Design and performance evaluation of a novel CO2 rooftop prototype
Air-to-air systems like rooftop units, combining mechanical ventilation and heating and cooling purposes, play a crucial role in the HVAC sector, mainly used in commercial buildings. In view of evolving regulatory frameworks on refrigerants, converting rooftop units from using synthetic refrigerants to natural refrigerants, becomes of great interest. This paper aims at presenting a study on a CO2 rooftop unit, from the thermodynamic evaluations of the refrigerant cycle performance to the design of the components for a 45kW prototype. Different configurations and control parameters have been evaluated thanks to a simulation model developed in Modelica: different strategies for refrigerant mass flow regulation, fan speed and different expansion devices have been taken into account. The selected compressor is regulated both through frequency control and piston shut-off. The refrigerant expansion process is done either through the use of an ejector, or through an electronic expansion valve used as high pressure valve to evaluate the best configuration according to the boundary conditions (air temperature and thermal load). The highest seasonal energy efficiency ratio was estimated to be 4.55 when ejector is used. In the paper it is also presented the layout of a prototype to investigate the real operation of the unit and validate the simulation model. The prototype can operate in heating and cooling modes and is able to test different configurations and control parameters.