Posted by: paul | August 20, 2009

Fuel cells compared with internal combustion engines

There is a great article at AutoBlogGreen comparing fuel cells with internal combustion engines.  Recommended reading!

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.

Posted by: paul | February 19, 2009

LA Times Hydrogen Article

The February 13 L.A. Times had an article today entitled Honda FCX Clarity: Beauty for Beauty’s Sake in which the author, Dan Neil, dismissed hydrogen for vehicles on two points: cost of the vehicle & the efficiency of generating hydrogen from electricity versus charging a battery from electricity.  Below is an email I sent to him in response to his article.  Hydrogen supporters know that there are other important points that I opted to leave out for brevity (such as the scalability of H2 vs. batteries).


Your calculations are right.  The big challenge for hydrogen from renewable resources is the efficiency of creating the hydrogen.  Batteries are a lot more efficient. [I should have said “charging batteries is a lot more efficient.” – paul]

But that isn’t the whole story.  Although I’ve been involved with hydrogen for awhile, I’ve only recently become an advocate of hydrogen for vehicles, because, like you, I thought batteries and hybrids were a better fit.  The bottom line is that there is a role for both h2 and batteries – they each have advantages and disadvantages, and simply looking at the conversion efficiency misses a lot.  In fact, if you are concerned about greenhouse gasses, particularly CO2, then hydrogen is much better, in large part because of the disastrous carbon footprint of our power supply. Hydrogen allows a gradual transition from fossil fuels to renewables in a way that batteries don’t. Using hydrogen from natural gas today is vastly better than using batteries off the current grid – which is something that virtually all analyses seem to ignore. (Most people seem to think that we can switch to 100% renewable power immediately.)  If you were to do a CO2 comparison of the Tesla (from the CA grid – which is cleaner than the rest of the US) and the FCX (from natural gas, which is where virtually all of it comes from today), you’ll find that the Tesla is much worse.  (By the way, comparing a tiny 2-seater to a family sedan with a spacious trunk is a bit dishonest, don’t you think?) Dr. Sandy Thomas wrote an excellent paper comparing different approaches – I encourage your to read my take at or, if you want the super-detailed analysis, you can read the original paper at

As for the price? The FCX is the first production fuel cell car.  You might as well talk about the cost of an 1888 Benz (the first production automobile – 20 years before the Model T made cars affordable). Or the **real** price of a Tesla (one that would fit 4 passengers and have an 11 cu ft trunk, of course).

I encourage you to give hydrogen a second look. It is not a silver bullet, but it is an important component of a sustainable future.

Posted by: paul | January 22, 2009

Heating Degree Days & Normalized Therms

This is a follow-up to a previous post, Measuring the Efficiency of a House, in which I discuss measuring the energy consumption of your house per heating degree day.

The website EnergyLens has some great articles on heating degree days and some issues with using heating degree days.  Very good stuff.  I can vouch for the truth in the concerns they raise.  I have noticed that my own house does not consume a consistent amount of energy per heating degree day.  I believe that the reason is that we get decent passive solar gain, and we also have solar thermal to heat our hot water.   What I’ve noticed is that we consume more energy per heating degree day in the month before and after the winter solstice than the other heating months.  This makes sense – because in mid-winter, oil has to provide a larger portion of our heating load.  In the other months, we get more energy from the sun that isn’t accounted for when I calculate the Heating Degree Days per Therm.  The article makes some recommendations for using heating degree days to avoid some of the issues – I agree with them, particularly using yearly data and comparing proportional, rather than absolute, differences.

I proposed the term “Heating Degree Days per Therm.”  EnergyLens uses the term “Weather Normalized Energy Consumption” for the general concept, since the idea is indeed a normalization process.  They refer to Normalized Kilowatt-Hours for electric consumption, so instead of Heating Degree Days per Therm, we should probably use Normalized Therms instead.  Much better than my goofy acronym, HDDpT, aka “headpit.” A google search for “normalized therm” shows this is a standard term.

Notice too that Normalized Therms are the inverse of Heating Degree Days Per Therm.  This seems like it shouldn’t matter (and it shouldn’t if you are comparing things by ratio) but it does matter when comparing absolutes.  In June 2008, two professors at Duke demonstrated that we should really measure vehicle efficiency in Gallons per Mile, not Miles Per Gallon.  It is a really interesting read. The same concept applies to home efficiency.  Normalized Therms (Therms per Heating Degree Day) are the corrolary to Gallons per Mile, so lets go with it.

