**1. Introduction**

Developing intelligent services to cover remote areas, Internet of Things (IoT) networks need to be deployed in places such as farms, forest, outlying islands, infrastructure piping, etc., where there is no electricity power grid. Acquiring power supply has always been a problem. Thus, this has led to demands for long-term reliable power supplies [1]. Secondary batteries are commonly used on IoT equipment. However, due to limited energy storage, connection to a power source for charging is still necessary for long operations. In addition to engine-driven generators, two types of power sources, namely, the energy-harvesting sources [2–5] and fuel cells (FCs) [6,7], can be used to charge. When installing energy-harvesting device, we need to consider the availability of the natural energy source and location suitability. Moreover, to meet steady demand, large enough capacity of the secondary batteries is needed to buffer the inherently uncontrollable variations in the natural energy resource [8–10]. Therefore, these power supplies are difficult to be portable. On the other hand, FCs demonstrate stable power generation needing relatively low battery buffering capacity. They can be installed quickly with the ability to work for a long time.

The FCs generate power by electrochemical reactions of different fuel types in different reaction modes. These are: polymer electrolyte fuel cell (PEMFC), direct methanol fuel cell (DMFC), direct formic acid fuel cell (DFAFC), direct ethanol fuel cell (DEFC), and solid oxide fuel cell (SOFC) [11–13]. Using liquid fuel, a DMFC has a higher fuel volume energy density so that the system is lightweight, easy to replenish, safe, and can be operated at room temperature. Therefore, DMFCs are suitable candidates portable power supplies for emergency deployments and IoT applications [14–16].

Facilitating an optimal environment for efficient electrochemical reactions, the balance of plant (BOP) of a DMFC can be an aggressive active type or a sub-optimal passive type. A passive DMFC maintains balance during electrochemical reactions without external forces. The architecture is simple. However, its electro-chemical efficiency is sensitive to the environment, the power generation is unstable, and the system durability is poor [17,18]. In an active DMFC, BOP components are installed to regulate the conditions for optimal reactions so that high power generation efficiency, stability, environmental tolerance, and long-term durability can be maximized for marketability. Therefore, the majority of DMFC products on the market are the active type [19]. Nevertheless, active DMFCs are disadvantaged by the complexity of the stack assembly process [20], the excessive number of BOP components, and complex in-system piping. Therefore, active DMFCs have low manufacturing yield, are difficult to miniaturize, and difficult to repair [21,22]. To ensure optimal operating conditions, the complexity of an active DMFC system is necessary. On the other hand, for simplicity, the passive DMFCs may be stable for only limited power output and operation time. However, for portability and long-term power generation reliability, a marketable semi-active DMFC design needs to be weighed between complexity and optimization. To address the complexity shortcomings of the active DMFCs, modularization would be one of the primary solutions. Additionally, several studies on the modularization of FCs, including stack modular design, combination of multiple stack modules, and control management of multiple stack modules have been proposed [23–27].

While a modularized stack can be individually installed, put in operation and serviced conveniently, there are great advantages to having multiple stack modules functioning collectively to increase power capacity, and to enhance the durability with added redundancies. Following this strategy, a novel architecture and control of the DMFC power station is developed with specifications suitable for IoT applications. Simplifying the system architecture, the functionalities of BOP components in the active DMFCs were reviewed. Only the necessary components were miniaturized and integrated onto the passive DMFC to form a compact semi-active DMFC stack module. The mechanism of a power station was reworked with much simplified wiring and piping to accept convenient multi-stack module plug-ins and to be operated collectively by one energy management system (EMS). Space for a larger fuel tank can, therefore, be made available for a longer period before refueling.

Although managing the fuel, heat and water of a DMFC for efficiency and durability is complicated and difficult compared with other types of FC [22,28], integrating controls of the electromechanical system with the EMS to accommodate the load demand can make improvements. Simultaneous controls of the limited BOP components in multiple DMFC stacks can maintain proper operating conditions of the semi-active DMFC power station effectively such that its environmental tolerance, endurance, and power generation stability of the system can approach that of an active DMFC. In verifying our new design, long-term evaluation of the system performance, the regulation of operating conditions by the BOP components, and the changes in stack characteristics is essential [29,30]. Therefore, a prototype of our power station was tested for 3600 h in actual outdoor environments throughout winter and summer under different weathers, temperatures, and humidity to verify its suitability for IoT applications in the field.
