General guidelines for selecting the appropriate power-plant cooling system are presented here
The main purpose of a power plant is to convert a source of energy into electric power. Most power plants rely on heat as the energy source (thermal power plants). Thermal power plants burn fossil fuel (coal, oil or natural gas) or use nuclear fission, and are based on the principle of heat engines. An engine is a thermodynamic cycle that produces a net power output by supplying heat from a high temperature reservoir (heat source), and rejecting heat to a low temperature reservoir (heat sink).
The heat sink is a fundamental element of any thermodynamic cycle in an electrical-power-generating plant. The point of interaction between the plant cycle and the heat sink is usually the main condenser. The critical phase in the power plant energy-conversion process is the conversion from heat into mechanical energy by means of the heat engine. A fundamental efficiency expression for this conversion is the Carnot (ideal) efficiency, given by Equation (1):
Where T1 is the temperature (in Kelvin) of the heat source and T2 is the temperature (in Kelvin) of the heat sink. The heat source temperature T1 is governed by the choice of the fuel and technological constraints on the combustion or reactor processes. The focus of this article is the selection of the cooling system which has a direct impact on the steam condensing temperature T2 (temperature of the heat sink) and is therefore related to the conversion efficiency and the steam turbine output.
Heat sink and cooling systems
The heat sink is one of the major components in the steam cycle, and its major purpose is to reject the heat duty of the steam condenser to the atmosphere. The cooling system performance is directly related to the steam turbine backpressure, which determines the steam-turbine generator output, in such a way that higher values of the steam turbine backpressure correspond with lower values of the steam turbine generator output and vice versa. The steam turbine backpressure is a function of the heat sink (or condensing) temperature, since saturated steam conditions typically exist at the low-pressure (LP) steam turbine exhaust.
Open cycle (or once-through) cooling systems usually have the lowest heat sink temperature and therefore the highest efficiency and output. Thus, these type of cooling systems have traditionally been the choice of power plant designers until the 1970s, particularly for power plants that can be located nearby a vast body of water (sea, lake or river). However, once-through systems have large water withdrawal requirements (up to 60 m3/s for a 1,000-MWe plant) and are characterized by significant environmental impacts (for example, thermal pollution and water intake entrainment). In view of increasingly stringent environmental laws and regulations and the decrease in cooling water availability, a significant shift in the type of cooling system has occurred in the last thirty years toward closed-cycle cooling systems.
The disadvantages of closed-cycle cooling lie mostly in the higher capital and operating costs. Moreover, lower thermal efficiency due to higher heat-sink temperatures leads to greater fuel use per unit of generated power and more waste heat produced. In addition, other environmental impacts including fogging, noise, land-use, drift, chemical blowdown, aesthetics and so on may be encountered. While water withdrawal can be decreased substantially with closed-cycle cooling systems, there is increased water consumption (by evaporation, drift, and so on) for wet evaporative cooling towers or reduced power output during hot summer operating conditions, particularly for the closed-cycle dry cooling systems.
Closed-cycle cooling systems for power plant heat sinks can be classified according to their water make-up requirements, with dry and wet cooling systems on opposite sides of the spectrum, as shown in Figure 1.
- Dry cooling systems: heat is rejected to the atmosphere using ambient air, so in theory, there is no make-up water required for the cooling system.
- Wet cooling systems: since most of the cooling is achieved by evaporation of water, wet cooling systems have the highest water needs and are the 100% reference with respect to water requirements for closed-cycle cooling systems.
- Hybrid cooling systems: any combination of a wet and dry technology that has an intermediate water requirement between a purely dry and wet cooling system. Typically, hybrid cooling systems consist of a dry section and a wet section.
- Dry cooling systems depend on the dry-bulb temperature, since the driving force for heat transfer is the difference in temperature between the process fluid and the air. Wet cooling systems depend on the wet-bulb temperature, since the driving force for heat and mass transfer is the difference in enthalpy between the water and air.
