NASA-Redox cell-stack shunt current, pumping power, and cell-performance tradeoffs

The NASA Redox energy storage system has been under active technology development since the mid 1970's. The hardware currently undergoing laboratory testing is either 310 cm/sup 2/ or 929 cm/sup 2/ (0.33 ft/sup 2/ or 1.0 ft/sup 2/) per cell active area with up to 40 individual cells connected to make up a modular cell stack. This size of hardware allows rather accurate projections to be made of the shunt power/pump power trade-offs. The modeling studies that have been completed on the system concept are reviewed along with the more recent approach of mapping the performance of Redox cells over a wide range of flow rates and depths-of-discharge of the Redox solutions. Methods are outlined for estimating the pumping and shunt current losses for any type of cell and stack combination. These methods are applicable to a variety of pumping options that are present with Redox systems. The results show that a fully developed Redox system will have acceptable parasitic losses when using a fixed flow rate adequate to meet the worst conditions of current density and depth of discharge. These losses can be reduced by about 65% if variable flow schedules are used. The exact value of the overall parasitics will depend on the specific system requirements of current density, voltage limits, charge, discharge time, etc.