Optimization Strategies of PEM Electrolyser as Part of Solar PV System Antti Kosonen Joonas Koponen, Kimmo Huoman, Jero Ahola, Vesa Ruuskanen, Tero Ahonen (LUT) Thomas Graf (IRD) 7.9.16
Introduction: Why hydrogen? Need for bridges between different energy chains Need for seasonal storage Main raw material for gas and liquid fuels High energy content as mass basis
Power to Gas Solar, wind Alkaline, PEM Zero emissions with renewable energy Seasonal storage Link between energy sectors Raw materials Flexible energy system Source: DNV KEMA Energy & Sustainability, Final Report: Systems Analyses Power to Gas, Deliverable 1: Technology Review, Jun. 13.
Water electrolysis The overall chemical reaction of low temperature water electrolysis without required thermodynamic energy value PEM (proton exchange membrane) H O l H g 1 Control range wider than alkaline 2 O g Lack of liquid electrolyte gastight thin polymeric membrane Minimum energy required to produce hydrogen that conducts H + protons o E HHV,H2 = 3.54 kwh/nm 3 = 39.4 kwh/kg Pressure difference between electrodes high pressure H 2 Table. Main characteristics of main electrolysis technologies. Characteristics Alkaline PEM SOE System efficiency (% HHV ) 68 77 62 77 Size (kw) 1.8 5.1 115 <15 Current density (A/cm 2 ) <.5 >1. <.3 Operating temp. ( C) 8 8 7 1 Min. load (%) 5 1 Ramp up (%/s).13 1 1 1 Stack lifetime (years) 1 Hydrogen purity (%) 99.5 99.9998 99.9 99.9999 Fig. Operating principle of PEM electrolysers.
Laboratory test system (1/2) Fig. Schematic of laboratory system for hydrogen production with solar energy.
Laboratory test system (2/2) Fig. Schematic of laboratory system for hydrogen production with solar energy.
Optimization strategies Control of electrolyser 1) Solar PV Most dynamic 2) Electricity price Static with fixed period 3) Frequency control Power capacity and dynamics depend on the market Power (kw) 4 3 2 1 1.6.16 [32.4 kwh] Power (kw) 6 5 4 3 2 1 5.6.16 [18.7 kwh] Hard to follow 2 4 6 8 1 12 14 16 18 22 24 Fig. Clear summer day with 5 kw p system. 2 4 6 8 1 12 14 16 18 22 24 Fig. Summer day with 5 kw p system when cloudiness varies.
PEM water electrolyser as a controllable load 8 8 7 Stack current (A) Stack current (A) 7 5 1 8 7 5 1 12:5 12:1 12:15 12: 12:25 12: 7 5 1 Fig. Cold start test of the PEM electrolyser. 5 1 15 Time (s) 8 7 5 1 Stack voltage (V) Stack voltage (V) Stack temperature ( C) 5 1 12:5 12:1 12:15 12: 12:25 12: Fig. Response of the studied commercial PEM electrolyser as the stack current reference is changed at t = s. Hydrogen outlet pressure from the stack was restricted to bar. Stack current (A) 8 7 5 1 1 5 Time (s) 1 8 7 5 1 Hydrogen outlet pressure (bar) Stack voltage (V) Practical limitations Increased H 2 pressure increased H 2 gas crossover into O 2 problems with low current densities min cold start + 1 min startup procedure (drying unit warm up, diwater circulation, H 2 flush, building up O 2 and H 2 stack pressures) Stack current limitations Max current slew rate (A/s) Up: W/s Down: 8 W/s Max cell voltage (2 V) PEM electrolyser in laboratory Input power 5.5 kw Current 7 A H 2 pressure 5 bar Safe control range: 25 1% @ bar 1% @ 5 bar
Analysis of solar PV dynamics Power (kw) 5.5 5 4.5 4 3.5 3 2.5 2 1.5 Instant s avg. 1.5 1 1.5 2 2.5 3 1 9 Solar PV Clear day Smooth variations Cloudy day Smooth variations Variable weather Rapid power changes up and down MPPT operation with test system Power rise: 175 W/s Power drop: 273 W/s 8 Temperature ( C) 19 18 7 5 Radiation (W/m 2 ) Solar PV system used in tests Total power 5 kw p Panels: 22 x 2 W p Inverter: 4.6 kw 17.5 1 1.5 2 2.5 3 Fig. Solar PV test condition that are used in the test with the PEM electrolyser.
Need for fast energy storage 15 Power (W) 1 5 5 1 15.5 1 1.5 2 2.5 3 1 Solar PV data filtering Fast changes are filtered away Moving average filters Practical implementation by inverter software (decreased energy production) by energy storage (all the solar PV production used) 5 Energy (Wh) 5 1 15 Table. Electricity storage requirements for solar PV system to slow down the power changes. Length of filtering (s) Power (W) Energy (Wh) 19 ± 18 ±1 15 1 ±5 25.5 1 1.5 2 2.5 3 Fig. Storage requirements if the solar PV power of the test system is filtered by moving average of seconds.
PEM water electrolyser drived by solar PV Current (A) Current (A) 7 5 Stack Setpoint.5 1 1.5 2 2.5 3 7 5 (a) Stack Setpoint.5 1 1.5 2 2.5 3 Stack Setpoint.5 1 1.5 2 2.5 3 (c) (d) Fig. Stack current of the PEM water electrolyser with different solar PV power reference. (a) Moving average of s. (b) Moving average of s. (c) Moving average of 15 s. (d) Without filtering. Current (A) Current (A) 7 5 7 5 (b) Stack Setpoint.5 1 1.5 2 2.5 3 Test condition H 2 pressure bar Control range 25 1% Tests 1) Moving average of s Follow visually without problem 2) Moving average of s Biggest problem when power is increased 3) Moving average of 15 s Biggest problem when power is increased 4) Without filtering Biggest problem when power is increased Current limit of 7 A reached scaling to fit the solar PV power (problem is not seen when the solar PV power is continuous)
Conclusion Role of demand response will increase in the future Water electrolysers have a remarkable role in 1% renewable energy system that is based mainly on solar and wind Control strategies of proton exchange membrane (PEM) electrolyser with a solar PV system is studied Commercial products are applied and their limitations and performance are tested and analysed According to the studies, a commercial PEM electrolyser can operate in a dynamic environment, but limitations and degradation in the performance should be taken into account Footer
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