Review of renewable energy-based hydrogen production processes for sustainable energy innovation

1 Introduction

Hydrogen is commonly believed to be one of the most abundant elements in the universe.The heat release per unit mass of hydrogen is calculated to be three times more that of gasoline.Furthermore,it is a clean,renewable source of energy and produces only water vapor on combustion.Therefore,hydrogen energy is a viable solution for future energy needs considering present environmental pollution issues due to combustion of fossil fuels; in fact,it is already a key component in several energy systems [1].It has been reported that the global hydrogen demand was 255.3 billion cubic meters in 2013,and will grow to 324.8 billion cubic meters by 2020 [2].Currently,more than 90% of the hydrogen in the world is produced from fossil fuels [3].Hydrogen energy can be utilized in various forms,including in a fuel cell,which can achieve high energy conversion efficiency [4].Moreover,hydrogen can be produced via various renewable energy production technologies.Thus,in this review,hydrogen production technologies based on different sources of energy,including wind energy,solar energy,and nuclear energy are introduced.Moreover,hydrogen production technologies based on biomass,including biomass decomposition via chemical,microbial,and electrolytic action are also discussed.In particular,the economic,technological,and environmental impacts of the different types of hydrogen production technologies are analyzed and comprehensively compared in our present study.The different hydrogen production technologies are summarized via the chart shown in Fig.1.

Fig.1 Renewable hydrogen production technologies

2 Hydrogen production technology based on water

Several technologies for hydrogen production using water are available,including water electrolysis,water thermolysis,photocatalytic water splitting,and thermochemical water splitting.These methods can be successfully performed using renewable sources of energy,including wind,solar,and nuclear energy.In particular,the thermal,radiant,or electrical energy generated via these renewable sources can be used for hydrogen production.The use of these renewable sources of energy for hydrogen production based on water is discussed in the following subsections.

2.1 Wind-driven hydrogen production using water

Utilizing the abandoned power of a wind farm to produce hydrogen can improve overall resource utilization of the wind farm,thus improving the grid-connected wind power output.In particular,this abandoned electricity of a wind farm can be converted into direct current through rectification using power electronic control systems,which can then be used to produce hydrogen in electrolytic cells.The hydrogen generated by the electrolytic cells can then be separated and purified,after which it can be compressed and stored in storage modules [5].A feasibility study on hydrogen production using wind power at a site in Ghardaia is conducted; the results of that study indicated that it was possible to improve system output by increasing the height of the wind turbine tower [6].In particular,30 MW and 100 MW wind farm clusters in Northeast China were considered as cases.The comparative analyses highlight that the building together scheme for wind farm clusters and hydrogen production systems is more economical [7].

2.2 Solar-energy-based hydrogen production using water

2.2.1 Photovoltaic electrolysis

A photovoltaic (PV) water electrolysis system comprises of PV panels,DC bus bar,AC grid,accumulator battery set,electrolyzer,and hydrogen storage canisters.This system can compensate for low reliability caused by intermittent instability of solar power generation and can provide stable and reliable power within a certain range.Fig.2 shows a structural diagram of the PV water electrolysis system.

Fig.2 Structural diagram of the PV water electrolysis system

2.2.2 Water thermolysis

Hydrogen production via water thermolysis based on solar energy involves using solar concentrators to directly collect solar energy to heat water to 2500 K,at which temperature it decomposes into H2 and O2.However,there are problems associated with this approach; the primary ones include achieving high temperature using a solar concentrator and effective separation of H2 and O2 at high temperature.To address these problems,Kogan proposed the use of a catalyst in water,which could allow for the decomposition of water in multiple steps,while considerably reducing the required heating temperature [8].

2.2.3 Photoelectrolysis and photocatalytic decomposition

Photoelectrolysis involves the application of heterogeneous photocatalysts at one electrode of a PV electrolytic cell,which is exposed to solar radiation.The photoanode of the cell absorbs sunlight because of which electrons are generated by the semiconductor at the anode.These electrons are then transmitted to the cathode via external current,which leads to H2 generation at the cathode.However,even using semiconductor materials with excellent properties such as double-interface Ga and As electrodes,only about 13% can be attained [9].While the principle of solar photocatalytic decomposition of water for hydrogen production is similar to that of solar photoelectrolysis,the anode and cathode are on the same particle,owing to which water decomposes into H2 and O2.Nevertheless,because the decomposition of water into H2 and O2 occurs simultaneously,the electron holes generated on the same particle easily recombine.In a study conducted at the University of Science and Technology of China,a “sandwich” structural material system based on quantum theory was designed to effectively inhibit the reverse reaction of O2 and H2 generated via photoelectrolysis back to water again [10].

