with identifying the suitable technologies for the islanded power system.
Due to the abundant wind and solar power potential of the island, wind turbines (WTs)
and photovoltaics (PVs) are chosen as the main energy sources, while electrochemical ES methods are strong
candidate solutions due to their high energy density and flexibility. The installed power of the conventional station
is 840 kW and covers 1221 MWh of annual demand. The capacity range considered for the optimization
analysis is up to 300 kWp for PVs, 800 kW for WTs and tens of MWh for the ES unit. The capacity for each of
the above RES and ES systems is calculated through optimization of the technical constraints and the
economic parameters included in the Net Present Cost (NPC) function. Afterwards, steady-state analysis
of the existing network topology is carried out. The static model is
developed in DIgSILENT simulator and the system’s behavior is investigated (voltage profile, loading
of lines and transformers). Moreover, this study discusses the issue of optimal placement for the RES and
ES units. The main criterion which is used for this evaluation is the improvement of steady-state voltage
magnitude and minimization of power losses. The final part of this study analyzes the system’s stability.
A dynamic model is constructed based on DIgSILENT built-in models. The results illustrate the ES’s
capability to manage the frequency and voltage variations under various cases of generation/load unbalance,
highlighting the importance of integrating them in systems with high RES penetration.


The results of this study show that a high RES penetration scenario is technically and economically
feasible and the proposed hybrid system consists of a 300 kW wind turbine and 100 kW solar
array. Larger capacities can be considered as well if higher penetration levels are desirable, according
to the operational strategy. Integration of a fast responding electrochemical
storage unit can improve short-term stability. Among the available ES technologies, lead-acid batteries
are the most economic solution. Moreover, distributed generation units should be placed close to the load
centers in order to improve voltage profile and minimize power losses.

Learning objectives
- Evaluation and comparison of suitable technologies for microgrid applications
- Cost optimization for hybrid renewable-based power systems
- Optimal placement of distributed generation units and its impact in the network
- Assessment of the dynamic behavior of stand-alone systems under different generation/load conditions and contingencies
- Evaluation of the effect of RES penetration and ES operation in dynamic stability