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Gas turbine efficiency
02-SEP-2005

how to maximize it

Gas turbines form the base of many cogeneration systems, so the efficiency of the turbine is fundamental to the efficiency of the overall plant. Here, David Flin goes back to basics with a look at how the turbines work and how their design affects performance.

A gas turbine functions by allowing the passage of expanding combustion gases through a series of turbine blades (see Figures 1 and 2). The efficiency of the turbine is measured by comparing power input to power output as measured by mechanical energy in the output shaft. Gas turbine efficiencies are usually given for ISO conditions at 15°C, 60% relative humidity and an atmospheric pressure equivalent to average sea level conditions. Variations in temperatures and relative humidities during operation of the turbine will result in changes to its efficiency.

The efficiency of a gas turbine – more correctly called the overall thermal efficiency – is the ratio of work done to the heat supplied. Efficiency is defined as:

Efficiency = 100 x K x (Tmax – Tmin)/Tmax

Tmax is the temperature of the gas at the inlet to the gas turbine, Tmin is the ambient temperature and K is internal losses.

As a result, there are three theoretical methods of increasing efficiency: increasing inlet temperature, decreasing ambient temperature and reducing internal losses.

Theoretically, a gas turbine could achieve efficiencies of up to 65%. At present, simple open-cycle turbines achieve efficiencies of about 40%. In addition, it is possible to use waste heat from the outlet of the gas turbine to improve efficiency of use. This is where the very high overall efficiencies from cogeneration come from.

The basic gas turbine cycle is shown in Figure 3. Air is compressed from point 1 to point 2. This increases the pressure as the volume of space occupied by the air is reduced. The air is then heated at constant pressure from point 2 to point 3. This heat is added by injecting fuel into the combustor and continuously igniting it. The hot compressed air at point 3 is then allowed to expand (point 3 to point 4), reducing the pressure and temperature and increasing the volume. This represents flow through the turbine to point 3’ and then flow through the power turbine to point 4. The combustion cycle is completed by decreasing the volume of air (point 4 to point 1) through decreasing the temperature, with heat being absorbed into the atmosphere.

This cycle is the simple gas turbine cycle, called the Brayton cycle. However, additional equipment and techniques can be used to increase the efficiency of the cycle. These modifications include: regeneration, intercooling and reheating.

Microturbines Typical microturbines have efficiencies of 25%–35%. When used in a CHP system, they can achieve efficiencies of greater than 80%. Microturbines have additional design considerations to take into account, however. As a general rule, microturbines have to be designed with the premise that once installed they will receive very little maintenance, except possibly an annual service. But they will need to be reliable despite this. In addition, microturbines need to be small, limiting opportunities for peripheral equipment, and they need to be quiet, limiting opportunities for additional rotating machinery. These factors tend to limit the opportunities to increase turbine efficiencies. There is a limit, for example, on the closeness of blade tips and the casing because of the potential variation in operating regimes. Methods of reducing blade fouling are limited to annual services.

In general, efficiency is influenced by:

• energy used by the air compressor – if less energy is used to compress the air, more energy is available at the output shaft
• temperature of the gas leaving the combustors and entering the turbine – the higher the temperature, the greater the efficiency
• temperature of the exhaust gas from the turbine – the lower the temperature, the greater the efficiency
• mass flow through the gas turbine – in general, higher mass flows result in higher efficiencies
• pressure drop across inlet air filters – increased pressure loss decreases efficiency
• pressure drop across exhaust gas silencers, ducts and stack – increased pressure loss decreases efficiency.

There has been considerable work done to improve the efficiency of gas turbines, mainly on increasing turbine entrygas temperatures and increasing the efficiency and capability of the compressor. Various methods have been used to improve efficiency in these areas. These include:

• using the exhaust gas to heat the air from the compressor – this is most effective in cold weather
• dividing the compressor into two parts and cooling the air between the two parts
• dividing the turbine into two parts and reheating the gas between the two parts
• cooling the inlet air – this is mainly used in hot weather
• reducing the humidity of the inlet air
• increasing the pressure of the air at the discharge of the air compressor
• regularly washing or otherwise cleaning the fouling of turbine and air-compressor blades.

