Centrifugal Turbines for Mini-Orc Power Systems

In the last two decades, the increase of fossil fuels cost, together with growing environmental consciousness, have shed light on the opportunities offered by waste heat recovery, and conversion of renewable energy sources. Organic Rankine cycle (ORC) power systems stand out in terms of reliability and cost-effectiveness, and have demonstrated their advantages compared to steam power plants [1, 2]. Since the first examples of implementations, ORC power plants proved suitable for the conversion of thermal energy into electricity even for very low power capacity, down to few kWe [3, 1, 2, 4]. These small systems are often referred to as mini-ORC (mORC) power plants. However, the most successful commercial applications have been deployed in a much larger power size, ranging from hundreds of kWe up to approximately 5 MWe, and these systems represent now the state of the art of this technology. Nonetheless, many researchers in the ORC field are still investigating the development of mORC modules for a variety of applications: the interested reader is referred to Ref. [5] for a thorough review. The expanding device constitutes the critical component of any small power system and, in the case of those working according to the Rankine cycle, this constitutes one of the motivations driving the interest towards the ORC concept. The adoption of molecularly complex and heavy working fluids, instead of steam, primarily allows fundamental simplifications of the turboexpander design [1, 2]. In case of mORC systems with power output lower than (say) 10 kWe, it is disputable which, among turbomachinery and volumetric machines, offers the best compromise in terms of efficiency, reliability and costs [6]. The vast majority of published works considers prototypes implementing positive displacement expanders and, according to the examples collected in Ref. [5], no author reports experimental mechanical efficiency higher than 65%, with expansion ratios in the range 2–6. This work treats the preliminary design of turboexpanders tailored to mORC power systems, stemming from two considerations: i) improving the expander efficiency is essential in order to gain higher system performance [7]; ii) turboexpanders arguably remain the only feasible choice if the expansion ratio is bound to be large, as in the case of high temperature applications. Following the work of Pini et al. [8], the expander architecture herein considered is the centrifugal or radial outflow turbine (ROT). The objective of this paper is to show the validity of this solution for cost-effective and compact multistage ORC turbines in the 10 kWe power range.