Proportional control of a chiller-dry cooler-combination: Efficient and robust

Energy efficiency improvements play a crucial role in achieving climate protection goals. This applies also to cooling processes (industrial and residential) which account for approximately 15% of electricity consumption in Germany. This study investigates the energy-saving potential of dry-cooled chillers through a simple but robust control approach: the three controllable actuators (compressor, cooling medium pump and dry cooler fan) are controlled proportionally using a single PI controller over the permissible range in order to meet the cooling demand under the specific boundary conditions, treating them as fluid machines with similar behaviors. The impact of this control method is compared to two conventional control strategies on the one hand and an optimal control on the other hand. In the latter case a model-based controller computes those setpoints for the three actuators for each time step under varying operating conditions that minimize total power consumption while meeting load requirements. Evaluation through simulations and experiments at a laboratory test facility reveals that the proposed proportional control strategy achieves up to 14% reduction in seasonal electricity consumption by this simple approach. This is approximately two thirds the savings which a model-based approach that is much more complex and difficult to implement in practice could reach.