Industrial process heat remains strongly dependent on fossil fuels. High-temperature heat pumps can reduce electricity demand and direct emissions by upgrading available waste heat, but their performance depends strongly on the temperature profiles of the heat source and heat sink. This study evaluates high-temperature heat pumps that absorb and reject heat over a temperature glide for industrial sensible-heat upgrading. Vapor-compression heat pumps (VCHPs), reversed-Brayton heat pumps (RBHPs), and an absorption-compression heat pump (ACHP) are compared for three heating scenarios with heat sink outlet temperatures of 150 or 200 C using a 100 C heat source.

Steady-state thermodynamic models were developed and validated for the VCHP and RBHP. Pure fluids and binary mixtures were optimized and evaluated through Pareto-based multi-objective screening and working-fluid family analysis. Thermodynamic performance was subsequently combined with environmental, safety, and practical criteria to select favorable working fluids. Preliminary compressor and heat-exchanger sizing, equipment-cost correlations, and levelized-cost-of-heat calculations were used to evaluate technical and economic performance at heating duties of 1 MW and 10 MW. Vendor data were used for the ACHP.

The VCHP achieved the highest and most consistent thermodynamic performance. Subcritical operation with a zeotropic mixture provided the largest benefit when the heat sink outlet temperature was 150 C and both the heat source and heat sink had a 50 K temperature glide. The zeotropic mixture advantage decreased at higher heat sink temperatures, where transcritical operation made pure fluids and low-glide mixtures more competitive. The RBHP performed most favorably when the heat-source-to-heat-sink temperature glide ratio matched its sensible heat-transfer profile, but generally had a lower COP and required larger high-pressure heat exchangers. Its internal heat exchanger provided a significant benefit mainly for the scenario with the largest heat-source-to-heat-sink temperature gap.

Equipment feasibility and economics improved substantially with scale. At 1 MW, several compressor and RBHP turbomachinery configurations required small, high-speed, or high-temperature designs with uncertain practical feasibility. At 10 MW, larger dimensions and estimated compressor efficiencies near or above 80% made centrifugal turbomachinery more credible and reduced specific investment costs. The VCHP generally achieved the lowest levelized cost of heat, while all evaluated heat pumps remained less expensive than direct electric heating under the assumed conditions. At 10 MW, the heat pumps were also competitive with gas-fired heating in most scenarios. Overall, temperature-glide heat pumps are most effective when the cycle, working fluid, process-temperature profiles, and heating duty are selected together. ... Read more

Industrial potato frying processes require large amounts of thermal energy, with the dominant share used for moisture evaporation from the product. This energy leaves the system as hot frying vapor and is typically rejected to the environment, while process heat is supplied by fossil-fired steam boilers. Recovering and upgrading this latent heat therefore represents a major opportunity to reduce external energy demand and improve the sustainability of industrial frying operations. This thesis investigates the feasibility of recovering frying vapor heat and returning it to the fryer as high-pressure steam using a closed-loop mechanical vapor recompression (MVR) system with water as the working fluid.

A structured methodology was applied, combining literature review, steady-state thermodynamic modelling, equipment sizing, and dynamic simulation. First, the energy distribution of the frying process was analysed, showing that evaporation-related heat dominates the total thermal demand and that the recoverable heat potential depends strongly on the vapor dew point. Based on this analysis, a complete system configuration was developed consisting of a vertical shell-and-tube condenser, a flash tank, and a three-stage water-injected screw compressor. The heat exchangers were sized to ensure sufficient heat-transfer capacity, reliable condensate drainage, acceptable pressure drops, and robustness against fouling. The flash tank was designed to accommodate the large specific volume of low-pressure vapor while providing adequate thermal and mass buffering. Water injection in the compressor limits discharge temperatures and enables high pressure ratios under near-saturated compression conditions.

For a representative industrial fryer with a heat load of 8161 kW and a vapor dew point of 87.5 °C, the reference configuration recovers approximately 4045 kW of latent heat from the frying vapor. After compression, 6572 kW of useful heat is returned to the fryer, with the remaining 1589 kW supplied externally. Under these conditions, the system achieves an MVR coefficient of performance of 2.63, demonstrating that a substantial fraction of the process heat demand can be met through internal heat recovery. Sensitivity analysis shows that vapor dew point, flash tank temperature, and condenser temperature approach strongly affect system performance, with vapor dew point being particularly critical: a decrease from 90 to 85 °C reduces recoverable heat by nearly 1 MW.

An improved configuration incorporating an additional vapor compression heat pump allows further cooling of the condensate and recovery of additional low-grade heat, enabling full coverage of the fryer heat demand without external steam. The resulting temperature levels are suitable for ammonia (R717) as a natural refrigerant. Finally, a dynamic model was developed to assess operability under realistic production disturbances. Simulations show that combined feed-forward and feedback control significantly improves transient performance, ensuring stable operation and consistent product quality. ... Read more

Global energy demand has seen a significant increase over the past 70 years, accelerating greenhouse gas emissions by placing excessive pressure on existing non-renewable energy sources. As wind and solar power are projected to account for a significant share of future electricity production, their intermittent nature introduces a major challenge to grid stability. Green hydrogen offers a promising solution as it can be produced during periods of excess electricity supply and combusted in land-based gas turbines during high electricity demand, thereby stabilizing any oscillations. Combustion of hydrogen is, however, not problem-free. The main challenge is its high flame speed, which can lead to flashback, an undesirable upstream flame propagation that can cause damage to the combustor hardware. Because conventional land-based gas burners are not designed for pure hydrogen combustion, new burner designs must be developed and tested. An example of such a burner is the FlameSheet™ burner, whose geometrical simplification, called a 2-D trapped-vortex burner, is investigated in the present experimental study.

