By Bob Ciminel
May 09, 2006
Generating electricity requires a lot of water, about 25 gallons for each kilowatt-hour. The most common thermal electric generating plants (fossil and nuclear) are only about 35% efficient. What that means is a 1,000 megawatt power plant has to dissipate over 2,800 megawatts in waste heat to the environment. In world of thermodynamics it's called the law of "there is no such thing as a free lunch." I won't bore you with the details, so you'll have to push the "I Believe button" and take my word for it.
There are two ways to get rid of that heat. Engineers call them open-cycle or closed-cycle cooling systems. An open-cycle system is simple. Find a nearby river or lake, pump the water through the power plant's condensers, and then send it back to its source.
A nuclear plant I once worked at on the Mississippi River used an open-cycle cooling system. Four pumps pulled one million gallons a minute out of the Big Muddy, pushed it through the condenser, and discharged it right back into the river. One million gallons a minute probably sounds like a lot of water, but it was a drop in the bucket. The Mississippi River carries about 288 million gallons per minute on a normal day.
A closed-cycle cooling system is one that does not draw water from the environment. The most common system uses a cooling tower. The towers can use fans, in which case they are called mechanical draft cooling towers, or they can use the tendency of heated air to rise, in which case they are called natural draft cooling towers. Those are the hyperbolic towers that gained notoriety during the Three Mile Island Accident in 1979. Natural draft towers are about 500 feet tall, made of concrete, and usually have a plume of water vapor rising out their tops. The media always shows a picture of a natural draft cooling tower whenever they want to say something bad about nuclear power, and for some reason they conveniently forget to explain that the cooling towers have absolutely nothing to do with the nuclear reactor.
Some power plants use open-cycle systems, but have built man-made reservoirs to absorb the heat and minimize the effects on the microenvironment around the plant site. I visited one in Texas recently that uses a 7,000 acre "pond." It provides enough cooling water for two 1,200 megawatt nuclear power plants and has enough capacity for more. A private reservoir has its advantages. Workers at the plant have a catch-and-release fishing rodeo every year and pull red fish out of reservoir that are as big as sharks.
Another option for open-cycle plants is to use natural water sources but to augment them with cooling towers, which reduces the temperature of the water going back to the environment. This is usually required when the local river commission places restrictions on the temperature of water returning to the river.
These days, very few power plants can be licensed for open-cycle cooling systems, and almost all new plants are built with cooling towers. However, cooling towers do not solve the problems associated with water consumption, which is the water evaporated by the cooling tower. Water loves to evaporate, and if you heat it, it evaporates more quickly. A natural draft cooling tower can evaporate anywhere from 10,000 to 20,000 gallons of water a minute. If we couldn't provide a source of makeup water, the cooling water system would eventually lose enough water that the pumps wouldn't run. That makeup water has to come from somewhere, and that's where plant designers really begin to innovate.
For example, there is a nuclear plant in Arizona that uses treated sewage water from Phoenix as its makeup source. The treated water is pumped more than 50 miles through huge concrete pipes to the plant's reservoir where it is used to provide makeup water for the plant's cooling water systems. This has resulted in a symbiotic relationship for both the city and the power station. It must be comforting for Phoenix's residents to know that every time they flush their toilets they are helping to generate their own electricity.
Over the past three years, a nuclear power plant in eastern Pennsylvania has been demonstrating another unique method for augmenting its cooling water needs. The plant uses natural draft cooling towers, but relies on the Schuylkill River (pronounced Sure Kill for you non-Pennsylvania Dutch types) for its makeup needs. Efforts to improve the water quality in the river resulted in restrictions on how much water the plant could take out of the river. The plant's owners designed and built a system to bring water from the Delaware River through a series of pumping stations, reservoirs, and pipelines. At full power, the plant's twin cooling towers need almost 30,000 gallons a minute for makeup.
It all came down to numbers. Restrictions on using the Schuylkill applied whenever the river temperature exceeded 59 degrees Fahrenheit and flow was below a certain value. The intent was to prevent the river's dissolved oxygen content from dropping below the value where aquatic life begins to suffer. When those conditions were met, the plant had to start the pumps on the Delaware River and bring in about 40 million gallons a day from the Delaware.
Necessity being the Mother of Invention, the plant's owners embarked on an unconventional way to bring additional water into the Schuylkill River and reduce the need to pump water all the way from the Delaware. They found the answer in the Anthracite Region, about 75 miles up the Schuylkill in a little mining town called Wadesville, once the home of the Wadesville Shaft and now the home of the Wadesville Pit.
As an underground mine, the Wadesville Colliery ceased operations in 1930. When mining stopped, the colliery owners allowed the mine to fill with water. In 1953, the Wadesville Pool was estimated to contain about 3.4 billion gallons of water. Strip mining began in 1940 and continued off and on to the present. The open pit over the old Wadesville workings is over 500 feet deep.
In 2003, the power plant owners received permission to pump up to 10,000 gallons a minute from the Wadesville Pool into the Schuylkill River, or about 40 percent of the plant's makeup water needs. A recent extension allows the plant to continue augmenting its water needs through 2007 with an increase to 12,000 gpm based on the minimal environmental impact the mine water is having on the local aqua sphere. Unlike most abandoned mine drainage, the Wadesville Pool is not acidic and has a pH between 6 and 8 (pure water has a pH of 7).
The Wadesville Pool water takes about four days to reach the power plant and it is monitored daily. The augmentation period is only six months long, typically when the Schuylkill is at low flows. The Wadesville Mine pool recharges itself during the six months it is not being pumped, so it is in effect a renewable resource. Another benefit of the program is the plant's owners contribute money to the Schuylkill River Restoration Fund based on the amount of water they do not use from the Delaware. In 2005, there was a reduction of 2.63 billion gallons of water withdrawn from the Delaware River and the plant contributed nearly $160,000 to the restoration fund. That is an environmental plus if there ever was one.
Plans are on the horizon to use more flooded and abandoned mines as cooling water sources for power plants. The University of West Virginia has been conducting research on the subject for a number of years. In those cases where the mine pool is too acidic, it would be possible to use it as a closed cooling system while leaving the water in the mine and only using its relatively cool temperature to absorb waste heat.
The Pennsylvania project is
a positive example of how technology and the environment can
exist in harmony and benefit both man and the ecosystem. Now
all we need to do is come up with a similar way for getting to
all that oil locked in the Green River oil shale!
He assumes informed readers will be able to tell the difference. Bob lives in Roswell, Georgia, and works for the Institute of Nuclear Power Operations. He is also a conductor on the Blue Ridge Scenic Railway.
Contact Bob at firstname.lastname@example.org