Hydrogen (H2) is an important substance for clean energy storage, but the financial feasibility of large-scale water electrolysis remains a challenge. A promising approach to improve the economics of industrial electrolyzers is the anodic production of hydrogen peroxide (H2O2) during alkaline water electrolysis. This dual production could attract H2O2 producers and increase the value of electrolyzer output. However, achieving high H2O2 selectivity is difficult due to competition with other oxidation products. Recent studies have shown that incorporating polytetrafluoroethylene (PTFE) on carbon fiber paper (CFP) electrodes can enhance H2O2 production. Xia et al. suggest that PTFE's hydrophobicity confines O2, driving the reaction toward H2O2. However, PTFE’s inert characteristic makes complete coverage of the electrode impractical. Vogel et al. further noted that the O2 bubble perimeter, which attracts more OH-, a key reactant for H2O2, could also increase H2O2 yield.

This project explores the effects of varying PTFE applications on carbon electrodes, focusing on three approaches: increasing the PTFE perimeter patterns (P1<P2<P3<P4), increasing the PTFE area patterns (A1>A2>A3>A4), and dip-coating the electrode in PTFE emulsion. The study uses a two-electrode system in a flow cell with a K2CO3 electrolyte, observing performance lifetime via chronopotentiometry and measuring H2O2 yield through permanganate titration. SEM and EDX are also used for electrode observation.

Results show that increasing the PTFE perimeter (P1 to P2) enhances H2O2 yield due to better O2 bubble formation, but further increases (P2 to P4) have little effect. Increasing the PTFE area patterns generally shortens operational lifetime and reduces H2O2 yield, with A2 and A3 showing similar results due to potentially non-optimal spacing. PTFE dip-coating leads to rapid performance degradation, confirming that PTFE’s lack of active sites makes it unsuitable for initiating reactions. Overall, optimizing PTFE surface area is improving H2O2 production in alkaline water electrolysis over than perimeter or dip-coating.

The unprecedented increase in mean ambient temperature due to immoderate CO2 emissions has shifted the energy policy towards the replacement of fossil fuels with renewable energy sources. Nevertheless, the intermittency of these technologies renders them unsuitable for reliable energy supply. A prominent solution is the utilization of renewable energy to produce green hydrogen that can be stored for later use.

The most environmental-friendly viable method for green hydrogen production is the electrolysis of water. Between the already existing types, alkaline electrolyser is the one with the highest technological maturity and lowest cost of hydrogen production. Despite that, the low energy efficiency, and the energy losses associated with the materials and the geometrical configuration make it difficult to produce hydrogen at competitive prices compared to fossil fuels. The connection with renewables exposes the electrolyser to variable loads that negatively affect the life of materials and the purity of hydrogen since the operating conditions like temperature and electrical current change constantly. Previous research work has shown that the energy losses owing to the electrode kinetics, diaphragm, and electrolyte resistivity are responsible for higher energy consumption than the thermodynamic minimum. These losses are a strong function of temperature and current density. This research project aims to investigate the performance of the electrolyser under fluctuating working conditions to get an insight into how energy consumption and efficiency are affected.

The research objectives are encountered by building a physical model that accurately predicts the electrochemical behavior of the cell under various circumstances and simulates the thermal response of the electrolyser. The numerous experiments performed at XINTC’s laboratory enabled the improvement of the model’s accuracy and the consideration of effects that are not easily confronted in a purely theoretical investigation. The results showed that the model accurately describes the experimental data of the cell with less than a 2% discrepancy. The model was also used for simulating the cell behavior under different pressures, electrodes, and diaphragm materials. The elevated temperatures and highly electroactive materials such as Nickel significantly reduce the voltage of the cell. Regarding the thermal model of the theoretical electrolysis system, sensitivity analysis of the current density, ambient conditions, and electrolyte flow was performed. The temperature across the system was uniform at an electrolyte volume flow of 5 Lt/min and the cooling load accounted for 20% of the electrolyser power. The thermal modeling of the real experimental setup was successful (<1.5°C deviation) at high current densities indicating the sound assumptions of the model. On the contrary, at low current densities, the discrepancy between the model and the experiments reached 10% due to the additional heat generation induced by the shunt currents. For that reason, a simplified electrical circuit analysis was formulated and included in the model. This led to reducing the deviation to less than 5% and at the same time calculation of the current efficiency in the absence of measuring devices was achieved.

