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.