The focal point for light going through a sphere is right at the surface of the sphere on the opposite side of the light source. So the focal point will rotate

The focal point for light going through a sphere is right at the surface of the sphere on the opposite side of the light source. So the focal point will rotate around the outer surface of the sphere as the sun moves throughout the day (and it also changes slightly each day in a perpendicular direction to the sun's daily movement). Assuming this is using some kind of steam engine, the water tank that gets heated by the focused light needs to follow this focal point. That's for efficient electrical generation, which I assume is the point of this thing. As far as pure heat, you're right. You could just have a big curved metal water tank wrapped around the bottom half of the sphere and you might get pretty good water heating capabilities without a complicated tracking system.