Modelling and experimental study of spray cooling systems for inlet air pre-cooling in natural draft dry cooling towers

Abstract

Geothermal energy is a promising renewable energy to be used for baseload electricity. Most potential sites with high geothermal temperature are mostly located in remote areas where water is limited. This is besides water scarcity and environmental protection, which made dry cooling systems a better alternative solution for heat rejection of power plants. Furthermore, natural draft cooling towers have the advantage of avoiding parasitic losses introduced by the fans at mechanical draft cooling towers. However, power plants utilizing dry cooling technologies experience a significant reduction in power generation during high ambient temperature periods. This reduction often goes along with the peak power demand which results in a great loss for the power plant owners. In certain instances, dry cooling tower performance can be enhanced during these periods by pre cooling of the inlet air by spraying atomized water into the inlet air.The present study introduces the use of spray cooling for inlet air pre-cooling in natural draft cooling towers. Spray cooling is investigated in this study due to its simplicity, low capital cost, ease of operation and maintenance, and capability of increasing power plant efficiency while consuming only a small amount of water compared to wet cooling towers or other evaporative cooling methods. Although spray cooling has found successful applications in process coolers and gas turbine inlet air cooling, the large scale applications in power industry have been limited.Several issues limit the application in power industry. The main one is the incomplete evaporation of water droplets which can cause corrosion and scaling of heat exchanger surface. Incomplete evaporation also increases operational cost due to water consumption. To the best of our knowledge, a detailed investigation of the spray cooling performance in natural draft dry cooling towers operating environment has not yet been performed. The aim of the current work is to optimise a spray cooling system for inlet air pre-cooling in natural draft dry cooling towers.In the present study, an Eulerian-Lagrangian 3-D numerical model was developed which is capable of simulating evaporating water sprays produced by real nozzles. In order to reproduce real nozzle characteristics in the simulation, a new adaptable method for hollow-cone spray representation in Eulerian-Lagrangian numerical models was developed. This allows real nozzles characterised during experiments to be included in the simulation, thereby correctly accounting for radial evolution of droplet size distribution and air/droplets momentum exchange. The CFD model was applied to calculate local droplet transport and evaporation, and spray cooling efficiency at different operating conditions for spray cooling systems optimisation under typical Natural Draft Dry Cooling Tower conditions.iii Experimental measurements from a wind tunnel test rig simulating Natural Draft Dry Cooling Tower inlet flow conditions have been performed in order to investigate droplet transport...

Experimental investigation on spray cooling using saline water for natural draft dry cooling towers

“…(Table 3) show that the area average temperature varies by less than 0.15%, the relative humidity less than 2.4% and the mass flow rate less than 0.02% at the outlet plane. The nozzle representation method developed by Alkhedhair [40] Table 5. It should be noted that only the H2O liquid material is set to evaporating species.…”

“…The same experimental setup developed by Alkhedhair [40] is presented below. The wind tunnel is divided into separate sub-systems which are the air system, the water supply system and the spray system in a test section.…”

“…In his thesis, Alkhedhair [40] studied the uncertainty related to the instrumentation at the wind tunnel. He found that the uncertainty in the calculated cooling efficiency ranged between ±1.5% and ±3.7%, depending on the ambient conditions by using the guide set out by the Joint Committee for Guides in Metrology [46].…”

“…However, there is a clear discrepancy between the experimental (Figure 69) and the numerical results (Figure 68), which can be explained by the method used to simulate the nozzle. In the 3D model, the nozzle was simulated by 4 concentric circles with the same mean Rosin-Rammler obtained by Alkhedhair [40] for all circles. A detail plot was setup comparing the different Rosin-Rammler distributions from the experimental results obtained by Alkhedhair [40] for the four injection circles was shown in Figure 70.…”

“…A computer model of the wind tunnel is shown inFigure 52andFigure 53shows a photo of the exit diffuser of the wind tunnel. The wind tunnel setup and associated instruments available in this laboratory are illustrated inFigure 54[40].…”