Direct (or Beam) Normal Irradiance (BNI): definitions and basics

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Generalities
BNI measurements: direct versus indirect
FAQ: Q1_I have tested two different requests using a ground albedo of 0.2 vs. 0.4. I was wondering why the resulting DNI is quite the same?

Generalities

Beam (or Direct) Normal Irradiance (BNI) is the "fuel" for Concentrated Solar Power (CSP) Systems. CSP systems use the complete solar spectrum, so only the broadband BNI is of interest.
One of the main characteristic of the BNI is to be highly variable, and so are the produced power and the generated revenues. That is why there is an increasing need for an accurate short-term BNI forecasting for integration of CSP into the electricity grid.
2 – 5 % uncertainties for good calibrated instruments,
Typical Meteorological Year widely used,
not many sites measuring DNI with high-quality instrumentation provide publicly available data. One international network of research-class stations : BSRN

reduction of direct irradiance:

Clouds: up to 100 %
Water vapor: 10-15 %
Aerosols: 20-25 % up to 100 %

	Theoretically, DNI is the irradiance received from the sun's disc only (smallest circle Fig. 2). Practically, CSP systems uses the direct irradiance from the sun's disc plus some circumsolar diffuse irradiance within a cone of 2.5° around the sun centre.
Fig. 2: Sun disc (smallest circle) and sun plus circumsolar disc (larger circle).

Fig. 1: Direct Normal Irradiance and CSP systems.

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DNI measurements: direct versus indirect

DIRECT MEASUREMENT (Fig. 3): The thermopile pyrheliometer is tracking the sun, and thus always tangent to sun rays. INDIRECT MEASUREMENT (Fig. 4): The principle is to measure both the Global and the Diffuse Horizontal Irradiation (GHI and DHI) components of the radiation, and to compute the BNI with the following formula: BNI = (GHI - DIF)/cos(γs), with γs the sun elevation at each instant.
	Fig. 3: Thermopile pyrheliometer	Fig. 4: one pyranometer for GHI and another one (with fixed shadowing or tracking shade-disc) for Diffuse Horizontal Irradiation (DHI) component.

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FAQ

Q1_I have tested two different requests using a ground albedo of 0.2 vs. 0.4. I was wondering why the resulting DNI is quite the same? F. S., Germany
A. The larger the ground albedo, the larger the part of the radiation which is reflected. Part of this reflected radiation may reach an inclined surface (reflected component) and the other part is scattered upwards to the atmosphere. The atmosphere itself reflects (the atmospheric albedo is not equal to zero) and part of this atmospheric reflected radiation contributes to the diffuse radiation received on a horizontal or inclined plane. The DNI is by definition the irradiation that is coming from the direction of the sun and received on a surface normal to the sun rays. Coming from the direction of the sun means that this component has not been reflected by the ground. Consequently, the ground albedo has no impact on the value of the DNI.

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