[ ENERGY / IN DETAIL ] [ ENERGY / IN DETAIL ] Power plant performance under extreme ambient conditions AUTHOR: Dawn Santoianni 22 in detail
WÄRTSILÄ TECHNICAL JOURNAL 01.2015 Fig. 1 Wärtsilä technology selected for the Antelope Station in Texas, U.S. was based on the need to deliver full power to the electric grid in less than five minutes in ambient conditions that ranged from -23 C to 46 C. Delivering reliable power in the world s most demanding climates requires technology that provides stable output despite ambient conditions. Wärtsilä power plants outperform gas turbines in areas where heat, humidity and altitude pose operational challenges. Harsh climatic conditions can pose a significant barrier to electrification plans and economic growth in developing countries. Hot climates, high elevations and excessive humidity put huge demands on power generating technology. Depending on the site conditions, a power plant s actual electrical output, efficiency, and fuel consumption can be quite different than its performance at design conditions. Ambient conditions (temperature, humidity and pressure) can vary dramatically with geographic location and by season. Summer temperatures in the Middle East and northern Africa frequently exceed 40 C (104 F) and some areas around the world experience large seasonal temperature swings of over 38 C (100 F), as shown in Figure 2. Ambient temperature, humidity and altitude affect the density of air, which can negatively impact power plant output and performance. As surging temperatures in detail 23
[ ENERGY / IN DETAIL ] [ ENERGY / IN DETAIL ] Hot and humid air is less dense than dry, cooler air. As the density of air decreases, more power is required to compress the same mass of air. This reduces the output of the gas turbine and decreases efficiency. Studies have found that gas turbine efficiency deteriorates by one percent for every 10 degree rise in temperature above ISO conditions [1]. This translates into a power output reduction of 5 to 10 percent, depending on the type of gas turbine. Gas turbine manufacturers use various techniques to cool inlet air and boost turbine output, including evaporative coolers and mechanical chillers. However, inlet air cooling requires additional power consumption, and the efficacy of cooling systems is highly dependent on the ambient humidity. Wärtsilä ICEs are less sensitive to temperature and humidity, retaining their rated efficiency and power output over a broader range of ambient conditions. Fig. 2 - In some regions of the world, average temperatures exceed 30 C. Image credit: Maps on the Web. usually correspond to peak electrical demand, a reduction in power output with high ambient temperatures and humidity can be problematic. High altitudes present engineering challenges. Air pressure decreases at higher altitudes, so the amount of air available for combustion is decreased. The air pressure at 1500 meters above sea level (masl) is 85 kpa (12.3 psi), significantly lower than standard gas turbine ISO reference conditions of 101 kpa (14.7 psi)*. At high temperatures, humidity or 24 in detail elevations, gas turbine output and efficiency can significantly degrade compared to standard reference performance. Wärtsilä internal combustion engines (ICEs) are designed to operate in even the most adverse conditions, setting the standard for efficiency and economic performance. How do temperature and humidity affect power plant output and efficiency? In gas turbines, power output is dependent on the mass flow through the compressor. * The standard reference conditions (prescribed by the International Organization for Standardization, or ISO) are the temperature and pressure conditions under which manufacturers evaluate generating capacity and efficiency. These ISO reference conditions differ depending on the technology. Standard reference conditions for gas turbines (ISO 3977) are 15 C (59 F) and 101.3 kpa (14.7 psia) while for combustion engines (ISO 3046) reference conditions are 25 C (77 F) and 99 kpa (14.4 psia). Performance in hot conditions The performance of simple cycle gas turbines, combined cycle gas turbines (CCGT) and Wärtsilä ICEs at varying ambient conditions was assessed using data from GT PRO [2]. Popular model heavy