|A cooling water intake structure at SCE&G’s Williams Power Plant on the Back River Reservoir in South Carolina. Photo courtesy of EPRI|
The controversial 316(b) rule is expected to be finalized in November
By Russell Ray, Managing Editor
For 41 years, the Environmental Protection Agency has been trying to impose new standards for cooling water systems at existing power plants in the U.S.
The new standards are aimed at reducing the mortality of fish and other aquatic life caused by water intake structures. After a decades-old legal battle between utilities and environmental groups, the EPA is expected to release the final rule in November.
Section 316(b) of the Clean Water Act would affect roughly 670 U.S. power plants. It would require plants that draw more than 2 million gallons a day and use 25 percent of that water for cooling to install the best technology available (BTA) to minimize the mortality of aquatic life. Losses occur when fish and other organisms become trapped (impinged) against water intake structures or sucked (entrained) into cooling systems and exposed to heat, pressure and machinery. The rule requires the best technology to mitigate what it describes as “adverse environmental impact” resulting from entrainment and impingement.
The measure would require some power producers to modify cooling water intake structures or construct new cooling towers.
“There isn’t one technology that’s best. The EPA has already made that conclusion,” said Doug Dixon, a technical executive with the Electric Power Research Institute. “The thing that probably comes closest to controlling impingement is modified traveling water screens. For entrainment control, there isn’t a best technology. It very much depends on where the plant is located. It’s site specific.”
EPRI has studied many methods for controlling impingement and entrainment, including the use of light, sound, barrier nets, screens, electric fields and closed-cycle cooling. In many cases, the best technology depends on the ecosystem around the plant, Dixon said. For example, some species of fish are repelled by light while other species of fish are attracted to light.
“Barrier nets are probably the cheapest method for controlling impingement.” Dixon said. “However, barrier nets can be fouled very easily by floating debris. In flowing rivers or estuaries, they can be almost impossible to maintain.”
There is a wide range of technologies and technology combinations available for controlling impingement. The most promising technologies are modified traveling screens and wedgewire screen systems. Modified traveling screens rotate continuously and are equipped with fish buckets, a fish return system and smooth mesh screens. “It’s smooth. It’s not going to cause injury via the contact to the fish,” Dixon said. The average cost of a modified traveling screen is between $300,000 and $400,000, which does not include the cost of shipping and installation. A typical 700-MW power plant is equipped with six or seven screens. “The cost of the screen is going to depend on the characteristics of that screen,” Dixon said. If a modified traveling screen is used to comply with the rule, the monthly maximum mortality of impinged fish cannot exceed 31 percent.
The draft rule, released in April 2011, contains a list of eligible technologies, including closed-cycle cooling. Although the rule does not mandate closed-cycle cooling, it does target plants using once-through cooling systems and could lead to more closed-cycle cooling retrofits at existing plants in the U.S. Closed-cycle systems use less water from rivers and bays and harm fewer fish. If there were a national requirement for closed-cycle cooling, the industrywide cost to comply could near $100 billion, according to a 2010 report published by EPRI.
|A modified traveling screen. Photo courtesy of HDR Inc.|
“It was exceptionally expensive,” Dixon said. “The cost was well beyond the monetized benefits that could be obtained.”
Despite pressure from environmental groups and the courts, the EPA decided not to mandate closed-cycle cooling for entrainment in the draft rule.
“The environmental groups would very much like to see all the existing once-through plants go to closed-cycle cooling,” Dixon said. “That’s what the 2nd Circuit Court of Appeals inferred to EPA back in 2007. They said to EPA ‘We don’t understand why you have not selected closed-cycle cooling as BTA.'”
The 316(b) rule was first enacted in 1972 when Congress passed the Federal Water Pollution Control Act Amendments. Since then, the rule has been suspended and rewritten several times in a long and drawn out legal battle between utilities and environmental groups.
In 1979, the EPA withdrew the regulation, but the agency was sued in the mid ‘90s by a coalition of environmental groups for not reenacting the regulation. The EPA was ordered to finalize the rule and developed it into three phases. But portions of the rule were remanded back to the EPA for reconsideration in 2007.
As a result, a draft rule for existing plants was published in April 2011. A final version was initially expected to be released in July 2012, but the release has been extended several times. The EPA expects to publish the final rule in November.
