Optimised solder interconnections in crystalline silicon (c-Si) photovoltaic modules for improved performance in elevated temperature climate

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy.The operations of c-Si PV modules in elevated temperature climates like Africa and the Middle East are plagued with poor thermo-mechanical reliability and short fatigue lives. There is the need to improve the performance of the system operating in such regions to solve the grave energy poverty and power shortages. Solder interconnection failure due to accelerated thermo-mechanical degradation is identified as the most dominant degradation mode and responsible for over 40% of c-Si PV module failures. Hence the optimisation of c-Si PV module solder interconnections for improved performance in elevated temperature climate is the focus of this research. The effects of relevant reliability influencing factors (RIFs) on the performance (thermo-mechanical degradation and fatigue life) of c-Si PV module solder interconnections are investigated utilising a combination of ANSYS finite element modelling (FEM), Taguchi L25 orthogonal array and analytical techniques. The investigated RIFs are operating temperature, material combination and interconnection geometry. Garofalo creep relations and temperature dependent Young’s Modulus of Elasticity are used to model solder properties, EVA layer is modelled as viscoelastic while the other component layers are modelled using appropriate constitutive material models. Results show that fatigue life decays with increases in ambient temperature loads. A power function model =721.48−1.343, was derived to predict the fatigue life (years) of c-Si PV modules in any climatic region. Of the various ribbon-contact material combination models investigated, Silver-Silver, Aluminium-Aluminium, Silver-Aluminium and Aluminium-Silver are the top four best performing solder interconnection models with low deformation ratios, , normalised degradation values, 1. Further findings indicate that only the solder layer demonstrates good miniaturisation properties while the standard dimensions for ribbon and contact layers remain the best performing geometry settings. Additionally, from the Taguchi robust optimisation, the Aluminium-Silver ribbon-contact material combination model (ribbon = 180μm, solder = 56μm, contact = 50μm) demonstrated the best performance in elevated temperature climate, reduced solder degradation by 95.1% and is the most suitable substitute to the conventional c-Si PV module solder interconnection in elevated temperature climate conditions – in terms of thermo-mechanical degradation. These findings presented provide more insight into the design and development of c-Si PV modules operating in elevated temperature climates by providing a fatigue life prediction model in various ambient conditions, identifying material combinations and geometry which demonstrate improved thermo-mechanical reliability and elongated fatigue life.Schlumberger Faculty for the Future Foundation (FFTF

Effect of operating temperature on degradation of solder joints in crystalline silicon photovoltaic modules for improved reliability in hot climates

Accelerated degradation of solder joint interconnections in crystalline silicon photovoltaic (c-Si PV) modules drives the high failure rate of the system operating in elevated temperatures. The phenomenon challenges the thermo-mechanical reliability of the system for hot climatic operations. This study investigates the degradation of solder interconnections in c-Si PV modules for cell temperature rise from 25 °C STC in steps of 1 °C to 120 °C. The degradation is measured using accumulated creep strain energy density (Wacc). Generated Wacc magnitudes are utilised to predict the fatigue life of the module for ambient temperatures ranging from European to hot climates. The ANSYS mechanical package coupled with the IEC 61,215 standard accelerated thermal cycle (ATC) profile is employed in the simulation. The Garofalo creep model is used to model the degradation response of solder while other module component materials are simulated with appropriate material models. Solder degradation is found to increase with every 1 °C cell temperature rise from the STC. Three distinct degradation rates in Pa/°C are observed. Region 1, 25 to 42 °C, is characterised by degradation rate increasing quadratically from 1.53 to 10.03 Pa/°C. The degradation rate in region 2, 43 to 63 °C, is critical with highest constant magnitude of 12.06 Pa/°C. Region 3, 64 to 120 °C, demonstrates lowest degradation rate of logarithmic nature with magnitude 5.47 at the beginning of the region and 2.25 Pa/°C at the end of the region. The module fatigue life, L (in years) is found to decay according to the power function L=721.48T-1.343. The model predicts module life in London and hot climate to be 18.5 and 9 years, respectively. The findings inform on the degradation of c-Si PV module solder interconnections in different operating ambient temperatures and advise on its operational reliability for improved thermo-mechanical design for hot climatic operations

A review of photovoltaic module technologies for increased performance in tropical climate

The global adoption and use of photovoltaic modules (PVMs) as the main source of energy is the key to realising the UN Millennium Development Goals on Green Energy. The technology – projected to contribute about 20% of world energy supply by 2050, over 60% by 2100 and leading to 50% reduction in global CO2 emissions – is threatened by its poor performance in tropical climate. Such performance discourages its regional acceptance. The magnitude of crucial module performance influencing factors (cell temperature, wind speed and relative humidity) reach critical values of 90 °C, 0.2 m/s and 85%, respectively in tropical climates which negatively impact module performance indices which include power output (PO), power conversion efficiency (PCE) and energy payback time (EPBT). This investigation reviews PVM technologies which include cell, contact and interconnection technologies. It identifies critical technology route(s) with potential to increase operational reliability of PVMs in the tropics when adopted. The cell performance is measured by PO, PCE and EPBT while contacts and interconnections performance is measured by the degree of recombination, shading losses and also the rate of thermo-mechanical degradation. It is found that the mono-crystalline cell has the best PCE of 25% while the Cadmium Telluride (CdTe) cell has the lowest EPBT of 8-months. Results show that the poly-crystalline cell has the largest market share amounting to 54%. The CdTe cell exhibits 0% drop in PCE at high-temperatures and low irradiance operations – demonstrating least affected PO by the conditions. Further results establish that back contacts and back-to-back interconnection technologies produce the least recombination losses and demonstrate absence of shading in addition to possessing longest interconnection fatigue life. Based on these findings, the authors propose a PVM comprising CdTe cell, back contacts and back-to-back interconnection technologies as the technology with latent capacity to produce improved performance in tropical climates