This research aims to analyse the non-ohmic resistance of multiple ion-exchange membranes (IEM) in series, for a better understanding of membrane resistances. These membranes can be found in series in energy storage systems such as acid-base flow batteries (ABFB).
The effect of different current densities on the ohmic and non-ohmic resistance of bipolar membranes (BPM), under forward and reverse bias were studied using electrochemical impedance spectroscopy (EIS). Hereafter, an anion exchange membrane (AEM), cation exchange membrane (CEM) and BPM were examined individually and in series using EIS, to compose a simplified equivalent circuit for a whole ABFB triplet.
For the effect of different current densities on the BPM, an exponential decline of the non-ohmic resistance of the diffusion boundary layer (DBL) was found as a function of the current density. Under forward bias, the DBL resistance becomes significant. The resistance of the water dissociation reaction (WDR) was minimal under all current densities. Flow experiments validate a depletion and enrichment of the DBL during water association and dissociation, respectively.
To compose a simplified equivalent circuit for an AEM, BPM and CEM in series, it was found that the specific resistances of all membranes can be summed at their respective frequencies. This made it possible to construct a simplified equivalent circuit for a whole triplet containing an AEM, BPM and CEM.

To anticipate for future demand of sustainable liquid fuels, Zero Emission Fuels B.V. develops a solar powered micro plant which produces methanol from water and carbon dioxide captured from outside air. An intermediate step in this process is the electrolysis of water to create hydrogen which reacts with carbon dioxide to methanol. This study encompasses an integration of the pressurised alkaline electrolysis system, in which problems observed in previous generations of the electrolyser have been solved and functionalities have been added. In addition to that, electrolyte samples are taken after different periods
of operation to investigate electrolyte deterioration over time.

A new Balance of Plant (BoP) has been designed, realised and tested. Eight new features to the system can be distinguished, two of which are included to add functionality: the degasser to decontaminate the carbon dioxide-rich
feed water and the pressure booster to replenish the consumed water at system
pressure. The other features are integrated to enlarge the operational envelope, the determination of which was central to the experiments conducted in this research.
Experiments are conducted to find and characterise the four limitations to the operational envelope: the relative valve opening duty (RVOD), flow stagnation, temperature control and crossover.

The RVOD experiments showed deviations from the modelled valve opening cycles, attributed to additional pressure drops, valve opening interference and smaller discharge volumes. Subsequently a corrected model is presented to improve valve cycle time predictions.
Flow stagnation was investigated at various current densities, pressures and temperatures using camera images and a characteristic temperature response to establish flow stagnation. The results showed a minimum volume flow rate decrease in comparison to the previous system, expressed in parameter 𝑋 which decreased from 3.4 to 0.74 A K bar^-1 cm^-2. The minimum volume flow rate decrease is attributed to the larger number of cells and the increase of the height difference between the stack and buffer tanks.
In the temperature control experiments, steady state temperatures were monitored at different current densities, with and without crossflow fan operation. This resulted in a temperature control map, in which the reachable temperatures for different current density are depicted. Furthermore, electrolyte mass flows are determined and pressure dependency of temperature control was investigated at low current densities, concluding that steady state temperatures are independent from pressures in the 10 bar to 50 bar regime. 
Hydrogen crossover was tested by taking gas samples at different operating conditions and subsequent gas composition analysis in a gas chromatograph; oxygen crossover was determined by an oxygen sensor implemented downstream the hydrogen exhaust. All steady state crossovers values were found to be below the safety limits, concluding that crossover is not limiting in the acquired system on all possible operating points. Overnight diffusion crossover experiments showed that maintaining the system under pressure overnight, keeps crossover values below the safety limit.
In addition to the operational envelope, complementary general characteristics such as power consumption and efficiency are presented and compared to industry and literature. On top of that, unexpected findings, design deficits and other relevant phenomena are described to complete the perspective on system performance and behaviour of the electrolysis system. Due to the electrolyte mist purged into the degasser and pressure booster, the electrolyte deterioration experiments are deemed inconclusive. Moreover, a demister is found to be an essential system feature to include in the next generation electrolyser, because both degasser and pressure booster were damaged by the electrolyte spill.

Electrochemical CO₂ reduction may present a solution to close the carbon cycle and to utilise CO₂ emissions. However, for this technology to have a significant impact, it has to be successfully implemented on an industrial scale. Numerical simulations can aid with the study of process parameters and reactor design.

The overall aim of this project is to develop and utilise a numerical model that can describe phenomena arising in the CO₂ electrolyser inside the flooded catalyst layer (CL). First, the general operation of the electrolyser is addressed, and this is extended for the effect of liquid flow rate, electrolyser length, and operating pressure. To investigate the performance and limitations arising at the large-scale, the model is scaled-up to describe a one meter long electrolyser. The study is concluded with two considerations that could improve the electrolyser performance. These points were addressed by developing a 2D numerical model of a gas diffusion-based CO₂ electrolyser in COMSOL Multiphysics. We assessed the performance of the electrolyser in terms of current density, reflecting rate of species formation, and of faradaic efficiency for CO (FE), reflecting selectivity towards the desired product.

Investigating the small-scale electrolyser we find that at high current density (200 mA cm^-2), the pH in the CL immediately increases by 3 units and further diagonally increases from pH 10.1the inlet to 12.2 around the outlet. When operating the electrolyser with excess of CO₂ supply, we find the CL to perform the best near the gas phase boundary (311 mA cm^-2, 95% FE), while the regions close to the electrolyte are underperforming (250 mA cm^-2, 89% FE). This shows that the performance in certain regions of the CL needs to be improved.

