Fuel Cell Vehicles

Like the tide, going in and out and so does the Fuel Cell Vehicle favorability.   Right now favorability is pretty well in the tank, but not completely.  The city of London is installing hydrogen (H2) fueling stations with the objective of encouraging their use.  California has a similar program, as does Germany.

Some of you may not be familiar with fuel cell cars because they have been  out of the spotlight recently.  The following is an overview of  the  fuel cell and the fuel cell car.   The cartoon below pictures most of the hardware needed.

Courtesy of http://www.imageproduction.nl

In the front of the vehicle is the Fuel Cell. Within the fuel cell, a chemical reaction generates electricity that powers an electrical motor propelling the vehicle.  More discussion about the fuel cell after we look at the accessories needed to make the fuel cell function.  At the rear of the vehicle is the main H2 storage tank.  It feeds a smaller H2 tank in the front of the vehicle.  Air is pulled in to the fuel cell—the oxygen (O2) in the air is what is needed to make the chemical reaction.   Also in the back is a battery pack.  The fuel cell shuts down, say, when the vehicle stops for a red light.   The batteries are there to get the vehicle moving until the fuel cell is back on-line.  The “power electronics” has several functions. One is to rectify the DC electricity made in the fuel cell to AC to power the motor.  This is accomplished through the use of an inverter.  It also manages the amount and the ratio of air and H2, the removal of water (the product of the chemical reaction), the cell temperature and the recharging of the battery pack.   Those are its main functions.

Lets look at how the fuel cell makes electricity.   Below is a simplified drawing of a fuel cell.

Curtesy of Wikipedia

The whole idea here is to react a molecule of O2 and two molecules of H2 to make  two molecules of water (2H2O) and to take some of the energy of this reaction away as electrical energy.  Remembering your chemistry labs, if these two elements are combined under normal circumstances, say by using a spark, water will be produced and a lot of heat is released.  That would be ok in an internal combustion engine, but not in fuel cell.  Fuel cells are more efficient in extracting the H2/oxygen reaction energy than is an internal combustion engine.  Following the steps 1 through 4 above we note that H2 and O2 are channeled into the fuel cell block on different sides with the H2 going to the anode and the O2 to the cathode.  A catalyst on the anode side splits the H2 into protons and electrons.  Between the anode and the cathode is a membrane. Protons can pass through the membrane into the cathode side where the O2 is.  The electrons are drawn to the positive charge on the cathode side but can only go there by an external route creating an electrical current.   The current is used to make the motor turn. The water is continuously removed from the fuel cell.

The polymer electrolyte membrane is typically Nafion ™. The temperature in the membrane ranges from about 50C to 100C.  The Nafion requires a moist condition to exist so it does not dry out and crack.  Cracks would allow the H2 molecule to go through to the cathode resulting in high temperature reactions that could severely damage the fuel cell.  Further, if the water is not removed as it is formed, it can reduce the rate of the chemical reaction which would limit electrical current production.

The makers of the fuel cell cars have target performance for the end product that includes top speeds, acceleration, life of the fuel cell and distance between refueling.

From the literature, it appears that they are nearing the 120,000 miles life for the fuel cell, although not there yet.  Acceleration is relatively modest but probably could be improved by using the battery pack assistance when needed.  The most difficult issue is the distance the vehicle will go without having to be refueled.  The problem is that H2 has a lot of energy per pound (kg) but not a lot of energy per cubic foot (liter).   The H2 fuel tank desired is about the same size at the fuel tank in today’s standard cars, which means you don’t lose trunk space for your suitcases, groceries, or what ever.   The tank must hold about 12.5 pounds (5.6kg) of H2 to pack in sufficient energy to travel about 350 miles (560km).  In order to get that much H2 by weight into a tank of that size, the H2 must be pressurized up to about 10,000 psi (700 bar).   Many of the refueling stations that have been installed in London, California, Germany, etc. make 10,000 psi H2 available.   The fill time is said to be about 5 minutes  and that would probably be acceptable to most owners.

There are a lot of things going for H2 powered fuel cell vehicles except the economics and H2’s physical characteristics. That will be discussed in the next posting.

cbdakota