Flexible and Scalable Photovoltaic Curve Tracer

Abstract

Current vs voltages (I-V) curves are needed to understand the electrical characteristics of photovoltaic (PV) materials. Whereas the researcher may be concerned with a temperature-compensated laboratory-grade setup for an individual cell, the practitioner may be interested in validating operation of a large PV plant. PV curve tracers exist on the commercial market for each of these market segments. However, for researchers working with small modules, the cell-level testers may not have adequate voltage and current range, or may not physical accommodate more than a single cell; units designed for large-scale PV plants may not have sufficient resolution or flexibility for low-power modules. Curve tracers for these two markets also tend to be specialized for the particular use-case and relatively expensive. The aim of this paper is to present a curve tracer that is based on off-the-shelf components but is flexible and scalable to accommodate a range of voltage and current levels, number of specimen, and connecting / disconnecting the specimen to other circuitry such as a DC/DC converter. As such, the system can be used for a few cells to large modules and can accommodate an arbitrary number of specimen, which is beneficial to perform comparative testing. It can also be used to validate the maximum power point tracking efficacy of a DC/DC power optimizer by disconnecting the converter, performing the I-V sweep, and then re-connecting to the converter. The system uses a Keithley 2461 Source Meter Unit (SMU) and one or more relay modules, all controlled by MATLAB. The SMU is responsible for generating the voltage sweep and measuring the resulting current. Using commercial equipment alleviates the user from having to custom design and built a curve-tracer. In addition, the SMU can be calibrated to ensure accurate and reliable data. If multiple specimen are to be tested, multiple relay modules can be added to enable multiplexing of many PV specimen to the SMU. Custom software running in MATLAB configures the relays; setups, triggers, and downloads data from the source meter; and saves the data and creates the plots.