This thesis regards the optimisation of the power conversion cycle of a hybrid power plant that combines geothermal and solar thermal resources. The aim is to find the optimal combination of various thermodynamic cycles and working fluids for a hypothetical hybrid power plant located in Moomba, South Australia.
This binary power plant uses geothermal heat from a hot rock resource assumed to produce geothermal brine at a temperature of 500°C and a flow rate of 500kg/s. The solar thermal resource is considered to maintain the solar collector fluid at a constant 350 °C. The ambient air temperature and its daily and annual variation are taken from meteorological data.
The supercritical and transcritical cycle are considered as options for the thermodynamic cycle. These cycles are suited to this comparatively low-temperature application due to their variable temperature heat addition. Five working fluids, all with critical temperatures and pressure well below those of water, are being considered: carbon dioxide (CO2), sulphur hexafluoride (SF6), ethane (C2H6), ammonia (NH3) and carbonyl sulphide (COS). Mixtures of fluids and their application are also considered.
The analysis is centred on computer code that simulates the power cycle of the power plant. Modelling equations of the thermodynamic cycles are contained in MATLAB scripts, while the thermodynamic properties of the fluids and their mixtures are calculated using the NIST/REFPROP software. The code outputs performance criteria, such as annual electricity generation, and plant requirements including heat exchanger sizing.
A transcritical cycle using COS as the working fluid has the highest annual electricity production of 900GWh. This cycle, however, is disadvantaged by higher capital costs caused by requirements for larger heat exchangers and solar collector area, compared to other cycle and fluid combinations considered. It is concluded that a financial analysis over the life of the power plant, which is outside the scope of this thesis, is required to determine whether the transcritical COS cycle can produce electricity more economically than a cycle that generates less electricity, but has a lower capital cost, such as a supercritical CO2 cycle.