2.3 Nuclear-assisted hydrogen production using water

Nuclear energy is considered as one of the primary renewable sources of energy that can be used to not only meet the energy demands of a country,but also ensure its national security [11].In particular,nuclear-assisted hydrogen production using water is realized by coupling an electrolyzer to a nuclear power plant,which,in itself,is another method of producing hydrogen via thermochemical reactions.In the nuclear-assisted hydrogen production process,the high heat provided by the reactor is used for thermolysis of water; to achieve this with a fourthgeneration reactor,processes such as I-S cycle,Cu-Cl cycle,Ca-Br cycle,or U-C cycle are used.These thermochemical cycles involve certain risks during the scaling-up of the process.Furthermore,the strong corrosivity at high temperatures also places significant demands in terms of materials and equipment used.At present,the Japan Atomic Energy Agency has completed a pilot test for hydrogen production via the I-S cycle,wherein a hydrogen production rate of 150 L/h was attained [12].In addition,researchers at Tsinghua University have established a laboratory-scale I-S cycle experimental system (60 L/h) that achieved long-term operation [11].

3 Hydrogen production technology based on biomass

As an environmentally friendly renewable source of energy,if we can achieve industrialization of hydrogen production using biomass,not only will this have a positive impact on energy use optimization,but also on reduction in environmental pollution,which is the primary cause of climate change.

3.1 Hydrogen production via gasification of biomass

Biomass gasification refers to the process of converting biomass-forming hydrocarbons into gaseous fuel using a gasification medium.Currently,gasification of biomass is typically realized via the use of catalysts,which accelerate the rate of gasification in the middle and reduce the temperature.Hydrogen production via gasification of biomass primarily includes three processes:biomass gasification,synthesis gas catalytic reforming,and hydrogen separation and purification.

Fig.3 Hydrogen production via gasification of biomass

In the mid-1970s,the supercritical water gasification (SCWG) technique for hydrogen production was first proposed [13].Biomass in supercritical water is converted through a series of complex thermochemical transformations,such as pyrolysis,hydrolysis,condensation,and dehydrogenation,to produce hydrogen,carbon monoxide,carbon dioxide,methane,and other gases; in addition,this technique does not require dry pretreatment and can reduce energy consumption.

3.2 Hydrogen production via pyrolysis and microbial action on biomass

Hydrogen production via pyrolysis of biomass is a process in which biomass is heated under the condition of insulating air or oxygen to convert it into hydrogen-rich gas.The resulting gas will also contains CO,CO2,CH4,and other hydrocarbons.Pyrolysis can be divided into different types based on the pyrolysis temperature,namely low-temperature slow pyrolysis,medium-temperature rapid pyrolysis,and high-temperature flash pyrolysis [14].Hydrogen production via microbial action can be divided into photosynthesisbased microbial hydrogen production and fermentationbased microbial hydrogen production methods.The former uses solar energy as the output energy,and photosynthetic microorganisms to decompose water or biomass to produce H2; this method has a low H2 yield,high operating cost,and is not viable for industrial production.In contrast,the latter,specifically dark fermentation,involves the conversion of biochemical energy stored in organic substances to other forms of energy in the absence of light.The bioreactors used for dark fermentation are simpler and cheaper that the ones for photosynthesis-based microbial hydrogen production because this process does not require solar input processing.Nevertheless,both these approaches based on microbial action suffer from technical bottlenecks,including low thermal efficiency,poor durability of the catalyst,and impurities in the final products.

