Systems and methods of long-duration energy storage and regeneration of energy-bearing redox pairs
Described herein are systems and methods of storing and delivering electrical using hydrogen at low-cost and for long-durations. The systems and methods use energy-bearing redox pairs that electrochemically bear energy through decoupled hydrogen and oxygen consumption and/or evolution reactions, which are typically associated with fuel cells. Each species of the energy-bearing redox pair is associated with a standard electrode potential within a water electrolysis voltage window for the electrolyte solution. Electrical energy delivery, hydrogen generation, electrolyte regeneration, or combinations thereof can be performed by logically or physically separated unit operations in a continuous manner, batch manner, or semi-batch manner facilitated by the energy-bearing redox pair.
1. A system for storing energy, the system comprising:
a first redox flow cell comprising:
a positive electrode side comprising a redox species dissolved in a liquid electrolyte solution;
a negative electrode side comprising a H + /H 2 half-cell, and
a proton permeable membrane separating the positive electrode and negative electrode sides, the first redox flow cell having a hydrogen generation mode and an electrical energy delivery mode;
a first electrolyte regeneration cell operatively coupled to the positive electrode side of the first redox flow cell, the first electrolyte regeneration cell comprising:
a reactor configured to react the liquid electrolyte solution comprising the redox species in a reduced state with an oxidizing agent to yield the redox species in an oxidized state and provide the redox species in the oxidized state to the positive electrode side of the first redox flow cell;
a second electrolyte regeneration cell operatively coupled to the positive side of the first redox flow cell, the second electrolyte regeneration cell comprising:
a photoreduction cell having a photo-sensitive reducing agent, wherein the photoreduction cell is configured to receive solar radiation and to react the liquid electrolyte solution comprising the redox species in an oxidized state with the photo-sensitive reducing agent to yield the redox species in a reduced state and provide the redox species in the reduced state to the positive electrode side of the first redox flow cell; and
a controller operatively engaged with the first redox flow cell, the first electrolyte regeneration cell, and the second electrolyte regeneration cell.
2. The system of claim 1 further comprising a circulation sub-system configured to transfer a first liquid electrolyte solution comprising the redox species in the oxidized state from the positive electrode side of the first redox flow cell to the photoreduction cell, and configured to transfer a second liquid electrolyte solution comprising the redox species in the reduced state from the photoreduction cell to the positive electrode side of the first redox flow cell.
3. The system of claim 2 wherein the second electrolyte regeneration cell further comprises a second redox flow cell comprising:
a negative electrode side comprising the redox species dissolved in the liquid electrolyte solution, the negative electrode side operatively coupled to the positive side of the first redox flow cell;
a positive electrode side comprising a H 2 O/O 2 half-cell; and
a proton permeable membrane separating the positive electrode and negative electrode sides, the second redox flow cell configured to reduce the redox species and yield O 2 .
4. The system of claim 3 further comprising one or more valves operatively aligned between the positive side of the first redox flow cell and both the photo reduction cell and the second redox flow cell.
5. The system of claim 2 wherein the circulation sub-system further comprises a first storage container configured to store a portion of the first liquid electrolyte solution and a second storage container configured to store a portion of the second liquid electrolyte solution.
6. The system of claim 1 wherein the controller is operably connected to the first redox flow cell and configured to select between the energy delivery mode and the hydrogen generation mode based on an energy-market condition.
7. The system of claim 6 wherein the energy-market condition comprises price of electrical energy supply, electrical energy demand, power grid health, H 2 price, H 2 demand, time of day, weather conditions, or a combination thereof.
8. The system of claim 1 wherein the redox species in the reduced and oxidized states comprise Fe 2+ and Fe 3+ , respectively.
9. The system of claim 1 wherein the oxidizing agent comprises oxygen.
10. The system of claim 1 wherein the reactor comprises a flow reactor.
11. The system of claim 1 wherein the redox species comprises iodine, vanadium, bromine, chlorine, or TEMPO.
12. The system of claim 1 further configured to operate in the energy delivery mode for a duration of 6 hours, 8 hours, 12 hours, 24 hours, or 48 hours.