When scaling-up the electrolyser to a length of one meter we find that the performance does not change dramatically when operating at excess of gas supply. However, if a high CO₂ conversion should be achieved, the long electrolyser shows a 10% decrease in FE, and CO₂ conversion compared to a small-scale electrolyser, at the same level of current density (115 mA cm^-2).  Analysing the current density locally, we find that difference between the inlet and outlet can be as large as 100 mA cm^-2. Next, we find that FE can fall to almost 50% around the outlet. This shows that when adding extra length to the long electrolyser, this extra length only adds a fraction of its potential performance.

The uneven utilization of the catalyst can be improved by varying the catalyst loading along the electrolyser length. This improves the FE by around 5% while using 40% less catalyst. We also find that while the current density is slightly lower, the amount of product generated per mass of catalyst has significantly increased. This shows that carefully engineering the catalyst loading can save the amount of catalyst needed and could potentially improve the cost-effectiveness of the CO₂ electrolyser.

In all cases the performance over the CL is unevenly distributed. To achieve a higher performance, research needs to find ways how to enhance the performance also in the poorly utilised regions of the CL. Scaling-up the electrolyser just by extending its length proves inefficient and inevitably leads to a lower performance. The beneficial buffering effect provided by the electrolyte at a small-scale does not translate to a large-scale. At this moment, performance of large-scale CO₂ electrolysers seems satisfactory only when operating at very low CO₂ conversion. From the investigated parameters that address the performance issues, higher operating pressure and smart catalyst loading seem only promising options, however, other options should be found to speed up the development.

Zero Emission Fuels B.V. (ZEF) is developing a microplant, which ultimately provides a pathway to produce a hydrocarbon in a sustainable manner. This microplant consists of several subsystems. One of which is an alkaline water electrolyser, in which water is split into oxygen and hydrogen under specific operating conditions.
As a mixture of the product gasses can be highly explosive, gas crossover in alkaline water electrolysis (AEC) systems is a point of concern in terms of safety. This explorative study aims to identify the main routes for crossover in AEC systems, as well as characterizing the ZEF AEC system. This characterization has been accomplished by studying the effect of process conditions and system geometry on gas crossover.

Based on a literature study, gas crossover is found to be the result of diffusion as well as convection. Both of these phenomena occur at the interconnects between the sides of the system, which is over the diaphragms and over the pressure equalization tube (PET) in case of the ZEF AEC system.

Gas measurements where conducted during over 194 hours of experiments. From this it was concluded that hydrogen crossover is the main point of concern in terms of safety. Operating pressure and temperature both very clearly affect hydrogen crossover. The effect of current density is also significant but obscured by the fact that temperature is an uncontrollable variable in the ZEF AEC system. From the steady-state data a window for safe operations has been framed.
Upon start-up of the system a spike in oxygen-side hydrogen concentration is consistently measured, which is a result of crossover by diffusion during system off-time. This spike in concentration also poses a threat in terms of safety.
Three different pressure equalization tubes have been tested to investigate the influence of interconnect geometry on gas crossover.

In order to identify the main route for hydrogen crossover a CSTR model of the oxygen side gas phase was developed, in which temperature and pressure effects were taken into account. The model showed that hydrogen crossover by diffusion could be held accountable for ~8% of the total amount of crossed-over hydrogen, which is in line with reports in literature for a mixed electrolyte cycle AEC system. The model also showed that crossover by solely diffusion through the PET is negligible compared to through the diaphragm.

From this, it was concluded that the ZEF AEC system behaves more like a system with mixed electrolyte cycles with respect to gas crossover levels. Recommendations are made in order to reduce hydrogen crossover, and enlarge the window for safe operation as a result.

The ongoing transition towards sustainable energy has resulted in a large interest in alternative sources. Proton exchange membrane fuel cells (PEMFC) use hydrogen to produce energy without harmful emissions, and they can be used in a range of applications, from mobility to industrial power generation.

One hurdle that still stands between PEMFC and widespread commercial adoption is the durability. The electrodes, catalyst and membrane of the fuel cells degrade due to the reactive environment inside the fuel cell (temperature, acidity and the presence of catalysts). This is worsened by situations with dynamic loads or start-stop cycling.

Another factor that can affect the degradation of fuel cells is the interaction between the fuel cell and the power electronics it interfaces with. The power electronics are necessary to boost and stabilize the voltages from the fuel cell, but they introduce current ripple into the fuel cell. The main objective of this research is to investigate how current ripple influences fuel cell degradation. The relevance is that the desired lifetime plays a significant role during the design of fuel cell systems, and quantitative information about the relationship between current ripple and fuel cell degradation can be used in the design process of the power electronics, where trade-offs need to be made between ripple amplitude, complexity and costs.

