A fundamental study on hybrid geothermal energy systems

Research Doctorate - Doctor of Philosophy (PhD)This thesis focuses on a fundamental study of hybrid geothermal energy systems, in which geothermal energy is hybridised with other energy sources, such as solar, biomass and fossil fuels, in an integrated technology platform. The motivation behind this study was to use hybridisation to reduce the cost and improve the thermal efficiency of geothermal resources, especially hot dry rock (HDR) resources. Once proven, the hybrid concept will offer a better approach to replacing fossil fuels with clean energy, thereby increasing the renewable energy share in the world’s current energy mix and reducing greenhouse gas emissions. The objective of this project was to undertake a systematic techno-economic assessment of hybrid geothermal energy systems for low emission power generation. The ultimate goal was to explore the potential benefits from the synergies between geothermal and other energy resources. The scope of the thesis, however, was limited to addressing mainly the surface engineering problems faced by geothermal energy systems, particularly Enhanced Geothermal System (EGS), by treating the geothermal energy as an “engineering approximation” to the complex reality where the subsurface geology of geothermal energy is subject to great uncertainties/difficulties. To achieve the broad objectives of this project, the research mainly focused on a theoretical investigation of the two main themes of hybrid geothermal energy systems: hybrid solar–geothermal energy systems and hybrid geothermal–fossil fuel energy systems. A comprehensive set of thermodynamic and economic analyses were undertaken. The thermodynamic studies were based on the first and second law of thermodynamics analyses, examining the technical feasibility of the hybrid systems. The proposed economic assessments were performed using established economic practices to: 1. Compare the capital and operating costs of hybrid geothermal energy systems. 2. Estimate the electricity production costs for each alternative technology option based on a plant cash flow analysis encompassing the total annual expenses and revenues. The cost of electricity approach, net present value and sensitivity analysis were employed as economic tools to examine the economic feasibility of the hybrid systems. In addition, figure of merit analyses were performed to evaluate the superiority or inferiority of the hybrid plants compared to two stand-alone plants, from both technical and economic viewpoints. Lastly, Australian case studies were performed to examine the hybrid system in more realistic technical and economic conditions. The process simulation package, Aspen HYSYS, was employed for all simulation purposes. Analyses of the hybrid solar–geothermal power plant showed that this plant was significantly affected by resource quality and weather conditions. Particularly on partially clear and partially cloudy days, the performance of the solar heating system was found to drop by 40 to 60%, significantly affecting the feasibility of the hybrid plant. Moreover, the inefficient air-cooling issues of stand-alone geothermal power plants in hot climates were found to be effectively mitigated by hybridising solar energy in the same power cycle. Generally, the net power output of the hybrid plant increased as the solar irradiance and/or geothermal reservoir temperature increased, while the power output decreased as the ambient temperature increased. The maximum net power output of the hybrid plant was achieved using a fully optimised operating mode. Under typical Australian climatic conditions, with an average ambient summer temperature of 31°C and a design-point solar direct normal irradiance of 1,000 W/m², a hybrid plant with a reservoir temperature of 150°C requires a solar aperture area greater than 14,000 m² (that is, a solar energy fraction larger than 52%) to thermodynamically outperform the stand-alone solar and geothermal plants. The economic analysis found that the solar-to-electricity cost of the hybrid plant was below $145/MWh. The case studies of the hybrid solar–geothermal power plant for three different Australian geographic locations indicated that the hybrid system produced much greater power output than the stand-alone geothermal plant. Moreover, the hybrid plant was found to be less subject to diurnal and seasonal fluctuations in the ambient temperature than the stand-alone geothermal plant. For the three locations, the average cost of solar-to-electricity of the hybrid plant was found to be only $101/MWh, while the hybrid plant electricity cost was found to be about 23% less than that of the stand-alone enhanced geothermal system (EGS) at $225/MWh. Analyses of the hybrid geothermal–fossil fuel power plant indicated that, in both a regenerative steam Rankine cycle (R-SRC) and non-R-SRC, the booster mode was the best hybridisation mode, outperforming both the fuel-saving and compensation modes. However, the effect of ineffective hybridisation—namely, when geothermal heat become thermodynamically useless in the hybrid geothermal-fossil fuel power cycle—was found in both the booster and fuel-saving modes for the hybrid system based on R-SRCs. This indicates the compensation mode as the best option in these cases. It was also found that hybrid geothermal–fossil fuel power generation is most effective when applied in a non-R-SRC, less effective in an R-SRC, and least effective in a highly efficient R-SRC (such as a large-scale contemporary fossil fuel power plant). Economic analyses showed that the two largest hybridisation costs were the well cost and cost of the geothermal pipeline system. The unit pipeline cost for the examined hybrid system was found to be approximately $10 to $32/km for each MWt of the transferred heat. With proper pipe insulation, geothermal energy can be converted to electricity in the hybrid system at a cost of around three to 15 cents/kWh depending on reservoir characteristics and hybridisation schemes. This corresponds approximately 33 to 87% less than the typical electricity cost of a stand-alone EGS plant at 22.5 cents/kWh. In addition, the feasibility of the hybrid system was found to be highly subject to economic policies, including carbon tax and Renewable Energy Certificates. The figure of merit analyses showed that an economically feasible GAPG system requires the use of HDR resources located no further than 20 km from the plants. Specifically, for a typical resource distance of 5 km GAPG system was found economically superior to the two stand-alone plants if the following combinations of reservoir temperatures and hybridisation schemes are met, namely 90 – 115°C for BT1/FS1, 110 – 140°C for BT2/FS2, 120 – 165°C for BT3/FS3, and 150 – 210°C for BT4/FS4. Based on figure of merit analyses, reference maps were developed to predict suitable conditions for which the hybrid plant outperforms the stand-alone plants