Hydrogen Infrastructure

I’m not totally against hydrogen. The sun is full of it. But it just doesn’t make sense to me as a fuel. According to Larry Burns of General Motors the development work on fuel cells is now to the point where it’s commercially viable except for the fact there is no hydrogen refueling infrastructure and no one has said anything about producing enough hydrogen to do anything.

BMW echoed the message on a commercial featuring the Hydrogen 7. They said that the car is ready for the world when the world is ready for it, presumably about the refueling issue. It only emits water vapor. Well that’s still about the only thing that anyone will talk about.

There’s a great article in the April 2008 issue of Fast Company (it’s like a car magazine) that talks about Iceland’s progress with hydrogen and energy in general, and how they’re exporting their know-how. That’s all fine and good but it appears to me that Iceland has three things going for it that the US and many other countries don’t have. Lots of installed geothermal energy sources, a very small population, and relatively small geographic area.

In a place where there is essentially energy from the ground to generate power, efficiency isn’t a deal breaker. But in just about every other part of the world, can someone explain to me where we are going to get the energy to make the hydrogen?

Maybe industry is doing this to show how impractical the idea is while getting green kudos from an uninformed public? Or maybe I’m uninformed and cynical. Time will tell.

Regenerative Braking Energy

Regenerative braking is a significant part of the increased efficiency of hybrid vehicles. Out of curiosity, I did a few back-of-the-envelope calculations to figure out how much energy is involved with stopping different vehicles from different speeds.

The basic equation is 0.5mv^2 where m is the vehicle mass and v is velocity. Let’s take a look at a few different vehicles and the amount of energy that might be recouped during a stop. Most of the following calculations are done in metric units.

Assuming a 3,300 lb (1,500 kg – how convenient) sedan coming to a stop from 50 mph (22.4 m/s). That works out to 376 kilo Joules (kJ).

For a 44,000 lb (20,000 kg) vehicle such as a refuse truck making a stop from 20 mph (say 9 m/s) that’s 810 kJ.

A tractor-trailer loaded to 60,000 lb (27,270 kg) at 65 mph (29.1 m/s) 23,092.5 would need to scrub off 23,093 kJ.

Checking Wikipedia for the energy content of gasoline and diesel yields estimates of (32-34.8 MJ/L and 40.3 MJ/L respectively. As an aside, ethanol has estimated energy content of 18.4 to 21.2 MJ/L, seemingly barely half of that of diesel.

Another figure I’ve come across is from a NextEnergy brochure about hydraulic hybrid vehicles can “capture and reuse over 70% of the energy normally wasted during braking”. If you could do that for the above 3 scenarios, each of those stops would regenerate 263 kJ, 567 kJ, and 16,165 kJ respectively.

However, assuming gasoline engine efficiency of 25% and diesel engine efficiency of 35% (my SWAG) you would need 1,052 kJ (263 kJ / 0.25), 1,620 kJ (567 kJ/ 0.35), and 46,186 kJ respectively to generate that power from fuel. In other words, to get the tractor-trailer in this example moving from a stop to 65 mph takes about 1/3 gallon of diesel fuel.

All this energy is currently just wasted to the atmosphere as heat from regular disc or drum brakes to bring things to a stop. Depending on a vehicle’s typical drive cycle, there is substantial potential savings in regenerative braking. Which is why they’re starting to be used on heavy vehicles that do a lot of stop-and-go driving such as refuse trucks and delivery vans.

It comes down to balancing space requirements, added weight, cost, the volume and maximum pressure capability of the accumulator, and fuel savings. I wonder what applications they’ll appear on next.