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| Owned and operated by FirstEnergy Corp., the Bruce Mans-field plant is the largest power plant in Pennsylvania with a capacity of 2,490 MW. The coal-fired plant uses about 70 million gallons of water a day from the Ohio River. |
By John Maulbetsch, Maulbetsch Consulting, and Jeff Stallings, Electric Power Research Institute
Electric power generation uses substantial amounts of fresh, brackish, or saline water, principally for cooling of thermal power plants. In freshwater withdrawals, electric utilities are second only to agriculture.
This use comes at a time of declining supply, particularly in regions of the West and Southwest in the U.S., where drought and population growth have led to present and projected water shortages. In these regions, opposition to power plant siting frequently focuses on the issue of water use. The use of alternative water-conserving cooling systems is frequently proposed and occasionally mandated as a condition for approval.
Moreover, the issue is no longer confined to water-short regions. In many instances, dry cooling has been proposed, or actually used, in regions where water is plentiful or on plants located near large bodies of water. These instances have been driven by regulatory concerns other than water conservation, such as the desire to reduce impacts to aquatic organisms, eliminate any discharge streams, or avoid problems with visible plumes or drift from wet cooling towers.
Some of the alternatives to the conventional cooling approaches offer significant opportunity for water conservation. However, these water savings normally, but perhaps not always, come at a price, in the form of more costly cooling equipment, power requirements, reduced plant efficiency, and limited plant capacity.
The Electric Power Research Institute (EPRI) recently conducted a study to present data, information, and methods that will enable utility decision makers to understand the performance and the economic and environmental trade-offs among the alternative cooling systems and to make the appropriate system choice for a variety of plant types and site locations.
COOLING SYSTEM TYPES
The EPRI study reviewed and compared four major types of cooling systems: once-through cooling, closed-cycle wet cooling, dry cooling (direct and indirect), and hybrid systems.
- Once-Through Cooling. Once-through systems withdraw water from a natural source (typically a lake, river, or ocean), use it to extract waste heat from the steam cycle, and then return it to the water body at a slightly elevated temperature. The systems consist of a steam condenser, typically of the shell-and-tube type, circulating water pumps, circulating water lines, intake and discharge structures, and in most cases, some water treatment equipment, typically chlorination for biofouling control. Through the early 1970s, once-through systems were the systems of choice for steam-electric plants, and today, in the United States, more than 1,200 generating units (about 40% of U.S. capacity) still use these systems. However, their use has been limited or prohibited on the basis of environmental concerns, including thermal discharge, cooling water intake issues of entrainment and impingement, and in-stream flow maintenance. They are now rarely considered for new plants, and, in fact, pressure has developed for the retrofit conversion of some once-through cooled plants to closed-cycle cooling.
- Closed-Cycle Wet Cooling. Closed-cycle (or recirculating) wet cooling systems are similar to once-through cooling in that the steam is condensed in a water-cooled, shell-and-tube steam condenser, but differ in that the heated water is not returned to the environment. Instead the hot water is conveyed to a cooling component, typically a wet cooling tower (other options include cooling ponds, spray-enhanced ponds, spray canals, etc.), where it is cooled and then recirculated to the condenser. Cooling towers are of two types: natural-draft and mechanical-draft. The cooling is accomplished by the evaporation of a small fraction (approximately 1 to 2%) of the water. Some portion of the circulating water flow is discharged as “blowdown” from the system back to the environment to control the build-up of suspended and dissolved solids brought into the cooling system with the make-up water.
- Dry Cooling. Dry cooling systems reject the heat of condensation directly to the atmosphere with no consumptive use of cooling water. Systems are of two types: direct and indirect dry cooling. Direct dry cooling systems utilize air-cooled condensers (ACCs) to which turbine exhaust steam is ducted from the turbine exit through a large horizontal duct to a lower steam header feeding several vertical risers. Dry cooling technology has been used for nearly 70 years, and was pioneered in regions as diverse as Western Europe, South Africa, and the Middle East. Since 1999, nearly 20 GW of new U.S. capacity has utilized dry cooling. Indirect dry cooling systems have a separate condenser, typically of the conventional shell-and-tube type. The heated cooling water leaving the condenser is then circulated to an air-cooled heat exchanger (ACHE) for ultimate heat rejection to the atmosphere. Indirect systems are more costly and less efficient than direct dry cooling system, because of the two-step heat transfer path to the atmosphere, the circulating water pumping power requirement, and the temperature rise of the cooling water as an additional temperature difference between the ambient air and the steam condensing temperature. These systems have seen limited use in Africa and the Middle East, but no indirect, all-dry systems are operating in the United States at this time.
