Christopher P. Garcia, Bei-jiann Chang, and Donald W. Johnson

QSS Group, Inc., Cleveland, Ohio

David J. Bents and Vincent J. Scullin

Glenn Research Center, Cleveland, Ohio

Ian J. Jakupca

Analex Corporation, Brook Park, Ohio

Round Trip Energy Efficiency of NASA Glenn

Regenerative Fuel Cell System

NASA/TM—2006-214054

January 2006
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Christopher P. Garcia, Bei-jiann Chang, and Donald W. Johnson

QSS Group, Inc., Cleveland, Ohio

David J. Bents and Vincent J. Scullin

Glenn Research Center, Cleveland, Ohio

Ian J. Jakupca

Analex Corporation, Brook Park, Ohio

Round Trip Energy Efficiency of NASA Glenn

Regenerative Fuel Cell System

NASA/TM—2006-214054

January 2006

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

NHA Annual Hydrogen Conference 2006

cosponsored by Air Products, BOC, Chevron, AQMD,

Solar Integrated Technologies, BMW, NASA, EPRI, and Linde

Long Beach, California, March 12–16, 2006
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

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This report is a preprint of a paper intended for presentation at a conference. Because

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Available electronically at http://gltrs.grc.nasa.gov

This work was sponsored by the Low Emissions Alternative

Power Project of the Vehicle Systems Program at the

NASA Glenn Research Center.
NASA/TM—2006-214054 1

Round Trip Energy Efficiency of NASA Glenn 

Regenerative Fuel Cell System 

Christopher P. Garcia, Bei-jiann Chang, and Donald W. Johnson 

QSS Group, Inc. 

Cleveland, Ohio 44135 

David J. Bents and Vincent J. Scullin 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Ian J. Jakupca 

Analex Corporation 

Brook Park, Ohio 44142 

Abstract 

NASA Glenn Research Center (GRC) has recently demonstrated a Polymer Electrolyte Membrane (PEM) based 

hydrogen/oxygen regenerative fuel cell system (RFCS) that operated for a charge/discharge cycle with round trip efficiency (RTE) 

greater than 50 percent. The regenerative fuel cell system (RFCS) demonstrated closed loop energy storage over a pressure range

of 90 to 190 psig. In charge mode, a constant electrical power profile of 7.1 kWe was absorbed by the RFCS and stored as 

pressurized hydrogen and oxygen gas. In discharge mode, the system delivered 3 to 4 kWe of electrical power along with product 

water. Fuel cell and electrolyzer power profiles and polarization performance are documented in this paper. Individual cell 

performance and the variation of cell voltages within the electrochemical stacks are also reported. Fuel cell efficiency, electrolyzer 

efficiency, and the system RTE were calculated from the test data and are included below. 

Introduction 

The hydrogen-oxygen RFCS has been promoted as one of 

the most favored energy storage technologies for solar electric 

power in aerospace applications, mainly due to its high (300 to 

1000 Whr/kg ) specific energy. A RFCS could provide much 

higher specific energies than any advanced battery system and 

potentially the highest storage capacity and lowest weight of 

any non-nuclear device. Due to irreversible heat generation the 

RTE, that is, the fraction of energy put into the system that 

actually gets delivered back to the user, is limited to about 

75 percent. 

Round trip efficiency is one of the most important attributes 

for the user since it dictates, along with the energy density, 

how much storage media, or storage installation size, will be 

required in order to accommodate user energy demands and 

how much power from the primary source will be required to 

replenish that storage. For many mechanical, and some 

magnetic and electrochemical storage devices, the round trip 

efficiencies can be as high as 80 percent. However, they do 

not compete with the RFC on energy content per unit weight 

basis. 

Performance estimates made for recent hydrogen oxygen 

PEM RFCS conceptual designs have predicted round trip 

efficiencies of roughly 50 to 60 percent depending on stack 

polarization performance and current densities applied. This 

paper presents electrochemical stack performance and round 

trip energy storage efficiencies from closed cycle hydrogen - 

oxygen PEM RFCS which is being operated as an energy 

storage device at NASA GRC. 

System Description

The RFC system is a completely closed loop test bed for the 

cyclic operation of fuel cell systems up to 60 kWh 

(5 kWe times 12 hr). The NASA GRC RFCS is shown in 

figure 1. Closed loop means the system with its 

electrochemical reactants and products is completely sealed; 

nothing goes into the system other than electrical power and 

there are no discharges or emissions from the system other 

than electrical power and waste heat. The RFC system can 

accommodate a fuel cell stack up to 5.25 kWe (100 A at 

52.5 V) capacity and an electrolyzer stack up to 15 kWe 

(150 A at 100 V). The system has a maximum operating 

pressure of 400 psig with hydrogen and oxygen storage 

capacities of 40 and 20 ft3

, respectively. The electrolyzer 

operates at the system pressure while the fuel cell reactant 

feeds and recirculation loops are regulated down to 50 psig. 

During the charge cycle the electrolyzer charges the reactant 

tanks to full pressure (400 psig) while running under a current 

profile that is either fixed, or shaped to simulate the output of 

a flat plate solar array. During the discharge cycle, the fuel cell 

delivers electrical power and product water from the stored 

reactants. 
NASA/TM—2006-214054 2

Figure 1.—PEM RFCS at NASA GRC. 

A DC power supply simulates the solar input to the 

electrolyzer and a DC load is used to sink the fuel cell current. 

The closed loop water supply provides source and cooling 

water for the electrolyzer, humidification, and product 

collection for the fuel cell. Data acquisition and control for the 

RFC is provided by a multithreading suite of NASA GRC 

developed software using the National Instruments 

LabVIEWTM programming language. A more detailed 

discussion of the NASA GRC RFCS can be found in 

references 1 to 4. 

Test Summary 

The test for round trip efficiency was performed July 14th 

and 15th of 2005. The test consisted of an electrolysis (charge) 

cycle and a fuel cell (discharge) cycle. The electrolysis cycle 

began with the hydrogen and oxygen reactant storage tanks 

each at a pressure of 90 psig, temperatures of 83 and 85 °F, 

respectively and the system fully charged with deionized 

water. Charging of the reactant storage tanks was completed 

over a time period of 3 hr and 45 min, with the hydrogen and 

oxygen storage tanks each reaching a pressure of 190 psig and 

temperatures of 86 and 88 °F, respectively. While charging the 

reactant storage tanks the RFC absorbed a constant electrical 

power input of 7.1 kW and the operating temperature of the 

electrolyzer was maintained at 140 °F. 

Hydrogen and Oxygen Reactant Storage Pressure Profile

80

100

120

140

160

180

200

0.000 1.000 2.000 3.000 4.000 5.000 6.000 7.000 8.000 9.000

Elapsed Time (hrs)

Pressure (psig)

H2 Pressure O2 Pressure

Figure 2.—Hydrogen and oxygen reactant pressure profile. 

The fuel cell cycle began with the hydrogen and oxygen 

reactant storage tanks each at a pressure of 190 psig and 

temperatures of 82 and 85 °F, respectively. Discharge or fuel 

cell mode was completed over a time period of 4 hr and 

22 min. During discharge the hydrogen and oxygen storage 

tanks were depleted to a pressure of 90 psig and the 

temperatures of the storage tanks rose to 85 and 87 °F, 

respectively. A plot showing the reactant pressure profiles 

during both the charge and discharge cycles is presented in 

figure 2. While discharging the reactant storage tanks the RFC 

delivered an electrical power output of 3.0 to 4.0 kW and the 

operating temperature of the fuel cell was maintained at 

135 °F. 

Results and Discussion 

During the charge cycle the electrolyzer operating pressure 

followed the pressure of the reactant storage tanks, varying 

from 90 to 190 psig. Initially, the electrolyzer operating 

temperature was set to 140 °F but was increased (to get better 

cell voltage performance) throughout the charge cycle to a 

maximum of 147 °F. While charging the reactant storage tanks 

the RFCS absorbed a constant electrical power input of 

7.1 kW and electrolyzer current was maintained at 75 A. The 

power versus time profile for the electrolyzer during the 

charge cycle is presented in figure 3. Integration of the area 

under this curve yields the total kilowatt-hours input into the 

RFCS during the electrolysis cycle. While absorbing 

27.23 kWhr of electrical energy the RFCS generated 

317 moles of gaseous hydrogen and 158 moles of gaseous 

oxygen, raising the pressure in reactant storage tanks from 90 

to 190 psig. 

During the discharging of the reactant storage tanks the 

RFCS delivered an electrical power output of 3.0 to 

4.0 kW, corresponding to a fuel cell current range of 60 to 

85 A and a stack voltage that varied from 51 to 48 V. The fuel 

cell operating pressure was 63 psig and its operating 

temperature was maintained at 135 °F. The power versus time 

profile for the fuel cell during the discharge cycle is presented 
NASA/TM—2006-214054 3

Electrolyzer Power Profile

0

2000

4000

6000

8000

10000

12000

14000

16000

9:36:00 AM 10:48:00 AM 12:00:00 PM 1:12:00 PM 2:24:00 PM

Time

Power (Watts)

EZ Power Input (Watts)

Figure 3.—Electrolyzer power profile. 

Fuel Cell Power Profile

0

500

1000

1500

2000

2500

3000

3500

4000

4500

9:36:00 AM 10:48:00 AM 12:00:00 PM 1:12:00 PM 2:24:00 PM

Time

Power (Watts)

Fuel Cell Power Profile

Figure 4.—Fuel cell power profile. 

in figure 4. Integration of the area under this curve yields the 

total kilowatt-hours delivered by the RFCS during the fuel cell 

cycle. While discharging the reactant storage tank pressures 

from 190 to 90 psig the actual energy output from the RFCS 

was 13.73 kWhr. 

The round trip efficiency (RTE) of the NASA GRC RFCS 

was calculated for this charge/discharge cycle which covered a 

pressure range of 100 psig. The total energy input to the 

system while the electrolyzer charged the reactant storage 

tanks was 27.23 kWhr. While discharging the reactant storage 

tanks the fuel cell was able to deliver 13.73 kWhr of energy. 

This resulted in RTE of 50.4 percent. Comparing the energy 

potential of the mass of hydrogen produced during electrolysis 

to the actual power delivered by the fuel cell yields a similar 

RTE. The electrolysis cycle produced 640 gms of hydrogen, 

which under ideal conditions could yield 25.42 kWhr of 

electrical energy. Despite the amount of potential energy 

available, the fuel cell was only able to deliver 13.73 kWhr of 

energy, resulting in a theoretically determined RTE of 

54 percent. 

Prior to beginning the charge cycle at de-rated power, the 

electrolyzer was ramped up to full power (to aid in getting the 

Electrolyzer Polarization 

0.0000

0.2000

0.4000

0.6000

0.8000

1.0000

1.2000

1.4000

1.6000

1.8000

2.0000

0 100 200 300 400 500 600 700 800

Current Density (ma/cm^2)

Average Cell Voltage (V)

Figure 5.—Electrolyzer polarization performance. 

Electrolyzer Cell Voltage and Efficiency Profile

1.5

1.52

1.54

1.56

1.58

1.6

1.62

1.64

10:04 10:33 11:02 11:31 12:00 12:28 12:57 13:26 13:55 14:24 14:52

Time

Cell Voltage (V)

0.88

0.9

0.92

0.94

0.96

0.98

1

Efficiency (%)

EZ11 (L) EZ24(H) EZ Avg CV EZ Eff

Figure 6.—Electrolyzer cell voltage and efficiency profile. 

stack to its operating temperature) and a polarization plot was 

generated. This plot, shown in figure 5, was taken at a system 

pressure of 70 psig. The polarization data shown is not at 

constant temperature, but over a range of operating 

temperatures that rose from 68 to 115 °F as current was 

ramped up. 

The average cell voltage of the electrolyzer throughout the 

charge cycle was 1.6 V which compared to the 1.48 V thermo 

neutral point, leads to an operating efficiency greater than 

92 percent. Figure 6 shows the electrolyzer high, low, and 

average cell voltages along with the operating efficiency 

versus time. 

Prior to beginning the discharge cycle at de-rated power the 

fuel cell was ramped up to full power (to aid in getting the 

stack to its operating temperature) and a polarization plot was 

generated. System pressure was 200 psig. Since the stack is 

actively cooled, the performance recorded was not at constant 

temperature but over a range of operating temperatures that 

varied from 68 to 140 °F roughly corresponding to load 

current applied. This polarization performance is shown in 

figure 7. 
NASA/TM—2006-214054 4

Fuel Cell Polarization

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 50 100 150 200 250 300 350 400 450 500

Cureent Density (mA/cm^2)

Average Cell Voltage (V)

Figure 7.—Fuel cell polarization performance. 

Fuel Cell Voltage and Efficiency Profile

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

10:04 11:16 12:28 13:40 14:52

Time

Cell Voltage (V)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Efficiency

FC39 FC50 FCavg Efficiency

Figure 8.—Fuel cell voltage and efficiency profile. 

During the discharge cycle a load current of 70 A was 

sustained, which corresponds to a stack current density of 

350 mA/cm2

. The average cell voltage of the fuel cell 

throughout the discharge cycle was 0.80 V which, compared 

to the 1.48 V thermo neutral point, leads to an operating 

efficiency of 54 percent. Figure 8 shows the fuel cell high, 

low, and average cell voltages along with the operating 

efficiency versus time. 

The change in Gibbs free energy and the change in enthalpy 

can be used to determine the maximum possible operating 

efficiency of the fuel cell; in this case the efficiency limit of 

the fuel cell was calculated to be 80 percent. Table 1 gives the 

thermodynamic values used to determine the efficiency limit, 

relative to the HHV, for a fuel cell operating at 135 °F. 

TABLE 1 

Form of water 

product 

Temp 

(°F) 

Δh f

mol

kJ

Δg f

mol

kJ

Max 

EMF 

(V) 

EFF 

Limit 

Liquid 135 –287 –231 1.20 80 % 

Fuel Cell Thermal Performance

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

7:12 8:24 9:36 10:48 12:00 13:12 14:24 15:36 16:48

Time

Heat (W)

100

110

120

130

140

150

160

170

180

190

200

Temperature (F)

wast heat CL111(out) CL115 (in)

Figure 9.—Fuel cell thermal performance. 

Electrolyzer Thermal Performance

0

200

400

600

800

1000

1200

10:48 11:16 11:45 12:14 12:43 13:12 13:40

Time

Heat (W)

100

110

120

130

140

150

160

170

180

190

200

Temperature (F)

Waste Heat DI102 XO142

Figure 10.—Electrolyzer thermal performance. 

The actual operating efficiency of the fuel cell, 54 percent, 

was much less than this ideal value because of voltage and 

current losses. The major factors that contribute to bringing 

down the open circuit and operating voltages of fuel cells are 

mass transport losses, ohmic losses, activation losses, and fuel 

crossover. It is evident from looking at the overall energy 

balance that the fuel cell was the major contributor to the 

overall system inefficiency. During fuel cell operation the 

irreversibility’s mentioned above appear as waste heat. 

Figure 9 shows the fuel cell coolant inlet and outlet 

temperatures during the discharge cycle. Also shown in the 

plot is the waste heat, calculated based on temperature rise and 

coolant flow rate. The amount of waste heat calculated 

accounted for the fuel cell inefficiency. 

Figure 10 shows a similar plot for the electrolyzer. As 

expected from its higher operating efficiency, the amount of 

waste heat generated was much lower than that of the fuel cell. 

For RTE comparison it is instructive to consider 

performance that was observed during a recent five day 

endurance run (ref. 5) where the electrolyzer input power 

profile was time-varied to duplicate the power delivery of a 

flat plate solar collector and the fuel cell was run at its 

maximum power. While charging the reactant storage tanks 
NASA/TM—2006-214054 5

during the endurance test, the electrolyzer operated with an 

efficiency between 88 and 92 percent, depending on the 

current input, which varied from 50 to 150 A. The fuel cell, 

drawing a constant current of 100 A, discharged the reactant 

storage tanks with an efficiency of 48 percent. In this case the 

fuel cell was operating at a current density of 500 mA/cm2

, 

and an average cell voltage of only 0.71 V per cell. This is 

0.09 V less than the 0.80 V per cell observed during the 

efficiency run, when load current was limited to 350 A. For 

the five day endurance run the RTE of the system (operating 

over the same storage tank pressure range as the July 14th and 

15th tests) was no better than 37 percent. Since the fuel cell 

stack was operating at a higher current density, the cell 

voltages were lower, resulting in a lower RTE. 

Obviously, running the RFC system at maximum capacity 

without de-rating has the advantage of delivering more power, 

but the RTE suffers. However, efficiency often gets traded 

away in favor of reduced mass in aerospace applications. 

Considering the entire charge/discharge cycle, the energy 

losses (hence, efficiency losses) can be attributed mainly to 

the fuel cell stack. These losses are exacerbated by running the 

stack at high current density in order to achieve rated power. 

This can be seen not only in the polarization curve (fig. 7) 

which shows the stacks average cell voltage, but also in the 

individual cell voltages which are captured in figure 11 and 

figure 12. Figure 11 is a snapshot of the stacks individual cell 

voltages taken during the five day endurance run, at a stack 

current of 100 A, which corresponds to a current density of 

500 mA/cm2

. Figure 12 shows individual cell voltages at a 

stack current of 60 A, taken during the July 14th and 15th 

efficiency run. Here the stack current of 60 A corresponds to a 

current density of 300 mA/cm2

. When current density is 

reduced, the cell voltages are much higher and the 

performance is more stable with less variation in voltage from 

cell to cell. This implies getting more power from the RFCS 

while trying to maintain a system RTE greater than 50 percent 

with the current fuel cell stack will require an increase in 

active area. This can be achieved through increased cell count, 

more active area per cell, or both. Increasing active area 

enlarges the stack which means the RFCS will increase in size, 

cost, and weight correspondingly. 

Conclusions 

NASA Glenn performance measurements on the RFCS 

indicate that contemporary hydrogen-oxygen PEM hardware 

operating in the current density regime of 300 to 500 mA/cm2

(fuel cell) and 500 to 900 mA/cm2

 (electrolyzer) can yield 

round trip energy storage efficiencies from 35 up to 

50 percent. Reducing stack current density improves 

efficiency at the expense of power throughput, with the fuel 

cell being the most sensitive component subject to the greatest 

variation. 

Fuel Cell Performance

0.6

0.65

0.7

0.75

0.8

0.85

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65

Cell Number

Voltage (V)

Figure 11.—Fuel cell performance at 100 A, 

65 psig, and 135 °F. 

Fuel Cell Performance

0.6

0.65

0.7

0.75

0.8

0.85

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63

Cell Number

Voltage (V)

Figure 12.—Fuel cell performance at 60 A, 

65 psig, and 135 °F. 

In the future the fuel cell stack will be replaced with more 

advanced stacks to achieve higher round trip efficiencies. The 

system has also recently completed a contiguous five-cycle 

endurance demonstration at rated pressure and power (ref. 5). 

Future plans include fully autonomous operation and 

demonstration of tens of charge/discharge cycles. Currently, 

the RFCS is programmatically part of the NASA Low 

Emissions and Alternative Power (LEAP) Program to further 

develop aerospace regenerative fuel cells for high altitude 

applications and as a surface power source for planetary 

missions. 

