Electric power technology requires dependable access to massive volumes of water. This need persists at a time of declining provide, when regions of the world are experiencing water constraints as a consequence of inhabitants growth, precipitation fluctuations, and altering demand patterns. Water constraints might affect future electricity era expertise choice, plant siting, and plant operation.
Though water wants are plant specific, for most pulverized coal-fired energy plants over 90% of water demand is drawn for cooling. Because of this, the Electric Energy Analysis Institute (EPRI) and other analysis organizations worldwide are in search of to optimize power plant water utilization by growing applied sciences to reduce the largest single use—cooling. Thermal energy plants can not function with out satisfactory cooling; steam from the electricity era turbine have to be cooled to reduce again strain on the turbine. Improved cooling allows for energy plants to function at an overall higher efficiency.
Most present power plant cooling methods within the U.S. are based on wet cooling applied sciences. Various dry cooling technologies that scale back water consumption are available and becoming extra prevalent. Nevertheless, these applied sciences usually include financial tradeoffs (larger capital expenses, increased operational/upkeep costs) and steam-condensing performance penalties.
Advanced cooling technology development, therefore, is targeted on analysis to enhance the efficiency of present cooling applied sciences and to discover strategies, designs, and functions that reduce the financial disadvantages and enhance the efficiency associated with different applied sciences. Different essential concerns for enhancements embody lowering the scale or footprint of cooling programs, using alternative coolants as an alternative of potable water, and enhancing condensation, evaporation, and smart heat switch mechanisms.
Power plant cooling applied sciences generally embody four different types: once-by way of cooling, recirculated wet cooling, dry cooling, and hybrid cooling.
As soon as-By way of Cooling
As soon as-by cooling (OTC) techniques withdraw water from a pure water body (comparable to a lake, river, ocean, or manmade reservoir). The water is pumped by means of the tubes of a steam condenser (see Figure 1 for a schematic) the place it is warmed about 100°F (87°C) relying on system design, after which it is returned to the unique source. The quantity withdrawn varies from 25,0000,000 gallons/MWh (9590 m3/MWh). Although not one of the water is consumed inside the plant, some consumptive loss results on account of evaporation from the receiving water physique due to the elevated temperature of the discharge. The amount of water lost attributable to evaporation is tough to precisely calculate due to site-specific elements (e.g., temperature differential, wind pace, ambient humidity), however it has been variously estimated as 0.5% of the withdrawn quantity, or 10000 gallons/MWh (zero.38.5 m3/MWh).
Determine 1. Water movement in as soon as-via cooling
OTC is just not with out environmental influence points. Withdrawal of water could 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 move through intake screens and enter a plant’s cooling system, the place they can expertise a high mortality price as a result of thermal and bodily stresses. The discharge of heated water may also lead to negative environmental impacts on the aquatic neighborhood, together with habitat.
Recirculating cooling techniques can scale back impingement and entrainment by as much as 90% or extra, however their price can make the choice problematic for some energy plants. Much less expensive protection expertise options (e.g., fish-pleasant touring water screens together with superb mesh, barrier nets, velocity caps, behavioral deterrence, wedge wire screens) can attain comparable performance depending on site-particular hydraulic, biological, and plant working traits.
Recirculated Wet Cooling
Recirculated wet cooling is just like OTC in that chilly water flows by way of the tubes of a steam condenser and steam condenses on the skin of the tubes. However, as an alternative of being returned to the supply, the heated water leaving the condenser is pumped to a cooling device corresponding to a tower, pond, or canal, where it is cooled by evaporation of a small portion of the water. The cooled water is then recirculated again to the condenser tube inlets (see Figure 2). Cooling towers contain 95% less water withdrawal than OTC systems, however usually are less efficient because of the higher parasitic load of the fans (mechanical draft towers) and the higher condensing temperatures compared to OTC.
Figure 2. Water circulation in a wet cooling tower
Cooling towers do not increase the temperature of the water supply, however they do eat more water than OTC. Water is lost via evaporation (vital 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 material), and drift (i.e., less than zero.0005% of the water is misplaced as droplets are entrained and carried out of the tower).
