Centrifugal Turbines for Mini-Organic Rankine Cycle Power Systems

Abstract

Organic Rankine cycle (ORC) power systems are rapidly diffusing as a technology for the conversion of thermal energy sources in the small-to-medium power range, e.g., from 150 kWe up to several MWe. The most critical component is arguably the expander, especially if the power capacity is small or very small, as it is the case for innovative high-potential applications such as waste heat recovery from truck engines, or distributed conversion of concentrated solar radiation. In these so-called high-temperature applications, the expansion ratio is very high; therefore, turbines are the expanders of choice. Recently, multistage radial-outflow turbines (ROT), a nonconventional turbine configuration, have been studied, and first commercial implementations in the MWe power range have been successful. The objective of this work is the evaluation of the radial-outflow arrangement for the turbine of high-temperature mini-ORC power systems, with power output of the order of 10 kWe. To this end, a method for the preliminary fluid-dynamic design is presented. It consists of an automated optimization procedure based on an inhouse mean-line code for the one-dimensional preliminary design and efficiency estimation of turbines. It is first shown that usually adopted simplified design procedures, such as that of the so-called repeating-stage, cannot be extended to minicentrifugal turbines. The novel methodology is applied to the exemplary case of the 10 kWe turbine of an ORC power system for truck engine heat recovery documented in the literature. The expansion ratio is 45. The preliminary fluid-dynamic design of two miniturbines is presented, namely, a five-stage transonic and a three-stage slightly supersonic turbine. The outcome of the preliminary design leads to two turbine configurations whose fluid-dynamic efficiency exceeds 79% and 77%, respectively. The speed of revolution is around 12,400 and 15,400 RPM for the five-stage and the three-stage machine, respectively. These results show that the ROT configuration may allow for compact and efficient expanders for low power output applications

Active subspaces for the optimal meanline design of unconventional turbomachinery

“…Consequently, and in order to enhance the robustness of the optimization, a gradientfree optimizer is commonly adopted, see, e.g., Refs. [12][13][14].…”

Combined Turbine and Cycle Optimization for Organic Rankine Cycle Power Systems—Part A: Turbine Model

“…In the ORC field, Fiaschi et al [6] have suggested the use of radial inflow turbines as advantageous solutions in the small power range due to their compact structure, higher enthalpy drop per stage and insensitivity to load variation at off-design. In the context of radial turboexpanders, novel architectures have been studied by Persico et al [7], Pini et al [8] and Casati et al [9], who have highlighted the specific flow features and advantages of centrifugal turbine configurations for high-temperature and small-power output applications. Axial turbines are generally adopted for high mass flow rates and in multistage configurations [10,11].…”

The impact of component performance on the overall cycle performance of small-scale low temperature organic Rankine cycles

“…High temperature heat sources such as exhaust gases from internal combustion engines require very high pressure ratios, which are achieved by a turbo-expander such as those presented in [5,6]. The relatively small enthalpy drop of organic fluids permits these pressure ratios to be achieved over a single stage, but at the expense of introducing supersonic flows within turbine.…”