NGV Calcs

As a gas, most people have difficulty perceiving how much fuel they are buying as they have been accustomed to buying by the litre or gallon. To make it easier for people to understand, the gas industry has devised an gasoline volume equivalent:   Gasoline Gallon Equivalent (GGE - USA) and the Gasoline Litre Equivalent (GLE - Canada).

Natural Resources Canada:

• How is natural gas sold? In a retail service station, natural gas is sold by mass in kilograms. With on-site fuelling, it is typically sold by volume in cubic metres or in units of energy called gigajoules. In terms of heating values,
  • 1 kilogram of natural gas is equivalent to 1.52 litres of gasoline
  • 1 cubic metre of natural gas is equivalent to 1.09 litres of gasoline
  • 1 gigajoule of natural gas is equivalent to 28.85 litres of gasoline

Pacific Gas and Electric Company:

Fuel Costs - An Energy Value Comparison As the adage goes, it is appropriate to compare apples to apples when evaluating fuel costs. As we have seen, liquid fuels are measured and sold by volume (gallons), not energy content. Natural gas is measured by volume or weight (standard cubic feet or pounds), but is sold by energy content (therms).

To begin making an economic comparison of fuels, it is easiest to compare the prices based on energy content - after all, energy is what you are purchasing, not volume or weight. This method of comparing fuels yields a gasoline or diesel gallon equivalent price for CNG, sometimes abbreviated as GGE or DGE. Let's look at a gasoline/CNG comparison, using retail prices: Gasoline in California is now standardized at 110,400 Btu/gallon, regardless of refiner or location. In March 1998, the retail price of gasoline averaged $1.07 per gallon in California, according to the California Energy Commission. PG&E sells CNG at its fueling stations for $0.78104 per Therm (plus a monthly customer charge of $13.42 per account - no vehicle limit). A THERM of CNG has an energy value of 100,000 BTUs. For comparison, how much does CNG cost for the equivalent amount of gasoline? The mathematical conversion is:

• Take the price of CNG, multiply it by the Energy value of liquid fuel, then divide by the Energy value of CNG.
• For this example, $0.78104 per Therm is the price of CNG, 110,400 BTU is the energy value of liquid fuel, and 100,000 BTU per therm of CNG is the Energy value of CNG.

The cost of CNG for the equivalent amount of gasoline is $0.86 per gallon. Therefore, based on our assumptions, compressed natural gas is retailing at $0.21 less than gasoline, on an gallon equivalent basis. If your assumptions stay the same regarding the energy content of the fuels you are comparing, you can simply use a conversion factor to compare price. Using the assumptions above: Cost per Therm CNG x 1.10 = cost for gasoline gallon equivalent. For diesel fuel, assuming a diesel energy value of 125,000 BTU per gallon, the conversion factor in the equation would be 1.25.

Standard Conversion Factor Adopted Nationwide.

Because CNG fueling dispensers measure the weight of the natural gas, which is then converted to an energy value based on a measurement of the composition of the gas, it is not possible to display an energy value-based liquid fuel gallon equivalent on the dispenser. However, dispensers often show a gallon gasoline equivalent display, using the national conversion factor. The national standard conversion factor of 1.276 is an approximation based on the average energy value of a gallon of gasoline and the average energy value of 5.66 pounds of natural gas.

Using the nationally accepted conversion factor, PG&E retails compressed natural gas for $0.996 per gasoline gallon equivalent. Note that because gasoline in California has a lower energy value than the national average, and natural gas in California has a higher energy value than the national average, the standard conversion factor overstates the cost of a gallon gasoline equivalent by about 16 percent.

In California, 1 gallon gasoline = 5.66 lb NG / 1.16 = 4.88 lb NG according to PG&E.

1 BTU (British Thermal Unit) is the amount of energy required to raised 1 lb water 1°F.  Depending upon the reference temperature, the BTU can vary slightly.

