Control and energy efficiency of PEM water electrolyzers in renewable energy systems

Joonas Koponen, Vesa Ruuskanen, Antti Kosonen, Kimmo Huoman, Markku Niemelä, Jero Ahola

/ Outline of the presentation

• Introduction

• Specific energy consumption tests

• Dynamic operation tests

• Conclusion

1. INTRODUCTION

/ Introduction

• Specific energy consumption of the PEM

stack is estimated based on measured

values from two measurement systems

• PEM electrolyzer performance and

dynamics are studied as a part of a solar

PV system

2. SPECIFIC ENERGY CONSUMPTION

/ Description of specific energy consumption tests

• Hydrogen storage (V = 0.7 m3) is filled progressively from 1.0–1.8 MPa,

then 2.0–2.8 MPa, and finally 3.0–3.8 MPa

• Corresponding hydrogen stack outlet pressures are 2.0 MPa, 3.0 MPa, and

4.0 MPa

• A total of five filling test campaigns were run

Requirement for electrical energy stays practically constant.

/ Specific energy consumption tests (1/2)

Higher differential pressure decreases the Faraday efficiency.

Two measurement systems:1) IRD2) cDAQ (NI)

/ Specific energy consumption tests (2/2)

• Specific energy effectively increases by a maximum of 0.2 kWh/Nm3 when

hydrogen stack outlet pressure is increased from 2.0 MPa–4.0 MPa

• The specific energy consumption increase is due to hydrogen gas crossover

(decreased Faraday efficiency)

• Ideal isothermal compression at 70 °C of hydrogen gas from 2.0 MPa to 4.0

MPa would only require 0.025 kWh/Nm3 of energy

3. INTEGRATION WITH SOLAR PV

/ Description of dynamic operation tests (1/2)

• Electrolyzer was operated with data from solar inverter MPPT power and

dynamics of 5 kWp power plant

• Ten hours of solar PV data at 1 s sampling time was selected for three

different test days:

a) Clear summer day

b) Summer day with variable weather conditions

c) Cloudy summer day

• Electrolyzer was operated in on-grid mode

electricity grid served as virtual battery

/ Description of dynamic operation tests (2/2)

• PEM electrolyzer is operated in current control mode where current

reference is calculated from the solar power reference and the measured

PEM stack voltage

• The calculated stack current reference is applied to power the electrolyzer

when the electrolyzer can be powered up to its minimum safe load

• Dynamic operation was preceded by a system preparation step

/ Integration with solar PV power production (1/2)

PEM electrolyzer operated at hydrogen outlet pressure of 2.0 MPa and oxygen outlet pressure 0.2 MPa.

Minimum safe stack current is 25% of 70 A.

/ Integration with solar PV power production (2/2)

Grid power balance stays within 200 W during the clear summer day.

Abrupt changes between cloudy and clear weather

Grid energy balance stays negative throughout.

Cloudy day still provides brief bursts of power to run the electrolyzer. Grid energy balance stays positive.

Solar PV power dynamics:• Max. +102 W/s• Max. 116 W/s

Solar PV power dynamics:• Max. +1116 W/s• Max. 1055 W/s

PEM stack dynamics are software limited:• +19 W/s

76 W/s

/ Conclusion

• Only a clear summer day provides power reference variations slow enough

for the studied PEM electrolyzer to follow

• E.g. a supercapacitor is required to minimize the electricity grid power

imbalance

• Higher differential stack pressure will decrease the Faraday efficiency

• Real hydrogen production can be maximized by controlling the hydrogen

outlet pressure as close to storage pressure as possible

/ Future work

• The contents of this presentation have been reported in greater detail in a

manuscript sent for peer review in International Journal of Hydrogen Energy

• Further, system integration of PEM water electrolyzers into PtX systems will

be analyzed in the SOLETAIR project

Thank you!

Questions? Comments?