Design-to-Resource (DTR) using SMC Turbine Adaptive Strategy : Design Process of Low Temperature Organic Rankine Cycle (LT-ORC)
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Low grade heat source has a huge amount of power generation potential from the industrial waste heat, and the untapped low temperature geothermal reservoirs. A suitable turbine for these heat resources is usually not available from the market. A turbine can be designed and developed but it requires the knowledge based engineering (KBE) in the turbine technology with high engineering development cost. It is not cost effective to follow the conventional turbine design and development pattern for each set of working fluid and operating condition for ORC system. Another solution is to select and adapt the off-the-shelf turbines for the given heat resources. The objective of this thesis is to define and develop the design-to-resource (DTR) methodology coupled with the SMC turbine adaptive strategy, named SMC-DTR approach. . The SMC strategy consists of three different analysis techniques; similarity analysis, meanline analysis, and computational fluid dynamics (CFD) method, to analyse the turbine performance for application and working fluid different from the original design. This strategy allows the design and optimization of an ORC system for low temperature resources by adapting the off-the-shelf radial inflow turbines for various heat resource condition. The SMC-DTR approach was applied in three different resources condition. The numerical study shows that the off-the-shelf radial inflow turbines are feasible to be adapted as ORC turbines but the turbine efficiency would deteriorate. The performance of automotive turbocharges deteriorates up to 20% whereas the gas turbine performance changes up to 10% if the working fluid is changed from air to refrigerants, including R134a, R245fa, and isobutane. Most of the off-the-shelf turbines are designed for sub-sonic expansion. If they are used for supersonic expansion, the turbine efficiency deteriorates when the expansion ratio increases. The rotor blade passage chokes, and the flow accelerates downstream of the rotor throat. This leads to the high swirl angle at the turbine outlet and high turbine loss in term of kinetic energy, which is reflected in the low turbine efficiency. The application of the SMC-DTR approach also shows that a radial inflow turbine with fixed nozzle vanes can be selected and adapted for two different resource condition: constant heat source and heat sink condition, and constant heat source condition with varying heat sink temperature. The turbine efficiency and the thermal efficiency of the thermodynamic cycle change up to 2% when the heat sink temperature changes from 10 to 23°C. However, a radial turbine with fixed nozzle vanes is not suitable for the heat source with varying flow rate. If the mass flow is reduced by 25% off the design value, the turbine performance and the thermodynamic cycle performance drop significantly. A variable nozzle turbine has to be selected for adaptation for the heat source with varying mass flow for optimal performance. The thesis also evaluated the suitability of off-the-shelf turbines as ORC turbine and proposed a number of design changes. Automotive turbochargers and gas turbines cannot be applied directly as the ORC turbines since they are originally designed for air. A mechanical system design has to be performed to be used with refrigerants.