A SYSTEM ANALYSIS OF PRESSURIZED ELECTROLYSIS FOR COMPRESSED
HYDROGEN PRODUCTION ASME Energy Sustainability Conference
July 2019Ryan Hamilton, Dustin McLarty
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• The Hydrogen Economy• Merits of Pressurized Electrolysis• Study Results
Outline
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21st Century Challenge: Connect renewable energy centers….
Source ORNL
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… to population centers via our electric and gas grids
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Hydrogen or Synthetic Gas: viable storage at point-of-production or point-of-use
California Hydrogen Business Council. 2015. The case for hydrogen (White Paper).
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Electrolysis in Hydrogen Production
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Electrolysis Technology
Source: NEL
Commercial electrolyzer system efficiencies
55 − 75% LHV
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Outline
• The Hydrogen Economy• Merits of Pressurized Electrolysis• Detailed Study Results
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Pressurized Electrolysis
• Relative to water pumps, hydrogen compressors are energy intensive, inefficient, expensive, and prone to failure
• High pressure H2 production could directly fuel vehicles, be inserted into pipelines, or stored in underground chambers
Pressure (MPa)Storage: 2-18Pipeline: 3-8Truck: 16-20Ammonia: 20-30Vehicle: 35-70
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Pressurized Electrolysis System
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Water Pump
𝜂𝜂 = 85%
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Heat Exchanger Network and Heating
• Renewable power can be used to pre-heat water to electrolysis temperature.
• Waste heat can displace heating.
• HEN assumes 10°𝐶𝐶 pinch difference.
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Electrolyzer
• Thermoneutral operation.• Technology, size, and production rate are general.
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Multistage Intercooled Hydrogen Compressor
• Max compression ratio of 8• 𝜂𝜂 = 45% − 88%
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• The Hydrogen Economy• Merits of Pressurized Electrolysis• Study Results
Outline
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Water Pre-heating Energy
• Pressurized gas streams lower the heating requirement.
• Heat capacitance between streams is closer at higher pressure.
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Thermoneutral Pressurized Electrolysis Energy Consumption
• For high pressure higher temperature electrolysis is better.
• Perfectly offset by reductions in heating requirement. Only 3% change
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Trade-off between pumping water and compressing hydrogen
Note 32x difference in scale!
Energy needed to compress hydrogen from electrolyzer pressure to storage pressure.
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Pressurized System Lowers Energy Consumption
• At 0.1 MPa hydrogen compressor is 16% of total.
• At 70 MPa water pump is 0.4% of total.
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Plant Efficiency
𝜂𝜂𝑒𝑒 =𝐿𝐿𝐿𝐿𝑉𝑉𝐻𝐻2
𝑊𝑊𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 + 𝑊𝑊ℎ𝑒𝑒𝑒𝑒𝑒𝑒 + 𝑊𝑊𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 + 𝑊𝑊𝑒𝑒𝑐𝑐𝑝𝑝𝑝𝑝
𝐿𝐿𝐿𝐿𝑉𝑉𝐻𝐻2 = 33.3𝑘𝑘𝑊𝑊𝑘𝑘𝑘𝑘𝑘
• Standardized electrical energy efficiency.
• Does not consider pressure of hydrogen.
• DOE technical target: 75%
44.4 𝑘𝑘𝑘𝑘ℎ𝑘𝑘𝑘𝑘
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Peak plant efficiency of 84.6% with electrolyzer operated at storage pressure
• Plant efficiency is independent of system temperature.
• Electrolyzer will have to operate above 88% eff. at 70MPa to DOE meet target.
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Plant Efficiency with Waste Heat
• An electrolyzer at atm. pressure will have to be 92% efficient to meet the plant target
• When operated at pressure an 800C electrolyzer will have to be 80% efficient
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Conclusions
• Plant electrical efficiency can reach 84% with pressurized electrolysis.
• Higher efficiency is expected with a high temperature plant.
• Waste heat significantly increases plant electrical efficiency and is necessary to meet DOE target.
• Pressurized solid oxide systems are most likely to meet target with the electrolyzer having to be 80% efficient.
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Questions ?
Thank You!
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Sensitivity of Plant Efficiency to Hydrogen Compressor Efficiency
Current electrolyzers cannot meet DOE target even with perfect hydrogen compression.
Current electrolyzers cannot meet DOE target even with perfect hydrogen compression.
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