practical-considerations-in-scaling-supercritical-carbon-dioxide ...

Proceedings of SCCO2 Power Cycle Symposium 2009 RPI, Troy, NY, April 29-30, 2009

"Practical Considerations in Scaling Supercritical Carbon Dioxide Closed Brayton Cycle Power Systems" Robert L. Fuller, William Batton BARBER-NICHOLS INC. 6325 West 55th, Arvada, Colorado, 80002 Phone: 303-421-8111, Fax: 303-420-4679, Email:[email protected];[email protected] Abstract - The supercritical CO2 closed Brayton cycle offers a high efficiency power generation system from relatively low temperature sources. The thermodynamic properties of CO2 including the high density make the turbomachinery and heat exchangers very compact. Unique challenges exist in the design of power systems using super critical CO2. The challenges to be addressed depend on the power level and the thermodynamic cycle. Several power levels and thermodynamic cycles are presented. Turbine and compressor design, power conversion technology, seals, bearings, and rotor dynamics are some of the items discussed. I. INTRODUCTION Supercritical carbon dioxide power cycles are being investigated as a way of making electric power with good efficiency at relatively low temperatures. Carbon Dioxide is unique in that the critical temperature and pressure are 87F and 1100 psi. Operating a gas compressor near the critical point reduces the compressor power requirement, increasing cycle efficiency. Operating near the critical point also means high fluid pressure and density which lead to engineering design problems that are unique. To prove out issues regarding supercritical CO2 power systems, several scaled systems are being designed. Many design problems have to be contemplated for scaled systems that are different than that for larger systems. These design problems and options are identified for scaling of 200kW, 3 MW, and 300 MW systems. A wide range of engineering skills are necessary to analyze the options when scaling these systems. These skill sets range from thermodynamic analysis to aerodynamics, to rotor dynamics, to stress analysis. Each skill is interdependent as seen in Figure 1 and leads, in many instances to an iterative design process. The machinery design solution is greatly accelerated by having a good experience base to draw upon. While the engineering design process has been done many times for low pressure Brayton cycles, the unique nature of CO2 leads to new design challenges that must be overcome for a successful design.

Figure 1. Turbomachinery Scaling Process Flow


II. Cycle Analysis Cycle analysis is an important first step in determining the suitability of supercritical CO2 for an application. Consideration has to be given to the unique properties of CO2. This includes a compressor inlet temperature of 90F to 100F and compressor inlet pressure of 1100 psi to 1500 psi. Compressor inlet temperatures of up to 140F have been contemplated, but with a reduction of cycle efficiency below that of competing technologies. Also of primary importance is the turbine inlet temperature. Supercritical CO2 cycles have shown to be advantageous for applications for turbine inlet temperatures as low as 500 deg F. Higher turbine inlet temperatures provide higher efficiencies but depending on the application, structural limitations of the high pressure turbine containment structure should be considered from a materials and safety perspective. To understand a method of scaling the supercritical CO2 cycle, consider the recompression cycle as shown in figure 2. This cycle was developed at Massachusetts Institute of Technology to overcome efficiency limitations in a standard recuperated Brayton cycle. This cycle was developed for a 300 MWe nuclear application and has the ability to produce over 45% efficiency with a 1022F turbine inlet temperature (1). Other cycles have been contemplated, primarily for nuclear applications (2).

Figure 2. Recompression Cycle Simple Diagram The cycle state points are listed in table 1. To scale the cycle from 300 MWe to 3 MWe and then to 200 kWe, the mass flow is reduced proportionally while keeping the pressures and temperatures equivalent. If the head remains constant across a turbine or compressor, and the mass flow changes, the turbo-machinery must adapt. In some instances the type of generator, compressor, or turbine technology must change. Also seals, bearings, and even rotor configurations must change as well to compensate for changing mass flow. If acceptable, the cycle design may have to be reconsidered in light of these technology choices with the goal of the scaling system within available technology choices.

Turbine

Re-compressor

Main Compressor

Inlet Pressure (psia)

2876

1116

1115

Exit Pressure (psia)

1145.8

3000

3000

Inlet Temperature (F)

1022

157

89.6

Table 1. Recompression Cycle State Point Information III. Turbomachinery Considerations: III (a). Generator Technology It is desirable for many applications to create either 50 or 60 Hz power. For large power plants this is a synchronous generator


operating at 3000 or 3600 rpm and operating at high voltage, 13.8 kV or above. This can typically be accommodated by multistage compressors and turbines operating on a single shaft. As the power level decreases it can be more difficult to provide efficient turbine and compressor machinery that can operate at synchronous operating speed. At this point several options exist to provide power at higher shaft speeds. The first option is to utilize a speed decreasing gearbox. This allows the turbine and compressor shaft to operate at higher speeds while utilizing a standard 1800 or 3600 rpm generator technology. Gearboxes are reasonably efficient, but require lubrication, extra seals, bearings, increased space, higher noise, and decreased reliability. Operating the gearbox in a hermetic type closed Brayton cycle would make it difficult to lubricate the bearings and gears with oil. The oil could tend to migrate or breakdown at supercritical CO2 conditions. The migration could introduce oil in areas such as the heater providing an opportunity to coke or otherwise inhibit proper heat exchanger performance. Another solution is to utilize 400 Hz power if it is suitable for the application. This would typically be for shipboard power applications where the distribution losses inherent in 400 Hz power are mitigated by the island nature of the load. This would allow consideration of synchronous machines operating at 12 to 24 krpm. While these higher speed machines are prevalent on aircraft at lower power levels, they would need to be developed for applications in the .5 to 20 MWe range where they may be desirable.

Still another solution is to utilize power conversion technology to take the high frequency generator output, convert this to DC voltage and then invert to 50 or 60 Hz. This technology has primarily been used for low voltage (