Energy Loss Analysis of Tandem PV Modules under Realistic Operating Conditions

Abstract

To complete the energy transition, a high efficiency for photovoltaic (PV) modules is desirable to reduce the needed material and surface area (per unit of generated electrical energy). The tandem PV technology has the potential to increase the efficiency of PV modules over 30%. In order to design efficient solar cells, a quantification of the different losses is important. Moreover, research in the losses of PV system has resulted in important insights for PV technology. This work introduces a comprehensive model for quantifying the different loss mechanisms in a PV system with tandem cells. The loss analysis model will be added to the PVMD Toolbox, which is a software developed at Photovoltaic Materials and Devices group at Delft University of Technology. This software can be used to simulate the energy yield of a PV system at any given location. In the loss analysis model, 17 losses are defined and divided into four categories (fundamental, optical, electrical and system losses). The developed model will be used to analyse the loss distribution under different operating conditions for four different PV modules. These different modules are a mono-facial crystalline silicon, a bifacial crystalline silicon, a two-terminal perovskite/silicon tandem, and a three-terminal perovskite/silicon tandem module. The design of these modules is based on a >29% efficient perovskite/silicon tandem cell, fabricated by HZB. The loss distribution of every module is simulated for Standard Test Conditions (STC) and for real word conditions at four geographical locations. Generally, we find that modules operating in tropical high irradiance climates have the lowest efficiency. For all locations, the difference in losses compared to STC follow similar trends. When the two-terminal perovskite/silicon module is simulated at STC, the loss distribution of the fundamental, optical, electrical, and system losses are 54.8%, 8.9%, 8.5%, and 0.1%, leaving a DC module efficiency of 27.7%. At real-world operating conditions, various differences can be found. The most significant differences are the thermalization, reflection, and recombination losses, which increase with 1.4%, 1.1%, and 0.5% respectively for the two terminal perovskite/silicon tandem module. Furthermore, the simulated two-terminal module has a higher efficiency than the three-terminal modules for all operating conditions due to lower mismatch losses. Additionally, this study was able to quantify the fill factor gain for two-terminal devices. Due to spectral variations, there can be a mismatch between the absorbed current in the top cell and bottom cell, which can lead to losses. However, this loss is partially compensated by an increase in fill factor. For example, a current mismatch of 7.0% is reduced to a power mismatch loss of 1.2%, due to an increase in fill factor. Therefore, the power mismatch should be used as an indicator for mismatch losses instead of the current mismatch. Finally, this study simulated different improvements on operating conditions. The results show that solar tracking does not only increase the in-plane irradiance of the PV system, but can also increase the efficiency. For example, dual-axis tracking can increase the efficiency with 1.1%. Also, the gain of active cooling is simulated and quantified. The increase of efficiency when cooling at 20oC compared to a PV system without cooling is around 0.4%, mostly caused by decrease in emission and recombination losses. Furthermore, the optimal perovskite thickness for real world conditions is found, by simulating different thicknesses for the perovskite layer. The results shows that the optimal thickness under STC (575 nm) is also optimal under real-world operating conditions. Finally, the optimal bandgap energies for reducing the fundamental losses are found for tandem cells. For all conditions (including STC), the optimal bandgap energies for the top and bottom cell are 1.73 and 0.94 eV respectively.