Maybe a solar power tower test.
You can see a tower at the center and its shadow.

The triangle shape presumably a result of leveling a triangular area to allow them to move the mirrors to any location they want during testing.

Why a triangle instead of a circle?
Presumably they just went with the easiest shape for data gathering.
As long as they have a symmetrical shape with the tower at the center and the corners at the planned perimeter of the array circle they can extrapolate data for any point of the circle.

The fact that they ended up putting windmills in that area later supports the idea that they were interested in using the area for renewable energy.

This is all just a guess though.