The project, carried out from 1 June 2020 to 30 September 2023, was executed by the Technical University of Munich and Efficient Energy GmbH. The research team, led by M. Eng. Philipp Nguyen and Prof. Dr. Christian Schweigler, developed a hybrid cooling concept that couples a turbo‑compressor machine using water as a natural refrigerant with an existing multi‑split air‑conditioning system. The aim was to reduce electrical energy consumption by exploiting free‑cooling opportunities while maintaining the required cooling capacity for a data‑centre.
The hybrid system integrates an eChiller that uses R718 and a conventional split unit that uses R410A. A reference split system, without the turbo unit, also uses R410A. In view of the high global warming potential of R410A, the project examined the substitution of R32 as an alternative refrigerant, noting that such a change would require mechanical adjustments and would alter the electrical load due to different thermodynamic properties. The thermodynamic behaviour of both subsystems was modelled in the Engineering Equation Solver (EES), and a hydraulic layout was designed to allow both parallel and serial operation of the two cooling generators. Control strategies were coded into a supervisory system that could switch between serial and parallel modes depending on ambient temperature and load demand.
A pilot installation was built at the university campus. The hydraulic network, measurement, control and regulation hardware were installed and commissioned. Functional tests confirmed that the hybrid concept operates as intended. The system was then subjected to yearly simulation studies for a typical data‑centre load profile. The results showed that serial operation of the turbo and split units delivers a higher annual performance coefficient (EER) ranging from 6.15 to 8.82, depending on the chosen strategy. Free cooling, enabled by the water loop, covered roughly 52 % of the annual cooling demand, while an additional 13 % was supplied by pre‑cooling the refrigerant charge. These figures translate into a substantial reduction in electrical energy use compared with the reference split system.
Emission calculations were performed for both the hybrid and reference systems. For the hybrid system with R410A in the split unit and R718 in the eChiller, the annual CO₂ emissions were 11 965 kg CO₂ a⁻¹, of which 11 935 kg came from electricity consumption and 30.23 kg from refrigerant leakage (0.67 % a⁻¹). Switching the split unit to R32 lowered the total emissions to 11 938 kg CO₂ a⁻¹, mainly by reducing the leakage contribution to 3.01 kg CO₂ a⁻¹. The reference split system emitted 18 600 kg CO₂ a⁻¹, with 18 510 kg from electricity and 90.69 kg from leakage; replacing R410A with R32 reduced this to 18 519 kg CO₂ a⁻¹. These results demonstrate that the hybrid concept not only improves energy efficiency but also offers a pathway to lower greenhouse‑gas emissions, especially when combined with low‑GWP refrigerants.
The collaboration between the university and Efficient Energy GmbH combined academic expertise in thermodynamics and control with industrial experience in HVAC system design. The project’s outcomes include validated simulation models, a functional pilot installation, and a set of optimized operating strategies that can be generalized to other data‑centre and office building applications. The findings are documented in a series of reports and publications, providing a foundation for future research and deployment of hybrid cooling solutions in the building sector.