BTUs, CFMs, and GGEs Demystified
In the world of Natural Gas and CNG it seems like everyone uses slightly different units of measure, and as you will see, nothing in the energy business is simple! We hope this page will help clear up any confusion, but if you want the simple “answer” just scroll to the bottom to see how to calculate the cost of CNG versus gasoline or diesel.
Units of Measure and Pricing
MMBTU – One million British Thermal Units or BTUs. Natural gas is generally bought and sold in MMBTUs and future prices are generally quoted in this unit of measure.
Therm – 1 Therm = 100,000 BTUs
SCF – Standard Cubic Foot is one cubic foot of gas at standard temperature and pressure (60 degrees F and sea level). Since both temperature and air pressure affect the energy content of a cubic foot of natural gas, the SCF is a way of standardizing. One SCF = 1020 BTUs.
MCF – An MCF is 1,000 cubic feet. One MCF = 1,020,000 btu’s. People often round to say that one MCF is the same as an MMBTU but one MCF is actually 1.02 MMBTUs.
BCF/TCF – Billion/Trillion Cubic Feet.
Henry Hub – Henry Hub (often abbreviated HH) is a natural gas pipeline hub in Erath, LA that interconnects with 13 interstate and regional pipelines. Most wholesale natural gas prices are quoted at this delivery point with an adder or discount based on local market dynamics and transportation cost. When you see the news reporting Natural Gas is at $3.50 that usually means 1 MMBTU, bought today, to be delivered to Henry Hub next month, costs $3.50.
Gasoline, Diesel and CNG
The energy content of liquid fuels like gasoline and diesel actually varies considerably between summer and winter and also depending on what sort of oxygenate it is blended with (10% ethanol gasoline has a fewer BTUs than gasoline reformulated with MTBE and both have fewer BTUs than pure gasoline). A summer gallon of gasoline will typically contain 114,500 BTUs while a winter gallon is 112,500 BTUs.
GGE – Gallon of Gasoline Equivalent is the typical way CNG is sold at public fueling stations and the typical way that CNG tanks are rated. One standard GGE = 114,000 BTUs which equals 126.67 SCF (126.67). Now, the sharp reader will immediately notice that if an SCF has 1,020 BTUs then 126.67 scf should be 129,000 BTUs so something isn’t adding up! That is something known as “lower heating values” or LHV (also called net calorific value). You can read all about this here, but for the purpose of understanding CNG you need to know that an SCF of Natural Gas only yields 900 BTUs of useable gasoline equivalent energy.
CNG compresses the gas to 3,600 psi (some older vehicles were compressed at 2,400 psi). At this compression level, one GGE requires 0.51 cubic feet of space in a CNG tank. So the interior space of a 20 GGE tank is approximately 10 cubic feet (think roughly 42″ wide, 18″ deep, and 18″ tall).
DGE – Diesel Gallon Equivalent is another way to rate CNG vehicle storage. Since Diesel has a higher energy content than gasoline (129,500 BTUs standard), 1 DGE = 1.136 GGE and 1 GGE = 0.88 DGE. Since most CNG metrics are in GGEs if you want to calculate how many cubic feet would be required for an equivalent number of DGEs, just divide by 0.88 (in terms of Standard Cubic Feet, a DGE = 126.67/0.88 or 143.94 SCF and so forth). The reverse is also true. If, for example, you want to convert a cylinder capacity from GGE to DGE, you can multiply by 0.88. So, for example, a 24 GGE cylinder holds about 21 DGEs.
A simple table of energy equivalents for alternative fuels may be found here if you want to learn more.
The Bottom Line
So where does all this get us? Here is a simple conversion table:
1 GGE = 126.67 scf
1 MMBTU of Gas = 7.74 GGEs
1 DGE = 143.94 scf
1 MMBTU = 6.81 DGEs
In addition to the cost of the natural gas itself, we need to account for the electricity costs used in compressing the fuel for your vehicle. This will add 10-20 cents per GGE depending on the efficiency of your compressor and your electricity rates.
Wikipedia does a nice job of explaining all of this in more detail and also provides links if you want to learn even more.