IEEE Electrification Magazine - March 2018 - 16

Renewable hydrogen
in the U.S.
transportation
sector is a high-value
pathway that will
both drive and
benefit from H2@
Scale's efforts.

long-term options are for hydrogen
production pathways.
it is hard to predict the implementation of long-term hydrogen production pathways, so here we focus on
near-term hydrogen production
through low-temperature electrolysis.
there are three primary technologies for low-temperature electrolysis
systems: alkaline, polymer electrolyte
membrane (peM), and alkaline membrane. alkaline systems are a proven
technology that is capable of scaling
to megawatt levels. peM systems have
seen the most growth in the past few
years in terms of size. alkaline membrane systems have the potential to be a very low-cost
solution; however, this technology is a few years away
from commercialization.
the peM electrolysis industry has seen growth for
two main reasons: 1) peM systems are more compact
and have a smaller footprint, and 2) peM systems
allow for a differential pressure between the hydrogen (cathode) and oxygen (anode) side of the electrolyzer stack. this differential pressure allows for some
work to be done by the stack to electrochemically
compress the hydrogen to higher pressures. current
peM stacks operate around 200-400 psig, but there
are smaller peM stacks that are approaching up to
6,000+ psig with electrochemical compression. electrochemical compression has the potential to eliminate mechanical compressors from hydrog en
stations altogether, but innovations in electrochemical compression are necessary to meet the needs of
transportation fueling stations.
the doe has set targets for stack and system efficiency for electrolyzers, which are easily compared with a
standard metric that measures the energy needed (kWh)
to make 1 kg of hydrogen. the 2020 doe target is
43 kWh/kg for stack energy use and 44 kWh/kg for system
energy use. currently, low-temperature electrolyzer efficiency is approximately 50-55 kWh/kg. to improve system efficiency, nreL is conducting research to integrate
and optimize electrolyzer systems. one system optimization problem is associated with the inverse relationship
between high power and high efficiency. on one hand,
larger systems operating at higher power levels are needed to absorb excess renewable electricity generation and
produce large volumes of hydrogen; on the other hand,
as electrolyzer stacks reach higher power levels, their
stack efficiency tends to drop. researchers at nreL track
the current state-of-the-art electrolyzer stacks and systems available on the market.
on top of the hydrogen stacks, an electrolyzer system
contains ac/dc power supplies, deionized water systems,
hydrogen cleanup or drying systems, instrumentation and

I EEE E l e c t r i f i c a t i on M a gaz ine / March 2018

controls, and safety and cooling systems. in general, the electrolyzer
stack consumes the most energy
in the hydrogen production process,
accounting for ~80% of the energy
consumed. the power conversion losses through the ac/dc power supplies
tend to have the second-highest
impact on system efficiency and could
be up to 15% of the energy consumed
by the system. hydrogen cleanup
through drying can also have an
impact on system efficiency.
a major benefit of low-temperature electrolysis is the ability of the
systems to ramp their energy consumption up and down quickly. an nreL analysis showed
that electrolyzer systems could change their electricity
demand within milliseconds of a set-point change. this
makes electrolyzer systems a great candidate to provide
grid services. also, their ability to handle highly variable
power profiles allows electrolyzer systems to be directly
coupled or closely coupled with renewable energy sources.
Building off of these small-scale experiments on electrolyzer control, nreL and the idaho national Laboratory (inL)
are studying the controllability of a larger-scale (250-kW)
electrolyzer, producing hydrogen for transportation fueling under dynamic grid conditions to
xx
demonstrate reliable, fast-reacting electrolyzer performance
xx
verify communications and controls needed for successful participation in grid services for potential
additional revenue generation by hydrogen-fueling stations
xx
validate enhanced grid management utilizing the
electrolyzer as a controllable load
xx
optimize operation decisions (e.g., whether or not to
participate in grid services) to maximize revenue and
reduce operating costs.
this project combines modeling, simulation, and hardware to validate system performance and quantify the
economic benefit. nreL and inL have validated the variable performance of multiple types and sizes of electrolyzers under many scenarios and operating conditions for
the following performance metrics:
xx
response time to power set-point changes
xx
settling time after set-point changes
xx
startup and shutdown time
xx
turndown capability.
however, commercial, off-the-shelf electrolyzers are
not currently equipped with the controls and interfaces necessary for dynamic integration with the grid and
renewables. nreL, with inL, has developed a front-end
controller for power and grid management commands
with a fast and slow loop. the fast loop is for grid services like voltage and frequency stability; the slow loop

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2018