Energy Vector Research Articles

The hydrogen storage challenge: Does storage method and size affect the cost and operational flexibility of hydrogen supply chains?

Hydrogen is seen as a key energy vector in future energy systems due to its ability to be stored in large volumes for long periods, providing energy flexibility and security. Despite the importance of storage in hydrogen's potential role in a zero-carbon energy system, many techno-economic analyses fail to adequately model different storage methods in hydrogen supply chains, often ignoring storage requirements altogether. Therefore, this paper uses a data-driven techno-economic analysis (TEA) tool to examine the effect of storage size and cost on three different 2030 hydrogen supply chain scenarios: wind-based, solar-based, and mixed-source grid electrolysis. For varying storage sizes and specific capital costs, the overall levelised cost of hydrogen (LCOH), including production, storage, and delivery to a constant demand, varies significantly. The LCOH ranges from €3.90–12.40/kgH2, €5.50–12.75/kgH2, and €2.80–15.65/kgH2 for the wind-based, solar-based, and mixed-source grid scenarios respectively, with lower values for scenarios with low-cost storage. This highlights the critical role of low-cost hydrogen storage in realising the energy flexibility and security electrolytic hydrogen can provide.

Hydrogenation of liquid organic hydrogen carriers: Process scale-up, economic analysis and optimization

Hydrogen has been proposed as energy vector because it can be produced from renewable energy sources and its use produces no carbon dioxide. Moreover, many industrial processes (e.g., steel industry, ammonia, and fertilizer industry, etc.) are also adopting renewable-produced hydrogen as raw material, replacing traditional fossil sources. In this context, the storage and transportation of hydrogen using liquid organic hydrogen carriers (LOHC) is interesting to deal with the discontinuous production of renewable energies.The work addressed the study of the hydrogenation process for three compounds, 3,5-dibenzyl toluene, 2-methyl indole and N-ethylcarbazole, on Ni and Ru-based catalysts. The reactor consisted of a three-phase slurry stirred tank reactor, which has been scale-up together with the rest of the process equipment units using Aspen Hysys simulation software. The hydrogen storage cost has been determined applying an economic evaluation to the overall process. The most important costs of the process are the reactor cost and the cost of raw materials (the liquid organic hydrogen carrier). The main design variables, plant capacity, reactor temperature and pressure, were optimized to reduce the storage cost. The liquid organic hydrogen carrier with the minimum storage cost at the optimum operating conditions is 3,5-dibenzyl toluene with a reactor configuration of two parallel tanks.

Cation-deficient perovskite Sr1-xTi1-yNiyO3-δ anodes with in-situ exsolution of Ni nanoparticles for direct ammonia solid oxide fuel cells

Ammonia is a carbon-free energy vector and has attracted great attention toward “zero-carbon emission” and “energy N-cycle” in the futuristic energy system. In this work, we applied perovskite SrTiO3 oxides as anode catalysts in NH3-fed solid oxide fuel cells (NH3-SOFCs) and investigated the effects of cation-deficient A sites and Ni-doped B sites on the conductivity and band structure by a series of characterizations. The results showed that appropriate A-site-deficient Sr0.9Ti1-yNiyO3-δ oxides as anodes improve the electrochemical performance in the NH3-SOFCs, while a negative effect was found for the B-site-deficient cases. Instead, the low valence Ni2+ arranged at B sites of Sr0.9TiO3-δ promote the reduction of Ti4+ to Ti3+, generating oxygen vacancies and then facilitating the electron migration in the reaction. Benefited from the ex-solution of Ni nanoparticles (NPs), the optimal powder density of the NH3-SOFC by electrolyte-supported YSZ is obtained over the Sr0.9Ti0.8Ni0.2O3-δ (STN0.2), reaching 287.1 and 262.7 mW·cm−2 in H2 and NH3 at 800 °C, respectively, presenting a high utilization rate of NH3. The activity of STN0.2 is 1.5-fold higher than that of NH3-SOFC adopting NiO/YSZ anode under the same conditions. The excellent electrochemical performance of STN0.2 is attributed to small flat-band potential, conduction band, band gap and high conductivity, resulting in a feasible bound electron transition to free electron. Combining distribution of relaxation time and the Bode plots analysis, we propose the preferential reaction mechanism and rate determining step.

