Increasing Microturbine Efficiency

Simple-cycle gas turbine engine electric efficiencies compare unfavorably with those of competing technologies in the relatively small sizes anticipated for microturbines (20 kW to 250 kW). Typically they offer LHV efficiencies in the mid teens to low 20's. In contrast, natural gas-fueled reciprocating engines in similar sizes yield electric efficiencies in the 26 to 32+% (LHV) range.

Recognizing this, microturbine developers have sought to raise engine efficiencies with the overall objective of meeting a current benchmark level of around 30%. Gas turbine designers traditionally employ a wide variety of design alternatives to raise both engine and overall system efficiencies. However, microturbine designers face relatively stiff system complexity and cost constraints. System improvements available to larger engines such as combined cycles are too costly to include in a microturbine sale. And designers must minimize both first cost and maintenance costs over the full life cycle of the machine.

This paper explores the design tradeoffs involved in efficiency-raising methods commonly used in microturbine development. These include improving component efficiencies, employing more aggressive engine operating conditions, and recovering heat through regeneration or recuperation. The advantages and disadvantages of various methods are examined along with their constraints and effectiveness. The impact on engine life is assessed.

Microturbine Characteristics

Microturbine engines typically employ metallic radial turbomachinery components. They operate at relatively low pressure ratios in the 3:1 to 5:1 range using one stage of compression and one or two turbine stages. In the latter case, the first turbine drives the compressor and the second free-power turbine drives the load. The engines are designed to produce low emission levels. For example, NOx emissions are expected to be below 9 ppmv (@15% O2) when using natural gas as a fuel and below 25 ppmv with liquid fuels. These levels have already been demonstrated in several cases. An example 70 kWe microturbine is shown in Figure 9.

Microturbine engines are packaged in several configurations. The most commonly discussed today is a cogeneration unit wherein the turbine drives an electric generator. An additional heat exchanger built into the exhaust stream is typically used to provide domestic hot water service. In a cogeneration application that can effectively use the waste heat, overall system efficiency can be quite high (80+%).

Microturbines are also being developed in configurations where the engine directly drives a centrifugal chiller compressor, a screw compressor for refrigeration systems, or an air compressor. Other direct drive applications are also possible.

Turbomachinery Performance

The turbomachinery components of a microturbine are obvious candidates when looking for system efficiency improvements. With time, compressor and turbine stage designs have continued to improve as fluid-dynamic designers employ new design methods and technologies. For example, the blading of a modern radial compressor stage is typically designed using a set of straight-line elements. As shown in Figure 1, these are arranged to create three-dimensional blade shapes called ruled surfaces, which can result in relatively efficient stage designs. However new techniques including three-dimensional Computational Fluid-Dynamic analysis capability, allow more complex blades shapes (called sculpted surfaces) to be employed that could offer opportunities for even greater stage performance.

Figure 2 illustrates the current "state-of-the-art" in compressor performance using today's design techniques. Radial compressors commonly used in microturbines would fall into the specific speed range of .7 to 1.0. As the figure shows, stage efficiencies of 87% to 89% could potentially be achieved for certain types of compressors in this specific speed range. However, compressors typically used in microturbines are more likely to top out at efficiency levels in the low 80's. Why?

First, compressors used in microturbines are relatively small, typically a few inches in diameter. Although "state-of-the-art" efficiencies might be possible with in large diameter designs, several factors significantly limit the achievable efficiencies of microturbine-scale compressors. Reynolds number effects impose aerodynamic limits. Clearance ratios are relatively large due to practical limits in the bearings.

To be economically viable, microturbines are subject to tight cost constraints, which essentially dictate the use of high-volume/low cost manufacturing techniques. For example, compressor impellers are cast rather than precision machined. Cast parts exhibit larger tip clearances, looser dimensional tolerances, and other characteristics that significantly reduce compressor performance.

The net result is a practical limit on improving the performance of the turbomachinery. As long as the turbomachinery components are well matched to the operating requirements of the microturbine in the first place, available improvements would only be incremental and would represent a relatively small overall system gain.

Engine Operating Conditions

Designers can employ more aggressive engine operating conditions like raising turbine inlet temperatures or increasing pressure ratios to raise system efficiency. As shown in Figure 3 for a typical simple cycle engine in this size range, either of these can result in substantial system improvements.

