To call water a hot commodity is an understatement. From controversial water trading to desalination, a slew of efforts are underway to solve water scarcity issues in many regions of the world. Some, like the massive undertaking by Israel to reuse wastewater and desalinate water from the Mediterranean Sea, are having an impact. But as population and urbanization continues to grow worldwide, so does water consumption, and, naturally energy use.
Water and energy are closely tied. Consider that thermoelectric power plants – which currently provide the vast majority of US electricity — consume a lot of water. In fact, the power industry is one of the largest water users in the United States.
Presently, in the US, coal plants are being displaced by natural gas plants. However, gas turbine efficiency is the lowest when the demand for power is the highest, during hot summer months. To offset this negative effect of high ambient temperature, gas turbine inlet air can be cooled via mechanical chillers.
A chiller is a heat-transfer device that uses refrigeration cycle to absorb heat from a process load, in this case heat from turbine inlet air. Absorbed heat cannot disappear but it can be transferred to the environment. Air-cooled chillers, as the name implies, disperse the absorbed heat into air via fans. Water-cooled chillers, disperse the absorbed heat through evaporation of water via cooling towers.
How to Choose
Which choice is better, a water or air-cooled chiller? The answer to that question isn’t easy.
Water-cooled chillers offer the most effective way to process heat; the net amount of power enhancement left for gas turbine is greater when compared to air-cooled chillers. However, they require a great deal of water. So they make sense where water is plentiful and inexpensive.
Air-cooled chillers do not consume water – that’s a plus. But they are less efficient than a water-cooled system. It takes a lot of energy to dissipate heat this way. Therefore, the net amount of power enhancement left for gas turbine is lower when compared to water-cooled chillers.
Air-cooled chillers also require a great deal of space – actual acreage — for the large air cooled radiators are required in the system. Air cooling is therefore typically a more expensive process, so it tends to be the secondary choice.
However, exceptions certainly exist, especially now in dry and drought-stricken areas.
In determining which makes the most sense economically, a power plant owner needs to balance out water costs associated with a water-cooled chiller system versus energy costs for an air-cooled chiller system.
Generally speaking, an air-cooled chiller is likely to win out if the power plant is located in say, southern California or Arizona, and a water-cooled chiller, if it is in a rainy area like upper state New York.
There is another consideration, as well, that a power plant developer should investigate before making a choice between the two systems. Local water discharge regulations might hamper use of water-cooled systems. Restrictions may exist, for example, on the corrosion inhibitors and other chemicals that are applied to the water loop in water-cooled chillers.
Of course, this discussion is not confined to the United States. Air-cooled chillers may make sense in many of the areas were people must conserve water. Just like in real estate, decisions for what type of chillers to use are about location, location, location.