Sunday, February 20, 2011

Why Not Natural Gas?

Increasingly I see people suggesting natural gas, specifically compressed natural gas, CNG, as a better option than EV's for personal transportation needs.  Increasingly I'm convinced it's not as easy as some would make it seem or even a good idea.  Yes we have large reserves of CNG here in the US, and hydro fracturing techniques, (fracking), are freeing up untapped reserves.  Fracking is not without it's problems, I won't go into details here but there are consequences that need to be closely looked into before we forge blindly ahead.  CNG has less energy density than gasoline so tanks need to be larger and provide less range, about the same as a battery powered Tesla Roadster, but with far lower performance.  CNG cars also have lower performance than conventional gasoline versions, the CNG Honda Civic GX has 113 hp compared to the 140 hp gasoline version.  Since it's a highly explosive gas under high pressure the tanks need to be very strong and well protected.  Since it's a gas it needs to be compressed to flow into a tank, which takes energy and time.  The Phill home fueling station will use about 6 kWh's of electricity to provide 100 miles worth of fuel.  An EV could go about 25 miles on those 6 kWh's of electricity.  Refilling a CNG vehicle is not that quick, it will take overnight with an expensive home filling system if your house has a NG pipeline connection.  The Phill would take about 12 hours for a full tank.
Phill Specs
A little more than half of all homes have NG, while almost 100% of US homes have electricity.  There are few public high pressure fast filling stations available, basically a non existent infrastructure that would need to be developed at high cost, while existing pipelines are failing.
The safety issue is significant.  If a CNG vehicle catches fire the potential for damage is much higher than with a gasoline vehicle.  Example: CNG Honda Explosion  Notice the gasoline vehicles next to the CNG vehicle also caught fire but did not explode.  More troubling are the high number of pipeline explosions that seem to occur, possibly with increasing frequency as older pipes fail.  A Google search of "Natural Gas Explosion" brings up a long list, many with shocking video footage.  This site seems to track some of the latest incidents Recent NG explosions  I'm quite happy that I don't have a NG pipe going into my house or even in my neighborhood.
Using CNG in an inefficient ICE vehicle at maybe 20% efficiency makes little sense when it can be use in combined cycle generating plants at 60% efficiency to charge EV's.  Additionally a few pipelines going to a few generating plants are much more efficient and easier to monitor and maintain than millions of pipelines going to millions of homes and public fill stations.  If we are going to use CNG lets not waste it in ICE's, let's use it to displace coal fueled electricity.

Another more detailed look at CNG for transportation:
CNG analysis
*Update:  A recent study suggests that NG may have a larger carbon footprint than previously thought.
NG's higher emissions
Industry insiders question the realities of NG production
Problems with NG production
Smaller NG reserves than previously thought
Hinchey on Gas Reserves 
Additional data on energy required to compress and transport NG by Rick Kermentz on Seeking Alpha:
 The collection network (from a group of wells) is generally low pressure. The gas is scrubbed of sulfur compounds, odorant added, and compressed to 1,500 psi for long distance transport, with additional compressor stations every 40-60 miles. The pressure is dropped backed down to typically 100 psi for city distribution, and often down to 3 psi for residential distribution. A high volume NG filling station probably would access gas at 100-200 psi.

To get a reasonable range in a CNG vehicle, the gas will be pressurized to 5,000-10,000 psi. Not only is there significant energy consumed compressing the gas, the thermal heat of compression is totally lost.

CNG Bus Fire
Bus Fire
CNG Bus Explosion
Bus Explosion