There is more than one efficiency definition for fuel cells. LHV cell efficiency is based on the lower heating value for hydrogen, which increases the published efficiency. This is an incorret number for fuel cells, and the HHV value should be used. The efficiency for the system of cells must include parasitic power and purge. Finally, if hydrogen is produced from hydrocarbons, the production and transportation efficiencies must be includud. The analysis below illustrates these effects.
All numbers are for 100% hydrogen.
(Single cell LHV efficiency = 57%)
Single cell HHV efficiency= 48%
Total energy input to Fuel Cell System = 418 kW chemical energy
Hydrogen in purge = 16 kW chemical energy (3% purge)
Hydrogen to Fuel Cells = 402 kW chemical energy
Air to Fuel Cells at 2x stoichiometric = 318 cubic feet per minute at 32°F and 1 Atm.
Power to compress air to 50 psig = 43.2 kW
Power to circulate cooling water @120 gallons per minute, and 30 feet of head = 1.3 kW
Fuel Cell outlet power = 402*0.48 = 193.0 kW
Fuel Cell System Net Power out = 193.0-43.2-1.3 = 148.5 kW
Net Fuel Cell System Efficiency = 148.5/(402+16)*100 = 35.5%
This is the true efficiency of the fuel cell system, when all parasitic power and purge are included. There is a huge difference between an advertised LHV efficiency of 57%, and the net fuel cell system efficiency of 35.5% based on HHV.
Hydrogen from reforming is at 65% efficiency (HHV) for a medium size plant, giving hydrogen at 350 psig.
For transportation, compression from 350 to 6000 psig will require about 15 kW, or 15/148.5*100 = 10% of outlet power from the fuel cell. (All hydrogen has to be compressed!)
The overall efficiency of Fuel Cell plus transportation is 133.5/418*100 = 32%.
The overall efficiency of Fuel Cell Systems, starting with natural gas at 350 psig, is only 32*65/100 = 20.7%.