|Fig. 1: When the temperature of intake versus discharge water is high, it can be detrimental to the river ecosystems that surround it. Source: M.Duboc, after Birkinshaw et al. )|
Nuclear energy accounts for over 70% of emission-free electricity generated in the US.  Although it does not produce greenhouse gasses, it does have an effect on the environment in other ways; primarily through the mismanagement of toxic waste and the negative effects of thermal pollution on local aquatic ecosystems.  Currently, all nuclear power plants in the US require water to power steam turbines and to cool the plants. The most cost-efficient way to do this is with a once through wet cooling system. This is when water from a nearby source is redirected and used as a cooling agent before being returned to its original source 10˚F to 20˚F warmer.  Returning the water at an increased temperature, however, can negatively affect the adjacent ecosystems through thermal pollution. The disparity in water temperatures can shock fish and other aquatic species, damaging the ecological diversity (see Fig. 1). Thus, other methods of cooling nuclear power plants are being explored, one of these is dry cooling. Dry cooling is not a new technology, it has been researched since the early 1970s, but due to the many disadvantages, it has not been employed in the nuclear energy sector. 
There are three different types of dry cooling; direct dry cooling, indirect dry cooling, and hybrid dry cooling. First, a direct system uses air-cooled condensers (ACCs) to condense steam from the turbine generator and recirculate that water through the system.  An indirect air-cooling system utilizes cooling towers; steam condenses in dome shaped steam condensers, after which hot cooling liquid is pumped through a cooled heat exchanger (ACHE) prior to recirculation.  Last, in hybrid wet-dry systems, components of both a traditional wet and dry cooling systems are combined to offset the mechanical and economic advantages and disadvantages of both.  These are advantageous because they reduce the water withdrawal and consumption when compared to traditional wet-cooling systems. This is often achieved by combining a recirculating wet system with cooling towers or ponds. 
There are many cost considerations that must be taken into account when analyzing the feasibility of using dry cooling as the primarily method to cool nuclear power plants.  First, dry cooling systems overhead capital cost is between 3 to 5 times more than wet cooling system. Second, the cost of retrofitting existing plants would be extremely expensive and very difficult since dry cooling requires large swaths of land. Since all current nuclear power plants rely on water, they are usually built near or on lakes, rivers, oceans, and reserves. Thus, it would be difficult to retrofit already existing plants because it would require purchase and rezoning of more land. Third, there are performance penalties due to the inefficiency of the system. Since dry cooling is less efficient than both direct and indirect wet cooling systems, during the hottest days of the month plants would need to reduce their electrical output to ensure no internal damage. However, this is when electricity is often in peak use and price, and replacement electricity is scarce and expensive. During these heat intensive periods, energy productive could decline between 2% to 4%, and brownouts could occur.
In the near-term, hybrid cooling, which uses combination of wet and dry cooling to offset the advantages and disadvantages of each, is the only feasible and realistic application of dry cooling.  In order for dry-cooling to be used as the primary cooling technique many of the problems above would need to be mitigated. However, the need for more research and innovation in this field is paramount as water becomes more scarce and aquatic ecosystems require more protection.
© Mady Duboc. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. Birkinshaw, M. Masri, and R. L. Therkelsen, "Comparison of Alternate Cooling Technologies for California Power Plants: Economic, Environmental, and Other Tradeoffs," California Energy Commission, 500-02-079F, February 2002.
"Cooling Water Issues and Opportunities at U.S. Nuclear Power Plants," Idaho National Laboratory, INL/EXT-10-20208, December 2010.
 W. Asvapoositkul and M. Kuansathan, "Comparative Evaluation of Hybrid (Dry/Wet) Cooling Tower Performance," Appl. Therm. Eng 71, 83 (2014).