Environmental Engineering Reference
In-Depth Information
The centrifugal air compressor at high speed seems the actual best choice for
FCS sized up to 100 kW [
14
]. The operation principle is the same of the
centrifugal blower, depending on transfer of energy from a rotating impeller to the
air. The pressure is created by converting the angular momentum determined by
the rotating impeller. Higher speeds permit high efficiency values to be reached.
The centrifugal air compressor is an oil-free compressor by design. The oil
lubricated running gear is separated from the air by shaft seals and atmospheric
vents. A high pressure air management system based on dynamic machines has
been successfully developed and applied to an 80 kWe FCS. It consists of a CEM
group, constituted by axial-radial liquid-cooled compressor coupled to a variable
nozzle radial turbine [
15
].
Positive displacement compressors offer more flexible compression ratio at low
fuel cell loads, moreover rotating devices offer the additional advantage of a not
fluctuating or flow rate with respect to alternative machines.
Within the class of rotating positive displacement particular interest has been
recently addressed toward scroll and screw compressors.
A scroll compressor uses two interleaved spiral-like vanes to compress air
stream. Generally one of the scrolls is fixed, while the other scroll is mobile and
orbits without friction in eccentrical way. Several air bladders are formed and
moved towards the center of the two spirals reducing their volume. The pressure
increases and then the air can be discharged. The operation of this kind of machine
results less noise and more smooth than alternative devices and reliable in the low
volume range [
16
]. Water in a scroll compressor could be used as both lubricant
and coolant, permitting nearly isothermal compression to be obtained [
17
]. The
advantages of scroll compressors have been then exploited in the development of
prototypes of compressor/expander modules to be used for designing air supply in
pressurized FCS [
18
].
Rotary screw compressors represent a different type of rotating positive dis-
placement and have been largely used in the last decades for industrial applications
and in particular for air conditioning and refrigeration systems. Their performance
is attractive in terms of efficiency, compactness, simplicity, and reliability.
A screw machine consists essentially of a pair of meshing helical lobed rotors
which rotate within a fixed casing that totally encloses them. As the mechanism
rotates, the meshing and rotation of the two helical rotors produces the internal air
compression by means of a series of volume-reducing cavities which capture the
inlet stream. Outlet stream is then discharged after volume reduction in the cav-
ities. Capacity control for these compressors is accomplished by variable speed
and variable compressor displacement.
Screw compressors can be either single or multistage machines, in dependence
of the final desired pressure level. In multistage option the air is compressed by
several sets of screws and can achieve pressure values higher than 0.5 relative bars.
Recently twin screw devices equipped with water injection have been inten-
sively studied in order to optimize their performance in view of a possible
application as BOP component suitable to feed cathode stream in FCS. The uti-
lization of water as cooling and sealing medium permits controlling both operating
