By Sean Bushart
Senior Program Manager, Electric Power Research Institute
Electric power generation requires reliable access to large volumes of water. This need persists at a time of declining supply, when regions of the world are experiencing water constraints due to population growth, precipitation fluctuations, and changing demand patterns. Water constraints could affect future electricity generation technology selection, plant siting, and plant operation.
Although water needs are plant specific, for most pulverized coal-fired power plants over 90% of water demand is drawn for cooling. As a result, the Electric Power Research Institute (EPRI) and other research organizations worldwide are seeking to optimize power plant water utilization by developing technologies to reduce the largest single use—cooling. Thermal power plants cannot operate without adequate cooling; steam from the electricity generation turbine must be cooled to minimize back pressure on the turbine. Improved cooling allows for power plants to operate at an overall higher efficiency.
Most existing power plant cooling systems in the U.S. are based on wet cooling technologies. Alternative dry cooling technologies that reduce water consumption are available and becoming more prevalent. However, these technologies often come with economic tradeoffs (higher capital expenses, increased operational/maintenance costs) and steam-condensing performance penalties.
Advanced cooling technology development, therefore, is focused on research to improve the efficiency of existing cooling technologies and to discover techniques, designs, and applications that reduce the economic disadvantages and improve the efficiency associated with alternative technologies. Other important considerations for improvements include reducing the size or footprint of cooling systems, utilizing alternative coolants instead of potable water, and enhancing condensation, evaporation, and sensible heat transfer mechanisms.
Power plant cooling technologies generally include four different types: once-through cooling, recirculated wet cooling, dry cooling, and hybrid cooling.
Once-through cooling (OTC) systems withdraw water from a natural water body (such as a lake, river, ocean, or manmade reservoir). The water is pumped through the tubes of a steam condenser (see Figure 1 for a schematic) where it is warmed about 10–30°F (8–17°C) depending on system design, after which it is returned to the original source. The amount withdrawn varies from 25,000–50,000 gallons/MWh (95–190 m3/MWh). Although none of the water is consumed within the plant, some consumptive loss results due to evaporation from the receiving water body because of the increased temperature of the discharge. The amount of water lost due to evaporation is difficult to accurately calculate because of site-specific factors (e.g., temperature differential, wind speed, ambient humidity), but it has been variously estimated as 0.5–2% of the withdrawn amount, or 100–400 gallons/MWh (0.38–1.5 m3/MWh).
OTC is not without environmental impact issues. Withdrawal of water can cause impingement and/or entrapment and mortality of fish and shellfish on intake screens, while smaller organisms (e.g., small eggs, larvae, juvenile fish, and shellfish) can pass through intake screens and enter a plant’s cooling system, where they can experience a high mortality rate due to the thermal and physical stresses. The discharge of heated water can also lead to negative environmental impacts on the aquatic community, including habitat.
Recirculating cooling systems can reduce impingement and entrainment by as much as 90% or more, but their cost can make the option problematic for some power plants. Less costly protection technology alternatives (e.g., fish-friendly traveling water screens including fine mesh, barrier nets, velocity caps, behavioral deterrence, wedge wire screens) can attain similar performance depending on site-specific hydraulic, biological, and plant operating characteristics.
Recirculated Wet Cooling
Recirculated wet cooling is similar to OTC in that cold water flows through the tubes of a steam condenser and steam condenses on the outside of the tubes. However, instead of being returned to the source, the heated water leaving the condenser is pumped to a cooling device such as a tower, pond, or canal, where it is cooled by evaporation of a small portion of the water. The cooled water is then recirculated back to the condenser tube inlets (see Figure 2). Cooling towers involve 95% less water withdrawal than OTC systems, but typically are less efficient due to the higher parasitic load of the fans (mechanical draft towers) and the higher condensing temperatures compared to OTC.
Cooling towers do not increase the temperature of the water source, but they do consume more water than OTC. Water is lost through evaporation (necessary to reduce the temperature of the water so it can be recirculated), blowdown (i.e., removal of a fraction of the recirculating water to manage the mineral content), and drift (i.e., less than 0.0005% of the water is lost as droplets are entrained and carried out of the tower).
