HSSR was designed as a dispatchable renewables plant to be constructed with 34-individual 1 MWe AC blocks, each of which would be configured with 2 MWh of advanced flow battery storage. This configuration was a key design element providing for maximum flexibility in energy storage and dispatch, maximum system reliability, and the potential to demonstrate multiple advanced energy storage and smartgrid technologies. For instance, one block could be configured with flow-battery storage, another with nanophosphate battery storage, and yet another with an onsite propane-fired fuel cell, allowing for multiple parallel demonstration projects. Any of these technologies could be deployed in a tightly controlled and disciplined manner, minimizing capital costs while reducing risk.

The system’s modular design allowed for a “plug-and-play” approach of virtually all DC-based electrical generation or storage technologies as they became available. Such an approach was intended to have created a new U.S. model for rational and disciplined technology commercialization and deployment. However, for budgeting, proforma, and system design reasons, the HSSR team assumed that the initial storage technology would be flow batteries. Given the modularity of the entire system, Xcel was also given the option of storage being provided for on a portion of the 34 1-MWe solar blocks