The structure of this study can be divided into two parts. The first part investigates the flashback behavior of the 2-D trapped-vortex burner for a range of equivalence ratios, hydrogen volume fractions, and unburned mixture temperatures. The results show increased flashback propensity with all three parameters, although the Reynolds number at flashback seems to be invariant with the last. The second part examines temperature profiles within the 2-D trapped-vortex burner, with a particular emphasis on their dependence on the Reynolds numbers, unburned mixture temperatures, and adiabatic flame temperatures. The thermocouple measurements reveal that the temperature profiles shift toward lower values with increasing Reynolds numbers and higher values with increasing adiabatic flame temperatures. To complement these findings, a one-dimensional heat transfer model is developed, which not only predicts the vertical temperature profiles in the remaining components but also identifies convection and conduction through the liner as the dominant and limiting heat transfer mechanisms. ... Read more

The need to reduce and mitigate the risk of global warming necessitates developing new methods or at least improving existing ones while maintaining a continuous, reliable and efficient supply of electricity. The increased share of power production from renewable sources like wind and solar brings challenges to grid stability due to their intermittent nature. Gas-fired power plants currently provide dispatchable generation; however, their continued reliance on fossil fuels presents a pressing environmental challenge. One potential solution to decarbonize gas turbines is the deployment of bioethanol, a renewable liquid fuel, that has the potential to have a closed carbon cycle.
Lean Premixed Pre-vaporized (LPP) technology has emerged as a promising solution, enabling the use of bioethanol in LPM systems by vaporizing the liquid fuel prior to injection. This study evaluates the use of pre-vaporized bioethanol in a combined cycle power plant by specifically examining the design and transient behavior of the fuel conditioning system required for bioethanol deployment. While existing research primarily focused on combustion behavior and steady-state performance, this study aims to fill the gap regarding the system integration, equipment design, transient operation, and the impact on the plant overall performance.
Initially, heat integration is conducted to optimize energy recovery within the combined cycle plant. Subsequently, preliminary design and sizing of major fuel conditioning equipment including a vaporizer, a superheater, and phase separator. The sizing of heat exchangers are performed using ASPEN Exchanger Design and Rating software. In the third phase, 1D dynamic models of these heat exchangers are developed using the finite volume (FV) method, chosen for its effectiveness in capturing complex transient thermal behaviors. Finally, these dynamic models are integrated with real combined cycle plant data to simulate startup.
Due to ethanol’s lower heating value compared to natural gas, a significantly higher mass flow rate was required to achieve the same heat input to the turbine. Intermediate-pressure (IP) steam was identified as the most suitable heating medium, minimizing heat exchanger size, cost, and installation challenges. A horizontally oriented shell-and-tube heat exchanger was selected for ethanol pre-vaporization due to its effectiveness in managing phase-change heat transfer. Results showed significant temperature gradients highlighting potential risks of localized heating, thermal stress and critical heat flux exceedance, necessitating further mechanical assessment.
Transient model results revealed that controlling heat input rates and managing the transition from external heating to IP steam are critical for maintaining stable fuel supply system operation during startup. Additionally, deployment of bioethanol required an external heating input for startup before IP steam became available. The use of IP steam as a heating medium introduced a measurable power penalty to the overall power output.
In conclusion, key recommendations are provided that include conducting detailed mechanical stress evaluations to address thermal stress concerns in the fuel conditioning system. Additionally, experiments on boiling heat transfer for ethanol under gas turbine operating conditions are essential to improve the accuracy of heat exchanger design. Furthermore, improving the transient model by incorporating real-time dynamic control strategies and relaxing simplifying assumptions such as accounting for pressure drop and void fraction will significantly improve the accuracy of transient predictions. Finally, conducting a pilot-scale test is recommended to validate the numerical model and confirm bioethanol’s feasibility, accelerating its deployment and marking an additional step toward decarbonized power generation.
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For centuries, combustion has played an important role in human development, yet as Giusti and Mastorakos notes [29], ”with some 100,000 years of development, combustion might be expected to be a mature technology. In fact, it is the least developed technology of modern engineering systems”. As the world transitions away from carbon-based fuels, hydrogen has emerged as a promising alternative due to its carbon free combustion. However, replacing conventional fuels with hydrogen introduces significant technical challenges. Pure hydrogen–air flames exhibit high laminar flame speeds and low ignition energies, making combustor hardware more susceptible to flame flashback. Flashback not only disrupts stable operation but can also result in damage and system failures. A thorough understanding of flame stabilization and flashback mechanisms is therefore essential.

In this context, the present work investigates a combustor geometry designed to generate a trapped vortex, a recirculation zone that enhances flame stabilization. Accurate numerical predictions of such phenomena require realistic thermal boundary conditions, yet these are often unavailable in experimental configurations and are commonly oversimplified in simulations. This study evaluates how different thermal wall boundary conditions modify flame stability and flow behavior in a premixed pure hydrogen–air trapped vortex combustor at lean conditions. A 2D and 3D simplified version of the FlameSheet™ burner, originally developed by Power Systems Manufacturing (PSM), is modeled in this study, using
PeleLMeX [24].