3.3 Hydrogen production via electrolytic method using biomass

Researchers at the Georgia Institute of Technology reported an electrolysis approach for directly producing hydrogen from almost all native biomasses using polyoxometalate (POM) solution as both a catalyst and charge carrier; their proposed approach has low energy consumption and is highly efficient,rendering it suitable for various applications.In their system,the biomass-POM solution is the electrolyte for the anode,while an aqueous solution of phosphoric acid is the electrolyte for the cathode.In this approach,biomass is decomposed and continuously oxidized into smaller derivatives with the final product of the oxidation being CO2 [15].This reaction process can be summarized as follows:

4 Economics of hydrogen production

4.1 Wind-driven hydrogen production

In terms of economic impact,the manufacturing cost of wind turbines can be significantly reduced.For example,for a wind farm with a capacity of tens of millions of kilowatt,if the grid-connected mode is adopted,it will cost about ¥100 billion.However,when the hydrogen production mode is adopted,the cost of the generating units alone could be reduced to ¥30 billion or less [16].Choi et al.[17]studied wind-driven electrolysis with a plant capacity of 28%; they calculated the generation cost for hydrogen to be $4.67/kg.Furthermore,Bertuccioli et al.[18]analyzed the cost of hydrogen production using proton-exchange membrane (PEM) and alkaline electrolyzers driven by wind to be $5.0/kg and $7.6/kg,respectively.Loisel et al.[19]evaluated the economics of a hybrid power plant consisting of an offshore wind power farm and hydrogen production-storage system in the French region of Pays de la Loire.Hydrogen production costs of selected projects would be between $3.5-$11.8/kg of H2 as a function of the application type.

4.2 Solar-energy-based hydrogen production

A solar PV panel connected to a PEM electrolysis system with a capacity of 1200 t/day was economically studied by Choi et al.[17]; they estimated the cost of hydrogen produced via their approach to be $8.98/kg.Furthermore,Giaconia et al.[20]considered compensating the heat demand for the S-I cycle using a combination of both solar and fossil fuel energies; they reported the cost of hydrogen as $7.53/kg with a daily production amount of 71 kg for 65% plant performance.Claudio et al.[21]presented a conceptual design and performed economic analysis for a hydrogen production plant based on thermochemical water splitting combined with a solar central receiver; they estimated a minimum hydrogen production specific cost of $3.19/kg in the long term.Boudries [22]assessed that the concentrating PV (CPV) electrolysis technique offers a significantly higher production rate than the PV electrolysis method at a lower cost; in particular,the hydrogen production cost via this method is $3.6/kg.In addition,Boudries [23]also conducted another study wherein hydrogen production was realized using a hybrid solar parabolic trough-gas power plant electrolysis system; the results for this study indicated that the hydrogen cost could be as low as $6.0/kg using the proposed method.

4.3 Nuclear-assisted hydrogen production

The cost of hydrogen production from a hybrid sulphur cycle (HyS) on a modular helium reactor (MHR) was assessed by Summers et al.[24]for a daily hydrogen production mount of 580 tons; the updated H2 production cost for such an integrated plant is $2.29/kg.Ozbilen et al.[25]studied the economics of hydrogen production using a hybrid thermochemical Cu-Cl cycle coupled to a supercritical water reactor (SCWR) via exergoeconomic assessment for a daily capacity of 125 tons for a 15-year plant lifetime; they calculated the cost of hydrogen production as $0.02 and $0.08/kWh in terms of the cost of thermal energy and electricity utilized,respectively,resulting in an updated production cost of $3.60/kg.The ideal input parameters of this previous study were applied to a modified Mg-Cl cycle,for which the hydrogen cost was estimated to be $3.87/kg [26].Furthermore,in a recent study considering the S-I cycle coupled with SCWR,the cost of hydrogen was estimated as $3.56/kg for large capacity application [27].

