light of the sun, to dry in the sun
烜 centers on the sun's light and its drying effect: the light of the sun, and drying something in that sunlight.
烜 combines 火 (fire) with 亘, which likely contributes the sound and may suggest spreading or extending, together expressing the sun's spreading light and heat.
Fire 火 beside 亘, which looks like a sun 日 over a horizon line 一, suggests the sun's fire spreading light and drying things.
For ケン, imagine a ken (a range of sight) lit by the sun's light: ken -> ケン, and the sun dries everything within that ken.