A case study presented by George Dafermos et al. in the context of transition projects towards renewable energy infrastructures:
“Kythnos is a small island in the Aegean sea in Greece. As is typical of islands in general, Kythnos is cut off from the national grid on mainland Greece. It has its own island grid, but this does not, however, have the capacity to electrify all settlements on the island. Thus, in the framework of two European Commission projects (PV-MODE, JOR3-CT98-0244 and MORE, JOR3CT98-0215), a microgrid was installed in 2001, which has since provided electricity for 12 houses in a small valley that is about 4km from the closest medium voltage line (Hatziargyriou et al., 2007: 80-82; Tselepis, 2010). The system, which was designed and implemented by the Athens-based Centre for Renewable Energy Sources and Saving (CRES), Kassel University and SMA, comprises 10kW of photovoltaic generators, a battery bank and a diesel genset. These are coordinated by intelligent load controllers, which were designed and installed by the National Technical University of Athens. The same team of engineers from the National Technical University of Athens provided the members of that community with training on how to operate the technological infrastructure. Being one of the very first pilot installations in Europe, the project has been frequently cited as an example of a cost-effective and environmentally sustainable way of providing a small community with electricity through a model of energy generation at the site of demand using renewable sources.
In more technical detail, the roll-out of the project was premised on the installation of a 1-phase microgrid composed of overhead power lines and a communication cable running in parallel. The grid and safety specifications for the house connections respect the technical solutions of the Public Power Corporation, which is the local electricity utility. The reason for such a decision was taken on the grounds that in the future the microgrid might be connected to the island grid. The power in each user’s house is limited by a 6 Amp fuse. The settlement is situated about 4km away from the closest pole of the medium voltage line of the island. A system house of 20m2 was built in the middle of the settlement in order to house the battery inverters, battery banks, diesel genset and its tank, computer equipment for monitoring and communication hardware.
The grid electrifying the users is powered by 3 Sunny-island battery inverters connected in parallel to form one strong single-phase grid in a master-slave configuration, allowing the use of more than one battery inverter only when more power is demanded by the consumers. Each battery inverter has a maximum power output of 3.6kW. The battery inverters in the Kythnos system have the capability to operate in both isochronous or droop mode. The operation in frequency droop mode gives the possibility to pass information to switching load controllers in case the battery state of charge is low, as well as to limit the power output of the PV inverters when the battery bank is full.
The users’ system is composed of 10kWp of photovoltaics divided in smaller sub-systems, a battery bank of nominal capacity of 53kWh and a diesel genset with a nominal output of 5kVA. A second system with about 2kWp, mounted on the roof of the system house, is connected to a Sunny-island inverter and a 32kWh battery bank. This second system provides the power for the monitoring and communication needs of the components. The PV modules are integrated as canopies into some of the houses.
To recap, the case of the implementation of the microgrid on the island of Kyhtnos illustrates a model of distributed energy that has enabled a small, isolated community to become energy-autonomous in an ecologically and economically sustainable fashion.”