Stack energy conversion efficiency

An overall DMFC stack energy conversion efficiency of 35% was achieved over a range of stack operating conditions of 0.46-0.57 V per cell. An extended life test over llOOh on a five-cell stack made of identical cell components and stack configurations was also performed. 

Figure 2.11 shows the stack energy conversion efficiency as a function of stack voltage and stack power output. The stack energy conversion efficiency is calculated by the voltage efficiency (= average cell voltage of a stack/1.21 V) times its... 

Figure 2.11 Stack energy conversion efficiency plotted against stack current (a) and stack power output (b) from the results of steady-state stack performance obtained at selected operating conditions listed in Table 2.2.

Most literature reports have addressed DM FC performance at the single cell level. More relevant for evaluating DMFCs as practical power sources is the performance obtained at the stack level, achieved under operating conditions appropriate for the complete power system to achieve acceptable energy conversion efficiency and with complete thermal and water balances. 

The results show that, at temperatures below 60 °C and an air feed stoichiometry below three, the cathode exhaust is fully saturated (nearly fully saturated at 60 °C) with water vapor and the exhaust remains saturated after passing through a condenser at a lower temperature. In order to maintain water balance, all of the liquid water and part of the water vapor in the cathode exhaust have to be recovered and returned to the anode side before the cathode exhaust is released to the atmosphere. Because of the low efficiency of a condenser operated with a small temperature gradient between the stack and the environment, a DMFC stack for portable power applications is preferably operated at a low air feed stoichiometry in order to maximize the efficiency of the balance of plant and thus the energy conversion efficiency for the complete DMFC power system. Thermal balance under given operating conditions was calculated here based on the demonstrated stack performance, mass balance and the amount of waste heat to be rejected. 

The amount of waste heat to be rejected from an operating stack constrains in a significant way the optimal DMFC stack operating conditions and therefore should be considered when designing the complete power system in order to minimize the size and weight and maximize the power and total energy conversion efficiency. 

Numerous demonstrations in recent years have shown that the level of performance of present-day polymer electrolyte fuel cells can compete with current energy conversion technologies in power densities and energy efficiencies. However, for large-scale commercialization in automobile and portable applications, the merit function of fuel cell systems—namely, the ratio of power density to cost—must be improved by a factor of 10 or more. Clever engineering and empirical optimization of cells and stacks alone cannot achieve such ambitious performance and cost targets. 

Another important point is that the production of electricity is one of our most efficient energy conversions. As elaborated upon earlier, the great losses commonly ascribed to the stack gases and cooling water are hardly losses at all the actual "losses" (really consumptions) are elsewhere in the plant—primarily in the boiler. 

Multiple Band Gap Semiconduc-tor/Electrolyte Solar Energy Conversion The strategy of stacking semiconductors with variant g s discussed with a goal to enhance the overall process efficiency. 44... 

Research in our laboratory and by Osa and Fujihira showed that it is possible to covalently attach monolayers of chromo-phores to metal-oxide semiconductor surfaces — with no compromise in quantum efficiency to energy conversion compared with dyes adsorbed from solution (9-11). The quantum efficiency for these systems (ratio of photo-generated current to photons adsorbed in the dye layer, ne/np) is quite low, in the range of 10 5 to 10 4 and argues against device applications of these simple modified electrodes without further improvements, such as linear, multielectrode stacks of dye-modified, semi-transparent electrodes (10). 

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