Design methodology for radial turbo expanders in mobile organic Rankine cycle applications

Abstract

h i g h l i g h t sWe describe a detailed design methodology for ORC radial turbo expanders. Toluene is selected as the working fluid for diesel engine waste energy recovery. A first turbine of 15.5 kW is designed but yields too small inlet blade heights. A second turbine for minimum power generates 34.1 kW with 51.5% efficiency. A third turbine for maximum efficiency produces 45.6 kW at 56.1% efficiency. a b s t r a c tFuture vehicles for clean transport will require new powertrain technologies to further reduce CO 2 emissions. Mobile organic Rankine cycle systems target the recovery of waste heat in internal combustion engines, with the exhaust system identified as a prime source. This article presents a design methodology and working fluid selection for radial turbo expanders in a heavy-duty off-road diesel engine application. Siloxanes and Toluene are explored as the candidate working fluids, with the latter identified as the preferred option, before describing three radial turbine designs in detail. A small 15.5 kW turbine design leads to impractical blade geometry, but a medium 34.1 kW turbine, designed for minimum power, is predicted to achieve an isentropic efficiency of 51.5% at a rotational speed of 91.7 k min À1 . A similar 45.6 kW turbine designed for maximum efficiency yields 56.1% at 71.5 k min À1 . This emphasizes the main design trade-off -efficiency decreases and rotational speed increases as the power requirement fallsbut shows reasonable radial turbine efficiencies and thus practical turbo expanders for mobile organic Rankine cycle applications are realizable, even considering the compromised flow geometry and high speeds imposed at such small scales.

Thermo-Economic Performance Analysis of a Regenerative Superheating Organic Rankine Cycle for Waste Heat Recovery

“…The volume flow ratio (VFR) is very important for turbine design, which has a significant influence on turbine efficiency and geometry [40]. This factor accounts for the effects of volume expansion and compressibility [28,39].…”

“…As a result, the system irreversibility increases and thermal efficiency reduces. A high built-in VFR needs a large variation rate of rotor blade height between the Energies 2017, 10, 1593 13 of 23 inlet and outlet, and manufacturing costs and difficulties increase [40]. Therefore, a moderate VFR which achieves a tradeoff between lower structural complexity and less under-expansion irreversibility is essential [39].…”

“…This factor accounts for the effects of volume expansion and compressibility [28,39]. Then, the parameter was investigated in other works to study the characters of the turbine in the ORC system [28,39,40]. The VFR is defined as the specific volume variation of working fluid across the turbine [28], and a moderate VFR is preferred in the ORC system.…”

“…Thus, in order to address Many efforts have been made with respect to working fluid selection and parametric optimization, considering thermodynamic or economic factors. However, very few similar works focus on discussing the limitation of volume flow ratio (VFR) [28,39,40], a factor which has a significant influence on turbine design and manufacture. In this study, a turbine is selected as expander in the investigated regenerative ORC system.…”

Similarity Theory Based Radial Turbine Performance and Loss Mechanism Comparison between R245fa and Air for Heavy-Duty Diesel Engine Organic Rankine Cycles

“…As the organic fluid properties are totally different from air, these researches are mainly divided into two technical routes. One route focuses on forward design, including preliminary design of geometry parameters and aerodynamic design of blade profiles [12][13][14][15][16][17][18][19][20][21]. Dolz et al [13] propose that the pre-design of turbomachinery must take real gas equations of state into consideration, because the specific energy deviation between real gas and perfect gas can be as large as 100%, which may lead to total wrong turbine preliminary design result.…”