Although water withdrawal is reduced, recirculated wet cooling systems have several cost- and energy efficiency-related disadvantages compared to OTC: 1) capital costs are typically twice as much as OTC, 2) they typically have higher parasitic load for the fans, and 3) they have a potential for power generation capacity reductions on hot days.
Dry cooling systems can be either direct or indirect. Direct dry cooling systems condense turbine exhaust steam in an air-cooled condenser (ACC) (see Figure 3). Indirect dry cooling systems utilize a cooling water loop to condense turbine steam in a conventional surface condenser or a contact condenser (i.e., Heller system). The cooling water, which has been heated by the condensing steam, is then recirculated to an air-cooled heat exchanger before being returned to the condenser.
Although dry cooling achieves significant water savings, the capital costs are up to five times more expensive than recirculating wet cooling. Also, the condensing temperature, in the case of direct dry cooling, or the cold water temperature, in the case of indirect dry cooling, is limited by the ambient temperature and humidity. As a result, dry cooling systems can produce up to 10–15% less power during the hottest days of the year, when the steam condensing temperature (and hence the turbine exhaust pressure) is substantially higher than it would be with wet cooling.
Hybrid cooling refers to cooling systems with both dry and wet cooling elements, which are used individually or together to achieve the best features of each: that is, the wet cooling performance on the hottest days of the year and the water conservation capability of dry cooling during the remainder of the year. Hybrid systems have the potential for more than 50% water savings compared to wet cooling towers (see Figure 4).
The drawback to hybrid cooling is that it can be more expensive compared to recirculated wet cooling towers alone, and significant amounts of water may still be needed, particularly during the summer. A hybrid system will also be subject to all of the operation and maintenance issues of both cooling systems (e.g., fan power, blowdown, cooling water treatment, freeze protection). Therefore, it is most suitable for sites where conservation is required, but some water is still available for partial evaporative cooling to shave hot-day efficiency penalties.
Advanced Cooling R&D
Research and development on advanced cooling technology for power plants is focused on several targets. For reducing water consumption in wet cooling systems, research is aimed at less evaporative loss in cooling towers, more efficient and compact liquid-cooled heat exchangers or condensers, and more efficient once-through cooling designs. For dry cooling systems, research has focused on reducing condensing temperatures by improving the air-side heat transfer coefficient without significantly increasing ACC size or air-side pressure drop (fan horsepower), and developing improved methods for control of flow-assisted corrosion inside the tubes.
In this arena, EPRI is pursuing early-stage, high-risk concepts and developing advanced technologies with game-changing potential for reducing freshwater withdrawal and consumption and improving energy conversion efficiency at existing power plants.
Since 2011, the Water Use and Availability Program within EPRI’s Technology Innovation Program has released three global Request for Information solicitations and conducted innovation scouting to help identify ideas with breakthrough potential. Among the 168 proposals received to date, 12 projects have been initiated, involving wet, dry, and hybrid cooling technologies. In addition, EPRI has recently consolidated ongoing research efforts into a Water Management Technology Program, which conducts advanced research across several fronts to improve power plant water use efficiency, decrease withdrawal rates, and reduce pollutant discharges.
A major focal point for future research is a new Water Research Center (WRC), at Georgia Power’s Plant Bowen, a 3500-MW coal-fired plant. This first-of-its-kind, industry-wide resource offers a pilot-scale infrastructure for conducting scaled-up, plant-based water research. The WRC provides electric generating companies, research organizations, and vendors with access to a field demonstration facility that has treatable water, monitoring and analysis facilities, and specialist staff. It is hoped that research conducted at the WRC will uncover insights on best practices for sustainable water management and meeting wastewater restrictions.
EPRI research on advanced cooling includes the following select technology investigations.
Thermosyphon Cooler System
Hybrid cooling systems typically incorporate conventional wet cooling towers and air-cooled condensers, with the latter operating the majority of the time and the former employed to mitigate performance penalties at high ambient temperatures. A novel hybridization concept, developed by Johnson Controls, applies a dry-heat-rejection technology, called thermosyphon cooling (TSC), which was originally developed for space conditioning in buildings. TSC units, consisting of an evaporator and an air-cooled condenser, pre-cool the hot water from the steam condenser prior to the wet cooling tower.
