5.3 Gas Turbines

Technology Overview

Gas turbines are broken into three main categories: heavy frame, aeroderivative, and microturbine. Commercially viable microturbines (MT) are available in the 27kW to 250kW range. The technology is largely based upon aircraft auxiliary power units and automotive style turbo chargers [1]. The combustion turbines (CT), including Heavy Frame and Aeroderivatives, are typically used in large-scale industrial and utility generation stations starting at the 200kW to 250MW level per unit [9]. Systems 15MW and above are often called utility grade turbines, rotating at a relatively slow constant speed driving a synchronous AC generator and are considered to be impractical for DG implementation [1]. Figure 3 provides an illustration for the typical cross section of a turbine and a commercially available microturbine.

A turbine, like an internal combustion engine, is essentially a large compressor. However, the turbine is a continuous flow/fuel-burning machine whereas an ICE relies on serial piston strokes to maintain air and fuel flow [1]. Air is heated, pressurized and then allowed to expand to ambient levels. Mechanical shaft energy is created when the expansion energy exceeds the heating and pressurization energies [9]. The shaft is then coupled to an appropriate electrical generator/inverter system. Turbine speeds range from 10,000-105,000 RPM and may require power electronics to produce utility grade power [10].

In this section, we will be comparing both microturbines (MTs) and combustion turbines (CTs) to the other DG technologies and to each other. First, we will look at the technology status, followed by overall efficiency, environmental and noise cosiderations, device cost, physical size, intended application, and potential for improvement for each technology.

Technology Status

MTs Ranked 3^rd

CTs Ranked 2^nd

In the 1930s, both English and German engineers produced viable gas turbine engines. Sir Frank Whittle patented a machine designed for jet propulsion in 1930. Simultaneously, Hans van Ohain and Max Hahn developed a German counterpart to this British jet engine. The 1930's era technology underwent a significant maturation process in the 1970's developing into the highly efficient cogeneration plants used today [11]. Overall turbine efficiency increases proportionally to increasing inlet temperature, and increasing the inlet temperature forces advances in metallurgy and materials [9]. Advances in materials, cooling passage design, and cogeneration have culminated in nearly 80% efficiency levels for large CTs. The aeroderivative units benefit from the higher inlet temperatures and from advancements in aircraft technology. Unfortunately, economical turbines that fall within the scope of this paper have not benefited from these advancements. The traditional gas turbine represents an 'economy of scale' system where efficiency is proportional to size. The large centralized utility grade turbines have benefited from these specific engineering solutions instead of the adapted smaller technologies[1]. While the inlet temperatures have steadily increased, the cooling passages, materials, and compressor geometry advancements of smaller CT and MT systems have not similarly kept pace with the larger units.

Microturbines have significantly reduced wear and internal frictional losses with the advent of air bearings. The shaft spins on a pressurized layer of air instead of oil. While this eliminates the 'oil' pump, any detergent related benefits and immunity to grit are lost increasing maintenance and filtering demands. Unlike traditional turbines, MTs are not required to use precise speed control mechanisms and therefore are able to spin are varying speeds somewhat more efficiently than CTs.

Significant numbers of large-scale advanced units have been sold and are in use. In fact, it is not uncommon for large CTs to run for a decade or more without major failures - unfortunately it will take more than a decade of 'experimental' MT and small CT installations before a cautious power industry readily adopts these as a DG solution. MTs have yet to make the significant inroads into the mainstream generation market and were once found mainly in off-grid combined heat and power (CHP) markets, peak shaving, and standby applications. Their role is gradually beginning to shift as grid reliability and transmissions costs become problematic [12].

Overall Efficiency

MTs Ranked 4^th

CTs Ranked 2^nd

A stand-alone turbine system has efficiency levels of 25-40% and may double with CHP systems [12]. Table 1 indicates the commercially viable CT and MT units. Note that CHP equipped units are typically twice as efficient as single cycle units through the use of heat recouperators, water heaters, etc.

Table 1. Commercially Available Turbines and Rated Efficiencies (compliments of GE, Bowman, and Capstone product information sheets for the product listed).

Re-engineered heavy-frame CTs also have much higher efficiency levels. Newer units designed for continuous duty are de-rated for operation at their maximum efficiency point. Figure 2 provides a general guideline for the various turbines available. The previously mentioned numerous commercial uninterruptible power supply (UPS) units and older style turbines populate the average efficiency band peaking near 30% at the 5MW mark. Current MT technology is less efficient than CTs, however both are still less efficient on average than ICEs and solid oxide fuel cells (not including cogeneration).

