An energy-savings evaluation method for adjustable-frequency drives on sea water cooling pumps on ships

Variable frequency drives (VFDs) are gradually beina recognized by maritime industries as one of the most effective tools for energy savings. Significant energy benefit can be realized from VFDs driven sea water cooling systems because the ambient sea water temperature varies greatly as ships travel through different sea areas. Before making a decision about which drive to choose, information is needed about the actual energy and cost savings. 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 sea water cooling pumps on ships. The ship main engine cooling system and navigation environment characteristics, including related marine equipment, pipe allocation, and ambient sea water temperature, etc., are taken into account. The process system that involves mechanical, instrument, and pump hydraulic, as well as electrical aspects of the drive is mathematically modeled so that the pump hydraulic formulas can be properly applied. Performance of the proposed method was validated through energy-savings analyses of an actual ship sea water cooling system. Effect of shipping routes on the energy savings was fully described. The analysis model developed in this paper can assist in performing the energy-saving calculations to permit checking of the economics of adjustable speed flow control for similar pumps on ships.