- Hybrid Cooling. Hybrid cooling systems are intended to exploit the virtues of both the wet and dry systems. In hybrid systems, both air-cooled and wet cooling equipment is available for handling the plant heat load as conditions dictate. The two major categories of hybrid cooling systems are plume-abatement systems and water-conservation systems. Plume-abatement towers are essentially all-wet systems that employ an ACHE, which provides some amount of dry cooling, but whose primary function is to provide a flow of heated dry air that can be mixed with the saturated exhaust plume from the wet portion of the system. Water-conservation systems have received increasing interest in recent years, although to date only a few are installed on U.S. power plants. They are intended to reduce the amount of water required for power plant cooling by using dry cooling during the cooler periods of the year and supplementing the dry capability with wet cooling during hotter periods when dry cooling systems cannot maintain a turbine exhaust pressure as low as is desired.
STUDY ASSUMPTIONS AND METHODOLOGY
The EPRI study developed case studies for three plant types (coal, nuclear, and gas-fired combined-cycle) at five different sites (identified below). Alternative cooling systems are configured and optimized for each site.
The study assumed that the comparison of alternative cooling systems would be made between “optimized systems.” An optimized system is normally, but not always, one that minimizes all the cooling-system-related costs over the life of the plant. The optimization (or cost minimization) must also include all costs affected by the choice and performance of the selected cooling system.
The costs in a system optimization consist of: (1) capital costs, (2) O&M costs, and (3) penalty costs. The capital costs encompass the major system components, such as a condenser and a cooling tower, and all the related elements such as the circulating water pumps, the circulating water lines, intake and discharge facilities, and water treatment facilities.
The operating costs include power costs for operating the circulating water pumps and cooling fans and the cost of make-up water to the cooling system. These elements are closely related to the initial design choice. Maintenance costs include the routine inspection and general maintenance activities associated with heat transfer and rotating equipment and water quality control for wet systems, periodic component and structural repair and replacement (mainly for wet systems), and periodic major surface cleaning for dry systems.
Penalty costs capture and quantify the influence that the choice of cooling system has on the efficiency and capacity of the plant.
In the EPRI study, basic cost information was developed for each of the major components of the alternative cooling systems, including surface steam condensers, mechanical-draft wet cooling towers, air-cooled condensers, and water treatment equipment. The information was based on budget price data provided by major vendors of the particular components. Where possible, information was obtained from more than one source. In addition, where possible, the costs were compared with information available from the open literature.
Because hybrid systems are still rare in the United States, no vendor cost information is available. For the purposes of this study, the costs for hybrid systems were estimated by combining the information obtained for the all-wet and all-dry systems.
The study sought to compare cost and performance of alternative power plant cooling systems over a range of meteorological conditions representative of sites across the United States. As a result, five sites in the contiguous 48 states were chosen to represent the range of ambient temperature and humidity that particularly affect the relative merits of the different systems. The site locations were:
- Site 1: Yuma, Arizona (hot, arid conditions, typical of many western and southwestern states)
- Site 2: Jacksonville, Florida (hot humid conditions, typical of many southeastern states)
- Site 3: Bismarck, North Dakota (arid conditions with extreme temperature ranges)
- Site 4: Burlington, Vermont (moderate cool and dry conditions)
- Site 5: St. Louis, Missouri (moderate warm and humid conditions)
For each site and plant, information was developed for the site, the plant, the business environment, and the regulatory environment. Important characteristics included: site elevation, meteorological conditions, water supply, plant type, capacity/heat rate, steam flow/heat load, cooling system requirements, regulated vs. merchant plant, water discharge/disposal, and air emissions.
The following cases were addressed:
- Closed-cycle wet cooling
- – Coal-fired steam plant
- – Nuclear steam plant
- – Gas-fired combined-cycle plant
- Direct dry cooling (ACC)
- – Coal-fired steam plant
- – Gas-fired combined-cycle plant
- Indirect dry cooling (ACHE)
- – Nuclear steam plant
- Hybrid cooling with direct dry section
- – Coal-fired steam plant
- – Gas-fired combined-cycle plant
- Hybrid cooling with indirect dry section
- – Nuclear steam plant
- Indirect dry cooling (ACHE)
FINDINGS
Results of the EPRI study included annualized cooling system costs for wet, dry-direct, dry-indirect, and hybrid systems at five sites for coal, nuclear, and gas-fired combined-cycle plants.
Findings for coal plant cooling system comparisons are shown in the Figure on page 126.
Other representative conclusions of the study are summarized below.
Water Consumption
Recirculating wet cooling systems with a mechanical-draft wet cooling tower significantly reduce (by a factor of 20 to 50 times) the amount of water drawn into a plant compared to plants using once-through cooling, but nearly all the water withdrawn for cooling purposes is evaporated in the process. Water-conserving systems, such as dry cooling using air-cooled condensers or hybrid wet/dry systems using parallel dry and wet condensing loops, can further reduce the water used for cooling.
Typical values for closed-cycle wet cooling systems range from 400 to 700 gallons/MWh.
For combined-cycle plants, because only about one-third of the energy is produced by the steam portion of the plant, the normalized water use on the basis of total plant energy production ranges from about 170 to 250 gallon per plant MWh. For hybrid systems, the amount of water used can be selected by design and is typically chosen to be 30 to 70% of the amount used in all-wet systems.