Although water withdrawal is lowered, recirculated wet cooling systems have a number of cost- and energy effectivity-related disadvantages in comparison with OTC: 1) capital costs are sometimes twice as a lot as OTC, 2) they sometimes have increased parasitic load for the followers, and 3) they’ve a potential for energy generation capability reductions on sizzling days.
Dry cooling methods might be both direct or oblique. Direct dry cooling systems condense turbine exhaust steam in an air-cooled condenser (ACC) (see Determine three). Indirect dry cooling programs make the most of 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 earlier than being returned to the condenser.
Determine three. Direct dry cooling
Although dry cooling achieves vital water financial savings, the capital prices are up to five occasions costlier than recirculating wet cooling. Additionally, the condensing temperature, within the case of direct dry cooling, or the chilly water temperature, in the case of oblique dry cooling, is proscribed by the ambient temperature and humidity. Consequently, dry cooling techniques can produce as much as 105% much less energy throughout the most well liked days of the yr, when the steam condensing temperature (and hence the turbine exhaust pressure) is substantially higher than it could be with wet cooling.
Hybrid cooling refers to cooling programs with both dry and wet cooling elements, which are used individually or collectively to realize one of the best features of each: that’s, the wet cooling efficiency on the most well liked days of the 12 months and the water conservation functionality of dry cooling through the remainder of the year. Hybrid methods have the potential for greater than 50% water financial savings in comparison with wet cooling towers (see Figure four).
Determine 4. Hybrid cooling
The disadvantage to hybrid cooling is that it can be more expensive compared to recirculated wet cooling towers alone, and significant quantities of water should be wanted, particularly during the summer time. A hybrid system will also be subject to all the operation and maintenance problems with each cooling methods (e.g., fan energy, blowdown, cooling water treatment, freeze protection). Subsequently, it is most fitted for websites the place conservation is required, however some water is still out there for partial evaporative cooling to shave scorching-day effectivity penalties.
Advanced Cooling R&D
Research and development on advanced cooling technology for energy plants is concentrated on a number of targets. For lowering water consumption in wet cooling techniques, analysis is aimed at much less evaporative loss in cooling towers, more environment friendly and compact liquid-cooled heat exchangers or condensers, and more efficient as soon as-by means of cooling designs. For dry cooling systems, analysis has targeted on reducing condensing temperatures by enhancing the air-aspect heat switch coefficient without significantly rising ACC dimension or air-aspect pressure drop (fan horsepower), and creating improved strategies for management of stream-assisted corrosion contained in the tubes.
In this arena, EPRI is pursuing early-stage, excessive-threat ideas and creating advanced applied sciences with sport-altering potential for decreasing freshwater withdrawal and consumption and improving vitality conversion efficiency at current energy plants.
Since 2011, the Water Use and Availability Program within EPRI’s Technology Innovation Program has launched three global Request for Information solicitations and carried out innovation scouting to assist determine ideas with breakthrough potential. Among the 168 proposals acquired so far, 12 tasks have been initiated, involving wet, dry, and hybrid cooling technologies. In addition, EPRI has lately consolidated ongoing analysis efforts into a Water Administration Expertise Program, which conducts superior analysis across several fronts to improve energy plant water use efficiency, decrease withdrawal charges, and cut back pollutant discharges.
A significant focal point for future analysis is a brand new Water Analysis Middle (WRC), at Georgia Power’s Plant Bowen, a 3500-MW coal-fired plant. This first-of-its-variety, trade-large resource affords a pilot-scale infrastructure for conducting scaled-up, plant-primarily based water research. The WRC offers electric generating corporations, research organizations, and vendors with entry to a discipline demonstration facility that has treatable water, monitoring and analysis amenities, and specialist staff. It’s hoped that analysis performed on the WRC will uncover insights on greatest practices for sustainable water management and assembly wastewater restrictions.
EPRI research on superior cooling contains the next select technology investigations.
Thermosyphon Cooler System
Hybrid cooling systems usually incorporate typical wet cooling towers and air-cooled condensers, with the latter operating nearly all of the time and the former employed to mitigate efficiency penalties at high ambient temperatures. A novel hybridization idea, developed by Johnson Controls, applies a dry-heat-rejection technology, called thermosyphon cooling (TSC), which was initially developed for house conditioning in buildings. TSC units, consisting of an evaporator and an air-cooled condenser, pre-cool the new water from the steam condenser prior to the wet cooling tower.
