The same total, two different skies

A resource figure quoted as global horizontal irradiance, so many kilowatt-hours per square metre per year, hides a split that matters for hardware. That total is the sum of two physically different streams. Direct or beam irradiance is the light that travels straight from the disc of the sun and casts a sharp shadow; it is what a lens can focus and what a sun-tracking mount is built to chase. Diffuse irradiance is the light scattered by cloud, haze and the blue sky itself, arriving from the whole dome at once; it casts no shadow, a tracker cannot aim at it, and a fixed panel simply banks it. Two sites can report the same total and behave completely differently, because one is mostly beam and the other mostly diffuse.

The diffuse fraction, the diffuse share of the total, is the number that captures this, and across the profiled sites it more than triples:

Selected from 44 profiled sites; full set in the public dataset. GHI and DNI are NASA POWER long-term-average annual totals (DNI is measured on the sun-facing normal, so it can exceed GHI at clear high-beam sites). Diffuse fraction is DIF / GHI on the horizontal. These irradiance-profiling sites are a companion set to the 40-market yield baseline, chosen to include the desert megaproject locations, with substantial overlap.

The gradient is not simply a map of how sunny a place is. Dubai and Abu Dhabi post high totals but a diffuse fraction near 45 per cent, because suspended dust and humidity scatter the abundant light, while cooler, drier Riyadh keeps a sharper 39 per cent sky. Jakarta receives more total light than Berlin yet delivers less usable beam, because in the humid tropics more than half of a generous resource arrives scattered.

The split, site by site

Splitting each total into its two components makes the hardware consequence visible directly:

Beam on horizontal is GHI minus diffuse, in kWh/m² per year; it is the horizontal-plane direct component, not the higher on-normal DNI. Values from the same NASA POWER profiles.

In the Atacama the direct beam outweighs the scattered light four to one, so a mount that tracks the sun harvests a great deal more than a fixed one, and concentrating solar, which can use only beam, is viable. In Berlin and Jakarta the scattered component equals or exceeds the beam, so there is far less directional light to chase: a fixed array already collects the diffuse half from the whole sky, and a tracker adds much less.

The tracker decision hides here

Two hardware choices turn on this ratio. A single-axis tracker adds roughly 15 to 25 per cent annual yield in high-beam desert climates, where the field literature and operating plants agree the gain is large and the payback clear; in high-diffuse climates the same tracker typically adds only about 5 to 12 per cent, because most of the light was never directional to begin with, and once its added cost, maintenance and failure modes are counted the case can collapse entirely. Concentrating solar power is the sharper version of the same rule. It can use beam only, and the CSP sites in this table sit at low diffuse fractions: Ouarzazate at 30 per cent, the Atacama at 19, Phoenix's American Southwest at 23. Read against those bands, the Gulf profiles are a finding in themselves: even Riyadh, the sharpest sky the set holds in the region at 39 per cent, sits above the one-third line below which a tracker is usually strong and above every CSP site in the table. On these numbers, no profiled Gulf sky is a clear case for either technology.

A bifacial module's rear face is fed by diffuse and ground-reflected light, exactly the light a tracker cannot use, so the two technologies suit opposite ends of the gradient, and pairing bifacial with a tracker in a high-beam desert stacks two effects that suit the same site. The buyer action is a single question: ask for the diffuse fraction or the DNI, not just the GHI, before accepting a tracker line-item. Below about a third, a tracker is usually strong; above about half it has to justify itself against a cheaper fixed array. A high headline GHI in a humid or dusty market does not support a tracker's beam assumption.

Three irradiance components, forty-four sites

The irradiance components are NASA POWER long-term-average monthly values for 44 profiled sites: global horizontal (GHI), direct normal (DNI) and diffuse horizontal (DIF), with the diffuse fraction computed as DIF divided by GHI and the horizontal beam component as GHI minus DIF. DNI is measured on the sun-facing normal and so is not directly additive with the horizontal GHI and DIF; the report keeps the two framings separate and labels each. The tracker and bifacial gain ranges are from the published field literature and NREL, not computed from these profiles; a real project should model its own mount with site data. The one-third and one-half diffuse-fraction decision lines are this report's own working bands, drawn from where those published gain ranges diverge, and are not published limits. And these long-term averages describe the typical sky, not a given year, whose beam-diffuse split swings with exactly the cloud and aerosol variability covered in parts 02, 03 and 06. Every value is public and recomputable from the named NASA product.