Combustion Engine Vs Gas Turbine 2p2x24

Combustion Engine vs Gas Turbine: Ambient Temperature The increasing need for flexible power across the world, often in harsh climates, makes power plant performance under varying conditions an important consideration in technology selection. As combustion engines are less sensible to temperature and humidity, Wärtsilä power plants outperform gas turbines in hot conditions. Depending on the technology and 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 can vary dramatically with geographic location and by season. For example, summer temperatures in the Middle East and northern Africa frequently exceed 40°C (104°F), while large seasonal temperature swings of over 38°C (100°F) occur in some locations. As surging temperatures usually correspond to peak electrical demand, reduction in power output at high ambient temperatures can be problematic. Gas turbines in particular can experience significant performance derating in hot, humid conditions.

Image credit: Maps of the World

How do ambient conditions affect power plant output and efficiency? Ambient temperature, altitude and humidity affect the density of air. Hot and humid air is less dense than dry, cooler air. In gas turbines, power output is dependent on the mass flow through the compressor. 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 conditions1. 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. Combustion enginesare less sensitive to temperature and humidity, retaining their rated efficiency and power output over a broader range of ambient conditions.

The performance of simple cycle gas turbines, combined cycle gas turbines (CCGT) and Wärtsilä combustion engines at varying ambient conditions was assessed using data from GT PRO. Popular model heavy frame industrial gas turbines were compared with similarly sized Wärtsilä engines, with capacities of 200 – 275 MW in simple cycle, and approximately 300 MW in combined cycle (see fact box at end of article 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 by stack to isolate the steam generating portion of the plant from the gas turbine. Figure 1 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 combustion engines operating in combined cycle. 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.

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Figure 1: Impact of ambient temperature on Wärtsilä Flexicycle and CCGT power output. The impact on plant efficiency is shown in Figure 2 for both combined cycle and simple cycle operation. At an ambient temperature of 40°C, CCGT efficiency decreases by 3.5 percent compared to ISO conditions. In a Flexicycle™ power plant using combustion engines, 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. As a result, Wärtsilä power plants offer a wide range of operational flexibility and reliable performance, even in harsh ambient conditions.

Figure 2: Efficiency of Wärtsilä engines compared with gas turbines at varying ambient temperatures.

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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).

Combustion Engine vs. Gas Turbine: Part Load Efficiency and Flexibility The performance of power plants at partial load has become a significant operational consideration for electric power grids worldwide. This technical comparison examines the range of output and the part load efficiency of combustion engines and gas turbines, and how Wärtsilä power plants deliver enhanced flexibility. 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 U.S. National Renewable Energy Laboratory (NREL) study demonstrated that high penetrations of wind and solar caused fossil-fueled plants to cycle more frequently (Figure 1).

Figure 1: Dispatch curve shows curtailment of coal and cycling of combined cycle plants to accommodate large load fluctuations from wind and solar sources during one challenging week in the U.S. Western Interconnection. Image credit: NREL (2013) The Western Wind and Solar Integration Study Phase 2 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 40 percent – so they 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 (CCGTs) which can respond to changes in load much faster than conventional steam power plants. The cycling of CCGT 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 is an important consideration for minimizing power system emissions, maintaining efficiency, and maximizing operational flexibility. The part load technical limitations and efficiency performance of combustion engines compared with gas turbines are explored below. 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 gas turbines is about 50 percent of full output because operation at lower loads can result in reduced combustion temperature, less conversion of CO to

CO2 and potential emissions permit exceedances. In combined cycle plants, the gas turbine outlet temperature must also be kept high to produce sufficient steam to power the steam turbine. To facilitate a wider range of gas turbine output, manufacturers have introduced control systems designed to extend emissions-compliant 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 CO2 but also boost steam production and thus output from the steam turbine, improving overall partload plant efficiency. As a result, some gas turbine models can achieve emissions-compliant turndown to about 40 percent of baseload power. Site-specific conditions including the environmental permit requirements, plant configuration and post-combustion emissions control systems will ultimately dictate the exact emissionscompliant turndown limit. For all practical purposes, combustion engine power plants do not have minimum load limitations and can maintain high efficiency at partial load due to modularity of design – the operation of a subset of the engines at full load. Part Load Efficiency Gas turbine manufacturers boast efficiencies of 55 percent or greater, but this is the efficiency at full, or baseload power. In reality, CCGT power plants often cycle frequently. Operation at partial load and turndown limitations can restrict the flexibility of CCGT plants. To compare the performance of CCGTs, simple cycle gas turbines, and Wärtsilä combustion engines at varying load, efficiency data was produced using GT PRO. 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 capacities of approximately 180 – 275 MW in simple cycle, and 235 – 310 MW when running in combined cycle mode (depending on ambient conditions). This assumes a 1x1 CCGT configuration (one gas turbine and heat recovery steam generator supplying one steam turbine), aircooled condensers and a by 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 25ºC (77ºF). The efficiencies of CCGTs drop below 50 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 30 percent efficiency at half load. The minimum environmental load of 50 percent for typical GT turndown and 40 percent for extended turndown is noted in Figure 2. For a 300 MW combined cycle plant, this means that the minimum emissions-compliant output is between 120 to 150 MW.

