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Optical modelling and performance analysis of a solar LFR receiver system with parabolic and involute secondary reflectors

01-10-2016, Balaji, Shanmugapriya, Srinivasa K Reddy, Sundararajan, T.

In this paper, a pilot scale solar Linear Fresnel Reflector of 154 m2 is designed and optically analyzed with two different profiles for the secondary concentrator. Compounded profiles of parabolic (PB) and involute (IN) shapes are compared for the secondary reflector geometry. Non-uniform intensity distribution of the solar disc with the flux transmission by the Monte Carlo Ray tracing method is used. Analyses are carried out with a 3D optical model and the combined optical performance of the Linear Fresnel Reflector (LFR) system with the parabolic secondary reflector is compared with that of the involute secondary reflector. The effects of truncating the secondary reflectors, optimizing the focusing distance of the absorber and the gap between the absorber and the secondary reflector, are investigated. Also the effects of errors caused by sun-tracking and contour of the mirror surface are studied. The efficiency of the Linear Fresnel Reflector system with the two models of secondary concentrators at different incidence angles of the solar beam are evaluated with Incidence Angle Modifier. Optical performance at different Direct Normal Irradiance (DNI) conditions is also performed. It is found that the Linear Fresnel Reflector system with Parabolic secondary reflector provides a higher optical efficiency of 62.3% with secondary efficiency of 83.3%. The Involute secondary on the other hand, provides an optical efficiency of 59.5% and secondary efficiency of 78.33%.

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Heat loss investigation of 125kWth solar LFR pilot plant with parabolic secondary evacuated receiver for performance improvement

01-03-2018, Reddy, K. S., Balaji, Shanmugapriya, Sundararajan, T.

Numerical investigation is carried out on the secondary reflector system of the pilot solar linear Fresnel reflector module of 154 m2 in Vallipuram, Tamil Nadu. Initially, optical investigation on the design of the Linear Fresnel Collector module and the secondary reflector system is carried out. The optimized flux thus obtained is applied as a boundary condition to the thermal loss analysis. Investigation of the total heat losses (convective, radiative and conductive) with three different boundary conditions namely, the constant flux, variable flux and non-uniform flux distribution is carried out. The heat loss study has been carried out when the absorber is under non-evacuated and evacuated state. The study has been carried out with the DNI ranging between 250 W/m2 and 1000 W/m2. Uncoated and selectively coated absorbers with the emissivity of 0.01–1 are analyzed. Comprehensive analysis on the influence of wind flow on the receiver system is carried out. The variation of wind velocity from the ground level is incorporated in the forced convection. Investigation on the effect of the wind speed (0 m/s–10 m/s) and the wind direction (0°–90°) on this second stage reflector system is analyzed. The deviation between the different flux boundary conditions is comprehensively analyzed under different DNI and wind conditions. The variable flux distribution shows less deviation of about 15% from the non-uniform flux distribution for the DNI of 1000 W/m2 and is found to be less than 5% for 500 W/m2 under evacuated conditions. To avoid the computational complications in applying non-uniform flux boundary conditions to the absorber for DNI greater than 500 W/m2, variable boundary flux condition can be applied as an equivalent flux condition by augmenting 15% non-uniform error percentage to the final heat loss value.

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Improvement of solar thermal plant efficiency by the use of secondary concentrators

01-01-2018, Sundararajan, T., Reddy, K. S., Balaji, Shanmugapriya, Chaitanya Prasad, G. S.

A solar collector field based on linear Fresnel reflectors has been set up with support from the Department of Science and Technology, Government of India. Fresnel mirrors coupled with secondary concentrator are employed to focus solar radiation on a coated stainless steel receiver tube of 70 mm diameter. The collector system directly generates superheated steam at about 45 bar and 400oC. The receiver tube is covered with borosilicate glass, with the annular space evacuated to reduce heat losses. Theoretical studies have been carried out to optimize the secondary concentrator profile and to assess the heat losses from the solar collector. Compound-parabolic, trapezoidal and segmented- parabolic configurations are considered for the secondary concentrator. It is shown that the segmented parabolic configuration can provide high optical efficiency as well as nearly uniform flux distribution on the receiver tube. Evacuation of the annular space between the absorber tube and the glass cover, significantly reduces heat losses and improves the temperature distribution within the absorber.