An analysis has been made concerning current ripple in fuel cells. Two current ripple types are mentioned in literature, low frequency current ripple and high frequency current ripple. Low frequency current ripple is created by the conversion from DC to AC by a single phase inverter, it has a frequency of 100Hz or 120Hz. High frequency current ripple is caused by the high frequency switching that occurs in any power converter. The link between these current ripple types and fuel cell degradation has mainly been investigated using experiments, but only for very specific parameters, a handful of frequencies and amplitudes. There is an absence of a quantitative analysis on the effect of different current ripple shapes on PEMFC degradation.

In this report, a model is developed which predicts the degradation of the PEMFC catalyst caused by current ripple. The model consists of two sub-models. The steady state degradation model predicts the decrease of the electrochemically active surface area (ECSA) due to the average current of the current ripple. It models the influence of electrochemical dissolution, chemical dissolution and Ostwald ripening on the ECSA over time. The ripple degradation model predicts the ECSA degradation as a function of the ripple amplitude and frequency, using an electrical representation of the fuel cell and experimental degradation data. The model can be used to help in the design process of power electronics that interface with fuel cells.

Global endeavors to reduce emissions in the shipping industry are accelerating the interest in fuel cell systems. This paper explores the application of different fuel cell types (LT-PEMFC, HT-PEMFC and SOFC) in combination with different fuels (LH2, LNG,MeOH and NH3) in expedition cruise ships. An impact model is developed for the first design phase. The goal of this paper is to evaluate the impact of the combination of fuel cell system implementation and operational profile on expedition cruise vessels. Impact is expressed in ship size, capital cost, operational cost and emissions. The model takes into account: fuel storage, on-board fuel processing, fuel cell system characteristics, balance of plant components, fuel cost over operational lifetime and all onboard emissions. In the research, seven different fuel cell systems and three different hybridization strategies are considered. For the six best performing combinations of fuel cell system and hybridization strategy, the range, endurance and capacity requirements are systematically varied to determine whether the best performing option depends on these requirements. Finally, hybrid option 2 (using diesel generators to support during long transits) combined with a methanol fueled LT-PEMFC system results in the lowest newbuild price. This option does comply with emission regulations and CO2 goals for 2030. Hybrid option 2 combined with an LNG fueled LT-PEMFC system results in the lowest total cost (newbuild price and fuel cost). This option does comply with emission regulations, but does not meet CO2 goals for 2030. When it is desired to reach this CO2 target, hybrid option 2 with methanol fueled LT-PEMFC is also recommended from a total cost perspective.

Power-to-gas (PtG) technology represents a promising route in providing long-term energy security and involves pairing volatile sources of renewable energy (solar and wind energy) with alkaline water electrolysis (AWE) to create a wide range of products based on intermediate hydrogen gas for end-users across various industries such as in fuels, chemicals and power sectors. However, PtG currently lacks the technical adaptability to match the intermittency of these sources caused by daily and seasonal weather patterns, and suffers from a low turndown ratio (TDR) as a consequence.

Zero Emission Fuels (ZEF) B.V. aim to adapt this process to create an autonomous, dynamically operated solar panel add-on known as a micro-plant that will produce methanol from CO2 and H2O in the atmosphere. In doing so, the company looks to extend the TDR, but requires the micro-plant and by extension, its AWE unit to operate in wider partial load ranges, in which lower current densities dominate. These cause a rise in gas crossover within the electrolyser, creating impure and potentially flammable H2-O2 mixtures in the outlet that hinder the functionality of the micro-plant. Specifically within the AWE unit, the risk of oxygen crossover has been qualitatively identified as a likely safety hazard. Therefore, the focus of this thesis was set on studying the feasible development of a prototype (gas scavenger) that can remove trace O2 within the techno-economic scope of ZEF’s micro-plant and electrolyser.

To determine a viable method that meets ZEF’s constraints on cost, size, weight and efficiency, industry techniques were studied that either purify hydrogen gas streams or destructively remove flammable H2-O2 mixtures. Based on a subsequent numerical estimation on each method’s limiting factors, a local combustion process in the form of a micro-combustor was identified to be the most suitable method.

To quantitatively estimate the safety risk crossover poses, two models were developed: a non-dimensional model to analyse how 50 bar pressure influences crossover during operation, and a two-dimensional, transient diffusion model to analyse the extent of trace O2 contamination overnight. Results indicate that to mitigate dangerous impurity levels, a gas scavenger needs to be implemented: (a) above a 0.1% difference in pressure between electrolyser half cells during operation, and (b) upon start-up to remove overnight gas accumulation that may not pose an immediate severe risk at 0.079 mol% contamination relative to amount of stored hydrogen, but has the potential to build-up on a monthly timescale.

Finally, the feasibility of the micro-combustor was preliminarily characterised at 50 bar and 80 ⁰C through flame simulations. The variability in low amounts of trace oxygen content show that the mixture is normally non-flammable. Further studies on geometry design and control strategies of the micro-combustor must consider how to achieve optimal ignition conditions, determined to be at richer fuel mixtures between 휙 = 1.03−1.40, and how to contain the flame within a well-defined region. Initial suggestions include using a small volume to induce ignition and relying on wall effects to promote flame quenching.

