Shallow geothermal system: feasibility and design practices in Melbourne, Australia

© 2018 Dr. Qi LuClimate change and the need to protect the environment have given rise to renewable energy targets, which drive the development of renewable energy technologies worldwide. For Australia, with a target to reduce greenhouse gas emissions by at least 26% below the 2005 level by 2030, it would be beneficial not only to produce ‘greener’ energy but also to reduce energy consumption through the use of energy efficient technologies. Considering that heating and cooling systems make up the majority of the energy consumed by commercial and residential buildings, a more energy efficient technology for Heating Ventilation and Air Conditioning (HVAC), is highly desirable. A Ground Source Heat Pump (GSHP) system represents one such technology. Under such a system, the thermal energy of the ground is transferred to the circulating liquid within the ground heat exchangers (GHEs); this energy is then extracted and enhanced by the heat pump to be delivered to the building via a distribution system, such as fan coil units. However, these systems are capital intensive mainly due to the high drilling cost associated with GHEs, which is hindering the wide adaptation of such systems worldwide. The aim of this research is to study the current design practices for GSHP systems and understand the suitability and accuracy of different design software, as well as assessing the financial and environmental feasibility of such systems in Victoria, Australia. This study is part of the Sustainable Energy Pilot Demonstration (SEPD) program, where around twenty properties in Melbourne are equipped with GSHP systems and their thermal performance is being monitored. The measured data under the SEPD program is generally of acceptable quality, however, due to imperfect insulation around temperature sensors, the ambient air temperature fluctuation seems to affect the measurements. To address this issue, an innovative framework utilising neural network is developed to correct the measured data. Once the data is collected and corrected accordingly, energy analyses are conducted in this thesis for selected properties. The majority of the monitored properties show acceptable performance (coefficient of performance, COP, around 3.7) while some experienced low energy efficiency (COP around 2.5). To investigate whether the poor thermal performance is due to an inferior heat pump or insufficient borehole length, an exergy analysis is performed. This analysis help to understand where the inefficiencies occur within the heat pumps. The results indicate that, among the studied heat pumps, they have relatively similar exergy efficiency and thus similar performance. A further examination of the collected data reveals the reason behind the different observed COP values, which is indeed associated with the interplay between borehole lengths and ground thermal properties, highlighting the importance of the latter in designing GSHP systems. The most common practice to design the borehole length of GSHP systems is through specialist design software. However, there is still a lack of understanding regarding the methodologies of the software employed and their validity due to the lack of relevant (open) literature. This study addresses this gap by conducting an in-depth examination of the methodologies utilised by various design software programs, performing several side by side analyses of different methodologies, and validating the design software generated outputs against experimentally measured data from four operational GSHP systems of the SEPD program. The side by side study reveals that there is a large variation in the software computed borehole design length (±20% from the average value). Such variation is contributed partially by the significant difference in the computed borehole thermal resistance, and partially by different software’s sizing algorithms. By studying the sizing algorithms being used in the design software (based on either Finite Line Source model with Eskilson’s G-function, or Cylindrical Source model with temperature penalty), it is observed that the Finite Line Source model with Eskilson’s G-function shows a better agreement with the measured experimental data. As the GSHP system is still considered as a relatively new technology in Australia, there has been a lack of information about its financial and environmental feasibility. To address this issue, two separate analyses were conducted utilising the collected data. The first one assesses the life cycle cost of GSHP systems using the actual documented costs from the SEPD program. By comparing to conventional heating and cooling systems, it is found that GSHP system would provide considerably more savings than other alternatives, including Air Source Heat Pump (ASHP) systems, for a design life of 40 years. Climate change was also factored into the economic analysis, with only minor effects observed. This analysis also highlights the importance of Government's role in promoting residential GSHP systems, which can significantly assist making GSHP technology an attractive option with tax credits as low as 8% on capital costs. The second analysis examines the lifecycle environmental impact of a GSHP system using a comprehensive ReCiPe method. The results suggest that by utilising a GSHP system, there would be an average reduction of 27-31% in environmental impact (measured by both GHG emission and ReCiPe score) when compared to other efficient conventional systems, such as an ASHP. This amount of reduction in environmental impact, for a single household, is equivalent to planting about a thousand tree seedlings for 10 years. When GSHP systems are widely adopted throughout the state/country (similar to US or western Europe), this amount of reduction in environmental impact would easily be multiplied by hundreds of thousands if not millions, making a huge contribution towards the long-term target of net zero greenhouse gas emission. The result of this research highlights the importance of the design process for any GSHP systems. With a properly designed GSHP system, it would save households on the long-term operational cost and help the society by reducing the environmental impacts. However, in order to achieve a proper design, a good understanding of the ground thermal properties together with the expected thermal loading is essential. In addition, for GSHP designers, an in-depth understanding of the analytical software used during the design process is also critical as these analytical software (if used properly) would help designers to optimise design and to minimize the capital cost of a GSHP system. This thesis provides such valuable information based on rare field measured data from a range of installed GSHP systems, and reveals the full potential (both financially and environmentally) of the wide utilisation of GSHP systems throughout Australia