By lowering 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 seventy five% with out sacrificing electrical output on the most popular summer days. Relative to different dry cooling choices, TSC technology promises easier, extra versatile, decrease-value 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 efficiency relative to different cooling configurations for conceptual 500-MW plants at five U.S. places with differing climates. Additionally, a pilot-scale system, incorporating a 1-MW equivalent dimension TSC unit and cooling tower, is being tested at the Water Research Heart. The mission will decide how much water may be saved by operating a TSC unit in series with a traditional wet cooling tower. Researchers may also determine the power penalty incurred and the most effective means for scale-up.
Dew-Level Cooling Tower
The chilly water return temperature of traditional recirculating wet cooling towers will be limited by the temperature and humidity of the ambient air. To address this difficulty, EPRI, in collaboration with the Gas Expertise Institute (GTI), is investigating an idea known as dew-point cooling to try to cut back the cold water return temperature additional. This know-how enhances the standard tower performance by constructing dry channels between wet channels within the tower, with a thin-walled fill material, and exploiting evaporative cooling on the wet aspect of the fill to cool the ambient air passing over the dry facet. This pre-cooled air is then used for contact evaporative cooling with the condenser water.
Dew-level cooling offers the potential to improve the water efficiency in addition to the general effectivity of thermal energy plants with conventional wet and hybrid wet-dry cooling towers. Preliminary evaluations indicate that tower fill replacements that permit the pre-cooling of ambient air may considerably scale back evaporative losses and makeup water necessities.
The Eco-WD Cooler (wet-dry cooling tower), developed by EVAPCO, has the potential to conserve water and power at power plants by employing an modern wet-dry cooling tower expertise.
This cooling tower technology works in wet-dry mode during the recent summer time months and in dry mode the remainder of the year. In wet-dry mode, scorching 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 know-how could be easily retrofit to plants at present using all-wet cooling.
Analysis at the moment underway on the WRC is gathering efficiency and operation knowledge under various loads and year-round weather conditions, and is demonstrating the cooler’s means to conserve water and energy and to scale back plume visibility.
Hydrophobic Condenser Tube Floor Remedy
The design of steam condensers is predicated on filmwise condensation, since condensing steam will type a water layer on the floor of the condenser tube. This movie of condensed water acts as an additional barrier to the heat switch process. Vital enhancement of the heat switch effectivity will be achieved by forcing the condensate to bead up into droplets, which might be swept off the floor by the steam movement, a process known as dropwise condensation. However, thus far, there has not been a dependable technique of generating dropwise condensation under industrial conditions for long durations, for the reason that required coatings and surface modifications deteriorate with use.
NEI Company has developed a hydrophobic floor treatment, known as SuperCNTM, which has proven potential for promoting dropwise condensation in industrial condensers. The remedy results in a durable, micron-thick coating on condenser tubes, resulting in dropwise condensation. Analysis currently underway is investigating the applying characteristics and customer incentives of the hydrophobic floor treatment expertise.
Outcomes point out that the hydrophobic coating might be utilized to the shell side of an current, in-place heat exchanger with a flow coating methodology in an economical approach. 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 situations of steady condensation and ammonia vapor conditioning. A cost–benefit analysis of the coating technology also instructed that potential savings are available from the applying of hydrophobic coating to floor condenser tubes.
Hybrid Dry/Wet Dephlegmator
The chief disadvantages of dry cooling systems are energy capability reductions and efficiency penalties during periods with hot temperatures. EPRI is sponsoring research on the University of Stellenbosch in South Africa to address this subject. The research is targeted on creating a new design for the a part of an ACC known as the dephlegmator, which provides a secondary condenser that facilitates vapor movement by the first condensers, and flushing them of any non-condensable gases.