frame industrial gas turbines were compared with similarly sized Wärtsilä engines, for capacities of 200 275 MW in simple cycle, and approximately 300 MW in combined cycle (see Table 1 for full load output of the specific models compared). For combined cycle operation, a 1x1 CCGT configuration was assumed with air-cooled condensers and a bypass stack to isolate the steam generating portion of the plant from the gas turbine. Figure 3 presents the net power plant output at varying ambient temperatures ranging from 10 C to 40 C (50 F to 104 F) for gas turbines and Wärtsilä ICEs operating in combined cycle (Flexicycle). CCGT output decreases by 15 to 18 percent at 40 C compared to ISO reference conditions, while the Wärtsilä Flexicycle plant output decreases by only 8 percent compared to reference conditions. The impact on plant efficiency is shown in Figure 4 for both combined cycle and simple cycle operation. At an ambient temperature of 40 C, CCGT efficiency decreases by 3.5
WÄRTSILÄ TECHNICAL JOURNAL 01.2015 Plant net output (MW) 320 310 300 290 280 270 260 250 240 230 10 C (50 F) 20 C (58 F) 30 C (85) 40 C (104 F) Ambient temperature GE 7FA.05 Combined cycle Siemens SGT6-5000F Combined cycle Wärtsilä Flexicycle Fig. 3 Wärtsilä Flexicycle plants experience less derating at high ambient temperatures than CCGTs. GE 7FA.05 Siemens SGT6-5000F Wärtsilä Combined cycle Simple cycle Combined cycle Simple cycle Flexicycle Simple cycle 10 C (50 F) 312 214 296 204 305 276 15 C (59 F) 306 209 289 198 304 276 20 C (68 F) 300 205 280 191 303 276 25 C (77 F) 292 200 270 185 301 276 30 C (86 F) 282 194 259 178 300 276 35 C (95 F) 271 188 248 172 292 269 40 C (104 F) 260 182 235 164 278 257 Table 1 - Plant net output at varying ambient temperatures. percent compared to ISO conditions. In a Flexicycle power plant, efficiency only drops by 1.1 percent at 40 C. All values represent net efficiency at the high-voltage grid side at sea level pressure. In simple cycle operation, Wärtsilä power plants demonstrate significant efficiency advantages over gas turbines. While simple cycle efficiency of a gas turbine is approximately 35 percent at 40 C, Wärtsilä efficiency is over 45 percent. The impact of ambient temperature on efficiency becomes even more pronounced when the plant is operating at part load. Operation at part load is becoming more and more common for gas-fired power plants due to cycling to follow renewable loads (see article Defining true flexibility, page 10). At 25 C, the efficiency of a CCGT drops from 55 percent at full load to 49 percent at half-load. The efficiency of a similarly-sized Flexicycle plant only drops 0.1 percent (49.1 percent efficiency at halfload). Although CCGTs are sometimes perceived as having unmatched high efficiency, in real-world situations including high heat and humidity, Wärtsilä engines offer equal efficiency and greater flexibility than gas turbines without the limitations of minimum turndown load. Performance at high altitudes Although high elevations are often associated with remote mountainous terrain, 140 million people worldwide live at altitudes above 2500 meters (8200 feet), including cities and densely populated in detail 25
[ ENERGY / IN DETAIL ] [ ENERGY / IN DETAIL ] Effiency (%) 60 55 50 45 40 35 30 10 C (50 F) 20 C (58 F) 30 C (85) 40 C (104 F) Ambient temperature GE 7FA.05 Combined cycle Siemens SGT6-5000F Combined cycle Wärtsilä Flexicycle GE 7FA.05 Simple cycle Siemens SGT6-5000F Simple cycle Wärtsilä Simple cycle Fig. 4 - Efficiency of Wärtsilä engines compared with gas turbines at varying ambient temperatures. areas in South America, Africa and Asia. In addition, it is not unusual for mining and other industrial operations to be located above 2000 masl. At these high altitudes, air density is lower than at sea level. Lower air density means less air is available for combustion and can create problems with startup and cooling. In gas turbines, the main impact of high altitude is the effect on the compressor inlet pressure and thus mass flow. (Figure 5) Gas turbine power output decreases proportionately with altitude, reducing by approximately 12 percent for every 1000 meters above sea level [3]. At 1500 masl, gas turbine output would be 18 percent less than its rated full capacity. While boosting inlet airflow with air injection (supercharging) can be used to prevent derating, it requires additional energy input, negatively impacts the overall economics of the plant and is seldom used in practice. Employing radiator cooling and turbocharging to achieve better performance at high altitudes, Wärtsilä 34SG engines can maintain full output to 2000 masl. The severe derating impact of altitude on gas turbines is evident in Figure 6, which shows the superior performance of Wärtsilä engines at high altitudes. 