Efficient utilization of waste heat from molten carbonate fuel cell in parabolic trough power plant for electricity and hydrogen coproduction

Direct steam generating parabolic trough power plant is an important technology to match future electric energy demand. One of the problems related to its emergence is energy storage. Solar-to-hydrogen is a promising technology for solar energy storage. Electrolysis is among the most processes of hydrogen production recently investigated. High temperature steam electrolysis is a clean process to efficiently produce hydrogen. In this paper, steam electrolysis process using solar energy is used to produce hydrogen. A heat recovery steam generator generates high temperature steam thanks to the molten carbonate fuel cell's waste heat. The analytical study investigates the energy efficiency of solar power plant, molten carbonate fuel cell and electrolyser. The impact of waste heat utilization on electricity and hydrogen generation is analysed. The results of calculations done with MATLAB software show that fuel cell produces 7.73 MWth of thermal energy at design conditions. 73.37 tonnes of hydrogen and 14.26 GWh of electricity are yearly produced. The annual energy efficiency of electrolyser is 70% while the annual mean electric efficiency of solar power plant is 18.30%.The proposed configuration based on the yearly electricity production and hydrogen generation has presented a good performance.     

Parameter study for dimensioning of a PV optimized hydrogen supply plant

Green hydrogen produced from intermittent renewable energy sources is a key component on the way to a carbon neutral planet. In order to achieve the most sustainable, efficient and cost-effective solutions, it is necessary to match the dimensioning of the renewable energy source, the capacity of the hydrogen production and the size of the hydrogen storage to the hydrogen demand of the application.For optimized dimensioning of a PV powered hydrogen production system, fulfilling a specific hydrogen demand, a detailed plant simulation model has been developed. In this study the model was used to conduct a parameter study to optimize a plant that should serve 5 hydrogen fuel cell buses with a daily hydrogen demand of 90 kg overall with photovoltaics (PV) as renewable energy source. Furthermore, the influence of the parameters PV system size, electrolyser capacity and hydrogen storage size on the hydrogen production costs and other key indicators is investigated. The plant primarily uses the PV produced energy but can also use grid energy for production.The results show that the most cost-efficient design primarily depends on the grid electricity price that is available to supplement the PV system if necessary. Higher grid electricity prices make it economically sensible to invest into higher hydrogen production and storage capacity. For a grid electricity price of 200 €/MWh the most cost-efficient design was found to be a plant with a 2000 kWp PV system, an electrolyser with 360 kW capacity and a hydrogen storage of 575 kg.     

Desalinated water and hydrogen generation from seawater via a desalination unit and a low temperature electrolysis using a novel solar-based setup

This study investigates the proficiency of employing solar energy in a novel setup geared towards simultaneous production of desalinated water and hydrogen wielding parabolic trough solar collectors (prime mover) in three solar radiation approaches; low radiation, high irradiation and no radiation. Targeted for coastal areas, this setup generates electricity using an organic Rankine cycle; utilizing its waste heat, a desalination unit applying humidification and dehumidification processes, yields desalinated water. Subsequently, hydrogen is produced through exploiting a proton exchange membrane electrolyser as a low temperature electrolyser fed by electricity and water. One of the cardinal points of this system is the production of hydrogen by means of electricity and desalinated water obtained from previous stages. With the purpose of determining the efficiency of this setup, a parametric study has been conducted grounded on the effect of important parameters on production rates and different efficiencies. Ensuing, multi-objective optimization is set forth by implementing a genetic algorithm in order to effectuate the optimal design state. The results indicated that the desalination rate in the three solar radiation approaches mentioned are 1.76 kg/s, 1.07 kg/s and 1.36 kg/s, respectively, and the hydrogen production rate are 4.33 g/s, 2.62 g/s and 3.54 g/s, correspondingly.     

