USA: Southeastern Pennsylvania Transportation Authority is working with Viridity Energy, Saft and Envitech Energy Inc to test an energy storage system at Letterly substation on the Market-Frankford metro line in Philadelphia.

Viridity Energy has selected Saft's Intensium Max20 lithium-ion storage batteries to capture and store excess power produced by Septa trains under regenerative braking.

A DC-to-DC converter and control system supplied by Envitech is used to transmit regenerated energy between the third rail and the lineside substation.

Viridity's software then controls the reception, storage and redistribution of energy back to the third rail supply. The supplier claims its software permits the reception and discharge of energy at a very rapid rate, up to 1·5 MW for short periods.

The pilot programme is due to be commissioned by the end of 2011. Costing $1·6m, the trial is being financed by a $900000 grant from the Pennsylvania Energy Development Authority and funds from FTA's Tigger programme to promote energy efficiency.

According to Viridity, the batteries could reduce Septa's energy bill by up to $135000 a year per substation, with cumulative savings of 1500 to 1600 MWh/year; in the longer term the technology may generate revenue from the sale of excess energy back to the local electricity grid.