Numerical and Experimental Investigation of a Velocity Compounded Radial Re-Entry Turbine for Small-Scale Waste Heat Recovery

Abstract

The energy industry must change dramatically in order to reduce CO2-emissions and to slow down climate change. Germany, for example, decided to shut down all large nuclear (2022) and fossil thermal power plants by 2038. Power generation will then rely on fluctuating renewables such as wind power and solar. However, thermal power plants will still play a role with respect to waste incineration, biomass, exploitation of geothermal wells, concentrated solar power (CSP), power-to-heat-to-power plants (P2H2P), and of course waste heat recovery (WHR). While the multistage axial turbine has prevailed for the last hundred years in power plants of the several hundred MW class, this architecture is certainly not the appropriate solution for small-scale waste heat recovery below 1 MW or even below 100 kW. Simpler, cost-effective turbo generators are required. Therefore, the authors examine uncommon turbine architectures that are known per se but were abandoned when power plants grew due to their poor efficiency compared to the multistage axial machines. One of these concepts is the so-called Elektra turbine, a velocity compounded radial re-entry turbine. The paper describes the concept of the Elektra turbine in comparison to other turbine concepts, especially other velocity compounded turbines, such as the Curtis type. In the second part, the 1D design and 3D computational fluid dynamics (CFD) optimization of the 5 kW air turbine demonstrator is explained. Finally, experimentally determined efficiency characteristics of various early versions of the Elektra are presented, compared, and critically discussed regarding the originally defined design approach. The unsteady CFD calculation of the final Elektra version promised 49.4% total-to-static isentropic efficiency, whereas the experiments confirmed 44.5%.

Concept and Design of a Velocity Compounded Radial Four-Fold Re-Entry Turbine for Organic Rankine Cycle (ORC) Applications

“…The examination of the Elektra turbine, as detailed in this article, adhered to design procedures (Figure 2) proven in the development of other turbomachines [7,49,50]. Commencing with the provided input data and boundary conditions (which will be described subsequently), we initially determined the preliminary flow path geometry of the turbine using our in-house 1D Turbine Design Tool (1DTDT).…”

“…Due to the potential of this concept, the authors first designed and investigated several compressed air Elektra demonstrators [49], equipped with two velocity stages. Based on the gained experience, the authors have currently developed a 7 kW el velocitycompounded radial four-fold RE turbine for application in an existing small CHP-ORC plant working with Hexamethyldisilioxane (MM).…”

CFD Simulation and Flow Study of a Velocity Compounded Radial Re-Entry Turbine - Elektra

“…Both geometries, V1 and V7, were manufactured and tested on the compressed air test bench of the OTH Amberg-Weiden. The specifications of the test bench are described in [3]. The efficiency is calculated according to Equation 1:…”

“…In need of turbines dealing with these low temperatures, spoken in a power generating context, new concepts have to be developed to reach reasonable efficiency levels. In previous papers the usage of the Elektra turbine was evaluated [2,3]. This turbine is velocity compounded and works according to the concept of a Curtis turbine, with the exception of only consisting of a single wheel with a radial re-entry.…”

CFD Flow Analysis of the Influence of Rotor Seals in a Cantilever Microturbine

“…This research is intensively underway. The ELEKTRA turbine has the advantage of a lower operational speed (leading to a cheaper electrical generator) and the aim is now to achieve an efficiency comparable to the "classic" turbine type presented in [9,10].…”