Direct methanol fuel cell-based power plant for autonomous underwater vehicles. Endurance increase and carbon dioxide management in a confined environment
- Autores: Antonio Villalba Herreros
- Directores de la Tesis: Ricardo Abad Arroyo (dir. tes.), Teresa Leo Mena (dir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2021
- Idioma: español
- Materias:
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- Resumen
Oceans and seas are and have always been vital for human beings. With approximately 71% of the Earth’s surface covered by oceans, their exploration is key to increasing our understanding, so we can more effectively manage, conserve, regulate and use ocean resources. However, by 2020 less than 20% of the ocean seafloor has been mapped using high-resolution instruments, a smaller surface than the already mapped and studied surfaces of the Moon or Mars.
Since the Challenger Expedition laid the foundations of oceanography in the period 1872-1876, the natural desire for knowledge of humans has motivated them to devise and build ingenious and sophisticated tools for the exploration of oceans. In this sense, Autonomous Underwater Vehicles (AUVs) stand out.
AUVs are robots that travel underwater without the need for operator intervention and are able to carry out missions autonomously. As instrument platforms, AUVs can include multiple sensors to collect the desired data ranging from temperature recorders to sophisticated sensorsas side scan sonars. Besides, their increasing capacity to take decisions autonomously makes them very versatile tools in many fields in both the civil and defence sectors. However, the development of AUVs is hindered by the limited endurance provided by the current power plants based on batteries or semi fuel cells. This concern is especially important in large AUVs.
One promising solution to extend AUVs navigation time is fuel cell technology. Despite their greater complexity, fuel cell-based power plants can store a greater amount of energy compared to lithium-ion batteries that are the standard power solution for AUVs today.
This Doctoral Thesis focuses in the study of Direct Methanol Fuel Cells (DMFC) to power AUVs. This kind of fuel cell has adequate characteristics that make them suitable for this use.
Methanol is liquid under ambient conditions of pressure and temperature, which facilitates its handling and storage. However, DMFCs produce CO2 as product of the reactions that occur in the fuel cell that must be treated. Depending on the treatment method the global energy density and specific energy of a DMFC-based power plant will vary.
This work studies three CO2 Treatment Methods (CO2TMs) to determine the most suitable depending on the working conditions: direct disposal, storage on-board as a pressurized gas and storage on-board embedded in an adsorbent material. Through a Multi-Criteria Decision Making (MCDM) analysis it was determined that storage on-board embedded in an adsorbent material is the most suitable CO2TM for large AUVs capable of reaching navigation depths up to 6000 m.
To study the benefits of powering an AUV with a DMFC-based power plant, a software tool was developed that allows to generate quick pre-designs of AUVs powered by such a power plant. The CO2TM considered consist in the storage of CO2 embedded in an adsorbent material.
A comparative study using real large AUVs that offer the longest endurance values commercially available as models showed that a DMFC powered AUV has the potential of reaching endurance values up to 90% longer than those of the model AUVs under the same operational conditions.
However, this result is highly influenced by the adsorbent material used, being Mg-MOF-74 the best adsorbent material found in the literature.
To advance in the knowledge of adsorption mechanisms, a CO2 capture setup was built. The experiments carried out allowed to study the impact on rate of accumulation of CO2 in the circuit of the adsorbent material under different ambient conditions. The results indicated a notable reduction of the rate of accumulation of CO2. These results are a first step of a future work to build an effective CO2 Capture System suitable for its use on board an AUV powered by a DMFC-based power plant.
As a conclusion this Doctoral Thesis reveals that the use of DMFCs to power AUVs has benefits in terms of longer endurance values than that currently available. Further investigations will allow the CO2TMs proposed to be studied in more detail to get even longer endurance values.
