Solar radiation instruments in energy and environmental engineering: A pyrheliometer is specifically designed to measure which component of solar radiation at the Earth’s surface?

Introduction / Context:
Solar radiation is decomposed into direct (beam), diffuse, and reflected components. Instrument choice depends on which component is required for design of solar thermal collectors, photovoltaic arrays, or daylighting analysis. Knowing the correct instrument ensures that collected data match the design calculations.

Given Data / Assumptions:

• Pyrheliometers are used with a solar tracker or are manually pointed at the sun.
• Measurements are typically expressed as W/m^2 on a plane normal to the solar beam.
• We distinguish this from pyranometers, which measure hemispherical (global or diffuse) radiation.

Concept / Approach:
A pyrheliometer has a narrow field of view and measures the direct normal irradiance (DNI), i.e., the beam component coming directly from the sun’s disc. It rejects diffuse sky radiation by optical geometry. In contrast, a pyranometer measures global horizontal irradiance (GHI) or, with a shading device, the diffuse horizontal irradiance (DHI). A sunshine recorder logs duration of bright sunshine but not irradiance magnitude.

Step-by-Step Solution:

Verification / Alternative check:
Solar resource standards (e.g., WMO/ISO) define pyrheliometers as direct-beam instruments; data are used to size concentrating solar collectors that require DNI.

Why Other Options Are Wrong:

• Bright sunshine period: Measured by sunshine recorders (Campbell–Stokes), not pyrheliometers.
• Diffuse radiation: Measured by a shaded pyranometer.
• Global horizontal irradiance: Measured by an unshaded pyranometer.

Common Pitfalls:
Confusing pyrheliometer with pyranometer; remember “helios” (sun) direct beam versus “pyran” (fire/heat over a hemisphere).

Final Answer:
Beam (direct normal) radiation