By reducing the heat load on the cooling tower, TSC hybrid systems have the potential to reduce annual evaporative losses, makeup water requirements, and blowdown volumes by up to 75% without sacrificing electrical output on the hottest summer days. Relative to other dry cooling options, TSC technology promises easier, more flexible, lower-cost integration at existing plants and in new builds in incremental, modular sections, with minimal plant outages required.
Ongoing design and modeling research is addressing issues of scale-up, cooling tower integration, and cost and performance relative to other cooling configurations for conceptual 500-MW plants at five U.S. locations with differing climates. Also, a pilot-scale system, incorporating a 1-MW equivalent size TSC unit and cooling tower, is being tested at the Water Research Center. The project will determine how much water can be saved by operating a TSC unit in series with a conventional wet cooling tower. Researchers will also determine the energy penalty incurred and the most effective means for scale-up.
Dew-Point Cooling Tower
The cold water return temperature of traditional recirculating wet cooling towers can be limited by the temperature and humidity of the ambient air. To address this issue, EPRI, in collaboration with the Gas Technology Institute (GTI), is investigating a concept called dew-point cooling to attempt to reduce the cold water return temperature further. This technology enhances the standard tower performance by constructing dry channels between wet channels in the tower, with a thin-walled fill material, and exploiting evaporative cooling on the wet side of the fill to cool the ambient air passing over the dry side. This pre-cooled air is then used for contact evaporative cooling with the condenser water.
Dew-point cooling offers the potential to improve the water efficiency as well as the overall efficiency of thermal power plants with conventional wet and hybrid wet-dry cooling towers. Preliminary evaluations indicate that tower fill replacements that allow the pre-cooling of ambient air could significantly reduce evaporative losses and makeup water requirements.
The Eco-WD Cooler (wet-dry cooling tower), developed by EVAPCO, has the potential to conserve water and energy at power plants by employing an innovative wet-dry cooling tower technology.
This cooling tower technology works in wet-dry mode during the hot summer months and in dry mode the remainder of the year. In wet-dry mode, hot water is initially cooled through air-cooled heat exchangers and further cooled through heat exchanger bundles sprayed with treated water. In dry mode, the spray system is turned off, and the system uses no water for evaporative cooling. In addition, the Eco-WD Cooler has a limited visible condensate plume in wet-dry mode and no visible plume in dry mode. The technology could be easily retrofit to plants currently using all-wet cooling.
Research currently underway at the WRC is gathering performance and operation data under varying loads and year-round weather conditions, and is demonstrating the cooler’s ability to conserve water and energy and to reduce plume visibility.
Hydrophobic Condenser Tube Surface Treatment
The design of steam condensers is based on filmwise condensation, since condensing steam will form a water layer on the surface of the condenser tube. This film of condensed water acts as an additional barrier to the heat transfer process. Significant enhancement of the heat transfer efficiency can be achieved by forcing the condensate to bead up into droplets, which can be swept off the surface by the steam flow, a process known as dropwise condensation. However, to date, there has not been a reliable means of generating dropwise condensation under industrial conditions for long periods, since the required coatings and surface modifications deteriorate with use.
NEI Corporation has developed a hydrophobic surface treatment, called SuperCNTM, which has shown potential for promoting dropwise condensation in industrial condensers. The treatment results in a durable, micron-thick coating on condenser tubes, leading to dropwise condensation. Research currently underway is investigating the application characteristics and customer incentives of the hydrophobic surface treatment technology.
Results indicate that the hydrophobic coating can be applied to the shell side of an existing, in-place heat exchanger with a flow coating method in a cost-effective way. The coating was shown to have significantly better abrasion and scratch resistance than a pristine stainless steel substrate. In testing, the coated tube maintained its high hydrophobicity after three months of durability testing, with alternating conditions of continuous condensation and ammonia vapor conditioning. A cost–benefit analysis of the coating technology also suggested that potential savings are available from the application of hydrophobic coating to surface condenser tubes.
Hybrid Dry/Wet Dephlegmator
The chief disadvantages of dry cooling systems are power capacity reductions and efficiency penalties during periods with hot temperatures. EPRI is sponsoring research at the University of Stellenbosch in South Africa to address this issue. The research is focused on developing a new design for the part of an ACC called the dephlegmator, which provides a secondary condenser that facilitates vapor flow through the primary condensers, and flushing them of any non-condensable gases.
