*2.6. Reliability and Availability*

As discussed in the previous points on the modularity and flexibility of the system, a fully integrated BIPV module with an integrated MLC is desired. The location of the MLC will be at the back of the PV panel or inside the metallic framework of the BIPV module. The major drawback of this choice is that the converter is difficult to reach after installation, making repairs or replacements undesirable or even impossible. The MLC lifetime should, thus, be comparable to the lifetime of the PV panel or even to the lifetime of the façade. The reliability of the converter becomes an important criterion to judge upon to enable this lifetime. A considerable amount of research is conducted to characterize the stresses of micro-inverters, considering mission profiles and degradation [59–61]. Commercial micro-inverter and power optimizers manufacturers use reliability as one of the key arguments to promote their products above competitors in the field and above the use of traditional string inverters. From Reference [62], the MTTF of string inverters is in the range of 20 years whereas micro-inverters report a MTTF of 300(+) years [63,64], certified by independent reliability test centers. Series power optimizers even report MTTF of 1000+ years, motivating that this is a consequence of the low amount of internal components, compared to string inverters [65].

No specific numbers have been found on parallel power optimizers. A higher reliability is, however, expected compared to micro-inverters as they only perform a DC/DC conversion, requiring less components and conversion steps. The converter reliability will, however, be lower compared to a series power optimizer, as a consequence of the higher voltage step-up, leading to more components and/or more complex circuit topologies [66,67]. For further calculations, the MTTF of parallel power optimizers is assumed to be 500 years.

From the above numbers, the converter reliability is plotted in Figure 3a for a time span of 40 years, corresponding to the desired lifetime of a BIPV façade. The string inverter performs the worst, leading to required replacements up to five times during the lifetime of a PV installation [59,68]. The replacements are not modeled in this study. The performance of micro-inverters and power optimizers is better, leading to an estimated reliability operational at 40 years of 87% for the micro-inverter, 96% for the series power optimizer (PO), and 92% for the parallel PO. These high numbers for distributed MPPT architectures have provoked skepticism within the PV industry and are criticized, as no or little field data is available to support these claims [62]. The numbers are mainly used to show specific trends, their numerical exactness is of secondary importance. Furthermore, the numbers that are reported in literature always referring to older converter designs that were installed several years ago. They can be used to derive a correct order of magnitude, but they do not take new designs with possible different failure mechanisms into account.

**Figure 3.** The predicted reliability of the different electrical architectures based on manufacturer reliability data. Note that repair is not modeled since the integrated converter is not servicable when installed in the module frame. The string inverter shows the lowest reliability but is installed on a servicable location which allows repair.

Even when special care is taken to have a reliable converter design, failures are inevitable. Hence, it is important that these failures only affect the converter and not the system as a whole. When a string inverter fails, the entire PV array becomes unavailable until the unit is repaired or replaced. As indicated in Reference [62], the independent and distributed architecture of MLC only affects one specific BIPV module. This leads to a reduced total power generation, but the system as a whole remains available. This is the case for micro-inverters and parallel power optimizers, if it is assumed that they are designed to fail safe. For series power optimizers, the malfunctioning of one device can lead to the malfunctioning of the entire string [69]. The string length of the leading commercial manufacturers is at least six and can go up to 25 (for a single phase grid connection). Taking these factors into account, the system reliability for the different cases is plotted in Figure 3b, showing that the parallel power optimizer approach leads to the highest system reliability. The reliability of the series power optimizer system has decreased due to the dependence on the correct functioning of the other optimizers within the string. Note that the MLC are assumed to be placed inside the metallic casing such that replacement is undesirable or even impossible. Specific failure modes of MLC and their consequences are investigated further on in this paper.

Furthermore, as reported by Reference [68], one of the main causes of PV inverter failure is related to inadequate protection from grid events, mainly surge voltages. To enhance the system's reliability, this factor needs to be taken into account.
