Feasibility analysis of ORC systems : thermo-economic and technical considerations for flexible design.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Geothermal energy conversion engineering is currently carried out nearly exclusively in-house by the technology providers, primarily Ormat and more recently Turboden and Exergy. Geothermal power has a long history of sustainable and economic baseload generation, but each new development requires a much more complex feasibility engineering process than other fossil fuel or renewable energy prospects. The development potential for geothermal energy is vastly greater than for wind or solar, but a critical gap exists in methodologies for the assessment of the technical and economic feasibility of a particular resource development at the exploratory stages of the project. This thesis contributes a sequential approach for carrying out the feasibility study for flexible design (FSFFD) for organic Rankine cycle (ORC) energy conversion technology. The FSFFD approach addresses all of the key ORC design choices to achieve the best performance within the constraints of the available component technologies and cost. The FSFFD approach involves three processes: 1) thermodynamic and economic feasibility studies for key components, 2) flexible design methodology for best resource utilization, and 3) a novel lifetime strategy for anticipating the geothermal resource degradation in the design of plant capacity. The feasibility studies are conducted to investigate the influence of the heat exchanger design and cycle configurations on the ORC cost and performance. Component selection and cycle configuration options are modelled to obtain the most profitable design considering thermodynamics, economics and technical aspects. The results show that the plate exchanger is the most economical exchanger for ORC systems, even though shell and tube are the current standard. The two-stage thermodynamic cycle configuration provides higher net electrical power output, and higher thermal and exergy efficiencies than the one-stage designs. However, the increased investment cost and the added technical complexity of two-stage designs can make these designs less feasible than one-stage designs. The FSFFD approach uses a two-stage design methodology. The first exploration methodology assesses design alternatives for a new binary geothermal power plant. This methodology is suitable for investigating the potential geothermal resource over which the binary geothermal power plant will be installed. The second development methodology obtains the cost-optimum design that is the best match to a heat resource. A breakdown of all typical costs of the geothermal plant projects is calculated in the exploration methodology, although it still deals with uncertainty costs in the preliminary stage, especially the drilling costs. The development methodology was tested with the experimental data from a lab-scale ORC system. The study of the lab-scale ORC system optimised the current ORC design with three ranges of heat input (condition 1, condition 2 and condition 3) – which was not clear to the original designers. The size of the current evaporator and condenser is significantly larger than the required heat transfer areas, but the size of the current gas-oil exchanger is significantly smaller especially under the low heat input conditions. Finally, a novel lifetime strategy is developed to optimise the binary geothermal power plant taking into consideration the resource degradation. The best design point is selected over the whole plant life considering the typical decrease of thermal input found by research of past developments. The results demonstrate that the initial geothermal resource temperature, pressure, and flowrate are not the best design point to develop the most profitable plant design, although current practice uses these values. This thesis also proposes how to improve the lifetime plant performance in two ways through operational parameters adjustments and adaptive designs. The results show that adjustment of mass flow rates of n-pentane and cooling air can maintain the performance over the whole plant life. After the half-life of the operation, the working fluid pumps need to be replaced to maintain the plant performance. The two adaptive designs discussed in this thesis are installing a recuperator and reducing the heat transfer area of preheater and vaporizer.