Experimental design, analysis and improvement of a Marnoch Heat Engine with heat recovery.

Abstract

This research examines the prototyping of a Marnoch Heat Engine (MHE) and conducts thermodynamic and heat transfer analyses of this heat engine with a waste heat recovery system. The heat source for the MHE can be low grade heat (about 365 K). During cold seasons an ambient temperature can be adequate for the heat sink to operate the heat engine. When the ambient temperature is high, the system can be connected to a dry cooler to achieve higher efficiency levels. A water/glycol mixture transfers heat from the heat source into the hot heat exchangers and removes the heat from the cold heat exchangers. Compressed dry air is used as the working fluid in the heat engine. This thesis develops prototyping of the MHE with two different mechanical configurations of the transmission system. Furthermore, thermodynamic analysis is carried out to calculate the heat engine power output, as well as energy and exergy efficiencies under various operational conditions. A heat transfer model is developed to predict the transient temperature behaviours of the heat exchangers for different flow regimes and temperatures. The results from the model are validated against the available data in the open literature and then compared with experimental results from the present MHE prototype. The average difference between the heat transfer model and the measured data from the MHE prototype is about 10 K. Life cycle assessment is applied to identify CO2 emissions of the MHE during the manufacturing and maintenance processes. The heat source and heat sink temperatures for this case study are assumed to be 281 K and 370 K, respectively. In a developed case study, it is found that the amount of CO2 released as a result of manufacturing and maintenance of the system is about 52 g CO2-quivalent per kWh. Results from the exergoeconomic analysis are used to determine the required changes in MHE design parameters for the purpose of improving the cost effectiveness of the overall system. Energy and exergy efficiencies of the MHE are evaluated under various operating conditions. It is shown that the maximum exergy efficiency of the MHE reaches, at most, 18% of the Carnot efficiency. The maximum value of the exergoeconomic factor is found to be 0.45, and that the heat exchangers are the major sources of exergy destruction within the system. To improve the system performance, the major sources of heat and mechanical losses are identified and addressed. This research proposes four new heat exchanger designs that can be applied to future designs of the MHE units. The strength of the effect of each proposed design on the system performance is also discussed.