Posted by: paul | February 23, 2009

More about Hydrogen & Electric Vehicles

The other day, while I was writing my reply to Dan Neil, fellow hydrogen blogger Greg Blencoe was writing a post entitled “Fallacy of energy efficiency argument against hydrogen fuel cell vehicles by plug-in battery advocates.”  Greg’s blog has a lot of great information about hydrogen vehicles and the views of the auto industry regarding hydrogen, plug-in hybrids, and 100% battery powered cars.

I happen to disagree with Greg on this one.  The energy conversion efficiency is important, and, in this case, batteries beat electrolysis, hands down.   It is true, as Greg’s article points out, that the earth has plenty of renewable resources that could be converted into energy.  However, harnessing those resources is another story.  Costs, environmental impacts, and the amount of time & resources needed for new site construction are going to limit the amount of energy we can generate from renewables for a long time. It will be a slow progression.  If we think building a hydrogen fueling infrastructure is daunting, converting to a 100% renewably powered infrastructure is even more so.  Having to oversize the infrastructure to support power conversion inefficiencies only makes it tougher.  And, the least expensive power is always the power you don’t need to buy, which is why energy efficiency should always be the first step in any program to reduce air emissions, move away from foreign oil, etc.

Rather than dismiss the conversion efficiency issue, we need to understand it, and respect the consequences it imposes. By the same token, battery enthusiasts need to recognize the limitations that batteries face.  The two groups need to work together, not in conflict, to identify the solutions that are best for each technology, and those that will benefit best from a combination.  (And while we’re singing kum-by-yah, we should invite the flywheel and supercapacitor advocates for some four-part harmony.)

In that spirit, here are some comparisons:

Battery benefits:

  • Conversion efficiency.  I’ve seen figures ranging from about 85%-90% efficiency in charging and discharging batteries.  The figure will vary by battery type.  We can ignore issues like the loss from converting from AC to DC, because the impact would be the same for generating hydrogen from electrolysis.  The conversion efficiency of batteries is about three times better than a electrolysis and fuel cell system.
  • Familiarity. All sorts of batteries are available today and everybody uses them.  Granted, a Tesla’s batteries are radically different from a standard automotive battery, but they’re not so different from the battery in your laptop.    Because batteries are familiar, it is harder for opponents to spread FUD.  Likewise, it is easier for advocates to gloss over inconvenient issues.
  • Simplicity.  Charging a battery is simple.  Just hook up a DC current and go to it.  Well, in truth, it is more complicated than that – you need to control the voltage and current, may need to monitor temperature, vary charging cycles, etc.  But, even with the complexities of modern charging, batteries are a lot simpler and need fewer components than hydrogen electrolysis, storage, and fuel cells. One of the most important aspects of the battery’s simplicity is that it takes minimal equipment to “plug in” a car for charging. However, the battery’s simplicity is also a liability, as discussed below, under hydrogen’s modularity.
  • Power Density.  Power density is a bit like the size of a water spigot.  A larger power density means more energy can flow out at once.  The higher the power density, the more power that will come out when you stomp on the accelerator. Claiming that batteries have a higher power density than fuel cells is not quite accurate, because it really depends on the specifics of the individual battery and the fuel cell, but the best batteries will beat the best PEM fuel cells.  (See Battery and Energy Technologies in Electropaedia for more information.)
  • Availability & Cost.  These actually aren’t real, but they are percieved to be advantages by many battery proponents.  The truth is, the battery technology doesn’t exist to provide sufficient power to run a sizeable vehicle (4 seater) for a reasonable range (300+ miles), so we don’t know what the cost would be.  On the other hand, the average US daily commute is 20 miles, so for c0mmuting, current battery technologies would be fine for many people.