Hybrid biological/inorganic photocathode for H2 production based on a NiFeSe hydrogenase immobilized on electrodeposited CuGaS2

In order to mitigate global warming and pollution problems, new energy vectors need to be developed towards a fossil fuel-free society. Hydrogen is a great candidate for the role, since it can be cleanly obtained from water splitting. However, finding an efficient method that allows diminishing the high potentials required for the reaction is still a challenge to overcome. In order to tackle this task, a combination of a redox biocatalyst and an inorganic semiconductor that harnesses sunlight to enhance the H2 photoelectrochemical production has been studied. For this endeavor, we have developed a photocathode based on the p-type semiconductor copper gallium sulfide and the Desulfovibrio vulgaris Hildenborough NiFeSe-hydrogenase where the photoexcited semiconductor transfers excited electrons to the enzyme upon visible irradiation. The system is capable of producing photoelectrochemical biocatalytic proton reduction to H2 without any sacrificial agent while providing an approximately 400 mV decrease of the applied potential.

Electrofuels in a circular economy: A systems approach towards net zero

Decarbonising the hard-to-abate sectors will be necessary in realising a future net-zero economy. Electrofuels store electricity as low carbon energy vectors such as hydrogen or methane which can be used in areas where electrification is not ideal, and as such can facilitate decarbonisation of sectors such as transport, agriculture, and wastewater treatment. In this study, the production of electrofuels was analysed at an industrial site by storing renewable electricity as green hydrogen produced using electrolysis. The analysis highlighted the need for scale in hydrogen production. The cost of hydrogen was calculated at €8.92/kg when a 122 kW electrolyser operated solely on curtailed electricity generated from the industry site was situated at a 65,000 person equivalent municipal wastewater treatment plant. A subsequent integrated and circular approach to electrofuels production was investigated. The oxygen by-product from electrolysis could be utilised for wastewater aeration and reduce the annual electricity usage at the wastewater treatment plant by 3.6%. Furthermore, the carbon dioxide in biogas generated from sewage sludge could be converted to methane through a Sabatier reaction (4H2 + CO2 → CH4 + 2H2O) as a means of carbon capture and utilisation. The hydrogen produced from the 122 kW electrolyser could convert only 40% of the total carbon dioxide (within the biogas) in a biomethanation process, again supporting the argument for larger electrolyser systems with increased hydrogen production. Pyrolysis of digestate to produce biochar was investigated as a negative emissions technology. If pyrolysis is coupled with anaerobic digestion of feedstocks within 10 km of the industry site savings of 42.7 kt CO2/a could be achieved. In essence, a circular economy approach to electrofuel production could integrate existing electrical, gas and water infrastructure, whilst treating waste, improving the environment, decarbonising agriculture, and storing energy in the form of new low carbon energy vectors for use in heavy transport. Such an approach is vital to progressing future net-zero strategies, however future emissions accountancy processes must adapt to facilitate the benefits of a circular economy.

Health and Safety Workshop Report (2022)

This report summarises the discussions that took place around the topic of “Ammonia as an Energy Vector – Health and Safety” during the Health and Safety Workshop of the 1st Symposium on Ammonia Energy hosted at Cardiff University from the 1st to the 3rd of September 2022. Several presenters from various companies and organizations participated in the discussions, and all provided some insight into the most concerning issues when using ammonia as an energy vector. The workshop, hosted by the Health and Safety Executive (HSE), promised to examine some important questions that need to be answered via R&D to tackle the unknowns of using ammonia in both the energy and transport sectors. The outcome of the workshop is this document, which will be put forward in the following H&S workshops scheduled to be taking place in future sessions of the Symposium, with the aim of coordinating research efforts undertaken by industries, governmental bodies and academic institutions that are targeted at solving the problems related to producing, carrying, distributing and employing ammonia as an energy source.

Maximizing the use of hydrogen as energy vector to cover the final energy demand for stand-alone systems, application and sensitivity analysis for the Canary Archipelago by 2040