Wet cooling systems
A closed-cycle wet cooling system (like evaporative cooling towers) is an indirect cooling system, which implies that the heat of condensation is first rejected inside the condenser (typically a steam surface condenser) using circulating cooling water, and in a second step, the hot cooling water is cooled in a cooling tower by rejecting the heat into the atmosphere.
An evaporative cooling tower cools water by a combination of heat and mass transfer. The water to be cooled is distributed in the tower by spray nozzles, splash bars, or film-type fill, which exposes a very large water surface area to atmospheric air. Atmospheric air is circulated by fans (mechanical draft cooling tower), natural draft currents (natural draft cooling tower), or induction effect from sprays (this technology is rarely used today). A portion of the water absorbs heat to change from a liquid to a vapor at constant pressure. This heat of vaporization at atmospheric pressure is transferred from the water remaining in the liquid state into the airstream.
Natural-draft (hyperbolic) cooling towers (Figure 2) have been used primarily for large power installations like coal-fired power plants, and in particular for nuclear power plants. The major advantage of natural-draft cooling towers is that the auxiliary power consumption is significantly lower, since there are no fans associated with the cooling tower, but they are characterized by a higher investment cost and a negative visual impact because of the tall structure.
For smaller-capacity power plants or in areas where the labor cost is high, mechanical-draft cooling towers are more frequently selected; the cooling tower cells can be arranged in a round configuration (see Figure 3) or in the typical standard rectangular configuration.
The difference between the leaving water temperature and the entering air wet-bulb temperature is the approach to the wet bulb or simply the approach of the cooling tower. The approach is a function of cooling tower capability, and a larger cooling tower produces a closer approach (colder leaving water) for a given heat load, flowrate and entering air condition. Thus, the amount of heat transferred to the atmosphere by the cooling tower is always equal to the heat load imposed on the tower, while the temperature level at which the heat is transferred is determined by the thermal capability of the cooling tower and the entering air wet-bulb temperature.
Evaporative cooling systems need water to make up for the evaporation, blowdown and drift. The total make-up water flowrate that is required for an evaporative cooling system or cooling tower in particular is the sum of the evaporation rate, blowdown rate and drift rate.
Usually the drift rate is expressed in terms of the circulating water flowrate and with the current high efficiency drift eliminators, the drift rate can be limited to 0.0005% of the circulating water flowrate.
The thermal performance of a cooling tower depends principally on the entering air wet-bulb temperature. The entering air dry-bulb temperature and relative humidity, taken independently, have an insignificant effect on thermal performance of mechanical-draft cooling towers, but they do affect the rate of water evaporation within the cooling tower.
Dry cooling systems
In a dry cooling system, heat is transferred from the process fluid (water or steam) to the air by means of extended heat exchanger surfaces or fin tube bundles. The performance of dry cooling systems is primarily dependent on the ambient dry bulb temperature of the air and no water is evaporated in the heat transfer process. Consequently, if no water is available for the cooling system, dry cooling is the way to go. Table 1 summarizes the advantages and disadvantages of dry cooling systems.
There are two major types of dry cooling systems: indirect and direct.
Indirect dry-cooling systems. With this type, the low-pressure (LP) steam turbine exhaust is not directly connected with the cooling system but with a condenser and the process fluid is water. Cold water is used in a separate condenser (surface condenser or direct-contact jet condenser) to condense the steam coming from the LP steam turbine. Examples of indirect dry cooling systems are the Heller system (Figure 4) and the indirect dry-cooling tower (IDCT). A natural draft tower or a mechanical draft configuration with axial flow fans can be used with indirect dry cooling systems.
Air-cooled heat exchangers can be classified as an indirect dry cooling system as well, although these are rarely used to cool the water for the steam-condensing process in a power plant.
Direct dry cooling system. The LP steam turbine is connected directly with the cooling/condensing system via the large-diameter steam duct and the process fluid is steam. This is the case in an air-cooled condenser (ACC).
Until recently, forced-draft, air-cooled condensers with single-row finned tubes arranged in an A-frame configuration have been the workhorse for direct dry-cooled power plants (Figure 5). However, high construction costs and operational issues associated with forced-draft installations have led to the development of the induced-draft, air-cooled condenser using the same single-row heat exchanger technology (Figure 6).
