Fuel cells for automotive use the Proton Exchange Membrane,or PEM for short.PEM uses a polymer electrolyte and is one of the furthest developed and most commonly used fuel cell systems today.The PEM system allows compact design and achieves a high energy to weight ratio.Another advantage is the relatively quick start-up when applying hydrogen.The stack runs at a moderate temperature of about 80°C (176°F) and has a 50-percent efficiency.The limitations of the PEM fuel cell are high manufacturing costs and complex water management systems.The stack contains hydrogen,oxygen and water and if dry,water must be added to get the system going;too much water causes flooding.
The system requires pure hydrogen;lower fuel grades can cause decomposition of the membrane.Testing and repairing a stack is difficult and this becomes apparent when realizing that a 150V,50kW stack to power a car requires 250 cells.Extreme operating temperatures are a further challenge.Freezing water can damage the stack and the manufacturer recommends heating elements to prevent ice formation.When cold,the start-up is slow and at first the performance is poor.Excessive heat can also cause damage and controlling the operating temperatures,as well as supplying enough oxygen requires compressors,pumps and other accessories that consume about 30 percent of the energy generated.
If operated in a vehicle,the PEMFC stack has an estimated service life of 2000-4000 hours.Start and stop conditions,induce drying and wetting,contribute to membrane stress.Running continuously,the stationary stack is good for about 40,000 hours.Stack replacement is a major expense.Even with these limitations,the fuel cell as propulsion system is in many ways superior to batteries.It reduces the need to carry large batteries,a necessity with a vehicle propelled by batteries alone.Figure 2 illustrates the practical travel range of a vehicle powered by a fuel cell compared to lead acid,NiMH or lithium battery.One can clearly note that lead and nickel-based batteries gets too heavy when increasing the size to enable larger driving distances.In this respect,the fuel cell enjoys similar qualities to the IC engine in that it can conquer large distances with only the extra weight of fuel.
Although the fuel cell assumes the duty of the IC engine in a vehicle,poor response time and a weak power band make onboard batteries necessary.In this respect,the FC car resembles an electric vehicle with an onboard power aggregate to keep the batteries charged.The battery is the master and the fuel cell becomes the slave.On start-up,the vehicle relies 100 percent on the battery and the fuel cell only begins contributing after reaching the steady state in 5-30 seconds.During the warm-up period,the battery must also deliver power to activate the air compressor and pumps.When warm,the FC provides enough power for cruising,and when accelerating or climbing hills both FC and battery provide power.During breaking,the kinetic energy is being returned to charge the battery.
The FC of a mid-sized car generates around 85kW,or 114hp,and the power couples to an electric motor of similar capacity.The onboard battery has a capacity of around 18kW and provide throttle response and power assist when passing vehicles or climbing hills.The battery serves a buffer similar to the HEV and does not get stressed by repeated deep cycling and fast charging,as is the case with the EV.Hydrogen costs about twice as much as gasoline but the high efficiency of the FC compared to the IC engine in converting fuel to energy gives the same net effect on the pocket book except less greenhouse gases and reduced pollution.
Hydrogen is commonly derived from natural gas and we ask why not burning natural gas directly in the IC engine instead of converting it to hydrogen through a reformer and then transforming it to electricity in a fuel cell to feed the electric motors?The answer is efficiency.Burning natural gas in a combustion turbine to produce electricity has an efficiency factor of only 26-32 percent,while using a FC is 35-50 percent efficient.We must keep in mind that the machinery required with the clean FC is far more expensive and requires added maintenance than simply using a burning process.
Durability and cost are other concerns with the fuel cell and here we have seen some encouraging improvements.The service life of a FC in car driven in normal traffic conditions has doubled from 1,000 hours to 2,000 hours.The target for 2015 is 5,000 hours,or the full life of a vehicle driving 240,000 km (150,000 miles).Another challenge is cost.The fuel cell costs substantially more than an IC engine and until mass-produced,pricing for a cost comparisons is impractical to make.As a simple guideline,the FC vehicles will be more expensive than plug-in hybrids,and the plug-in hybrid cost more than a regular gasoline powered car.
It is conceivable that the fuel cell will never become the engine of choice that experts had hoped and there could be similarities with the failed attempt to fly airplanes on a steam engine in the mid 1800s.It is,however,everyone’s desire that the fuel cell will succeed,and taxpayers may one day have to pay to open the markets similar to subsidizing the electric car.It is also conceivable that governments might in the future mandate the use of fuel cells for environmental reasons.Fuel cells could also become the energy source of choice once the supply of fossil fuel gets dangerously low.Meanwhile,we hope that the development of the fuel cell will continue and become a replacement for the polluting internal combustion engine.
