Counters and Totalizers

The counters and totalizers include various operating times, production and consumption levels, and number of system startups. The definition of each indicator is described in the following paragraphs.

System Startups

A system startup is the process of going from an inactive stack to an active stack, i.e. providing power. The definition of system startup is when the stack current has increased from 0 A to above 5 A, i.e. 5 A is the threshold for the stack being active. Each system startup is associated with some degradation of the fuel cell. The startups are extracted by counting events where the current is increased from 0 A to above 5 A.

When the stack has been inactive for some time, there is air present on the anode side. The air needs to be replaced with fresh hydrogen during startup.

The filling of hydrogen into the anode will create some internal current that can lead to corrosion and thereby irreversible degradation. The corrosion currents are reduced by quickly purging the hydrogen into the anode as well as applying a load during startup to draw down the cell voltage. These events are called air-air startups and are extracted by counting the startups where the average cell voltage is below 0.1 V.

Operating Time

Active operation of the fuel cell is another cause of performance degradation.

The active operating time (Runtime) of the fuel cell stack can be calculated by looking at when the stack has supplied more than a certain threshold level of current (5 A). The active operating time is derived by finding the instances where the current rises above and falls below the threshold level.

To give an overview of the usage profile of the systems, the time spent in each of the operating modes (Table 2.2) can be investigated. The mode operating times are calculated by finding the instances where a certain mode is entered (changes from a different mode) and exited (changes to a different mode). An example of the mode logs and mode times of a specific stack is shown in Fig. 2.2. The figure shows the system modes versus the date and the total time spent in each of the system modes during the total operating time of an example system. The ‘unknown’ mode is not an actual operating mode, but simply indicates that no other system mode has been logged during a time interval. This might be caused by a number of things, including transport time to the system site, missing or corrupted data points, or downtime in relation to service of the system. The figure clearly indicates that the normal state of the system is in standby mode and only a small fraction of the time, is the system actually actively operating.

The fuel cells have an optimum operating temperature approximated by a linear relationship with the produced current (i): Topt = a·i+b. The time the fuel cell stack spends at non-optimum temperatures can also have an influence on the performance level. This is split into two indicators: under- and over-temperature:

Tunder≤Topt−∆T (2.1)

Tover≥Topt+ ∆T (2.2)

where ∆T defines the tolerable deviation from the optimum operating tempera-ture. The under/over-temperature operating times are extracted for each of the systems modes (Runtime_undertemp_{mode}andRuntime_overtemp_{mode}) as well as for the active operating time (Runtime_overtempand

Runtime_undertemp).

2015-012015-052015-092016-012016-052016-092017-012017-052017-092018-01

Fig. 2.2: Example system operating modes. Left: dots indicate instances of the different system modes versus date. Right: Total time spent in each mode.

Production and Consumption

Another way of quantifying the usage of the stacks is to calculate the amount of energy and charge it has produced over its lifetime. The energy produced by the fuel cell stack is the integral of the fuel cell power as such

E= Z tend

0

V(t)·I(t)dt (2.3)

for the sampled measurements, trapezoidal integration is used to approximate the energy:

whereLis the number of samples in the dataset. Similarly, the produced charge, which is the integral of the produced current, is calculated by

Q=

The charge can then be used to approximate the consumed hydrogen and oxygen, respectively. The reaction levels are calculated from (2.6) and (2.7), whereKH2 andKO2 are empirical constants.

˙

m_H2=K_H2·Q (2.6)

˙

mO2=KO2·Q (2.7)

Table 2.3: Counters and totalizers

KPI Description

Startups Number of startups of each stack Airair_startups Number of startups of each stack when

there is no hydrogen at the anode Runtime Operating time above current threshold Runtime_{mode} Operating time in each mode

Runtime_overtemp Operating time at above optimum tem-peratures above current threshold Runtime_overtemp_{mode} Operating time at above optimum

tem-peratures for each mode

Runtime_undertemp Operating time at under optimum tem-peratures above current threshold Runtime_undertemp_{mode} Operating time at under optimum

tem-peratures for each mode

Charge_produced The amount of electric charge produced Energy_produced The amount of electric energy produced Oxygen_reacted The amount of reacted oxygen

Hydrogen_reacted The amount of reacted hydrogen

Extracted Values

The list of KPIs are summarized in Table 2.3 and the extracted KPIs for each stack in the available systems’ data is shown in the boxplot of Fig. 2.3. This provides an overview of each stack in the fleet of backup power systems and the distribution of each KPI for the fleet. This is the basis for detecting abnormally performing stacks, which is presented in Chapter 3.