System-level power-to-gas energy storage for high penetrations of variable renewables

According to outlooks by the IEA and the U.S. EIA, renewables will become the largest source of electricity by 2050 if global temperature rise is to be limited to 2 °C. However, at penetrations greater than 30%, curtailment of wind and solar can be significant in even the most flexible systems. Energy storage can reduce curtailment and increase utilisation of variable renewables. Power-to-gas is a form of long-term storage based on electrolytic production of hydrogen. This research models the co-sizing of wind and solar PV capacity and electrolyser capacity in a jurisdiction targeting 80% penetration of variable renewable electricity. Results indicate that power-to-gas can reduce required wind and solar capacity by as much as 23% and curtailment by as much as 87%. While the majority of charging events last less than 12 h, the majority of the total annual stored energy comes from longer-term events. Additional scenarios reveal that geographic diversity of wind farms reduces capacity requirements, but the same benefit is not found for distributing solar PV.     

Model development and analysis of a novel high-temperature electrolyser for gas phase electrolysis of hydrogen chloride for hydrogen production

In this study, a high temperature electrolyser for the gas phase electrolysis of hydrogen chloride for hydrogen production is proposed and assessed. A detailed electrochemical model is developed to study the J-E characteristics for the proposed electrolyser (a solid oxide electrolyser based on a proton conducting electrolyte). The developed model accounts for all major losses, namely activation, concentration and ohmic. Energy and exergy analyses are carried out, and the energy and exergy efficiencies of the proposed electrolyser are determined to be 41.1% and 39.0%, respectively. The simulation results show that at T = 1073 K, P = 100.325 kPa and J = 1000 A/m², 1.6 V is required to produce 1 mol of hydrogen. This is approximately 0.3 V less than the voltage required by a high temperature steam electrolyser (based on a proton conducting electrolyte) operating at same condition (T = 1073 K, P = 101.325 kPa and J = 1000 A/m²), suggesting that the proposed electrolyser offers a new option for high temperature electrolysis for hydrogen production, potentially with a low electrical energy requirement. The proposed electrolyser may be incorporated into thermochemical cycles for hydrogen production, like CuCl or chlorine cycles.     

Thermoeconomic analysis and artificial neural network based genetic algorithm optimization of geothermal and solar energy assisted hydrogen and power generation

The study aims to optimize the geothermal and solar-assisted sustainable energy and hydrogen production system by considering the genetic algorithm. The study will be useful by integrating hydrogen as an energy storage unit to bring sustainability to smart grid systems. Using the Artificial Neural Network (ANN) based Genetic Algorithm (GA) optimization technique in the study will ensure that the system is constantly studied in the most suitable under different climatic and operating conditions, including unit product cost and the plant's power output. The water temperature of the Afyon Geothermal Power Plant varies between 70 and 130 °C, and its mass flow rate varies between 70 and 150 kg/s. In addition, the solar radiation varies between 300 and 1000 W/m² for different periods. The net power generated from the region's geothermal and solar energy-supported system is calculated as 2900 kW. If all of this produced power is used for hydrogen production in the electrolysis unit, 0.0185 kg/s hydrogen can be produced. The results indicated that the overall energy and exergy efficiencies of the integrated system are 4.97% and 16.0%, respectively. The cost of electricity generated in the combined geothermal and solar power plant is 0.027 $/kWh if the electricity is directly supplied to the grid and used. The optimized cost of hydrogen produced using the electricity produced in geothermal and solar power plants in the electrolysis unit is calculated as 1.576 $/kg H₂. The optimized unit cost of electricity produced due to hydrogen in the fuel cell is calculated as 0.091 $/kWh.     

A concept of combined cooling,heating and power system utilising solar power and based on reversible solid oxide fuel cell and metal hydrides

In energy systems, multi-generation including co-generation and tri-generation has gained tremendous interest in the recent years as an effective way of waste heat recovery. Solid oxide fuel cells are efficient power plants that not only generate electricity with high energy efficiency but also produce high quality waste heat that can be further used for hot and chilled water production. In this work, we present a concept of combined cooling, heating and power (CCHP) energy system which uses solar power as a primary energy source and utilizes a reversible solid oxide fuel cell (R-SOFC) for producing hydrogen and generating electricity in the electrolyser (SOEC) and fuel cell (SOFC) modes, respectively. The system uses “high temperature” metal hydride (MH) for storage of both hydrogen and heat, as well as “low temperature” MH's for the additional heat management, including hot water supply, residential heating during winter time, or cooling/air conditioning during summer time.The work presents evaluation of energy balances of the system components, as well as heat-and-mass transfer modelling of MH beds in metal hydride hydrogen and heat storage system (MHHS; MgH₂), MH hydrogen compressor (MHHC; AB₅; A = La + Mm, BNi + Co + Al + Mn) and MH heat pump (MHHP; AB₂; A = Ti + Zr, BMn + Cr + Ni + Fe). A case study of a 3 kWe R-SOFC is analysed and discussed. The results showed that the energy efficiencies are 69.4 and 72.4% in electrolyser and fuel cell modes, respectively. The round-trip COP's of metal hydride heat management system (MHHC + MHHP) are close to 40% for both heating and cooling outputs. Moreover, the tri-generation leads to an improvement of 36% in round-trip energy efficiency as compared to that of a stand-alone R-SOFC.     

