Direct vs. Diffuse Irradiance

Germany is one of the world's top solar PV producers despite being cloudier than most of the American South. Meanwhile, concentrated solar power (CSP) plants exist almost exclusively in deserts. The difference comes down to which type of sunlight each technology can use.

Sunlight reaching the ground arrives in two forms. Direct normal irradiance (DNI) is the beam component: sunlight traveling in a straight line from the sun. Only DNI can be focused by mirrors or lenses, which is why CSP requires it. Diffuse irradiance is sunlight scattered by clouds, aerosols, and atmospheric molecules, arriving from all directions. Flat-plate solar PV panels capture both direct and diffuse light, which is why they still generate power on cloudy days (at reduced output). On a clear day in the desert, roughly 80% of irradiance is direct. Under overcast skies, diffuse can reach 100%.

Compare two locations. Phoenix, Arizona: annual DNI of about 2,700 kWh/m2, total (direct + diffuse) about 2,100 kWh/m2 on a flat panel. Hamburg, Germany: annual DNI of about 900 kWh/m2, total about 1,050 kWh/m2 on a flat panel.

Evaluate CSP viability. CSP needs DNI above roughly 2,000 kWh/m2/year to be economical. Phoenix qualifies. Hamburg does not, with DNI below half the threshold.

Why can Hamburg still be viable for solar PV despite such low DNI?

PV captures diffuse light. Hamburg's total irradiance (1,050 kWh/m2) includes both direct and diffuse. PV panels use all of it. Combined with lower panel costs, PV works in cloudy climates where CSP cannot.

This distinction is why PV has spread globally while CSP remains confined to the world's sunniest regions.

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