Ponceau 2R Doped Poly (ST/MMA) as Fluorescent Solar Collectors and Evaluation Effect of Matrix on Their Field Performance ()
1. Introduction
The luminescent solar collector (LSC) appears to be promising for low cost solar energy conversion into electrical energy. The luminescent solar concentrator (LSC) is a light-transparent plate like glasses or polymers with high refractive indices incorporating with a luminescent dopant which has a broad absorption band in the visible light region. Operation of the LSC is based on the absorption of solar radiation in a collector containing a fluorescent species in which the emission bands have little or no overlap with the absorption bands [1]. The emission light is trapped by total internal reflection and concentrate at the edges of the collector where a photovoltaic cell facing the edge of plate converts it into electricity. Because, polymethylmethacrylate (PMMA) and polystyrene (PS) bear all properties required for good LSC base material (highly transparent and high refractive index) and it is easy to prepare as sheets of desired shape and size. Therefore, doping of it with organic dyes may be a most useful combination for use in luminescence solar concentrators. Many experimental reports have detailed consideration of the preparation of LSCs doped with different luminescent dyes and the principles of LSCs dye photodegradation and optical efficiency has been discussed [1].
The performance of a LSC is determined by the spectroscopic properties of the dye doped in its matrix. Ponceau 2R is the one type of azo dyes which are important colorants and it is characterized by the presence of one azo group (-N=N-) and constitute the largest class of dyes having extensive application in textiles, papers, leathers, gasoline, additives, foodstuffs and cosmetics [2,3]. Moreover, the ponceau 2R has optical properties help in improving the efficiency of LSC such as the high extinction coefficient, strong fluorescent properties and high photo stability.
The purpose of this paper is to copolymerize styrene and methylmethacrylate to obtain a product with properties between those of the individual homopolymers. The effect of type of substrate on photostability and fluorescence properties of ponceau 2R has been measured before and after exposure to sun light to attain the best conditions under which these LSCs will have high photostability and enhance the solar cell conversion efficiency.
2. Experimental
2.1. Preparation of LSCs
The polymerization of MMA (Aldrich), ST (Aldrich) and MMA/ST was performed using benzoyl peroxide as initiator (1 wt%) at 443 K for 7 hours [4,5]. The ST/MMA percentage in copolymer is 100/0, 70/30, 50/50, 30/70, and 0/100 volume%. The formed polymer left for several hours at room temperature to evaporate the solvent off. Plate samples of thickness (
0.02 cm) and ponceau 2R (Aldrich) concentration of 0.374 mM/L were prepared by casting method where the dye was homogeneously diffused in the precast polymers. The chemical structure of the investigated dye is shown in Figure 1.
Shimadzu (1000-V9,4 Build 287 DSCQ) differential scanning calorimeter was used to determine the glass transition temperature of LSCs. Samples were scanned under nitrogen at a ramp rate of 10˚C/min and temperature ranging from room temperature to 773 K. FTIR spectra were measured on Infrared Spectrophotometer (FTIR-Nicolet 6700) Thermo Scientific German in wave number range (400 - 4000 cm-1).
The outdoor testing for LSCs was carried out for 9 weeks, from March to May (summer of 2011) in Dammam, Eastern region of KSA in a range of solar intensity (80 - 1200 W/m2). The intensity of solar light was measured by solar parameter (ISO-TECH.ISM410). The absorption spectra were recorder using UV Spectrophotometer (UV-1800 SHIMADZU) in the wavelength range (200 - 900 nm) at room temperature. The fluorescence spectra have been recorded using Perkin Elmer LS 55 Fluorescence Spectrofluorometer at excitation wavelength 500nm. The absorbance and emission spectra were recorded for the samples in the form of rectangular discs of area (3 × 1 cm2) and thickness of 0.02 cm.
2.2. Photovoltaic Cell Coupled with PMMA/2R LSC
A commercial solar cell of (2 × 1 × 0.5 cm2) was used in this work. Silicon oil was used as optical matching between LSC and the solar cell. The LSC with dimensions of 2 × 1 × 0.02 cm3 was used. The LSC was tested under
Figure 1. The molecular structure of ponceau 2R.
direct sunlight illumination on April (2012). The efficiency of cell was measured before and after coupling with PMMA/ponceau 2R LSC.
The open-circuit voltage (Voc) and the short-circuit photocurrent (Isc) were measured at direct sunlight illumination intensity of 850 W/m2 using Digital Voltmeter Sanwa Model CD 800 and Digital Electrometer Sanwa Model CD 800, respectively.
3. Result and Discussion
3.1. Characterization of LSCs
3.1.1. Thermal Analysis (DSC)
DSC was used to determine the glass transition temperature of LSCs. Figure 2 illustrates the DSC plots of LSCs of different ST/MMA percentage. The figure shows that the glass transition temperature Tg have been decreased with increasing percentage of ST. This indicates that the thermal stability of LSC decrease by increasing styrene content [6]. The LSC of homo PMMA has the highest Tg (122˚C) compared with the others matrixes. The higher thermal stability of PMMA/ponceau 2R system refers to the possibility of using it to prepare some solar collectors.
3.1.2. FT-IR
Figure 3 shows the IR spectra of LSCs of different ST/MMA percentage. As shown in this Figure the disappearance of aliphatic C=C bands (1680 - 1600 cm−1) characterizing MMA from all PMMA matrixes indicate the complete polymerization [6]. The shift appears for C-O ester in all PMMA matrixes to lower wave number with increasing the percentage of ST provides evidence of complete copolymerization. The shift occurring in C=O ester in homo PMMA and copolymers to higher wave number indicating amphoteric nature of this matrix [7]. The appearance of -N=N- (1460 - 1400 cm–1) and C-H band of aromatic ring (750 cm–1) for ponceau