A 2MWh, prototype system was completed in November 2003 for PacifiCorp in Utah and handed over in March 2004. It operated as an unmanned site since then. The original intent was to use the VRB-ESS™ to defer capital expenditure line upgrades by shaving of the peak demand during certain periods of each day. The VRB-ESS was selected against other technologies for its ease of use, flexibility, and life as well as the fact that the area was very environmentally sensitive one and the VRB-ESS had no impact and very low risk. The site used the original SEI cell stacks and VRB Power Systems plant design. Availability was over 96% (one PCS card failure due to lightning strike in a 4 year period). Ambient temperature ranges over -25 to +55C so this has been a very challenging site for any technology. The system cycled daily over its full charge discharge range for over 1600 cycles. Some cell stack degradation was evidenced in the last year.

This was the first VRB installed system in the USA. It was a prototype from which a great deal was learned. Performance up time has in the words of EPRI been “incredible”. Design however showed two areas of weakness both of which have been addressed. The designs used a DC bus voltage of 300V instead of preferred 750V. This meant greater current and hence lower efficiency. Additionally the building was uninsulated with a vented roof which at 50 Celsius was a bad decision requiring a lot of cooling. This therefore lowered efficiency.

The new Prudent Energy designs operate with these two oversights corrected and 65 to 75% efficiencies can be achieved on a round trip basis. In addition newer cell stack designs have overcome the previous degradation issues.