• Energy Research

Abstract To solve the problem of large time shifts between renewable energy supply and user demand, power-to-H2 is a well-known option. In this framework, previous studies have shown that the direct coupling of a photovoltaic array with an electrolyzer stack is a viable solution. However, these studies assumed perfectly known operating parameters to optimize the setup. Moreover, they focused on maximizing hydrogen and minimizing the energy loss, while the cost was not addressed. We have performed an optimization including uncertainty quantification (i.e. surrogate-assisted robust design optimization) for several locations with the Levelized Cost Of Hydrogen (LCOH) as objective. This paper provides the least sensitive design to uncertainties and shows which parameters are most affecting the variability of the LCOH for that design. The robust design optimization illustrates that the mean and standard deviation of the LCOH are non-conflicting objectives for the robust designs of all considered locations. The optimal robust design is established at the considered location with the highest average yearly solar irradiance, achieving a mean LCOH of 6.6 €/kg and a standard deviation of 0.72 €/kg. The discount rate uncertainty is the main contributor to the LCOH variation. Therefore, installing a PV-electrolyzer system in locations with a high average yearly solar irradiation is favorable for both the LCOH mean and standard deviation, while de-risking the technology has the highest impact on further reducing the LCOH variation. Future works will focus on including accurate probability distributions and adding batteries to the system.

While the optimization of the internal combustion engine (ICE) remains a very important topic, alternative fuels are also expected to play a significant role in the reduction of CO2 emissions. High energy densities and handling ease are their main advantages amongst other energy carriers. Ammonia (NH3) additionally contains no carbon and has a worldwide existing transport and storage infrastructure. It could be produced directly from renewable electricity, water and air, and is thus currently considered as a smart energy carrier and combustion fuel. However, ammonia presents a low combustion intensity and the risk of elevated nitrogen-based emissions, thus rendering in-depth investigation of its suitability as an ICE fuel necessary.In the present study, a recent single-cylinder spark-ignition engine is fueled with gaseous ammonia/hydrogen/air mixtures at various hydrogen fractions, equivalence ratios and intake pressures. A small hydrogen fraction is used as combustion promoter and might be generated in-situ through NH3 catalytic or heat-assisted dissociation. The in-cylinder pressure and exhaust concentrations of selected species are recorded and analyzed. Results show that ammonia is a very suitable fuel for SI engine operation, since high power outputs could be achieved with indicated efficiencies higher than 37% by taking advantage of the promoting effects of supercharging and hydrogen enrichment around 10% by volume. High NOx and unburned NH3 exhaust concentrations were also observed under fuel-lean and fuel-rich conditions, respectively. While hydrogen enrichment promotes the NH3 combustion efficiency and helps reducing its exhaust concentration, it has a promoting effect on NOx formation, assumedly due to higher flame temperatures. Therefore, it is recommended to take advantage of the simultaneous presence of exhaust heat, NOx and NH3 in a dedicated after-treatment device to ensure the economic and environmental viability of future ammonia-fueled engine systems.

This study explores the combustion of single iron particles, emphasizing the phenomenon of an unmixed surface during the liquid-phase combustion regime. While previous single-particle combustion experiments have advanced the understanding of the combustion process, the exact configuration of the liquid phases remains unclear. In addition, insights derived from ex situ microstructure analysis are limited by uncertainties introduced during particle cooling, which can alter the internal structure. To address this, we utilized an electrodynamic levitator and laser ignition to study suspended iron particles heated to a high initial temperature (between 1820 K and 2092 K). The combined use of a high-speed color camera and a luminance acquisition system enables high-resolution in situ imaging and luminance tracking. A distinct “unmixed surface period” is observed, during which two immiscible liquid phases – pure iron (L1) and iron oxide (L2) – coexist at the particle surface. Initially, the surface is fully covered with L2, followed by the appearance of moving L1 spots, likely driven by a Marangoni flow. This period concludes with the formation of a core–shell structure. These findings provide new insights into the dynamics of liquid-phase oxidation in iron combustion, as the unmixed surface configuration might influence both the rate-limiting mechanism and evaporation dynamics. Such observations contribute to improving numerical models, particularly in capturing the initial stages of the liquid-state combustion. Moreover, particle size analysis indicates that smaller particles deviate further from a fully external-diffusion-limited regime, underscoring the role of alternative rate-limiting mechanisms in their combustion behavior.

