CO2 gas cooler performance on-site.

The efficiency for most CO2 supermarket refrigeration systems rely on a cost optimal rejection of the heat to the ambient air. This is true for a larger part of the year, even for systems with heat recovery. A critical part of this process is the heat exchanger in the form of an air-cooled gas cooler and the control of the pressure inthis component. In order to gain a better understanding of the relation between the selection of the gas cooler relying on manufacturer data and the performance in the actual installation over the year, an analysis has been performed on data obtained from a Danish supermarket. In addition to the measuring points in the advanced control system, the supermarket has been equipped with coriolis mass flow meters for accurate measurement of the refrigerant flow. By mapping the performance at design conditions and more importantly at off-design conditions, a better understanding of the parameters for a cost effective selection of the gas cooler is obtained. The next step is a generic model of the gas cooler performance calibrated with the data in order to aid future simulations of system performance. A draft for this work is presented. The performance of a standard gas cooler in a small supermarket installation has been mapped. The mapping shows a wide range for dt or the difference between the air inlet and CO2 outlet temperature over the full range of the load. The mean value for the dt is higher than expected from the manufacturer data. The average values for intervals of mass flow and inlet air temperature enable a fit for the UA value of the heat exchanger. The UA value predicts mean values for dt with high accuracy for both sub and transcritical operation. The accuracy is dependent on cyclic variations of the load and the geometry of the heat exchanger. Future work aims at generalising these results to other geometries and flow regimes.