Figure 2: Part Load Efficiency of Wärtsilä Engines Compared with Gas Turbines Unlike gas turbines, Wärtsilä engine 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 power plant 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 300 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 the constraints of turndown limitations or efficiency impacts.

Reciprocating Engine vs Gas Turbine: Advantages of Modularity Forget the conventional wisdom that “bigger is better” – modularity in power plant design enhances flexibility and reliability. Reciprocating engine power plants comprised of multiple generating units complement renewable energy without sacrificing efficiency. And unlike large combined cycle gas turbines, reciprocating engine power plants have a wide range in load turndown making them ideally suited to distributed energy applications. Over the course of a century, the trend in the electric power industry had been toward ever increasing generating unit sizes and plant capacities. Centralized power plants were built using custom engineered technology of massive size. Conventional wisdom was that “bigger is better” as the capital costs per unit of capacity and production costs declined with increasing unit size, delivering economies of scale driven in part by improved steam turbine efficiencies. Steam turbine units averaging 400 megawatts (MW) or more

capacity were common by the 1980s. The push for higher outputs and efficiencies directly led to the development of combined cycle, necessitating larger gas turbines with higher firing temperatures that enabled exhaust gas heat recovery to drive a steam turbine. While in the 1950s the firing temperature of gas turbines was around 800ºC and average turbine size was around 10 MW, by the 1990s advanced gas turbines had firing temperatures exceeding 1300ºC and averaged over 100 MW. However, large power plants required considerable on-site construction and assembly, and could not easily adjust load to meet fluctuating demand. As the use of renewable energy sources such as wind and solar increased, the need for more flexible power that could quickly start, ramp to full load, and operate at part load became more apparent. Smaller-scale generating units that can be operated in parallel and deployed as needed to match the changing power requirements began to serve an important function for the stability of electric transmission grids. Using modern manufacturing and engineering, small electric generating units are modularized to suit current and future needs of any power project. This shift toward “economies of numbers” provides reliability, siting, and efficiency benefits. What is Modularity? Many manufacturers of power generating equipment promote the concept of “modularity” in product portfolios and plant architecture. Modularity refers to electric generating equipment that is prefabricated in a factory environment and packaged so as to shorten the timeframe needed to plan, engineer and construct a power plant. Because gas turbines were expensive relative to other technologies, the concept of prepackaging the gas turbine within a power plant and shortening delivery times became essential to capture market share. Prefabricated power generation modules are self-contained components of the system that are designed to interface with other power plant components. Engine modules, fuel delivery systems, emissions control equipment, heat recovery steam generator (HRSG) modules, generators, pipe racks and other auxiliary systems are some of the components that can be prefabricated and delivered in modules to the power plant site. Standardizing the design of these components offers significant manufacturing cost savings, shorter lead times for equipment delivery, and speeds on-site assembly and commissioning of the plant. Modularity also offers simplified maintenance features and quality benefits, as components are manufactured in a controlled environment and factory tested. Modular design has been deployed for a range of generating sources, from small nuclear reactors to solar farms to gas engines. Even gas turbine power plants, which have traditionally required significant on-site assembly, have begun to be designed in a modular fashion to shorten construction time. Aeroderivative gas turbines in particular, have been designed with modular architecture and range in size from about 15 to 120 MW. Because they have lower exhaust gas temperatures than industrial gas turbines, aeroderivatives are not typically operated in combined cycle mode. Modularity can encom more than just design and prefabrication – it can extend to plant sizing and the efficient use of a subset of the plant capacity to provide a range of load response. Plant Configuration and Operational Modularity Interconnecting different modules offers flexibility of design to meet a variety of project needs. Because generating units are incrementally sized, a wide range of plant capacities and fuel options – including multifuel use – can be designed. The control systems and interfaces between modules are designed to accommodate fuel selection, expected operations and emissions permit limits. Reciprocating engines are ideally suited to modular use, as sets of 4 – 30 MW engine units can provide a range of incremental part load power without sacrificing efficiency. For example, a Wärtsilä power plant that has 28 modular 34SG reciprocating engine units, each sized at approximately 10 MW, can deliver a range of output from just a few MW to over 270 MW. By operating only a subset of the engines at full load to produce the desired output, high efficiency is maintained. Further, because the startup time for reciprocating engines is within minutes, the power plant can quickly adjust load by bringing additional engine sets online to meet changes in electric demand.