Thermodynamic development and design of a concentrating solar thermochemical water-splitting process for co-production of hydrogen and electricity

A concentrating solar plant is proposed for a thermochemical water-splitting process with excess heat used for electricity generation in an organic Rankine cycle. The quasi-steady state thermodynamic model consisting of 23 components and 45 states uses adjustable design parameters to optimize hydrogen production and system efficiency. The plant design and associated thermodynamic model demonstrate that cerium oxide is suitable for thermochemical water-splitting cycles involving the co-production of hydrogen and electricity. Design point analyses at 900 W/m² DNI indicate that a single tower with solar radiation input of 27.74 MW and an aperture area of 9.424 m² yields 10.96 MW total output comprised of 5.55 MW hydrogen (Gibbs free energy) and 5.41 MW net electricity after subtracting off 22.0% of total power generation for auxiliary loads. Pure hydrogen output amounts to 522 tonne/year at 20.73 GWh/year (HHV) or 17.20 GWh/year (Gibbs free energy) with net electricity generation at 14.52 GWh/year using TMY3 data from Daggett, California, USA. Annual average system efficiency is 38.2% with the constituent hydrogen fraction and electrical fraction being 54.2% and 45.8%, respectively. Sensitivity analyses illustrate that increases in particle loop recuperator effectiveness create an increase in hydrogen production and a decrease in electricity generation. Further, recuperator effectiveness has a measurable effect on hydrogen production, but has limited impact on total system efficiency given that 81.1% of excess heat is recuperated within the system for electricity generation.     

Design,modelling and optimal power and hydrogen management strategy of an off grid PV system for hydrogen production using methanol electrolysis

Hydrogen used as an energy carrier and chemical element can be produced by several processes such as gasification of coal and biomass, steam reforming of fossil fuel and electrolysis of water. Each of these methods has its own advantage and disadvantage. Electrolysis process is seen as the best option for quick hydrogen production. Hydrogen generation by methanol electrolysis process (MEP) gained much attention since it guarantees high purity gas and can be compatible with renewable energies. Furthermore, due to its very low theoretical potential (0.02 V), MEP can save more than 65% of electrical energy required to produce 1 kg of hydrogen compared to water electrolysis process (WEP). Electrolytic hydrogen production using solar photovoltaic (PV) energy is positioned to become as one of the preferred options due to the harmful environmental impacts of widely used methane steam reforming process and also since the prices of PV modules are more competitive.In this paper, hydrogen production by MEP using PV energy is investigated. A design of an off grid PV/battery/MethElec system is proposed. Mathematical models of each component of the system are presented. Semi-empirical relationship between hydrogen production rate and power consumption at 80 °C and 4 M concentration is developed. Optimal power and hydrogen management strategy (PHMS) is designed to achieve high system efficiency and safe operation. Case studies are carried out on two tilts of PV array: horizontal and tilted at 36° using measured meteorological data of solar irradiation and ambient temperature of Algiers site. Simulation results reveal great opportunities of hydrogen production using MEP compared to the WEP with 22.36 g/m² d and 24.38 g/m² d of hydrogen when using system with horizontal and tilted PV array position, respectively.     