Fossil fuel based energy systems are the main contributor to global warming. As new regulations come into place, a transition towards a more sustainable energy system must be realised. Hydrogen usage is regarded as the key enabler of such systems. The heart of these hydrogen-based energy systems (HESs) is the hydrogen back-up system (HBS) which is responsible for the production, transport and storage of hydrogen. This study investigated how the power-to-gas efficiency of a HBS is governed by various sources and services. Two services are defined for the HES; Service 1: Constant hydrogen supply for the chemical industry. Service 2: Interruptible hydrogen supply as a fuel for power back-up. Both services can use a wind farm or the power grid as the source. A fully transient mathematical model of the system was built in Matlab Simulink. This model uses a rectifier and a PEM electrolyzer to convert the source power into hydrogen. A transport system, consisting of a pressure control station (PCS) and a 50 kilometre transmission pipeline, delivers the hydrogen to the salt cavern. Here, the hydrogen is injected into a 1000 metre deep cavern by a PCS and an injection pipeline. After a discharge pipeline, a third PCS is installed to adapt the hydrogen temperature and pressure for another 50 kilometre long transmission pipeline. A final PCS adapts the flow properties to the desired service values. All components are able to fully capture the intermittent nature of the cases. Historical data of the spot prices for power and wind speeds of 2017 are used for the sources and services. The resulting production and consumption profiles for 2017 were used to size the HBS for each case, ensuring full use of the cavern capacity without over- or underproduction. Simulations of the defined cases showed that the caverns only completed one full charge-discharge cycle during 2017, showing that all cases demand seasonal storage. The power-to-gas efficiency of both the grid-powered cases is 58%. The wind-powered cases show efficiencies of 44% and 41% for the chemical industry and the power back-up cases, respectively. The main conclusions as to why these difference are present are described below. Wind-powered HBSs lead to roughly 24 – 28% more power losses due to a mismatch between the wind farm power output and the PEME operational power input limits. The most significant losses occur during low wind speeds when the wind farm output is between zero and the minimum PEME operating power. The PEME efficiencies are in the range of 67.5 - 70.1%, making it the largest contributor to the total HBS losses. The transport and storage systems for all cases have an efficiency of roughly 95.0 - 95.5%. The efficiency of wind-powered PEMEs is 2.3 - 2.6 percentage points higher compared to grid-powered. The service has no significant effect on the efficiency as the service pressure for all cases is for large part of the year lower than the transport and storage pressure, resulting in negligible power requirements for the transport system.

Zero Emission Fuels B.V. (ZEF) is developing an integrated micro-plant to produce methanol from captured CO2 and green Hydrogen produced via alkaline water electrolysis. The electrolysis unit represents one of the largest financial and energetic costs in the system and is required to operate at 166 mA/cm2 under 2 V. In order to decrease the costs, ZEF is looking to synthesize highly efficient nickel-based electrodes to avoid using expensive noble-metal ones.
The objective of this work was the synthesis of efficient and stable nickel-based electrodes. Two electrodes were synthesized through electrodeposition (Raney nickel and NiFe) and one was synthesized through hydrothermal treatment (NiFe-LDH) and were compared to a RuO2-containing (Permascand) electrode and to a smooth nickel electrode. On/off cycles for 2.5 hours were carried out to measure the stability of the electrodes. Additionally, performance tests in a pressure range of 1-5 bar were carried out to measure the effect of increasing pressure. The experiments were carried out in an in-house designed and built zero-gap alkaline electrolysis cell.
Raney nickel, with a measured roughness factor of 150 was the best performing of the synthesized electrodes. After the stability tests, it was able to produce 166 mA/cm2 at 1.9 V (1.73 and 2.12 V for Permascand and smooth nickel, respectively). Raney nickel as anode material presented significant degradation. Raney nickel is the most promising material for HER, none of the synthesized materials presented significant stability for OER.
From the nickel-iron electrodes, NiFe with a roughness factor of 19.5 presented the best performance of the synthesized materials with 2.04 V for the mentioned current density. NiFe presented significant degradation, especially as anode material. NiFe presented a relative high performance considering its low electrochemical active surface area attributed to the presence of highly efficient active sites. NiFe-LDH was quickly degraded during the tests as cathode and anode material.
The pressure tests showed an inverse relationship between voltage and pressure. This hints that a decrease in bubble size with pressure is the cause behind the decrease in voltage. A simple model, based on experimental data and thermodynamic considerations, estimated that operating the cell at 50 bar reduces the voltage by 0.25 V compared to operation at 1 bar. The estimated reduction in voltage at a pressure of 50 bar would allow to operate the electrolysis cell under 2 V even with smooth nickel mesh as electrode.

