Systems And BoP - Technical Basics

Question 1

Formula for Thermodynamic efficiency.

Answer 1

E°/E°_H

Question 2

Formula for voltage efficiency.

Answer 2

E_Z/E°

Question 3

Formula for cell efficiency.

Answer 3

E_Z/E°_H

Question 4

Formula for utilisation of the fuel gases in the cell.

Answer 4

U_t = mdot_fuel,used/mdot_fuel,in = ((M/zF)I)/mdot_fuel,in

Question 5

Formula for stack efficiency including gas usage.

Answer 5

Cell efficieny * u_t

Question 6

Difference between stack efficiency and faraday efficiency.

Answer 6

Faraday efficiency also considers gas usage but is metrologically easier

Question 7

Formula for Faraday efficiency.

Answer 7

I/(zFMdot_i) where Mdot_i is the Molar flow of the fuel

Question 8

Why is it that fuel cells with higher operating temperatures are more efficient, even though this is theoretically according to thermodynamics inaccurate?

Answer 8

This is because the determining factor here is the kinetics.

Question 9

What is utilisation?

Answer 9

It does not refer, as typically with efficiency, to the ratio of the performance of the system, but to that of the work. Ultimately, it represents the time integral over a longer period of operation. For stationary plants, for example, one year makes sense in this case to compensate for seasonal fluctuations. The utilization rate strongly takes into account the partial load conditions that occur during the operating period.

Question 10

Utilisation formula.

Answer 10

W_used/W_in

Question 11

What is the Faraday‘s law connecting the measured current in the FC and the amount of the reacted substance?

Answer 11

m = (QM)/(Fz) = (ItM)/(F*z)

Question 12

What does the Clausius-Clapeyron equation do and what is the formula?

Answer 12

If a the p of vapour is known at a certain temp, it helps predict it at another temp, but only if the enthalpy of vaporisation is constant in that temperature range.
ln(p2/p1) = (dH_V/R)((T_2 - T_1)/(T_1T_2))

Question 13

Formula for saturation limit.

Answer 13

x_s = (p_s/(p-p_s))*0,622
Where p is the ambient pressure and p_s is the steam partial pressure / saturation Vapor saturation
x>x_S means condensation and the opposite is evaporation

Question 14

Why is a low discharge of water in FC strived towards.

Answer 14

Water discharge is partly used for cooling because of high enthalpy of vaporisation but too high water discharge leads to the filling of the cell.

Question 15

What is the Kelvin equation for Vapor pressure depending on the curvature of a surface?

Answer 15

p = p* * e^((2gammaV_M)/(rRT))
Where p = partial pressure over flat surface, p* = partial pressure over curved surface, gamma = surface tension, V_M = molar volume of liquid and r = droplet radius

Question 16

Where is laminar flow used and where is turbulent flow used?

Answer 16

FC uses laminar flow due to its minimal flow resistance. Heat exchangers use turbulent flow because it allows for better heat transfer through convection, but leads to higher pressure loss.

Question 17

What is the relation between viscosity of ideal gases and pressure and Temp.?

Answer 17

• Pressure-independent course of the viscosity of ideal gases in a wide range
• Temperature dependence with sqrt(𝑇)

Question 18

What is the dependence of viscosity on temperature in Newtonian liquids?

Answer 18

Log(eta) = log(eta_0) + (E_A/(R*T))

Question 19

Why are predictions on the transport of heat through radiation difficult?

Answer 19

Heat transport by means of radiation already plays an essential role in the operating temperatures of the SOFC. Predictions on the transport of heat through radiation are difficult because the emissivity values of the metallic materials change greatly over time due to oxidation.

Question 20

How does thermal conductivity change with temperature in metals?

Answer 20

For metals, the thermal conductivity decreases with increasing temperature, as well as the electrical conductivity.

Question 21

How does thermal conductivity change with temperature in non-metals?

Answer 21

In non-metallic materials, however, the heat is transported by phonons, which also provide the electrical conductivity. The mobility of the phonons increases with temperature, whereby here, in contrast to the metallic materials, the conductivity increases with the temperature.

Question 22

What re the criteria for the design of wall thicknesses of pipelines?

Answer 22

• Internal pressure
• Handling capacity during transport and installation
• Bend between supports
• External damage risks (mechanical, corrosion)
• Type of pipe connection
• Traffic loads
• Impeded thermal expansion