Flow Energy Storage

December 12, 2012

The principal problem of most renewable energy sources is that their energy supply is not constant. Hydroelectric and tidal power plants are renewable energy sources with fairly constant output, but you can't get solar energy at night, and you can't get too much of it on cloudy days. Wind turbines will supply power at night, but only when the wind blows. You need to store energy so it will be available for use later.

Nineteen megawatts peak by day, but zero megawatts at night. A photovoltaic system near Thüngen, Bavaria, Germany. (Photograph by OhWeh, via Wikimedia Commons).

In a previous article (Flow Batteries, July 18, 2012), I reviewed the flow capacitor, an interesting energy storage technology for load balancing in renewable energy systems. A flow capacitor stores electric charge in a fluid from which power can be later extracted. The flow capacitor is actually a variation of a similar device called a flow battery.

A standard electrochemical battery stores energy in the potential energy of the chemical reactions it contains. In most batteries, the electrodes participate in these reactions, but there are some reactions that involve only the electrolyte solutions, so the electrode materials are not consumed. In a flow battery, you can separate the charge/discharge portion from the energy storage portion. Your energy can be stored in a storage tank.

Scientists at Harvard University's School of Engineering and Applied Sciences are developing flow battery systems under an innovation grant from the U.S. Department of Energy through its Advanced Research Projects Agency–Energy (ARPA-E) program. ARPA-E is presently funded at $130 million to advance innovative energy technologies.[1]

The Harvard team, led by Materials Science professor, Michael Aziz, previously developed a hydrogen-chlorine regenerative fuel cell, which can function also as a flow battery (see figure).[2] Chlorine, of course, is a difficult chemical. It will even dissolve platinum, a common electrode material, by forming chloroplatinic acid (H₂PtCl₆). The HCl system was chosen over the typical hydrogen-oxygen system of fuel cells, since the chlorine system is much more efficient, the principal reason being that the chlorine reaction involves just a single electron.[3]

This HCl cell used a carbon electrode coated with a (Ru_0.09Co_0.91)₃O₄ catalyst at the chloride side, and a platinum electrode at the hydrogen side. The proton exchange membrane (PEM), typically Nafion, conducts H⁺ ions (generally called, but not precisely, protons) in both directions.

The peak power density was greater than a watt per square centimeter of electrode area, and the cell could be run with 90% efficiency at 0.4 W/cm².[2] It was possible to produce cells having less than 0.15 milligrams of Ru per square centimeter of electrode area;[2] but the presence of the expensive metals, Ru and Pt, along with the corrosive nature of chlorine, would be impediments in commercial use of such cells.

A hydrochloric acid (HCl) regenerative fuel cell/flow battery. HCl is electrolyzed by a voltage to form Cl2 on the anode side of the cell, and H2 on the cathode side. In reverse mode, reaction of the gases regenerates HCl and gives a voltage. (Image by the author, rendered with Inkscape).[2]

The key to a commercial flow battery is its electrolyte system, and development of a better chemical system is what the Harvard team intends to do. They are collaborating with Sustainable Innovations, LLC,, a commercial electrochemical system developer.[1] Their focus is on an undisclosed class of small organic molecules that are non-toxic and inexpensive.[1] The current class of flow batteries have problematic chemical systems.

One type of flow battery uses vanadium, but vanadium is an expensive metal, selling for more than twenty-five times the price of iron. Sodium-sulfur is another flow battery chemical system, but high temperature is required to keep the components in their molten state, and the components are corrosive. All this adds cost and complexity. The type of flow battery envisioned by the Harvard team is one that could be placed in a home to locally buffer solar energy.[1]

The US Department of Energy has a serious commitment to battery technology. It's set a 5/5/5 goal for the development of batteries within five years that have five times greater storage capacity at a fifth the price, and it's created a Joint Center for Energy Storage Research at Argonne National Laboratory. This center will be funded at $120 million over its first five years, and it will have the mix of scientists and engineers that made research and development organizations, such as Bell Labs, so effective in the past.[4]

References:

1. Greener storage for green energy, Harvard School of Engineering and Applied Sciences Press Release, November 28, 2012.
2. Brian Huskinson, Jason Rugolo, Sujit K. Mondal and Michael J. Aziz, "A High Power Density, High Efficiency Hydrogen-Chlorine Regenerative Fuel Cell with a Low Precious Metal Content Catalyst," arXiv Preprint Server. June, 13, 2012.
3. Hydrogen-Chlorine Fuel Cell, Harvard University Web Site.
4. Patrick Thibodeau, "DOE wants 5X battery power boost in 5 years," Computer World, November 30, 2012.