Fossil fuel-based economies must undergo a deep transition for complete decarbonization. To this end, it is widely recognized that all economies must move towards the electrification of energy end uses. Even though there is a part that cannot be electrified, at least at affordable costs; clear examples are heavy road transport, maritime and air transport, and some industrial processes. As a result of the limitations of electrification in certain energy end uses, the potential use of hydrogen as an alternative energy carrier has been examined in recent decades. Hydrogen is seen as a viable option for the medium to long term. Specifically, efforts are being made in the case of the Canary Islands to move towards a carbon-free energy mix. In fact, the aim is to advance the economy decarbonization by 10 years over the date foreseen for both Europe and the rest of Spain. To this end, the HOMER program has been used to analyze the possibility of producing the necessary hydrogen in a scenario applied to the Canary Islands in which the uses of this energy vector have been maximized. This research considers two possible scenarios, together with a sensitivity analysis under the different uncertain conditions associated with the used technologies. The results have been the estimation of hydrogen production costs for high demands, about 230,000 tH2/year, in 2040 for an isolated archipelago under two totally different generation scenarios. The first scenario is totally renewable, while the other is based on nuclear generation by means of high-temperature SMRs. The results show, that it would be possible to produce at a cost in the range of 1 €/kgH2 using nuclear technology and around 4 €/kgH2 using renewables, with uncertainty cost ranges of around 40%, i.e. costs between 0.85-1.48 and 3.29–4.99 € kgH2 respectively.

Economic and environmental valuation of green hydrogen decarbonisation process for price responsive multi-energy industry prosumer

Carbon neutrality is one of the main goals in current power system planning and operation. Many different actions and solutions are presented and implemented so far, incorporating renewable energy sources at all levels of the system. With the raising prices of natural gas and its negative impact on the environment, particularly emphasised over the past two years, new technologies and energy vectors are emerging as potential for its replacement. High importance of the hydrogen energy vector, especially in the decarbonisation of the industry sector, is being put forward due to its advantages of zero greenhouse gas emissions, its capacity for storing energy and capability to balance the production of renewable energy sources. This paper brings a detailed mathematical model of a price driven, demand responsive, multi-energy industry facility, as a logic first implementer of hydrogen technologies due to its high and multi-energy consumption nature. The systematic analyses are conducted over a set of scenarios of local production, considering different hydrogen technologies as well as range of natural gas and electricity prices. The findings of the paper conclude that hydrogen technology implementation into a realistic industrial consumers processes results in zero local emissions production, high level of autonomy and resistance from the market disturbances. When compared with classic industrial layouts, the overall CO2 footprint is reduced from around 30% to around 85%, depending on the scenario. The sensitivity analysis has proven that hydrogen layouts are comparable to natural gas layouts in terms of total costs, showing that hydrogen options result in lower cost in the range from 25% to 58%, depending on the observed scenario.

Treatment of Effluent Containing p-Cresol through an Advanced Oxidation Process in a Batch Reactor: Kinetic Optimization

The present research is related to the study of p-cresol oxidation reaction in aqueous phase. Firstly, the conventional advanced oxidation process (AOP) in a lab-scale batch reactor was used, seeking to identify the most impacting process variables and then to propose an optimization approach for ensuring the complete p-cresol degradation and the highest total organic carbon (TOC) conversion. In the AOP with the use of hydrogen peroxide as the oxidizing agent, the oxidation reaction was optimized with the aid of a factorial design, and a maximum TOC conversion of 63% was obtained. The Lumped Kinetic Model (LKM) was used to describe the profile of residual TOC concentration due to chemical species, which were categorized into two groups (refractory and non-refractory compounds). The model was able to satisfactorily describe the profile of the residual fractions of these two classes of organic compounds and allowed estimating the related kinetic constants (k) at two different temperatures, namely (a) 3.19 × 10−1 and 2.82 × 10−3 min−1 for non-refractory and refractory compounds at 80 °C and (b) 4.73 × 10−1 and 5.09 × 10−3 min−1 for the same compound classes at 90 °C, while the activation energy (Ea) of the process was 42.02 and 62.09 kJ mol−1, respectively. The kinetic modeling of organic pollutants oxidation in liquid effluents would allow to perform in situ seawater treatment on vertical reactors installed in offshore platforms and to properly release treated water into the oceans. In this way, ocean contamination caused by the exploration on offshore platforms of oil and natural gas, the main energy sources and vectors in the current world, may be remarkably reduced, thus favoring a more eco-friendly energy production.