The study begins with a 2D cold flow simulation performed using DNS. This computation is analyzed in light of a previously performed 3D study of the same geometry, with the same numerical solver and following the same numerical approach [25]. Differences are acknowledged and explained. After examining the baseline flow behavior, six reactive cases are then simulated, varying the main flow Reynolds number (12,000 and 8,000) and the thermal boundary conditions applied to the liner walls (adiabatic, isothermal at 300 K, and a fitted temperature profile based on experimental data). Results show that wall thermal conditions significantly affect flame stability and structure. Time averaged fields reveal that
thermal boundary conditions also influence recirculation zone size and peak flame temperatures. The 2D simulations, while limited in capturing full 3D turbulence, offer valuable insights into flame wall interactions and highlight the importance of accurate thermal boundary conditions in predicting flashback propensity and flame stabilization in hydrogen combustors. ... Read more

The use of hydrogen in gas turbines is promising to help with the energy transition since there are no carbon-based pollutants released during its combustion. However, the transition from fossil fuels to hydrogen in gas turbines involves significant challenges. Hydrogen flames exhibit higher flame speed, making burners more prone to flashback, which in turn can cause damage by propagating upstream into the premixing sections. To avoid this type of failure, it is necessary to understand the different mechanisms that can cause flame flashback in gas turbines and develop strategies to help reduce the flashback tendency of hydrogen flames. The FlameSheet™ burner, originally developed by Power Systems Manufacturing (PSM), is a two-stage burner that can be used with fuels having high content of hydrogen. The geometry of the main stage includes a 180-degree bend (U-bend). This feature, which abruptly changes the flow direction of the air-fuel mixture, creates a trapped vortex which helps in stabilizing the flame in the main stage. Such a design makes the burner more resistant to flame flashback.

In this thesis, a numerical study is done on an academic trapped vortex burner, inspired by the FlameSheet™ design using Ansys Fluent. Initially, a cold flow simulation of the flow inside the main stage of the burner was performed using RANS. This was followed by a parametric study (using RANS) of the cold flow patterns while varying several parameters like the shape of the inner liner tip, channel height ratio and the distance between the tip and outer wall of the U-bend. From these results, useful insights were gained by analyzing and comparing the velocity profiles in different parts of the U-bend. To gain more information from some of the relevant cases, large eddy simulations (LES) were used. With the LES results, two-point correlations and cumulative distribution functions of the velocity field were made to describe the cold flow behaviour in the main stage of the burner. These results confirmed that the tip shape and its distance from the U-bend are significant parameters since they affect the velocity profile in the boundary layers of the flow. This change in the velocity profile of its boundary layer can alter the flashback tendency of hydrogen flames in the burner. Along with these cold flow simulations, a reactive simulation was also performed with the original geometry of the trapped vortex burner to understand the flow features inside, in the presence of a flame. Super adiabatic flame temperature and non-uniform diffusion of hydrogen into the flame were observed at various points in the flame. ... Read more

In the further advancement of the energy transition towards renewable energy sources, a significant role can be played by energy storage systems. A relatively new energy storage technology is the reversible solid oxide cell. A reversible solid oxide cell is an electrochemical device that can operate as an electrolyser to produce a fuel from electricity, and as a fuel cell to produce electricity from fuel. At Delft University of Technology, a hydrogen and oxygen fuelled solid oxide fuel cell and gas turbine integrated system has been designed. In order to redesign this solid oxide fuel cell and gas turbine integrated system into a reversible solid oxide cell system, the design of a solid oxide electrolyser system is required. One of the main challenges when designing a solid oxide electrolyser system is the heat requirement of the system. However, little research on the design of solid oxide electrolyser systems has yet focussed on thermal optimization of the system. In this thesis, three solid oxide electrolyser systems, displaying different heat integration layouts, were designed and examined. The results show that the highest energy and exergy efficiencies, respectively 75.52% and 75.42%, are obtained by the solid oxide electrolyser system that evaporates the water in the system using the heat that is generated by the hydrogen and oxygen storage compression. This solid oxide electrolyser system is combined with the solid oxide fuel cell and gas turbine integrated system to design a reversible solid oxide cell system. A round-trip energy efficiency, based on the lower heating value of hydrogen, of 48.40% is reached by this reversible solid oxide cell system. ... Read more

Decarbonization is an important step to achieve the goals set by the Paris Agreement. Greenhouse gas emissions should be reduced to zero, and therefore, the reliability of fossil fuels should be reduced. This causes a shift of interest towards more renewable solutions. However, the intermittent nature
of renewable energy sources such as solar and wind energy leaves a gap in the energy supply. Currently, this gap is still filled by natural gas, but biofuels could potentially help in decarbonizing this gap. Biofuels could be prevaporized and used as an alternative fuel within existing natural gas-fired power plants. Ethanol is an interesting biofuel as it has a relatively low boiling point, meaning that it is relatively easy to evaporate. Next to that, ethanol has a similar Wobbe Index (WI) compared to natural gas, meaning that it could be potentially used with only minor adjustments to the gas turbine. This research will focus on implementing ethanol as an alternative fuel for the Killingholme power plant, a
600 MW power plant in an open cycle gas turbine (OCGT) configuration that Uniper operates in the United Kingdom.

This research focused on the effect of ethanol on the process design and combustion characteristics, where the process design was only briefly touched upon. It was found that ethanol should be heated to a temperature of 467 K to be in vapor form at the relevant gas turbine conditions. A process design was
made for the baseload operation, where the required heat for the evaporation process was provided by the flue gas flowing out of the gas turbine. Next to that, the use of ethanol requires slightly higher volume flow rates, meaning that the pipes and fittings should be adjusted to keep the desired fuel pressure. The effect of ethanol on combustion characteristics was researched by a kinetic modeling study and a CFD study focusing on fuel-air mixing, where the results will be compared to methane. A RANS study was performed for the CFD study, which showed that the use of ethanol results in a better quality of mixing. From the kinetic modeling study, it was found that ethanol has a lower autoignition
delay time than methane. This will probably not lead to the autoignition of the fuel-air mixture in the mixing section, but it could lead to periodic flashes in regions close to the recirculation zones within the burner. It was also found that ethanol has a 78% higher laminar flame speed than methane. Next to that, it was found that ethanol has an effective Lewis number of 1.56 at the relevant gas turbine conditions, whereas methane has an effective Lewis number close to unity. From the laminar flame speed and the effective Lewis number, it was concluded that the use of ethanol results in an increase in turbulent flame speed. The increased turbulent flame speed and better quality of mixing of ethanol suggest a decrease in flame length. The decrease in flame length and increase in turbulent flame speed lead to higher flashback risks, but it is expected that this will be within the flashback margin of the burner. Based on the kinetic modeling study and the fuel-air mixing study, it was concluded
that ethanol will have NOx emissions similar to methane and that the driving energy source of the combustion dynamics will probably shift to higher frequencies.

