On the Design and Analysis of a Pyrheliometer Comprising a Convex Lens

A novel pyrheliometer, based on a solar irradiance concentration method, has been designed, constructed and calibrated. An analytical model of the instrument was developed and incorporated for the design and investigation of the instrument performance. The apparatus contains a small receiver that intercepts and absorbs the solar beam irradiation near the focus of a convex lens, by which the flux is concentrated. The irradiated receiver area is isothermal owing to its small dimensions and high thermal conductivity. Therefore, a single thermo-couple is sufficient to measure the temperature difference between the receiver irradiated area and its surroundings. The solar irradiative flux is evaluated by substituting the recorded temperature difference into, either, a linear experimental calibration chart or into a linear characteristic formula that was derived analytically. The instrument calibration was performed at the Israeli Institute of Standards, using an EPLAB normal incidence pyrheliometer as a reference. The new instrument is inexpensive, simple and portable. Its accuracy is suitable for routine field measurements of direct solar beam irradiation. The analytical model is typical to many solar radiation collection problems and could be considered as useful analytical tool, beyond the specific purpose for which it was developed.