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2 GRC Transactions, Vol. 35, 2011 Evaporative Cooling Enhancement at the Steamboat Complex and Condenser Performance Research and Development Efforts Uri Kaplan, Zvi Reiss, and Bob Sullivan Ormat Technologies, Inc. Keywords Ormat Technologies, Ormat, Organic Rankine Cycle, Galena 3, geothermal power plant, Thermal Performance Analysis, evaporative cooling, fogging system, water-assisted air cooling Abstract Preliminary results from testing that took place through the peak summer months at the Galena III Power Plant facility in Reno, Nev. to develop a new Evaporative Cooling Enhancement system aimed at reducing the air temperature entering the geothermal condenser and increasing power plant performance. With this concept, water is discharged into the atmosphere through spray nozzles that mist into the air entering condenser tubes. During the tests, the Evaporative Cooling Enhancement system s affect on temperature reduction of inlet air and power output increase were measured using multiple variables. The results of the Galena 3 Evaporative Cooling Enhancement tests will be used in concept and system design of future geothermal power projects. Introduction Due to scarcity of water supplies and permitting concerns associated with the use of potable water for power plant cooling, most of Ormat s power plants are air-cooled. These air-cooled units are simple to operate and require no water, but they are less thermodynamically efficient during the heat of the day, especially during the summer months. The performance is very dependent on the dry bulb temperature of the air. The reduction in plant performance during the summer season is very significant at plants located in desert areas such as Nevada or southern California (summer days can reach and be in excess of 95ºF) and plant electric output can drop significantly on hot summer days. The dry desert climate is ideal for evaporative cooling as the relative air humidity and wet bulb temperature is very low. Adding a small amount of water to the air by spraying fine mist into the air stream causes the water droplets to evaporate, increasing the relative humidity while reducing the dry bulb temperature. This cooling mechanism is successfully applied in desert coolers and other processes to decrease air temperature in hot and dry areas. The objective of water-assisted cooling is to improve performance of air-cooled facilities while maintaining minimal water use. The water cooling reduces the dry bulb temperature during the hot summer days when plant performance is most severely restricted and electricity rates are the highest. Evaporative cooling allows air traveling to the air condensers to be cooled closer to the wet bulb temperature, which is frequently 25 or more degrees lower, and thereby increase the efficiency and output of the facilities. Improved performance of air-cooled facilities is especially important as lower temperature geothermal resources are developed and plant output becomes more sensitive to the difference between heat source and heat sink temperatures. Ormat has not only been engaged for some time in research and development of water-assisted air cooling, but also in improving performance of air-cooled condensers in general. Field testing has taken place at multiple facilities in various areas of air condenser performance including water-assisted air cooling. Field testing on multiple facilities provides more accurate conclusions. Ormat is uniquely situated to benefit from field testing due to a large fleet of operating air condensers. Background The concept of improved power plant cooling by spraying water into the air before entering the condenser is not new. Ormat has been testing different concepts at multiple power plants in Nevada and California for some time. In all three areas, the test concluded the added water efficiently cools the air and increases plant output. However, operational and maintenance problems have prevented the continuous operation of this system. Through field testing, Ormat found the wind speed and direction imposed significant impacts to water-assisted cooling efficiency and to air cooler performance in general. At the Mammoth facility, multiple technologies were tested including misting jets, wetted media and combinations of both. There are concerns about the use of misting jets due to potential corrosion or deposits resulting from the evaporation of the treated effluent. Wetted media restricted air 1315