1 BTU = 1055.056 J (Joule)
1 calorie = 4.1868 J
1 MBTU = 10³ BTU (1,000 BTU)
1 MMBTU = 10⁶ BTU (1 million BTU)
1 Therm = 10⁵ BTU (100,000 BTU)
1 MBTU = 10 Therm
1 MCF = 10³ CF (1000 Cubic Feet)
1 MMCF = 10⁶ CF (1 million Cubic Feet)

When you convert from CF of natural gas (or of any fuel) to BTUs, you can use either the lower heating value (LHV) or the higher heating value (HHV). The higher heating value (HHV) includes the energy contained in the latent heat of evaporation of the moisture contained in the burned fuel (actually bringing all products of combustion to 25°C/77°F). Since most processes do not recover this heat, the LHV of the fuel is often used (actually bringing all products of combustion to 150°C/302°F). For natural gas, the ratio between the two is about 1.1.  If you're NOT condensing the steam in the exhaust gases back in liquid water, you need to use LHV in your calculations.

On the average, because the heating value of natural gas varies from gas well to gas well, the energy density of natural gas can vary from 970 BTU/CF to 1130 BTU/CF HHV or 880-1030 BTU/CF LHV. Because it is such a nice round number, 1000 BTU/CF HHV is often used in energy calculations. Therefore, 1 MCF contains about 1000 MBTU HHV in very round numbers.

The US DoE's Transport Energy Book has useful conversion data, which likely uses data from Center for Transportation Analysis (CTA):  Lower and Higher Heating Values of Gas, Liquid and Solid Fuels.   It is similar to data from the AFDC Property of Fuels Table.

Using CTA data,

However, Wikipedia's Gasoline Gallon Equivalent article refers to the EPA's Fuel Economy Impact Analysis of RFG, which uses 114,000 BTU/gallon LHV (Winter Maximum) as the basis for their GGE calculations.  The US DoE recommended data in the Transport Energy Data book as the basis.  However, in the USA, the Department of Commerce in each state determines the basis for pricing so there is no national standard for energy content.  Presumably, the natural gas utilities will also incorporate their own pipeline gas analysis in the GGE calculation as well.  The Wikipedia's GGE article also uses 900 BTU/CF for natural which is close to the rule of thumb value of 1000 BTU/CF HHV.

Using Reformulated Gasoline,  113,602 BTU/gallon / 983 BTU/CF = 115.57 CF/gallon .  Therefore, 1 GGE = 115.57 CF of natural gas.

Similarly, 113,602 BTU/gallon / 20267 BTU/lb = 5.605 lb/gallon.  Therefore, 1 GGE = 5.605 lb of natural gas.  This is slightly lower that the 5.66 lb suggested by PG&E.

1 gallon = 113,602 BTU
1 BTU = 1/113,602 gal
1 MMBTU = 1/113,602 x 10⁶ gal  = 8.80 gal

In terms of Gasoline:

Using CTA data,

However, Wikipedia's Gasoline Gallon Equivalent article uses 129,500 BTU/gallon LHV as the basis for their GGE calculations, which is reasonably close to the CTA's Low-Sulfur diesel value.  The US DoE recommended data in the Transport Energy Data book as the basis.  However, in the USA, the Department of Commerce in each state determines the basis for pricing so there is no national standard for energy content.  Presumably, the natural gas utilities will also incorporate their own pipeline gas analysis in the DGE calculation as well.  The Wikipedia's GGE article also uses 900 BTU/CF for natural which is close to the rule of thumb value of 1000 BTU/CF HHV.

Using Low-sulfur diesel,  129,488 BTU/gallon / 983 BTU/CF = 131.73 CF/gallon .  Therefore, 1 DGE = 131.73 CF of natural gas.

Similarly, 129,488 BTU/gallon / 20267 BTU/lb = 6.389 lb/gallon.  Therefore, 1 DGE = 6.389 lb of natural gas.

1 gallon = 129,488 BTU
1 BTU = 1/129,488 gal
1 MMBTU = 1/129,488 x 10⁶ gal  = 7.72 gal

1 litre = 3.785 gal, therefore 1 DLE = 6.389/3.785 = 1.688 lb

1 DLE = 1.688 lb x 2.204 lb/kg = 3.721 kg

Often, CNG is advertised in terms of GGE.  So 129,488 BTU/gallon / 113,602 BTU/gallon = 1.1398 or  ~1.14 GGE = 1 DGE

Similarly, a Diesel Litre Equivalent (DLE) relative to a Gasoline Litre Equivalent (GLE) is also 1.398 GLE = 1 DLE

In Terms of Diesel: