A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Renewable Hydrogen: Integration, 

Validation, and Demonstration 

K.W. Harrison and G.D. Martin 

Prepared for the 2008 NHA Annual Hydrogen Conference 

Sacramento, California 

March 30 – April 3, 2008

Conference Paper 

NREL/CP-581-43114 

July 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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RENEWABLE HYDROGEN: INTEGRATION, VALIDATION, 

AND DEMONSTRATION 

K. W. Harrison 1

G. D. Martin 1

1. Introduction 

Renewable energy (RE) sources such as photovoltaic (PV), wind, biomass, hydro, 

and geothermal can provide clean and sustainable energy for our nation. Several 

of these options are already cost-competitive and are contributing nearly 10% of 

the U.S. electricity supply. Limiting greater penetration of some of these 

renewable energy sources, however, is their inherent variability and seasonal 

energy production. 

One solution to this problem is to produce hydrogen through the electrolysis of 

water and use the hydrogen in a fuel cell or internal combustion engine to produce 

electricity during times of peak demand or as a transportation fuel. Currently, this 

approach is hindered, in part, by the difficulty of producing hydrogen from these 

RE sources in a cost-competitive manner. In addition to the ongoing efforts to 

reduce the cost of RE technologies and to lower the capital requirements for 

electrolyzers, these renewable electrolysis systems must be optimized and tailored 

to realize the most cost-competitive option for electricity and hydrogen 

production. 

Producing hydrogen with domestic renewable resources reduces the impact of 

greenhouse gases. Wind energy is currently the lowest cost renewable energy 

source, so it’s the leading near-term candidate. Wind also is a variable energy 

source. In the mid- to long-term it may be beneficial to produce hydrogen when 

electricity created by the wind is not needed, and then add generation from 

hydrogen when electricity demand is high. Energy storage systems have the 

potential for addressing electric system integration issues inherent with variable 

wind energy resources, thereby enabling higher amounts of wind power on the 

electric system. This all aligns with the nation’s interest in developing and 

demonstrating advanced hydrogen technologies to reduce our dependence on 

foreign energy resources, improve our air quality, and ultimately support our 

long-term economic viability. 

2. Background 

Today, nearly every commercially available electrolyzer is designed for gridconnected operation; therefore, they incorporate power electronics (PE) to convert 

alternating current (AC) from the grid to direct current (DC) required by the 

electrolyzer cell stack. These power converters can represent 25% to 30% of the 

total cost of the electrolyzer. Power converters are also required for the RE 

 

1

 National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 

1 
source. For example, variable-speed wind turbines use PE to convert the variable 

frequency, variable voltage that is produced at the generator to DC; when 

connected to the grid, this voltage must be converted back to AC at grid 

frequency (60 Hz). 

Hydrogen production, via electrolysis, is highly dependent on the delivered cost 

of electricity. This research includes analysis of hydrogen storage options and 

electrolyzer component-level studies to help reveal areas to reduce the overall 

cost of hydrogen production. These studies aim to develop and optimize new 

controls, subsystems, and power electronics configurations for renewable 

electrolysis systems. Experimental testing provides feedback to the analysis to 

verify system efficiency improvements. Ultimately, the results of this research 

will reduce capital costs of renewable electrolysis systems through reduced 

energy designs, lower cost materials, and improved performance with RE sources. 

3. Technical Targets 

By addressing the technical barriers of capital costs, system efficiency, and 

integration with renewable energy sources, the National Renewable Energy 

Laboratory (NREL) is working to achieve the Department of Energy’s (DOE’s) 

cost targets for distributed and central electrolytic hydrogen production. 

Distributed Electrolysis

• By 2012, reduce the cost of distributed production of hydrogen from 

distributed electrolysis to $3.70/gge of hydrogen (delivered) at the pump. 

• By 2017, reduce the cost of distributed production of hydrogen from 

distributed electrolysis to less than $3.00/gge of hydrogen (delivered) at 

the pump. 

Central Electrolysis

• By 2012, reduce the cost of central production of hydrogen from wind 

electrolysis to $3.10/gge of hydrogen at the plant gate ($4.80/gge 

delivered). 

• By 2017, reduce the cost of central production of hydrogen from wind 

electrolysis to less than $2.00/gge of hydrogen at the plant gate (less than 

$3.00/gge delivered). 

Component-Level Analysis

A capital cost component analysis is being conducted as part of the renewable 

electrolysis work at NREL. The component cost analysis aims to break down the 

cost of system-level components within each subsystem to help determine where 

further research could help reduce the system capital costs. The goal is to have a 

manageable, yet detailed enough, analysis to identify key areas where cost might 

be reduced or efficiency increased in the integrated renewable system framework. 

In the first stage, inquiries to various manufacturers of commercial and researchlevel electrolyzer systems will be used to create a breakdown of the various 

2 
components as a percentage of total cost. Rather than looking at exact unit costs, 

the data will be viewed as parts of a whole to identify the largest opportunities for 

improvement. A normalization of inputs and outputs will occur to eventually fit 

this into the integrated system cost analysis. Identification of components with 

greatest cost reduction potential or that serve redundant purposes within an 

integrated renewable system framework can be used to make hydrogen production 

more economical and efficient. 

4. Integration 

Essentially, the entire renewable electrolysis system is limited by the constraints 

imposed by the PE interfaces between components. There are a number of 

weaknesses with this configuration, namely a redundancy of PE leading to 

increased cost, potential for failure, and a lack of ability to match RE power 

output to electrolyzer stack power requirements. One goal of testing an 

electrolyzer stack closely coupled with a wind-turbine generator or PV array is to 

determine the effect of fluctuating power output on electrolyzer stack and system 

operation. The presence of a battery bank serves to fix the common bus voltage as 

well as smooth the power input to the electrolyzer stack, thus somewhat defeating 

the purpose and adding to system cost. 

10 kW PV

NREL has designed, constructed, and begun testing a DC/DC power converter to 

be used with the 10 kW PV system at NREL’s Distributed Energy Resources Test 

Facility (DERTF). The focus of this power converter design is to reduce cost, 

increase flexibility of energy input, and to bring the system closer to commercial 

viability than previous designs using unique controllers and software. An off-theshelf programmable logic controller (PLC) is interfaced with an off-the-shelf 

insulated-gate bipolar transistor (IGBT)-based PE module to step-down the DC 

voltage from the PV array to that required by the electrolyzer stack. The system 

was designed to be low-cost, easily programmable, and tuned using commercially 

available components. The new design will take advantage of many of the recent 

advances in semiconductor power electronics including higher power density and 

reduced thermal resistance. Closed-loop current-control to the electrolyzer stacks 

and maximum power point tracking (MPPT) of the PV array will provide 

improved performance and efficiency. 

10 kW Wind

Using knowledge of both the performance characteristics of a 10-kW permanentmagnet wind turbine and electrolyzer stack, we developed a solution that replaced 

the two separate PE interfaces with a single one that takes varying magnitude and 

frequency AC directly from the generator output and provides acceptable DC 

power to the electrolyzer stack, thereby reducing the cost and increasing the 

robustness of the wind turbine-electrolyzer stack link. 

3 
Figure 1 Composite graph of improved power capture of second 

generation power electronics design 

Figure 1 shows the increase in energy capture (blue shade) from the first 

generation PE design (red line) to the second generation (green line). In 2008 we 

plan to complete the testing with additional modifications to further improve the 

performance and capture some (if not all) of the red shaded area. The single point 

of control allows matching of the wind turbine and electrolyzer stack electrical 

characteristics, thereby increasing the energy capture of the wind turbine. 

100 kW Wind

Some energy from a 100-kW variable-speed wind turbine will be “picked off” 

from the existing power controller which already produces a DC bus between 750 

and 800 V. That voltage is too high for the electrolyzer stacks and PE is being 

designed to make the DC-DC conversion. The 30-kW alkaline electrolyzer stack 

requires a maximum of approximately 150 Vdc. Thus, a DC-DC converter 

topology with a sufficient power rating is required to perform the conversion. To 

address this, we designed an isolated DC-DC converter using a full bridge, single 

phase inverter coupled with a rectifier and energy storing inductor. A 10-kHz, 30-

kVA transformer connects the inverter to the rectifier, providing electrically 

isolated energy transfer. This circuit (Figure 2) was simulated using Matlab 

Simulink software, and a simple feedback control system was integrated in the 

simulation. 

0

2000

4000

6000

8000

10000

12000

14000

0 5 10 15 20 25 30 35 40

Calculated Wind Speed (mph)

Power (Watts)

0

5

10

15

20

25

30

35

40

45

Hydrogen Flow Rate (scfh)

Gen II - DC Power Available Wind Power Gen I - DC Power Hydrogen Flow rate (scfh) Available Wind Power Curve Fit

Planned 

Increased Energy 

Capture from 

Gen2 to Gen3 

Increased Energy 

Capture from 

Gen1 to Gen2 

Gen1 Energy Capture 

4 
Figure 2 Simulation schematic illustrating the electrical topology of the fullbridge DC-DC converter. 

5. Validation 

NREL is currently completing testing of a polymer electrolyte membrane (PEM)-

based electrolyzer module. The system, model EP1 (Figure 3), is installed at 

NREL’s National Wind Technology Center (NWTC) in the Hydrogen Test 

Facility (HTF), which is adjacent to the DERTF. The electrolyzer is designed and 

built by Giner Electrochemical Systems, with support from the DOE, to 

demonstrate differential high-pressure (~1000 psig) PEM electrolysis with 

improved stack efficiency and cost. The electrolyzer is being tested to provide 

independent verification of stack efficiency towards the DOE’s Joule Milestone 

EE GG 1.1.01.1, which states: “Complete lab-scale electrolyzer, test to determine 

whether it achieves 64% energy efficiency, and evaluate systems capability to 

meet $5.50/gge hydrogen cost target, untaxed at the station, and with large 

equipment production volumes [e.g., 500 units/year].” 

Figure 3 Giner's PEM-based Electrolyzer Module 

installed at NREL's Hydrogen Test Facility. 

5 
The electrolyzer stack contains 28 cells connected in series. The average stack 

voltage was found to be 52.8 V resulting in an average cell voltage of 1.89 V. 

The stack efficiency was determined to be 67% when the pressure and 

temperature compensated Nernst potential equal to 1.27 V/cell (50.8°C, 65 atm) 

was divided by the actual cell voltage 1.89 V/cell. 

%67)100( 89.1

27.1

ActualStack Potential

Stack Ideal Potential Stack Efficiency = = =

1.1. Test Procedures 

NREL has developed a draft document that provides recommended tests, 

procedures, and specifications for qualification tests intended to evaluate 

electrolytic hydrogen-production equipment under varying power input. The 

intent of this recommended practice is to extend the basic testing arrangements 

provided by the International Organization for Standardization (ISO), “Hydrogen 

generators using water electrolysis process — Part 1: Industrial and commercial 

applications.” The tests provided are aimed at evaluating hydrogen generator 

operation and performance at lower power levels, like those that exist when 

powered with RE sources. 

Nearly all of the commercial electrolytic hydrogen generators available today are 

powered entirely by regulated AC sources from the utility grid. The DC required 

by the stack is derived by rectifying the grid AC with on-board PE. The majority 

(~85%) of energy required by a hydrogen generator system is consumed as DC by 

the hydrogen-producing stack. A hydrogen generator powered in part or wholly 

by a RE source will be subjected to the intrinsic time variable nature of the 

source. Power variations from RE sources present challenges in quantifying 

performance not typically experienced with a grid-connected system. 

For example, to enable the qualification of the hydrogen generator at varying 

stack power levels, an external or software-controlled adjustment of stack DC 

power is required (Figure 4). Typical ways to accomplish this adjustment might 

be to use internal software with operator access to control the power supply set 

point or to use a trim potentiometer to limit the DC power supply to the stack. 

Stack voltage and current transducers will verify the various power level set 

points. 

6 
Figure 4 System monitoring transducer placements 

6. Demonstration 

A cooperative research and development agreement (CRADA) was created 

between Xcel Energy and NREL to investigate ways to improve the system 

efficiency of producing, delivering, and using hydrogen from renewable 

resources. The wind-to-hydrogen (Wind2H2) demonstration project aims to 

quantify system-level efficiency improvements and cost reductions by designing, 

building, and integrating dedicated RE-to-electrolyzer stack PE. The goal of the 

project is to enable closer coupling of RE-generated electricity and the 

electrolyzer stack to produce hydrogen, which is then compressed and stored for 

future use to produce electricity via a hydrogen-fueled internal combustion engine 

for grid-connected peaking power applications, or, in the future, as a vehicle fuel. 

The project will use two wind turbine technologies: a Northern Power Systems 

100-kW (NW100a) wind turbine and a Bergey 10-kW wind turbine. Both wind 

turbines are variable speed in that the blades’ speed varies with wind speed. 

Variable speed wind turbines produce AC that varies in magnitude and frequency 

(known as “wild” AC) as the wind speed changes. The energy from the 10 kW 

wind turbine will be converted from its wild AC form to DC, then used by one of 

the 6-kW PEM electrolyzer stacks to produce hydrogen and oxygen from water. 