This research challenge proposes to develop a novel hybrid (dry/wet) dephlegmator (HDWD) that will exchange the typical all-dry dephlegmator unit in an ACC. The HDWD consists of two stages; the working mode of the second stage can be controlled in response to changing ambient circumstances. Throughout periods of low ambient temperature, when air cooling is adequate, the second stage is operated dry. During hotter durations, deluge water is sprayed over the plain tubes, and the second stage is operated as an evaporative condenser.
It is believed that this know-how has the potential to extend power manufacturing on the hottest days as in comparison with standard ACCs. It might additionally use much less makeup water than wet cooling tower systems and less water than presently used by dry cooling with evaporative pre-cooling of the inlet air.
The EPRI venture aims to further develop the design idea, perform modeling and experimental investigations of varied options, and conduct technical and financial feasibility research.
Collaborative Alternatives with EPRI
In 2013, EPRI and the National Science Foundation (NSF) launched a joint solicitation to advance dry and dry-wet hybrid cooling applied sciences for power plant applications. The project is a US$6 million joint collaboration, which aims to attract the top talent to energy plant cooling innovation and fund 5 to 10 tasks. Award notifications will likely be introduced in early 2014.
EPRI can also be collaborating with its home and international utility members on its superior cooling analysis. For example, in 2013, Eskom hosted an EPRI tour of the utility’s dry cooling services in South Africa. The EPRI staff visited working indirect (Kendal) and direct (Matimba) dry cooling programs in addition to new state-of-the-art ACC techniques beneath development at Medupi and Kusile. These experiences provided by Eskom served as an vital benchmark in setting standards for researching the next era of dry cooling technologies.
Case Study: Eskom’s Utility of Dry Cooling Expertise
In South Africa, with its traditionally scarce water sources, Eskom has been a technological chief in dry-cooled coal-fired energy 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 technique of constructing new dry-cooled plants at Medupi and Kusile. In 2010011, the Eskom fleet consumed a complete of 327 million m3 of water for power generation. With out innovative, efficient cooling systems in place, the consumption would have been 530 million m3.
The 4800-MW Medupi power station, which is now beneath development, will become the world’s largest direct-dry-cooled energy plant as soon as it’s positioned into operation.
Matimba Power Station has six units with a complete installed capability of about 4000 MW. Using direct dry cooling, the plant reduces water consumption to about zero.1 L/kWh (0.1 m3/MWh). This stage is roughly 19 instances less than an equal wet-cooled power plant. Matimba makes use of about three.5 million m3 of water per year, in comparison with an equal wet-cooled energy plant, which might use 50 million m3.
Medupi Power Station, at the moment below development, will surpass Matimba as the most important direct-dry-cooled plant. Medupi could have six items with a complete put in capability of roughly 4800 MW. The footprint of the ACC at Medupi is 108 × 669 m, or the equal of 10 soccer fields. The design at Medupi incorporates a number of lessons learned at Matimba, including extended spacing between the ACC and turbine corridor to reduce impacts from wind.
Kendal Energy Station has six items with a total installed capability of about 4116 MW utilizing oblique dry cooling. The plant employs six natural-draft dry-cooling towers, each 165 m tall. Water from a typical floor condenser is circulated to the towers, the place it enters a series of heat-alternate elements at the bottom of the cooling shell. Air enters the underside periphery of the towers, is heated by passing over the heat-trade parts, and rises in the tower, pulling in cooler ambient air from the underside. The system does not require followers. Water consumption for the ability plant is about 0.08 L/kWh (0.08 m3/MWh).
Eskom calculates a number of significant energy and price penalties for dry cooling. One penalty entails elevated energy demand for cooling followers. At every of Matimba’s six items, the dry cooling system uses 48 fans which might be 30 ft (~10 m) in diameter. Fan operation corresponds to an auxiliary energy demand of 72 MW, or 2% of the plant’s complete generating capacity. In addition, era performance at a dry-cooled plant is sensitive to meteorological circumstances. Specifically, excessive ambient temperature and high winds can result in reductions of generating capability of as much as 105%.
Weakening the Energy/Water Relationship
As power and water demand develop, there may be a tremendous incentive to scale back the water required for energy generation. Cooling water is currently the biggest draw of water for thermal power plants, so it has been the focus of a large amount of water-saving technology development; the tasks mentioned in this article symbolize solely a sampling of the continuing efforts globally.