Capital Costs
Cooling system capital costs for a specified plant at a given site cannot be determined in the absence of a full consideration of performance issues and of the project’s economic and business objectives. The choice of a larger, higher-capacity cooling system will result in higher capital costs, but will provide higher plant output and more efficient operation over the life of the plant.
The capital and operating costs should include the equipment, labor, and expendables costs for all plant elements influenced by the choice of cooling system, such as the cost of water and water supply, treatment, and discharge/disposal facilities.
Wet Systems
In the case of wet systems, the optimized system is determined by the balance between capital cost and the operating power cost for the tower fans and the circulating water pumps. Because wet cooling systems are typically sized to achieve design backpressure at the “1% wet-bulb” condition, the heat rate and capacity penalties are not an important consideration in the design optimization.
Water consumption is essentially the same for all wet system designs at a given site. Therefore, the cost of water is not an important factor for the selection of an optimum wet system design. It is an important factor in comparisons between wet and dry systems.
Dry Systems
The EPRI study considered two types of dry systems: direct dry cooling using an air-cooled condenser, and indirect dry cooling using an ACHE paired with a conventional shell-and-tube steam condenser.
For similar applications, ambient conditions, and design points, the indirect system is more costly with higher capital cost, higher operating power requirements, and greater performance penalties than the corresponding direct system. Direct comparisons were made only for the cases of the coal-fired plant, where the annualized cost ratios were approximately 1.6 to 1.7 at all sites.
Hybrid Systems
As in the case of the all-dry systems, two types of hybrid system were considered: a direct system in which the dry portion uses an air-cooled condenser, and an indirect system in which the dry portion uses an ACHE.
The optimization of a hybrid system with parallel wet and dry steam condensing loops introduces additional complexity. The relative capability of the wet and dry systems is the primary determinant of the system cost. This comparison, in turn, depends both on the amount of water available for cooling, and the value of plant output during the hottest hours of the year compared to the average value over the entire year.
As in the case of the all-dry systems, the indirect hybrid has higher capital and operating costs than the direct system.
Performance
For optimized designs under nearly all conditions, wet cooling systems are not only the least expensive but result in the highest plant output and efficiency. The important issues to be determined are how much more expensive are dry cooling systems, how great are the effects on plant output and efficiency, and what is the cost of those effects.
For hybrid systems, the performance penalty, as well as the system cost, varies depending on the amount of water available. For annual water use ranging from approximately 30 to 80 percent of the amount required for an all-wet system, the costs of the direct hybrid are typically less than those for the all-dry system. For the indirect systems, the hybrid system cost can approach or exceed the cost of the all-dry system. A more complete analysis is required to fully understand the trade-offs for indirect systems.
Water Conservation
The use of either dry or hybrid cooling can result in a large reduction in the amount of water used by a plant. Depending on the plant design and the water required for uses other than cooling, the dry-cooled plant will save from 95 to 75 percent of the water used by a wet-cooled plant. For the cooling alone, recirculating wet cooling at a 500-MW coal-fired steam plant consumes approximately 5,000 to 7,000 acre-feet per year, which is saved by the use of dry cooling. The savings come at a cost of about $7 to $10 million per year depending on the meteorology at the site, which represents a cost of $1,000 to $2,000 per acre-foot of water saved.
References
EPRI. 2012. Economic Evaluation of Alternative Cooling Technologies. Report No. 1024805. Palo Alto, CA. January.
EPRI. 2004. Comparison of Alternate Cooling Technologies for U.S. Power Plants: Economic, Environmental, and Other Tradeoffs. Report No. 1005358. Palo Alto, CA. August.
The authors wish to acknowledge the contributions of the late Chuck McGowin in the completion of this study.
Authors
John Maulbetsch is a private consultant specializing in power plant cooling systems with particular emphasis on water conserving technologies. He worked at EPRI for 23 years, managing research activities in the Environmental Control Systems area and in Exploratory Research. He has SB, SM, and PhD degrees from MIT.
Jeff Stallings is a senior project manager in EPRI’s Generation Sector. He is a registered Professional Engineer with a bachelor’s degree in chemical engineering from Princeton University, a masters in international studies from Johns Hopkins, and an MBA from University of California at Berkeley.
Water Management Research
EPRI recently initiated a Water Management Technology program, which evaluates the performance, operability, reliability, and cost of advanced cooling technologies designed to reduce water withdrawal and consumption while minimizing the impact on plant thermal efficiency. EPRI is also taking a leadership role in the new Water Research Center (WRC), being developed by Georgia Power in collaboration with EPRI and 16 electric generating companies. The Center is a first-of-its-kind industrywide resource for conducting power company water research. Located at Georgia Power’s Plant Bowen, the WRC will provide insights on best practices for sustainable water management and meeting wastewater restrictions. Electric generating companies, research organizations, and vendors will have access to full-scale infrastructure, treatable water, monitoring and analysis facilities, and specialist staff to enable plant-based water research. Research projects will include advanced cooling water technologies, biological and inorganic wastewater treatments, zero-liquid-discharge options, solid landfill water management, and water conservation.
