10MW-Class sCO2 Compressor Test Facility at University of Notre Dame

Abstract

The compressor is a key component in closed-loop Brayton Cycles and advanced electrothermal energy storage systems. The use of sCO2 as the primary working fluid had many advantages for these systems. However, due to the unique operating conditions and fluid properties, there remains significant challenges for the development of high efficiency compression systems with sCO2. Detailed experimental measurements from sCO2 compressors are extremely difficult to obtain, given the small size and very high power requirements. This has limited the majority of current experimental results to very small scale, single stage, centrifugal compressors. Larger, multi-stage axial compressors are of significant interest for sCO2 systems, but have not been subject to experimental investigations. The present communication describes the design and salient characteristics of a new, 10MW-class closed-loop sCO2 or CO2 compressor test facility. The 10MW drive system allows for a physical scale that allows testing of both axial and centrifugal compressor types with flow passages large enough to enable detailed experimental measurements., including surveys through the flow passage, steady and unsteady performance measurements, and aeromechanical measurements on vanes or blades. INTRODUCTION The use of supercritical CO2 as the working fluid in closedloop Brayton Cycles and advanced electrothermal energy storage systems has shown great promise in delivering electricity with high efficiency, flexibility of heat source, and reduced power-plant size and cost [1, 2]. However, a number of new technology advancements must be realized in order to make sCO2 cycles commercially viable. One of the major components is the compressor, which provides the pressure increase needed in the cycle. Some characteristics of sCO2 in regard to its application in a compressor differ from those seen with air or gas which is widely used for turbo compressors. Higher density inside the compressor, overall higher operating pressure ranges, and drastic change of fluid properties near the critical point each present unique challenges for compressor design. Recent experimental studies of sCO2 compressor in compressor test loops [3-8] or in power cycle loops [9-15] have successfully demonstrated the operation of sCO2 compressors in closed loops test environment. However, due to the small demonstration scale or due to limited available driving power, all of them were designed with a centrifugal type compressor with small scale where efficiency must be sacrificed resulting in low overall cycle efficiency. Also studies on detail flow DOI: 10.17185/duepublico/73975 2 measurement were not suitable due to the very small flow passages through the compressor. With this background, Notre Dame Turbomachinery Laboratory and Echogen Power Systems has designed a 10 MWclass sCO2 compressor test facility to be built at the University of Notre Dame. The test compressor is driven by a 10 MW variable speed motor with a speed increasing gear box. A water/Glycol cooled heat exchanger absorbs added energy from the test compressor. The closed loop is designed to reach steady operation where the addition of energy through the drive motor and the absorption of energy through cooling flow are equal. A CO2 inventory management system with CO2 tank and supply system was designed to supply CO2 from initial operation to test operation. The choice of a 10 MW size sCO2 compressor test facility has various merits in terms of scale. It allows use of commercial hardware for many of the components of the test compressor and facility [16]. Also it enables the facility to test the multistage axial compressor as well as centrifugal compressors with flow passages wide enough to allow a detailed flow field investigation through various flow measurement technologies. The experiments will include detailed measurements that will significantly advance our understanding of the design, performance, efficiency, and operability of sCO2 compressors for advanced power systems. At the time of this writing, the construction of the drive train with a variable speed motor, a gear box, a torquemeter and relevant control systems is complete and has been fully commissioned. Also, a unique data acquisition/control system was developed and tested. This includes the ability to conduct real-time post processing and control with 100s or 1000s of steady and unsteady acquisition channels. The design of the closed CO2 test loop is funded by the U.S. Department of Energy and will be complete in mid-2021. Initial sCO2 axial compressor tests will begin in 2021. TEST COMPRESSOR : 3-STAGE AXIAL COMPRESSOR The first compressor to be tested at the facility is a three stage axial compressor. Its design condition is in Table 1 and its predicted map is shown in figure 1. Table 1: Design point of 3-stage axial compressor # of compressor stages 3 Inlet total pressure MPa 2.77 Inlet total temperature ̊C 97.94 Mass flow rate kg/s 116 Pressure ratio 2.706 Design point aero power MW 9.09 Design speed rpm 19,800 Figure 2: p-h diagram of the 3 stage test compressor in test loop vs full stage compressor operating point. The 3-stage test compressor is a scaled version of first 3 stages of a 100 MW class multi-stage axial compressor designed for a sCO2-based energy storage system. The objective of the program is to study and ultimately demonstrate a high efficiency, multi-stage, axial sCO2 compressor. Pressure ratio of the full stage compressor is 10.27. The first three stages of the full machine will be tested in the designed test facility with a design-point pressure ratio and mass flow rate of approximately 2.6 and 116 kg/s, respectively.