Question 11.AE.8: A PEM fuel cell has the parameters: Performance: output powe......
A PEM fuel cell has the parameters:
Performance: output power=P_{rat}=2400 W^1), output current=I_{rat}=92 A^1), DC voltage range=V_{rat}=25 to 50 V, operating lifetime: T_{life}=1500 h^2).
Fuel: composition=C=99.99% dry gaseous hydrogen, supply pressure=p=10 to 250 PSIG, consumption=V=37 SLPM^3).
Operating environment: ambient temperature=T_{amb}=3°C to 30°C, relative humidity=HR=0% to 95%, location=indoors and outdoors^4).
Physical: length x width x height=56 x 50 x 33 cm, weight=W=26 kg-force. Emissions: liquid water=H_2O=1.74 liters maximum per hour.
1) Beginning of life, sea level, rated temperature range
2) CO destroys the proton exchange membrane
3) At rated power output, SLPM≡standard liters per minute (standard flow)
4) Unit must be protected from inclement weather, sand and dust.
a) Calculate the power efficiency of a PEM fuel cell in two different ways.
b) Find the specific power density of this PEM fuel cell expressed in W/kg-force.
c) How does this specific power density compare with that of a lithium-ion battery?
Hints:
1) The nominal energy density of hydrogen is 28 kWh/(kg-force), which is significantly larger than that of gasoline (12.3 kWh/kg-force). This makes hydrogen a desirable fuel for automobiles.
2) The (weight) density of hydrogen is γ_{hydrogen}=0.0899 g-force/liter.
3) The oxygen atom has 8 electrons, 8 protons and 8 neutrons.
a) Atomic-weight method: Atomic weights of hydrogen (H), oxygen (O), and water (H_2O) is 1, 16, and 18, respectively. If 1.74 liters of water are emitted then the weight of hydrogen consumed is weight_{H2}=(2/18)(1.74 kg-force/h)=0.19333 kg-force/h and the corresponding hydrogen input energy provided is E_{H2}=(0.19333 kg-force/h) (28.kWh/kg-force)=5.41 kWh/h which corresponds to an input power of P_{in}=5.4133 kW. The output power is P_{out}=2.4 kW resulting in a power efficiency of η=P_{out}/P_{in}=0.443.
Hydrogen-density method: The input power of hydrogen is P_{in}=(γ_{hydrogen})(standard flow/h)(E_{H2}/kg-force)=(0.0899.10^{-3} kg-force/liter)(3760 liters/h)(28 kWh/kgforce)=5.59 kWh/h, or P_{in}=5.59 kW yielding a power efficiency of η=P_{out}/ P_{in}=0.43.
b) The specific power density per unit of weight is power density=P_{out}/ weight=2400 W/(26 kg-force) =92.3 W/kg-force. However, some publications cite PEM fuel cell stacks with power densities over 1,500 W/kg-force. The disadvantage of a fuel cell is that braking energy cannot be stored as can be done by a battery.
c) The power density of a lithium-ion battery is 300 W/kg-force.