By Richard Clubb, Project Manager at Enercon Services, Inc.
Throughout the country, utilities have been closely monitoring the Environmental Protection Agency’s (EPA) Section 316(b) rule requirements and compliance timeline to determine how the regulation may affect their existing facilities. In preparing for the rule to be issued on June 27, 2013, the industry can’t help but read the newspaper articles from states with more aggressive environmental policies sounding an alarm that closed-loop cooling retrofits may cost utilities billions of dollars. So, should utilities be idly waiting for the EPA’s draft rule to be issued in June, should they start budgeting for significant capital expenditures to ensure compliance, or is there somewhere in between where utilities can cautiously prepare for the new regulations?
The background for this regulation comes from Section 316(b) of the Clean Water Act (CWA), which requires “that the location, design, construction, and capacity of cooling water intake structures reflect the best technology available for minimizing adverse environmental impact.” Without getting into too much detail, this means that facilities which draw in more than 2 million gallons of water daily (MGD) and use at least 25 percent of the water withdrawn exclusively for cooling purposes need to take steps to reduce their impact on the aquatic environment. In specific, facilities are required to minimize the mortality of aquatic organism due to impingement and entrainment. The EPA provides a description of impingement and entrainment mortality CWA §316(b) Existing Facilities Proposed Rule Qs & As document, stating:
Impingement happens when fish and other organisms are trapped against screens when water is drawn into facility’s cooling system. The injuries often prove fatal within a few days, because, for example, the fish lose gills and cannot breathe. Young or small fish are most susceptible to being killed by impingement. Entrainment happens when organisms are drawn into the facility. Once inside of the facility, entrained organisms are exposed to pressure and high temperatures, which kill them. Very young organisms, usually at the egg or larvae stage, are most susceptible to death by entrainment.
Impingement mortality is a relatively straightforward concept to understand as it involves fish of a size that can be viewed by a casual observer; however, entrainment is much more challenging as egg or larvae are difficult to observe and even more difficult to understand in adult equivalent terms (i.e., only one fish may survive to become an adult from tens or hundreds of thousands of eggs). Moreover, since the general public does not distinguish between impingement of fish and entrainment of eggs and larvae, the local paper publishes articles claiming that millions of fish are killed by a power plant when in fact the number of equivalent adult fish is many orders of magnitude below this number.
|Turkey Point Power Plant uses a closed-loop system of 36 interconnected canals for cooling. The overall length of the canals is 168 miles.|
The question then arises, if the eggs and larvae entrained are so small that they are difficult to see, how is a power plant supposed to be able to effectively screen them from the intake flow? One historical answer to this question is to reduce the amount of water drawn in by a power plant by installing a closed-loop cooling system. A closed-loop cooling system rejects heat to the atmosphere via cooling towers instead of relying on the source water body. This in fact is how ENERCON became involved in review of CWA §316(b) compliance technologies. The company had a history of providing engineering services related to cooling tower installations and modifications to existing closed-loop cooling systems and was asked by several utility clients to review the feasibility of converting a once-through cooling power plant to closed-loop cooling. 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 (and thus the power plant’s ability to produce power) 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.
Conversion to closed-loop cooling requires tie-in of piping to and from the cooling towers to the existing circulating water system. Depending on how congested the area is between the existing intake structure and the turbine building, tie-in may be a minor or significant design obstacle. The existing circulating water piping can be more than 10 feet in diameter and is typically located underground amongst fire protection piping, electrical ductwork and other underground utilities, so even areas that appear uncongested on the surface may difficult to retrofit. It’s also important to evaluate the impact changes to the circulating water system would have on condenser and overall power plant operation post a closed-loop conversion.
There are many additional design constraints that are evaluated when determining if a closed-loop cooling retrofit is feasible or appropriate for a given site. If mechanical draft cooling towers are being considered, it’s important to account for the power required to operate the cooling tower fans and to pump the water up the cooling tower. If natural draft cooling towers are being considered, it’s important to review zoning restrictions to determine if the tall structures (some greater than 500 feet tall) can be permitted at that location. Cooling towers drawing make-up water from a saltwater or brackish water source will need to evaluate the particulate emissions from the cooling tower drift against local air quality standards to ensure closed-loop operation is feasible. Cooling tower plume, drift, water treatment and blowdown all need to be reviewed to determine any ancillary impacts cooling towers may impart on the local area. If conversion to closed-loop cooling is feasible, construction timelines, cost estimates and permit limitations all need to be reviewed to provide the utility with a clear understanding of what would be necessary should closed-loop cooling be required at that power plant. In summary, conversion to closed-loop cooling involves considerable uncertainty relative to cost and feasibility and often effectively derates the facility due to associated additional power losses.