Recommendations for further research are the gas turbine’s start-up, ethanol contaminants, extended CFD study of the burner, and the blending of ethanol and natural gas. Further research could complement the already promising results from this report, and eventually, this could lead to combustion tests where ethanol will be used as fuel. ... Read more

This report analyses the improvement potential of zeotropic refrigerant mixtures on the performance of vapour compression heat pumps integrated into industrial dryers. The report reviews the scope of total global energy consumption and the percentage of this attributable to industry to demarcate application area.

It proposes that by targeting industrial process heating, significant reductions to energy consumption and carbon emission by industry can be achieved. Heat pumps are introduced as the preferred system to accomplish this for temperatures up to 200 ◦C. A performance limitation is identified in the form of heat transfer between process streams that reject and receive heat at non-constant temperatures.

Zeotropic refrigerant mixtures are introduced in heat pumps to improve the interaction with non-isothermal heat sources and-sinks. The considered mixture compounds are limited to future-proof refrigerants and the mixtures are limited to binary ones. Humid air encountered in industrial drying is targeted as one such non-isothermal process stream with a large potential for waste heat recovery. The most prominent industries that use dryers are identified and evaluated as to provide representative operating conditions at which a heat pump integrated dryer would have to work. Heat pump cycles are described both on a cycle scale as well as per component and the most important design aspects and considerations are presented. The methods used to allow heat pumps to interact favourably with temperature glides are presented namely zeotropic mixtures and trans-critical heat pump cycles.

A modeling strategy is proposed to simulate heat pump integrated dryers in large numbers in order to investigate the effects of dryer inlet and outlet conditions as well as refrigerant mixture composition both in terms of constituents and mixing ratio. The total process flow diagram is given and divided into definable thermodynamic states for both the (humid) air as well as the refrigerant. Governing equations for each component are presented and the calculation method for the thermodynamic states is discussed in terms of known state variables and used thermodynamic libraries. Finally a novel heat pump cycle is proposed, motivated by a desire to decouple the glide matching in the evaporator from that in the condenser, that attempts to use the fractionation risk present in zeotropic mixtures as an advantage instead. The criteria placed on refrigerant are presented both within the context of the future-proof limitation, legislative limitation as well as those introduced by the process requirements.
The most promising refrigerant candidates from literature are presented for applications in the defined process conditions. Finally a selection is made of 13 refrigerants namely water, ammonia, (iso)butane,
(iso)pentane, methane, ethane, propane, CO2, propylene, ethylene and hexane. The thirteen refrigerants are combined into 78 refrigerant pairs and modelled using the described modeling strategy. The modelling reveals that using zeotropic mixtures does improve the COP for many refrigerants when compared to their pure cycle performance. For drying at 180 ◦C two heat pumps are proposed at different outlet temperatures. The cycles use 87.5%mol Isobutane mixed with Ethane and 87.5%mol NH3 mixed with Propane for a high temperature and lower temperature outlet respectively. COPs of 3.38 and 3.44 were calculated with PRs of 16.53 and 6.84 for the Isobutane-based and NH3-based cycles respectively. A lower drying temperature of 120 ◦C was explored and here CO2-based mixtures were identified as highly desirable refrigerants due to their non-toxic, non-flammable nature as well as low global warming and ozone depletion potential. The mixtures 87.5%mol CO2-Isopentane and 90%mol CO2-Isobutane were proposed, both feasible with single stage compression and possessing a COP of 3.96 and 4.02. Zeotropic mixtures improved the COP in at least 48 out of 78 possible refrigerant combinations when compared to the pure cycle COPs and allowed the dampening of flammability and toxicity where the pure refrigerant possesses those properties.

It is clearly demonstrated that future-proof binary zeotropic mixtures increase the performance of VCHP-integrated dryers across all relevant temperature ranges, by as much as 21.47%. The highest COPs are found when zeotropic refrigerant mixtures are used together with trans-critical operation. It is concluded that zeotropic mixtures improve the COP of vapour compression heat pump integrated dryers and should be utilised for non-isothermal processes like drying. ... Read more

Today's electricity supply falls short of current demands, leading to the utilization of gas turbines in both ground based and avionic infrastructures. Nevertheless, these often rely on carbon-based fuels, resulting in escalating CO2 emissions. However, adopting hydrogen as a fuel eliminates carbon emissions. Aside from zero carbon emissions, hydrogen has a higher energy density by weight compared to conventional fuels. This makes it an distinct option for applications requiring efficient energy storage and delivery. Due to its wide flammability range and low ignition energy, hydrogen can combust in scenarios where traditional fuels might not. This unique characteristic, while advantageous in certain contexts, requires detailed study to ensure safe and efficient combustion in gas turbines. However, the combustion of hydrogen inherently results in elevated flame temperatures, thereby generating increased NOx levels. Furthermore, hydrogen's high mass diffusivity translates to a reduced Lewis number. Consequently, it becomes vital to grasp the local dynamic characteristics of the flames, particularly at stable and flashback points. Understanding the thermo-physical behavior of hydrogen flames, especially at stable and flashback points, therefore, requires experimental studies to reflect real life complexities. These tools can offer insights into turbulence-flame interactions, flame stabilization, and emission formation mechanisms.


