Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
This paper presents and discusses the emergence of two
distinct classes of energy conversion systems based on
thermodynamic vapour-phase heat engine cycles undergone by
organic working fluids, namely organic Rankine cycles (ORCs)
and two-phase thermofluidic oscillators (TFOs). Each type of
system has its own distinctive characteristics, advantages and
limitations. ORCs are a more well-established and mature
technology, are more efficient, especially with higher
temperature heat sources and at larger scales, whereas TFOs have
the potential to be more cost-competitive, in particular at lower
temperatures and at smaller scales. Specifically, ORC systems
are particularly well-suited to the conversion of low- to mediumgrade
heat (i.e. hot temperatures up to about 300 – 400 °C) to
mechanical or electrical work, and at an output power scale from
a few kW up to 10s of MW. Thermal efficiencies in excess of
25% are achievable at the higher temperatures, and efforts are
currently in progress to develop improved ORC systems by
focussing on advanced architectures, working fluid selection,
heat exchangers and expansion machines. Correspondingly, TFO
systems are a more recent development aimed at the affordable
conversion of low-grade heat (i.e. hot temperatures from 20 –
30 °C above ambient, up to about 100 – 200 °C) to hydraulic
work for fluid pumping and/or pressurisation. Ultimately, TFOs
could emerge at scales of up to a few hundred W and with a
thermal efficiency of the order of a few % points. The two energy
conversion systems are complementary, and together have a
great potential to be used for distributed power generation and
improved energy efficiency, leading to primary energy (i.e. fuel)
use and emission minimisation. Relevant applications and fields
of use include the recovery of waste heat and conversion to
useful work including mechanical, hydraulic or electrical energy,
or the effective utilisation of renewable energy sources such as
geothermal, biomass/biogas and solar energy.
