Energy Efficiency

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Energy efficiency is defined as the higher heating value (HHV) of hydrogen divided by the energy consumed by the electrolysis system per kilogram of hydrogen pro-

Table 3. Efficiency of selected electrolyzers in the market.

Energy required system (kWh/kg)

System efficiency (%)

Production pressure

Ref.

AccaGen SA

74.5-52.4

52-74

Up to 200 bar

29

Hydrogenics

53.4

73

363 psi

37

IHT

46.7-51.2

84-76

Up to 32 bar

29

PIEL division of ILT Technology s.r.l.

77.9

50

3 bar

31

duced. See Eqs. 5 and 6 for details on this calculation. HHV is used as opposed to the lower heating value (LHV) because in commercial electrolyzers in 2005, the water electrolyzed is in a liquid state. To further clarify why HHV is used, the reaction of the formation of water is:

At 25 °C and 1 atm, the heat of formation of liquid water, or the energy released when water is formed in the reaction above is 39 kWh kg-1 of hydrogen. This value is the higher heating value (HHV) of hydrogen. The heat of formation of steam is 33 kWh/kg of hydrogen, and is the lower heating value (LHV) of hydrogen. The electrolysis reaction is the opposite of the formation of water reaction:

As a result, the amount of energy needed to create hydrogen from liquid water using electrolysis is 39 kWh kg-1. The reason this distinction is important is because if using the lower heating value the efficiency of electrolyzers is misrepresented. If LHV is used to calculate electrolyzer efficiencies, the maximum hydrogen system efficiency is 33/39 or approximately 85%. Thus a 100% efficient electrolysis system on an LHV basis is actually thermodynamically impossible if you are electrolyzing liquid water. That is to say that an electrolyzer that converts every kWh of input energy into hydrogen energy will have only 85% efficiency, even though there are no losses.41

The energy efficiency of several electrolyzers is shown in Table 3. The energy efficiency ranges of commercial systems ranges from 47-77 kWh/kg (83-51%). An efficiency goal for electrolyzers in the future has been reported to be in the 46.9 kWh kg-1 range, or a system efficiency of 83%.42 This 83% includes compression of the hydrogen gas to 6000 psig. currently most electrolyzers reach a pressure ranging from 0-500 psig for the power requirements presented, with a few research stage electrolyzers reaching pressures in the 3000-6500-psig range. So most electrolyzers would need additional energy input beyond what is presented in the table below to compress to fueling pressures.

Note that the above values are energy requirement of the entire electrolysis system, excluding any additional compression beyond what the stack produces. This is an appropriate way to calculate system efficiency. As an example, the electrolyzer

Solar Powered Electrolysis Water
Fig. 7. Hydrogen costs via electrolysis with only electricity costs considered.

stack alone for a Hydrogenics system requires 46.8 kWh kg-1 (4.2 kWh Nm-3), which corresponds to 83% efficiency when you divide the HHV of hydrogen by the electrolyzer power requirement. However, when you include the rectifier and auxiliaries the energy requirement becomes 53.4 kWh kg-1 (4.8 kWh Nm-3) or 73% efficient.

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