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Session: Parallel Session: System Design, Optimization and Applications I
Session starts: Thursday 22 September, 10:00
Presentation starts: 10:20
Room: Senaatszaal

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Matthew Orosz (MIT)
Alexander Fanderl (TUM)
Christian Muller (TUM)
Harold Hemond (MIT)

Abstract:
The lack of suitable expanders hinders the use of ORCs (organic Rankine cycle engines) for solar power or waste heat recovery at small (kilowatt) scales. Purpose-built turbomachinery is cost prohibitive at these scales. Positive displacement machines derived from HVAC scroll compressors are generally inexpensive alternatives and are available in various displacements. A drawback of HVAC scrolls as expanders, however, is their inbuilt volume ratio, usually ~3, which is only useful with typical ORC fluids at thermal resource temperatures of ~100°C, i.e. much lower than the potential of concentrating solar and some waste heat sources. Kane addressed this problem by using modified HVAC scrolls and operating two ORCs in series, driven by solar thermal and diesel exhaust gas at 150°C [1]. This solution has the drawback of requiring the duplication of equipment needed to match the fluid and machine volume ratios. We report here on three alternative approaches using a single ORC operated from a 150°C thermal source and various configurations of positive displacement expanders: a pair of HVAC scrolls in series, an HVAC machine incorporating a purpose-built high volume ratio scroll, and a piston expander. The compounding of HVAC scroll expanders is greatly facilitated by removing the limitation of asynchronous generation at grid frequencies. Without this constraint, testing at MIT on a 3kWe ORC test bench confirms that HVAC scrolls of suitable displacements can be paired and loaded separately for effective two-stage expansion of the working fluid R245fa to a volume ratio of 9, albeit with the added complexity of recombination of electrical outputs. Starting from the geometric framework of HVAC scrolls [2], we created a new scroll design derived from an examination of the tradeoffs between volume ratio, scroll size, number of turns, and spiral equations for constant and varying wall thicknesses. We also examined the potential of applying the ubiquitous IC engine, reconfigured for two-stroke operation as an expander using redesigned cam profiles to alter the valve gear timing; this approach has the advantage of being low-cost with standard volume ratios of 8-10. For these several approaches machine characteristics were related to a thermodynamic model to derive machine speeds (RPM) at mass flow rates corresponding to a few kilowatts of power output. Sizing, specification, and performance models are developed in Matlab and EES, prototype expanders are designed, and performance characteristics are compared on the MIT 3kWe test system using R245fa at 150°C.