In this research, an exploration was undertaken to understand the local kinematics and dynamics of Hydrogen and DNG flames, with emphasis on stable states and those approaching flashback conditions. Particle Image Velocimetry (PIV) experiments were employed on a Bunsen burner setup, facilitating the capture of the jet flames' velocity fields. Both low-speed and high-speed recordings were captured by high-speed camera, providing distinct insights into flame dynamics. Flame front detection was achieved using Mie-scattering, capitalizing on the differential seeding particle densities between the unburnt and burnt regions. Intensity differences between these regions were meticulously captured with a bilateral filter, leading to the successful extraction of the flame front. This extracted front was subsequently distinguished via segmentation and superimposed onto the velocity field. Low-speed recordings offered a generalized perspective on flame turbulence characteristics through cold flow validation, while high-speed recordings unveiled specific dynamics, inclusive of flame curvature, local flame and displacement speeds, and both normal and tangential velocities and stretches. It consistently holds the 1-D unstretched flame speed, even as the Reynolds number increases, aligning with the respective flashback points and stable conditions of the flames. As a result, in-depth comparison of DNG and $H_2$ fuels in terms of flame dynamics and kinematics were discerned. ... Read more

Variable green energy sources were sought to supply the growing demand for energy without impacting the environment. Wind and solar energy are at the forefront of the energy transition. However, their intermittent nature poses a challenge that necessitates the development of energy storage technologies.
Several technologies were studied over the years including pumped hydro energy storage, compressed air energy storage, electrochemical energy storage, and pumped thermal energy storage.
Pumped thermal energy storage provides a mean to store excess electrical energy in the form of heat by employing a heat pump cycle during the charging process and a heat engine cycle during the discharging process. There are multiple variations of pumped thermal energy storage cycles of which Rankine and transcritical cycles. Unlike Brayton cycles, Rankine and transcritical cycles typically operate at low-temperature levels < 200o𝐶 which facilitate their
integration with low-temperature waste heat in order to boost their round trip efficiencies even beyond 100%. Therefore, the focus of this thesis was to model and optimize Rankine and transcritical thermally integrated small scale pumped thermal energy storage.
Critical to the storage system’s efficacy are the performances of its key components: the compressor and expander. Geometric models were developed and validated for these components, facilitating the determination of their isentropic efficiencies and their variation with pressure ratio and rotation speed.
Additionally, a small-scale pumped thermal energy storage system model was developed to study the system performance. The choice of working fluid and operating parameters were guided by both a simplified optimization scheme and a multi-objective approach utilizing the Non-Dominated Sorting Genetic Algorithm-II (NSGA-II). This yielded 𝑅13𝐼1 as the optimum working fluid with source temperature 𝑇𝑠𝑜𝑢𝑟𝑐𝑒 = 80o𝐶 and storage temperature 𝑇𝑠𝑡𝑜𝑟𝑎𝑔𝑒 =140o𝐶. This configuration led to a transcritical heat pump charging cycle and a subcritical heat engine discharging cycle employing pressurized water as the storage medium.
Consequently, the compressor and expander geometric models were integrated into the pumped thermal energy storage system model working with 𝑅13𝐼1 at the source and storage
temperature mentioned earlier which yielded a round trip efficiency of 𝜂𝑟𝑡 = 93%, energy density 𝜌𝑒𝑛 = 12𝑘𝑊ℎ𝑟/𝑚3, and exergy efficiency 𝜓𝑒𝑥 = 29%.
Finally, a case study was demonstrated with the integrated cycle employed in a Dutch household solar system where dual electric and thermal panels are employed for the electrical and heat energy input in the pumped thermal energy storage. The system was developed for a 1𝑘𝑊 electrical input from the solar system to be stored for further use during off peak hours.

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The rise in temperature attributed to human CO2 emissions and the escalating energy needs of society necessitates the development of clean energy production. Solar and wind energy, both renewable sources, have emerged as cost-effective alternatives to conventional fossil fuel systems. They now account for a substantial portion of the world's electricity generation (IEA, 2022). However, their intermittent nature poses a challenge to their reliability. To overcome this, the implementation of grid-scale energy storage systems is crucial. Such systems can store excess energy produced during peak periods and release it during low-generation or high-demand periods, ensuring a stable and dependable power supply to the grid.

Pumped Thermal Energy Storage (PTES) is one such type of promising grid-scale storage solution based on the concept of storing electricity in the form of heat. These systems are not reliant on rare earth metals, are not restricted by geographical location, and are relatively economical over their lifetime. They employ a heat pump cycle for charging and a heat engine cycle during times of discharge. Often, in the thermodynamic modelling of PTES systems, a fixed value of turbomachinery efficiency is assumed. This approach holds well for the first estimate of performance, but for better accuracy and further analysis, meanline models could be used to arrive at the efficiency value and preliminary geometric design. Hence, this work presents a method for developing meanline models for centrifugal compressors and radial inflow turbines. Modelling techniques and guidelines from the literature are noted and presented here. The accuracy of these models is dependent mainly on the loss models used. Using suitable models selected from the literature, a fair agreement was found between the meanline model's prediction and experimental data from open literature, validating the methodology.

