compressor 1 a heat rejector 2 an expansion means 3 and a heat absorber 4 connected in a closed circulation system fig2 shows a transcritical co 2 cycle in a temperature entropy diagram the compression process is indicated as isentropic from state a to b the refrigerant exit temperature out of the heat rejector c is regarded as constant specific work specific cooling capacity and coefficient of performance are explained in the figure as mentioned above there is a mathematical expression for high optimum high side pressure in a trans critical vapour compression system the expression is as follows the optimum pressure is achieved when the marginal increase of capacity change of h c at constant temperature equals u03b5 times the marginal increase in work change of h b at constant entropy perturbation of the high side pressure is in principle a practical approach to use the equation above by mapping the energy efficiency or a parameter which reflects the energy efficiency as function of high side pressure it is possible to establish the point where the marginal increase of capacity equals u03b5 times the marginal increase in work various parameters can be used as reflection for the energy efficiency the temperature difference between refrigerant and heat sink at the cold end of the heat rejector 4 is often denoted.