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Experimental tests on storage systems in a distributed generation microgrid

pubblicazioni - Memoria

Experimental tests on storage systems in a distributed generation microgrid

Energy storage systems are generally present in distributed generation (DG) grids and are particularly important in renewable energy generators installations in order to decouple energy availability and demand and for the stability of a microgrid in islanding operation.

The paper presents the results of an experimental activity on different typologies of storage systems, based on electrical energy storage and fuel storage. The various storage technologies have been characterized in laboratory tests and afterward have been installed and connected to a low voltage grid test facility used for distributed generation studies and analysis. The test facility consists of conventional and innovative renewable energy generators, co-generators plants, fuel cells, controllable loads, traditional and innovative electrical energy storage systems including electrochemical and mechanical equipments, such as a lead acid battery, ZEBRA™ batteries, a high speed flywheel, a Vanadium Redox Flow battery. The distributed resources can be connected at different points of an automated LV grid working in radial, ring and meshed configuration. Experimental activity and tests results are described in the paper. In particular storage systems characterisation tests and performances verification at different operating and ambient conditions are reported, including systems reliability. Tests performed in test facility were also focused on energy storage systems management in a DG grid, demonstrating possible applications and evaluating systems performances both in islanding operation and connected to the main grid. In particular overall systems managing tools have been developed and tested for grid operation. The experimental results concerning a diagnostic tool for on-line estimation of the state of health (SOH) of VRLA and ZEBRA™ batteries are also reported. In addition to electrical energy storage systems, hydrogen storage systems have been designed and tested, coupled to PEM fuel cell and connected to the DG test facility. Hydrogen storage systems developed in our laboratories use metal hydride technology (La-Ni or Mg based) that allows hydrogen storage at low pressures. Different storage tanks with hydrogen capacity from 150 to 6500 Nl have been designed and tested. In particular a storage system with a volume of about 30 l and filled with 50 kg of metal hydride (LaNi based), operating at a pressure between 1 to 6 bars and in a temperature range between 10 and 70°C, characterised by a hydrogen storage capacity of 6500 Nl (energy storage of about 19 kWh) has been developed. The same hydrogen capacity can be obtained with a 30 l compressed bottle operating at about 200 bar. The storage system has been previously tested in a stand-alone configuration verifying hydrogen charge and discharge cycles characteristics at different pressure and temperature conditions and evaluating negligible performance degradation after about 100 cycles. Afterward hydrogen storage system has been connected to a 3 kW PEM fuel cell in order to demonstrate that fuel cell heat losses can be recovered and used by the storage system during hydrogen discharge phase. Several tests at different fuel cell load conditions have been tested; storage system allowed operating fuel cell at nominal load for about 3 hours. The work has been financed by the Ministry of Economic Development with the Research Fund of the Italian Electrical System under the Contract Agreement established with the Ministry Decree of March 23, 2006. KEYWORDS Distributed generation, electrical energy storage, hydrogen storage, fuel cell, renewable energy, test facility, peak shaving, power quality, SOH – State Of Health.

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