*4.1. Compactness*

The structural strength of BIPV modules is achieved via extruded aluminium profiles, and the MLC is preferably installed inside these cavities. In BAPV modules, the MLC is installed at the back of the PV panel, but in the case of BIPV, this space is occupied by thermal insulation material.

In general, a flat and long design is preferred to fit inside the framework. Currently commercially available MLC converters do not have this form factor, making it impossible to fit in this cavity. The implementation of wide-bandgap components such as Silicon Carbide (Sic) or Gallium Nitride (GaN) in the converter topology can help to achieve the required power density goals by an increased switching frequency, leading to smaller passive devices. The avoidance of transformers or inductors can lead to further size reductions, usually accomplished via switched capacitor circuits [105]. However, switched capacitor circuits can suffer from a lower reliability given that capacitors are one of the dominant components that lead to device failures [59]. Especially when MultiLayer Ceramic Capacitors (MLCCs) are used, a careful design is important due to their dominant short-circuit failure mode [102].

#### *4.2. Wide Power and Input Voltage Range*

In order to meet the scalability and flexibility requirements, it is preferable that the converter is compatible with a wide variety of PV panel types (mono-, polycristalline, and thin film) and sizes. The type of panel will already dictate part of its electrical characterisitics, but this is also influenced by the active surface, leading to a higher or lower amount of active cells. Apart from the standard 60 or 72 cell panel size, strongly deviating designs can occur, depending on the architect's needs. Currently, commercial and research projects still focus on standard 60 or 72 cell PV modules, leading to designs that work for 20–50 V input voltage. This voltage range needs to be widened on both ends for future converter designs. The low input voltages might, however, lead to lower efficiencies due to the higher step-up [66,67]. Therefore, BIPV modules with a higher output voltage are more attractive from a power electronics point of view. This allows for simpler and smaller topologies to be used.

Furthermore, to optimize the energy-yield of the installation, the converters need to cover a wide input power range, preferably from 10–100% of the nominal peak power. The challenge is to maintain a decent efficiency over the entire operating range as typically the efficiency curve strongly decreases on the low-power operating range. Interleaved converters can help to achieve this goal by controlling the amount of phases based on the input power, thereby allowing a flatter efficiency curve [106].

#### *4.3. Temperature Range and Cooling*

Experimental results from previous studies indicate that the temperature in and around façade BIPV panels can strongly exceed 100 °C, depending on the installation and type of grid ventilation [53]. Measurements inside the framework where the converter will be placed indicate temperatures around 80 °C, leading to high thermal stresses for the converter components. As temperature and temperature cycling are one of the major stressors for power electronic components [59], cooling or heat-spreading will be required to keep the component temperature low enough to meet the reliability requirements. However, the physical dimensions of the heat sink need to be limited such that the size does not conflict with the compactness requirements. An alternative gaining importance in space-constrained applications is evaporative cooling using ultrathin heat pipes [107]. Active cooling using forced air flow is not preferred as the lifetime of fans is limited due to wear-out of the bearings [62], whereas active liquid cooling is considered to be expensive and difficult to implement in the system. BIPVT systems, where the generated heat of the PV panel is partly recovered in the building via forced convection, seems to gain lots of research interest [12]. No studies have been carried out that check whether this is also an effective approach to cool integrated electronics such as the MLC.
