As renewable energy generation increases in popularity, solar photovoltaic (PV) power is becoming more widely used around the world. PV systems are most commonly used to provide solar energy to the electric grid. Recent advancements in PV power systems include the integration of energy storage with PV systems to increase their reliability and cost-effectiveness [1
]. Additionally, both policies and technological advancements are focused on reducing power consumption by increasing efficiency at the consumer level, such as switching to LED lighting [2
]. At the same time, the concept of wearable devices is also gaining attention, in particular, devices like smart watches, fitness trackers, and smart clothing [3
]. At the intersection of these trends is the concept of using PV cells to power wearable applications. However, there are a number of challenges of using PV power for wearables that can be overcome with effective design of the power conversion system.
For grid-connected systems, PV modules are typically connected in series strings with one main central converter to control and process the power. With the series string configuration, it is well known that imbalances in the PV cell powers can result in extremely low system efficiency [4
]. Imbalances or mismatch can occur among the PV cells due to manufacturing variance, dust accumulation, partial shading, aging, etc. [8
]. Thus, great effort is taken to ensure that PV cells are well-matched within modules, to keep the surface free of dust, and to locate modules on roof tops or open locations to prevent partial shading.
Because PV modules are typically connected in series, PV systems are generally thought to be most appropriate for applications where lighting is expected to be uniform. However, this work focuses on enabling PV power for emerging applications where non-uniform light intensities are expected over the PV modules. The key to enabling PV power for non-uniform lighting applications, like wearables, is to reinvestigate the PV module and power converter configuration. Here, a parallel connection, rather than a series one, and the concept of differential power processing (DPP) are utilized to enable PV power for wearable applications.
As mentioned, series-connected PV modules are typically connected to a power converter, which controls a PV string using maximum power point tracking (MPPT) to optimize output power [10
]. In the string, all cells must operate at the same current, but this current is limited by the PV cell receiving the lowest amount of light. Thus, all PV cells cannot operate at their own maximum power point (MPP) when PV cells receive different light intensities and, as a result, output power decreases severely. This power reduction problem commonly occurs in PV systems with long strings of PV cells controlled by one central converter [5
Various converter architectures have been proposed to overcome the output power reduction problem under unbalanced light conditions. Previously, module-integrated converters (MICs) [13
] and direct current (DC) optimizers [15
] were introduced, where each PV module provides power through a power converter that operates each module individually and independently. An example MIC architecture is shown in Figure 1
a. Each PV module can operate at its own MPP regardless of the light imbalance among PV modules. However, the PV module is connected to the grid through the MIC, so the input–output voltage step-up ratio is relatively high. Alternatively, DC optimizers can be used, as shown in Figure 1
b. The PV modules are controlled by the DC optimizers, but their outputs are connected in series such that the voltage ratio from the DC optimizer output to the grid is lower than for MICs. MICs and DC optimizers are full power processing (FPP) converters [18
], where all the power passes through the converter and losses are proportional to the total PV power.
More recently, differential power processing (DPP) converters [20
] were introduced as a solution that allows for independent MPPT for each PV module, while decreasing the total input power to the power converters. One of the series DPP configurations for series-connected PV modules is shown in Figure 1
c, referred to as series DPP. The majority of the existing literature is for series DPP, where the fundamental connection of the PV modules is in series. Series DPP converters are able to compensate for even extreme light differences, but in some cases the DPP converters process a significant amount of power to achieve individual MPPT [27
], which is a limitation of the architecture.
An alternative DPP architecture is to use parallel DPP converters, as shown in Figure 1
d. Based on the operating characteristics of PV cells, the voltage characteristics are less sensitive to extreme light differences than the current characteristics [28
]. This indicates that when extreme light differences are expected, parallel connection is more advantageous for maximizing the power from each cell. Some potential converter topologies for parallel DPP converters have been explored in Reference [31
], but the design, implementation, and control methods still need in-depth investigation.
This work focuses on the system design and analysis of a PV-powered wearable bag application to charge battery-powered consumer devices [32
]. This paper explains and analyzes the operation of the target parallel DPP system, including the battery load impedance characteristics, PV modules, and system control considerations, in Section 2
. Section 3
shows detailed simulation results of the system to determine the most effective DPP configuration and control strategy. The findings are summarized and concluded in Section 4
A PV-powered wearable application utilizing parallel DPP converters and MPPT for improved power output was explored through mathematical analysis and detailed simulation. The target of the PV-powered bag application is to charge the battery of a portable device. The system consists of four PV modules, each independently controlled by a parallel DPP converter. Two parallel DPP configurations are compared: with and without a front-end converter. The parallel DPP system with a front-end converter uses higher-voltage PV modules such that the power loss in the DPP converters is very small under nominal irradiance conditions. However, it cannot always achieve MPPT when the DC bus voltage is low; further, it is unable to charge the battery load in some lower-irradiance cases. The parallel DPP design without a front-end converter uses lower-voltage PV modules, such that power loss in the DPP converters is slightly higher in nominal irradiance conditions; however, it shows higher system efficiency at lower DC bus voltages and is able to charge the battery over a wider range of irradiance conditions. These results identified the parallel DPP system without a front-end converter as the more effective design for the PV-powered bag application.
Independent MPPT capability of the DPP converters and proper charging of the battery load was also verified through the simulation results. Three PV MPPT algorithms (constant voltage, P&O, and VRC) were identified as viable control strategies for parallel DPP systems and their performance was compared. With well-calibrated MPPT parameters, VRC showed the best performance under a sudden irradiance change to uneven lighting across the four PV modules. However, each MPPT algorithm has implementation trade-offs and the algorithm selection depends on the specific application requirements. With these findings, effective power converters can be developed and implemented for PV-powered wearable applications charging Li-ion battery loads. The parallel DPP architecture and control schemes can be extended to other PV applications where uneven lighting is expected over multiple PV modules, such as mobile PV power systems for camping or military applications, building-integrated PV systems, and PV modules installed in vehicles.