My GoSun heated up under clouds... what happened?

Even when overcast, there's often some about of radiation making it through the clouds. The GoSun's compound parabolic reflectors can focus even trace amounts of light onto the solar vacuum tube, where is can be trapped and slowly build to high temperatures. Furthermore, the tube's highly absorptive coating of aluminum nitride can capture a broad spectrum of light, including ultraviolet which can often pierce through clouds when visible light can't!