After completing a review of closed-loop cooling, many clients ask the apparent question, “What other technologies are available to reduce impingement and entrainment?” Many technologies exist that reduce impingement or prevent it from happening, including modified screens capable of returning fish to the water body via a fish return system or increasing the size of the intake structure to reduce the approach velocity to less than 0.5 fps (the velocity at which most fish can swim away from the cooling water intake). While more difficult to reduce than impingement, there are also several technologies that can be applied (alone or together) to reduce entrainment mortality. The primary entrainment reduction technologies can be broken down into five categories: 1) flow reduction from the source water body, 2) fine mesh traveling water screens, 3) relocate intake offshore, 4) boomed mesh barrier surrounding the intake and 5) cylindrical wedgewire (CWW) screens.
Closed-loop cooling is not the only available means to reduce flow from the source water body. Dependent on the intake water temperature and the design of the power plant, variable speed pumps can be utilized to adjust the intake flow on a monthly or even weekly basis. In conjunction with biological monitoring, use of the pumps can be adjusted to the minimum required flow rate for power plant operation during times of high entrainment. While undesirable, a power plant may also select to down power or take an outage during months of high entrainment to reduce mortality. In all of these circumstances, the net result is reduced intake flow through the intake structure, which in turn reduces the entrainment mortality.
|Natural draft cooling towers rely on the “chimney effect” to draw air through the cooling tower to provide cooling. As such they do not require mechanical fans to induce airflow; however, the towers are necessarily very tall and wide at the base to draw in the required airflow. Photo courtesy of Ad Meskens via Wikimedia Commons.|
Power plants with traveling water screens typically use a mesh with 3/8″ openings that does little to reduce entrainment of eggs and larvae. Research has been conducted on the effectiveness of reducing the mesh size significantly (termed fine mesh) to allow for screening of eggs and larvae. Once screened, eggs and larvae can then be returned to the source water body via a fish return system similar to that used for impingement reduction. Fine mesh traveling water screens offer an attractive retrofit solution, but must be fully evaluated against debris loading at the site and the potential need to increase the size and/or number of intake bays to reduce through screen velocity. A careful review of entrainment and impingement mortality during the screening and return process is also necessary.
Intake locations for power plants vary from shoreline to miles offshore. Since the EPA evaluation baseline is for that of a shoreline intake, many power plants may be able to extend their intake to an area with less biological density and, in doing so, reduce biological impacts versus a shoreline intake (i.e., biological density may be reduced in deeper and cooler water with reduced light penetration). By surveying and mapping the local concentrations of aquatic organisms, one can determine if and where a location offshore exists with measurable entrainment reductions due to less biological density. An engineering review of the feasibility and cost of extending the intake to this location can then completed, including a determination of whether the cooler water being drawn in from offshore would benefit power plant performance.
In areas with relatively still water, a boomed mesh barrier can be deployed out from the intake structure. The mesh openings are specified such that the eggs and larvae are kept outside the barrier in the source water body. The boomed barrier length is enlarged to keep the through screen velocity at a minimum. Different cleaning mechanisms exist for a boomed mesh barrier; however, given the sizeable area enclosed by the barrier, debris loading and storm flooding conditions must be thoroughly reviewed before deployment.
In contrast to a boomed mesh barrier, CWW screens work best in areas with a steady ambient flowrate. CWW screens are different from a typical shoreline intake in that they allow aquatic organisms to travel past the screens without being drawn into the power plant. The two ends of the CWW screen are typically solid and divert the ambient flow of water around the screens. As an aquatic organism is carried around the screen, they are restricted from entering the screen by their ability to fit through the screen spacing, their momentum carrying them downstream and their ability to actively maneuver away from the screen’s hydraulic zone of influence.
Given a defined ambient flow rate and depth, the size and number of CWW screens necessary can be determined such that the through screen velocity is less than that of the ambient flow rate. This reduction in through screen velocity can increase the effectiveness of the screen by further allowing aquatic organisms to bypass the screens. Debris loading and frazil ice conditions can restrict use of CWW screens; however, an airburst system can be employed that discharges water and air through the screens for cleaning. CWW screen systems can also be installed with a bypass gate such that a power plant can revert to use of traveling water screens under a severe blockage condition.
There are certainly other entrainment reduction technologies available; however, these provide a good starting point for considering what options may be available for a particular power plant. As with closed-loop cooling, entrainment reduction technologies vary both in effectiveness and in difficulty of installation based on the site-specific conditions at the power plant.
The EPA estimates that roughly 670 power plants and 590 manufacturers and will be affected by the updated CWA §316(b) rule. Out of these 1,260 facilities, many will be converted to closed-loop cooling or will be shut down, many will be determined to be in compliance in their existing condition, and the remainder will need to implement a technology that falls somewhere in between. It is important for each utility to consider all options available and avoid making any decisions assuming either the worst or best case scenarios.
Returning to the original question: what can a utility do to cautiously prepare for the new regulations? A utility can begin efforts to determine the likelihood that their plants will require a compliance technology and, if a technology is required, start work to fully explore all compliance options to determine the most cost effective solution for each specific power plant.
Importantly, the utility also gains an accurate assessment of the projected cost and timeline for implementation of the compliance technology. Given the extremely wide variance in the cost of CWA §316(b) compliance, it’s important to avoid using an industry average cost of compliance, but instead use the time available to consider all options and determine an accurate cost of compliance for each power plant.
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