So start calculating the Normalized Therms for your house, make some changes, and see if they improve.  I’ll post some numbers for our house at the end of this heating season.

And check out Energy Smackdown, a competition to reduce energy consumption.  Cool idea!

The 1:5:10 EcoTip blog posted similar thoughts last month. (The blog’s name is because each tip is one thing to do that takes 5-10 minutes.)

Posted by: paul | January 22, 2009

Steven Chu

The excellent astrophysics/physics/science/politics/miscellania blog Cosmic Variance has a posting today about newly-confirmed Secretary of Energy Steven Chu addressing the National Labs (Chu comes from Lawrence Berkely National Lab).  These notes from his presentation are very encouraging.

I haven’t found much else out there on the web providing significant details about Chu’s positions.  He makes a video statement on the Obama-Biden transition website (including a statement about a Manhattan Project style effort for transitioning to renewable energy), there is a posting on a Wall Street Journal blog, and a throwaway piece in Business Week – that seems to be about it. Please let me know if you have insights into his views on energy, other than his obvious interest in biofuels.

Posted by: paul | January 18, 2009

Where is the hydrogen?

The DOE’s “Electricity Advisory Committee” submits a report on Energy Storage Technologies and hydrogen isn’t even mentioned once?  What is up with that? !

Posted by: paul | January 17, 2009

Why Fuel Cell Vehicles?

Because hydrogen fuel cell vehicles offer the single best option to dramatically reduce carbon dioxide contributions from automobiles.

That is a bold statement. And although I’ve supported hydrogen for stationary and small portable applications for years, only in the past twelve months have I really understood how critically important hydrogen is to reducing green house emissions from automobiles.

Dr. C. E. (Sandy) Thomas, of H2Gen Innovations, developed a model that convinced me.This model compares projected greenhouse gas emissions throughout the 21st century for a transition to four different vehicle options: gasoline-electric hybrids (e.g., Toyota Prius), gasoline plug-in electric hybrids (e.g., Chevy Volt), ethanol plug-in electric hybrids (e.g., the Volt using ethanol for the gasoline), and fuel-cell vehicles (e.g., Honda Clarity FCX). The graph below summarizes Thomas’s results. It clearly shows that fuel cell vehicles offer the only option to significantly reduce greenhouse gas emissions below 1990 levels:

Greenhouse Gas Emissions for Various Vehicle Types

Greenhouse Gas Emissions for Various Vehicle Types

I recommend reading Dr. Thomas’s full paper. It is quite readable if you are conversant in greenhouse gas and new vehicle concerns. You can also watch a webinar of Dr. Thomas presenting his paper at the National Hydrogen Association’s website.

Of course, if the model’s assumptions are wrong, then the results are wrong. So, since learning of Dr. Thomas’s results, I’ve been seeking evidence to either support or discredit his assumptions and results.  So far, I haven’t found anything to discredit his model, but I have found some that corroborate it, although indirectly.

Dr. Thomas’s model is the only one I have seen that incorporates a gradual transition from current automobiles to the replacement technology. His is also one of the few to consider the current electric supply and realistically introduce more renewables and nuclear power over time. For comparison, consider the results from a model Dr. Mark Jacobson of Stanford University has developed:

Rankings of Vehicle and Energy Source Combinations

Rankings of Vehicle and Energy Source Combinations

Jacobson’s model looks at a wide range of socio-economic factors. Furthermore, Jacobson compares different renewable energy sources, while Thomas combines all renewable and nuclear sources. But most importantly, Jacobson looks only at 100% renewable solutions. He completely ignores the reality of the current grid and the fact that, at best, we face a long, slow transition to a grid powered largely by renewables. In fact, our current grid is so coal-dependent that electric vehicles today result in higher CO2 emissions than similar gasoline vehicles! As a result of considering only 100% renewably sourced energy, Jacobson’s results place battery electric vehicles powered by wind power as the best option. Fuel cell vehicles powered by wind come in a close second. Within the article, Jacobson points out that he did not perform calculations for fuel cell vehicles using the other renewable sources, but one could easily do so using a simple conversion, since their impact differs from batteries by a constant factor. In other words, right next to each battery solution in Jacobson’s graph, there is a similar fuel cell solution. So Jacobson’s model tells us that in an ideal, 100% renewable scenario, battery-powered vehicles are slightly better than fuel cell vehicles for any given resource. (This is because charging and discharging a battery is about three times more efficient than producing hydrogen though electrolysis and using it in a fuel cell.) He leaves it to the reader to recognize that in a more realistic scenario, fuel cells will fare better.

