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 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 1MWare 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.