Reference 

1. D.J. Bents, V.J. Scullin, B.J. Chang, D.W. Johnson, and 

C.P. Garcia “Hydrogen-Oxygen PEM Regenerative Fuel 

Cell Energy Storage System,” 2004 Fuel Cell Seminar, 

San Antonio, TX, Nov. 1–5, 2004, NASA/TM—2005-

213381. 
NASA/TM—2006-214054 6

2. V.J. Scullin, D.J. Bents, et al, “Closed Loop H2/O2 Fuel 

Cell Test Facility at NASA Glenn,” Proc. Hydrogen and 

Fuel Cell Futures Conference,” Perth, Australia, Sept. 12–

15, 2004. 

3. B.J. Chang, D.W. Johnson, C.P. Garcia, I.J. Jakupca, 

V.J. Scullin, and D.J. Bents “Regenerative Fuel Cell Test 

Rig at Glenn Research Center,” Proc. First International 

Energy Conversion Engineering Conference (AIAA, 

ASME, IEEE), Portsmouth, VA, Aug. 17–21, 2003, 

AIAA–2003–5942, NASA/TM—2003–212375. 

4. D.J. Bents, V.J. Scullin, B.J. Chang, D.W. Johnson, and 

C.P. Garcia “PEM Hydrogen-Oxygen Regenerative Fuel 

Cell at NASA Glenn Research Center,” Proc. 8th ANSE 

Electrochemical Power Sources R&D Symposium, 

Portsmouth, VA, July 21–24, 2003. 

5. D.J. Bents, V.J. Scullin, J.E. Freeh, D.A. Jacqmin, B.J. 

Chang, D.W. Johnson, C.P. Garcia, and I.J. Jakupca 

“Regenerative Fuel Cell Development at NASA Glenn 

Research Center,” to be presented at 2005 Fuel Cell 

Seminar, Palm Springs, CA Nov. 14–15, 2005. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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E–15411

WBS–22–066–20–04

12

Round Trip Energy Efficiency of NASA Glenn Regenerative Fuel Cell System

Christopher P. Garcia, Bei-jiann Chang, Donald W. Johnson, David J. Bents,

Vincent J. Scullin, and Ian J. Jakupca

Fuel cells; Regenerative fuel cells; Hydrogen/oxygen

Unclassified -Unlimited

Subject Category: 44

Prepared for the NHA Annual Hydrogen Conference 2006 cosponsored by Air Products, BOC, Chevron, AQMD, Solar

Integrated Technologies, BMW, NASA, EPRI, and Linde, Long Beach, California, March 12–16, 2006. Christopher P.

Garcia, Bei-jiann Chang, and Donald W. Johnson, QSS Group, Inc., 21000 Brookpark Road, Cleveland, Ohio 44135;

David J. Bents and Vincent J. Scullin, NASA Glenn Research Center; and Ian J. Jakupca, Analex Corporation, 1100

Apollo Drive, Brook Park, Ohio 44142. Responsible person, Christopher P. Garcia, organization code DR, 216–433–3933.

NASA Glenn Research Center (GRC) has recently demonstrated a Polymer Electrolyte Membrane (PEM) based

hydrogen/oxygen regenerative fuel cell system (RFCS) that operated for a charge/discharge cycle with round trip

efficiency (RTE) greater than 50 percent. The regenerative fuel cell system (RFCS) demonstrated closed loop energy

storage over a pressure range of 90 to 190 psig. In charge mode, a constant electrical power profile of 7.1 kWe was

absorbed by the RFCS and stored as pressurized hydrogen and oxygen gas. In discharge mode, the system delivered

3 to 4 kWe of electrical power along with product water. Fuel cell and electrolyzer power profiles and polarization

performance are documented in this paper. Individual cell performance and the variation of cell voltages within the

electrochemical stacks are also reported. Fuel cell efficiency, electrolyzer efficiency, and the system RTE were

calculated from the test data and are included below.