26 in detail Economic implications Gas turbine performance derating has direct economic investment implications. Oversizing a gas turbine to compensate for derating results in additional capital investment costs, with an estimated cost increase of 18 percent for a gas turbine at 1500 masl compared one at sea level [4]. Efficiency degrading under challenging ambient conditions also increases fuel costs and emissions. Life-cycle cost analysis has shown that ambient conditions of 35 C and an altitude of 500 masl would increase the cost of a 200 MW CCGT unit by 2.0 million, and would have to run an additional 1000 hours to produce the same output as a similarly-sized Wärtsilä power plant [5]. Wärtsilä has power plants operating in the harshest climates around the world. The recently inaugurated Sasol gas engine power plant located south of Johannesburg, South Africa is powered by 18 Wärtsilä 34SG generating sets operating at an elevation of 1500 masl [6]. Wärtsilä s highest elevation power plant serves the Mina Pirquitas mining project in Argentina, operating at an altitude of 4100 masl [7]. The ability to efficiently deliver load despite ambient conditions along with the operational flexibility advantages of rapid startup and quick ramping makes Wärtsilä power plants ideally suited for integrating renewable energy sources. Technology selected for the Antelope Station in Texas, U.S. (20 Wärtsilä 34SG engines) was based on the need to deliver full power to the electric grid in less than five minutes in ambient conditions that ranged from -23 C (-10 F) to 46 C (115 F) at 1020 masl [8]. As global energy needs expand into areas with challenging climatic conditions, Wärtsilä technology provides the most costeffective solution for reliable power. References [1]Farouk, N., L. Sheng, and Q. Hayat. Effect of Ambient Temperature on the Performance of Gas Turbines Power Plant. International Journal of Computer Science Issues, 10:1. 3 January 2013. Web. 28 January 2015. <http://www.ijcsi.org/papers/ IJCSI-10-1-3-439-442.pdf >. [2] GT PRO. Thermoflow Products - Combined Cycle. Thermoflow Inc., n.d. Web. 27 Jan. 2015. <http:// www.thermoflow.com/combinedcycle_gtp.html>. [3]Meher-Homji, Cyrus B., Mustapha A. Chaker, and Hatim M. Motiwala. Gas Turbine Performance Deterioration. Proceedings of the 30th Turbomachinery Symposium. 2001. Web. 27 Jan. 2015.
WÄRTSILÄ TECHNICAL JOURNAL 01.2015 Fig. 5-140 million people worldwide live at altitudes above 2500 meters. Photo: El Alto by Danielle Pereira, licensed under Creative Commons. 100 Percent of full output 95 90 85 80 75 70 65 60 0 500 1,000 1,500 2,000 2,500 3,000 Altitude (meters above sea level) Wärtsilä 34SG SC Wärtsilä 34SG Flexicycle Industrial gas turbine Fig. 6 - Wärtsilä engines maintain output even at high altitudes while gas turbine output degrades significantly above 1000 masl. [4] Recips give gas turbines a run for their money. Modern Power Systems. Global Trade Media, 5 February 2002. Web. 27 January 2015. <http:// www.modernpowersystems.com/features/ featurerecips-give-gas-turbines-a-run-for-theirmoney/ >. [5]Back, Andreas. Lifecycle cost knowledge will impact power plant investment decisions. In detail Wärtsilä Technical Journal, 2010: 9-13. [6]Overton, T. Top Plant: Sasol Gas Engine Power Plant, Sasolburg, South Africa. POWER Magazine. Access Intelligence, 01 September 2013. Web. 27 January 2015. <http://www.powermag.com/ sasol-gas-engine-power-plant-sasolburg-southafrica/?printmode=1 >. [7]Voyles, Bennett. On Top of the World. Twentyfour7. Wärtsilä. 1/2010: 55-58. [8]Finn, Dennis, Anna Jarowicz, and Chauncet Thomas. Providing fast wind following response. In detail Wärtsilä Technical Journal, 2012: 26-30. in detail 27