Startup-shutdown cycles

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

At the Weizmann Institute in Israel, the closed-cycle CO2 reforming process of natural gas has been adapted and modified to solar energy as the primary heat input. The use of gaseous CO2 eases the daily startup / shutdown cycles. A lab-scale facility with 5-10 kW power was successfully operated. A scale-up plant designed for 480 kW heat input to the reforming reaction and 250 kW heat output in the methanation step is being tested since 1994. Catalysts are rhodium and ruthenium. So far 150 cycles have been completed [3]. The solar power absorbed was 97 kW, a methane conversion rate of 70 % maximum was achieved. A computer code has been developed for loop design and optimization purposes [9]. 

Based on Celtec -P1000 MEAs, Schmidt et al. [95, 96] reported a similar daily startup-shutdown cycling test (12 h of operation at 160 °C followed by 12 h of shutdown). After a period of 6500 h with 260 cycles under mild conditions (160 C and Ha/air), an average voltage drop of 300 pV per cycle was observed, corresponding to a performance loss of ca. 11 pV/h. 

Figure 8. Startup/shutdown cycling tests (Hong Kim, etal. 2010).

Figure 10. Result of startup/shutdown cycling 1500 cycles.

Hong Kim, J. et al. 2010 Effects of Cathode Inlet Relative Humidity on PEMFC Durability during Startup-Shutdown Cycling Journal of The Electrochemical Society, vol. 157, no.l, 104-112. 

A long-term durability test with a optimized reP-25-based membrane showed 4,000 h of continuous operation under moderate conditions 500 mA cm" with a cell temperature of 80-85°C, H ( 100% RH)/Oj(dry) under ambient pressure (Gubler et al. 2004). The final lifetime reached 7,900h, where the membrane degradation was accelerated by several controlled/uncontrolled startup/shutdown cycles. 

Performance response times (e.g., screen refresh rates, cycle times, and critical control response times), Mean-Time-To-Failure (MTTF), system remedial action, power failure recovery, startup, shutdown... 

Durability is a fundamental and necessary feature for PEM fuel cells to see a wide diffusion as a practical power source in any application field. The requirements for fuel cell lifetime vary with the specific application, in particular a duration of at least 5000 h is mandatory for use on cars, while even longer periods are needed for bus and stationary employments (for most applications an acceptable degradation rate is considered to be comprised in the range 2-10 pV/h [51]). However, the wide variability of operative conditions usually encountered in automotive applications, such as dynamic driving cycles, startup/shutdown phases, and freeze/thaw, makes also the target for car very difficult to be met with the current technologies. 

Swathirajan S, Merzougui B, Yu PT Fuel cell and method for reducing electrode degradation during startup and shutdown cycles. US patent US 2008/0166599 Al... 

Many factors affect the mechanical design of evaporator systems, particularly of the calandrias. The two most important are the temperatures and pressures to which the equipment will be subjected. Not only are the temperatures and pressure during normal operation important, but upset, startup, shutdown, dryout, cycling, pulsating pressure, and safety relief requirements are equally important. Other considerations include external loadings from supports or piping and vibrations transmitted from external sources. Wind loadings and earthquake loads must also be considered. Anticipated life expectancy and future service should be considered. 

How to start up and shut down a stack impact its lifetime significantly. The most important thing is to minimize or better prevent the formation of H2/air boundary at the anode, followed by shortening the time at OCV. Several methods can achieve those goals. One method invented by the Author only resulted in 25 mV voltage loss (at 0.6 A cm ) after 20,000 times of startup and shutdown cycles. 

The temperature difference between any adjacent points (points separated less than 2-v/ from each other, R = mean radius, t = thickness) during normal operation and during startup and shutdown operation does not exceed SJIRa, where Sg is the alternating stress for the specified number of startup and shutdown cycles. 

Thin walls increase tube life because secondary stresses are minimized during thermal cycling on startups, shutdowns, and upsets. Accordingly, the tubewall thickness should be minimized consistent with meeting the tensile strength requirement. For many applications, the minimum sound wall (MSW) can be as low as 0.25 inches. 

Degradation and durability of a PEM fuel cell or stack can be affected by many internal and external factors, including fuel cell design and assembly, operating conditions (e.g. humidification, temperature, cell voltage), impurities or contaminants in the feeds, environmental conditions (e.g. subffeezing or cold start), and operation modes (e.g. startup, shutdown, potential cycling). 

Ease of installation and commissioning is another reason for gas turbine use. A gas turbine unit can be tested and packaged at the factory. Use of a unit should be carefully planned so as to cause as few start cycles as possible. Frequent startups and shutdowns at commissioning greatly reduce the life of a unit. 

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