Enabling hydrate-based methane storage under mild operating conditions by periodic mesoporous organosilica nanotubes

Biomethane is a renewable natural gas substitute produced from biogas. Storage of this sustainable energy vector in confined clathrate hydrates, encapsulated in the pores of a host material, is a highly promising avenue to improve storage capacity and energy efficiency. Herein, a new type of periodic mesoporous organosilica (PMO) nanotubes, referred to as hollow ring PMO (HR-PMO), capable of promoting methane clathrate hydrate formation under mild working conditions (273 K, 3.5 MPa) and at high water loading (5.1 g water/g HR-PMO) is reported. Gravimetric uptake measurements reveal a steep single-stepped isotherm and a noticeably high methane storage capacity (0.55 g methane/g HR-PMO; 0.11 g methane/g water at 3.5 MPa). The large working capacity throughout consecutive pressure-induced clathrate hydrate formation-dissociation cycles demonstrates the material’s excellent recyclability (97% preservation of capacity). Supported by ex situ cryo-electron tomography and x-ray diffraction, HR-PMO nanotubes are hypothesized to promote clathrate hydrate nucleation and growth by distribution and confinement of water in the mesopores of their outer wall, along the central channels of the nanotubes and on the external nanotube surface. These findings showcase the potential for application of organosilica materials with hierarchical and interconnected pore systems for pressure-based storage of biomethane in confined clathrate hydrates.

Numerical study on the use of ammonia/hydrogen fuel blends for automotive spark-ignition engines

The importance of new alternative fuels has assumed great relevance in the last decades to face the issues of global warming and pollutant emissions from energy production. The scientific community is responsible for developing solutions to achieve the necessary environmental restriction policies. In this context, ammonia appears as a potential fuel candidate and energy vector that may solve the technological difficulties of using hydrogen (H2) directly in internal combustion engines. Its high hydrogen content per unit mass, higher energy density than liquid hydrogen, well-developed infrastructure and experience in handling and storage make it suitable to be implemented as a long-term solution. In this work, a virtual engine model was developed to perform prospective simulations of different operating conditions using ammonia and H2-enriched ammonia as fuel in a spark-ignition (SI) engine, integrating a chemical kinetics model and empirical correlations for combustion prediction. In addition, specific conditions were evaluated to consider and to understand the governing parameters of ammonia combustion using computational fluid dynamics (CFD) simulations. Results revealed similar thermal efficiency than methane fuel, with considerable improvements after appropriate H2-enrichment. Moreover, increasing the intake temperature and the turbulence intensity inside the cylinder evinced significant reductions in combustion duration. Finally, higher compression ratios ensure efficiency gains with no evidence of abnormal combustion (knocking), even at high compression ratios (above 16:1) and low engine speeds (800 rpm). Numerical simulations showed the direct influence of the flame front surface area and the turbulent combustion velocity on efficiency, reflecting the need for optimizing the SI engines design paradigm for ammonia applications.

Pathways towards Achieving High Current Density Water Electrolysis: from Material Perspective to System Configuration.

Hydrogen is a clean, flexible, powerful energy vector that can be leveraged as a promising alternative to fossil fuels. Additionally, green hydrogen production has been recognized as one of the most prevalent solutions to decarbonize the energy system. Water electrolysis studies have increased throughout the decade as higher industrial interest comes into play. The catalyst, system design, and configuration act in a congenial manner to deliver high-performing water electrolysis. Despite performance targets peaking at high current densities, the current status of water electrolyzer technologies would require more research efforts to achieve such goals. This work presents a comprehensive review of how catalysts and electrolyzer designs can be enhanced to attain high current density water electrolysis. Modification strategies of catalysts, advances in characterization and modelling, and optimizing system designs are highlighted. Furthermore, this paper aims to elucidate the future research direction of water electrolysis to bridge the laboratory-to-industry gap.

Economic and environmental optimal design of a multi-vector energy hub feeding a Local Energy Community

One of the main ways to reach the important targets of the European Commission in the energy field set by 2030 is to implement sustainable energy systems to cover electrical and thermal energy demands, as well as transportation needs, in urban areas. In the aforesaid scenario, multi-vector energy hubs consist of installing small and medium size power plants, mainly fed by renewable sources, and charging infrastructures for electric vehicles in sustainable urban districts. The optimal design of such systems can be faced from different points of view and using different methods, one of the most adopted being the development of mathematical programming algorithms typically aimed to minimize global costs and emissions. The contribution of this paper is to propose an innovative methodology to optimally design a multi-vector energy hub able to supply electricity and thermal energy, for space heating and cooling, to a set of buildings within a sustainable district acting as a local energy community. The developed mixed integer linear programming model is characterized by very detailed models of the technologies and by a multi-objective function which is proposed to evaluate the impact of the optimal solution on global costs and emissions. The proposed mathematical model is very comprehensive as it involves a wide range of energy technologies with detailed modelling by considering the impact of external ambient conditions and energy vector temperature values on the plant conversion efficiency, thus showing high replicability potential in real contexts. In the case study, the model is applied to a university campus in the North of Italy.