Today’s renewables, wind and solar power, have a fluctuating nature, making the grid less stable.However, with the increasing share of intermittent sources of renewable power, novel options have to be created to stabilize the power grid. One of these options is energy storage via the conversion of excess power to hydrogen, during periods of high generation from wind and/or solar. In periods of power shortages hydrogen is converted back to power. In the literature fossil fuelled turbine cycles have been studied extensively, furthermore multiple fuelcell turbine cycles have been modelled. However, models with hydrogen and oxygen fuelled turbine cycles and fuel cell turbine cycles are scarce, leaving opportunity for further optimization of modeled hydrogen and oxygen fuelled cycles. The application of pressurized H2/O2 can lead to several improvements over conventional thermodynamic cycles and fuel cells. In this work, a number of high efficiency hydrogen and oxygen fuelled thermodynamic cycles, based upon the Graz cycle and the Toshiba Reheat Rankine cycle, both a coupled closed Brayton cycle with a Rankine cycle, are investigated. urthermore, the following thermodynamic improvements are proposed to fit the Graz cycle: an increased TIT (turbine inlet temperature), the use of a reheat combustor (the inclusion of a second combustor), condensate preheating and hybrid cooling (a combination of open loop and closed loop cooling) of the high temperature turbine blades. All upgrades together lead to an efficiency of 75% LHV. The rise of efficiency of the individual upgrades summed up is lower than the rise off all upgrades together. This is due to the fact that adding reheat and increasing TIT introduce extra high temperature turbine blade cooling needs and losses, which an improved cooling method counteracts. For this reason, the cooling method used is rather influential. Therefore, two cooling methods, open loop cooling and hybrid cooling, used in the Graz and Toshiba cycle respectively, are studied in this thesis.Next to the thermodynamic improvements, an electrochemical improvement is studied, by adding fuel cell systems to the turbine cycles. Multiple fuel cells combinations are studied: a single low temperature solid oxide fuel cell (SOFC) and a low temperature, intermediate temperature and high temperature SOFC in series. Moreover, the type of fuel cell cooling is investigated, both cooling by adding steam to the cathode and recirculating excess oxygen in the cathode are studied. The addition, of a triple SOFC cooled by oxygen recirculation, to an upgraded Graz cycle leads to a potential efficiency of 84% LHV. This efficiency is reached at a relatively low fuel cell fuel utilization of 58%. In this study the turbine cycle is not fully adapted to the fuel cell system or vice versa. Both systems are adapted to each other, leading to the unique triple fuel cell system operating in series, which is more easily coupled to a turbine cycle and can operate at an overall higher SOFC utilization, compared to a single fuel cell. However, from the fuel cell side 2 of the 3 fuel cells are less electrochemically favorable, as only one of the three fuel cells can operate in the best conditions. The overall system efficiency of the combined multiple fuel cell in series system with a turbine cycle, however is most efficient.

In the oil and gas industry the occurrence of slug flow in flowlines and risers can cause operational problems. Such flowline-riser systems are used to transport the oil and/or gas from the location of the wells to a production platform, where the fluids might be separated into single phases. Slug flow conditions imposefluctuations in the production rate, which may lead to the flooding of the separators, trips of compressors or pumps, and to increased loads on the supports of the pipeline and piping sections at the production platform.Therefore, measures have to be taken to mitigate the slugging, which can be a reduction of the production rate, making adjustments to the pipeline system, or adding surfactants to the flow. For vertical flow in production wells, the use of surfactants is a proven technology. Here the creation of a foam through adding asurfactant can increase the production life time, as the accumulation of liquid, which typically occurs when the reservoir pressure has decreased at the end of field life, is prevented. Far less is known yet about the effect of using surfactants for the mitigation of hydrodynamic slugs in a nearly horizontal flowline or the mitigation of severe slugs in risers. From a previous Master’s Thesis project by Pronk [1], in which lab experiments were carried out in the same flowline-riser facility as used in the present study, it was concluded that growing slugs could be suppressed by adding a surfactant to create foam. The surfactants also influence the characteristics of the severe slugging cycle, but they are not able to fully suppress it. Re-analysis of the measurement data by Pronk hasturned out that instead of growing slugs, severe sluggingwas measured and therefore that severe slugging can be suppressed using surfactants and no conclusions can be drawn on growing slugs or hydrodynamic slugsfrom her research. There is limited literature on the effect of adding a surfactant to hydrodynamic slug flow in horizontal flowlines, but there are some indications that hydrodynamic slugs can be suppressed, though requiring a higher concentration of the surfactant than vertical flow. Suppressing slug flow using surfactants should eliminate the fluctuations in the flow and can lead to a lower pressure drop. For this research experiments have been carried out in the Severe Slugging Loop at the Shell Technology Centre Amsterdam. This flow loop consists of a horizontal flowline of 50 m, a U-turn and a 15 m section. From there onward the pipe starts declining at an angle of -2.54° over a length of 35 m. Thereafter there is a short horizontal section of 5 m before arriving at the base of the riser, which has a height of 16.8 m and an internal diameter of 32 mm. The other sections have an internal diameter of 50.8 mm. For this research the configuration was used where air and water are connected through a Y-sprout at the start of the flow line. The air is supplied as pressurized air of 6 barg. The water is supplied by a pump, and it is separated from the airat a section at the top of the riser and recirculated through the system. The used surfactant is the household detergent Dreft. At several points along the flowline and riser the flow conditions have been measured using pressure indicators and at two locations along the flowline the pressure difference over 3 m has been measured using differential pressure indicators. Through the inspection windowat the end of the flowline the flowis recorded by using a GoPro camera. Furthermore, over a distance of 40mthe acoustic energy of the flow has been measured by a Distributed Acoustic Sensing (DAS) system. The DASmeasurements have been used to determine the flow behaviour of the flow. In the case of slug flow, the velocity of the slugs was deduced from the DAS data. Adding a 2000 ppm concentration of the surfactant Dreft Original has resulted in the full suppression of hydrodynamic slug flow. This has been verified using the pressure data, the video data and the DAS data in combination with the velocity tracking tool. The waves that were present without surfactant are decreased to a much smaller wave height when the surfactant is added; therefore, the flow pattern is changed from slug flow to wavy stratified flow. The severe slugging cycle can also be suppressed using 2000 ppm concentration of the surfactant Dreft Original. At the same flow conditions, the gas is able to lift more liquid through the riser. The measurements by the DAS system were quantified using a velocity tracking algorithm. The measurement data can now be used to accurately measure the slug velocity and to determine whether slug flow or stratified flow is present in the system. This measurement method is ready for scale-up. When the concentration of surfactant is sufficient to suppress slug flow, the flow can be described from visual observations as a threephase flow, containing gas, foam and liquid. The foam absorbs the stresses imposed by the other phases, because of its non-Newtonian fluid properties. The waves are no longer able to grow until they reach the top of the pipe and are diminished into slow moving dampened waves of the foam layer.

