The projects that comprise ARPA-E’s DAYS (Duration Addition to electricitY Storage) program will develop energy storage systems that provide power to the electric grid for durations of 10 to approximately 100 hours, opening significant new opportunities to increase grid resilience and performance. Whereas most new energy storage systems today deliver power over limited durations, for example to alleviate transmission congestion, stabilize voltage and frequency levels, or provide intra-day shifts of energy, the extended discharge times of DAYS projects will enable a new set of applications including long-lasting backup power and even greater integration of domestic, renewable energy resources.

Project teams will seek to develop storage systems that are deployable in almost any location and charge and discharge electricity at a target fixed cost per cycle. Projects will fall into two categories: 1) DAYS systems that provide daily cycling in addition to longer duration, less frequent cycling and 2) DAYS systems that do not provide daily cycling, but can take over when daily cycling resources are either filled or depleted. DAYS projects will explore a new design space in electricity storage that allows for strategic compromise of performance to achieve extremely low costs. The program also seeks to establish new paradigms for increasing stored energy and extending duration of stationary electricity storage systems.

Stationary electrical energy storage plays several important roles in the U.S. electricity system, and these are expected to grow as the grid continues to evolve. Today, pumped storage hydroelectric (PSH) power provides more than 95% of stationary electricity storage capacity on the U.S. grid. With PSH, electricity drives pumps to fill a reservoir that is later emptied through turbines when extra energy is needed. Although the value of PSH is widely recognized, there have been few new installations in the United States in the last three decades, largely due to the difficulty of permitting new sites and financing large projects. Electrochemical batteries, particularly Li-ion, have recently experienced a rapid growth in deployment on the grid, but their high cost of energy limits their viability in long-duration applications.

Long-duration energy storage systems address grid needs beyond those covered by daily cycling. Such systems could provide backup power for several days or allow integration of even larger amounts of intermittent renewable sources like wind and solar. In the near term, such systems could help shape the output from individual wind and solar installations, improving the reliability of these resources and thus greatly increasing their value to the grid.

If successful, DAYS projects will provide new forms of long-duration stationary electricity storage systems that enhance grid resiliency, provide low-cost energy capacity, support grid infrastructure, and enable a greater share of intermittent renewable resources in the generation mix.