Let’s see if I’m doing this right…
I think on a small scale cryogenically stored gasses are a non-starter. Liquified natural gas requires so much infrastructure to create, contain, and transport safely and efficiently that it pretty much only works on levels of ‘oil tanker’ in size. Too bad, it would be nice if it would just condense like propane…
So how about compressed natural gas? There’s a bit of infrastructure being built out for use in transportation. I’m curious how it compares to gasoline either for vehicle propulsion or home scale electricity production (like those <10kw dual fuel generators but with CNG instead of propane).
Gasoline Gallon Equivalent
GGE is the measure of how much non-gasoline fuel it takes to equal one gallon of gasoline in energy storage. As far as I can tell, this is purely a measure of BTU’s and does not consider comparative costs of extraction, refinement or storage.
According to the US DOE [State & Alternative Fuel Provider Fleets: Fuel Conversion Factors to Gasoline Gallon Equivalents], CNG measured in gallons at 3600PSI is 0.287. For reference diesel is 1.155*, and LP(liquid propane) is 0.758.
GGE for natural gas is also a bit of a moving target since the mass of CNG you can stuff in a tank at a given pressure is widely variable on temperature- of the ambient air, of the gas as supplied, and even by the rapid temperature and volume changes that depend on how fast the tank is filled.
There are 4 often cited storage tank types, so named types 1-4. I’m going to use the type 1 numbers, as that’s your standard deep-drawn steel industrial tanks. They are heavy, and the other 3 tank-types are some variation of lighter weight aluminum core with kevlar/carbon composite wrapping.
Just for kicks I’m going to imagine converting my F150 to CNG, which currently holds about 22 gallons of gasoline. I get a maximum range of roughly 400 miles on a full tank. Let us also assume engine power outputs are 1:1 between CNG and gasoline.
Cenergy solutions [CNG Tanks | CNG Cylinders | CNG Storage by Cenergy Solutions] type-1 tank with a storage capacity of 133 liters @ 3600psi has a gge of 11.8 and a weight of 295 pounds.
So about the same fuel quantity in GGE would require two of those
- 22gal gasoline * 6lb/gal = 132lbs (tank weight not included)
- 295lb type-1 11.8gal CNG tank * 2 = 590lbs
So by weight we’re about 4.4 times heavier for CNG. I can see now why it’s only been trucks and large construction equipment converted at quantity to CNG thus far.
BTU conversion and kWh?
So as far as I can tell, I should be able to use the GGE number as a direct conversion between energy storage of gasoline and that of CNG.
If so, then 114,000 BTU/gal of gasoline * 0.287 = 32,718 BTU/gal @ 3600psi
- 114,000 BTU = 33.41kWh
- 32,718 BTU = 9.589kWh
At time of writing the national average of CNG is $2.11, gasoline is at $3.39, and diesel is #3.89 [Average Prices].
So that is saving 62% of cost per gallon, somewhat tempered by increased fuel weight and possibly engine efficiency (which I haven’t researched yet)
By weight and by volume CNG is a bit of a bummer for such applications where it’s a concern, but for large vehicles that can handle the weight and perhaps smaller scale fixed-location storage the cost is quite a bit less even if there are increased losses in weight, space, storage/transfer complexity, and whatever comparative efficiencies there are between internal combustion engines for either transport or electrical power generation.
* Which is less than I thought it would be. I guess that says a lot for the ability of diesel engines to extract more of that energy efficiently, between increased compression ratio, longer engine life, and so on. Certainly diesel provides better performance to industry than a mere ~12% increase in power output by GGE.