However, the strict cost constraints of a microturbine again have an impact on design choices. Turbine designs are limited to metal components without the advanced features found in more expensive gas turbine engines such as air-cooled blading. Therefore raising pressures and temperatures quickly reaches limits that exact a steep price in engine component life. Raising temperature is especially detrimental due to the creep-life and stress constraints of the materials used in microturbines

Figures 4 and 5 illustrate the manufacturer's published limits for a material commonly used in turbine components, Inconel 713. The first figure compares tensile and yield strength properties of this material with increasing temperature. As a general indication note how significantly these strengths decrease in the temperature range of interest (microturbine turbine inlet temperatures in current designs vary between 1600°F and 1850°F [870°C and 1000°C]).

Figure 4

Figure 5

Of equal relevance to the turbine designer are the creep-rupture characteristics of the material describing the time-to-failure under particular stresses at various temperatures such as shown in Figure 5. Of course actual component lives will vary according to the particular stresses encountered and temperatures experienced based on their design. And a designer will use better measures of expected material limits such as Larson-Miller time-temperature parameters to guide design decisions. However, these figures do illustrate the critical impact of increased operating temperatures in this range.

Increasing the pressure ratio has the detrimental effect of raising stresses in the system, again reducing engine life. Reducing these stresses is one reason why some microturbines use two turbine stages in their design, essentially splitting stresses between them. Also the designer can use somewhat higher turbine inlet temperatures and still preserve long engine life.

An additional disadvantage of higher operating pressures is the need for higher fuel delivery pressures which raises the power needed by the fuel gas booster (required in at least half of the U.S. natural gas service area), a parasitic loss to the system.

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Heat Recovery

The most promising way to raise engine efficiency is to recover heat by using a regenerator or recuperator as part of the engine cycle. And indeed, a recuperated cycle has been adopted by all of the microturbine designers, although the degree of heat recovery varies considerably. Figure 6 shows the impact of recuperation for a typical simple cycle gas turbine engine. Note the dramatic effect in the range of low-pressure ratios employed by microturbines (3:1 to 6:1).

Figures 7 and 8 summarize the performance impact in balancing turbine inlet temperature, pressure ratio, and recuperator effectiveness for a typical engine. Notice the critical impact of recuperator effectiveness. An engine design with an effectiveness of only 85% would require a turbine inlet temperature of 1800°F and a pressure ratio of 4:1 to reach an engine efficiency of 33% LHV. However, if a 91% effective recuperator is used, a 1600°F turbine inlet temperature at a pressure ratio of about 3:1 will suffice. Needless to say, an engine running under the latter conditions will enjoy a significantly longer life and reduced maintenance costs.

Recuperators are relatively large and heavy components where more effectiveness means more surface area and thus more weight and volume. Fortunately, the cogeneration, chiller, refrigeration, air compression and other market opportunities for today's microturbines are stationary applications. Within reasonable limits, it doesn't matter how heavy the system is! Therefore, a microturbine designed specifically for stationary applications is free to incorporate a very effective recuperator. Figure 9 shows an example microturbine design that incorporates a high-effectiveness recuperator. The recuperator is contained in the bright metal enclosure in the upper portion of the machine that also acts as ducting for inlet air and engine exhaust.

But recuperator life and cost represent another difficulty. The working environment to which a recuperator is subject includes large thermal gradients and load swings. Recuperator inlet temperatures are high, but considerably less than turbine inlet temperatures. Also, if the designer raises the turbine inlet temperature and pressure ratio to improve efficiency, recuperator inlet temperature also increases, but not as much. This is because the turbine expansion ratio increases with a corresponding increase in the ratio of turbine inlet and outlet temperatures. Therefore, metallic recuperators will still be applicable even if ceramic turbine components are used in the future to allow greater operating temperatures without degrading overall engine life.

Producing a recuperator that can reliably withstand these has generally meant high unit costs. The challenge to the designer is to employ a recuperator technology that offers low cost and exceptional durability.

Conclusion

Efficiency and life will be critical performance metrics for customers evaluating microturbines. Constrained by severe engine life penalties for high operating temperatures unless relatively cheap ceramic turbine components emerge someday in the future, efficient long-life microturbine designs will be dependent on high recuperator effectiveness. Fortunately, new breakthrough recuperator technologies have been achieved which will allow certain microturbine designs to reach and even exceed the important 30% efficiency benchmark.