The induced-draft ACC has the following advantages compared to the traditional forced-draft ACC:
- Weight of the supporting steel structure is reduced by approximately 50%
- Reduced length of the large-diameter exhaust steam duct
- Shorter delivery time
- Reduced air inlet height and total height of ACC
- Number of steel structure pieces reduced by 60
- Pre-assembling easier (more ground pre-assembly)
- 25–30% less man hours for construction; reduced construction duration
- Less vibration issues due to elimination of fan bridge and consequently longer lifetime for gearboxes and fans
- ACC performance is less sensitive to wind effects since the bundles reduce the aerodynamic effect of the wind on the fans
- Reduced hot air recirculation since the exit velocities are significantly higher
Today, more and more power plants rely on dry cooling technologies, because of the large make-up water requirements of wet cooling towers. One of the drawbacks of dry cooling systems is the performance hit under hot ambient conditions.
A hybrid cooling system
The parallel condensing system, shown in Figure 7, is a hybrid cooling system that consists of an air-cooled condenser and a steam surface condenser that is typically cooled by a mechanical draft wet cooling tower, with the steam coming from the low-pressure (LP) steam turbine being condensed in parallel by the air-cooled condenser and surface condenser.
This type of hybrid cooling system was first introduced by GEA (now Enexio) for a relatively small 31 MW waste-to-energy plant (Exeter Energy facility) in Connecticut. It utilizes whole and shredded tires as primary fuel. Later, a larger version was implemented for gas-fired power plants in Argentina (Tucuman). The largest system in the world started commercial operation in 2010 for the Comanche III unit in Pueblo, Colorado — a 750-MW coal-fired power plant that is owned and operated by Xcel energy.
Hybrid cooling systems
A hybrid cooling system consists of a dry section and a wet section. One recently commercialized hybrid cooling system is described in the box above (“A hybrid cooling system”).
It is important to note that the water use of a hybrid cooling system will depend on the mode of operation: if the wet section is used at its maximum capacity to get the maximum performance and turbine output, the water consumption will be the highest.
The make-up water usage of the system shown in Figure 7 can be controlled by adjusting the capacity of the wet section, simply by shutting down the fans of the wet cooling tower in such a way that the dry section will handle a larger portion of the total condensing heat duty.
One of the unrecognized features of hybrid cooling systems is the high level of control flexibility with respect to water usage and electric output. In a traditional wet cooling system and for a given condensing heat duty, the make-up water usage cannot be changed because the water usage is determined by the ambient conditions and the heat load. With a hybrid cooling system, the make-up water consumption can be changed over a very broad range for a given design and in most cases with a limited impact on the electric output of the steam turbine generator.
In a hybrid system with a relatively small wet section (consisting of a cooling tower and surface condenser), the steam turbine backpressure can be reduced significantly at hot ambient conditions compared to a 100% dry cooling system, so the power plant can continue to operate at full load with a value of the turbine backpressure that can be significantly lower than would be the case for an all-dry cooling system. When it gets colder, the capacity of the dry section will increase in such a way that the wet section does not need to operate at all because the dry section will be able to handle 100% of the condensation heat duty.
The significant performance improvement of the hybrid cooling system (Figure 7) compared to an ACC during hot ambient conditions is illustrated by the graph shown in Figure 8. As can be noticed, the hybrid cooling system shows a significantly lower steam turbine backpressure during hot ambient conditions while the wet section could be shut off if the dry-bulb temperature is lower than 20°C, if a set point of 150 mbarA would be selected, for example.
Selection: dry versus wet
If only capital cost has to be considered, wet cooling is usually the preferred system since it is more efficient from a thermal engineering point of view — for a given design ambient condition, the heat sink temperature will always be lower for a wet cooling system.
However, wet cooling systems are characterized by a large water consumption rate (water loss by evaporation, drift and blowdown). Energy and water are essential interdependent resources. As the world population is increasing, demand for energy and water is on the rise. Competing demands for water supply are affecting the value and availability of the resource. Operation of some power plant facilities has been curtailed due to water concerns, and siting and operation of new energy facilities must take into account the value of water resources.
