The importance of energetic flexibility of distributed energy resources grows with the share of renewable generation in the power grid. However, the quantitative description and aggregation of flexible resources is challenging. This work proposes the use of zonotopes, a subclass of polytopes, to approximate flexibility. It is shown how optimal zonotopic approximations of flexibility can be computed efficiently for different objectives, and that the aggregation of those sets is tractable with regard to memory and computational complexity for long planning horizons and large populations of systems. In addition, we describe synergistic behavior exhibited by the aggregation of flexibility and illustrate that zonotopes can partly capture these synergy effects.