An essential function of energy storage is to provide load flexibility, meaning its charging and discharging cycles must adjust to match the net load curve. As a result, the turbomachinery would need to operate under off-design conditions to meet these demands. Therefore, this report introduces an approach to extend the PTES model by Radi (2023) for off-design operation based on turbomachine performance.
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With variable renewable energy on the rise, large-scale energy storage is needed to assure grid stability. Thermal energy storage (TES) is a new player in the large-scale energy storage landscape, but might prove a cheap, safe and reliable option, with low permitting barriers. TES is not geographically limited, generally compact and has a low levelised cost of storage (LCOS) which comes at a cost of a lower round-trip efficiency compared to other grid-scale energy storage methods. Thermal energy storage is most suited for intraday and multiday storage.
Within thermal energy storage, several types can be distinguished based on the design of the thermodynamic cycle. In this thesis, these types are analysed and compared on basis of technology, cost, and miscellaneous characteristics. An overview of cost and characteristics of current relevant thermal energy storage (TES) storage materials and TES companies is given. A Rankine-type TES, as is used in the E2S project, is generally cheaper and easier to implement, at a cost of a reduced round-trip efficiency. Rankine-type TES have a round-trip efficiency of about 40% as it is limited by the Rankine-cycle efficiency.
The TES studied in the scope of this thesis relies on evaporation of water and a steam turbine to recover power. The work of the thesis comprises of a thermodynamic and hydraulic optimization study regarding the heat transfer during conduction, convection and boiling inside the TES. To this end, a steady boiling model is constructed based on empirical and phenomenological correlations. It is found that during steady state, boiling phenomena incur higher heat transfer coefficients for low to medium wall superheat in comparison to a model that does not regard boiling phenomena. Conversely, lower heat transfer coefficients are attained for high to very high wall superheat in comparison to a model that does not regard boiling phenomena. It is hypothesised that this phenomenon will incur higher overall boiling heat transfer during transient operation.
An optimization study is conducted with the aforementioned boiling model to find the optimum combination of TES parameters for a given power and capacity, with the optimization parameter being the combined material cost of steam generator tubing and storage material. Several design guidelines are concluded. Most notably is found that pressure drop plays a very minor role in TES optimization and therefore a lower pressure drop may be chosen. Furthermore, critical parameters and bottlenecks of the TES are identified in a sensitivity analysis.
Lastly, integration of the TES in a steam cycle is evaluated. The effect of variations in pressure and temperature output of the TES are analysed, and the total round-trip efficiency and its performance during part-load operation are determined. Various qualitative changes are discussed, and a quantitative estimate is made of the sizing of a start-up TES, which may be used for cold, warm and hot start of the steam turbine. As the TES is to be implemented in a thermal power plant, an estimate is made of the sizing and cost of such an implementation using the data from the optimisation study. Various aspects of this plant are evaluated such as system response, start-up time and a techno-economical analysis of multi-day operation. ... Read more

Hydrogen is a promising fuel for both reducing greenhouse gas emissions and being used as an energy carrier for renewable energy sources. The adaption of hydrogen instead of natural gas as a fuel in gas turbine combustors introduces additional challenges regarding flame stability. One instabilities is boundary layer flashback. This phenomenon occurs if the flame speed exceeds the local flow velocity. Then the flame can propagate upstream, which can result in equipment failure. Lean premixed hydrogen mixtures are more prone to boundary layer flashback because hydrogen flames have both a smaller quenching distance and higher flame speed compared to natural gas. Therefore, operating at 100% hydrogen fuel content is an enormous challenge. Active control strategies are a promising strategy to prevent and withstand possible occurrences of boundary layer flashback in gas turbine combustors. In this work, the detection, prevention, and counteraction of flashback are investigated in unconfined Bunsen burners. After an experimental analysis of suitable sensors, a tracking controller is therefore designed and different control design approaches evaluated.

Both the increase in flame fluctuations and in flame angle are reasonable as indicators for a flame moving towards flashback as the flow velocity is reduced. However, the complex nature of a flame makes it challenging to measure these effects in a flame. Four types of sensors, i.e. an ion sensor, a thermocouple, a photo-detector and a microphone, were investigated for their ability to detect flashback and to find precursors that indicate the onset of flashback. A Bunsen burner equipped with the different types of sensors was used in an experiment to determine the most suitable sensor type. Of the investigated sensors, the thermocouple is the most promising sensor for both flashback detection and use the temperature signal as a control variable. The temperature is a precursor for the onset of flashback since the temperature increases with increasing flame angle. Also, by placing multiple thermocouples in $360^oC$ configuration on the burner rim, it is possible to estimate the position, where the flame entered the burner.

A complete fault-tolerant framework to control a flashback event is proposed. This framework increases the flashback resistance of the system and it counteracts possible flashback events, which is considered as a fault in the system. A supervisor block has two functions. Firstly, the supervisor uses a fault detection method, known as trend checking, to detect flashback. Secondly, it decides which strategy should be applied, based on the temperature measurements. The fault-tolerant control strategy is threefold. First, the prevention controller closes the loop, so it can steer away from flashback conditions by rejecting disturbances and tracking a desired reference path using the temperature data. Different tuning methodologies based on the traditional proportional and integral (PI), linear-quadratic-integral (LQI) and pole placement regulator (PPR) are explored for this purpose. The robustness of the proposed controller was verified through loop shaping interpretation. However, flashback could be consider as an stochastic process that still could be triggered. Therefore, the second task is to detect present flashback events and apply counteraction to oppose the fault. A design is proposed by the injection of pressurized air upstream of the burner rim to push back the flame by diluting the boundary layer and increasing the flow speed. A proof-of-concept is validated in the experimental set-up as a possible and feasible counteraction. The third task is a final safety measure. A safety switch would be applied to shut off the fuel supply to fail safe after a potential unsuccessful counteraction attempt. The fault-tolerant control framework was simulated in Simulink for a demonstration purpose, where the models were identified from experimental data.

The proposed fault tolerant control framework is intended to increases the flashback resistance of the system by being more robust towards disturbances and flashback events that would normally require a restart of the system. Its potential to do so has been confirmed by simulations. However, further research on the prediction of flashback and controller implementation are still required to run burners on 100% hydrogen without flashback. ... Read more

The energy transition from fossil fuels to renewable energy sources is crucial for climate resilience. In particular, hydrogen combustion in gas turbine combustors is expected to play an essential role in the energy transition. Hence, a fuel-flexible gas turbine combustor operating with a wide range of natural gas and hydrogen fuels is necessary. Low Swirl Burner(LSB), developed at Lawrence Berkeley National Laboratory (LBNL), is one of the promising solutions for fuel-flexible gas turbine combustors. However, hydrogen combustion in LSB at lean conditions is accompanied by several challenges, especially flame flashback. Flashback results in damage and shutdown of gas turbines. Recent experimental studies performed on LSB revealed the occurrence of flame flashback at a high volume percentage of hydrogen, but the exact cause of the flashback in LSB is yet to be discovered.

