An Energy-Savings Evaluation Method for Variable-Frequency-Drive Applications on Ship Central Cooling Systems

Variable-frequency-drive (VFD) applications are gradually being recognized by maritime industries as one of the most effective tools for energy savings. Significant energy benefit can be realized from VFD-driven central seawater cooling systems because the ambient seawater temperature varies greatly as ships travel through different sea areas. Before making a decision about which drive to choose, information about the actual energy and cost savings is needed. A simple and fast method that is able to effectively determine the energy savings for closely estimating the economics for their application is essential. This paper aims at presenting a fast method for calculating energy savings that would result from installing VFDs on the seawater cooling pumps on ships. The ship main engine cooling system and navigation environment characteristics, including related marine equipment, pipe allocation, ambient seawater temperature, etc., are taken into account. The process system that involves mechanical, instrumental, and pump hydraulics, as well as the electrical aspects of the drive, is mathematically modeled so that the pump hydraulic formulas can be properly applied. The performance of the proposed method was validated through the energy-savings analyses of an actual ship seawater cooling system. The effect of shipping routes on the energy savings was fully described. The analysis model developed in this paper can assist in performing the energy-savings calculations to permit the checking of the economics of adjustable-speed flow control for similar systems in industrial applications.