Auxiliary systems configuration

In addition to high-profile fuel cell applications such as automotive propulsion and distributed power generation, the use of fuel cells as auxiliary power units (APUs) for vehicles has received considerable attention (see Figure 1-9). APU applications may be an attractive market because it offers a true mass-market opportunity that does not require the challenging performance and low cost required for propulsion systems for vehicles. In this section, a discussion of the technical performance requirements for such fuel cell APUs, as well as the current status of the technology and the implications for fuel cell system configuration and cost is given. 

Figure 11.23 A schematic diagram showing some of the options for sputtering system configurations including dc or rf power supplies (but not both), and the possible addition of a substrate bias and a filament or auxiliary anode for a triode configuration.

These examples and those in Scheme 2.6 illustrate the key variables that determine the stereochemical outcome of aldol addition reactions using chiral auxiliaries. The first element that has to be taken into account is the configuration of the ring system that is used to establish steric differentiation. Then the nature of the TS, whether it is acyclic, cyclic, or chelated must be considered. Generally for boron enolates, reaction proceeds through a cyclic but nonchelated TS. With boron enolates, excess Lewis acid can favor an acyclic TS by coordination with the carbonyl electrophile. Titanium enolates appear to be somewhat variable but can be shifted to chelated TSs by use of excess reagent and by auxiliaries such as oxazolidine-2-thiones that enhance the tendency to chelation. Ultimately, all of the factors play a role in determining which TS is favored. 

Another approach, developed in our laboratory, consists of the compartmentalization of the sensing layer25"27. This concept, only applicable for multi-enzyme based sensors, consist in immobilizing the luminescence enzymes and the auxiliary enzymes on different membranes and then in stacking these membranes at the sensing tip of the optical fibre sensor. This configuration results in an enhancement of the sensor response, compared with the case where all the enzymes are co-immobilized on the same membrane. This was due to an hyperconcentration of the common intermediate, i.e. the final product of the auxiliary enzymatic system, which is also the substrate of the luminescence reaction, in the microcompartment existing between the two stacked membranes. 

The modular design of the HyPM fuel cells allows scaling for higher power requirements using a variety of configurations, such as series and parallel systems. Potential applications for the technology include vehicle propulsion, auxiliary power units (APU), stationary applications including backup and standby power units, combined heat and power units and portable power applications for the construction industry and the military. 

Fig. 16.3. Cyclic voltammogram of 13.6 U PP2A from Upstate (blank) and 0.3 mM catechol monophosphate+13.6 U PP2A from Upstate (CMP+PP) at lOOrnVs-1 using a single-drop configuration on a horizontally supported screen-printed two-electrode system, with graphite as working and Ag/AgCl as reference/auxiliary electrode. Reprinted from Campas et al. [86], with permission from Elsevier.

The system efficiency is lower than the stack efficiency due to power requirements for auxiliary components and due to power conversion. A well-designed system should not use more than 10% of the fuel cell output power for auxiliary components. The efficiency of DC/DC or DC/AC converters is relatively high (typically >90%) but their number and configurations must be optimized for given application. 

Asymmetric Alkylations. The use of nitrogen derivatives of carbonyl compounds (imines, imides, amides, sultams, oxazo-lines) is often the most efficient procedure for achieving a-alkylations. Chiral auxiliaries bearing heteroatoms in a 1,2-relationship appear to work best, as they have chelation sites for the metal cation. High levels of asymmetric induction can thus be achieved due to the system rigidity. Cyclic ketones have been alkylated via the lithiated enamine formed from L-f-leucine f-butyl ester (eq 1). High enantiomeric excesses and predictability of absolute configuration make this method attractive. 

---