ENVIRONMENTAL IMPACT IN QUESTION
The rule requires that “the location, design, construction and capacity of cooling water intake structures shall reflect the best technology available for minimizing adverse environmental impacts” resulting from the drawing of fish and larvae into and through the cooling systems of power plants, where mechanical and thermal stresses and the trapping of fish against screens can cause high mortality rates.
|A cooling water intake structure at Alabama Power’s Plant Barry on the lower Mobile River. Photo courtesy of EPRI|
But new research indicates there is no evidence that impingement and entrainment of aquatic life at U.S. power plants cause a meaningful loss of fish and other aquatic organisms. A peer-reviewed article authored by Lawrence Barnthouse and published in the May issue of Environmental Science & Policy found there is no scientific evidence that shows a reduction in entrainment and impingement would lead to measurable improvements in fish populations.
“Any impacts caused by impingement and entrainment are small compared to other impacts on fish populations and communities, including overfishing, habitat destruction, pollution, and invasive species,” wrote Barnthouse, whose research was sponsored by EPRI.
The EPA has not performed a single study that shows entrainment and impingement impact fish populations any more than commercial fishing, according to Barnthouse.
“Adverse impacts have been implicitly or explicitly defined as entrainment and impingement per se, irrespective of whether any adverse changes in populations can be demonstrated or predicted,” Barnthouse wrote. “The rarity of documentation of such impacts, after 40 years of operation of large power plants, some of which have been conducting extensive monitoring programs for several decades, provides substantial evidence that impacts related to entrainment and impingement are generally small.”
The 316(b) rule does not provide a definition for “adverse environmental impact.” The term has long been understood by the scientific community to refer to adverse changes in the abundance and productivity of fish and other aquatic life.
ENERCON Services Inc. provides engineering services related to cooling tower installations and modifications to closed-loop cooling systems. The company was asked by its clients to review the feasibility of converting a power plant using once-through cooling to closed-cycle cooling.
|A fine mesh screen. Testing of these screens was performed at Alden Research Laboratory in Worcester, Mass. Photo courtesy of EPRI.|
Sounds simple enough, but after conducting several feasibility studies it was obvious that each specific site evaluated had a different set of physical constraints and design considerations that may or may not challenge the feasibility of a closed-loop cooling retrofit, appreciably affect the implementation cost and result in vastly differing levels of plant performance post retrofit.
Closed-loop cooling relies on a source of heat rejection different from the existing water body drawn from in once-through cooling. In most cases, this means a cooling tower (or series of cooling towers) are used to reject heat to the atmosphere. Other closed-loop cooling designs are available and typically involve a cooling pond or cooling canals, possibly equipped with power spray modules; however, the space required to produce the necessary cooling capacity restricts where these options are available. Similarly, cooling towers can be either wet (relying on air and evaporation for cooling) or dry (relying solely on air cooling); however, dry cooling towers have limited applicability for retrofit use given their reduced capacity for cooling. In either circumstance, cooling towers reject heat to the atmosphere and their ability to cool varies with the ambient weather conditions. In particular, wet cooling towers ability to cool varies in relation to the wet-bulb temperature, which is a combination of dry-bulb temperature and humidity.
The ability to reject the design heat load is a basic requirement of any facility generating electricity using a steam cycle. Steam traveling through the turbines is converted to water in the condenser, which in turn transfers heat to the circulating water system. When warm and humid weather conditions impact a cooling tower’s ability to cool the circulating water, the condenser’s cooling efficiency is reduced. The loss of power due to a reduced ability to cool the circulating water is termed an operational power loss. In circumstances where a power plant was designed with a relatively small condenser accounting for a source of cold once-through cooling (i.e., ocean intakes, great lakes, reservoirs, etc.), the power plant retrofit to closed-loop cooling would be operating at reduced capacity or potentially shut down for periods of time in the summer when energy use is typically at its highest demand. Conversely, in circumstances where a power plant was designed with a large condenser to allow once-through cooling from a relatively warm water body, a power plant may be able to install cooling towers with little to no operational power losses.
PLANNING FOR 316(B)
Some power producers may be reluctant to invest too much time and money into 316(b) compliance because of the uncertainty surrounding the final rule. However, many industry observers say the EPA won’t make significant changes in the final rule. There are some low-risk actions power producers can take to position their plants for the new requirements once they are finalized.
There’s virtually no risk in making sure that both the plant’s owners and operators fully understand the state of planning at their facilities and how that subjects them to the fundamental changes that loom.
Operations & Maintenance staff should be involved in these discussions and planning early on.
If a plant is scheduled for system upgrades or changes in the next year or two, those plans should be shelved for now. Making those changes along with those required by 316(b) may be more sensible in cost and functionality.
Nathan Henderson of Stantec and Richard Clubb of ENERCON Services contributed to this report.
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