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Redox-Flow Cell Stores Renewable Energy as Hydrogen

36 points by jonbaer 7y ago

acidburnNSA 7y ago
Good to see progress in large-scale energy sources. As always we ideally would like to know current and prospective costs per MWh of storage, per MW of capacity, round-trip efficiency, raw material mass flows and recyclability, and space requirements to fully judge. I understand that these metrics are not always reliably available in new issue tech. We should make a worksheet or something to help navigate these complex issues. At least more people are talking about hydrogen as an energy carrier these days as opposed to a raw energy source, which was a huge misconception in the hydrogen excitement bubble of ~2005. And a nitpick: headline should be about storing intermittent energy, not renewable energy. I personally hate the term renewable because not all renewable energy is low-carbon (e.g. most biofuels) and not everything that's sustainable for hundreds of millions of years is considered renewable (e.g. nuclear with breeder reactors or seawater uranium extraction).
throwaway154213 7y ago
Redis, Redux, Redox, and now Redox-Flow. My brain is getting a little confused now.
twic 7y ago
So how does this thing work? > A redox-flow battery, in essence a reversible fuel cell, is typically made up of a positive and negative electrolyte stored in two separate tanks. When the liquids are pumped into the battery cell stack situated between the tanks, a redox reaction occurs, and generates electricity at the battery’s electrodes. > By comparison, the new invention has only one electrolyte, comprised of an iron salt (rather than the more commonly used vanadium) dissolved in acid. When hydrogen ions react with the iron salt (Fe2+), hydrogen gas is produced at the platinum-coated carbon cathode in the battery stack. > “We introduce iron as a middleman, so we can separate electrolysis into two reactions,” says Wang. Doing so allows one to control where and when to reverse the reaction to produce electrical energy to supply to the grid. “The system gives you flexibility... you could do the regeneration during evening time when electricity prices are at a peak,” he says. > Regenerating Fe2+ in the reverse reaction also allows for the continuous production of hydrogen gas, he says. “And because the hydrogen-iron cell uses about half the voltage of a traditional electrolyzer, you can generate hydrogen at a much cheaper cost if you do everything right.” It sounds to me like there are two things going on here. Firstly, they have a redox cell which has one electrolyte, in which iron ions move between the II and III redox states. Secondly, they have a way to shuffle electrons between iron and hydrogen, so they can take energy from the iron in the redox cell and use it to make hydrogen. So, over short timescales, the system just works like a redox battery. No hydrogen is produced or consumed. But if the battery gets full, or they want hydrogen for other purposes, they can produce hydrogen, and in doing so, regenerate the iron. Is that right? If so, what are the reactions here? What is the composition of the electrolyte, and what stops the iron ions decaying to the lower-energy state? How do they use iron ions to make hydrogen from water?
AtlasBarfed 7y ago
So electrolysis has a thermodynamic efficiency currently at best at 80%. Fuel cells are 40-60% for the regeneration of the electricity. I've heard that pumping water for gravitational potential energy is 90% end-to-end though. I don't see a thermodynamic efficiency metric, so that's not encouraging.

36 points by jonbaer 7y ago

Good to see progress in large-scale energy sources. As always we ideally would like to know current and prospective costs per MWh of storage, per MW of capacity, round-trip efficiency, raw material mass flows and recyclability, and space requirements to fully judge. I understand that these metrics are not always reliably available in new issue tech. We should make a worksheet or something to help navigate these complex issues. At least more people are talking about hydrogen as an energy carrier these days as opposed to a raw energy source, which was a huge misconception in the hydrogen excitement bubble of ~2005. And a nitpick: headline should be about storing intermittent energy, not renewable energy. I personally hate the term renewable because not all renewable energy is low-carbon (e.g. most biofuels) and not everything that's sustainable for hundreds of millions of years is considered renewable (e.g. nuclear with breeder reactors or seawater uranium extraction).

throwaway154213 7y ago

Redis, Redux, Redox, and now Redox-Flow. My brain is getting a little confused now.

twic 7y ago

So how does this thing work? > A redox-flow battery, in essence a reversible fuel cell, is typically made up of a positive and negative electrolyte stored in two separate tanks. When the liquids are pumped into the battery cell stack situated between the tanks, a redox reaction occurs, and generates electricity at the battery’s electrodes. > By comparison, the new invention has only one electrolyte, comprised of an iron salt (rather than the more commonly used vanadium) dissolved in acid. When hydrogen ions react with the iron salt (Fe2+), hydrogen gas is produced at the platinum-coated carbon cathode in the battery stack. > “We introduce iron as a middleman, so we can separate electrolysis into two reactions,” says Wang. Doing so allows one to control where and when to reverse the reaction to produce electrical energy to supply to the grid. “The system gives you flexibility... you could do the regeneration during evening time when electricity prices are at a peak,” he says. > Regenerating Fe2+ in the reverse reaction also allows for the continuous production of hydrogen gas, he says. “And because the hydrogen-iron cell uses about half the voltage of a traditional electrolyzer, you can generate hydrogen at a much cheaper cost if you do everything right.” It sounds to me like there are two things going on here. Firstly, they have a redox cell which has one electrolyte, in which iron ions move between the II and III redox states. Secondly, they have a way to shuffle electrons between iron and hydrogen, so they can take energy from the iron in the redox cell and use it to make hydrogen. So, over short timescales, the system just works like a redox battery. No hydrogen is produced or consumed. But if the battery gets full, or they want hydrogen for other purposes, they can produce hydrogen, and in doing so, regenerate the iron. Is that right? If so, what are the reactions here? What is the composition of the electrolyte, and what stops the iron ions decaying to the lower-energy state? How do they use iron ions to make hydrogen from water?

AtlasBarfed 7y ago

So electrolysis has a thermodynamic efficiency currently at best at 80%. Fuel cells are 40-60% for the regeneration of the electricity. I've heard that pumping water for gravitational potential energy is 90% end-to-end though. I don't see a thermodynamic efficiency metric, so that's not encouraging.