However, there is a trade-off with all of these methods of increasing efficiency. They all increase costs, and some reduce the available power output of the gas turbine. The final design choice will be the most appropriate compromise that balances cost, power and efficiency for each specific application.

These different approaches can be broken down into five main categories:

• increased inlet temperature
• regeneration
• compressor intercooling
• turbine reheat
• steam/water injection.

INCREASING TURBINE INLET TEMPERATURE

The most obvious way of increasing the efficiency of a gas turbine is to increase the inlet temperature. Efficiency is related to both the inlet and outlet temperatures of the turbine; the higher the difference between the two temperatures, the greater the thermal efficiency of the turbine. There is an absolute limit to how low the outlet temperature can go, so increasing the inlet temperature is an obvious method of increasing efficiency.

However, increases in inlet temperature have already reached the point where the temperatures are actually higher than the melting point of some of the metals used in the turbine. Cooling of the first rows of turbine blades is therefore imperative, and any further increases in inlet temperature will require improvements in cooling techniques. This could involve increased use of steam cooling, increased flow of the cooling fluid, or increased effectiveness of heat transfer with the cooling fluid.

In addition, different materials with improved heat-resisting properties and improved thermal barrier coating can also assist in allowing the elevation of the inlet temperature.

Nonetheless, the cost involved in increasing the efficiency of the turbine through raising the inlet temperature is becoming increasingly prohibitive. The materials involved are adding to the cost, while the addition of ever-more complex cooling techniques gives rise to more expensive production and more areas that need maintenance, which adds to O&M costs. It also introduces more items that can fail, potentially leading to greater outage time for the turbine.

The pressure ratio has to be increased alongside increases in turbine inlet temperature otherwise the turbine exit temperature will drop, resulting in a reduction in the combined cycle efficiency.

REGENERATION

Regeneration is the internal exchange of heat within the cycle. In the gas turbine cycle, the gases leaving the turbine are at a relatively high temperature. This temperature is higher than the temperature at the compressor outlet. Therefore, a regenerator (a surface-type heat exchanger) is used to preheat the compressed gases by using heat from the exhaust gas. This reduces the amount of fuel required by the combustor. Regeneration involves the installation of a heat exchanger (recuperator) through which the turbine exhaust gases pass. The compressed air is then heated in the exhaust gas heat exchanger before the flow enters the combustor, preheating the gas before it enters the combustion chamber, thus reducing the amount of fuel required (see Figure 4).

Use of a regenerator can increase the simple-cycle efficiency. However, the relatively high cost of such a regenerator is a disincentive to its use. Regeneration can improve the efficiency of simple gas turbines by 5%–6%. However, use of a regenerator reduces specific power output as a result of additional pressure losses in the regenerator.

According to the Managing Director of OPRA Gas Turbines, Frederick Mowill, gas turbines have exit temperatures of about 560°C. This heat can be of great use in providing energy to produce process steam or heating. The use of a recuperator reduces this exit temperature to about 330°C. As a result, while the efficiency of the gas turbine is increased by the use of a recuperator, the available energy from the heat of the gas exhaust is reduced. Operators have to decide which is more important for their specific needs.

COMPRESSOR INTERCOOLING

Another method of increasing the overall efficiency of a gas turbine is to decrease the work input to the compression process. The effect of this is to increase the net work output. This can be achieved by cooling the gas passing through the compressor. Intercooling involves compressing the fluid to an intermediate pressure, then passing it through a heat exchanger, or intercooler, where it is cooled to a lower temperature at essentially constant pressure. The fluid is then passed through another stage of the compressor, where its pressure is increased. This is followed by another intercooler process, and then another staging of the compressor, until the final pressure is achieved (see Figure 5). The overall result is a lowering of the net work input required for a given pressure ratio.

Intercoolers can be air-cooled heat exchangers but are more commonly water-cooled. The output of a gas turbine is increased with an intercooler.

