Distributed heat conversion technologies based on organic fluid cycles for a high-efficiency and sustainable energy future

Abstract

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.