Binary liquid metal–organic Rankine cycle for small power distributed high efficiency systems

Abstract

There is a common interest in the distributed power generation: generally for the combined production of electrical and thermal energy and often, although not necessarily, in association with renewable energies as heat sources for the prime mover. For example, in the field of distributed concentrated solar power generation of small size, the gas engine technology now seems to be prevailing (Stirling engines operating at maximum temperatures of 600-800 C, with peak net efficiencies at 20-30% and power up to several kilowatts are commonly considered). Organic Rankine engines, fed by biomass, in the power range of about 1 MW are actually a standard. From a strictly thermodynamic point of view, the binary cycle technology, accomplished by alkaline metal Rankine cycle as the topping cycle and a Rankine cycle with organic fluid as the bottoming cycle, could be an advantageous alternative. By their very nature, Rankine cycles have good thermodynamic qualities and, potentially, their thermodynamic performance, for the same maximum and minimum temperatures, could be better than that of a gas cycles. This paper discusses the possibility of adopting binary cycles with a power level in the order of tens of kilowatts. Following an overview of the characteristics of alkaline metals and a look at the possible organic fluids that can be employed in Rankine engines at high temperature (400 C), assuming a limit condensation pressure of 0.05 bar, the thermodynamic efficiency of binary cycles was evaluated and the preliminary sizing of turbines was discussed. The results (e.g. a net cycle efficiency of around 0.46, with maximum temperature of 800-850 C) appear encouraging, even though setting up the systems may be far from easy. For instance, there are difficulties due to the extremely high volumetric expansion ratios of bottoming cycles (400-600, an order of magnitude larger than those of the topping cycles with alkaline metals that we considered), which are moreover associated with a very low minimum pressure and elevated number of revolutions of the turbomachinery (50,000-200,000 r/min). Without doubt, the design tends to be easier as the power levels increase and the minimum condensation pressure for the bottoming cycle rises. Although the authors know of no activity in progress on binary cycles at present, the interesting prospects suggest the topic deserves further study and research.

Titanium tetrachloride as novel working fluid for high temperature Rankine Cycles: Thermodynamic analysis and experimental assessment of the thermal stability

“…These considerations and studies support the idea of adopting TiCl 4 in high temperature RCs (see also Ref. [28]). …”

“…Halides have been considered as suitable working fluids in heat pipes, in the temperature range from 200 to 400°C [26][27][28][29][30], as well as in Rankine Cycles [28]. Indeed, Titanium tetrachloride would be particularly desirable in the latter application, thanks to its high molar weight and low molecular complexity.…”

“…3 is well suited in case of a variable temperature heat source, the presence of bleedings would improve the cycle performances whenever a constant or nearly constant temperature source is exploited. This can be the case of heat obtained from a fuel combustion, concentrated solar power plants, fluidized beds or more complex cycle arrangements such as binary plants [15,28,31].…”

Energy, exergy analysis and working fluid selection of a Rankine cycle for subsea power system

“…Fluids of water, carbon dioxide, organics or even alkali liquid metals [18,19] can be utilized in a Rankine cycle. With different application purpose, the working conditions for the Rankine cycle are all quite different.…”

High-Efficiency Small-Scale Combined Heat and Power Organic Binary Rankine Cycles

“…The mixture, according to the manufacturers, is utilizable up to 400 • C, [32] The p-terphenyl, the most common isomer among the terphenyls, has an high critical temperature (about 640 • C) and it is probably as thermally stable as the eutectic mixture of diphenyl and diphenyl oxide. Terphenyls, for their high boiling point and thermal stability, were considered and thoroughly studied as cooling and moderating fluids in some particular nuclear reactors, ( [33], Chapter 17, p. 411).…”