Friday, February 7, 2014

Efficiency of EVs and Hydrogen Powered Vehicles.

If you wanted to power your car on renewable energy, would you be better off with an electric vehicle (EV), like the Nissan LEAF, or a hydrogen fuel cell vehicle (HFCV) like the Honda FCX Clarity?  There seems to be two different opinions on this.  Among the car companies, Nissan and Tesla seem to believe that electric vehicles are the way to a green future, whereas Honda and Toyota are firmly planted in hydrogen fuel cell vehicle camp.

To be sure, both technologies have advantages and disadvantages. For me the best part of hydrogen is a quick fillup and a long range, 240 miles in the case of the Honda FCX Clarity.  The best part of electric cars is their highly efficient simple operation. Battery. Electric motor. Done.  On the downside, hydrogen seems to have an infrastructure problem and probably very high costs.  With EVs, the problems are all about the battery limitations causing issues with cost, range, weight, and life.

But let’s set the larger picture aside for a moment and focus on efficiency.  Let’s narrow the focus still further and talk about powering both vehicles from solar panels mounted, say, for example, on the roof of a home.  How does the efficiency of an EV compare to a HFCV?

The Vehicles

Well the EV part is straightforward because the EPA lists the efficiency of each model sold in the USA.  Let’s make the Nissan LEAF our exemplar for this discussion.  The EPA says the LEAF requires 30 kWh/100 miles.

The HFCV is more complicated and comes in two parts.  The first is the efficiency of the HFCV at using hydrogen.  The second is the efficiency of making the hydrogen by electrolyzing water using electricity.

The efficiency of the HFCV is easy.  Let’s make the Honda FCX Clarity our exemplar for this discussion. The EPA lists the efficiency of the Honda as 60 miles/kg of hydrogen.  So if you purchase 1 kg of hydrogen, you can go 60 miles.

The Electrolyzer

The difficult part is how much electricity does it take to make a kilogram of hydrogen.  Let’s take a couple of different approaches to answering that.

First Try:  In 2010 Honda introduced a new version of its solar powered hydrogen generator, shown in the picture.  It is a very cool piece of technology that uses a high differential pressure PEM electrolyzer that can make high pressure hydrogen directly without the need for a compressor (Gizmag, “Honda’s next gen solar-powered hydrogen fuel station for home use”, 2010).  That is a big deal because it saves on both electricity and noise.  The station slowly makes 0.5 kg hydrogen overnight (8 hours) from grid electricity and transfers it to the car, i.e. no hydrogen storage in the home, saving on tanks.  It really sounds like overnight home charging of EVs more than anything.  But that 0.5 kg made overnight is only going to make the car travel 30 miles, compared to a full overnight charge on the LEAF EV allowing it to travel 84 miles.  But I digress, ...back to efficiency.

The press release doesn’t give the energy efficiency of the station directly, so we need to do a little inferring.  The 6 KW array is supposed to supply enough power to produce 0.5 kg/day.  Well a 6 KW array will produce about 24 kwh/day (more in the summer, less in the winter).  So that means it takes 24kWh/0.5 kg or 48 kWh/kg.

Second Try: A 2009 report from NREL (National Renewable Energy Laboratory) entitled “Hydrogen Resource Assessment” by Milbrandt and Mann researched the efficiency of commercially available electrolyzers.  The number NREL uses in their analysis is 58.8 kWh/kg which is based on the average of four different electrolyzers.  The number does not seem to include the energy needed to compress the hydrogen to, say, 5000 psi, which would raise the number higher.  The number is about 25% more than the approximation from the Honda Solar hydrogen generator which is interesting because Honda claims their 2010 system is 25% more efficient than their previous model. So perhaps the 48 kWh/kg is a good approximate number.

Third Try:  From chemistry, it is possible to calculate the energy needed to convert liquid water into hydrogen and oxygen gas.  The details of the process are described in a book called  “Solar Hydrogen Generation: Toward a Renewable Energy Future” on page 57.  It takes 39 kWh/kg to convert liquid water into hydrogen and oxygen gas.  This value is known as the higher heating value.  But when a fuel cell in a car converts hydrogen gas back into water, the water comes out in the form of steam.  So it is only possible to recover the so-called lower heating value of 33 kWh/kg.  So even with no losses in the system, this process is only 85% (33/39) efficient just because we start with liquid water in the electrolyzer but end up with steam coming out of the tailpipe of the HFCV.  So 39 kWh/kg is the lowest possible amount of electricity that could be used to by the electrolyzer.

And the Final Answer Is

So we have three values, 39, 48, and 58.8 kWh/kg.  Let’s give the benefit of the doubt to the HFCV and choose the middle number of 48 kWh/kg.  The actually number is probably a little higher, closer to the NREL number, but perhaps Honda has built a better mousetrap.

Armed with that number, it is now possible to calculate the efficiency of the HFCV.
48 kWh/kg / 60 miles/kg = 0.8 kWh/mile =

HFCV 80 kWh/100 miles
BEV   30 kWh/100 miles

That means it takes a whopping 2.6× more electricity to power a HFCV than an EV.  If both are powered by the sun, the HFCV would need 2.6× more solar panels.  As a practical matter, if you own two HFCVs, the 12 KW solar array will be much too large to place on the roof of a typical home.

So given this difference, why are the car companies and the EPA still interested in HFCV?  The efficiency is poor. The cars are shaping up to be very expensive (even by EV standards).  Oh, and there is this infrastructure problem.  Sure, EVs have a little bit of infrastructure to deal with too, but for less than  $2000, you can at least install an EVSE (“charger”) in your home and drive to and from work.  Filling up with hydrogen will likely need to be done in the old gas-station model.  Hydrogen stations will need to build in clusters starting, say, in South California, and working their way out.

Hydrogen does have its advantages.  In addition to being able to make it from water, it can be made from steam reforming of natural gas.  It can even be made from coal, if need be.  So there is some flexibility there that helps address the energy security needs.

Exiting from the tailpipe of HFCV is nothing but water, so local pollution issues are not a problem.  That is definitely a good thing.

An if, and it’s a big if, the hydrogen infrastructure could be put in place, a HFCV could be operated in the same way as a gasoline car.  Drive 240 miles and stop at the nearest hydrogen station to fill up again.

And perhaps if gasoline cars are used as a baseline for comparison, rather than EVs, the HFCV doesn’t look so inefficient.  Of course it isn’t much of a step forward either.

Efficiency for the Win

In the end though, in matters of energy, efficiency generally wins out.  For each little bit of improvement in efficiency, there is just that much less coal to dig up, or oil to pump out, or solar panels to buy, making the more efficient system worth paying little more for up-front.  Of course, in the case of EVs versus HFCV, maybe the more efficient EV system is actually less expensive, but we will not know for a few more years once HFCV start being sold, perhaps in 2015.

Let’s put it this way, if you can get a 6 KW solar PV array on the roof of your home, it can be used to power either one HFCV, or two EVs plus the house itself.  That makes EVs a very big win indeed.

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