Performance evaluation and analyses of novel parabolic trough evacuated collector tubes with spectrum-selective glass envelope

Evacuated collector tubes (ECTs), the key components of the parabolic trough collector, suffer considerable heat loss at high operating temperature. Based on the characteristic of uneven distribution of solar irradiance around the absorber tube, novel ECTs covered with infrared (IR)-reflectors on the partial area of glass envelope were proposed to effectively reduce the heat loss and enhance the performance. Two kinds of ideal IR-reflectors, namely, the first one with a fixed cutoff wavelength, the second one with a variable and optimal cutoff wavelength were proposed. A real material of transparent conductive oxide (TCO) film was also used as an IR-reflector in novel ECTs for the analysis. The mathematical models relying on spectral parameter calculation method were established for comprehensively investigating the overall performance of the novel ECTs. Moreover, the influences of TCO film reflectivity and emissivity on the ECTs were investigated. Results demonstrated that the novel ECTs with TCO films, the first ideal films, and the second ideal films achieved obvious superior thermal performance compared with the conventional ECTs. Their relative heat loss reductions at an absorber temperature of 600 °C reached 18.7, 25.3, and 43.8%, the heat-collecting efficiencies were relatively enhanced by 7.2, 10.8, and 16.7%, respectively.