Gas-fired efficiency in part-load and pulse operation

[ ENERGY / IN DETAIL ] [ ENERGY / IN DETAIL ] Gas-fired efficiency in part-load and pulse operation AUTHOR: Dawn Santoianni Gas-fired power plants are the most responsive and flexible generating assets in electricity markets, adjusting output to balance system demand and loads. But technology choice matters for efficiency in part-load and pulse operation. The efficiency and economic performance of Wärtsilä power plants during cycling operation is far superior to gas turbines. 16 in detail Preferential access for renewables and feed-in tariffs often result in fossil-fueled generation curtailing output to allow for wind and solar loads. This periodic adjustment of output is called cycling. A recent assessment by the California Independent System Operator (CAISO) in the U.S. demonstrated the need for significant cycling over two hours during the morning peak (8 MW) and the end of the work day (13,5 MW) to meet demand when solar and wind are not producing (Figure 1) [1]. This challenge is not unique to California, as many countries are adopting energy policies to achieve 2 percent or more from renewable sources. Surges in wind and solar output in Germany recently peaked at 59 percent of electric generation during a single day, causing conventional power plants to ramp down significantly. Simple cycle gas turbines have traditionally served as peaking units because they can be started within minutes and ramped up and down quickly to meet spikes in demand or sudden changes in electric system loads. They also have lower efficiencies less than 4 percent so they

WÄRTSILÄ TECHNICAL JOURNAL 1.215 Load, Wind Solar Profiles High Load Case January 22 44, 4, 8, 8, 8, Load & Net Load (MW) 36, 6, 32, 5, 4, 28, 3, 24, 2, 2, 1, : 3: 6: 9: 12: 15: 18: 21: : Load Net Load Wind Solar Wind & Solar (MW) Fig. 1 - CAISO load profile demonstrates need for rapid, short duration (pulse) generation to accommodate load fluctuations from wind and solar sources. Image credit: Combined Cycle Journal. operate during when electric demand peaks and the price of electricity is high. With the expanding need for more flexible power, capacity that was designed for continuous, baseload operation is often being used to provide load-following and even peaking electric service. This is particularly true for combined cycle gas turbines (s) which can respond to changes in load much faster than conventional steam power plants. The cycling of plants presents other issues however, including increased thermal and mechanical stress on plant components and load turndown limitations. The performance of cycling power plants at part load and for short duration pulsed output is an important consideration for minimizing power system emissions, maintaining high efficiency, and maximizing operational flexibility. This article explores the partial load limitations and efficiency performance of internal combustion engines (ICEs) compared with gas turbines. Minimum environmental load A technical constraint for partial load operation of gas turbine power plants is the minimum environmental load, also called the minimum emissions-compliant load. This is the lowest output at which the generating unit can operate and still meet environmental limits for nitrous oxides (NOx) and carbon monoxide (CO) emissions. The minimum environmental load for most s is about 5 percent of full output. To facilitate a wider range of gas turbine output, manufacturers have introduced control systems designed to extend emissionscompliant turndown while minimizing efficiency impacts at part load. While the exact methods for turndown optimization vary from manufacturer to manufacturer, the control systems use variable guide vanes to decrease compressor mass flow and sequential firing (reheat) to produce higher combustion temperatures at low loads. Higher combustion temperatures not only enhance the conversion of CO to carbon dioxide (CO2) but also boost steam production and thus output from the steam turbine, improving overall part-load plant efficiency. As a result, some gas turbine models can achieve emissions-compliant turndown to about 4 percent of baseload power. in detail 17

[ ENERGY / IN DETAIL ] [ ENERGY / IN DETAIL ] Effiency (%) 6 55 5 45 4 35 3 25 2 Minimum load for Flexicycle steam turbine Extended gas turbine turndown Typical minimum GT load destriction due to emissions 15 Flexicycle running in simple cycle mode 1 5 Minimum load for Wärtsilä reciprocating engines due to emissions 1 2 3 4 5 6 7 8 9 1 Percent of full load GE 7FA.5 Combined cycle, 25 C GE 7FA.5 Simple cycle, 25 C Siemens SGT6-5F Combined cycle, 25 C Siemens SGT6-5F Simple cycle, 25 C, 25 C Wärtsilä Simple cycle, 25 C Fig. 2 - Part load efficiency of Wärtsilä engines compared with gas turbines. For all practical purposes, ICE power plants do not have minimum load limitations and can maintain high efficiency at partial load due to modularity of design operating of a subset of the engines at full load. Impacts of cycling: part load efficiency Gas turbine manufacturers boast efficiencies of 55 percent or greater, but this is the efficiency at full output. Cycling and operation at partial load negatively affects efficiency. To compare the performance of s, simple cycle gas turbines, and Wärtsilä ICEs at varying load, efficiency data was produced using GT PRO [2]. The gas turbines chosen for comparison were based on popular heavy frame industrial models well-suited for combined cycle operation that could also be used in simple cycle operation as peaking units. Similar sized units are compared, with 18 in detail capacities of approximately 18 275 MW in simple cycle, and 235 31 MW when running in combined cycle mode (depending on ambient conditions). This assumes a 1x1 configuration (one gas turbine and heat recovery steam generator supplying one steam turbine), air-cooled condensers and a bypass stack to isolate the steam generating portion of the plant from the gas turbine. Figure 2 shows efficiency curves for plants operating at summer ambient conditions of 25 C (77 F). The efficiencies of s drop below 5 percent between 55 to 65 percent of full load. In simple cycle mode, the degradation of gas turbine efficiency is more pronounced, with gas turbines dropping to less than 3 percent efficiency at half load. The minimum environmental load of 5 percent for typical GT turndown and 4 percent for extended turndown is noted in Figure 2. For a 3 MW combined cycle plant, this means that the minimum emissions-compliant output is between 12 to 15 MW. Unlike gas turbines, Wärtsilä ICE power plants have near full range capability of emissions-compliant turndown. As load is decreased, individual engines within the generating set are shut down to reduce output. The engines that remain operating can generate at full load, retaining high efficiency of the generating set. Flexicycle efficiency is above 48 percent all the way down to 23 percent of full load (69 MW). Beyond the minimum load for the Flexicycle steam turbine, the engines will operate in simple cycle mode. Thus, the output of a 3 MW Flexicycle plant can be turned down to only 18 MW. As a result, Flexicycle power plants provide a much wider range of output flexibility than gas turbines without

WÄRTSILÄ TECHNICAL JOURNAL 1.215 1 Plant Load (% of Full Load) 8 6 4 2 6 6 12 18 24 3 36 42 48 54 6 Time from full load (minutes) Fig. 3 - Two-shifting (4 2 4 pulse production) for plant compared with. The fast startup and shutdown of the Flexicycle plant reduces fuel consumption and minimizes non-productive operating hours. the constraints of turndown limitations or efficiency impacts. Impacts of cycling: pulsed operation A key characteristic of flexibility is cycling to produce short-duration or pulse load. Pulsed loads are produced in response to the sudden loss of a power generator, reduced output from wind and solar sources, or spikes in demand. Pulse generation improves system reliability and power quality by stabilizing the electric grid. Power plant owners want to optimize operation to minimize fuel consumption and maximize revenue. The startup time, time to ramp up output to full load, minimum load restrictions, and the efficiency at part load determine the amount of fuel consumed (energy input) and the length of time the plant operates each day. By producing output during peak demand times, the power plant earns revenue from high electric tariffs. Ramping up and down should be as fast as possible without risking damage to the power plant equipment. As explored deeper in other articles in this issue, s have technical constraints that affect their startup time, ramp rate, and minimum load for maintaining hot conditions. Cycling raises concerns about increased air emissions and thermomechanical stress on plant equipment [3]. Wärtsilä engines have lower exhaust gas temperatures (36 C) compared with a gas turbine (6 C) and thus lower steam temperatures, enabling quicker startup and ramping to full load. Typically, a load following ramps up slowly during early morning hours, operates at full load during morning peak demand, curtails output during midday hours to a minimum operating load at which hot conditions are maintained (4 to 5 percent of full load), then ramps back up again to full load for the evening peak. The plant is shut down at night and then ramped up again in the early morning hours the next day. This double pulse load profile, also called two-shifting or two-cycling for a typical compared with a Wärtsilä Flexicycle power plant is shown in Figure 3. The duration of each pulse is 4 hours, with a curtailment in between pulses of 2 hours (4 2 4 pulse production). The shaded area in Figure 3 shows when the power plant would be receiving revenue from pulse production. As can be seen in Figure 3, the slower startup time and the minimum load limit increase the total time the plant is operating and thus its overall energy (fuel) consumption and operating expenses. While a typical requires about 6 minutes to reach full load in the morning, a Wärtsilä power plant starts within a few minutes. Energy use The cumulative energy input of a 2 MW Wärtsilä power plant compared with a similarly sized for two-shifting operation (4 2 4 pulse production) is shown in Figure 4. The amount of energy input is a function of the efficiency of the power plant and the total operating time. The rapid Wärtsilä start eliminates the need for a load hold point to maintain hot conditions, reducing the overall operating time. This in turn reduces the amount of energy (fuel) input to generate revenueproducing load. As a result, the cumulative energy input of a Flexicycle plant is five in detail 19

[ ENERGY / IN DETAIL ] [ ENERGY / IN DETAIL ] Cumulative energy input (MWh) 2 18 16 14 12 1 8 6 4 2 6 12 24 36 48 6 Time from full load (minutes) Shorter pulses and more cycling decrease the efficiency of s and increase the amount of energy input (fuel) per MWh of electricity produced. This impacts the plant s profitability by increasing operating and fuel costs. Figure 6 illustrates the disparity in income per pulse for a 2 MW Flexicycle power plant compared with a of similar size for the most common pulse outputs. While the Wärtsilä power plant is profitable for any pulse duration or cycling, a plant is not profitable for short duration pulse operation. Fig. 4 - Two-shifting (4 2 4 pulse production) cumulative energy input for Wärtsilä Flexicycle plant compared with. The fast start of a Wärtsilä power plant reduces total fuel consumption. Cash flow (EUR) 8 6 4 2 2 4 6 8 Fig. 5 - Two-shifting (4 2 4 pulse production) cash flow for plant compared with based on startup costs, fuel costs and O&M costs in Table 1. percent less than a over the same operating period. Profitability The cash flow for 4 2 4 pulse production is shown in Figure 5 for a 2 MW plant. The cash flow projection takes into account startup costs, fuel costs, and O&M costs (see Table 1). The shaded area indicates negative cash flow the power plant is costing more money to operate than revenue earned. As can be seen in Figure 5, a does 2 in detail 1 6 12 24 36 48 6 Time from full load (minutes) not become profitable until the end of the second four-hour pulse. The decline in cash flow after the first pulse is because the gas turbine continues to run at the minimum hold point and burns fuel less efficiently until it ramps back up again for the second pulse during the evening peak. There may be several types of pulse load output that a power plant produces over the course of a year including one-hour, two-hour, four-hour, and eight-hour single pulses, as well as 2 2 2 pulse operation. Pulse efficiency Pulse efficiency is the net efficiency for the duration of the operating period, including startup, shutdown and part load operation. As discussed above, the efficiency of a plant at part load can be dramatically different than the baseload efficiency. ICEs can reach full load and curtail output to zero within minutes. Based on modular architecture, Wärtsilä power plants maintain full load efficiency at part load by running only a subset of the engines. The impact of pulsed operation (cycling) on efficiency is seen in Figure 7. While the has higher efficiency during baseload operation at full output, the Flexicycle plant achieves higher overall efficiency for all pulses less than 8 hours long. Impacts of pulse operation on efficiency is most pronounced for pulse duration of two hours or less. Wärtsilä power plants are efficient and economic over a wide range of pulsed loads, providing ultra-flexible capacity for meeting the challenges of renewables integration and changing demand. References [1] Integrating Renewables May Call for Some Combined Cycles to Start Twice Daily, Increasing Emissions. CCJ Online Combined Cycle Journal. CCJ Online Inc., 21 Oct. 212. Web. 27 Jan. 215. <http://www.ccj-online.com/integratingrenewables-may-call-for-some-combined-cyclesto-start-twice-daily-increasing-emissions/>. [2] GT PRO. Thermoflow Products - Combined Cycle. Thermoflow Inc., n.d. Web. 27 Jan. 215. <http:// www.thermoflow.com/combinedcycle_gtp.html>. [3] Kumar, N., P. Besuner, S. Lefton, D. Agan, and D. Hilleman. Power Plant Cycling Costs. Rep. no. NREL/SR-55-55433. Golden, CO: National Renewable Energy Laboratory, 212.

WÄRTSILÄ TECHNICAL JOURNAL 1.215 1 Pulse income 5 5 8-hr 4-hr 2-hr 1-hr 4-2-4 2-2-2 EUR 1 15 2 25 3 Fig. 6 - Income per pulsed operation for a 2 MW power plant. The plant is profitable for a wide range of pulse operation while the plant is not profitable. 55 Pulse effiency 5 Effiency (%) 45 4 35 3 25 Baseload 8-hr 4-hr 2-hr 1-hr 4-2-4 2-2-2 Fig. 7 - Net efficiency over pulse duration for plant compared with. Wärtsilä plants are more efficient than s for short duration pulse operation. Base values for technologies Wärtsilä plant Electricity and fuel prices Full load efficiencies (%) 45.7 simple cycle 49.7 Flexicycle 55. 1 hour pulse 75 EUR/MWh el Startup time (minutes) 6 6 2 hour pulse 7 EUR/MWh el Shut-down time (minutes) 2 3 4 hour pulse 65 EUR/MWh el O&M costs (EUR/MWh) 5 3 8 hour pulse 6 EUR/MWh el Startup costs (EUR/MW) 5 Non-pulse hours EUR/MWh el Baseload generating cost 59.7 simple cycle (EUR/MWh el) 55.3 Flexicycle 48.5 Fuel price 25 EUR/MWh Table 1 - Cost inputs for calculation of pulse cash flow and income, based on nominal 2 MW power plant. in detail 21