TURBINE REHEAT

Another method of increasing overall efficiency is to keep the gas temperature in the turbine as high as possible. This can be achieved by continuous heating of the gas as it expands through the turbine. Continuous heating as such is not practical, and reheat is carried out in stages (see Figure 6). Gases are allowed to partially expand before being returned to the combustion chamber, where heat is added at constant pressure until the limiting temperature is reached. The use of reheat increases the turbine work output without changing the compressor work or the maximum limiting temperature. Using the turbine reheat increases the whole cycle output. However, the final turbine exhaust temperature is above the outlet turbine temperature without reheat. As a consequence, reheating is most effective when used in conjunction with regeneration, as the quantity of heat exchanged in the regenerator can be greatly increased.

If a gas turbine has a high-pressure and a low-pressure turbine at the back end of the machine, a reheater, usually another combustor, can be used to reheat the flow between the two turbines, giving an increase in efficiency of 1%–3%.

INJECTION OF STEAM OR WATER

Steam or water injection is a method by which the output power of a gas turbine cycle can be increased. This has several effects: it increases the flexibility of the gas turbine during part load operations and significantly decreases emissions of carbon monoxide and unburned hydrocarbons.

Steam injection can be carried out using saturated or superheated steam. The introduced steam is usually injected into the combustion chamber. However, water injection is a more common method of increasing efficiency than steam injection. Water is typically injected into the system at the compressor outlet to increase the mass flow rate. The compressed air temperature falls as a consequence of water injection, but this temperature reduction can be minimized through use of a regenerator. No more fuel is consumed in this temperature compensation because the process uses heat from the gas turbine exhaust that would otherwise be wasted.

Water can also be injected at the compressor inlet. This has the following advantage compared with injection at the compressor outlet:

• Because air at the inlet air duct is at about atmospheric pressure, there is no need to use a high pressure pump.
• The inlet air temperature is atmospheric, which means there is no need to warm up the spray water to prevent thermal shock.
• There is usually a long distance between the inlet air duct and the compressor inlet, so by the time the atomized water reaches the compressor, it will be thoroughly mixed with the air, so it is a homogenized mixture of water and air that will be introduced into the compressor. This reduces impact damage and corrosion effects on compressor components.

ADDITIONAL METHODS OF INCREASING EFFICIENCY

Several other methods of increasing efficiency are also employed.

Heat recovery

Gas turbines generate a large volume of very hot air. This exhaust is also high in oxygen content compared to other combustion exhaust streams because only a small amount of oxygen is required by the combustor relative to the total volume available. Depending on how much thermal energy is required, the turbine exhaust may be supplemented by a duct burner.

A duct burner is a direct-fired gas burner located in the turbine exhaust stream. It has a very high efficiency due to the high inlet air temperature and is used to boost the total available thermal energy. The turbine exhaust boosted by the duct burner is directed into the HRSG.

Turbine exhaust can also be ducted directly into hot air processes, such as kilns and material drying systems. This is the least costly first cost, as there is no boiler or steam drying system to purchase. Turbine exhaust can also be ducted directly into absorption chillers for large cooling loads.

The system will also include a diverter for those times when the waste heat is not required. The diverter vents the turbine exhaust to the atmosphere. This substantially reduces the system efficiency because only the electrical energy output of the turbine is being used.

The higher the electrical efficiency of the turbine the lower the available thermal energy in the exhaust. Newer turbines with recuperators and larger turbines tend to have higher efficiencies.

Inlet air cooling

Another method of increasing turbine efficiency is through inlet air cooling. This is most effective in hot, dry climates because in effect it reduces the ambient temperature entering the turbine. Gas turbines operate with a constant volume of air, but the power generated depends on the mass flow of air. Warm air is less dense than cold air, resulting in lower power output. Warm air is also harder to compress than cold air, taking more work from the compressor and thus increasing internal losses.

Inlet air cooling is sensitive to ambient conditions at the site, and thus selection of the correct inlet air cooling system is site specific. For a typical gas turbine, it is possible to cool the inlet air from 15°C to 5.6°C, increasing fuel efficiency by about 2%.

There are many technologies that are commercially available for turbine air inlet cooling. These technologies can be divided into the following major categories:

• evaporative: wetted media, fogging and wet compression/overspray
• chillers: mechanical and absorption chillers with or without thermal energy storage
• LNG vaporization
• hybrid systems.

