Energy plays a fundamental role in societies impacting everything from basic human needs like lighting, cooling and heating to complex industrial processes that significantly influence social development, economic growth, and national security particularly in terms of access and affordability for all citizens. The world’s electricity demand grew by 2.2% in 2023 and is expected to rise at a faster rate by an average of 3.4% in 2026 (IEA, 2024). With the global climate warming, it is important to reduce our societal impact on Earth by using clean energy sources. One of the clean sources is geothermal energy which has a great potential to reduce dependency on the fossil fuels. Geothermal energy sources can deliver both, heat and electricity. Tanzania has a huge geothermal potential that has not yet been used and has only been explored to a limited extent. This thesis attempts to identify the geothermal potential of southwest Tanzania with appropriate drilling zones by assessing the main components of a geothermal system reservoir which are temperature, permeability and fluid flow at Songwe Tanzania. It is one of the identified potential development areas with very little information available.
The work flow begins with chapter 2 where a geological and thermal numerical model is set up to simulate the temperature at depth. The thermal model considers pure conductive heat flow to achieve a first idea of the temperature distribution in the different geological layers.
Chapter 3 describes the conducted field study at the thermal spring areas as well as the laboratory analysis of the samples in order to understand the geothermal fluid source by using geochemical modelling, obtain the reservoir temperature through geothermometer calculation and locate the up-flow and the outflow zones of the geothermal field.
In chapter 4 numerical simulations of fluid and convective heat flow are performed to allow understanding the heat transport by fluids and the hydrogeological behaviour of the geothermal reservoir by varying thermal and physical parameters.
Chapter 5 shows possible drilling locations for a first geothermal well in the study area, whereby areas of different productivity are distinguished.

@en

The main objective of this thesis is to understand the environmental risks related to geothermal operations. The aim is to provide an integrated approach from both natural and social sciences and to perform this in three different country settings. These countries are Indonesia, Turkey and the Netherlands and they were selected because of their different geothermal system types. The integrated approach in this study results in a research process that requires a broad range of measurement and analysis techniques and a clear understanding of the natural and social disciplines. From the natural sciences approach this report studies the environmental risks through a geochemical characterization of the geothermal fluids, whereas the social sciences approach studies the risk perception on geothermal operations. The outline of the process followed is visualized in Figure 0.1. From the natural sciences approach, an extensive geochemical data set is used to characterize the geothermal fluids and their environmental risk. This contains approximately 750 sample measurements from three countries that were collected through partners and third parties. The samples are characterized with help of two ternary diagrams (Na-K-Mg and Cl-SO4-HCO3), their salinity (in TDS), pH and correlation coefficients between commonly occurring elements in fluids, such as Ca, Si and F. These fluid properties help to define the maturity, origin and characteristics of geothermal fluids at a broad range of locations in each of the three countries. Through this analysis we found a strong relation between the fluid classification and the sample type (well or spring) in Indonesia. Besides, a relative high influence of volcanic activity on the geochemistry was observed. The Turkish and Dutch samples are dominated by their geothermal system types that are respectively carbonatic and clastic sedimentary systems. The concentrations of nine toxic gases and elements dissolved in the geothermal fluids are analysed and compared to guideline values to determine the relative environmental risks in the three countries. The toxic gases are H2S, CO2 and CH4 and the toxic elements Al, As, Cd, F, Hg and Pb. Their effects on the environment differ but all of them affect the health of humans, flora and fauna when they contaminate groundwater or the atmosphere. From the risk analysis in the three countries it was found that the risk of excessive H2S pollution is highest in Indonesia, for CO2 in Turkey and for CH4 in the Netherlands. The contamination risk with toxic elements differs largely per element but often occurs in specific locations, for example with high volcanic impact. For the social sciences approach, a survey was distributed to measure how the public perceives the risks of geothermal energy. The population for this survey is people affiliated with the geothermal industry because of their prior knowledge on the subject. They are asked to indicate their (risk) perception through several statements and factors to which they indicate their level of agreement. The result indicates that the perceived risks were generally higher in countries with a higher risk, like Indonesia. However, there were several interesting exceptions to this rule in which the perceived risks were low whereas a relatively high environmental risk was identified, like for potential CH4 pollution in the Netherlands.

High temperature aquifer thermal energy storage (HT-ATES) can potentially solve the mismatch between heat supply and demand. It can provide a large scale seasonal heat storage solution. Thereby it enables an increase in full load hours of the base heat source, which can benefit project performance on both costs and emissions. However, the limited number of successful pilot projects indicates the technology has not escaped its state of infancy. There is a gap from concept to implementation, which is signified by the disagreement of experts on performance drivers and barriers of HT-ATES. This research aims to narrow the described knowledge gap, by improving identification of HT-ATES performance drivers and barriers. Thereby it strives to improve decision making of HT-ATES implementation, and further enhance future HT-ATES application in heating projects. The broad scope of research demands both a diagnostic and design-orientated approach, and fits seamlessly with a multi-criteria decision analysis. The analysis entails the stages of creating, evaluating, comparing and ranking of case-specific scenarios. Parametric variation changes the conditions for HT-ATES implementation across the scenarios. A simulation model is developed and connected to a groundwater model to apply the parametric variation, to create the different scenarios, and consequently to produce the quantitative information for further evaluation. During the stages of creating, evaluating, comparing and ranking, the methodology systematically produces new results on the opportunities and risks introduced by HT-ATES, and additionally on the HT-ATES performance drivers and barriers. The results show that HT-ATES enables the opportunity of improving project performance with respect to the internal rate of return and emissions. Groundwater impact remains the greatest risk, but it can be minimised with smart decision making. To support the decision maker and to overcome the risk of groundwater impact, the research proposes several performance-enhancing, non-explicit guidelines. The guidelines focus on realising an HT-ATES implementation, where project performance with respect to internal rate of return, emissions and groundwater impact are balanced. Thereby they explain the major HT-ATES performance drivers and barriers. The guidelines are summarised below. The decision maker is recommended to .. 1. .. minimise the uncertainty, through thorough subsurface characterization before implementation. Secondly, to focus on aquifers with a minimum depth of 200 [m] and a minimum hydraulic conductivity of 5 [m/d] 2. .. assure network return temperatures during peak demand are below expected storage temperatures 3. .. not consider project life-times exceeding 20 years 4. .. assure yearly maximum base source heat production is always lower than yearly consumer heat demand 5. .. to strive for a flat demand curve and apply peak-shaving, by means of, for example, variable heat prices Currently, the guidelines have the purpose of giving direction to the decision maker, but they will become more explicit once the methodology is improved, and the uncertainty and number of assumptions in the model is decreased.