Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 689

primarily due to cheaper cooling. There is a lot of market potential for SMES due
to its unique application characteristics, primarily in transmission upgrades and
industrial power quality [3]. However, one of the greatest concerns for SMES is
its reliability over a long period of time.

4.9 Hydrogen energy storage system

HESS is the fi rst of the three energy storage systems discussed in this chapter.
HESS is the one of the most immature but also one of the most promising energy
storage techniques available. As an energy storage system, HESS acts as a bridge
between all three major sectors of an energy system: the electricity, heat and trans-
port sectors. It is the only energy storage system that allows this level of interac-
tion between these sectors and hence it is becoming a very attractive option for
integrating large quantities of intermittent wind energy. There are three stages in
HESS: create hydrogen; store hydrogen; use hydrogen (for required application).

4.9.1 Hydrogen production
There are three primary techniques to produce hydrogen: extraction from fossil
fuels; reacting steam with methane; electricity (electrolysis). However, as pro-
ducing hydrogen from fossil fuels is four times more expensive than using the
fuel itself, and reacting steam with methane produces pollutants, electrolysis has
become the most promising technique for hydrogen production going forward.
An electrolyser uses electrolysis to breakdown water into hydrogen and oxygen.
The oxygen is dissipated into the atmosphere and the hydrogen is stored so it can
be used for future generation. Due to the high cost of electrical production, only a
small proportion of the current hydrogen production originates from electrolysis.
Therefore, the most attractive option for future production is integrating electroly-
ser units with renewable resources such as wind or solar. In order to achieve this, an
electrolyser must be capable of operating: with high effi ciency; under good dynamic
response; over a wide input range; under frequently changing conditions [2].
Recently a number of advancements have been made including higher effi cien-
cies of 85%, wider input power capabilities, and more variable inputs. A new pro-
ton exchange membrane (PEM) has been developed instead of the preceding
alkaline membranes. This can operate with more impure hydrogen, faster dynamic
response, lower maintenance, and increased suitability for pressurization [2].
However, a PEM unit has lower effi ciency (40-60%) so some development is still
required. Electrolysers are modular devices so the capacity of a device is propor-
tional to the number of cells that make up a stack. The largest commercial systems
available can produce 485 Nm 3 /h, corresponding to an input power of 2.5 MW.
The lifetime of an electrolyser is proving diffi cult to predict due to its limited
experience. However, research has indicated that the electrolyser unit will have the
shortest lifespan within HESS. Some have predicted a lifespan in the region of
5-10 years but this is only an estimate [2].

4.9.1.1 Cost of hydrogen production
The estimated costs to produce power using an electrolyser are extremely var-
ied. Predictions are as low as € 300/kW [25] up to € 1100/kW [ 2 ]. ITM Power in