Saturday, November 24, 2012

Volt Versus CNG Civic

Tom an avid reader of the Green Machine sent me an email and asked why not use natural gas to generate electricity and then use the electricity to power a plug in car like the Tesla.  Well I would never allow anyone to use good electricity to fuel a Tesla but I will do the analysis of using grid electric power generated from natural gas to propel a Chev Volt versus a CNG powered Honda Civic.

The CNG Civic can be bought for $26,305 and averages 33 MPG between city and highway driving.  The Volt can travel a maximum of 40 miles on batteries and uses 0.4 kwh (DC) per mile.  To help the Volt let’s assume the power station is a combined cycle power station with 57% efficiency (AC) and that 10% of the AC power is lost getting the AC power to the plug in the motorist’s home.  Another 8% is lost getting the AC power as DC power in the battery.   This means 0.48 kwh of AC power must be generated at the power station for each mile the Volt travels.   2,873 BTUs (LHV) of natural gas are needed to generate this amount of power for one mile of travel in the Volt.  The Civic needs 3,485 BTUs per mile of travel and the compression of the CNG grosses this up by 5% to require 3,668 BTUs of natural gas from the utility per mile traveled in the Civic.

Clearly the plug in Volt uses less natural gas than the CNG civic to travel a mile (2,873 versus 3,668 BTUs).  Now let’s see how much CO2 is saved per mile by driving the Volt versus the CNG Civic.  The math is tedious but I calculate the saving to be approximately 0.1 pounds of CO2 per mile.  The Volt costs $39,145.  Let’s ignore taxes and grants and simply compare the cost to society of the Volt versus the CNG Civic.  The added first cost is $12,840.  Let’s also assume that the cost of purchased electricity per mile is the same as the cost of purchased compressed gas and that the motorist spends a similar amount for an electric charger as they would for a gas compressor.  This is the case if purchased electricity is 10 cents per kilowatt hour and natural gas is purchased at $1.20 per therm higher heating value.  The chargers and compressors each cost about $2,000.

If we amortize the added first cost over 8 years and assume the motorist drives 12,000 miles a year, we have an added cost of per year of $1,605 to save 1,200 pounds of CO2.  This is a cost of $2,675 per US ton of CO2 emissions avoided.  In Europe a US ton of CO2 trades for about $8 and in California the recent Cap and Trade auction yielded approximately $12 per US ton.  Australia has a carbon tax of approximately $20 per US ton.

By this example it is simply far too high a collective carbon tax to convert natural gas to electricity and use this electricity as a vehicle fuel when a simple CNG vehicle is the far more economic alternate.  The government grants given to buy a Volt or other electric cars should rather be given to folks to buy a CNG car.


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