energies
Article Performance Analysis of an Updraft Tower System for Dry Cooling in Large-Scale Power Plants
Haotian Liu ID , Justin Weibel and Eckhard Groll *
Ray W. Herrick Laboratories, School of Mechanical Engineering , Purdue University, West Lafayette, IN 47906, USA; [email protected] (H.L.); [email protected] (J.W.) * Correspondence: [email protected]; Tel.: +1-765-494-7429
Academic Editor: George Tsatsaronis Received: 13 October 2017; Accepted: 7 November 2017; Published: 9 November 2017
Abstract: An updraft tower cooling system is assessed for elimination of water use associated with power plant heat rejection. Heat rejected from the power plant condenser is used to warm the air at the base of an updraft tower; buoyancy-driven air flows through a recuperative turbine inside the tower. The secondary loop, which couples the power plant condenser to a heat exchanger at the tower base, can be configured either as a constant-pressure pump cycle or a vapor compression cycle. The novel use of a compressor can elevate the air temperature in the tower base to increases the turbine power recovery and decrease the power plant condensing temperature. The system feasibility is evaluated by comparing the net power needed to operate the system versus alternative dry cooling schemes. A thermodynamic model coupling all system components is developed for parametric studies and system performance evaluation. The model predicts that constant-pressure pump cycle consumes less power than using a compressor; the extra compression power required for temperature lift is much larger than the gain in turbine power output. The updraft tower system with a pumped secondary loop can allow dry cooling with less power plant efficiency penalty compared to air-cooled condensers.
Keywords: power plant dry cooling; updraft tower; air-cooled condenser; vapor compression; thermodynamic feasibility analysis
1. Introduction Utility scale power plants must reject a huge amount of heat based on the second law of thermodynamics. The heat is not useful for power production, and must be rejected to the ambient at the lowest possible temperature to maximize the power plant efficiency. Typically, for each megawatt of electricity generated, one to two megawatts of heat must be rejected [1]. The most common method of rejecting heat from the condenser of the power plant Rankine cycle uses cooling towers. Water cooling towers are widely used in industry and provide reliable long-term operation. The return water temperature from the cooling tower to the condenser can be well controlled by the cooling tower water temperature. However, processing heat rejection using cooling towers consumes a large amount of water. The United States Geological Survey (USGS) data shows that water usage in thermoelectric power generation was 45% of the total national water use in 2010 [2]. In water-stressed arid regions, there is not enough water available to simultaneously satisfy the needs of power production and other critical functions. In areas with physical access to water sources, cooling towers are co-located with rivers or lakes. Nevertheless, heat rejected to the environment may cause temperature increases that negatively affect the local ecological system. From an energy perspective, the cooling towers do not recover or utilize any of the waste heat that is rejected. As a consequence of these disadvantages, government agencies have changed policy to limit open-loop water cooling for power plants [3–5].
Energies 2017, 10, 1812; doi:10.3390/en10111812 www.mdpi.com/journal/energies Energies 2017, 10, 1812 2 of 23
Thus, there is a significant need for alternatives to existing water cooling approaches that reduce or eliminate the water consumption associated with power plant heat rejection. Several different methods for power plant heat rejection have been investigated in recent decades that utilize the waste heat stream, such as a water canal system [6], a spray pond system [7], or rejecting the heat to an algae pond to produce biofuel [8,9]. However, water canal systems or spray pond systems still consume significant amounts of water when rejecting heat to the environment. In order to solve the water consumption problem, this paper concentrates on alternative dry cooling methods, a topic of recent significant technological interest due to regional water shortages [10]. One dry cooling method that is becoming increasingly adopted in newly constructed utility scale power plants is the use of air-cooled condensers (ACC) [11]. There are two general types of dry cooling systems using air-cooled condensers: direct and indirect. Indirect dry cooling systems use secondary water loops; the steam from the power plant is condensed by water in the secondary loop, which is then cooled within the air-cooled condenser. In the direct system, the power plant steam is ducted directly to the air cooled condenser with either mechanical draft or natural draft. Air-cooled condensers with a mechanical draft system are used in most of the dry cooling power plants [11]. In mechanically-driven direct air-cooled condensers, the power plant exhaust steam exiting the turbines passes through a system of air-cooled heat exchangers, typically finned-tube heat exchangers arranged in an A-frame configuration [12]. The exhaust steam condenses on the inner tubes walls, flows downward to a condensate receiver tank by gravity, and then is pumped back to the boiler. On the floor of the condenser, large mechanically-driven axial flow fans force the cooling air through the condenser. The power consumption