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Duration Addition to electricitY Storage

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.
For a detailed technical overview about this program, please click here.  

Antora Energy

Solid State Thermal Battery

The Antora Energy team will develop key components for a thermal energy storage system (solid state thermal battery) that stores thermal energy in inexpensive carbon blocks. To charge the battery, power from the grid will heat the blocks to temperatures exceeding 2000°C (3632°F) via resistive heating. To discharge energy, the hot blocks are exposed to thermophotovoltaics (TPV) panels that are similar to traditional solar panels but specifically designed to efficiently use the heat radiated by the blocks. The team will develop a thermophotovoltaic heat engine capable of efficiently and durably converting high-temperature heat into electricity. It will seek to double panel efficiency through new materials and smart system design, potentially enabling a cost effective grid storage solution.

Brayton Energy

Improved Laughlin-Brayton Cycle Energy Storage

The Brayton Energy team will develop a key component to enable a cost-competitive Laughlin-Brayton battery energy storage system that combines thermal storage and innovative turbomachinery to generate power. When the system is charging, an electrically driven heat pump will accumulate thermal energy in a high temperature thermal energy storage medium. During discharge, electricity is produced by heating a gas using the stored thermal energy and sending it through the generation turbine that drives an electric generator. Brayton Energy's innovation lies in its reversing, counter-rotating turbine design, in which each turbomachinery stage is designed to act as both as a compressor and turbine, alternating between charging and discharging cycles. This approach greatly simplifies the Laughlin-Brayton battery system, improves its efficiency and operability, and reduces the capital cost.

Echogen Power Systems (DE), Inc.

Low-cost, long duration electrical energy storage using a CO2-based Pumped Thermal Energy Storage (PTES) system

The Echogen Power Systems team will develop an energy storage system that uses a carbon dioxide (CO2) heat pump cycle to convert electrical energy into thermal energy by heating a "reservoir" of low-cost materials such as sand or concrete. During the charging cycle, the reservoir will store the heat that will be converted into electricity on demand in the discharge or generating cycle. To generate power, liquid CO2 will be pumped to a supercritical pressure and brought to a higher temperature using the stored heat from the reservoir. Finally, the supercritical CO2 will be used to expand through a turbine to generate electricity during the discharge cycle.

Form Energy, Inc.


Form Energy will develop a long-duration energy storage system that takes advantage of the low cost and high abundance of sulfur in a water-based solution. Previous MIT research demonstrated that aqueous sulfur flow batteries represent the lowest chemical cost among rechargeable batteries. However, these systems have relatively low efficiency. Conversely, numerous rechargeable battery chemistries with higher efficiency have high chemical costs. The solution requires low chemical cost, high efficiency, and streamlined architecture. The team will pursue several competing strategies and ultimately select a single approach to develop a prototype system. Focus areas include developing anode and cathode formulations, membranes, and physical system designs.

Michigan State University

Scalable Thermochemical Option for Renewable Energy Storage (STORES)

The Michigan State University team will develop a modular thermal energy storage system that uses electricity from sources like wind and solar power to heat up a bed of magnesium manganese oxide (Mg-Mn-O) particles to high temperatures. Once heated, the Mg-Mn-O will release oxygen and store the heat energy in the form of chemical energy. Later, when additional power is needed, the system will pass air over the particle bed, initiating a chemical reaction that releases heat to drive a gas turbine generator. The low cost of magnesium and manganese oxides will enable the system to be cost competitive.

National Renewable Energy Laboratory

Economic Long-Duration Electricity Storage by Using Low-Cost Thermal Energy Storage and High-Efficiency Power Cycle

The National Renewable Energy Laboratory team will develop a high-temperature, low-cost thermal energy storage system using a high-performance heat exchanger and Brayton combined-cycle turbine to generate power. Electric heaters will heat stable, inexpensive solid particles to temperatures greater than 1100°C (2012°F) during charging, which can be stored in insulated silos for several days. To discharge the system, the hot particles will be fed through the fluidized bed heat exchanger, heating a working fluid to drive the gas turbine attached to a generator. The electricity storage system is designed to be deployed economically anywhere in the United States.

Primus Power

Minimal Overhead Storage Technology for Duration Addition to Electricity Storage

The Primus Power team will work with the Columbia Electrochemical Energy Center to develop a long-duration grid energy storage solution that leverages a new approach to the zinc bromine battery, a popular chemistry for flow batteries. Taking advantage of the way zinc and bromine behave in the cell, the battery will eliminate the need for a separator to keep the reactants apart when charged, as well as allow all the electrolyte to be stored in a single tank, instead of multiple cells. This reduction in "balance of plant" hardware will reduce system cost.

Quidnet Energy Inc

Geomechanical Pumped Storage

The Quidnet Energy team will develop a modified pumped hydro energy storage system that stores energy via high-pressure water in the subsurface. To charge, the team will pump water into confined rock underground, creating high pressures. When energy is needed later, the pressure forces water back up the well and through a generator to produce electricity. The Quidnet team will demonstrate the reversibility of this process and the ability to translate it across multiple types of geography within the U.S.

United Technologies Research Center

High-Performance Flow Battery with Inexpensive Inorganic Reactants (P.400.0618)

The United Technologies Research Center team will develop an energy storage system based on a new flow battery chemistry using inexpensive and readily available sulfur and manganese based active materials. The team will employ innovative strategies to overcome challenges of system control and unwanted crossover of active materials through the membrane. The affordable reactants, paired with the unique requirements for long-duration electricity discharge, present the opportunity for very low cost energy storage.

University of Tennessee

Reversible Fuel Cells for Long Duration Storage

The University of Tennessee, Knoxville team will develop an energy storage system based on an innovative electrolyzer/fuel cell combination. Typically, fuel cells produce water from hydrogen and oxygen. The Tennessee team will instead use the fuel cell to produce hydrogen peroxide, a liquid that can be stored. When extra power is needed on the grid, the fuel cell will produce peroxide and electricity. Available electricity then can be used to convert the peroxide back to hydrogen and oxygen during the charging cycle, which can be stored for future use. The benefit of using peroxide rather than water is higher efficiency in both charging and discharging the system.
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