A real-time alorithm for determining R744 systems optimal high pressure

In this paper the optimal energy efficiency and heat rejection pressure problem in single stage refrigeration carbon dioxide vapour compressor units operating in transcritical conditions is addressed. A real-time model based optimisation algorithm for determining the optimal (or quasi-optimal) pressure value is developed as a more efficient and robust solution than literature approximated solutions to the optimisation problem. The problem is solved based on a model estimating the system performance and on the system boundary conditions measurements. The system model is obtained by an on-line artificial neural network system identification technique and the optimisation problem is solved by a particle swarm optimisation technique. The proposed algorithm performance is dynamically tested by simulation considering the performance of a carbon dioxide heat pump water heater with supply water temperature control. It appears to successfully approximate the optimal heat rejection pressure values showing an average daily pressure deviation of 0.9 10^5 Pa over a two year simulation period. The corresponding cumulated energy consumption penalisation is 1.1%.