Comparative performance analysis of a grid connected PV system for hydrogen production using PEM water,methanol and hybrid sulfur electrolysis

This paper presents comparative performance analysis of photovoltaic (PV) hydrogen production using water, methanol and hybrid sulfur (SO₂) electrolysis processes. Proton exchange membrane (PEM) electrolysers are powered by grid connected PV system. In this system design, electrical grid is considered as a virtual energy storage system (VESS) where the surplus of PV production can be injected and subsequently taken to support the electrolyser. Methanol (ME) and hybrid sulfur (HSE) electrolysis are compared to the conventional water electrolysis (WE) in term of operating cell voltage. Based on the experimental results reported in the literature, semi-empirical models describing the relationship between the hydrogen production rate and the electrolyser cell power input are proposed. Furthermore, power and hydrogen management strategy (PHMS) is developed. Case study is carried out to show the impact of each type of electrolysis on the system component sizes and evaluate the hydrogen production potentialities. Results show that the use of ME allows to produce 65% more hydrogen than with using WE. Moreover, the amount of hydrogen produced is almost double in the case of HSE. At Algiers city, based on a grid connected PV/Electrolyser system, it is possible to produce about 25 g/m² d and 29 g/m² d of hydrogen, respectively, through ME and HSE compared to 15 g/m² d of hydrogen when using WE.     

Design and analysis of a solar tower power plant integrated with thermal energy storage system for cogeneration

In the current study, a solar tower–based energy system integrated with a thermal energy storage option is offered to supply both the electricity and freshwater through distillation and reverse osmosis technologies. A high‐temperature thermal energy storage subsystem using molten salt is considered for the effective and efficient operation of the integrated system. The molten salt is heated up to 565°C through passing the solar tower. The thermal energy storage tanks are designed to store heat up to 12 hours. The temperature variations in the storage tanks are studied and compared accordingly for evaluation. The effect of operating temperatures on the freshwater production and overall system efficiency is determined. About 24.46 MW electricity is generated in the steam turbine under sunny conditions. Furthermore, the storage subsystem stores heat during sunny hours to utilize later in cloudy hours and night time. The produced power decreases to 20.17 MW in discharging hours due to temperature decrease in the tank. The electricity generated by the system is then used to produce freshwater through the reverse osmosis units and also to supply electricity for the residential use. A total flowrate of 240.02 kg/s freshwater is obtained by distillation and reverse osmosis subsystems.     

Simulation-based sensitivity analysis of an on-site hydrogen production unit in Hungary

This article examines the additional profit that can be achieved with the integrated operation of an on-site electrolyser, a hydrogen tank, a photovoltaic system, and a wind power plant based on Hungarian data from 2019. The results of the optimisation show that the system economically reduces the volatility of weather-dependent renewable production, so there is a promising demand-side management potential in coordination. We found that the operating profit is highest in April at EUR 19,416, 18,932 in July, and lowest at EUR 17,075 in January. The production curve of photovoltaic capacities is better matched to fuel demand, so increasing the share of solar energy results in lower balancing activity but higher profits. Increasing the size of the hydrogen storage and electrolyser, with constant hydrogen demand and prices, will cause a convergent increase in profits, however above a 10 kg storage capacity or 350 kW electrolyser capacity there is no substantial profit increase. In the case of the economically optimal asset size, there is a slight competition between the electricity market and the hydrogen distribution activity. The choice between the two activities depends on current electricity and hydrogen prices and the cost of unmet hydrogen demand.     

