Study On The Characteristics Of A Nanofluid-Based Direct Absorption Medium-temperature Solar Collector With Concentrator

The demand for medium-temperature solar collection is enormous in industrial process heating, low-temperature power generation and air conditioning. It is a kind of energy saving and clean heating method, which can be used to replace the traditional fossil fuel combustion. Due to the bad durability of selective absorption coating under high temperature and distortion caused by the nonuniformity of radiative heat flux in the circumferential direction, the high manufacturing and maintenance cost of commonly used surface solar collector become the main obstacle in business applications.In this paper, a novel nanofluid-based direct absorption solar collector (NDASC) with concentrator was proposed and studied. The NDASC employs transparent all-glass evacuated tube in which nanofluid acts as both a solar absorbing medium and heat-transfer fluid. On the basis of studying the spectral absorption properties of CuO/oil nanofluid, the mechanism of direct absorption of nanofluid and heat transmission in collector tube was investigated and the temperature distribution and the collection characteristics of the NDASC with nonuniform sunlight concentration were studied. The main work of this paper is as follows:Firstly, CuO/oil nanofluids with four different concentration were prepared, the spectral absorption properties and the basic heat physical, such as viscosity and conductivity were studied by theoretical analysis and experimental test. According to the spectral energy distribution of the solar radiation, the average absorption coefficients of nanofluids was calculated.Then, on the basis of the analysis of the optical properties of concentrator and the mechanism of absorption and heat transmission of nanofluid, the mathematical model of NDASC with parabolic trough concentrator was established and the temperature distribution and the collection characteristic in the collector tube were simulated. The effects of concentration of nanofluid, radiation intensity, inlet temperature were analyzed and the optimum of concentration was obtained. The thermal performances of NDASC and IASC with concentrator were compared. It was found that the temperature distribution of NDASC was more uniform and its wall temperature of tube was much lower. For the simulated condition, the NDASC could achieve high efficiency within a certain temperature range.A comparative experimental system of NDASC and IASC with parabolic trough concentrator was built and the collection characteristic in typical working condition was test. The results showed that the highest instantaneous efficiency of NDASC was up to 62% and the instantaneous efficiency decreased with inlet temperature increased. When inlet temperature was below 124.2℃, the efficiency of NDASC was higher than IASC, which proved the feasibility of NDASC.The optical properties of compound parabolic concentrator (CPC) was analyzed and the radiative heat flux on the surface of collector tube. Then the temperature distribution and collection characteristic of NDASC with CPC were simulated. It was showed that the NDASC performed better than IASC when the inlet temperature was below 106.7℃ and the concentrating ratio was 3.66.