The advancement of renewable energy technologies is critical due to the unsustainable nature of currently used energy sources and the need to meet increasing energy demands. A broad and diverse energy plan is important for long term energy independence and stability. Geothermal energy sources including hydrothermal systems and enhanced geothermal systems (EGS) should play a key role in this energy plan. Geothermal energy is the most reliable, currently developed, schedulable alternative energy source. It is estimated that improvements in these systems have the potential to economically access at least 200,000 exajoules of energy to the U.S. within 50 years [1]. The progress in developing this field is challenging in part due to the complex geochemical and geophysical properties of the subsurface environment. This dissertation reviews geothermal energy system limitations resulting from current well monitoring technologies and introduces microelectromechanical systems (MEMS) sensors as a means of optimizing well performance. Harsh environment MEMS offer an ideal alternative to macro-scale sensors due to their harsh environment material compatibility, the ability to couple MEMS with wireless RF transmission systems, and their small footprint.
In this work, harsh environment MEMS encapsulation materials are tested in simulated geothermal environments to determine their survivability. The sensors must be encapsulated to provide protection from oxidation, erosion, surface roughening, and other chemical attacks. Encapsulation materials such as silicon carbide, sapphire, vitreous carbon, and poly-crystalline diamond were tested in multiple simulated geothermal environments to determine a suitable protective layer for these MEMS devices. Once appropriate materials are determined, two temperature sensors are designed, optimized, fabricated, and tested. The first sensor improves upon existing out-of-plane MEMS capacitive temperature sensor devices by utilizing harsh environment materials. The second sensor is a novel in-plane design optimized to linearize capacitive output. These sensors are tested up to over 650 ◦C.