Researchers from The University of Queensland’s Geothermal Energy Centre of Excellence (QGECE) have developed hybrid cooling tower technology that reduce water consumption in thermal power generation, while also being modular for faster construction.
All thermal power plants (including geothermal, solar thermal, biomass, coal-fired and even nuclear) produce waste heat as a by-product. The role of the cooling tower is to dissipate the waste heat, in order to allow the plants to operate efficiently.
There are two types of cooling towers: wet or dry. Wet cooling towers evaporate water into the air flowing through the tower. Dry cooling towers transfer heat from the power plant directly into the air.
Wet cooling towers are one of the largest consumers of water in power generation, due to evaporation loss, blow down water loss and drift water loss. A wet cooling tower for a 350 MWe coal-fired power plant, for example, consumes 5.5 billion litres of water per year.
In contrast, dry cooling towers save significant amounts of water by keeping the working fluid separated from the cooling air. However, these towers require more heat exchangers, and are also less efficient when ambient temperatures are higher.
For thermal power plants located in remote, arid areas where water is less available, dry cooling may be the only viable option. However, the higher capital costs and reduced efficiency during hot days are a concern.
The QGECE hybrid tower is engineered to solve the intermittent high temperature efficiency issue. Its design allows the tower to be built in dry, wet or hybrid modes, depending on the environment and the availability of water supplies.
The hybrid mode uses limited water to achieve high performance. In this mode, the cooling tower uses small amounts of water to pre-cool the ambient air streaming into the tower, reducing the air temperature toward the ‘wet bulb temperature’, the minimum temperature that can be reached by evaporative cooling.
The cooler air then passes through the heat exchangers, extracting more heat.
Numerical modelling indicates that a power plant using this system can increase net power output during periods of high ambient temperature by up to 20%. The hybrid system also allows the tower to be smaller, and operate in a larger range of ambient temperatures.
Another innovation by the QGECE researchers is the application of natural draft to its small scale cooling towers. While natural draft is used to enhance the performance of large towers, the design of smaller cooling towers means that crosswinds can drastically reduce overall plant efficiency.
The QGECE studied the mechanics of crosswind interaction with cooling towers, and developed windbreak walls, which divert crosswind flow through the heat exchangers. This increases the heat transferred, improving plant efficiency during crosswind conditions.