As the demand for electrical power increases, so do the threats of global warming and power source depletion. To counter non sustainable power generation, environmentally friendly energy sources have been increasingly investigated and developed. Although globally beneficial, common plants such as wind or tidal farms show no promise for remote townships such as Birdsville, Queensland. Here geothermal power has been in operation, extracting heated brine from Hot Sedimentary Aquifers to heat a cycle working fluid within a Binary Rankine Cycle. To increase consistency and reduce maintenance costs, replacement of the existing plant with the UQ ORC Mobile plant is being considered.
Before the plant can be implemented at Birdsville, analytical investigations are required into the projected outputs and efficiencies of both the existing and mobile plant. This thesis aimed to use thermodynamic relationships as well as an industry based literature review to break down the individual fluid cycle components, determining system outputs and efficiencies if installed at Birdsville. This involved splitting calculations into two main areas, the first to determine the cycles performance if pump and turbine systems are maintained, the second investigates the potential advantages of open-ended upgrading of the existing system. Pinch point analysis was implemented to help design heat exchanger systems, using alternate fluids and working fluid flow rates for the second focus area. Both Matlab and Refprop (fluid property database) were used to quantify the analysis, examining net-work production, thermal efficiencies and a heat exchanger area based objective function to aid cycle comparisons.
Results of the analysis show that installation of the UQ ORC system using the existing pumps and turbine is not efficient. In this case, the output power is 48.5kW with an objective function of 2.75m2/kW. This performance is below the current output of the existing system at its design point, showing a 40.5kW reduction in net-power production. The decreased outputs were due to the two fluids not being coupled correctly, therefore not exchanging heat efficiently. By increasing the working fluid flow rate, it was found that the output power could be improved to 98.7kW, an addition of 103% compared to the flow rate restricted design. Increasing this flow was also found to reduce the objective function to 1.58m2/kW, a property which would be very beneficial if a cost analysis was completed. One key relationship was also found between net-work production and working fluid flow rates, showing that a maximum is found after superheater removal.
Once common fluids were compared to refrigerant R245fa (currently operating within the mobile plant), isobutane was found to be the only option capable of producing similar net-work at reduced flow rates. This was at the cost of doubled upper pressure, a parameter which is of particular concern considering the current issue of pipe leaks compiled within the 1.8% lower flammability limit of isobutane, potentially causing safety breaches. An iteration of zeotropic mixtures between R245fa and isobutane was found to fill the gaps between each fluids design points, allowing for comparable cycle results between a working fluid flow rate of 4.5 and 6kg/s.
Through further investigations into superheater inclusion, heat exchanger losses and a cost based analysis, an optimum solution is capable of being determined. If successful, the UQ Organic Rankine cycle may prove to be the start of a range of power stations for regional Queensland.