Techno-economic assessment of hydrogen production from seawater

Population growth and the expansion of industries have increased energy demand and the use of fossil fuels as an energy source, resulting in release of greenhouse gases (GHG) and increased air pollution. Countries are therefore looking for alternatives to fossil fuels for energy generation. Using hydrogen as an energy carrier is one of the most promising alternatives to replace fossil fuels in electricity generation. It is therefore essential to know how hydrogen is produced. Hydrogen can be produced by splitting the water molecules in an electrolyser, using the abondand water resources, which are covering around ? of the Earth's surface. Electrolysers, however, require high-quality water, with conductivity in the range of 0.1–1 μS/cm. In January 2018, there were 184 offshore oil and gas rigs in the North Sea which may be excellent sites for hydrogen production from seawater. The hydrogen production process reported in this paper is based on a proton exchange membrane (PEM) electrolyser with an input flow rate of 300 L/h. A financially optimal system for producing demineralized water from seawater, with conductivity in the range of 0.1–1 μS/cm as the input for electrolyser, by WAVE (Water Application Value Engine) design software was studied. The costs of producing hydrogen using the optimised system was calculated to be US$3.51/kg H₂. The best option for low-cost power generation, using renewable resources such as photovoltaic (PV) devices, wind turbines, as well as electricity from the grid was assessed, considering the location of the case considered. All calculations were based on assumption of existing cable from the grid to the offshore, meaning that the cost of cables and distribution infrastructure were not considered. Models were created using HOMER Pro (Hybrid Optimisation of Multiple Energy Resources) software to optimise the microgrids and the distributed energy resources, under the assumption of a nominal discount rate, inflation rate, project lifetime, and CO₂ tax in Norway. Eight different scenarios were examined using HOMER Pro, and the main findings being as follows:The cost of producing water with quality required by the electrolyser is low, compared with the cost of electricity for operation of the electrolyser, and therefore has little effect on the total cost of hydrogen production (less than 1%).The optimal solution was shown to be electricity from the grid, which has the lowest levelised cost of energy (LCOE) of the options considered. The hydrogen production cost using electricity from the grid was about US$ 5/kg H₂.Grid based electricity resulted in the lowest hydrogen production cost, even when costs for CO₂ emissions in Norway, that will start to apply in 2025 was considered, being approximately US$7.7/kg H₂.From economical point of view, wind energy was found to be a more economical than solar.     

Design and simulation of a solar-hydrogen system for different situations

In recent years, hybrid photovoltaic–fuel cell energy systems have been popular as energy production systems for different applications. A typical solar-hydrogen system can be modeled the electricity supplied by PV panels is used to meet the demand directly to the maximum extent possible. If there is any surplus PV power over demand, and capacity left in the tank for accommodating additional hydrogen, this surplus power is supplied to the electrolyser to produce hydrogen for storage. When the output of the PV array is not sufficient to supply the demand, the fuel cell draws on hydrogen from storage and produces electricity to meet the supply deficit.     

Development of an ambient temperature alkaline electrolyser for dynamic operation with renewable energy sources

A comparison is made between the ambient and conventional temperature alkaline electrolysers in terms of operational system, voltage efficiency and corrosion rates. The capital, operational and maintenance costs are reduced by reducing auxiliary equipment as well as auxiliary utilities in the ambient temperature alkaline electrolyser. Also, since auxiliary electricity consumption is reduced, the alkaline electrolyser is capable for dynamic, continuous and fast-response operation with renewable energy sources. The ambient temperature alkaline electrolyser is capable for wider operational range and faster response time when powered by wind energy sources. Although the voltage efficiency for hydrogen production is increased by about 12% at the conventional operating temperature, corrosion rate of the electrode is increased by a factor of about 6.3. The voltage efficiency for hydrogen production, however, is increased by about 12% by employing electrocatalyst in the ambient temperature alkaline electrolyser, and there is benefit of enhancing lifetime durability of the electrode as well as cell components at relatively lower operating temperature.     

Flexible production of green hydrogen and ammonia from variable solar and wind energy: Case study of Chile and Argentina

We report a techno-economic modelling for the flexible production of hydrogen and ammonia from water and optimally combined solar and wind energy. We use hourly data in four locations with world-class solar in the Atacama desert and wind in Patagonia steppes. We find that hybridization of wind and solar can reduce hydrogen production costs by a few percents, when the effect of increasing the load factor on the electrolyser overweighs the electricity cost increase. For ammonia production, the gains by hybridization can be substantially larger, because it reduces the power variability, which is costly, due to the need for intermediate storage of hydrogen between the flexible electrolyser and the less flexible ammonia synthesis unit. Our modelling reveals the crucial role in the synthesis of flexibility, which cuts the cost of variability, especially for the more variable wind power. Our estimated near-term production costs for green hydrogen, around 2 USD/kg, and green ammonia, below 500 USD/t, are encouragingly close to competitiveness against fossil-fuel alternatives.     

