Production of renewable hydrogen and electrofuels

 

How long is the service life of an industrial-scale electrolyzer?

Commercial electrolyzer technologies today have service hour lifetimes in the range of 60,000-90,000 hours with routine maintenance before needing major overhauls (e.g., PEM membranes).

See: Future cost and performance of water electrolysis: An expert elicitation study, Schmidt, Gambhir, et al, International Journal of Hydrogen Energy, 2017.

Aside from power to the electrolyzer stack, are there other significant energy inputs to electrolyzer operations?

It takes between 50 and 60 kWh of electricity to make a kilogram of hydrogen. That energy requirement includes ancillary equipment such as pumps and coolers. However, there can be significant additional energy requirements to compress hydrogen to high pressures (e.g., light duty vehicles take up to 10,000 psi [700 bar] hydrogen). Energy for compression adds another 1.5-6 KWh of energy depending on the efficiency and compression level.

Hydrogen can also be liquefied for more efficient bulk transportation over long distances, which can add 8.3-12 kWh/kg to the energy requirement beyond the energy used to make the hydrogen.

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Are there inherent or potential releases of toxic materials to the environment that are associated with the electrolyzer lifecycle—manufacture, operations, and decommissioning?

For operations utilizing PEM, there are no toxic materials released to the environment. The materials are recyclable. For liquid potassium hydroxide electrolyzers, post treatment of waste electrolyte is needed.

Oregon electrolyzer start-up manufacturer HydroStar USA responds to this question relating to their technology:

Our manufacturing processes are simple, standard machining like drilling, welding, and other machining processes as well as either 3D printing or injection molding of plastic parts.  Our devices can use recycled 304 Stainless Steel reducing global material waste. The manufacturing site can be setup regionally thus reducing the logistics issues and significant transportation costs.  Our electrolyte is benign and therefore does not have any issues when used or decommissioned.  Our devices are largely made up of 304 SS tubing and 3D extruded plastic parts, all which can be recycled.  The only potential issues of potential releases or toxic materials would be in the actual recycling process.

Does electrolyzer manufacture entail use of materials likely to become scarce, expensive, and constraining, thereby constraining growth of the renewable hydrogen industry?

Some expensive or limited materials are used in today’s electrolyzers such as platinum, but work continues on finding substitutes with more commonly available materials (e.g., nickel) that can lower the cost with little or no reduction in efficiency. Progress is being made on continuing reductions in the use of expensive materials in electrolyzers.

 

One of the challenges in the past with the process of electrolysis of water has been fouling of electrodes.  Has this problem been solved with modern electrolyzers?

Today’s electrolyzers are expected to have a minimum of ten-year lifetime with routine maintenance. Fouling is primarily controlled by purifying feedstock water into the electrolyzer stack. Work continues to improve both membrane materials in PEM electrolyzers and electrodes in alkaline electrolyzers.

 

What periodic servicing do electrolyzers require?  Does servicing require extended outages?  What percentage of the time is industrial-scale electrolyzer equipment typically available?

Regular maintenance requirements are about one day per year.

 

How does renewable hydrogen production efficiency vary with scale?  That is, are there reasons why renewable hydrogen may not make sense to produce at very small scale, or conversely, why it may not be economic at very large scale?

The efficiency of hydrogen production is largely unaffected by scale. Electrolyzer efficiencies are normally quoted at their rated input level, but the efficiency at power levels below the rated level can actually be higher. Commercial electrolyzers are quoted in a range of 40-60 kWh/kg (55-85%).

The capital cost of electrolyzers tend to drop rapidly with scale—the larger the project, the lower the cost. Electrolyzers of 100+ MW scale were in service from the 1920s and although most commercial electrolyzers are in the 1-20 MW scale range today, 100 MW electrolyzers are in development by various manufacturers as demand for them increases.

 

It is commonly stated that renewable (i.e., biogenic) natural gas is not scalable to play a significant role in decarbonizing the economy.  Are there factors that similarly limit renewable hydrogen’s potential to play a significant role in decarbonization?

No. The limiting factor may be the rapidity with which manufacturers can expand their electrolyzer manufacturing capacity.

 

Oxygen is a co-product of electrolyzer operation.  Is any of the oxygen produced utilized or is it routinely vented to the atmosphere?

Oxygen is routinely, but not always vented to the atmosphere. Because oxygen can be separated from air at relatively low cost, economics generally dictates that it be vented. Where there are local uses of oxygen such as in wastewater treatment, it may have an industrial use. Electrolyzers are used in submarines and spacecraft to supply breathable oxygen.

One future use that may have significant value in decarbonizing the industrial and power sectors is oxy-fuel combustion—burning combustible fuels in pure oxygen rather than air.  Because air consist of 78% nitrogen and 21% oxygen, oxy-fuel combustion dramatically reduces exhaust flow and increases the concentration of combustion products, facilitating capture an reuse of combustion products (e.g., carbon dioxide for carbon sequestration or carbon monoxide for alkane synthesis).  It also prevents the formation of nitrogen oxides, which is the major potential air pollutant from the combustion of hydrogen.

 

What is the outlook for reversible electrolysis devices (electrolyzer/fuel cells); i.e., single devices capable of both charging by making H2 and O2 and discharging to make electricity and H2O?

Reversible devices do exist, but it in most applications there is more value to optimizing the operation for either hydrogen production (electrolyzers) or electricity production from hydrogen (fuel cells).