In my blog ‘Identifying Potential Sources of Power Generation’ I discussed the concept of heat sources and heat sinks. Most power plants use the ambient air temperature as their heat sink a.k.a. the cold end of their process. But there are actually two ambient temperatures that can be useful as a heat sink for a power plant. I’m going to try not to get too lost in weeds in this post, so if you want more information about this, you can look up psychrometrics. Anyway, the first one is just the regular air temperature known as the dry bulb temperature. This is the temperature that will come into play for air cooled power plants. The second air temperature that is useful is called the wet bulb temperature. This temperature is based on a combination of the Dry bulb temperature and humidity. The reason that it is important is because this is the temperature that water will evaporate at.
In a water cooled power plant, water is sent through the top of cooling tower (basically a large box with metal mesh inside) and then as it falls some of it evaporates. The cool water that comes out of the cooling tower usually has a temperature a few degrees higher than the wet-bulb temperature but usually several degrees colder than the dry bulb temperature. The cooling water then acts as the plant’s heat sink. One advantage of the water cooled system is that your heat sink will be at a lower temperature and therefore your process will be more efficient. Another advantage is that water is much better at holding and absorbing heat than air is. This also makes your process more efficient. The main downside of a water cooled power plant is that it is often difficult to get water rights to keep them running. A key component of a water cooled plant is that some of the water has to evaporate in the cooling tower. So that water has to be replaced by new water from somewhere.
For an air cooled power plant, the heat sink will be the higher dry bulb temperature. Of course in the opposite of the water cooled scenario, the air cooled plant has lower efficiency. Because air cooled power plants are less efficient it also means the condensers have to be much larger to get the same effect. Air cooled power plants take up much more land area than their water cooled cousins. They also have a lot of moving parts with all of the fans that blow the air. All of those fans require periodic maintenance. The good things about air cooled power plants are that you can put them anywhere even in the middle of a desert, and you don’t have to worry about things freezing.
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Air Cooled Condenser Photo Credit: http://www.gdareno.com/wp-content/uploads/2015/10/CoolingTower.jpg