Wärtsilä modular power plant comprised of multiple reciprocating engine generating units provides output versatility. Modularity in architecture provides limited operational modularity for gas turbines, however. This is particularly true of gas turbine power plants due to the size of units, limited number of units, and efficiency tradeoffs for simple cycle versus combined cycle. Industrial gas turbines for power generation may be 100 – 350 MW apiece and have limits on the lower range of output at which they can operate. This minimal load, or “turndown” percentage, is bounded by emissions limits. When the turbine operates at low load, the compressor airflow may not be enough to conversion of carbon monoxide (CO) into carbon dioxide (CO2) in the combustion chamber. Although manufacturers offer enhanced design features such as inlet guide vane control, gas turbines are generally constrained to a turndown of 30 to 40 percent of full load to meet emissions regulations. A simple cycle power plant with two gas turbines can adjust plant output down to about 15 to 20 percent of full load by operating only one turbine.

Large gas turbine power plants (1x1 configuration shown) are limited in their turndown capability. Combined cycle operation introduces more complexity into the operating parameters of the plant. Modular architecture for combined cycle gas turbine (CCGT) power plants consists of one to four gas turbines, HRSGs for each gas turbine, and a common proportionally-sized steam turbine. Modularization is intended to provide turnkey power plant solutions with reduced on-site assembly and higher part load efficiencies because of the steam turbine output. The steam turbine s for about one third of the total plant output and is inflexible capacity because of the time required to achieve necessary steam conditions and for the steam turbine to warm up. The lower load limit is affected by the turbine exhaust temperature, which must be high enough to generate sufficient steam pressure in the HRSG to power the steam turbine. The typical configuration of a 2x1 CCGT plant which has two gas turbine/HRSG units supplying one steam turbine may have the ability to operate one of the gas turbines independently of the other, depending on emissions criteria. Emissions compliant turndown for CCGT plants is usually 40 to 50 percent of full load. For example, a combined cycle power plant design based on 200 MW gas turbines (in the typical 2x1 configuration) has a rated output of over 600 MW, limiting turndown ability to about 300 MW. A Flexicycle power plant based on modular reciprocating engine units does not have similar restrictions on load turndown because sufficient steam pressure can be developed by operating only 25 percent of the generating units. Modularity using reciprocating engine units has other operational benefits over gas turbine power plants. The “economies of numbers” provides reciprocating engine power plants built-in redundancy in case of unit outages or maintenance without significantly affecting overall full plant output. Modular engine technology also allows siting to complement distributed renewable energy sources in areas that lack transmission infrastructure to large power stations, providing better matching to changing grid needs. Expanding power needs in the future can be met with the addition of more engine units and ancillary modules, rather than the construction of a new power plant. Reciprocating engine technology provides the needed versatility and flexibility of load to compensate for variable smaller, distributed scale of renewable sources which may only be a few MW. And importantly, using small modular reciprocating engines to provide flexible load allows larger combined cycle plants to operate at full output, taking advantage of their high efficiencies at full load and reducing electric system costs.

Combustion Engine vs Gas Turbine: Startup Time Power plants that can be dispatched within minutes are important assets for balancing electric system loads and maintaining grid reliability. The generating technology affects the time required for a power plant to startup and reach full load. While combined cycle gas turbines can take over 30 minutes to start, combustion engine power plants can start and reach full load in less than 10 minutes – providing flexible, quick-start capability.

Increasing penetration of renewable energy sources presents challenges for transmission grid operators to maintain electric reliability despite the intermittency of wind and solar power. This variability is managed with redundant generating capacity that can quickly respond to fluctuations in demand, and has predominately been served by coal and gas-fired units that are synchronized to the grid but operating at part load. Flexible power generation that can be rapidly brought online reduces the inefficiency of relying on part load operation. System operators, such as PJM, California ISO and ERCOT define such “quick-start” or “nonspinning” reserve as generation capacity that can be synchronized to the grid and ramped to capacity within 10 minutes. Whereas conventional steam cycle generators (based on the Rankine cycle) can take more than 12 hours to reach full load, internal combustion engines can be dispatched in minutes. The two primary internal combustion engine technologies utilized for power generation are reciprocating combustion engines and gas turbines. The differences between the two technologies affect startup time and their suitability to provide flexible power.

