Patricia Alegría

In the current climate and energy context, it is important to develop technologies that permit the use of renewable energy sources that cannot currently be harnessed. The contribution of renewables to global electricity production has increased by 8% in the last decade, but geothermal energy, the only renewable source that does not depend on the weather and which is totally stable and continuous, contributed with less than 3% in 2021. There are areas where boosting the use of this renewable source is particularly important, due to its availability and the enormous dependence on fossil fuels, as is the case of the Canary Islands, especially in Lanzarote, where there is a large Hot Dry Rock Field. This Ph.D. Thesis proposes the use of thermoelectric generators to transform the heat from the shallow geothermal anomalies on the island of Lanzarote directly into electricity, since the use of conventional geothermal power plants would not be possible because they would require drilling into the rock and huge installations that would damage the environment. But all thermoelectric generators existing in the literature use moving parts to circulate heatcarrying fluids, which entails auxiliary consumption and the need for maintenance, losing the intrinsic advantage of thermoelectricity. Moreover, there are hardly any experimental studies and even fewer installed in field. In order to bring this proposal to reality, this thesis has succeeded in developing and installing in field two thermoelectric generators for geothermal energy that operate without moving parts thanks to phase change in their heat exchangers. They are robust generators that do not require maintenance nor auxiliary consumption, as well as producing a minimal environmental impact, as they have a reduced visual impact, do not produce noise, and the use of water as working fluid makes them completely harmless. The first of these, for areas with medium temperature geothermal anomalies, has two prototypes, each consisting of a copper thermosyphon on the hot side, and 10 and 6 thermoelectric modules respectively, with their corresponding exchangers on the cold side formed by a finned dissipator with four heat pipes with fins in their condensation zone. It was first designed and characterised in the laboratory and then installed in a borehole at 170 °C in the Timanfaya National Park, constituting the first thermoelectric generator for geothermal energy without moving parts installed and tested in field, where it has been in uninterrupted operation since August 2020, and has achieved a peak output power of 39.99 W (2.49 W per module), and an annual energy generated per unit of occupied surface area of 180.16 kWh/m2. After demonstrating the feasibility of this technology for transforming surface geothermal anomalies in Lanzarote into electricity, the next challenge was to develop a generator adapted for high temperature areas. This thermoelectric device has a single stainless steel thermosyphon and 8 thermoelectric modules with heat exchangers, similarly to the medium temperature case. Tests were carried out in the laboratory at various heat source temperatures varying between 4 and 8 thermoelectric modules. It was experimentally determined that installing more modules decreases the efficiency per module, but for the number of modules tested the total power increases, so the field installation was carried out with 8 modules. After the good results in the laboratory, it was possible to transport it to the Timanfaya National Park and it was satisfactorily installed in a borehole with gases coming out at 465 °C. This generator has an output peak power per module of 4.5 W, and an average power per module of 4.12 W, 80% higher than in the generator for medium temperature, which means an energy generated per year of 382.59 kWh/m2, demonstrating the great potential of the developed thermoelectric generators. With the results of the two generators installed in field, a computational model was developed based on the finite difference method and the thermalelectrical analogy, which was adjusted by means of the experimental data and which is highly reliable. This model is capable of simulating the output power of the thermoelectric generators with relative errors of less than ±10%, and of reproducing the temperatures on the faces of the thermoelectric modules, as well as the energy generated throughout the year with an error of less than 1.6% in the medium temperature prototype and 0.5% in the high temperature one. Finally, using this computational model, an estimated calculation of the electricity generation potential was made with the available surface data of geothermal anomalies in the Timanfaya National Park, where it could generate between 3.61 GWh and 11.35 GWh per year, with an average LCOE of 4.8 cente/kWh. Furthermore, this can be extrapolated to the rest of the island, since more anomalies have been detected outside this area and which, if calculated accurately, could be exploited by a larger scale installation of thermoelectric generators such as those developed in this doctoral thesis.