On a sunny and windy autumn day, scores of energy traders are glued to banks of screens in offices ranging from a tiny basement to some of the plushest buildings in the Danish city of Aarhus.

Most of them are in their 20s or 30s and form a new generation seeking to make a buck by predicting the wild swings in electricity prices caused by a few more hours of sunshine or a fresh blast of wind. Europe's unprecedented shift toward renewable energy helped quadruple activity last year in the region's biggest market for power delivered on the same day.