1. Introduction
In recent years, Photovoltaic (PV) power capacity has increased more than any other type of generation technology. In 2018, the addition of PV power installed capacity of 100 GW was higher than all other technology types combined, and now accounts for 505 GW globally [
1]. PV power generation is heavily dependable on the environmental conditions—solar irradiation and temperature. In order to increase PV system flexibility and to provide more dispatchable energy, integration of battery systems has been considered as a viable solution. Historically, the high cost of this storage technology has been the main barrier for its deployment. However, the declining cost of battery systems in recent years has enabled its commercialization. Lithium-ion is predominated technology type due to its merits suitable for a PV application—a fast response, scalability and low self-discharge. Its cost has declined for an average of 23% per year from 2010 to 2015, as reported in [
2]. It is expected that continuous reduction in cost will further continue, which is then reflected in the expected increase of installed PV-battery systems. Precisely, 55% of annual energy storage deployments are expected to be coupled with PV systems by 2023 [
3]. In such a case, system architecture and its impact on overall system performance are becoming an important topic.
In general, PV-battery power processing units consists of the three main components. Those are (1) PV panels representing power generation unit; (2) battery representing a storage unit and; (3) power electronic interface representing power conversion unit. Depending on the number of power electronic components and their type of connection, two main system configurations are available DC- and AC-coupled configurations. The main difference between two lies in the point of connection (POC) for the battery unit which can either be connected in the DC-link or at the point of common coupling (PCC).
System configuration does not only directly influence the operation, but also the cost, efficiency, lifetime and, consequently, reliability of the PV-battery system. In terms of operation, DC-coupled configuration has lower operational flexibility, as the total power delivered to the load is limited by the inverter capacity [
4]. In cases of high load demand, PV and battery unit power capacity may be sufficient, but inverter capacity will limit the amount of the delivered power. In AC-coupled configuration, a greater amount of power can be delivered to supply the load, where both the PV and the battery inverter can supply the load at the same time. Cost of AC-coupled configuration is higher due to the additional DC/AC conversion unit which makes higher complexity of the system design and its balance. As reported in [
5], DC-coupled configurations yield on average 1% lower total cost than AC-coupled configurations. However, in the case in which battery is integrated into the already existing PV system, the cost of the DC-coupled configuration could be higher. Already existing DC/AC conversion unit may need to be modified to accommodate multiple DC connections as well as a bi-directional DC/AC inverter. This would then additionally increase the installation cost of such a system. On the other hand, the AC-coupled battery system can be easily added to the existing PV system at the PCC, as shown in
Figure 1. As a general rule, the efficiency of a system decreases as the number of power conversion units increases. Hence, the AC-coupled configuration has lower efficiency due to the additional DC/AC conversion unit. Nonetheless, as stated in [
5], higher efficiency of power electronic units nowadays has caused the difference in system efficiency to become smaller compared to the early stage of development of the PV-battery systems.
In [
6], the residential PV-battery systems were studied from a control point of view. A techno-economic analysis of a PV-battery system is investigated for the installation site in Greece [
7] with DC-coupled configuration. Similarly, the benefits of connecting battery to a PV system in a DC-coupled configuration are investigated in [
8], while AC-coupled configuration is investigated in [
9]. Two configurations are compared in [
10], where the performance of the PV-system connected in each of the configuration is further analyzed. However, most of the research on comparison of the two configurations is done from the economic point of view. In [
11], the installed cost benchmark is proposed and the authors focused on researching potential cost-reduction opportunities of the PV-battery system. Similar research is conducted in [
5] where the main focus is also put on the cost-effectiveness of a such system.
However, an important aspect that has not yet been researched is related to the lifetime and reliability of the PV-battery system connected in the two aforementioned configurations. This evaluation is necessary as it gives information on differences in the reliability of the two configurations resulting from the different number of components and their electrical loading. In general, information about the reliability of the system and its components is critical for adequate system operation and related economic profitability. In [
12], reliability of the inverter unit for the DC-coupled configuration is analyzed. However, the reliability of the remaining system components, such as the DC/DC converters, have not been investigated. Thus, it is necessary to investigate each of the reliability-critical system components. If such an approach is used, it will yield information on which part of the power electronic interface is prone to failure the most. Moreover, by comparing the reliability of the two systems, additional information on the choice of the adequate configuration type for PV-battery system is obtained. Considering that, a reliability benchmark for PV-battery system connected in DC- and AC-coupled configuration is here presented.
With respect to that, an overview of the PV-battery system configurations is provided in
Section 2 along with the implemented energy management strategy. The power converters electrical and thermal loading in the DC- and AC-coupled configurations is investigated in
Section 3. The procedure for reliability evaluation is presented in
Section 4. Reliability analysis is carried out with a case study of the real on-site measurement data in
Section 5 where the reliability of the components and systems in DC- and AC-coupled configuration are compared. The conclusion of the carried work is presented in
Section 5.