In this study, numerical modelling of low swirl premixed burner with turbulent flow and high hydrogen content is performed. RANS simulations are performed initially for the non-reacting flow and are validated with the experimental measurements. Flamelet combustion model with Zimont Turbulent Flame speed correlation is employed to model the reacting flow. The flame front position obtained from the Zimont correlation closely agrees with the experiments.
Further, Algebraic Flame Surface Wrinkling (AFSW) correlation is compared with the Zimont correlation for hydrogen-enriched flames. Based on the comparison, it is concluded that the AFSW correlation did not lead to a significant difference. However, both the correlations failed to model the flashback process. Therefore, the potential mechanism responsible for the flame flashback in LSB is examined. It has been found that none of the standard flashback mechanisms is accountable for the flame flashback in LSB. Furthermore, the effect of different flame shapes reported in the experiments is evaluated. It has been observed that ’M’ shape flame has more propensity to flashback than ’V’ shape flame. Then, a shear layer and turbulent wakes in the premix nozzle of the LSB were noticed. Hence, an increase in flame speed due to the local enrichment of thermo-diffusive unstable hydrogen flame and all the aforementioned effects might lead to a sudden flashback in LSB.

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The growing energy demand and climate change poses a need for alternative energy generation in terms of renewable resources. Renewable energy resources are characterised by their intermittent behaviour. A backup power supply is required that can deliver electricity when the supply from the renewables is not sufficient. Gas turbines operating with hydrogen is an attractive option, since hydrogen combustion has zero carbon emissions and can be used as an energy storage when the supply of renewable energy sources are abundant. However, hydrogen combustion poses several challenges. A hydrogen flame has a higher flame temperature than natural gas, leading to more NO𝑥 production. To reduce these NO𝑥 emissions, gas turbines operate in lean premixed conditions. This creates a risk of flame flashback, in particular boundary layer flashback, which can lead to severe damage to the gas turbine.
Recent research performed on boundary layer flashback revealed that two flame configurations, i.e. unconfined and confined, showed fundamentally different flashback phenomena. Unconfined flame flashback refers to the situation where an initially stable flame anchored at the burner rim eventually moves into burner tube. When a flame is partially of completely surrounded by walls and propagates and then starts propagation along the wall, it is called confined flame flashback. Previous studies have focused on one of the two flashback processes at a time. However, the transient flashback process between unconfined and confined flame flashback is not well understood. Research has shown that hydrogen is much more prone to flashback compared to natural gas. This has been attributed to the difference in flame speed between natural gas and hydrogen, but the exact reason for the difference in flashback behaviour between natural gas flames and hydrogen flames has yet to be found.
In this study, a quartz Bunsen burner is used to investigate the flashback phenomena of turbulent premixed hydrogen-natural gas-air flames. To gain more insight in the flashback phenomena, three experiments have been performed. First, flashback maps are obtained to determine the flashback limits of the quartz Bunsen burner. Secondly, the influence of a flame on the flow was investigated using turbulent statistics. Finally, both unconfined and confined flashback are visualised, thereby capturing the transient flashback process between these two configurations. This has been done for a stoichiometric natural gas flame and a lean hydrogen flame. Laser diagnostics like Particle Image Velocimetry (PIV) and Miescattering are used to obtain the turbulent flow statistics and to visualize the instantaneous flashback process. The results show that regions with negative velocity fluctuations in the unburned mixture are the predominant physical mechanism for the start of unconfined flashback and for the transient flashback process of the flame propagating into the burner. However, the start of a flashback event depends on the combination of several parameters: the bulk velocity, the position of the flame front before it interacts with a region with negative velocity fluctuations, the magnitude of the negative velocity fluctuations and whether a region with positive velocity fluctuations is absent after interaction of the flame with the region consisting of negative fluctuations. So, unconfined flashback is rather a statistical phenomena, where the chance of the occurrence of a flashback event is increasing for a decreasing bulk flow velocity. Experiments showed that the transient process between unconfined and confined flashback is very short and fast. After a distance of approximately 5 mm upstream of the burner rim, a backflow region starts to develop in front of the flame, which denotes the start of confined flashback. The time needed for a natural gas flame propagating upstream from the burner rim to reach a confined configuration is approximately 17 ms and only 5.6 ms for the hydrogen flame. The suggested physical mechanisms leading from unconfined flashback to confined flashback are the convex shape of the flame towards the reactants during upstream flame propagation and the reduced cross-sectional flow area of the burned gases at the flame tip. Due to the created backflow in front of the flame and the abovementioned mechanisms, the upstream flame propagation is strongly enhanced, which explains why the flashback propensity for confined flames is much higher than for unconfined flames. The experiments showed that the hydrogen flame propagates closer to the wall than the natural gas flame, indicating higher backpressure effects. The hydrogen flame is thermaldiffusive unstable and the convex shape of the flame tip during flashback strongly enhances the local flame speed and thus the upstream propagation velocity. In contrast, the hydrodynamic instability encountered in the natural gas flame only retards the flow in front of the flame tip, but does not affect the flame speed. This might explain the difference in flashback behaviour between natural gas and hydrogen flames. ... Read more

At present, there is a worldwide awareness and concern about how global warming could impact daily lives and the need to switch to renewable sources of energy. Commercially, various green energy technologies such as hydro, wind, solar, nuclear fission, etc. are being used to harness their stored energy and convert it into electricity. However, amongst these, the natural sources of energy are limited in the operation by their intermittent nature. This potentially leads to the instability of the power grid system. Hence, it is difficult to replace the existing conventional fossil-fired energy generation systems with their renewable counterparts, unless and until, flexibility is introduced to damp the intermittency
of renewable sources of energy.

