Monday, February 21, 2011

More on Hydrogen Fuel Cell Vehicles

I drive a SVU. While I don't need to all that often, I have to tow a boat weighing up to 7 tons every now and again. That is hard to do with a bicycle or Mini Cooper. There are plenty of people that drive big SUV's because they can, not because they need to. Many parents want their kids driving bigger vehicles because they are safer than smaller vehicles. Whether for need, status or piece of mind, big vehicles make up a large portion on the vehicles on American roads. Hydrogen fuel cells are tailor made for these vehicles.

Hydrogen is the lightest element in the universe. That makes it hard to contain. There has been a lot of research to improve our ability to contain hydrogen well enough to make it a viable fuel. Pressurized hydrogen appears to be the best way to contain this fuel. Liquid hydrogen works great, but it has to be kept at a very low temperature which means refrigeration energy or super insulated containers. Under pressure, hydrogen is a lot less expensive to store.

One kilogram of hydrogen is equivalent to one gallon of gas, energy wise. The volume of one kilogram of liquid hydrogen to equal one gallon of gas is 4 time greater than gasoline. The U.S. Department of Energy has funded research and development of high pressure hydrogen storage tanks for vehicles. The carbon fiber tanks lined with high density polyethylene, can store 15 gasoline gallon equivalents in roughly 60 gallons of space at 10,000 PSI (700 bar). At 10,000 psi the energy density is close to that of liquid hydrogen. The weight of the filled hydrogen tank is less than the weight of the filled gasoline tank. At 3000 psi (200 bar)the volume of equivalent energy would be 3.5 times greater. It is the required volume that is the issue.

The reason I used 10,000 psi and 3000 psi has to do with the energy efficiency in delivering the hydrogen. High pressure hydrogen electrolysis produces hydrogen at roughly 3000 psi. No extra work has to be done to compress hydrogen if 3000 psi provides reasonable vehicle range for its tank size. While US natural gas pipelines are currently rated for only 1200 psi, compressing from 1200 psi to 3000 psi is a lot cheaper than to 10,000 psi. Looking into my crystal ball, I think I see nominal 3000 psi hydrogen pipelines in our future.

A compact car would be difficult to put a large hydrogen tank into. A SUV would handle a bigger tank(s) with greater ease. Even a big SUV has limits. 100 gallons of fuel space, about 14 cubic feet, or four - one foot diameter tanks eight feet long, would be a realistic maximum. That tank configuration would be equal to just over seven gallons of gas. Since the Fuel cell is more efficient (40 MPG) than an internal combustion engine (20 MPG), the range of the vehicle with a low pressure hydrogen fill would be about 285 miles. Should you want to make a long road trip you could pay the extra cost for a 10,000 psi full up and get a range 3.5 times greater, or 1000 miles. That is just an example. With only two tanks you would have about a 140 mile low pressure range and a 500 mile high pressure range. The actual low pressure range, since it would generally be for in city driving, would be higher with regenerative breaking.

I approximated a little, but these numbers are in the ballpark. For comparison the Chevy Equinox FCV gets 43 gasoline equivalent miles per gallon and a 200 mile range with about 4.5 kilogram tank. There are other considerations. Current fuel cells have a 50,000 mile life before overhaul. The overhaul is basically replacing the PEM, which is the Proton Exchange Membrane. So instead of ten $50, oil changes you would have one $1000 PEM change every 50,000 miles. I am guessing of course on the costs. PEM using a platinum catalyst are more expensive than non-precious metal catalyst membranes. At around $550 per square meter, the platinum PEM material is much cheaper than many think. So those numbers may not be too far off.

The biggest problem is the hydrogen infrastructure. Compressed natural gas proponents brag about their existing infrastructure. It is there, but you have to look to find it. The four tank system I am talking about may seem like over kill, but until the hydrogen infrastructure catches up, it is good choice for rural users more inclined to have home hydrogen refueling stations. More on that later, but there is a huge difference between 3000 psi and 10,000 psi home fueling systems.

A few links:

This first one shows a insulated liquid hydrogen prototype with a 650 mile range. Remember the 10,000 psi tanks are about the same size since they do not have the thick insulation. With the liquid hydrogen, the fuel boils off in about 6 days, not a good thing.

This one is a low volume home hydrogen fueling station designed in the UK. It is an older design that I list mainly to show the relative cost.

This is a Honda solar home refueling system that product 1/2 gasoline equivalent gallons (0.5 kilograms) per day. Standard home electric versions can produce more in even less space.

This is a US government estimate for a Home Hydrogen Refueling (HHR)appliance. The estimated cost per kilogram is around $4 per kilogram which with the added fuel efficiency of the fuel cell is similar to $2.00 a gallon gasoline.

Note: All estimates based on nominal 60% fuel cell efficiency.

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