4.4 Biomass-based hydrogen production

Lv et al.[28]evaluated a downdraft biomass oxygen gasification approach with CO-shift at atmospheric pressure and determined the hydrogen production cost to be $1.69/kg.Inayat et al.[29]designed a heat integrated flow sheet for the production of hydrogen from the empty fruit bunch of palm oil trees using a steam gasification approach in a fluidized bed with in-situ CO2 capture.At temperature of 1150 K,an S/B of 4 and sorbent/biomass ratio of 0.87 was obtained,for an H2 yield of 0.0179 kg/h and cost of $1.91/kg.Moreover,Abuolu et al.[30]studied the use of biogas for hydrogen generation via PEM and alkaline electrolysis,high temperature steam electrolysis (HTSE),dark fermentation,and H2S electrolysis technologies for micro-scale plant capacities; they reported that considering full capacity operation and same electricity price across the approaches,the lowest hydrogen generation cost was for H2S electrolysis,while the highest cost was for the dark fermentation approach.Moneti et al.[31]analyzed 1 MWth indirectly heated gasification with catalytic filter candles,water gas shift (WGS) at 400 °C,WGS at 200 °C.The hydrogen production cost was estimated as $9.4/kg.

Overall,the lowest costs for hydrogen production were realized using nuclear-assisted and biomass-based hydrogen production technologies whereas the highest costs for the same were realized using solar-energy-based and winddriven hydrogen production approaches.Considering this,the latter technologies are still not a feasible option compared with conventional fossil-fuel-based hydrogen technologies.However,biomass-based hydrogen production approaches might be promising for future hydrogen production.

Table 1 Cost ranges for hydrogen production via different production techniques

Hydrogen production technique Cost range ($/kg)Wind-driven hydrogen production 3.5-11.8 Solar-energy-based hydrogen production 3.19-8.98 Nuclear-assisted hydrogen production 2.29-3.87 Biomass-based hydrogen production 1.69-1.91,9.4

5 Environmental impact of hydrogen production

The life cycle assessment (LCA) of a system provides an evaluation of its environmental effects and energyconsumption benefits [32].In this work,we primarily analyze the environmental impact of several well-developed hydrogen production technologies,including winddriven hydrogen production,hydrogen production via PV electrolysis,nuclear-assisted hydrogen production via thermochemical cycle and water thermolysis,and hydrogen production via gasification of biomass.In addition,hydrogen production via biomass electrolysis was discussed because it is a good candidate for hydrogen production,because it is easier to scale than other hydrogen production technologies.

5.1 Wind-driven hydrogen production

Cetinkaya et al.[32]performed a comprehensive LCA of five hydrogen production methods,including steam reforming of natural gas,coal gasification,water electrolysis via wind and solar electrolysis,and thermochemical water splitting with a Cu-Cl cycle; their results show that the carbon dioxide equivalent emission due to hydrogen production using wind power was 970 gCO2/kgH2.Furthermore,Ghandehariun et al.[33]conducted an LCA of wind-based hydrogen production in Western Canada,and estimated total greenhouse gas (GHG) emissions from a wind-based hydrogen production plant to be 680 gCO2/kgH2.Reiter [34]primarily studied the effect of the life cycle of renewable energy generation on the efficiency of hydrogen production; they used GABI5 software to analyze their collected data,which yielded a GHG emission equivalent of 600 gCO2/kgH2.

5.2 Hydrogen production via PV electrolysis

Cetinkaya et al.[32]also used the LCA method to study the comprehensive benefits of PV power generation systems for hydrogen production; their evaluation results showed that the GHG emission equivalent for such systems is 2412 gCO2/kgH2.Dufour et al.[35]analyzed the life cycle effects of various hydrogen production systems,and concluded that the life cycle energy consumption of PV power generation for hydrogen production was 77.864 MJ/kgH2 with a GHG emission rate of 6674 gCO2/kgH2.

5.3 Nuclear-assisted hydrogen production via thermochemical cycle and water thermolysis

Ozbilen et al.[36]showed that a GHG equivalent of 860 gCO2/kgH2 was released using nuclear thermal hydrogen production technology based on the S-I cycle.Olli et al.[37]performed an LCA analysis of a similar system,but their system was based on the Leontief matrix model; their results show that the life cycle GHG emission equivalent of the system is 412 gCO2/kgH2.Furthermore,an LCA of one of the proposed method for hydrogen production—the high temperature electrolysis of water vapor—is presented; the results of this analysis are presented in terms of the global warming potential (GWP) and acidification potential (AP) of the system.In particular,the GWP for the system is 2000 gCO2/kgH2 [38].

