A renaissance for alkaline fuel cells - Fuel Cell Markets

INNOVATION
faster kinetics of the peroxide reaction on the oxygen electrode
results in lower irreversible losses. In practice it means that in
strong alkali, the oxygen electrode has gained some extra
100 mV. This can be seen in the Pourbaix diagram for the oxygen–hydrogen
electrodes 8 .
● System efficiency: While the efficiency of the typical low-temperature,
low-pressure AFCs discussed earlier is by no means
remarkable, the overall system efficiency is. The latest AFC generator
designs from Astris Energi are a case in point. It was possible,
owing to the inherent simplicity of the AFC architecture,
to engineer a system with only three moving parts: an electrolyte
circulation pump, an air pump or blower and a cooling
fan. The entire budget for the ancillaries is 5% of the rated
power, which also includes stack parasitic losses and losses due
to venting and purging. This translates to less than 2.5% of the
energy of the incoming fuel. There is no compression or
humidification of gases; the pumps must overcome only the
dynamic resistance of the moving fluids. The system efficiency
rises to more than 60% at partial load, and remains above 50%
throughout the operating range from 20 to 120% of full power.
● Operating lifetime and durability: AFCs are rugged performers
made of plastic, carbon and base metals (i.e. they do not require
fragile graphite components or other exotic materials). Good
operating lifetime is a given. The UTC Orbiter, for example, had
a rated life of 2000 h, though in tests it exceeded 10 000 operating
hours and individual stacks were tested beyond 15 000 h.
Meanwhile, terrestrial AFCs have a rated life of 2000 h (Astris),
with individual cells and electrodes tested to 5000–8000 h.
● Failure modes and safety: AFCs tend not to fail catastrophically,
but rather decline gradually in performance terms. Most manufacturers
declare the end-of-life when performance falls below
a predetermined level, usually caused by the corrosion and
agglomeration of a catalyst and loss of repellence of the gas-diffusion
electrodes. However, as long as the electrodes are in the
wet state, fuel and oxidizer never meet and there is no danger
of internal stack failure. Even if the electrolyte is removed
before the reactants, wet electrodes will remain impervious to
gases for many hours. This inherent safety should be considered
an advantage over solid-electrolyte cells – PEMFCs and
solid-oxide fuel cells (SOFCs) – where cracks, pinholes or
hotspots in the electrolyte may lead to dangerous flash
through, internal fire and stack destruction.
● System design, balance-of-plant and materials of construction: One
look inside an AFC generator built with the philosophy of
“good performance at low cost” reveals inexpensive plastic
parts, fittings, pipes and tubes. What’s more, the inherent simplicity
of the balance-of-plant is striking. The fuel-delivery circuit
uses a low-pressure regulator that feeds hydrogen to the
stacks and a simple provision for purging of impurities from
the stack. Fuel may be recirculated but often is “dead-ended” in
the stack. A pump or a blower provides an air supply to the
stacks in excess of the stoichiometric value. The electrolyte circuit
with a pump, heat exchanger(s), filter and a cooling fan is
usually the most complicated subsystem.
● Capital cost: Five years ago, the cost of source materials for an
AFC stack was just over €1100/kW. More recently, perform-
ance has been enhanced and the cost of materials brought
down to €430–460/kW 9 . Meanwhile, Astris Energi reported a
source material cost of $275/kW in 2004. With further evolution
in performance, increased production volumes, reduced
labour content and enhanced yields, it is conceivable that the
cost of stacks will drop below the $400/kW level in annual
quantities of 10 000 kW. If appropriate investment is forthcoming,
the authors believe that, given sufficient volumes, with
AFC stacks in the few hundred dollars range, the complete system
selling price can break through the magic barrier of
$1000/kW, with a healthy margin over the manufacturing cost.
● Operating cost: While AFC technology has the potential to
become the cheapest low-temperature fuel cell – and apart
from high-temperature SOFCs, probably the cheapest fuel cell
on the market – it also comes with a very low operational cost.
Versatile performance is achieved with the least expensive
industrial-grade hydrogen (99.9%) and even with unpurified,
raw electrolytic hydrogen. In the platinum-free embodiment
with circulating electrolyte, AFCs are insensitive to trace carbon
oxides, and inert to most other impurities. Cost of the consumables
(soda lime and, in some cases, potassium hydroxide)
may add some 0.2–2 cents/kWh 1 .
Commercial opportunities
Right now, fuel cells of all types are being corralled into applications
where they can potentially displace incumbent powergeneration
schemes. As a result, the approach to market entry is
largely based on the cost dynamic that the fuel cell can achieve
compared with the existing technology of choice. In most
cases, comparing only the capital or purchase cost is not currently
sufficient to prove the business case positive for fuel cells.
In many cases, the benefits of fuel-cell operation must be
proved based on a total cost of ownership owing to higher reliability
and elimination of peripheral equipment (e.g. because
of quiet or emission-free operation). The value proposition
delivered by AFCs is similar to other low-temperature fuel cells,
namely PEMFCs, with the added advantage that both capital
cost and operations cost are considerably lower. Initial applications
are expected to be as backup power sources and in light
mobility, though declining AFC price points could in time open
up additional markets.
● Backup power: The list of applications that must be either shutdown
in a controlled manner or operated for long periods without
their normal power supply is a long one and includes
mechanical systems (such as elevators) and all manner of hardware
in telecommunications and computing networks. Today,
the majority of backup schemes rely heavily on banks of leadacid
batteries, sometimes supplemented by a diesel or gasoline
generator. Fuel cells that can be produced economically will be
able to deliver increased performance and offer a savings to the
operator for a variety of backup power applications owing to
decreased maintenance, simpler installation and enhanced
reliability. There is an incredible advantage to having a power
supply that is fully independent from the primary supply. In
times of extended power outage, for example, additional gas
cylinders can be delivered to power the backup power system,
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