Hydrogen benefits:

  • CO2. The CO2 impact of battery cars is vastly higher than that of hydrogen cars today and will continue to be so for the foreseeable future. This is primarily due to the amount of coal we use to generate electricity and the fact that hydrogen can be efficiently created from natural gas.  See Why Fuel Cell Vehicles.
  • Driving range.  Today’s plug-in Priuses get about 40 miles per charge.  Today’s best battery powered car, the tiny (but very cool) two-seater Tesla, gets about 220 miles per charge.  Today’s best fuel cell cars are getting over 400 miles per fill up.  And they are four passenger vehicles with storage!
  • Production from fossil fuels. It is hard to imagine that production from fossil fuels could be an advantage, but it is.  The bottom line is that ramping up renewable energy production will be a long road, and we’ll be using fossil fuels for quite some time.  Hydrogen can be efficiently extracted from fossil fuels, most notably natural gas.  As mentioned above, this allows today’s hydrogen vehicles to have a lower carbon footprint than today’s battery or plug-in hybrids.
  • Modularity.  With a battery, the same unit is used to convert electricity to chemical energy, store the energy, and convert it back to electricity.  While this simplicity is in some ways an advantage, it also is a burden.  To double the storage capacity of a battery bank, you need to double the number of batteries.  With hydrogen, you have the complexity of separate systems to produce the hydrogen, store it, and use it.   But, to double the storage capacity of a hydrogen system, you only need to double the tanks (or increase the pressure you’re using, if it is gaseous).  The modularity of the system provides great flexibility. The hydrogen can from a variety of sources – the car doesn’t care if the H2 is from electricity, direct-from-solar, natural gas, or otherwise.  Different storage technologies can be used to best meet the application.  For example, a small commuter car might use gaseous hydrogen while a heavy-duty pickup truck might use metal hydride storage.  There are even options in the final step of using the hydrogen to power the vehicle.  While most hydrogen vehicles use fuel cells, some, including the BMW Hydrogen 8, burns the hydrogen in an internal combustion engine.  The flexibility that hydrogen’s modularity offers is why hydrogen is a potential fit for a huge range of applications, from video cameras to powering large industrial sites.
  • Energy density.  There are two common types of energy density.  One is the amount of energy stored per unit volume (e.g., how much energy is in a gallon).  The other is the amount of energy stored per unit mass or weight (e.g., how much energy is in a pound).  Hydrogen advocates will point out that, by either measurement, hydrogen is hundreds of times better than the best batteries.  The bottom line is that batteries are big and heavy, and hydrogen isn’t!  However, the numbers that people use are typically for storage only.  Of course, to use the hydrogen, you also need a fuel cell (or engine), so a fair comparison needs to include both the storage and the fuel cell.  As with Power Density, given the modularity of hydrogen systems, you can’t give a single fixed number for Energy Density.   A vehicle that needs a lot of power might have a large fuel cell (resulting in a lower energy density, but a higher power density), while a vehicle that doesn’t need much power would have a smaller fuel cell stack (higher energy density, but lower power density).   Given a battery system and a fuel cell system of similar power densities, the fuel cell system will have a much greater energy density (probably by a factor in the tens, rather than the hundreds).  That is why fuel cells offer a better driving range.
  • Fill/charge time.  The time to charge batteries is measured in hours, while the time to fill a typical hydrogen vehicle tank is minutes.

Given the differing advantages of the two approaches, it seems clear to me that hydrogen is a much better fit for long-distance driving, so it would probably make sense as the primary family car.  Batteries make a lot of sense for shorter distances, so for those with two (or more) cars, it might make sense for at least one to be a battery car.  However, given the realities of the amount of carbon (and other pollutants) created to produce electricity, if air quality or climate change are your primary concern, the hydrogen vehicle may still be a better choice.   If you want to power your car entirely on renewable energy today and you’re willing to buy the equipment, you know how valuable each watt is, and you’ll definitely want the energy conversion efficiency that the battery offers.

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Responses

  1. Really interesting article, thanks for sharing!

  2. I really enjoy the facts and comparisons you make here. One of the better honest comparisons of BEV vs FCV.

    I think your summary paragraph is right on.

    Definitely subscribing to your RSS feed.

  3. Because of the advantages and disadvantages of each technology so eloquently outlined here, I believe the ultimate green car of the future to be the plug-in hybrid fuel cell vehicle. Today there are a couple of prototypes such as the Chevy Volt Hydrogen and the Ford Flexible Series Edge with HySeries Drive.

    I know people like to pit hydrogen cars against electric cars as natural rivals, but it makes sense to me to take advantage of the best of both worlds and put them into one vehicle that can go the first 40 miles on all electric but also has a range of 400 miles.


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