associated with using air-cooled condensers can be classified in two categories: the mechanical fan power consumption required to drive the air through the heat exchanger and the penalty in power generation efficiency compared to cooling tower heat rejection due to the higher condensing temperature. Based on a report from the California Energy Commission [13], the fan power consumption is usually 1 to 1.5% of the total power output. The reported efficiency penalty on the power generation ranges from 5% on normal days to 20% on hot and windy days; wind reduces the ACC heat transfer rates due to reduced fan performance or recirculation of the hot air [14]. The condensing temperature and the power generation efficiency penalty is strongly influenced by the environmental temperature, and hence is affected by the location. In the studied case, the location of the power plant had an average temperature 13.8 ◦C. A solar-driven updraft tower concept was originally proposed over 100 years ago and only later studied for solar power generation purposes [15,16]. A solar updraft tower comprises a tall tower chimney with a large solar collection area surrounding the base of the tower, and gas turbines at the inlet. Air is heated by the sun in the collector. Air at the top of the tower is cooler due to the height above sea level. The temperature and density difference between the heated air and the ambient air at the top of the tower establishes the driving buoyancy force to draw air into the tower without any mechanical draft. Solar energy is thereby converted to kinetic energy that can be extracted by the gas turbines. A 50 kW experimental plant was built in Manzanares (Spain) in 1981 and operated for two years [17]; the operating principle, construction details, and preliminary test results were described and analyzed [18]. This plant had a tower height of 194.6 m and a vertical axis single-rotor turbine configuration with four blades. The experiment showed that the updraft tower would operate in real-world conditions and the performance agreed with model predictions [19]. A number of different theoretical modeling efforts have followed this first experimental demonstration to analyze various parameters’ influence on solar updraft tower system performance. Padki and Sherif developed a simple analytical model for a constant-diameter tower by using a momentum balance and assuming the Boussinesq approximation was valid to calculate the air velocity and kinetic energy at the outlet; the power output was assumed equivalent to the kinetic energy. Bernardes et al. studied the power output using a model that balanced the pressure potential and theoretical maximum volume flow rate at the tower inlet under a no-load condition to obtain the pressure drop and the volume flow rate at the updraft tower inlet; the same method was adopted by Koonsrisuk Energies 2017, 10, 1812 3 of 23 and Chitsomboon and Zhou et al. [20–22]. Pastohr et al. calculated the temperature and flow field in the solar updraft tower numerically [23]. The simulation results were validated against experimental data and showed a good agreement. Fluri and von Backström compared different modeling approaches and layouts for solar updraft turbines, and confirmed the assumptions of previous researchers that the turbine efficiency of a solar updraft tower is approximately 80% [24,25]. Nizetic and Klari evaluated the influence of turbine pressure drop on the updraft tower; the turbine pressure drop factors were in the range of 0.8 to 0.9 [26]. While solar updraft towers use solar energy to heat air at the tower base, an updraft tower could also operate in an analogous manner using power plant waste heat as the energy source. Namely, the solar updraft tower collector is replaced by a heat exchanger at the tower base. A secondary loop is needed to couple the waste heat source to the heat exchanger at the tower base to heat the air. A constant-pressure pump secondary loop with a condenser (i.e., tower base air heater) and an evaporator (i.e., power plant steam condenser) can be used as the secondary loop that transfers the heat from power plant steam to atmospheric air. The condenser surface area is distributed around the tower base and rejects heat to the buoyant-driven airflow. The updraft tower system with a constant-pressure pump cycle is one possible design. This technology is known as the natural draft dry cooling tower and has been analyzed in the literature and applied in large power plants. Zhao et al. developed a 3D numerical model to assess the cooling performance of a natural draft system with vertical radiators and validated the model against published data [27]. Zou et al. combined the solar updraft tower concept with a natural draft cooling tower and analyzed this solar-enhanced natural draft cooling scheme [27,28]; they concluded that a solar updraft tower coupled to a power plant can significantly increase the updraft tower turbine power output, but requires a larger heat transfer area to reject the power plant condensing heat. In this paper, an innovative indirect dry cooling method is analyzed that uses an updraft tower, combined with a secondary loop with vapor compression cycle, to reject the power plant condenser heat and increase the energy recovered from the turbine. The system cooling efficiency and performance is benchmarked against air-cooled condensers to evaluate the thermodynamic feasibility. A traditional natural draft cooling system couples the updraft tower to the power plant using a constant-pressure pump cycle; this requires a