PowerWindow, a built-in Luminescent Solar Concentrator (LSC), is composed of a window glass with thulium doped coating and a CIGS PV-cell strip attached to one glass edge. LSCs can absorb part of the incoming sunlight and re-direct it to the edges, where PV-cells can produce electricity. Light transport efficiency is then determined by several characteristics of the glass waveguide and thulium as luminescent particles. The main objectives of this work were to find the most relevant characteristics of a LSC affecting its optical efficiency, and to optimize such parameters for a high performance.
In the first part of this work, LightTools, a ray-tracing software, was tested to model a LSC based on red-dyes. Simulation results were compared with experimental measurements using a sample of PMMA doped with red-dye (Lumogen F 305). The optical efficiency of the red-dye model was 4.1%, and LSC sample had 4.5% of efficiency (measured in a previous thesis work [Overbeek, 2015]), meaning 0.4% of marginal error. Optical losses were identified and quantified; from them, specular transmission resulted in 25.5% losses, and 0.1% error compared to measurements. In this respect, LightTools was proven to be a useful tool for modelling LSCs and quantify accurately the optical efficiency. With these results, the ray-tracing method proposed was extended for thulium based LSCs.
Two thulium LSC models were simulated in LightTools: A glass doped with thulium particles (model 1), and pure glass with thulium coating (model 2, PowerWindow design). From the tested variables, the mean free path (MFP) length, related to the luminophore concentration, is one of the main parameters affecting the optical efficiency and transmission losses. Moreover, the MFP ratio showed high relevance in the optical efficiency since it describes how large is the path length for photons in the emission range compared to blue (absorbed) photons.
In both models, it was shown that large MFP ratio (ℛ = 10), high quantum yield (QY= 1), small area (25 cm²), and 5 mm of glass thickness provide optimum results. With these parameters, the optical efficiency of model 1 was 11%, and model 2 about 13%. However, a large scale model 2 of 1 m² had maximum optical efficiency below 1% with light transmission above 80%, and concentration factor of 1. Considering the optical efficiency of this large scale LSC model, and high performance CIGS PV-cells, the energy yield was estimated as only 1.4 kWh per year in the Netherlands. LSC performance might be improved by adding thin film layers in the glass, such as anti-reflection coating and selective filters to allow visible light and reflect NIR photons into the glass.