3 Kaplan, et al. flow and resulted in significant maintenance to remove mineral deposits. The extensive testing also revealed knowledge about warm air recirculation. Mitigation measures for reducing recirculation impacts can be included easily in water-assisted design, as well as overall air condenser performance. The testing also verified shortcomings to relying solely on computer modeling for air condenser air flow effects. Properly designed field testing is important to verify computer modeling. Recent Progress Ormat has performed additional full-scale testing at the Galena 3 Geothermal Power Plant at the Steamboat complex in Reno, Nev. The Galena 3 power plant is an air-cooled 30 MW facility. The project purpose was to test the evaporative cooling concept on a relatively large scale and address all operational and maintenance issues preventing continuous operation of the previous tests. The Galena 3 test started June 2010 and ran through September A test protocol had been formulated to test the system in a large range of operational conditions and to address a list of non measurable parameters related to the proper system operation and its influence on the host facility (Galena 3 plant) and the environment. The principle of cooling an air stream by spraying it with water is well known. This practice does not require further theoretical or practical proof. The goal of the test was to obtain real figures of system performance, provide calibration data for CFD modeling, identify optimum operation parameters and issues that may hinder operation and cause operation and performance problems. The test and analysis results will then be the basis for future project design parameters where similar systems will be installed. The primary tested and measured parameters include: Ambient conditions temperature, relative humidity Total water spray flow rate Water pressure Water quality Wind velocity and direction Spray nozzles type Spray nozzle quantity and location Working conditions and performance of the air condenser units The following issues, related to system performance, have been evaluated and analyzed: System efficiency relative to actual temperature reduction of air compared to the theoretical maximum temperature reduction in the working conditions. A wide range of working parameters were tested in various ambient conditions (temperature and relative humidity). During the analysis process, it was attempted to determine the system efficiency as function of the operating parameters. The performance of mist eliminators and their influence on the fogging system performance. Efficiency of evaporation (i.e. the percentage of sprayed water that evaporates and the percentage accumulated as droplets on the condenser fins). Scaling and deposits on the spray nozzles and condenser fins. Water treatment and filtering efficiency in preventing the above scaling. Thermal Performance Analysis Figure 1 represents the test results. Parameters on the graph: Relative Water Flow is the actual measured water flow divided by the maximum water flow the ambient air is capable to absorb (flow required for full saturation); And, Relative Air Temperature Reduction is the actual measured condensing temperature reduction divided by the difference between Dry and Wet Bulb, i.e., it represents the effective cooling efficiency. The maximum cooling effect obtained in the tests is close to 50% of the difference between Dry and Wet Bulb. The graph shows that the fogging effect in many cases is close to the ideal process of evaporative cooling with no water losses. Linear approximation of the test data shows that the average cooling effect is 84% of the ideal. In other words, in order to get some specified cooling effect we need to provide 20% extra flow of water than is estimated by the ideal model. Relatively large measured water usage efficiency does not mean that the water losses are small; rather, it is explained by wetting of the air cooler tubes in the tests in addition to the evaporative cooling of the air assumed in the theoretical model, the actual cooling mechanism includes direct evaporation on the tubes of air cooler and increase of the heat transfer coefficient by direct contact with water, which amplified the cooling effect. In order to verify the influence of the heat transfer wetting on the fogging system performance, the data on Figure 1 was divided into three groups according to the difference between Dry and Wet Bulb, i.e., the maximum cooling potential attainable with the evaporative cooling process under given ambient conditions. If the only mechanism in the studied process was the evaporative cooling then the influence of the ambient conditions on the data would not be observed, which is not the case for the data presented. As can be seen, there exists correlation, though not very pronounced the higher the cooling potential the higher the average cooling efficiency. This implies that the aforementioned processes related to the wetting of the heat transfer surface play an important role in the condensing temperature reduction. Figure 1. Fogging System Performance. 1316

4 Kaplan, et al. Water Quality and Tube Fin Impacts During the test period the water quality was stable with average values listed in Table 1. Table 1. Average water quality values. Parameter Units Average results Method / instrument ph 8 Mettler Toledo SG8-SevenGo pro Conductivity [SS/cm] 100 Mettler Toledo SG8-SevenGo pro TSS [mg/l] 7 Hach DR2700 spectrophotometer under method TDS [mg/l] 65 Portable ph meter CaCO3 ppm 40 EDTA titration The water does not contain significant potential scaling elements and the quality regarding TDS is better than most drinking water. However, there are some elements such as calcium carbonate that might cause scaling to accumulate on the fins. Fogging system s water droplets may contain potential scaling salts such as calcium carbonate. Salts may form scale on the air cooler fins and lead to heat transfer reduction. Water droplets emerge from the nozzles and can: Evaporate before contact with the fins. Salt may impact and deposit on the fins or travel away with the wind. Contact with the hot fins and immediately evaporate thus the salt will form scaling on the fins. The probability that the water will drip away is very low due to relatively high temperature, dry conditions and wind speed (beneath the fan). Contact the relatively cool supporting elements, working fluid pipes or mist eliminators and drip. Salt may form scaling on the element or remain soluble and drip away. The first two scenarios, with the addition of airborne dust, can lead to fin deposits and may damage the fins heat exchange efficiency. During the testing, scaling appeared on the lowest fins and, although scaling formation is not negligible, it s far from being a catastrophic or significant. Water is the driving force for fins scaling. The majority (about 80-90%) of the scaling comes from particles traveling in air, and the minority comes directly from the water. The scaling contains mainly Si minerals and does not contain carbon minerals. Scaling kinetics was not measured and will be dependent on many variants such as wind, dust composition, etc. Scaling could be treated in two approaches: prevention and cleaning. Prevention could be achieved by a physical barrier such as mist eliminators or by further water treatment such as RO. Cleaning could be achieved by fins soaking in ph 3-4 solution such as citric acid. Conclusions The wind parameter is one of the most important factors influencing the performance of an air-cooled power plant. When employing a fogging system, the wind factor, direction and speed also play a key part in the air condenser s overall performance. Factors that are used in determining plant location: Weather data (wind velocity, prevailing winds direction, ambient temperature and relative humidity) must be collected and analyzed prior to a plant location decision. For accurate measurement, multiple locations for instruments should be considered. Continued collection of data is important during operation to monitor changes in weather patterns and impacts due to construction of a new feature that may impact wind direction and speed, thus a standard weather station should be installed in every plant or plant cluster. The distance between two parallel air-cooled condensers must be calculated to avoid mutual influence, as hot discharge air from the windward air condenser may impact the intake air of the leeward air condenser. The axis of the air condenser should be parallel to the prevailing wind for best air cooler performance and reduced mutual air condenser influence. Obstacles (heat exchangers, tanks, control room, etc.) near the air condenser can cause leeward low pressure, resulting in local hot air recirculation. Other obstacles and plant design that act to alter wind behavior can improve air condenser performance. CFD modeling should be calibrated with field testing. 1317

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