The prana from the sun is indeed strong but the practice is no longer viable in this age. The sun's radiation has changed compared to the past.
My group and I sustained irreversible damage to eyes after practising solar yoga (HRM Sungazing). Don't stare into the sun, no matter the perceived benefits.
There are other ways to do this. Besides, the published technique is incomplete.
You also need grounding by absorbing earth's energy through the soles. The excess prana from the sun moves into the earth this way and will not remain stagnant in the body.