Heatpipes are promising devices for geothermal energy extraction owing to their effectiveness to transport heat. The goal of this research is to validate an analytical heatpipe model with experiments and to investigate the difficulties in designing and constructing geothermal heatpipes. There is a lack of literature and research concerning the operation, performance limits and construction of heatpipes suitable for geothermal heat extraction. A prototype heatpipe is designed based on specifications for geothermal energy extraction and constructed in a laboratory set-up with sensors and data acquisition. The prototype setup collects experimental data and is used to evaluate important parameters, requirements and practical design difficulties. This research shows the difficulties in designing a geothermal heatpipe taking into account fluid choice and physical limitations as well as complications in constructing a properly sealed heatpipe under the influence of repeated heating and cooling. Furthermore it shows the limitations of the analytical model by comparing the model predictions with experimental data.

Heat pipes are typically used in the semiconductor industry. This means that the scale of these heat pipes is typically in the order of centimeters. Zijm suggests that heat pipes could be used for geothermal applications, but literature is lacking. To further investigate the geothermal application of heat pipes, a large scale heat pipe is built. This thesis gives an insight in the typical design challenges that one faces when constructing a heat pipe of this scale. The heat pipe that is constructed can support a heat flow of 10 kW. The heat pipe is constructed from mainly 54 millimeter copper and glass pipes. The evaporator section is 1.5 meters long and facilitates controlled electric heating. Then follows a 4 meter long adiabatic section. The condenser section is 2.5 meters long. The total length of the heat pipe is 9 meters. The individual sections are held together by EPDM connectors. The dimensions of the heat pipe are compared to the operational limits posed by the Engineering Sciences Data Unit. Then the thermal resistances of the heat pipe sections are calculated and afterwards validated. The interfacial thermal resistance between the electric heaters and evaporator wall was reduced by applying thermal conduction paste to the band heaters. It was found that at coolant flows of 1000 l/h and higher, the vapour temperature in the heat pipe drops significantly. The drop in temperature facilitates a higher heat flow through the heat pipe. Also, the resistance across the evaporator and the condenser section gets smaller for higher coolant flows. The results found are supported by theory and formulae from the Engineering Sciences Data Unit.

The Intergovernmental Panel on Climate Change (IPCC) believes that multiple solutions must be deployed simultaneously to reduce the emission of greenhouse gases into the atmosphere. Carbon, Capture & Storage (CCS) is an unavoidable technique within this portfolio as an intermediate solution. CCS requires transport of CO2 through pipeline systems and into wells. There are still large uncertainties on the thermodynamics of the CO2 in flow through valves. The CO2 flow undergoes significant changes in pressure, temperature and phase distribution when it passes this control valve. Therefore, the behaviour of the CO2 flow flowing through a control valve is examined in this study. The simulation of CO2 in a 3D valve including phase transitions is complex. Furthermore, few validation experiments are available. As a first step, more simple nozzles are simulated. In these devices, the same processes occur and validation data is available. These simulations are validated with experimental data by Nakagawa et al. to examine the accuracy. Three types of models (isenthalpic, Euler and Enhanced Mass Transfer (EMT)) are implemented in increasing levels of complexity to investigate the differences between these models and to consider when complexity is needed or simplifications are valid. The validation cases show experimental pressure data of high-pressure CO2 flow through converging-diverging nozzles with phase transitions. The results showed that the EMT model matched the experimental data best. A substantial similarity was achieved regarding the pressure data. The mass transfer mechanism, however, needed adjustments in its coefficients to match the experimental data. Finally, after finding the right values, the EMT shows the best technique for modelling flashing or cavitation. In short, the overall consequences of the transition in a valve are substantial and must be considered. The behaviour of the high-pressure CO2 flow is heavily influenced by flowing through a valve. Substantial amounts of vapour are formed, but only after the throat. This is the same for choking condition, which is achieved in the diverging section of the nozzle. The large expected drop in temperature due to pressure reduction has also been noted. Although a high degree of similarity between the results of the model with experimental data is obtained, there is room for improvement regarding the model. A flaw was discovered in the handling of the thermodynamic properties of the fluid near critical points. Also, the surface tension has not been considered, but might have a substantial influence. Next steps in the research are 2D and 3D simulations of actual valves, but require experimental validation data.