larger heat transfer area to reject the condensing heat. In order to increase the heat rejection rate to the ambient air, the secondary fluid temperature can be increased at the tower base using a modified system with a conventional vapor compression cycle as the secondary loop. In this context, the constant-pressure pump cycle can be considered a special case of the vapor compression cycle by eliminating the compressor (pressure ratio equal to one) and using a liquid pump instead of the expansion valve. Results and design insights gained through study of the vapor compression secondary loop system can generally be applied to the traditional constant pressure pump system. In this paper, a quantitative, model-based analysis is used to evaluate the system performance and compare with traditional methods of power plant cooling. This paper includes a detailed thermodynamic feasibility analysis of the components and system performances of an updraft tower dry cooling system. State-point thermodynamic models are developed for the power plant, updraft tower, and secondary loop, and are coupled together to analyze the system performance. The simulation model is built in Engineering Equation Solver (EES), which has libraries for refrigerant properties and can solve coupled systems of equations [29]. The analysis considers two secondary loop systems, a constant-pressure pump secondary loop as well as a vapor compression secondary loop. Parametric studies of the vapor compression secondary loop system are first carried out to determine the influence of the tower height, tower diameter, secondary loop condenser depth, and secondary loop evaporator heat transfer area on turbine power output and the required compressor work. A realistic updraft tower system design is formulated based on the results of these parametric studies. For this design, the constant-pressure pump secondary loop system performance is evaluated and compared to the vapor compression secondary loop system. Lastly, the penalty in power generation efficiency using a dry cooling updraft tower system is compared to a power plant using air-cooled condensers. Energies 2017, 10, 1812 4 of 23
Energies 2017, 10, 1812 4 of 23 2. System Description and Modeling Approach Energies2. System 2017, Description10, 1812 and Modeling Approach 4 of 23 The updraftThe updraft tower tower dry cooling dry cooling system system comprises comprise threes majorthree major subsystems: subsystems: the power the power plant, secondaryplant, loop,2. andsecondary System updraft Description loop, tower. and Figure updraftand Modeling1 presents tower. ApproachFigure a schematic 1 presen illustrationts a schematic of the illustrati dry coolingon of system the dry with cooling a power plantsystem connectedThe with updraft toa power an tower updraft plant dry connected towercooling using system to an a updraft vaporcomprise tower compressions three using major a vaporsecondary subsystems: compression loop. the The secondary power secondary plant, loop. loop transferssecondaryThe waste secondary heatloop, loop from and transfers theupdraft power wastetower. plant heat Figure steam from 1 condenser thepresen powerts a toplant schematic the steam heat exchanger condenserillustration atto of thethe the updraftheat dry exchanger cooling tower base. This heatsystemat the will withupdraft increase a power tower the plant base. air temperatureconnected This heat to will an at the updraftincrease base tower and,the air dueusing temperature to a thevapor density compression at the difference, base secondary and, drive due airflowloop.to the up throughThedensity thesecondary tower.difference, loop The drivetransfers kinetic airflow energywaste up heat through of thisfrom air the stream powertower. canplantThe thenkinetic steam be energycondenser converted of this to to theair electric heatstream exchanger powercan then using turbinesatbe the insideconverted updraft the towertotower electric core.base. power TheThis driving heatusing will turbines buoyancy increase inside forcethe airthe eliminates temperaturetower core. the atThe need the driving forbase mechanical and, buoyancy due to fans force the found eliminates the need for mechanical fans found in conventional dry cooling using air-cooled in conventionaldensity difference, dry cooling drive usingairflowair-cooled up through condensers. the tower. TheThe kinetic power energy generated of this by air the stream turbines can then can assist becondensers. converted Theto electric power generatedpower using by theturbines turbines inside can theassist tower in compression core. The driving of the refrigerantbuoyancy forcein the in compression of the refrigerant in the secondary loop. Detailed descriptions of the component models eliminatessecondary theloop. need Detailed for descriptionsmechanical offans the found compon inent conventional models developed dry cooling to predict using the behaviorair-cooled of developed to predict the behavior of this dry cooling system are introduced in this section. The coupled condensers.this dry cooling The systempower generatedare introduced by the in turbinesthis section. can The assist coupled in compression thermodynamic of the refrigerantmodels are insolved the thermodynamicsecondaryusing Engineering loop. models Detailed Equa are solvedtiondescriptions Solver using (EES). of Engineering the component Equation models Solverdeveloped (EES). to predict the behavior of this dry cooling system are introduced in this section. The coupled thermodynamic models are solved using Engineering Equation Solver (EES).