Startup time is a significant metric for flexibility, but comparison of different technologies and designs is complicated by the way startup time is measured by different manufacturers. The startup time quoted can be from push of the start command or from ignition. In the case of gas turbines, this difference in “start” definition can be as much as 20 minutes. Further, it is important to differentiate between time to achieve full load versus partial load. Gas Turbine Power Plant Startup During startup, the gas turbine (GT) undergoes a sequence of increasing compressor spin to reach firing speed, ignition, turbine acceleration to self-sustaining speed, synchronization, and loading. There are numerous thermo-mechanical constraints during startup of the GT, including limits on airflow velocity through the compressor blades to prevent stall, vibrational limits, and combustion temperature limits to prevent turbine blade fatigue, with the significant parameter being the turbine inlet temperature. In combined cycle operation, the heat recovery steam generator (HRSG) imposes additional thermal limitations on the gas turbine power plant, as the high temperature environment subjects HRSG components to thermal stress. The HRSG is directly coupled to the gas turbine, so changes in turbine exhaust gases induce flow, temperature, and pressure gradients within the HRSG. These gradients must be carefully controlled to prevent adverse impacts such as material fatigue, creep (damage caused by high temperatures) and corrosion. In order to avoid impacts, it takes longer to start the HRSG from cold conditions than from hot conditions. The definition of “hot” conditions varies by manufacturer, but is generally defined as within eight (8) to 16 hours of HRSG shutdown. As a result, the amount of time elapsed since last shutdown greatly influences startup time. Once-through HRSGs are used by some manufacturers to overcome the startup thermal and pressure limitations that exist with steam drums. CCGTs are also subject to purge requirements to prevent auto-ignition from possible accumulation of combustible gases in the gas turbine, HRSG and exhaust systems. The purge is required before the unit is restarted. Purge times depend on the boiler volume and air flow through the HRSG, and are typically set to about 15 minutes. This purge time adds to the overall start time. In addition, the steam turbine can restrict the GT loading rate if the steam temperature leaving the HRSG exceeds steam turbine limits. To avoid this, temperature matching using GT holds as the load is increased may be necessary. In order to enable faster startup, CCGT manufacturers have attempted to decouple the gas turbine startup from the HRSG and steam turbine warm-up. Process- and equipment-enhanced start options have been developed that can be used under hot start conditions. A “purge credit” allows the system purge to be completed at shutdown, eliminating the requirement for a redundant purge at next startup. The purge credit can only be used in some HRSGs that have no duct burners and where the GT is fired on natural gas only. By dampers can be used to restrict the exhaust gas flow to the HRSG. However, pollution control equipment for nitrogen oxides (NOx) and carbon monoxide (CO) are typically integrated within the HRSG and environmental regulations for these emissions may prohibit the startup of the GT without the HRSG. Another method for decoupling the HRSG and steam turbine from the GT exhaust gas usesspray water attemperators or air attemperators to control the steam temperature so that gas turbine loading is not limited for temperature matching. This enables parallel loading of the gas turbine and steam turbine. Although hot start conditions for CCGTs vary somewhat by manufacturer, maintaining energized electrical systems, purge credit, and steam temperature control enable CCGT startup times of about 30 to 35 minutes from initiation of the start sequence. This is about half the time for conventional hot start that would require purge and gas turbine holds. In simple cycle, published start times for gas turbines are about 10 to 15 minutes. Combustion Engine Power Plant Startup A combustion engine power plant can start and ramp to full load very quickly due to rapid ignition of fuel within the cylinders and the coordinated starting of multiple generating sets. Wärtsilä combustion engine power plants employ high efficiency lean-burn technology that can reach full load in as little as two (2) minutes under “hot start” conditions. To meet “hot start” conditions, cooling water is preheated and maintained above 70ºC, engine bearings are continuously prelubricated, a jack up pump supplies prelubrication to the generator bearings, and the engine is slow turning (cycling). The Wärtsilä 34SG power plant requires only 30 seconds to complete startup preparations, speed acceleration, and synchronization to the grid. Loading to full power occurs rapidly in just 90 seconds. Startup time is not affected by the amount of time the unit had been previously shut down. The 50SG power plant takes seven (7) minutes to reach full load. Under cold startup conditions, the 34SG power plant can reach full load in 10 minutes and the 50SG in 12 minutes. Combustion engine power plants also have combined cycle advantages as sufficient steam pressure can be generated with only a subset of the engines operating.

Rapid Startup for Flexible Power Generation Figure 1 shows a startup time comparison of the Wärtsilä 34SG and 50SG power plants with simple cycle and combined cycle gas turbine plants from manufacturers GE, Alstom, and Siemens. All startup times are measured from operator initiation of the start sequence. As can be seen from the graph, Wärtsilä power plants provide quick start ability under 10 minutes, which meets system operator requirements. Unlike CCGTs, hot start conditions in a Wärtsilä power plant can be maintained regardless of how long the engines had previously been inactive.