5.4 Hydrogen production via gasification of biomass

Susmozas et al.[39]used an energy crop,namely poplar,as a material for hydrogen production via the biomass gasification method; their evaluation results show that the net life-cycle GHG emission of this biomass gasification system was 405 gCO2/kgH2.In another study,the use of a downdraft gasifier (DG) was considered and analyzed for hydrogen production using data taken from the literature; based on their analysis,they calculated the emissions of the DG system to be 896.61 gCO2/kgH2 [40].Furthermore,Hajjaji et al.[41]used the LCA method for hydrogen production via biomass gasification,and obtained the life cycle GHG emission equivalent of 5590 gCO2/kgH2 for the bio-waste gasification hydrogen production process.

Table 2 Summary of GHG emission range of the hydrogen production approaches

Type of hydrogen production methods GHG emission range ( gCO2/kgH2)Wind driven electrolysis 600-970 Nuclear-thermochemical cycle/Water thermolysis 412-860 Gasification of Biomass 405-896.61/5590 PV electrolysis 2412-6674

The environmental impact of selected hydrogen production technologies are summarized in Table 2 in terms of different GHG emission ranges.Gasification of biomass and nuclear-thermochemical cycle/water thermolysis hydrogen production methods have the lowest environmental impact,whereas the PV electrolysis approach has relatively more impact.In contrast,the environmental impact of hydrogen production via wind-driven electrolysis is also not significant.It is quite evident that all renewable energy-based approaches for hydrogen production are more environmentally friendly than fossil-based hydrogen generation.

5.5 Hydrogen production via biomass electrolysis

A hydrogen production system that uses wheat straw as a raw material is considered as the research object.The energy consumption and environmental emissions during the manufacture and construction of system equipment,recovery of production equipment,and recycling and reuse of pollutants generated at various steps in the process were not be considered.The current study determines the boundary of the hydrogen production from biomass including four processes:(1) Biomass acquisition process,(2) Biomass transportation process,(3) Biomass pretreatment process,and (4) Electrolytic hydrogen production process.Because hydrogen production via biomass electrolysis has not been scaled up thus far,the data for biomass collection and transportation in the initial stages is drawn from that of hydrogen production by gasification of biomass.Considering the absorption of CO2 during biomass growth,the calculation results are as follows:

Table 3 Environment impact of the life cycle of the selected hydrogen production system in terms of GHG pollutant emissions (kg/H2)

Pollutant Acquisition Transportation Pretreatment Production Total CO2 -27.7656 0.9395 1.9733 24.2327 -0.6201 SO2 0.0042 0.0029 0.0151 0 0.0222 CH4 0.0031 0.0001 0 0 0.0032

Per our calculations,under the same conditions,the GWP for the system is 0.3779 mPET2000,while the energy consumption is 96.4324 MJ/kgH2.In contrast,the GWP for hydrogen production via gasification of biomass is 2.7 mPET2000 with an energy consumption of 128.72 MJ/kgH2.Thus,both energy consumption and GHG emissions are lower for this approach compared with other technologies; therefore,it has great potential as a hydrogen production technology.

6 Conclusion

Hydrogen energy is a clean source of energy that can play a significant role in meeting the future energy demands of the world in an environmentally friendly manner.Thus,recent environmentally viable approaches for hydrogen production that can allow for a smooth transition to hydrogen economy have been under intensive research for feasibility and cost-effectiveness compared with polluting fossil-based hydrogen production approaches.

Based on the results from various studies in the literature,it is clear that all renewable energy-based approaches for hydrogen production are more environmentally friendly than fossil-fuel-based hydrogen generation approaches.However,the cost of hydrogen production using renewable energy needs to be further reduced in order to be applied on a large scale.Among the hydrogen production technologies based on renewable energy,hydrogen production technology based on biomass has certain advantages over others both economically and environmentally.In particular,under the same evaluation conditions as other studies,the environmental impact of hydrogen production via biomass electrolysis is smaller than the other approaches,making it a potential candidate for an environmentally friendly hydrogen energy production technique with low energy consumption.

Acknowledgements

This work was supported by the Beijing Science and Technology Major Project (Grant No.Z171100002017021); CHN Energy Science and Technology Innovation Project (2017B1BE00100).