The energy from the 100-kW wind turbine will be captured from its existing 

controller, which already powers a variable DC bus of roughly ±400 V. That 

DC A

Power 

Grid AC 

Loads 

V, I, pf, 

Harmonics

V

O2/H2Ov 

BPR

Phase 

Separation 

& Drying

Hydrogen 

Generator 

F 

Desiccant 

R

H2

Waste

H2O 

Input 

RE 

Source 

A 

V 

BPR

Stack F 

Ti

Phase 

Separation

To

Adjustment: 25% to 100% 

full scale stack current 

H2

Product P F

DP

NV1 

NV2 

NV3 

NV4 

IC

H2/H2Ov

HV1 

H2/H2O

1

v 2

7 
voltage is too high for the electrolyzer stacks, and new PE will be designed to 

make the necessary conversion. 

Information on the Wind2H2 is located on the NREL Web site at 

http://www.nrel.gov/hydrogen/proj_wind_hydrogen.html. 

One goal of this work is to characterize system performance and develop PE 

converters and control algorithms to optimize the production of hydrogen from 

wind power. Baseline testing of the Xcel/NREL Wind2H2 project is providing 

data about system performance under steady power provided from the utility 

(Figure 5). 

Figure 5. System configuration for utility connected baseline testing 

Power for the stacks of two HOGEN 40RE PEM electrolyzers (Proton Energy 

Systems) and one alkaline electrolyzer (Teledyne Energy Systems) will be the 

focus of much of the research. The project will examine issues surrounding the 

integration of both technologies as well as how to operate electrolyzers of 

different product pressures in parallel. The demonstration project will reveal 

integration and operational issues as well as identify opportunities for 

improvement and other potential benefits for consumers. 

8 
AC-DC 

Converter 

Bergey 10 kW 

Wind Turbine 

10 kW PV 

array 

DC-DC 

Converter 

Figure 6 System configuration for 10-kW stand-alone wind turbine to 6-kW 

electrolyzer stack testing. 

In 2007, the Wind2H2 construction project began normal operations focused on 

baseline (i.e., utility- or grid-tied operation). A second generation AC to DC 

power converter was designed, modeled, built, and tested to provide power from 

the 10-kW wind turbine to one or both of the PEM electrolyzer stacks (Figure 6). 

Other key research goals include: 

• Creating synergies from the co-production of electricity and hydrogen 

• Addressing the variable nature of wind power by storing hydrogen for 

later use, creating a ready source of electricity for when the wind doesn't 

blow or the demand for electricity is high 

• Producing hydrogen for use in vehicles 

• Comparing multiple electrolyzer technologies to gauge their efficiencies 

and abilities to accommodate the variable input power of RE sources 

• Achieving efficiency gains though unique, integrated AC-to-DC and DCto-DC power converters between the wind turbines, PV array. and the 

electrolyzer stacks. 

Future work will include sensor calibrations and hardware to allow direct 

communications with the various devices for automated unattended operation. 

The ~800 Vdc from the NW100 wind turbine to 150 Vdc power converter is 

currently being designed to power the 30-kW alkaline electrolyzer stack. 

9 
10 

7. Summary 

Two PE systems were designed and tested using a small 10-kW wind turbine and 

10-kW PV array connected to a 6-kW electrolyzer stack. A larger 30-kW PE 

converter is being designed to perform a DC-DC conversion, using power from a 

100-kW wind turbine to power the alkaline electrolyzer stack. 

We began testing the electrolyzer from Giner Electrochemical Systems by verify 

the stack voltage efficiency. The stack is operated at a nominal 250 Adc resulting 

in an average stack voltage of 52.8 Vdc. The stack efficiency was determined to 

be 67% when the pressure and temperature compensated Nernst potential equal to 

1.27 V (50.8°C, 65 atm) was divided by the actual cell voltage 1.89 V. System 

efficiency testing is currently being completed. 

The Xcel/NREL Wind2H2 project is running under grid-connected power to 

establish baseline performance levels. Both the 10-kW wind turbine and 10-kW 

PV power electronics were tested on the system as well. Improvements in energy 

capture are being pursued for both of the power converters, while the design of 

the larger 30-kW device is being completed. 
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Renewable Hydrogen: Integration, Validation, and Demonstration 

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14. ABSTRACT (Maximum 200 Words)

This paper is about producing hydrogen through the electrolysis of water and using the hydrogen in a fuel cell or 

internal combustion engine generator to produce electricity during times of peak demand, or as a transportation fuel. 

15. SUBJECT TERMS

hydrogen; renewable hydrogen; electrolyzer; grid; fuel cell; integration; electrolysis; polymer electrolyte membrane; 

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