PEM's SOFC's cont.....
The alternate to tanked/ stored hydrogen onboard would be hydrogen reformed onboard from a fuel reformer/processor. This would allow the PEM powered vehicle to operate in extended ranges and be a solution to the hydrogen storage problems. This however, adds additional complications. The reforming process of extracting hydrogen from hydrocarbon fuels changes some of the intial advantages of the PEM. The reformer to make this hydrogen (a seperate piece of equipment from the cell) runs at a substantilly higher temperature than the PEM fuel cell. This means, similar to the SOFC, although the PEM cell itself does not require the time to start up, the reformer will. Suddenly the face of the playing field changes. We have two technologies that will operate or better said begin to operate in the same proximity of time. In addition to this, the PEM in combination with the reformer faces additional difficulties. You've come up with one answer to storage but you've created a pandora's box of others. The fuel reforming for a PEM is complex and expensive. Expensive not only because the reformer needs to gasify the fuel, it also needs to seperate the fuel streams, i.e. pull the hydrogen gas out of the other gasses that will be present when the fuel is gassified. Sulphur would have to be extracted if present, CO, etc. Not only that, but the reformer itself utilizes energy to operate.
Other problems.
1. Climate - PEM fuel cells are limited in the areas of the world they can be used. The cells cannot freeze as there is moisture present in the membrane. This moisture would expand and ruin the cell if it were to. A PEM always has to be kept above freezing level. Either through constant operation, or by adding supplementary heat to the cell when it is not in operation yet reamins exposed. GM seems to be working hard on one soolution as is Ballard im sure. I think GM is adding some sort of antifreeze to its fuel stream. When not in operation and in the cold the there would be antifreeze in the membrane which would inhibit expansion damaged to the membrane cause by freezing. GM claims to have solved this down to temperatures of -20 degrees so far, others, have not yet announced any advancements. Take Ballards busses in Vancouver for instance, they are not allowed to be parked inside the bus barn because of the hydrogen. They take hoses hooked up to heaters and put them in the cell compartment to provide constant protection.
Sulphur. PEMS have little to no tolerance at all to sulphur. Sulphur in a fuel stream at very low levels will contaminate the cell.
CO (Carbon monoxide) PEM's as well have little tolerance to Carbon monoxide, again, it poisons the cell membrane and kills it.
Companies developing SOFC's have taken a different approach to the fuel cell problem. Instead of choosing the "ideal" solution that satisfies the need for "immediate" immediate power, they have chosen a solution that works in the world today. A solution that works with todays infrastructure, its fuel system, and have turned around and looked for ways to make this technology suitable for mobility and methods to make it suitable for use in an "on demand world".
Disadvantages of a SOFC
1. Start up time - as a SOFC works at a higher temperature than a PEM the amount of time required to get the cell to a temperature required to facilitate the production of electricity is greater.
2. Sealing of the cell. When used for applications that require frequent start-ups SOFC's have a problem with expansion and contraction. Warming a cell slowly and cooling it down slowly is one way to elleviate sealing problems, but when you rapidly heat or rapidly cool the cell the coefficient of expansion and contraction between the cell materials and its supporting structure etc differs. The difference between the two coefficients cause breaks in the seals of the cell. If you have a fundamental understanding of how a cell works its like this. The anode compartment and the cathode compartment are two different chambers, now when hydrogen enters the differing atoms take alternate routes to get to the other compartment. One atom can actually penetrate the cell membrane the other cannot. Think of it as sifting gravel through a screen. The smaller particles that fit through the sreen can get past, the larger particles however cannot and take an alternate route to the otherside. It is this splitting of routes that causes an electric current. Now if the cell has cell sealing problems and it faces rapid start up and shut down the differeing coefficients of expansion between the material cause this seal to crack. Take a dish of mud and allow one to dry slowly and take the other one and dry it rapidly and you will see the difference. Now because the cell seperates these two comparments any failure in the membrane will cause an effect like getting a hole in that screen and both different atoms taking the same path to the otherside of the cell. It is by isolating the poitive and the negative ions and making them take seperate patsh to get to the otherside of the cell that current is made. If there is a hole that both ions can get through then you no longer have seperate paths and no longer have current. |
