Design, Analysis, and Construction of a Novel Closed-Loop Supercritical Carbon Dioxide (SCO2) Reciprocating Inline Piston Expander

A small-scale supercritical Carbon Dioxide (sCO2) power cycle operating on the principle of a closed-loop Brayton cycle is currently under construction at The University of Texas at San Antonio (UTSA). The advantage of using sCO2 as a working fluid is that it is highly efficient to generate power, and it can be paired with various heat sources because of its near ambient critical temperature property. This sCO2 power cycle is being built to explore the potential alternative energy source research at the institution. This power cycle consists of four principal components: a compressor, a heating source, a piston expander to produce power, and a heat sink to dissipate excess heat. A two-cylinder, two-stroke, inline, reciprocating piston engine was conceptualized as the expander for this facility's design to produce power at a kilowatt-scale. Piston expander was selected instead of a turbine due to its relatively low cost to design and fabrication. The current manuscript discusses the design and analysis of this sCO2 piston expander in detail. Also, various preliminary testing performed to validate some design choices helped in finalizing the engine's design. A small-scale sCO2 engine was designed, fabricated, and tested to help understand the design and dynamics of the piston expanders and validate the novel piston engine's design. A valve spring test was done to determine a spring to be used in the valvetrain with the right spring rate. The small-scale sCO2 engine test and the valve spring tests are discussed in the manuscript. The small-scale sCO2 engine failed during testing, but it was an overall success because it helped fix the design flaws in the sCO2 engine. The use of sCO2 for the power cycle generates 4.5 kW of power using the piston expander at inlet pressure and temperature of 17.23 MPa (2500 psi) and 385.15 K (185 °F) at 0.2 kg/s of flow rate. The facility is designed to run for a period of 5 to 8 minutes per test run, producing total energy of up to 0.5 kWh.