Study On Optical Characteristic Of Di-Methyl Silicon Oil Applied In Cooling For Liquid-Immersed Concentration Photovoltaic(CPV) Systems

In recent years, the concentration photovoltaic(CPV) systems were concerned for their high power generation efficiency and low cost price. Cooling concentrating solar cells with flowing di-methyl silicon oil was efficient. However as the running time and concentrating ration increases, the power generation of CPV system reduce because of the optical characteristic of di-methyl silicon oil decreases. According to this problem, accelerated life test by an external-loop airlift circulation receiver was proposed to study the optical characteristic of di-methyl silicon oil working in the system for a long time with the following aspects.1. An external-loop airlift circulation receiver was built and the flow characteristics were also investigated by experiments. The results showed that gas holdup was effected by superficial gas velocity, viscosity and temperature. The superficial liquid and gas velocity was positive relationship. There was a uniform temperature distribution of axial distribution of temperature of the light received surface and the uniform degree and superficial gas velocity was positive relationship.2. The effect of aging factors on optical characteristic of di-methyl silicon oil were studied. The results showed that the optical characteristic of di-methyl silicon oil decreased lead a fall on power generation efficiency for heat, UV light and oxygen. For the EQE of silicon cells, decrease on visible light is the main reason for power generation efficiency down, and decrease on UV and near infrared band did little effect on power generation efficiency.3. The changing mechanism of optical characteristic of di-methyl silicon oil working in CPV system for long time was studied by instrument analyzes. The results showed that rearrangement and photo oxidation reaction reacted under the coupling effect of heat, UV light and oxygen. Ring form siloxane compounds generated from rearrangement reaction and the optical characteristic of products were lower than silicon oil, but it was not the main reason for the falling on optical characteristic of di-methyl silicon oil. The linear siloxane compounds with chromophore and auxochrome generated from photo oxidation reaction which was the mainly reason to decrease the optical characteristic.4. By referencing CPV standard IEC62108, the analysis method of service life of silicon oil was carried out and the recovering methods of aging silicon oil were also proposed. The results showed that the decrease of spectral transmittance of silicon oil by the order of UV, visible light and near-infrared band, and the decrease velocity of Jnp increased as the reaction time grown. When Jnp is lower than 0.871, the service life of silicon oil working in the CPV system with silicon solar cells ended. By experiment method, the UV accumulation that silicon oil accumulated when Jnp reached at 0.871. By these values, the service life can be calculated. Extraction was more suitable than vacuum distillation for engineering application, and methanol is an ideal extracting.5. The silicon oil composites were prepared by adding light stabilizer. The performance of composites was investigated. The results showed that the aging effect on optical characteristic of itself can be inhibited by composites. During accelerated life test, the viscosity of composites changed little, the solar cells were not damaged by composites, and the contents of light stabilizer were still in the effective range.