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

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 ℃, 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 ℃), 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 ℃) 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.