FigureFigure 1. Schematic 1. Schematic illustration illustration of of an an updraft updraft tower tower systemsystem with with a avapor vapor compression compression secondary secondary loop loop
for dryfor cooling dry cooling in large-scale in large-scale power power plants. plants. Figure 1. Schematic illustration of an updraft tower system with a vapor compression secondary loop 2.1. forPower dry Plantcooling Model in large-scale power plants. 2.1. Power Plant Model A simplified power plant model was developed based on realistic power plant components and 2.1.Aoperating simplified Power Plant conditions. power Model plantA block model diagram was describing developed the based key components on realistic of power the power plant plant components is shown and operatingFigureA conditions. simplified 2. The power power A block plant plant diagram modelis a three-pressure-s was describing developed thetage ba keysed coal-fired on components realistic Rankine power of thecycleplant power componentswith plantreheat is andand shown Figureoperatingregeneration.2. The power conditions. The plant primary A is block a three-pressure-stage steam diagram line fromdescribing the coal-firedboiler the key passes components Rankine through cycle aof high-pressure the with power reheat plant and(HP) is regeneration. turbine.shown The primaryFigurePart of 2. steamtheThe HP power line turbine from plant outlet the is boiler asteam three-pressure-s passes is reheat throughedtage in the acoal-fired high-pressure boiler andRankine passes (HP) cycle through turbine. with reheata Part two-stage ofand the HP turbineregeneration.intermediate outlet steam Thepressure is primary reheated (IP) steamturbin in the linee and boiler from two and the five-stage boiler passes passes low through pressure through a two-stage (LP) a high-pressure turbines. intermediate The (HP) other turbine. pressure portion (IP) of the HP turbine outlet steam floes to the feed-water heater that preheat the return water. turbinePart and of twothe HP five-stage turbine low outlet pressure steam (LP)is reheat turbines.ed in The the otherboiler portion and passes of the through HP turbine a two-stage outlet steam intermediateFurthermore, pressure portions (IP) of steamturbin aree and extracted two five-stage at the IP low turbine pressure and (LP)the LP turbines. turbines The to preheatother portion return floes to the feed-water heater that preheat the return water. Furthermore, portions of steam are extracted ofwater the inHP additional turbine outletfeed-water steam heaters. floes toThe the steam feed-water exiting theheater LP turbinesthat preheat flows intothe returnthe condenser. water. at the IP turbine and the LP turbines to preheat return water in additional feed-water heaters. The steam Furthermore,There are eight portions feed-water of steam heaters are extractedto preheat at the the re IPturn turbine water and from the the LP condenser.turbines to Fivepreheat feed-water return exitingwaterheaters the in LP areadditional turbines located flows feed-waterupstream into of theheaters. the condenser. condensa The steamte There pump exiting are and eight the three LP feed-water downstreamturbines flows heaters of intothe topump. the preheat condenser. the return waterThere from are the eight condenser. feed-water Five heaters feed-water to preheat heaters the re areturn located water upstreamfrom the condenser. of the condensate Five feed-water pump and threeheaters downstream are located of the upstream pump. of the condensate pump and three downstream of the pump.
Figure 2. Schematic diagram of power plant model.
Figure 2. Schematic diagram of power plant model. Figure 2. Schematic diagram of power plant model.
Energies 2017, 10, 1812 5 of 23
A simplified model is built to predict the condensing heat transfer rate of the power plant for given operating conditions, and to subsequently couple the model with the secondary loop model in the system model. Pressure drop in the piping is neglected. The condensation pressure and temperature are assumed to be known inputs based on baseline power plant operation with a water cooling tower. The outlet state of the condenser (and hence feed-water heaters) are saturated water. The power plant model is built as the system of equations formed from balancing energy and mass for each component. For the boilers, the enthalpies of the inlet and outlet steam are known for both heating and reheating processes. For the turbines, the power output is calculated based on the inlet properties, the outlet pressure, and a known isentropic efficiency: Energies 2017, 10, 1812 5 of 23 hin − hout ηs,turb = where hout,s = h(P = Pout,s = sin) (1) A simplified model ish builtin − toh outpredict,s the condensing heat transfer rate of the power plant for given operating conditions, and to subsequently. . couple the model with the secondary loop model in the system model. Pressure drop Win =them pipingout(h inis −neglected.hout) The condensation pressure and (2) temperature are assumed to be known inputs based on baseline power plant operation with a water For thecooling feed-water tower. heaters,The outlet itstate is assumedof the condenser that (a thend hence heater feed-water temperature heaters) isare the saturated saturated water. temperature at the given pressure;The power plant to model model theis built regeneration as the system cycleof equations performance formed from and balancing preheat energy of the andcondensing mass flow, it is also assumedfor each that component. all heaters For the have boilers, a constant the enthalpies terminal of the inlet temperature and outlet steam difference are known (TTD) for both that is used to heating and reheating processes. For the turbines, the power output is calculated based on the inlet calculate theproperties, outlet temperature the outlet pressure, of the an heater:d a known isentropic efficiency: