  To reduce the carbon footprint of computing and stabilize electricity grids,
there is an increasing focus on approaches that align the power usage of IT
infrastructure with the availability of clean energy. Unfortunately, research
on energy-aware and carbon-aware applications, as well as the interfaces
between computing and energy systems, remains complex due to the scarcity of
available testing environments. To this day, almost all new approaches are
evaluated on custom simulation testbeds, which leads to repeated development
efforts and limited comparability of results.
  In this paper, we present Vessim, a co-simulation environment for testing
applications and computing systems that interact with their energy systems. Our
testbed connects domain-specific simulators for renewable power generation and
energy storage, and enables users to implement interfaces to integrate real
systems through software and hardware-in-the-loop simulation. Vessim offers an
easy-to-use interface, is extendable to new simulators, and provides direct
access to historical datasets. We aim to not only accelerate research in
carbon-aware computing but also facilitate development and operation, as in
continuous testing or digital twins. Vessim is publicly available:
https://github.com/dos-group/vessim.
