Waste-heat recovery and power generation with reciprocating motion
Creators: name: Kirmse, Christoph Joachim Wolfgang
Publisher: name: Imperial College London
Year: 2018

description: The utilization of renewable and waste heat from industrial processes is an important step towards the reduction of emissions and the increase in the efficiency of energy systems. This heat is available at different and mostly low grade temperatures (ca 100 °C to 550 °C) and various mass flow rates. The current work focuses on the understanding of the most important parameters that determine the potential efficiency of generalised heat engines from a technology agnostic perspective, followed by the development of modelling frameworks that correspond to two specific engines that can make use of heat sources at low temperature levels. The efficiency and power output of generalised heat engines when optimising different objective functions have been examined. These are the power output and the ecological- criterion value. The efficiency remains within upper and lower bounds, when varying the heat capacity of the heat source and heat sink, as well as the contact time between the external heat reservoirs and the working fluid. The corresponding power output variations are considerably higher than those observed for the efficiency. The power output reaches a maximum for values of the heat capacity of the heat source or sink larger than those of the working fluid or for contact times of the heat-exchange processes that are short or of approximately equal length. From these technology-agnostic considerations and the resulting limits that are imposed on the expected performance of real engines, we proceed to consider two specific technologies in the context of waste-heat recovery and power generation. The common feature of these technologies is that they involve reciprocating motion, either as part of a dedicated component (i.e., expander) or inherently as part of the overall operation of the entire device. The first engine, called Up-THERM, is a two-phase thermofluidic oscillator with low investment costs. A dynamic non-linear model framework of the Up-THERM has been developed. The dominant fluid or thermal effect in each engine component is described by a first-order differential equation. The temperature profile along the heat-exchanger walls has been validated experimentally. After the validation a parametric study has been performed examining the effects of five geometric parameters and the heat-source temperature on the engine’s performance. It is found that the heat-source temperature should be high for high power outputs, the volume of the gas spring small and the diameterof the displacer cylinder should be at its nominal value or somewhat larger. The Up- THERM engine has been compared with organic Rankine cycle (ORC) engines in terms of technical and economical performance. While for low temperature heat-sources the ORC engines have a higher power output, for higher heat-source temperatures this becomes comparable between the two engines. Due to the lower investment costs, the costs per unit power become lower for the Up-THERM engine at high heat-source temperatures. The second engine is an open cycle hot air Ericsson engine. It uses two reciprocating- piston cylinders as compressor and expander, inter-linked by a heat exchanger. It is particularly suitable for heat-sources at higher temperatures with small mass flow rates. The engine is described by a system of 12 equations. Pressure losses across valves, heat losses in the cylinders, friction and mass leakage are considered as loss mechanisms. Pres- sure losses and heat losses together account for 99% of the total losses. An optimisation using neural networks has been performed at five different heat-source conditions. Three operational and four geometric parameters are varied to maximise the power output of the engine. The net power output scales nearly linearly with the mass flow rate of the heat source. The thermal efficiency is constant at around 15% for a heat-source tem- perature of 350 °C and mass flow rates between 0.025 kg/s and 0.1 kg/s. The exergy efficiency increases with decreasing mass flow rate from 3.1% to 6.4%. For a heat-source mass flow rate of 0.1 kg/s the net power output increases from 1.9 kW at 250 °C to 7.2 kW at 350 °C and 48 kW at 450 °C. Higher heat-source temperatures also result in higher thermal efficiencies, but lower exergy efficiencies. Comparing the Ericsson to an equivalent ORC engine, which is a mature waste-heat recovery technology, reveals that the former can operate at lower thermal heat inputs, which allows operation over a wider range of applications with different heat-source mass flow rates. For comparable heat inputs and heat-source temperatures the power output and thermal efficiency of the Ericsson engine are higher than those of the ORC engine. Therefore, the Ericsson engine is an attractive alternative to existing waste-heat recovery technologies.