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Carbon dioxide based refrigeration systems are considered to have a particularly high potential for becoming efficient heat energy producers. In this paper, a carbon dioxide system that operates in the subcritical region is examined. The modelling approach is presented, and used for operation optimisation by way of non-linear model predictive control techniques. Assuming that the heat is sold, it turns out that the system has negative operational cost. Depending on the choice of objective function, daily revenue can vary from about 7.9 [eur] to 11.9 [eur].