Single acting uniflow condensing engine system for low grade heat recovery.

Abstract

Low grade heat with temperatures of 50°C - 150°C from renewable or industrial sources (eg. waste heat) constitutes a substantial and unused energy resource. Several barriers to low grade heat recovery exist but rising energy costs are increasing the importance of its development. The most common technology currently used for low grade heat recovery is the Organic Rankine Cycle. This is a heat engine which uses organic refrigerants at high pressures for recovery of low temperature heat with high energy density. The atmospheric condensing steam engine has been proposed as a potential alternative technology for low grade heat recovery with safety and sustainability benefits arising from its use of water as a working fluid and operation at atmospheric pressure and below. This technology has reduced energy density but offers advantages for applications on smaller power scales (1kW – 200kW) where simplicity is favoured, such as in domestic systems or in remote and rural communities.
This thesis has continued the development of the condensing engine technology by building on previous work conducted at the University of Southampton. Specifically, this research has built and tested a novel single acting uniflow design for improved performance. This allows increased steam evacuation through ports on the cylinder wall resulting in a larger pressure driving force for power production. With an effective temperature difference of around 30^oC and without steam expansion the engine reached a maximum power output of 5.4W, assuming 10% mechanical losses, and thermal efficiency of 2.5%. Assuming no losses between boiler and cylinder, this equated to a second law efficiency of around 40%. This constitutes comparable power output to the previous model under similar operating conditions and an increase of 25% in maximum efficiencies. Second law efficiencies were also comparable to Organic Rankine Cycle systems of a similar scale. Future condensing engine thermal efficiencies as high as 9% were predicted through analysis of the data.
A novel heat recovery and re-use concept within the condensing engine system itself was also investigated theoretically and experimentally in this thesis with the aim of improving efficiency. Theoretical modelling confirmed that as much as 75% of thermal energy input would be available for recovery during condensation, assuming dry steam. Modelling showed that if this heat was re-used within the engine system, to drive a second sub-atmospheric stage, it could operate at reduced steam expansion without compromising efficiency. This would allow improved power output and stability. Experimental testing demonstrated suitable proof of concept of heat recovery on the condensing engine using a two-stage condenser arrangement with intermediate flat plate heat exchanger for latent heat recovery. A maximum heat recovery efficiency around 60% was achieved empirically with cooling water outlet temperatures of 65^oC - 90^oC without affecting the system pressure required for engine operation, maintained around 0.2bar.

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Identifiers

ORCID for Curtis Reece Howell: orcid.org/0000-0003-0494-8766

ORCID for Andrew Cruden: orcid.org/0000-0003-3236-2535

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