Sensor Integration in PV Devices

Abstract

The on-field performance and lifespan of PV modules are affected by the interaction of numerous parameters, among which temperature, strain, and humidity. Among these parameters, only temperature is occasionally measured in PV installations, and such monitoring is often external to the module. This limits its use and does not provide accurate information on the conditions at cell level. The integration of different sensors on PV cells can improve the performance of PV devices and enable the early detection of faults and unwanted operating conditions, examples of which are PV-module delamination and the occurrence of hotspots, respectively.

This thesis reports on the design, fabrication and testing of four types of sensors, as an initial step within the PVMD group towards sensor integration on PV cells. The devices considered for this work are the following: an aluminum-based resistive temperature sensor, a Boron-doped poly-silicon piezoresistive strain sensor, and two polyimide-functionalized humidity sensors – one capacitive, the other thermoresistive. Furthermore, these devices are combined into multi-sensing platforms with the aim to simultaneously sense sets of parameters (e.g., strain & temperature) to eventually compensate for their reciprocal interference.

The results of the electrothermal characterization of the non-laminated temperature and strain sensors reveal an overall linear response of the devices, with an average TCR of around +3.75*10-3 °C-1 and -3.7*10-3 °C-1, respectively; this translates into a change in resistance of approximately 22.2-22.5% over the temperature range under study (30 °C-90 °C). Meanwhile, the characterization with a climate chamber of a fabricated capacitive humidity sensor shows a significant increase in capacitance: when relative humidity changes between 20% and 80% at 30 °C, the measured capacitance rises from 6.86 nF to 7.20 nF – equivalent to a 5% increase. Furthermore, capacitance measurements performed at constant humidity ratio and varying temperatures indicate a substantial cross-sensitivity of the humidity sensor under test, resulting in an approximately linear capacitance increase between 20 °C and 40 °C with an average regression slope of 10 pF/°C.

Since the tested capacitive humidity sensor and piezoresistive strain sensor show a substantial response to temperature, temperature compensation is needed to ensure reliable and accurate measurements of both humidity and strain. To this regard, a simplified compensation based on the simultaneous interrogation of the resistive temperature sensor and capacitive humidity sensor is successfully performed, which highlights the importance of integrated multi-sensing platforms.