Posted by: paul | January 17, 2009

Measuring the Efficiency of a House

We all know a simple way to measure the efficiency of our cars: Miles per Gallon, or MPG. It is easy to do – just measure how many miles you travel between fill-ups, note how many gallons it takes to fill up, and divide the miles by the number of gallons. So, if you reset your trip odometer when you fill up, then the next time you fill up you’ve gone 200 miles, and you put 10 gallons in, then you were getting 20 MPG (200 miles / 10 gallons).

But how do we measure the efficiency of a house?

Gallons per Year, Cords per Year, etc.

The easiest measurement is simply how much fuel you use in a year. If you burn oil you’ll measure gallons per year, if wood then cords, if natural gas then cubic feet.

This is a fine way to compare two houses in the same area for the same year, but what if you want to compare the same house in different years? Perhaps you added an addition, or did some insulation work. Sure, you can compare the total fuel consumption before and after, but if one year was warmer than the other, your comparisons really aren’t valid.

Heating Degree Days

We need a standard way to compare how warm or cold a given year is. One way to do that is to use heating degree days. A heating degree day is a statement of how many degrees below 65 F a day’s average temperature is. So, if the average temperature on Monday is 40 degrees, you would say there were 25 heating degree days on Monday. Heating degree days accumulate over time, so if the average temperature on Tuesday was 45, then you would say that there were 45 heating degree days on Monday and Tuesday.

NOAA‘s National Climate Data Center has heating degree data for each state. There are lots of ways to get this data, but for now, one of the easiest ways is from here. Pick the years you are interested in and find your state. The top line states how many heating degree days there were for each month. The second line is a running total, so the total for the heating year is the value for June on the second line. (Heating years run from July to June.) The third line compares the running total for that year to the average value – so a number less than 100 indicates that the year so far is warmer than average, while a value greater than 100 means it was colder than average. Below is a snippet of the data (for Maine) in 2006/2007. We see that there were 7689 heating degree days, and that particular heating year was only 96% of the average. (There are ways to get more accurate data for your region, rather than a statewide average. I’ll address that in a future post.)

2006/2007 10 81 365 103
2006/2007 10 91 7586 7689
2006/2007 28.6 97.8 96.2 96.0

Heating Degree Days per Gallon

If we know how many heating degree days were in a year and we know how much fuel we used in a year, then we can calculate how many heating degree days per gallon (assuming the fuel is oil). For example, if you lived in Maine and used 700 gallons of oil during the 2006-2007 heating season, then your house “got” 11 heating degree days per gallon (7689/700). Or, if you burned 10 cords of wood, then your house “got” 769 heating degree days per cord (7689/10).

That is closer to something like Miles per Gallon, but it doesn’t let us compare the house that burns oil to one that burns wood. If you burn both, then you can’t even compare years.

BTUs and

To compare oil, wood, gas, etc, we need a common unit. In the US, the most common unit used in the heating industry is the BTU. A BTU is a pretty small unit of energy when it comes to heating a house, so an easier number to use is the therm, which is 10,000 BTU. We can convert our heating degree days per gallons, cords, etc into heating degree days per therm by dividing by the following numbers: (data from Fuel and Energy Conversion and Equivalence Chart)

Fuel Divide by:
Cubic foot of Natural Gas 0.0103
Cubic foot of Propane 0.025
Gallon of #2 Heating Oil 1.38
Cord of Wood 200
Ton of Wood Pellets 165
Kilowatt-hour of Electricity 0.0341

So, how many heating degree days per BTU did our two examples get?

The oil-heated house (which got 11 heating degree days per gallon of oil) got 8 heating degree days per therm. (11/1.38  =  8 ) Our cordwood heated house got 4 (769/200  =  4) heating degree days per therm. So the oil heated house is performing better – perhaps it is better insulated or smaller.


So there you go – a way to compare the same house across multiple years, or multiple houses by calculating the Heating Degree Days per Therm. Lets call it HddPT (“head pit”?) and make it a standard value when we buy and sell homes, when we compare them, etc.

This concept is nothing new or earth-shattering. Many heating oil companies essentially calculate your HddPT , and compare it to the number of heating degree days to date to know when to schedule your next delivery.

To calculate it, you just look up how many heating degree days there were, divide that by how much fuel you used, and then divide that by the conversion factor for your fuel above. That is,

HddPT = (heating degree days / amount of fuel ) / conversion factor

Wouldn’t it be great if it were as common a number as MPG?