This research project proposes to develop a novel hybrid (dry/wet) dephlegmator (HDWD) that would replace the conventional all-dry dephlegmator unit in an ACC. The HDWD consists of two stages; the operating mode of the second stage can be controlled in response to changing ambient conditions. During periods of low ambient temperature, when air cooling is sufficient, the second stage is operated dry. During hotter periods, deluge water is sprayed over the plain tubes, and the second stage is operated as an evaporative condenser.
It is believed that this technology has the potential to increase power production on the hottest days as compared to conventional ACCs. It would also use less makeup water than wet cooling tower systems and less water than currently used by dry cooling with evaporative pre-cooling of the inlet air.
The EPRI project aims to further develop the design concept, perform modeling and experimental investigations of various options, and conduct technical and economic feasibility studies.
Collaborative Opportunities with EPRI
In 2013, EPRI and the National Science Foundation (NSF) released a joint solicitation to advance dry and dry-wet hybrid cooling technologies for power plant applications. The project is a US$6 million joint collaboration, which aims to attract the top talent to power plant cooling innovation and fund five to 10 projects. Award notifications will be announced in early 2014.
EPRI is also collaborating with its domestic and international utility members on its advanced cooling research. For example, in 2013, Eskom hosted an EPRI tour of the utility’s dry cooling facilities in South Africa. The EPRI team visited operating indirect (Kendal) and direct (Matimba) dry cooling systems as well as new state-of-the-art ACC systems under construction at Medupi and Kusile. These experiences provided by Eskom served as an important benchmark in setting standards for researching the next generation of dry cooling technologies.
Case Study: Eskom’s Application of Dry Cooling Technology
In South Africa, with its historically scarce water resources, Eskom has been a technological leader in dry-cooled coal-fired power plants for more than 30 years.
The utility operates both the world’s largest direct-dry-cooled (Matimba Power Station) and indirect-dry-cooled (Kendal Power Station) plants. All fossil-fueled new-build Eskom power plants are dry cooled, and the utility is in the process of constructing new dry-cooled plants at Medupi and Kusile. In 2010–2011, the Eskom fleet consumed a total of 327 million m3 of water for power generation. Without innovative, efficient cooling systems in place, the consumption would have been 530 million m3.
Matimba Power Station has six units with a total installed capacity of about 4000 MW. Using direct dry cooling, the plant reduces water consumption to about 0.1 L/kWh (0.1 m3/MWh). This level is approximately 19 times less than an equivalent wet-cooled power plant. Matimba uses about 3.5 million m3 of water per year, compared to an equivalent wet-cooled power plant, which would use 50 million m3.
Medupi Power Station, currently under construction, will surpass Matimba as the largest direct-dry-cooled plant. Medupi will have six units with a total installed capacity of approximately 4800 MW. The footprint of the ACC at Medupi is 108 × 669 m, or the equivalent of 10 football fields. The design at Medupi incorporates a number of lessons learned at Matimba, including extended spacing between the ACC and turbine hall to minimize impacts from wind.
Kendal Power Station has six units with a total installed capacity of about 4116 MW utilizing indirect dry cooling. The plant employs six natural-draft dry-cooling towers, each 165 m tall. Water from a standard surface condenser is circulated to the towers, where it enters a series of heat-exchange elements at the base of the cooling shell. Air enters the bottom periphery of the towers, is heated by passing over the heat-exchange elements, and rises in the tower, pulling in cooler ambient air from the bottom. The system does not require fans. Water consumption for the power plant is about 0.08 L/kWh (0.08 m3/MWh).
Eskom calculates a number of significant energy and cost penalties for dry cooling. One penalty involves increased power demand for cooling fans. At each of Matimba’s six units, the dry cooling system uses 48 fans that are 30 ft (~10 m) in diameter. Fan operation corresponds to an auxiliary power demand of 72 MW, or 2% of the plant’s total generating capacity. In addition, generation performance at a dry-cooled plant is sensitive to meteorological conditions. In particular, high ambient temperature and high winds can result in reductions of generating capacity of up to 10–15%.
Weakening the Energy/Water Relationship
As energy and water demand grow, there is a tremendous incentive to reduce the water required for energy generation. Cooling water is currently the largest draw of water for thermal power plants, so it has been the focus of a large amount of water-saving technology development; the projects discussed in this article represent only a sampling of the ongoing efforts globally.
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