|Fig. 1: An aerial shot of the nuclear power plant at Diablo Canyon in California with its once-through water discharge clearly visible on the left hand side of the photo. (Source: Wikimedia Commons)|
A standard thermoelectric power plant works by using energy from a source nuclear, coal, natural gas, solar etc. to boil water, with the resulting steam rotating a turbine, which results in electricity. However, after the steam is pushed through the turbine, the plant needs a way to re-condense the water so the process may repeat. Traditionally there are a handful of cooling mechanisms, many of which include running colder water through the system to dissipate the heat. The methods of sourcing and disposing the water used differentiates the systems, and they each have their benefits and costs, usually pitting economic burden versus environmental harm. One method in particular poses a tremendous ecological risk, the once-through cooling system.
There are three main categories of cooling systems for power plants: dry-cooling, closed loop, and once-through. The dry-cooling process does not require water at all, but instead relies on air to condense the steam after it passed through the turbine. Given the relative heat capacities of air and water (with water able to hold over 4 times as much heat as air), the process is not as efficient.
Closed-loop systems use cooling towers, which are large open-air container tanks, to expose water to the environment after it is used to cool the system. The air gradually cools the water, and then the water can be reused, minus any water that evaporated. In a system with enough cooling tower capacity, the inefficiencies of a dry-cooling system do not hold, since water can still quickly absorb the excess heat from the steam before gradually losing the heat to the atmosphere. The downside is the water lost to evaporation, which needs to be replaced and removed from the original source.
Finally, a once-through cooling system sucks water in from a river, lake, or the ocean, cycles the water through the system to condense the steam, then is shot right back into the original water source. This system is cheapest to operate because it both uses water and does not require the additional infrastructure of cooling towers.
Altogether, power plants in the US use on average 4.35 × 108 cubic meters/day of waters for the cooling process.  To put it in perspective, that is 43.8 percent more water than flows through the Mississippi River at St. Louis every day (3.024 × 108 cubic meters/day). 
As of 2008, 43 percent of US thermoelectric plants (including 86 percent of plants drawing water from the ocean) utilized some form of once-through cooling, 54 percent used closed-loop systems, and only two percent used dry-cooling.  The result: three times the amount of water cascading over Niagara Falls is used to cool power plants every minute. 
When the water from a once-through system returns to the original source, it can have temperatures up to 37°C hotter than the surrounding water.  This has been shown not only to scorch unsuspecting marine life, but also to contribute to algae blooms and ruin coral reefs, further harming the ecosystem. In Southern California alone (including the nuclear power plant at Diablo Canyon shwon in Fig. 1), this has been estimated to cost the fishing industry $9 million per year.  With only 20 of 258 fish species important to the fishing industry, this number is wholeheartedly under representative of the full impact. For another metric, as of 2005, an estimated 247,000 acres of coastal and estuary habitat had been ruined, with nearly all organisms lost, due to the output of once-through systems. 
While dry-cooling is not always practical due to the inefficiency of using air or in the case of nuclear power cannot be used thanks to certain safety risks associated with the reactors, closed-loop systems have a lot of potential to mitigate some of the environmental flaws of a once-through system. With the implementation of the cooling towers to utilize air to dissipate some of the heat from the coolant and allow water to be reused instead of being damagingly discharged back into the ecosystem, it seems the best proven balance between economic cost and environmental cost. To the extent that government subsidies can be used to depress the cost barrier and encourage new facilities to adopt the closed-loop system, decreasing the 43 percent share of once-through systems would be a step in the right direction of the planet.
© Jamie MacFarlane. 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.
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