In the Netherlands most houses and buildings are connected to a gas network since the nineteen seventies to heat our houses and cook. In march of this year the new Dutch government announced that to meet the climate accord of Paris all dutch household should be of the gas net in 2030. To do this multiple alternatives to heat homes are available and one of those which has proven to be feasible over the last years is district heat. In a district heating network hot water is pumped round a city through a network of pipe lines to houses and other buildings. The network is split into a primary and a secondary network. In the primary network the heat is transported from the source, and in the secondary network the heat is pumped from a substation to the customers. The heat usually is rest heat of a STEG- or waste incineration power plant. With the increasing demand for heat due to the disconnection of the gas network, and at the same time the search for a more sustainable heat source for the district heating network like geothermal heat the prices and costs rise rapidly. This makes the heat loss in the system a more important issue also from a financial perspective. The current installed networks are all series 1 single pipe lines of ST/PUR/PE, this means the inner pipe through which the water flows is steal, followed by an insulating PUR layer which is covered by a protective PE layer. To decrease the heat loss from the pipe lines Nuon is looking at two possible options. Series 2 single piping which has a thicker insulating PUR layer and second the twin pipeline where both the steal supply and return lines are
embedded into one insulating PUR layer and a protective PE shell. The heat loss calculations for the buried pipelines in the ground are done using empirical formulas found in literature. These are checked numerical using a pde-solver. Comparing both results it was the concluded the empirical formulas give a good approximation of the heat loss. It also showed the heat loss is for a great deal depended on the temperature in the pipe and the ground temperature. In current heat loss calculations done by Nuon the temperature gradient of the water entering the network and leaving the network in not taken into consideration. By using the ambient temperature and a formula formulated in literature an estimation of the ground temperature at the buried depth of the pipelines can be made. By dividing the network into a number of pieces with length 푑푥 and an individual temperature 푇 the temperature gradient over
the entire network is taken into account. Collected data by Nuon concerning the user demand and the ambient temperature makes it possible to simulate what is the mass flow through the system at any given time and to analyze how the system responds to a demand fluctuation. By analyzing what happens in the networks in term of heat loss during a diurnal heat curve the results show that the heat loss stays nearly constant during a day. There are however big differences between different days in different times of the year. Analyzing what happens over the course of a year especially in the summer during times of very low demand the system under performs. A main reason for this is caused by the
minimum required temperature of 70 degrees Celsius at the customers due to salmonella regulations. This causes a lot of extra mass flow of hot water pumped around the system which heats up the return flow. This is also clearly notable from the efficiency of the network which drops tremendously in the summer. Comparing the difference in heat loss for the series 1, series 2 and twin system the results where as expected an decrease in the amount of heat lost, by 14.6% for series 2 and 39.70% for twin compared to series 1. The overall image of what happens actually stays the same with high return temperature and relative high heat loss and low efficiency in summer. There are a few options to further improve the performance of the system with a few percent by changing the inlet temperature or the location of the bypass valve. Financially speaking the twin system is also the better choice of the three. Compared to series 1 for series 2 the investment costs will increase because the materials
and instalment costs will increase. For the twin system the prices of the materials will increase and the placements of the welds will become more expansive however only half the number of pipe lines and joints is needed, so in total the prices will stay nearly the same compared to series 1. There is a lot of discussion on whether or not the maintenance costs will increase for the twin system, arguments for both cases are given. However the maintenance costs are so small compared to the costs of the heat loss that in every case the twin system is clearly the better choice.