More details on gas turbine inlet air cooling can be found in COSPP July–August 2004.

Reduction of leakage flow

One area of potential loss of efficiency lies in the leakage of flow through the gas turbine. This typically comes from inevitable leakage around the tips of the blades and vanes, as well as from other areas that require sealing.

Reduction of leakage is a compromise between reducing the leakage flow and allowing the leakage flow ‘necessary’ to avoid high temperature increases caused by disc friction or heat conduction or both. The most common method of optimizing blade design to achieve the best compromise is through the use of a blade tip shroud. These restrict gas leakage flow across the blade tip by using knife-edge seals designed to rub into a honeycomb seal material that is brazed onto the shroud blocks. However, centrifugal forces on the rotating stages of a gas turbine are very large. For example, an F-class blade weighing about 3.6 kg will exert a pull of over 440,000 N at operating speed. Typically, shrouds account for about 10% of the weight of a blade.

Another method of reducing flow leakage at blade tips is through the use of abradable material. The obvious method of reducing this flow leakage is to reduce the clearance between the blade tip and the casing. Reducing this clearance can result in the blade tips rubbing against the casing. By applying an abradable coating, some rubbing can be tolerated. The abradable coatings are designed to release fine wear debris while causing no wear on the blades. A balance needs to be struck between efficiency losses due to gas flow leakage and those due to friction effects.

Aerodynamic design of all components

The optimization of the aerodynamic design of all components can help reduce internal losses. However, there is an optimization process involved. For example, reducing pressure loss in the combustor will increase efficiency but may also result in a smaller stability range. It may also influence the possibility of having efficient cooling systems, especially in the first turbine stage.

Inlet air filtration

The operating conditions within a gas turbine can make particulate matter and chemical impurities in the air and fuel damage the blades of the turbine, either by collecting and sticking to the blades or by corroding and eroding them, which reduces their effectiveness and degrades the efficiency of the gas turbine. Consequently, the cleanliness of the air and fuel used has a major impact on the amount of maintenance required. There are two basic solutions to this: coating the turbine blades so that they can better resist this degradation; and preventing the impurities from entering the turbine in the first place.

Inlet air filtration will prevent a proportion of the impurities from entering the turbine. However, filtration also generally results in a pressure drop across the filtration system, resulting in a loss of efficiency. As a general rule, the more effective the filtration system is at removing particulate matter, the greater the pressure drop across the system and hence the greater the drop in efficiency. Designing a filtration system to maximize the first and minimize the second is a complex compromise. Precise selection will depend very heavily on site-specific operating conditions. More details on air inlet filtration can be found in COSPP March–April 2005.

Particles that stick to blades and vanes cause blade fouling. These interfere with the smooth flow of the air stream, resulting in a reduction in the mass flow through the turbine. Fouling also degrades the effective pressure ratio, also impacting on efficiency.

Fouling has to be washed away by spraying a liquid from a set of nozzles installed upstream of the inlet. The liquid follows the air stream, and mechanical movements and chemical action by the washing liquid removes the deposits. Washing can be carried out either on-line or off-line. On-line washing is not as effective as off-line washing, but has the advantage that the turbine is still generating power. On-line washing has also been criticized for causing erosion damage.

It should be noted that inlet air filtration doesn’t increase efficiency; it limits and reduces the loss of efficiency from particulate degradation.

AREAS OF DEVELOPMENT

There are several areas of research being carried out with regard to aerodynamic improvements, particularly with relation to the use of unsteady effects. One major field is the potential of stator clocking. Numerical analyses have predicted an efficiency increase of about 0.5% for an optimal clocking position. Experimental investigations in compressors and turbines have shown promising results, especially for axial turbines.

Another aspect of the unsteady flow phenomenon that is under investigation is the axial spacing between the rotor and the stator row blades. The potential increase in efficiency has been estimated at about 0.5%.

However, approaches based on periodic unsteady effects increase the risk of vibration excitation of the blades and vanes. Further investigation into these is needed before they can be used in industrial applications.

Another research activity is in 3D optimization. The vane and blade shapes combined with 3D side-wall contouring have been shown to have a great impact on secondary flow losses. Optimizing these can lead to a significant efficiency increase.

