Comparative study on the performance of natural draft dry, pre-cooled and wet cooling towers

Abstract

Most geothermal and concentrated solar thermal (CST) power plants are dry cooling due to the lack of water sources. Unfortunately, the performance of dry cooling is particularly reduced when the ambient air is hot, which is because dry cooling relies mainly on convective heat transfer to reject heat from the working fluid rather than water evaporation as is in wet cooling systems. This is even worse for natural draft cooling towers as their air flow is created by means of buoyancy effects due to the difference in air density between the inside and outside of the towers. The heat rejection capacity of natural draft dry cooling towers is significantly reduced at high ambient temperatures because the driving buoyancy force is small, resulting in small air flow through the cooling towers. To offset the low performance of dry cooling, pre-cooled cooling systems which combine dry and wet cooling are proposed. The wet cooling system only operates at high ambient temperatures to assist dry cooling. This paper is to investigate the variations in annual performance of three proposed natural draft cooling towers, i.e., a Natural Draft Dry Cooling Tower (NDDCT), a pre-cooled NDDCT and a Natural Draft Wet Cooling Tower (NDWCT). The water consumption of the pre-cooled NDDCT is compared with that of the NDWCT. The present study finds that: the performances of the NDDCT, the pre-cooled NDDCT and the NDWCT are significantly affected by the ambient conditions in operation; the NDWCT provides the highest heat rejection rate, followed by the pre-cooled NDDCT and the NDDCT; the pre-cooling enhancement can go up to 46% by increasing the NDDCT heat rejection rate from 93 MW to 136 MW in the hottest month. The use of the pre-cooling system is season-dependent (usually for hot summers); the pre-cooled NDDCT consumes 70% less water for every MW heat rejection than the NDWCT.