Sizing and operating power-to-gas systems to absorb excess renewable electricity

Various configurations of power-to-gas system are investigated as a means for capturing excess wind power in the Emden region of Germany and transferring it to the natural gas grid or local biogas-CHP plant. Consideration is given to producing and injecting low concentration hydrogen admixtures, synthetic methane, or hydrogen/synthetic methane mixtures. Predictions based on time series data for wind generation and electricity demand indicate that excess renewable electricity levels will reach about 40 MW and 45 GW h per annum by 2020, and that it is desirable to achieve a progression in power-to-gas capacity in the preceding period. The findings are indicative for regions transitioning from medium to high renewable power penetrations. To capture an increasing proportion of the growing amount of excess renewable electricity, the following recommendations are made: implement a 4 MW hydrogen admixture plant and hydrogen buffer of 600 kg in 2018; then in 2020, implement a 17 MW hybrid system for injecting hydrogen and synthetic methane (with a hydrogen storage capacity of at least 400 kg) in conjunction with a bio-methane injection plant. The 17 MW plant will capture 68% of the available excess renewable electricity in 2020, by offering an availability to the electricity grid operator of >97% and contributing 19.1 GW h of ‘green’ gas to the gas grid.     

Economics of solar-based hydrogen production: Sensitivity to financial and technical factors

This study examines the sensitivity of the levelised cost of hydrogen (LCOH), produced from solar photovoltaic (PV) electricity, to four factors that strongly influence the economics of green hydrogen: electrolyser efficiency, PV capacity factor, nominal interest rate and inflation rate. The authors' aim was not to calculate an absolute value for the LCOH, which varies according to location and economic circumstances, but to examine its sensitivity to these critical parameters of the economic model. This approach facilitates comparisons between potential solar hydrogen projects to select the location with the lowest LCOH. Direct coupling of a PV power plant to proton exchange membrane (PEM) electrolysis, without storage, was assumed, along with a base-case scenario with nominal interest rate 7%, inflation rate 2%, electrolyser efficiency 75% and PV capacity factor 22%. To account for the rapidly evolving electrolyser market, a learning-rate model was employed to estimate for the cost of routine end-of-life replacement of the electrolyser. Finally, the effect of grid-assisted operation on the LCOH was considered. The results demonstrated clearly the importance of careful site selection to achieve high PV capacity factor, which was more influential than foreseeable increases in electrolyser efficiency. Moreover, examination of the mutual sensitivities between the four critical parameters showed that high capacity factor is a good hedge against high inflation rates.     

Hydrogen storage for off-grid power supply

The use of intermittent renewable energy sources for power supply to off-grid electricity consumers depends on energy storage technology to guarantee continuous supply. Potential applications of storage-guaranteed systems range from small installations for remote telecoms, water-pumping and single dwellings, to farms and whole communities for whom grid connection is too expensive or otherwise infeasible, to industrial, military and humanitarian uses. In this paper we explore some of the technical issues surrounding the use of hydrogen storage, in conjunction with a PEM electrolyser and PEM fuel cell, to guarantee electricity supply when the energy source is intermittent, most typically solar photovoltaic. We advocate metal-hydride storage and compare its energy density to that of Li-ion battery storage, concluding that a significantly smaller package is possible with metal-hydride storage. A simple approach to match the output of a photovoltaic array to an electrolyser is presented. The properties required for the metal-hydride storage material to interface the electrolyser to the fuel cell are discussed in detail. It is concluded that relatively conventional Mischmetal-based AB₅ alloys are suitable for this application.     

Performance assessment of a direct steam solar power plant with hydrogen energy storage: An exergoeconomic study

Power generation and its storage using solar energy and hydrogen energy systems is a promising approach to overcome serious challenges associated with fossil fuel-based power plants. In this study, an exergoeconomic model is developed to analyze a direct steam solar tower-hydrogen gas turbine power plant under different operating conditions. An on-grid solar power plant integrated with a hydrogen storage system composed of an electrolyser, hydrogen gas turbine and fuel cell is considered. When solar energy is not available, electrical power is generated by the gas turbine and the fuel cell utilizing the hydrogen produced by the electrolyser. The effects of different working parameters on the cycle performance during charging and discharging processes are investigated using thermodynamic analysis. The results indicate that increasing the solar irradiation by 36%, leads to 13% increase in the exergy efficiency of the cycle. Moreover, the mass flow rate of the heat transfer fluid in solar system has a considerable effect on the exergy cost of output power. Solar tower has the highest exergy destruction and capital investment cost. The highest exergoeconomic factor for the integrated cycle is 60.94%. The steam turbine and PEM electrolyser have the highest share of exergoeconomic factor i.e., 80.4% and 50%, respectively.