Zero Gradient Control for R-744 Refrigeration Cycles

This paper proposes a method called Zero Gradient Control, which is used to control the heat rejection pressure and the rotational speed of the fans of the gascooler of R-744 refrigeration cycles nearly energy optimal. The basic concept is to develop algebraic terms for the partial derivatives of the COP with respect to the heat rejection pressure and the speed of the fans. These equations are evaluated online with measurement signals and directly fed to decentralized controllers with setpoint zero. The control structure is the same for both sub- and transcritical operation, requires little parameterization and can be applied to a wide range of systems. Using Zero Gradient Control, there is no need to calculate setpoints, which simplifies the mathematical effort in the implementation. Measurements for various scenarios validate the method even in the case of atypical system behavior or gascooler fouling.