Due to rising concerns about climate change, a lot of research is currently underway with respect to the development of new technologies which can contribute to the decline of atmospheric CO₂, and will allow further penetration of renewables into the energy mix. A promising technology which is currently actively researched is the electrochemical reduction of CO₂ (ERC). This technology utilizes otherwise polluting and unwanted CO₂ and converts
it into value-added products under the influence of an electrical current. The process can therefore be designed as an energy storage mechanism since electrical energy is stored as chemical bonds. In this research, ERC towards formic acid has been investigated from two perspectives.

First, the feasibility of the commercial production of formic acid compared to other products of ERC was investigated. The electrochemical production of the most common reduction products have been compared based on production costs, energy storage capabilities, toxicity and manageability. Due to the relatively low energy consumption for 2-electron products, namely formic acid, carbon monoxide and oxalic acid, it is found that these products have the most promising business case. Additionally, ERC to formic acid is best studied compared to other products, and high selectivities are commonly reported. Formic acid and methanol are liquid at atmospheric conditions, which is beneficial as relatively large amounts of energy per unit volume can be stored without the need of additional compression or cooling. This will also allow for easy transportation. As hydrogen carrier, formic acid has the advantage that it can be decomposed in H2 and CO₂ near room temperature. In the second part of this research, the use of numerical modeling to study the reduction of CO₂ in an electrochemical cell towards formic acid/formate at elevated CO₂ pressures is presented. The model investigates to impact on the cathodic half-cell of a cell designed for the reduction of CO₂ in aqueous electrolyte solutions at a constant temperature of 25℃, simultaneously assuming non-limiting conditions with respect to the anodic half-cell. The modeled part of the cell has been divided in three main regions, namely the bulk, cathode surface region and the electrode surface, which are discussed separately. The bulk is assumed to be the region of equilibrated concentrations which are constant in time, as they are not dynamically influenced by any mass transport phenomena. Reactants are supplied from the bulk to the electrode surface and products are removed vice versa via the cathode surface region, which is a thin region in the vicinity of the electrode. The transfer of species within this region and the chemical reactions between the species, form a system of diffusion-reaction equations. This system is solved numerically using appropriate boundary conditions. The actual reduction of CO₂ occurs on the electrode surface, and the kinetics of the electrochemical reactions towards HCOO^-, CO and H₂ are described using Tafel-type kinetics.

The electrochemical model has been verified and compared with experimental data, and despite various simplifications has proven to be predictive of the electrochemical reduction of CO₂. It is found that the potentially beneficial effects of an elevated CO₂ pressure on both the production rate and selectivity, as experimentally observed, can be reproduced with reasonable accuracy. The CO₂ concentration at the electrode surface is identified as the main limiting factor for achieving both a high selectivity towards formate and a higher production rate on formate producing metals. The model shows that with an increased CO₂ pressure the amount of CO₂ dissolved into the solution is increased significantly, resulting in a higher concentration of CO₂ at the electrode and less mass transfer limitations.

Zero Emission Fuels B.V. develops a standalone System that produces renewable methanol from air and sunlight. In this thesis the steady-state model of this System is replaced with a Simulation Tool that can predict its behaviour for every minute of a year. The goal is to verify how technical design parameters influence methanol production and to determine if the business case target of 6.7 mole methanol per day is achievable. This thesis will contribute to the understanding of the System, speeds-up the design process and makes it possible to see the impact of fluctuating weather conditions throughout the year at different locations on earth. The System as it was at January 1, 2018 is implemented in the Simulation Tool and its production determined for Tucson, Arizona in the year 2005. The result is an average daily production of 3.0 mole methanol with fluctuations from 1.0 mole during a cloudy winter day to 4.2 mole during a sunny summer day. The minute by minute production is found to be influenced by all System design parameters. They must thus be designed together to make sure the System performs optimally. The Alkaline Electrolysis uses the most energy of all subsystems. Four options to improve the System are identified. These are 1) improving the power the Solar Panel produces, 2) improving the efficiency of each subsystem, 3) improving how the System is controlled and 4) integrating heat. A selection of these improvements shows that it is possible to produce 4.8 mole methanol per day. The observed seasonal and daily fluctuating methanol production demonstrates the importance of the dynamic model developed in this thesis. Compared to the steady-state model it gives a more realistic view of methanol production and can be used to improve System design. However, to reach the business case target more technical improvements are needed. It should be realised that the design decisions will be based on technical, economical and political arguments.