CONCENTRAÇÃO REGIONAL DA INDÚSTRIA DA CERÂMICA VERMELHA NA PARAÍBA, BRASIL (2006 – 2020)

This work analyzed the regional concentration of the red ceramic sector, from 2006 to 2020. It carried out a statewide analysis of the sector's conjuncture, its relation to firewood consumption and the regional concentration was determined through the indicators: Concentration Ratio CR(k ), Herfindahl-Hirschman Index (HHI), Theil entropy (E) and the Gini coefficient (G). The results showed that Borborema has the highest number of red ceramic companies when compared to the other mesoregions of the state. The CR(k) rated with moderately low concentration. The HHI for municipalities and microregions showed low concentration. The municipal, micro-regional and meso-regional E indices of Paraíba indicated deconcentration. G showed weak to average inequality for municipalities and microregions and weak inequality for mesoregions. Thus, it was concluded that the ceramic sector in Paraíba is not concentrated and that the supply is associated with raw material stocks and the energy vector of firewood.

Self-recuperative liquid air energy storage system: A new sustainable approach for uninterrupted power supply

Liquid air energy storage (LAES) system is an emerging but promising candidate solution to the intermittency and weather/climate dependability issues of renewable energy. It is also envisioned as an energy vector for its multi-faceted potential applications, especially in the energy supply chain systems, and its ability to satisfy off-terrain demands. During low energy demand, the air is liquefied and later regasified to generate electricity when needed. However, the low round-trip efficiency of the conventional approaches (∼50%) is a challenge. The present study proposes a novel self-recuperative process whereby stored liquid air is used as a refrigerant instead of external mechanisms. Four slightly modifying configurations have been studied; the best case recuperates with the cold box and the compression heat during the discharging half cycle before power recovery. The case studies are simulated using Aspen HYSYS®. This excellent integration results in a round-trip efficiency of 75.8% and a specific energy consumption of 0.071 kWh/kgLA. The proposed process consumes 39.8 MW power, 58% less than the base case, resulting in 0.10 $/ton of levelized cost (production). The scheme opens up multiple pathways for liquid air, from further integration opportunities to its service in a continuous power generation mode as a transport medium and energy source in remote/off-grid locations. However, a dynamic and control strategy study should be conducted.

A comprehensive multi-node multi-vector multi-sector modelling framework to investigate integrated energy systems and assess decarbonisation needs

The pathway towards decarbonised energy systems involves massive changes in adopted energy vectors, installed technologies, networks roles, and interaction capabilities. To investigate the combination of these effects, this work presents the OMNI-ES modelling framework (Optimisation Model for Network-Integrated Energy Systems), which offers a comprehensive approach to analyse multi-node, multi-vector, multi-sector energy systems. It adopts a detailed temporal and spatial resolution and implements multiple conversion options between energy vectors (electricity, hydrogen, natural gas, biomethane, biofuels, e-fuels, …). The formulation solves the energy vector balances at each time step, taking into account sources, sinks, conversion processes, and storage systems. CO2 flows are also tracked, allowing the introduction of CO2 emission constraints that account for all contributions (fossil and biogenic, direct and indirect) and mitigation measures (capture, re-use, sequestration). In the article, OMNI-ES is applied to investigate an Italian scenario for 2050, adopting a regional (NUTS-2) resolution. The model output yields the cost-optimal energy system configuration that is capable to support the demand with net-zero CO2 emissions. Results show that the need for CO2 balance closure calls in several technologies, including massive renewable power generation (up to 20 times today’s capacities), storage systems (batteries, hydrogen, pumped hydro), biogenic sources (residual biomass and biomethane), and CO2 capture (both on fossil and biogenic sources). Networks emerge as critical elements, as the need to transport energy vectors saturates the expected capacities of grid infrastructures, especially in the case of hydrogen.

