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Batteries vs. Fuel Cells
Jeremy Faludi, 30 Jan 05


In the continuing race to be the best energy-storage and retrieval system, which will win? Batteries, or fuel cells with hydrogen fuel?

The last few years have seen boundless hype for fuel cells, but remember, no fuel cells are affordable or robust enough for vehicle use yet, and are barely affordable enough for high-end building use. Most people think that battery technology is maxed out, with no breakthroughs to be made, but most people have not been paying close attention. And worse, the people setting research budgets haven't been paying close attention, following the herd down fuel cell boulevard.

High-pressure hydrogen gas has an energy volume-density of about 400 watt-hours per liter, not counting pressurized container or fuel cell volume (which in a car-size system would probably cut the system energy density by 1/3 to 1/2). Status-quo lithium ion batteries have an energy density of 150 Wh/L, but emerging thin-film lithium polymer batteries created in labs by Solidcore, Cymbet, NASA Glenn Research Center, Voltaflex, ITN, and others have energy densities up to 900 Wh/L. They also claim to have "virtually unlimited re-charge capability", unlike status-quo batteries which degrade after several years of normal use.

Not only that, these batteries are robust. For instance, Solidcore shows that you can drive a nail through one of their batteries, and not only will it not leak or explode, it will still function. Even better, the batteries are flexible as a plastic membrane. This makes them useful for applications fuel cells will never see: the manufacturers list them as being useful for clothing-embedded electronics, medical implants, self-powered smart cards, and other small-scale uses. Perhaps best yet, most of these batteries do not require any toxic ingredients for their production. And although the technologies are still nascent now, the ability to manufacture them roll-to-toll should make them inexpensive at mass-production scales.

Energy carriers don't just store energy, though, they're also used to transport it. And for all the fragility of our current electrical grid, it has extremely good transmission efficiency--an average transmission loss of about 3% from power plant to your door. In order to achieve this level of efficiency with H2/fuel-cell infrastructure, you would have to transport the hydrogen using less than 3% of the energy being transported. However, power plant generation efficiencies are bad enough that a good method of "cracking" H2 out of water could give fuel cells a net advantage.

It's hard to say which technology will win the race for best energy carrier. Millions of dollars are being spent on fuel cell research and very little is being spent on battery research, but batteries had a head start. ...and if we already knew the answers, it wouldn't be research, would it?

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The entry makes a good point that batteries are not yet dead. In fact, for many, if not all, applications, batteries will out-do fuel cells for the foreseeable future.

But if we're talking about use in vehicles - which the entry seems to indicate in its second paragraph - then thin-film lithium polymer batteries seem an inappropriate technology. Roll-to-roll manufacturing can only be used for very small batteries. Note that the examples given in the entry (e.g., medical implants or smart cards) are all small-scale.

This leads one to question whether the quoted engery density of 900 Wh/L is for very large or very small batteries. Obviously, the later. The next question, then is to what degree do battery energy densities scale with batter size. Anyone have an answer?

Posted by: Ross on 30 Jan 05

Where can we learn how to make these??

Posted by: barry on 30 Jan 05

Ross, they should be able to keep the same energy density in a massive array. Picture a stack of paper, but each page is a battery. Volvo has already shown with their 3CC that you can use a massive array of small batteries to replace a small array of big batteries.

More importantly, be aware that existing batteries are in fact stacks of dozens to hundreds of cathode/anode/electrolyte layers on the inside. Thin-film batteries just make the layers thinner, so you'd have thousands instead of hundreds of layers, and there'd be no need for the blotter layers that keep the liquids under control in existing batteries.

Posted by: Jer on 30 Jan 05

Battery performance metrics are many. They have thermal operating max and mins to meet. They have power density requirements to be sure, but that is soooo basic. Equally critical is how they handle a rapid charge or discharge at thermal extremes. Then there is the matter of design life. How many cycles until they are not good, in other words. Not to be forgotten, ever, is supply chain "toxicity" or "hazard exposure". How many people realize what happens if the electrolyte from a lithium battery comes in contact with human mucosa? Ugggh dead tissue overnight. Finally, lets not forget about recycle and design for dis-assembly choices. Life cycle and supply chain thinking: unfortunately not well taught in the worlds engineering and business schools.

Posted by: John Laumer on 31 Jan 05

True, John, but if you read my article i do mention their purported "virtually unlimited" cycles, and if you take a minute to look at any of the links, you'll see that they have much better thermal-extreme performance than lead-acid or normal LiIon batteries.

Posted by: Jer on 31 Jan 05



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