CNG/LPG Fuel Cells for Cars

Open Access Article Originally Published: September 23, 2009 And Available from EV World

The hydrogen initiative is stalled. The hydrogen fuel cell cars work fine but no good solutions have been found to the problems of where to get the hydrogen, how to deliver it and how to store it. 95% of our hydrogen is made from natural gas, which is abundant on earth and already distributed at 1/3rd of the price of gasoline.

Three recent breakthroughs have made natural gas a very interesting fuel:

1. Ceramic fuel cells that can make electricity from natural gas at 60% efficiency.
2. ANG: Adsorption stores natural gas at low (500 psi) pressure in compact tanks.
3. A glut of cheap natural gas caused by new shale drilling/extraction techniques.

The fuel cell breakthrough is particularly important because it means a car can generate its own electricity more efficiently than a massive power plant! Big plants typically average 30% efficiency, so a 60% NG fuel cell hybrid is twice as efficient as an electric vehicle charged from the grid.

Fuel Cell in the Engine Bay

That means half as much fuel is consumed. Twice as efficient as an electric car is saying a lot because electric cars are already three times more efficient than conventional cars. This is because internal combustion engines are less than 30% efficient verses 90% for electric motors.

Natural gas fuel cell cars are thus about six times more efficient than today’s cars. Using 1/6th as much fuel means pollution is also 1/6th . But NG is inherently very clean. and has 30% lower carbon content and virtually no sulfur, mercury, volatiles, and Nox so pollution is way less than 1/6th.

Since NG fuel cells have a warm up time, the hybrid batteries must have enough capacity for all-electric operation until warm up is complete. After warm up, the fuel cell keeps the batteries charged and the batteries provide power for peak loads and acceleration and recapture energy on braking.

A Prius uses 16.8 kW for continuous 70 mph driving on a level road. The fuel cell must be able to supply this much power for steady driving. Natural gas is already distributed by pipeline to homes all over the US and UK, so home refueling is possible. Compressed Natural Gas (CNG) is already used to run five million vehicles worldwide. Pump prices for CNG are about one third of the price of gasoline in spite of the expensive ($350k), 3600 psi pumps and fittings currently used for delivery.

The pipeline cost of natural gas is only 1/4th of the cost of crude oil with the same energy content. If much simpler, 500 psi Adsorbed Natural Gas refueling is adopted, prices could be reduced even further.
Cost per mile for a NG fuel cell hybrid would currently be only 1/18th of present cars but could be reduced even further with low pressure ANG refueling! ANG fuel tanks contain activated carbon “sponges” that adsorb 160 times their own volume of natural gas. They can be made from Corn cobs , which have a network of nanoscale passageways that remain after carbonization. One gram of this material has as much adsorbing surface area as a football field.

When natural gas is adsorbed on a carbon surface it ceases to act like a gas. Dense storage at low pressure makes it possible to hide the much smaller tank inside the car's frame. Even if we kept the existing CNG high pressure storage, the tripled efficiency would allow fuel cylinders only 1/3rd as large as present CNG tanks.
So an NG fuel cell hybrid is a lot like a Chevy Volt with a fuel cell replacing the range extender (engine/generator) and a much smaller battery. Its battery only needs to be large enough to run the car during warm-up of the fuel cell, currently about 15 miles. The Chevy Volt's 40-mile battery is rumored to cost $5000, so the NG car's 15-mile battery would cost $3125 less. Incidentally, at these battery prices a 400-mile range pure electric car would need $50,000 worth of batteries!

Clearly, small batteries with range extenders are the way to go until we have a significant battery breakthrough. Pure electrics have other problems too: A 110v, 20A household plug can only supply 2.2 kW which means that, unless you add 220v service, 10 hours of home charging will only take you 10 x 2.2 x 4 mi/kW = 88 miles. Natural gas today is primarily a non-renewable, fossil fuel.

But people have already begun selling renewable gas into the pipeline. Landfills, manure piles and sewage plants that used to release significant amounts of methane into the atmosphere are now selling it as green gas. Biomass and garbage can also be gasified to add to the supply.

The energy balance of grass bio methane production is 50% better than annual crops now used. Though the US power grid uses significant hydro power and other renewables, CO2 emissions are still almost twice as much per kilowatt-hour as a 60% efficient NG fuel cell. In 2007 the US power grid emitted 605 grams/kWh.
A NG fuel cell emits only 327 grams. At 4mi/kWh that translates to about 151 grams per mile for a grid charged car verses 82 for the NG fuel cell car.

Someday the grid could be cleaned up so that electric cars charged from it are cleaner than NG fuel cell hybrids. EIA data makes it easy to track our progress towards this goal: In 1996 we emitted 627 grams of CO2 per kWh and by 2007 this was reduced to 605 grams.

That’s a 2-gram per year decrease. If we continue at that rate, it will take 139 years to equal what we can do now with a NG fuel cell. Recent years show even less progress. There was no improvement between 2006 and 2007. Plugging into the grid is, unfortunately, a bit like plugging into a lump of coal.

Infrastructure expansion also favors natural gas. Gas pipelines cost half as much to build as ugly overhead electric transmission lines of the same power capacity. Gas also has one fourth the transmission loss of electricity and much cheaper energy storage.

Depleted gas fields and salt caverns are already storing 4.1 Tcf of gas in the US. At 60% efficiency this could produce 1,970 gigawatt-hours of electricity. A very cheap battery! Fuel cell developers are in a race to commercialize suitable fuel cells. The first products using NG fuel cells are home CHP electricity generators that use their waste heat to make hot water. The fuel cells in these units produce only 2 kW but they can start up from an idle state in 5 or 6 minutes.

Scaling up to 15 kW and adapting to the tough environment of a car could take years. Another company is developing a fuel cell range extender that is fueled by methanol. Methanol has only half the energy density of gasoline but, because of the high efficiency, fuel tanks would still be smaller than current gasoline tanks. “Price at the pump” is the one thing that seems to get voters excited. Reducing fuel cost/mile by a factor of 18 with a fuel that is 97% from North America while using corncobs should generate some excitement. The hydrogen initiative should be immediately redirected to focus instead on a fuel that is plentifully available, transportable and storable.

Finally if the Governments of the US, UK Europe and the rest of the world wish to allow some kind of demarcation, as its a transport fuel (and thus subject to some form of road pricing tax) then LPG is already available as both a transport fuel and heating fuel. It is also subject to the tax as well. LPG whilst a more complex molecule could still be developed as the fuel cell of choice by the motor industry.