Design of Variable Wall Thickness Geometry and Performance Mapping of Scroll Expanders for Distributed Power Generation

Low-temperature heat sources such as industrial waste heat, solar, and geothermal are more suitable for small-scale power generation rather than utility scale. In order to maximize the electricity generated from low-temperature heat sources, an efficient expansion device for small-scale power output is necessary. This research work has focused on evaluating the use of a scroll expander and improving its geometrical design for the power output range of 1-25 kWe. The first part of the work focusses on modeling the performance of a scroll expander using two non-dimensionless parameters, namely, specific speed and specific diameter. Performance of a scroll expander was modeled using mass and energy balance governing equations and analyzed for three different expander inlet temperatures (150°C, 200°C, and 250°C) for pure fluids and zeotropic mixtures. The scroll expander geometry was developed using empirical equations and losses due to leakage, mechanical, and expansion were modeled. The performance indicators such as expander efficiency, specific speed and specific diameter were determined using the thermodynamic properties and mass flow rates at the exit state. Several hundreds of scroll geometries with different aspect ratios were modeled for each volume ratio. The expander efficiency was then plotted as a contour function of specific speed and specific diameter. An expander efficiency of 76-77.8% was achieved at an expander inlet temperature of 150°C for both propane and R433C. However, the zeotropic mixtures required smaller volume ratio and scroll size than pure fluids for achieving the same performance. The next part of this work focusses on addressing the challenges of modeling the variable wall thickness scrolls from state-of-the-art approaches. A novel approach to modeling scroll geometry with variable wall thickness was developed and presented. Three different scroll designs with decreasing, increasing, and constant wall thicknesses were modeled using this approach. The performance of all three different designs was modeled and analyzed for R433C and propane at an expander inlet temperature, pressure, and rotational speed of 150°C, 7 MPa, and 3000 RPM. The decreasing wall thickness design yielded the highest expander efficiency of 77% and 77.8% for propane and R433C respectively, while increasing wall thickness design exhibited the lowest efficiency of 72% and 73.5% for propane and R433C respectively. Finally, the dynamic performance of supercritical organic Rankine cycle was modeled to evaluate the impact of the expander’s off-design performance on the cycle. The scroll expander’s off-design performance was characterized as a function of pressure ratio, where an optimum expander efficiency is achieved at an optimal pressure ratio. This optimal pressure ratio is where the expansion losses are minimum, thereby leading to better expander efficiency. The cycle operating conditions such as the expander inlet temperature and pressure were optimized for maximizing cycle energy efficiency using different optimization methods. The optimized expander temperature and pressure are 159°C and 7.1 MPa with a maximum cycle efficiency of 8.3% for propane, while R433C achieved a maximum cycle efficiency of 8.6% at the inlet conditions of 159°C and 6.3 MPa. Dynamic performance of the cycle was also modeled for different days using solar radiation as the heat source