A Bidirectional, Transformerless Converter Topology for Grid-tied Energy Storage Systems

Grid-tied energy storage systems provide stabilization to the electric grid that is necessary to accommodate the growing percentage of electricity generated by intermittent renewable energy sources such as wind and solar. Existing conversion systems between the storage media and the utility lines are large, costly and only 95%-98% energy efficient. In this ARPA-E project, Opcondys, Inc. will build, test and ready for market an innovative converter using a modified switched multiplier topology that will allow connection to utility transmission lines without intervening step-up transformers. It requires the use of a novel, photonic, wide bandgap power switching device called the Optical Transconductance Varistor. This is a fast, high voltage, bi-directional device which reduces the number of circuit elements required for charging and discharging the storage element. The MSM topology overcomes voltage limitations of other topologies now in use. Its 100 kHz operation increases efficiency to 99% compared to 95-98%, reduces the size of passive elements by 50%, and, because of the optical control, mitigates EMI issues. The elimination of step-up transformers further reduces system size and ensures cost is lower than existing systems. The reduced size and cost and increased energy efficiency of the proposed MSM converter will make adoption of grid-tied energy storage and, therefore, more renewable energy generation possible. This increases energy security while reducing carbon emissions. It also increases domestic production of energy, reducing reliance on foreign sources.