Control of Non-linear Marine Cooling System Michael Hansen, Jakob Stoustrup, Jan Dimon Bendtsen Abstract— We consider the problem of designing control laws for a marine cooling system used for cooling the main engine and auxiliary components aboard several classes of container vessels. We focus on achieving simple set point control for the system and do not consider compensation of the non-linearities, closed circuit flow dynamics or transport delays that are present in the system. Control laws are therefore designed using classical control theory and the performance of the design is illustrated through a simulation example where it is compared to a reference control design. I. INTRODUCTION In recent years the attention to energy efficiency in the shipping industry has increased as a consequence of fluctuat- ing oil prices [1] and a growing focus on CO 2 , NO x and SO x emissions from maritime transportation [2]. This has led to several initiatives within the shipping industry to bring down the energy consumption in ocean-going vessels, ranging from waste heat recovery systems to energy optimization of subsystems [3]. In this paper we consider design of control laws for a cooling system found aboard several classes of ocean-going container vessels. The system in question is used for cooling the main engine and auxiliary components and currently makes use of a very simple control method. In the current control, the pumps in the cooling system are operated in three steps based on the temperature of the sea water and the load on the main engine [4]. The result is that the pumps in this type of cooling system are used excessively, and that operating conditions are unlikely to be optimal in particular for the main engine auxiliary components. This leaves a significant potential for energy savings by improving the existing controls, not only by lowering the power consumption of the pumps, but also by ensuring optimal operating conditions for the main engine auxiliary components and thereby improving their energy efficiency. The focus in this paper is on the latter, which means that the control design aims at achieving the desired set point temperatures for main engine auxiliary components, rather than achieving optimal energy efficiency for the pumps. A model for the cooling system was derived in [5] and is adopted here for the control design and for simulating M. Hansen is with A.P. Moeller - Maersk A/S, MMT - Innovation, Esplanaden 50, DK-1098, Copenhagen, Denmark and is also affiliated with the Department of Electronic Systems, Section for Automation and Control, Aalborg University, Fredrik Bajers Vej 7, DK-9320, Aalborg, Denmark (e- mail: michael.hansen1@maersk.com). Jakob Stoustrup and Jan Dimon Bendtsen are with the Department of Electronic Systems, Section for Automation and Control, Aalborg Univer- sity, Fredrik Bajers Vej 7, DK-9320, Aalborg, Denmark (e-mail: {jakob, dimon}@es.aau.dk) the compensated system. The control laws derived in this paper serves the purpose of improving the performance of the cooling system compared to the current control method. The remainder of the paper is structured as follows: In Section II we give a short description of the system and present the model. In Section III we present the control strategy and derive control laws for the system. Section IV presents simulation results for the control design and conclusions are given in Section V. II. MODEL The cooling system consists of three circuits; a sea water (SW) circuit, a low temperature (LT) circuit and a high temperature (HT) circuit. This is illustrated in Fig. 1 where q LT and q SW are volumetric flows in the LT and SW circuits, while q HT is the volumetric flow to the HT circuit. Central coolers Sea water circuit Low temperature components Main engine Low temperature circuit High temperature circuit TT TT q LT q SW q HT Fig. 1. Simplified system layout. The SW circuit pumps sea water through the cold side of the central coolers for lowering the temperature of the coolant in the LT and HT circuits. The HT circuit only contains the main engine of the ship, while the LT circuit contains all the main engine auxiliary components in a parallel configuration. In the current control scheme, the SW circuit pumps are operated in three steps depending on the temperature of the sea water and the operating mode of the vessel. This setup is designed such that the SW circuit provides sufficient cooling, even when the sea water temperature is high, which means that under most operating conditions the SW circuit generates excess cooling. Similar to the SW pumps, the LT pumps are also controlled in 2011 IEEE International Conference on Control Applications (CCA) Part of 2011 IEEE Multi-Conference on Systems and Control Denver, CO, USA. September 28-30, 2011 978-1-4577-1063-6/11/$26.00 ©2011 IEEE 88