A Pumped Thermal Energy Storage (PTES) system, which uses a combination of a heat pump and heat engine to store electricity in form of heat and convert the heat back to electricity, could efficiently provide intraday to multiday flexibility. However, the literature reveals that this technology is not mature enough and requires enhanced research and experimental work.

The scope of this thesis includes a conceptual design, steady-state modelling, and optimization of a waste heat integrated PTES, alternatively called Compressed Heat Energy Storage (CHEST) system. Two configurations of the CHEST system were analysed: (i) CHEST with latent heat storage, and (ii) CHEST with sensible heat storage. A combination of R1233zD as a working fluid and sunflower oil
as a sensible thermal energy storage medium was selected and a Phase Change Material (PCM) as a latent heat storage medium. Towards this end, a sensitivity analysis is performed to examine the effect of various design parameters on the performance of the system. It is found that CHEST systems are not suitable for high-temperature storage applications (> 200°𝐶). Further, the effect of superheating in a heat pump, and the effect of waste heat source temperature on system performance is investigated.

An optimization study is conducted on CHEST with a sensible storage configuration to use a single heat exchanger for the charging and discharging cycle during sensible heat transfer with a storage medium. A
detailed comparison between the two configurations have been performed. Most notably a trade-off has to be set between the performance and economic viability of the system.

Finally, the integration of the optimised sensible heat storage configuration with a solar-powered alkaline electrolyser is evaluated. A 30 MW of heat from the electrolyser unit is considered to be recovered with the help of the CHEST system. Towards the end, a preliminary design of the CHEST components (heat exchangers and turbomachinery) is provided to estimate the overall sizing and performance characteristics of the components. ... Read more

The issue of climate change has fostered the innovation of new technology in an attempt to curb the rising emissions, with the energy sector being one of the biggest contributors of carbon emissions. Renewable Energy has shown promise in reducing the emissions by up to 32 %. However the problem of intermittency of
renewable energy sources is still a big reason why they aren’t more mainstream. Therefore, hydrogen has been seen as an energy carrier to provide integration amongst the various sectors of energy generation, transportation, household heating and industrial use. The use of hydrogen as a fuel for re-electrification and power production can be done via IC engines, gas turbines and fuel cells.
This thesis is interested in the possible applications of re-electrification of hydrogen using a system consisting of an SOFC and a gas turbine. It aims to design a small lab scale, test-setup of such a power cycle. The setup
would consist of a small gas turbine and a solid oxide fuel cell.
To that end, this thesis introduces the importance of hydrogen as an energy storage medium followed by the advantages of the SOFC-GT systems in providing higher thermal efficiencies. This is followed by the introduction of the H2-O2 power cycles, namely the Graz and the Toshiba cycles. From previous research done at TU Delft, a basic schematic of the system has been shown, consisting of an SOFC, a combustor, and gas turbine, heat exchangers and an ejector for re-circulative cooling of the SOFC. This is the setup that has to be
designed and so the theory behind the components is presented.
Firstly, the theory behind solid oxide fuel cells, and their modelling is presented. The cooling of the SOFC has been looked into in greater detail. This is followed by the theory of combustion and turbulence modelling.
Two combustor types, a swirl based and a micromix have been considered. Out of these the micromix has been chosen because of its superior mixing capabilities. Finally, the theory behind ejectors, heat exchangers and
sizing of turbomachinery is presented. This concludes the theoretical background of the study. A 0-D model of the SOFC is made based on the Sunfire/Staxera Mk-200 ISM with the 3YSZ KeraCell 11 by Kerafol. This model is then used as a basis of design for the entire thermodynamic H2-O2 cycle. The designed thermodynamic cycle has an overall efficiency of 67.28 %. LHV of hydrogen.
The micromix combustor design is introduced next. A conceptual micromix geometry is presented following which, 2 different configurations are scaled and designed using the theory of jets in cross flow. The two configurations are compared after performing a CFD analysis using the k−ε model with equilibrium chemistry for combustion. The comparison is based on mixing performance, pressure drop and temperature contours. Following the design of the combustor, the supplementary components such as the heat exchangers and the
ejector are designed. The turbine is selected as a small radial turbocharger with a pressure ratio of 8 and is sized using the Balje diagrams. material recommendations are given for the combustor and high- temperature
heat exchangers. The final H2-O2 thermodynamic cycle is presented with the updated component designs. The final thermal efficiency is found to be 64.09 % LHV.
Finally, a basic control scheme is developed for the system and the start-up procedure is briefly discussed. ... Read more

The detrimental influence of nitric oxides (NOx) on humans and the environment has been widely discussed by researchers.
The dominant part of nitic oxides emissions comes from combustion - majority of NOx is being produced by reaction of nitrogen and oxygen at high temperatures.

This thesis project is carried out in cooperation with Siemens Energy, one of the gas turbines manufacturers.
The gas turbine combustor design requires extensive work in many fields: in addition to temperature distribution and flow field prediction, modelling of acoustics and emissions is necessary. As NOx are influenced by many variables, detailed sensitivity analyses of design features is required. Since that gas turbine producers are developing their own tools for emissions predictions.

The aim of this study is improvement and assessment of the existing tool used by Siemens Energy to predict NOx emissions by simulating the combustor system (both the flow and chemical kinetics).
As NOx emissions depend on the air-fuel mixing quality, the tool is equipped with a Monte Carlo-based turbulent mixing model. The combustor flow is divided into small parts - particles, that react and exchange properties between each other according to the Curl's turbulent mixing model.

The tool was improved by adding useful features (such as prescribed unmixedness in the flame front). After that, the Siemens Energy axially staged combustor was modelled (with different equivalence ratios, pressure levels and pilot fuel flows). In the end the tool was validated - the simulation results were compared to the experimental ones.
... Read more