Evaporative or wet cooling systems generally are the most economical choice for closed-cycle cooling systems where an adequate supply of suitable make-up water is available at a reasonable cost. However, wet cooling systems suffer from certain environmental issues like plumes (which may result in fogging and icing conditions in areas close to the power plant), drift (PM10 emissions), blowdown (concentrated cooling water) and risk for Legionella, so dry cooling systems are certainly more environmentally friendly, and are considered as a “green” technology. So, if there is not enough make-up water available for wet evaporative cooling or if there are environmental concerns (drift, plume, and so on), dry cooling is a feasible alternative.
The next question is then how to select between a direct dry (ACC) or indirect dry (Heller) system. From a capital investment point of view only, air-cooled condensers are somewhat less expensive than Heller systems. However, Heller systems can be beneficial in the following cases:
- The power plant is designed around a fast start-up concept: air-cooled condensers have a large volume compared to the condenser volume of an indirect dry cooling system, so the start-up time of a Heller cooling system can be much shorter
- The power plant is located in regions that are vulnerable to high wind velocities or wind gusts — the performance of indirect dry cooling systems is less impacted by wind as compared to the traditional A-frame air-cooled condensers; however, induced-draft air-cooled condensers are less sensitive to the wind as well, as stated above
- Plot area restrictions: indirect dry cooling systems are easier to arrange on the site plot plan compared to ACCs; the ACC cells are typically arranged in a rectangular plot area that is located close to the turbine building. Heller systems are characterized by a high degree of flexibility when there are restrictions in plot area or location
- Low noise limit: if there is a low noise level to be guaranteed at the site boundary, Heller systems are preferred; moreover, because of the flexibility in the arrangement of the indirect dry cooling tower cells, a location can be selected that is further away from the critical noise reception point
- Retrofit of an existing wet cooling system to a hybrid or 100% dry-cooled system: if the power plant was originally equipped with a wet cooling system, it is much easier to add an indirect dry cooling add-on to the circulating water system or to convert it to a 100% indirect dry cooling system — connecting the water piping of the indirect cooling tower to the existing circulating water piping is rather straightforward — connecting a large diameter steam duct of an ACC to the steam turbine exhaust with a steam surface condenser already in place (typically located in close proximity of the steam turbine exhaust), can be very challenging
A very promising technology is hybrid cooling, since it combines the best features of both worlds, particularly if there is a little bit of make-up water available. The yearly water consumption can be reduced significantly compared to 100% wet cooling, and the investment cost can be reduced significantly compared to 100% dry systems for a given thermal duty to be handled. The wet section only needs to be engaged during hot summer operating conditions and will allow operators to increase the performance of the cooling system significantly. During moderate or cold operating conditions, the dry section can handle 100% of the heat duty, which allows plant operators to save water and avoid having to deal with the negative effects of plume coming from the cooling tower in winter. n
Luc De Backer is vice president of Technology with Enexio US (Enexio Power Cooling Solutions, 300 Union Blvd, Suite 350, Lakewood, CO 80228; Phone: +1 303-987-4027; Fax.: +1 303-987-0101; Email: email@example.com, Website: www.enexio.com). De Backer started his professional career in Belgium with Hamon Thermal Europe in 1995 and developed his expertise in the single-row tube technology for air-cooled condensers. He moved to the U.S. in 2000 and became a member of the ASME to co-author the ASME PTC 30.1 performance test code for Air-Cooled Steam Condensers. He was also the technical chairman for the Heat Exchange Institute (HEI), issued the first HEI standard for air cooled condensers. De Backer focused on wet cooling systems as the technical specialist for Bechtel. In 2008 he established the center of excellence for hybrid cooling systems for GEA in Phoenix, Arizona. He is also founder of Advanced Cooling Solutions, a partnership company that specializes in cooling systems for power plants. De Backer holds a Ph.D. in chemistry, an M.S. in chemical engineering and an M.S. in chemistry, all obtained from the Free University of Brussels, Belgium. Currently he lives with his family in San Tan Valley, Arizona.
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