Figure 2.   Efficiency of Small Gas Turbines and their possible efficiency levels. Note, these levels were achieved at the max efficiency point, not max power point. The 'A' indicates the Aeroderivative units, while the 'R' and unlabeled points indicate the Heavy Frame industrial turbines [13].

Environmental and Noise Considerations

MTs Ranked 3^rd

CTs Ranked 2^nd

Gas turbines are a fossil fuel powered DG technology as such they emit environmental pollutants. The figure below illustrates how excess air present in the combustion process drives down NO_x emission levels. By design, many turbines already satisfy the EPA's 'low' and 'extra low' emissions rating. Minimizing the peak firing temperatures by running a lean mix and injecting water or inert gas into the combustors has been an effective method in reducing NO_x emissions by 85% [14]. The reduced greenhouse gas emissions make CTs and MTs more appealing than ICEs in emissions sensitive locations.

Gas Turbines in an uncontrolled state are deafening to the point on impracticality. Some DG systems will be placed in residential areas, and the constant hissing/roaring sound of a jet engine is not desirable. Muffled single cycle systems are typically 80-85dB at 30ft and further reduction is difficult without significantly impacting system efficiencies. CHP systems offer reduced ambient sound levels as a result of using the energy grade exhaust heat for cogeneration [1]. Consequently, quiet mixed residential and light commercial areas mandate significantly quieter levels.

Figure 3. NO_x Emission Levels for different turbines [11].

Device Cost per kW

MTs Ranked 3^rd

CTs Ranked 2^nd

MT cost is tied directly to presence of accessories - heat recouperators, CHP devices, etc. The initial cost of the unit is substantially higher than CT, and without the accessories, the MT quickly become an expensive device to operate [12]. The CT is typically readily available and will usually take up to two weeks to install. CTs are also fairly robust. The operations and maintenance for these units is also less than for a microturbine. In installations where substation real estate is very expensive and distributed generation sites are small, the capital cost of the MT and CT may even out [10]. The initial costs estimated in Table 2 illustrate the large disparity between MT and CTs. This disparity is so large that CTs are the most cost competitive technology along with ICEs, while MTs can only beat out fuel cells in a cost comparison.

Physical Size

MTs Ranked 3^rd

CTs Ranked 2^nd

Turbine rotational speed is directly tied to the diameter of the rotor vanes. Since the object of the turbine is to basically have the tip of blades spinning at approximately the speed of sound, the 3" diameter turbine found in MTs will need to spin at 110,000RPM. A CT will spin much slower as the turbine diameter is larger. The actual units are much larger then their turbine diameter with the exhaust housing consuming most of the space. Microturbines typically employ small heat recouperators which add little volume. Moderately sized turbines (200kW- 10MW) require more space for their recouperators and cogeneration equipment. Table 2 provides a quick reference of commercial units. However, this does not include any cogeneration equipment, transformers, etc.

Table 2. Size, Cost, Weight comparison table for MT and CT [1].

As with cost, a power density disparity exists between MTs and CTs. Though difficult to compare because of the different power levels, the MT averages about 1 cubic foot per kW while the CT averages about .85 cubic feet per kW. Though CTs have a slightly higher power density than MTs, both technologies occupy approximately 4 times less area per kW than a similar fuel cell installation.

Intended Application

MTs Ranked 4^th

CTs Ranked 1^st

CT and MT and their respective applications are dictated by the available turbine rating. Microturbines have not yet broken the MW level, and commercially have not exceeded 400kW. However, where fuel costs are high, it does not appear economical to operate small MTs until their fuel efficiency increases. With regards to the 5MW case, a CT would be better suited for the application with regards to cost, efficiency, and proven design. Fuel and necessary infrastructure are no different than those required by ICE and fuel cells.

Potential for Improvement

MTs Ranked 2^nd

CTs Ranked 3^rd

Advancement in aircraft engines and significant improvements in materials have allowed the turbine to reach current efficiency levels. Large advanced CTs have reached a level of maturity where further research is not going to yield an appreciable gain in efficiency. Further maturation will bring the smaller CTs to a maximum of 45% fuel efficiencies. Larger purpose built CTs already have maximum inlet temperatures of 2500^oF or more, employ advanced metallurgy, and have optimized CHP. Aside from increasing their firing and inlet temperatures, these large utility sized CTs have reached a plateau. Smaller commercial CTs and MTs have not widely benefited from these improvements. Laboratory advancements have driven the firing point temperature beyond 2800^oF, increased the air cooling methods and improved turbine blades [14]. Realizing that MTs and CTs are younger technologies than ICEs, they can be expected to best ICEs in many aspects on a long enough time scale.

 

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