The homogeneous charge compression ignition (HCCI) engine can be run on a large range of fuels if the appropriate operating conditions are chosen. This can improve the efficiency of biofuel production from low-value biomass by suppressing the need for the transformation process to obtain products that are compatible with spark ignition or compression ignition engines. A simple biochemical process that includes acidogenic fermentation and produces a mixture of various esters can take advantage of this flexibility. However, the behavior of this mixture under HCCI conditions needs to be characterized. It can also have a great impact on the HCCI operating limits and its successful implementation. Using an HCCI engine, we investigated how the operating limits are modified by the combustion characteristics of three of these esters: ethyl acetate, ethyl propionate, and ethyl butanoate. This paper reports the experimental results for each of these products and for ethanol taken as the reference fuel. It also analyzes their effects on the ignition timing and the combustion rate. For the selected operating conditions, stable HCCI operations on a large range of equivalence ratios were obtained for every fuel The difference in specific heats of the air/fuel mixtures and in the ignition kinetics both contributed to the ignition characteristics. Ethanol ignites earlier, which leads to a low upper limit, whereas the late ignition of ethyl acetate shifts the operating zone upward due to smoothed high loads but unstable low loads. As a consequence, these low-grade products can be used in an HCCI engine. Fuel blends of these products may take advantage of the different combustion characteristics to extend the HCCI zone. Still, the range of this extension is difficult to estimate and the research of the optimal fuel blend composition will, therefore, remain the focus of future work.

AbstractMicro Gas Turbines (mGT) appear as a promising technology for small-scale (up to 500kW) Combined Heat and Power (CHP) production. However, their rather low electric efficiency limits their profitability when the heat demand decreases. Hot liquid water injection in mGTs –particularly within the micro Humid Air Turbine (mHAT) cycle– allows increasing electric efficiency by making use of the flue gas residual heat in moments of low heat demand.Based on simulations performed on a Turbec T100 mGT –modified to operate as an mHAT– installed at the VUB, this paper presents an analysis of the economic profitability of such facility running on real network conditions. The study is performed assuming typical electricity and heat demand profiles for a domestic consumer. 25 natural gas and electricity price combinations have been taken into consideration, along with two types of domestic customers –with higher and lower heat demands. Results show that the profitability of the mHAT with respect to the equivalent CHP facility increases with higher electricity and lower natural gas prices. In particular, given a certain number of CHP running hours and a natural gas price, there is a threshold for the electricity price above which the net income of the mHAT unit is always higher than that of the corresponding CHP unit. In addition, water-cleaning costs for the mHAT case appear to constitute only 1 to 2.5% of total running costs.

Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21-30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0-30 vol.%) at elevated pressure (1-10 bar) and temperature (298-473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N2H2 formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH2+NH=N2H2+H, NH2+NH2=N2H2+H2, N2H2+H=NNH+H2 and N2H2+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.

Wind and solar energies present a time and space disparity that generally leads to a mismatch between the demand and the supply. To harvest their maximum potentials, one of the main challenges is the storage and transport of these energies. This challenge can be tackled by electrofuels, such as hydrogen, methane, and methanol. They offer three main advantages: compatibility with existing distribution networks or technologies of conversion, economical storage solution for high capacity, and ability to couple sectors (i.e., electricity to transport, to heat, or to industry). However, the level of contribution of electric-energy carriers is unknown. To assess their role in the future, we used whole-energy system modelling (EnergyScope Typical Days) to study the case of Belgium in 2050. This model is multi-energy and multi-sector. It optimises the design of the overall system to minimise its costs and emissions. Such a model relies on many parameters (e.g., price of natural gas, efficiency of heat pump) to represent as closely as possible the future energy system. However, these parameters can be highly uncertain, especially for long-term planning. Consequently, this work uses the polynomial chaos expansion method to integrate a global sensitivity analysis in order to highlight the influence of the parameters on the total cost of the system. The outcome of this analysis points out that, compared to the deterministic cost-optimum situation, the system cost, accounting for uncertainties, becomes higher (+17%) and twice more uncertain at carbon neutrality and that electrofuels are a major contribution to the uncertainty (up to 53% in the variation of the costs) due to their importance in the energy system and their high uncertainties, their higher price, and uncertainty.