*2.2. Energy Management System (EMS)*

Figure 3 is the functional block diagram of the semi-active DMFC power station. The DMFC stack system is located on the top consisting of a fuel tank and a set of four DMFC stack modules. The lower half is the function block diagram of the EMS controlling the collective operating conditions and status report communication.

**Figure 3.** Semi-active DMFC power station functional block diagram.

The EMS has a system handler at the core of the multiple sets of FC reaction controller. FC reaction controllers coordinate the BOPs in the stack modules to work, while the FC voltage regulators keep the modules properly loaded to maintain the electrochemical reactions and manage the energy storage into the battery. The switches between the stack modules and the regulator determine the operating mode of the corresponding module, whether it is offline for maintenance or online in parallel power generation. Li-ion batteries act as energy buffer between generation and output demand. The output voltage regulator serves the output demand. There is a core supervisor monitoring the status of the batteries to control the inhibition of the power generation and the output power drive so that batteries are protected from over charge or discharge.

When DMFC stacks are activated, they can be switched online and offline independently, and their working voltages and currents can change rapidly as a variable energy source. In order to have their electricity output connected in parallel for stable power generation, the dynamic energy management control is designed to maintain the stability of their reaction voltages to the setpoint *Vsp* by the FC voltage regulator. Given the characteristics of the MEA, the generator current estimator estimates *Ies* as the target current to draw from stacks. It also adjusts the current estimate based on the error between the present FC voltage *VFC* and the setpoint voltage *Vsp* so that *VFC* converges to *Vsp*. In the inner loop, the current feedback controller measures the actual output current *Io* and quickly adjusts the direct current to direct current (DC/DC) converter so that *Io* tracks *Ies*. Meanwhile, all electricity generated, namely *Io* drawn from the stack, gets pumped into the battery for storage.

The right side of Figure 4 shows the output voltage regulator taking energy from the Li-ion battery to drive the output demand. Wherein, the output converter is a boost DC/DC converter controlled by the system handler. When enabled, it draws from the battery voltage to drive the output voltage. The output would be turned off when disabled to prevent over drawing on the batteries. An eFuse module which prevents excessive output current and reversed current against load variation improves the reliability of the power station.

**Figure 4.** The fuel-cell (FC) voltage regulator and the output voltage regulator block diagram.

The electrochemical reaction conditions of the DMFC stack modules are maintained by their individual BOPs under the coordinated control of the FC reaction controller as shown in Figure 5a. The reaction voltage calculator determines the appropriate stack reaction voltage setpoint *Vsp*, based on the ambient conditions, the stack temperature and the characteristics of the DMFC stack module, for the FC voltage regulator to follow. The fuel supply calculator determines the fuel consumption to control the dosing pumps to replenish according to the sensor-less approach equation, Equation (1). Assuming only a portion of the fuel consumed by the DMFC stack module is effectively converted into electricity *IFC* and the rest is the temperature dependent crossover, thus derived the fuel consumption estimator according to sensing measurements.

$$S = K\_T \int\_0^{\Delta t} I\_{FC} \, dt + K\_\mathcal{c} \cdot \Delta t \tag{1}$$

where *S* is the total fuel consumption over a given period Δ*t*, *KT* is the fuel consumption coefficient resulting in effective power generation represented by the DMFC output current *IFC*, and *Kc* is the fuel consumption rate resulting in the crossover. Both *KT* and *Kc* coefficients are determined from the MEA size, the stack module characteristics, and the ambient temperature. The backbone of the controls is to maintain the proper heat generation for temperature management. The fuel consumption is estimated by Equation (1) to determine the appropriate fuel supply to maintain, to increase, or to decrease current operating temperature.

**Figure 5.** (**a**) Functional block diagram of an FC reaction controller; (**b**) the state machine of the FC process controller.

Figure 5b illustrates the state machine of the FC process controller explaining the logic behind the controls. When the power station is turned on, it first enters the idle state. When the enable signal is triggered, it enters the initial state that the controller controls the fuel supply and stack voltages to increase the stack temperature progressively. When the initial process ends, the state enters the normal operation state. If it yields an abnormal result, then the process enters the recovery state. In the normal operation state, the stack module continues to generate power, perform self-tests, and periodically switches to the activation state. If an error occurs, then the process switches to the recovery state. When the disable command is received, it switches to the shutdown state for safe turn off. While operating in the recovery state, the process attempts to resolve the stack problem. If it succeeds, the controller returns to the initial state to restart or the controller enters the fail state to stop any operations on this stack. In the activation state, the process blocks the stack current, stops supplying oxygen and waits 60 s. Afterwards, the controller returns to normal operation to generate electricity again. This process refreshes the MEAs, the reaction efficiency of which deteriorated after long usage.

The system handler of the EMS sets the system operating modes according to the commands received from the communication as well as the Li-ion battery voltage monitored. The power output is disabled when the battery voltage is less than 3.5 V, and it will be reactivated only until the voltage exceeds 3.7 V. The station power generating is enabled when the battery voltage falls below 3.8 V and will be disabled when the charged voltage rises above 4.1 V. The overall power balance of whole DMFC power station is described by Equation (2)

$$P\_{\rm BAT} = P\_{\rm FC} \cdot \eta\_{\rm Reg} - P\_{\rm Lead} \cdot D\_{\rm Lead} - P\_{\rm EMS} \tag{2}$$

where *PBAT* denotes the power charging the batteries, *PFC* is the total FC power generation, η*Reg* is the efficiency of the FC voltage regulator, *PLoad* is the power station output power to the external load, *DLoad* is the working duty of the external load, and *PEMS* is the power consumption of EMS. With the battery buffering and the output voltage regulator, *PLoad* is designed up to 12 W exceeding *PFC* and to be suitable to drive IoT applications.
