Energy demand is constantly increasing and the use of fossil fuels causes an accumulation of carbon dioxide (CO2) which is an important environmental problem that needs to be solved. A promising solution to this problem would be the electrochemical reduction of CO2 to useful products, using the surplus of electricity from renewable sources. This would be a way of storing this excess of electricity in chemical bonds. However, this has not reached high efficiency and selectivity needed for establishing its use. For this process, the CO2 is usually dissolved in an aqueous electrolyte. This thesis proposes the new approach, using Deep Eutectic Solvents (DES) which would capture a higher concentration of CO2 helping to make the whole process more efficient. However, not all salt mixtures reach the eutectic point and therefore, the solvents formed are a low-transition temperature mixtures (LTTMs) of the selected salts.

Experimental work was performed to find out if these solvents can be used for electrochemical carbon dioxide reduction. Non-reported LTTMs were synthetized; they were formed with mixtures of the hydrogen bond donor citric acid (CA), fructose (F) and diethanolamine (DEA) with the hydrogen bond acceptor tetrabutylammonium chloride (TBA-Cl) and different quantities of water, which was found necessary to carry out electrochemistry since the solvents formed were too viscous, and this water has an important effect in the electrocatalytic reduction of CO2 as proton source and charge carrier.

These solvents were characterized electrochemically, performing studies to find out on which metallic surface they behave better (wider working potential window, low degree of decomposition and adsorption) and how different water quantities affect this process. These studies were performed using cyclic voltammetry. From this, it was seen that the solvents are more stable during electrochemical process in presence of copper. Moreover, the solvent formed by DEA:TBA-Cl:H2O in the molar proportions 1:1:1,375 was found to have interesting features that resemble CO2 reduction. As result, this solvent was further analysed using electrochemical in-situ Fourier-Transformed Infrared (FTIR) spectroscopy in surface-enhanced attenuated total reflectance configuration. With this, it was seen that the carbon dioxide was captured by the solvent and that there are visible changes when applying potential. Remarkably, the data shows the formation of a dimer OCCO on the Cu electrode surface, stabilized by the solvent.

This project shows for the first time the electrochemical reduction of CO2 in LTTMs, evaluating different solvents, metals and water quantities. Determining that the carbon dioxide reduction is possible with the solvent DEA:TBA-Cl:H2O in the molar proportions 1:1:1,375 in a copper polycrystalline electrode.
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At the moment, the worldwide demand for air conditioning is rapidly growing, and it is expected to exceed the demand for space heating by the 2060s. However, traditional refrigerants such as CFCs and HCFCs are regulated or phased out by the Montreal and Kyoto protocol. Secondary loop refrigerant systems use less of these harmful refrigerants since they make use of a distribution fluid (for example water) as transport medium between the chiller and the coolers. The efficiency of these systems can be improved by using a phase change material as a secondary refrigerant.

Tetra-n-butylammonium bromide (TBAB) is a promising phase change material for air conditioning applications. The phase change can take places at a temperature up to 12.5 ºC, which allows for an increase in the evaporation temperature. Furthermore, due to its phase change, the TBAB slurry has an up to 4 times larger cooling capacity than chilled water. This can be utilized in order to reduce the flow rates in the system.

At his moment, the TU Delft has in collaboration with Hollander Techniek installed a small pilot air conditioning system in the sports hall 'De Jachtlust' located in Twello, the Netherlands. The system has a capacity of approximately 3.5 kW. It has a single 300 L storage tank of and it is equipped with sensors to monitor the performance of the system. In this study, an existing model of a secondary air conditioning system is improved and extended taking into account the design parameters of the installation in Twello. This model is validated using the experimental data from the actual system.

The simulations predict that using a 36.5 wt% TBAB solution increases the COP of the system from 2.96 to 4.00, while the energy consumption reduces by 24.8 %. This reduction is mainly due to a 30.5 % decrease in the power consumption of the compressor. At the same time, the generation side pump consumes 204 % more electricity due to adhesion of the produced crystals to the heat transfer surface. The performance of the TBAB can still be improved by lowering the initial TBAB fraction to 35.0 wt% or by further optimizing the control strategy for the crystal production.
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