On loss characterisation and minimisation in high-performance reciprocating-piston expanders
Creators: name: Simpson, Michael
Publisher: name: Imperial College London
Year: 2021

description: Reciprocating-piston expanders are being considered for a range of clean-energy applications, including small- to medium-scale organic Rankine cycle (ORC) engines for conversion of low-grade heat to power, and compressed air and pumped thermal energy storage (CAES, PTES) to help integrate intermittent renewables. At large scale, turbines remain the dominant technology, but at scales of 10 – 100s of kilowatts, reciprocating-piston expanders can offer high efficiency at large pressure ratios, robust part-load performance, and affordable manufacture. Further advantages are available in compressed air energy storage applications, where the potential exists to operate a single reciprocating-piston machine as both compressor and expander. Although there is a wealth of experience in reciprocating-piston machines for internal combustion engines and compressors, quantitative understanding of the particular loss mechanisms which influence reciprocating expanders is less well-established, as a consequence of operating without combustion, and at different temperatures and valve timings. This thesis describes the development and validation of a lumped-mass piston expander model by means of experimental data and computational fluid dynamics (CFD) simulations. The model replicates the cylinder pressure and temperature to a high degree of accuracy, and captures trends in gas-wall heat transfer, though with slight discrepancies in instantaneous magnitudes. The model accounts for frictional losses, and a time-resolved exergy calculation has been introduced to attribute losses to particular thermodynamic and mechanical processes occurring within the device, at both design conditions and part-load operation. Surrogate-model-based optimisation approaches have been demonstrated to optimise an expander geometry and valve timings. By considering a broad range of possible operating conditions and geometries, a set of performance maps are developed to provide insights into the relative magnitudes of the different loss mechanisms that should be considered within the design process. The role of piston expanders in ORC systems is investigated here in multiple waste heat recovery applications, such as recovering heat from stationary internal combustion engines. The piston expander is compared to and contrasted with screw expanders for a broad range of possible ORC working fluids. Unlike previous studies, the expansion device and working fluid selection are carried out simultaneously by means of an integrated optimisation process, ensuring that performance interactions between the expansion device and working fluid properties are captured. When capturing heat from the exhaust gas of the stationary internal combustion engine, a two-stage screw expander is found to provide the highest power output and efficiency, with acetone found to be the optimal working fluid, while a single-stage piston expander offers the lowest specific investment cost and payback time. The relative merits of piston expanders and small-scale radial-inflow turbines for a fluctuating heat source are also assessed, for low-temperature (&lt;100 °C) and high-temperature (400 – 600 °C) waste heat. At low temperatures, radial-inflow turbines are found to offer both higher isentropic efficiency and lower cost per unit power, while at high temperatures the reciprocating-piston expander is found to be the more economical option, demonstrating lower specific costs as well as a greater robustness to part-load conditions. Building on this finding, an in-depth assessment is carried out into two competing configurations of stationary internal combustion engines paired with ORC engines for building heat and power needs, using reciprocating-piston expanders for the ORC engine in each case. As current ORC engines are limited to small-scale production and tend to face high integration costs due to the custom nature of each installation, building energy consumption data from a set of thirty supermarkets is used to explore the economic feasibility of a common internal combustion and ORC engine design that could offer greater economies of scale. In the first configuration considered, the ORC engine recovers heat from both the exhaust gas and cooling jacket water of the stationary engine, while in the second, heat is extracted only from the exhaust gases, with the condensing temperature of the ORC engine deliberately raised well above ambient in order to allow heat to be rejected from the ORC engine's condenser to the building heating system. The former is found to offer considerably higher power and lower specific investment costs, but the economic returns are inconsistent across the set of buildings, while the second configuration demonstrates favourable payback periods of 3.5 – 7.5 years for all thirty of the buildings considered, offering much greater scope for larger manufacturing volumes and associated reductions in the unit price and competitiveness of these systems. The design and optimisation of a set of reciprocating-piston engines for small-scale CAES systems is performed. An initial feasibility study identifies two overall architectures for the CAES system as being of particular interest, and these are assessed from a technoeconomic standpoint. The on- and off-design performance of appropriately sized reciprocating-piston machines are simulated in order to quantify the realistic technical and commercial performance of these systems. Valve timing strategies are proposed for efficient charging and discharging of the preferred system configuration in fixed- and variable-speed operation, with variable-speed part-load operation found to deliver higher efficiency than fixed-speed even with fully variable valve timing. Consideration is given to mitigation of the main loss mechanisms within the system and the piston machines in particular, with a strong sensitivity to piston ring leakage identified for high-pressure stages. Based on broad parameter space exploration and specific case studies of piston expander operation, this work aims to highlight favourable opportunities for high-efficiency reciprocating machines and demonstrates the use of a lumped-mass model for determining relevant loss mechanisms and identifying optimal designs at component- and system-level, with a view to accelerating appropriate deployment of these machines for selected clean-energy applications.