Benefits assessment of a dual-intake-port sliding vane rotary expander replacing the high pressure throttling valve of a trans-critical CO2 refrigeration unit

Refrigeration sector is responsible of the 7.8 % of the worldwide greenhouse gases emissions pushing participants to COP 28 to commit a reduction close to 70 % by 2050 of the 2022 CO2 emissions level. To this aim, the adoption of the CO2 as working fluid is particularly suitable. In refrigeration compression system, to the valve throttling is associated the most severe cycle irreversibility wasting up to the 20 % of compressor input work. To overcome this issue, the replacement of these devices with volumetric expanders is one of the most effective solutions. Despite the huge interest about this topic, a knowledge gap is observed on impact the expander would produce on the refrigeration plant. In fact, the valve replacement with an expander must keep the same cooling performances as well as the same thermodynamic properties on most relevant variables the refrigeration unit had before the substitution. To reach this result, the availability of detailed model of the refrigeration unit is needed, able to represent the whole complexity of the processes occurring (propagative, inertial and volumetric properties). To fill this gap, in the present paper a novel comprehensive model of the unit was developed and experimentally validated against experimental data collected on a fully instrumented test bench in which an industrial refrigeration unit (18 kW as refrigeration power) was operated. Once the model resulted validated, it was used to predict the plant behaviour if the throttling valve was replaced by a Sliding Rotary Vane Expander (SVRE). Concerning the expander design, a further innovation was considered referring to a dual intake port technology DIP. To assess the introduced benefits, an expander model validated in previous works developed by the authors was used. Adapting this model for the application at hand and integrating it with the plant whole model, it was found that an improve up to 20 % the whole efficiency with respect to the baseline case was achieved for the maximum considered external temperature (31 ◦C-33 ◦C).