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.
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 and evaporation, and spray cooling efficiency experimentally and for the CFD model validation. Based on a literature review, nine promising high pressure, hollow cone nozzles for inlet air pre-cooling were selected. Spray characterisation of the different nozzles was conducted in UQ’s wind tunnel at different atomization pressures and environmental conditions. The nozzle characterisation was performed utilizing a 2D-Phase Doppler Particle Analyser and a high speed photography system. In addition, measurements of streamwise development of droplet size and velocity, and airflow temperature and humidity were performed for different droplet sizes, velocities, injection rates, spray cone angles and spray patterns under different air velocities and ambient conditions selected to represent typical Natural Draft Dry Cooling Tower operating conditions. The wind tunnel measurements have been used for the validation of the CFD simulation model using the new more realistic hollow-cone nozzle representation approach. The simulated droplet evaporation, transport, and spray cooling efficiency were validated by comparing the predictions of the main features of the airflow and the spray (droplet axial velocity, Sauter mean droplet diameter, outlet air dry bulb temperature) with the experimental measurements downstream the nozzles at various inlet air conditions and spray characteristics. Overall, good agreement was obtained between CFD predictions and experimental measurements yielding an average deviation below 5.3% for all parameters compared. The experimental results indicated that spray dispersion is a major factor affecting nozzle and spray cooling systems performance. A modified spray cooling efficiency was introduced to separate the spray dispersion inﬂuence from the cooling efﬁciency.
Finally, the validated CFD model has been used to identify the complex interaction between inlet air (velocity, temperature and relative humidity) and spray (droplet size, velocity, flow rate and cone angle). Two injection approaches were studied. In the first approach, droplets with a single size distribution were injected into the airflow at one single injection location in order to identify the effect of major spray characteristics parameters on droplet transport, evaporation and spray cooling efficiency. In the second approach, different droplet size distributions were injected at the breakup length using the newly developed nozzle representation. The second approach was carried out to investigate the effect of different droplet size distribution spray patterns (as observed during experiments) and major spray characteristics parameters on spray effectiveness. These simulations have provided new insight into the interactions between the spray, air flow, and cooling effectiveness. Air velocity, droplet velocity, and droplet size distribution play significant roles in droplet evaporation, transport, and spray cooling efficiency. In small size distribution, it is not always beneficial to decrease droplet size to enhance evaporation due to the compromise effect of evaporation rate and spray dispersion. However, high droplet velocities are a very effective solution that allows small droplets to penetrate deeper and distribute better within the airflow to mitigate the low Stokes number of small droplet size. Moreover, the validated CFD model has provided insight into some experimental observations (i.e. local droplet velocity increase and higher air cooling in the lower region of the duct).
The study demonstrates the feasibility of utilizing spray cooling systems in natural draft dry cooling towers. Complete evaporation of water droplets emitted into airflow under NDDCTs typical condition at a hot and dry ambient condition is an achievable event but compound and trade-offs have to be made as it depends on many factors including: inlet air (velocity, temperature and relative humidity), spray (droplet size, velocity, flow rate, cone angle and configuration). The developed CFD model using the new spray initiation technique provides a good predictive design tool for spray cooling systems optimisation to enhance NDDCTs during high ambient temperature periods.