A novel multi-level predictive management strategy to optimize phase-change energy storage and building-integrated renewable technologies operation under dynamic tariffs

In the evolving energy sector, where buildings are recognized as dynamic components of energy networks and smart grids, the implementation of new regulations and guidelines is crucial to optimize the interaction between buildings and the grid. The imperative to reduce building energy consumption facilitates the promotion of new technologies that rely on renewable energy generation and innovative materials. As technology has progressed, the multitude of energy vectors involved has made controlling energy systems increasingly challenging. This poses a barrier to the widespread adoption and implementation of cutting-edge technologies. In this framework, this paper explores an energy-efficient solution using an integrated photovoltaic/thermal collector and an active phase-change material storage system. The study optimizes the integration of technologies through a resistance capacitance model, assessing the impact on thermal comfort, energy savings and costs. A novel cascade methodology, combining particle swarm optimization search with model predictive control, is designed to select the optimal mode of operation for the proposed technologies. The economic feasibility of the proposed system is analyzed across different tariff structures, whereas the interaction with the grid is evaluated using energy flexibility key performance indicators. The energy and economic performance, as well as the flexibility of the system are assessed through a proof-of-concept conducted in an office building scenario. The results demonstrate an increase in energy efficiency, with savings ranging from 9% to 28% compared to a suitable baseline scenario, and a significant energy shift from on-peak to off-peak periods, potentially accounting for up to 46% of the total building load. This energy flexibility enables the grid to receive reduced demand during morning hours.

Thermoeconomic and life cycle assessments for a trigeneration system: A case study

ABSTRACT The current need to diversify the global energy matrix implies introducing renewable energy sources and evaluating commercially available solutions. This study assesses a trigeneration system from thermoeconomic and environmental viewpoints, employing the Theory of Exergy Cost and Life Cycle Assessment. The location of the theoretical study case is the state of Pernambuco, Northeast Brazil. The trigeneration system operates on biogas and is comprised of four subsystems: biogas purification, molted carbon fuel cell, combined cycle, and absorption refrigeration system. The energy services produced are electricity, heat (hot water), and cooling. The results of the thermoeconomic analysis revealed that the energy system requires an electricity tariff of at least US$ 106.76/MWh to generate revenue due to the high costs associated with the purchase of biogas and capital costs of the equipment for biogas purification and fuel cell. The life cycle assessment identified that the biogas purification step presented the highest environmental impacts due to the material composition of the absorption and regeneration columns, which is interesting as this is the most critical step of the process. LCA results indicate that although methane is a vital energy vector for energy transition, further studies are required on the technical and environmental aspects of its use in energy systems.

Sustainable Additives for the Production of Hydrogen via Sodium Borohydride Hydrolysis

Finding stable solutions for hydrogen storage is one of the main challenges to boosting its deployment as an energy vector and contributing to the decarbonization of the energy sector. In this context, sodium borohydride (NaBH4) has been largely studied as a hydrogen storage material due to its significant advantages, such as low pressure, stability, and high hydrogen storage density. The development of catalysts and additive materials for the on-demand hydrolysis of NaBH4 for hydrogen release is a key research area. This work studies the effects of non-toxic and environmentally friendly additives for the hydrolysis process in terms of yield, lag time, hydrogen generation rate, and gravimetric density. Specifically, four additives, including sodium carboxymethylcellulose (CMC), polyacrylamide (PAM), sodium dodecyl sulfate (SDS), and β-cyclodextrin (BCD), were studied for their application in the storage and release of hydrogen. The best results were provided by the use of sodium carboxymethyl cellulose and polyacrylamide. In the first case, a hydrolysis yield of 85%, a lag time of 70 s, a hydrogen production rate of 1374 mL·min−1·gcat−1, and a storage capacity of 1.8 wt% were obtained. Using polyacrylamide as additive, a hydrolysis yield of almost 100% was achieved, although it required a significantly higher time period for complete conversion.

Technoeconomic evaluation of offshore green ammonia production using tidal and wind energy: a case study

ABSTRACT The climate crisis is driving policy makers, energy companies and chemical industries to accelerate the transition from conventional high-carbon ammonia to low-carbon and zero-carbon (green) ammonia. Ammonia is produced on a vast scale globally and is predominantly used for fertilizers but has a potential use as an energy vector. Utilizing variable renewable energy (typically solar or wind) for green ammonia production requires expensive energy storage. Tidal stream energy is a novel energy source for green ammonia production and could be an ideal energy source given its high resource availability and predictability. The objective of this paper is to determine if tidal stream energy is a useful energy resource (technically and economically) for offshore green ammonia production. This idea is presented through a case study on the Pentland Firth, a globally excellent tidal stream location. Using 30 years of hourly wind and tidal data, the addition of tidal stream capacity to wind capacity is shown to decrease the hydrogen storage requirement by 96% and eliminate the fuel cell requirement and thus reduce the levelized cost of ammonia (LCOA) by 12%.