There is also work being carried out on new and improved cycles, such as ‘isothermal’ compression, involving cooling in the compressor, sequential combustion, bottoming cycles and semi-closed cycles.

One of the main factors complicating all this research is that none of the actions can be considered in isolation. Each effects the functioning of the gas turbine as a whole. For example, a change in the load distribution inside the turbine will influence the hot gas pressure, the exit pressure of the cooling system, the cooling air mass flow, the balance of mass flow inside the secondary air system, and the extracted mass flow from the compressor.

One also has to look at the efficiency of the gas turbine across a wide operating range and compare this with the likely operating regime that the turbine will be under. If the turbine is a base-load engine that will operate within fairly tight conditions, it can be optimized for those specific conditions. If, on the other hand, it has to operate under differing load conditions, then it may be better for the turbine to have a lower design efficiency that is more stable over the whole operating range.

Development of specific gas turbines The size of industrial gas turbines has grown. They can generate up to 200 MWe at 50 Hz. Turbine entry temperatures have risen to up to 1260°C, and pressure ratios have increased to up to 16:1. Examples include: The ABB GT 13E2, rated at 164 MWe gross output on natural gas, has an efficiency of 35.7%. The pressure ratio is 15:1. The combustion system is designed for low NOx production. The dry NOx-emission levels are less than 25 ppm with no water or steam injection on natural gas. The turbine entry temperature is 1100°C, and the exhaust temperature 525°C. The turbine has five stages, and the first two rotor stages and the first three stator stages are cooled. The roots of the last two stages are also cooled. Siemens’ model V84.3, rated at 152 MWe, has an efficiency of 36.1%. The turbine has a pressure ratio of 16:1. Each chamber has six burners designed for low NOx emissions. The turbine entry temperature is 1290°C and the exhaust temperature 550°C. The turbine has four stages, and the first three rotating stages are air cooled. The effectiveness of the cooling is improved by intercooling the cooling air after it is drawn out of the compressor. GE and European Gas Turbines have jointly developed the MS9001F 50 Hz engine, which generates 215 MWe at an efficiency of 35%. The engine uses an 18-stage compressor with an overall compression ratio of about 20:1. The gas turbine has three stages, the first two being cooled. Gas temperature at entry into the turbine is 1288°C.

In the long term, it is expected that the increasing costs for fossil fuels will allow techniques that are currently considered not to be cost effective to become economically viable. Most of these techniques will drive the gas turbine thermal cycle to more closely resemble the Carnot cycle. This introduces the possibility of using intercooling in the compressors and increasing the level of sequential combustion in the turbine.

The increasing cost of fossil fuel is also likely to drive moves towards increased use of alternative fuels, which will require greater fuel flexibility from turbines. This will in turn require turbines to be able to operate across a broader range of conditions, making it necessary to reconsider the balance of very high efficiencies at specific conditions with high efficiencies across a range of conditions.

It is perhaps worth noting that a report from the EU states: ‘Natural gas combined-cycle technology has been continuously improved over the last 20 years... Further increases of efficiency depend on gas turbine development, such as increased turbine inlet temperature and blade cooling technologies. The watersteam cycle in a combined cycle is almost at its physical limits, and only small improvements in efficiency are expected through the improvement of components such as pumps and steam turbine blades.’

DURABILITY IS STILL AN ISSUE

WEFA’s report Banking on Advanced Gas Turbines, Prospects for a Financial Meltdown states that around 60%–70% of the nonfuel cost of a typical gas turbine is consumed in the repair and replacement of components in the hot gas path. The report goes on to say that the worldwide OEM hot-section replacement business exceeds US$1 billion annually, and that the profit margins for the parts are reported to be greater than the margins on the original turbine.

The report says that the hot gas path components giving the most trouble are:

• turbine blades and vanes
• combustion liners
• end caps
• fuel nozzle assemblies
• turbine stationary slides.

Increasing inlet temperatures will only exacerbate this situation, and maintenance issues look set to become an increasing concern and a significant limiting factor on efficiency improvement.

David Flin is a freelance energy journalist based in the UK.
e-mail: cospp@jxj.com

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