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Sustainable hydrogen production from offshore marine renewable farms: Techno-energetic insight on seawater electrolysis technologies Rafael d'Amore-Domenech, and Teresa J. Leo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06779 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Sustainable Chemistry & Engineering

Sustainable hydrogen production from offshore marine renewable farms: Techno-energetic insight on seawater electrolysis technologies Rafael d’Amore-Domenech*†, Teresa J. Leo† †

Dept. Arquitectura, Construcción y Sistemas Oceánicos y Navales, ETSI Navales, Universidad Politécnica de Madrid, Avenida de la Memoria 4, Madrid 28040, Spain

Abstract Hydrogen production with offshore marine renewable energies may have an important role in the future as an energy vector and as a fuel. In this regard, this work reviews all the technologies capable of performing electrolysis at sea. The review includes a thorough description and explanation of all known possible damages to the different electrolysis technologies caused by the impurities that may be present in water sourcing from the sea. In addition, this work studies three different hypothetic plants based on the reviewed technologies, to produce hydrogen at 350 bar for its transportation in compressed state. The study is aimed to make an energetic and environmental comparison. The results show that low-temperature electrolysis technologies are currently the best possible candidates regarding both sustainability and durability, with an estimated specific energy to produce hydrogen at 350 bar of 175 MJ/kg under a steady state operation.

Introduction Today the world is undergoing an environmental and energetic crisis due to the intensive use of fossil fuels1. As a result, people of heavily populated areas are starting to develop respiratory afflictions2. The vast emission of greenhouse effect gases, is subjecting cold regions to great stress, quickly destroying ecosystems and menacing wildlife3. In this sense, there are two possible pathways for the mitigation of this problem: diminishing our economic activity or changing our energy use towards a more sustainable one4. Regarding the latter, it is necessary to increase the renewable energy proportion in our mix5. Despite all this, most industrialized and emerging economies still prefer to use fossil fuels because they are cheap, their conversion to usable energy is mature and there are still enough known reserves to power Earth’s economic activity for, at least, some more decades6. However, the aforementioned emissions problems, along with the high volatility on fossil fuel prices, and the awareness of potential lack of availability of such fuels7 due to political and diplomatic instability are creating the perfect scenario for a change8. Most of the infrastructures of any developed country are already electrified9, meaning that substituting thermal power plants by renewable energy sources would solve most of the problem10. However, completely emissions-free renewable energies are frequently intermittent and difficult to convey to mobility without first being stored into batteries11. Battery solutions have not only been studied for road vehicles, but also for waterborne12 and airborne ones13, although in the two last ones without the same level of success. A study performed on 201714 revealed that, according to executives from major car companies, battery solutions on road vehicles are transient, whereas hydrogen solutions are believed to be more permanent in the long run. The reason to think in that *

E-mail: [email protected] Telephone: +34 91 067 6270

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direction, despite fuel cells being less energy efficient than batteries, is based on the limitations of the electric infrastructures and the enhanced flexibility regarding operation of fuel cells15. In countries like Germany or the Netherlands, there are strategic plans to implement hydrogen as a fuel in their economy16, 17. The reason behind such measures is founded on the fact that hydrogen can be produced with domestic resources, boosting independence from fossil fuel providers18, while being completely emissions free. This last fact is true whenever hydrogen is produced using an emissions-free source19. One example of this is coupling renewable energies to water electrolysis to split water molecules into hydrogen and oxygen. As a result, only two resources are needed: an area to harness renewable energies and a source of water. Sometimes, farmable land is too valuable to be used for purposes other than agriculture, especially in densely populated areas20. In addition, intensive consumption of inland water could lead to catastrophic results like the disaster of the Aral Sea21. In this sense, sustainable hydrogen production22 has to ensure the long term availability of all the sources employed in its production, namely, energy and water, without compromising the wellbeing of the society23 or the environment24. For these reasons, marine offshore hydrogen production coupled with marine renewable energies could be a sustainable answer to the future needs of industrialized countries20. The main challenges for an electrolysis plant in an offshore environment are the high variability in energy production inherent to renewables25, motion in floating platforms26 and also the lack of availability of fresh water27. In this context, even though there are already existing technologies capable of performing seawater electrolysis28, 29, it is yet unclear which one is the fittest for being coupled with a marine renewable farm in an offshore situation30. In Figure 1, all marine renewable energies are listed, accompanied by a qualitative graphic comparison regarding different key aspects, providing an outlook of their status and perspectives31-36. Geographic availability refers to what degree the renewable resource is or not location-specific: the highest score corresponds to worldwide availability. Resource predictability measures the anticipation of accurate forecasts given by existing prediction models. The anticipation goes from some days in advance, for wind and waves, to years in advance for tidal energy. There are resources that are inherently variable, namely, wind, waves, and tides. The less variable is the resource, the higher is the score. Global resource potential refers to the estimated theoretical maximum power that could be harnessed worldwide37. Technology readiness level (TRL) refers to the maturity state of harnessing technologies. The best score corresponds to the technologies with full commercial status, i.e., TRL=9. Levelized cost of energy (LCOE) determines how expensive it is to produce each unit of energy throughout the life cycle of the technology: the cheaper, the better. Currently, the best marine renewable resource is offshore wind, because it holds the lowest LCOE among the different renewable energies and presents best perspectives for its global growth in the short term. For this reason, offshore wind will be the target renewable energy of this study.


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Geographic availability

SALINITY GRADIENT

kPa

MARINE CURRENT

Levelized Cost of Energy

Resource predictability

Technology Readiness Level




Resource variability

Global resource potential











Geographic availability Resource predictability






WAVE




THERMAL GRADIENT




TIDAL



OFFSHORE WIND

Resource variability Global resource potential

Figure 1: Outlook of marine renewable energies.

Single electrolysis cells are usually stacked to operate at greater voltages and reduce mass and volume of the resulting device19. A single electrolysis cell for splitting water is composed of two electrodes, an electrolyte, a water input, a separator to prevent the gaseous outputs from mixing, and two gaseous outputs. Hydrogen is produced at the cathode and oxygen at the anode. However, in saline electrolysis, chlorine and other hazardous species are produced at the anode if no special measures are taken38. Electrolysis technologies are classified in the literature regarding the type of electrolyte they bear19. In this regard, the technologies capable of performing electrolysis at sea30 are Direct electrolysis of seawater, Alkaline electrolysis, Proton Exchange Membrane (PEM) electrolysis and Solid Oxide (SO) electrolysis. Hydrogen production plants at sea will not only have to produce hydrogen but also will have to increase its density for its exportation since, at 293.15 K and 1 bar of pressure, 1 m3 only contains 82.6 g of hydrogen39. Three possible physical transformations are possible to increase its density: compression, liquefaction, and cryo-compression40. None of these options have at present commercial status for large-scale maritime shipping. However, the first two options, compressed and liquid, offer the best prospects of application in the maritime sector, since major companies of 3 ACS Paragon Plus Environment


the sector are already making efforts in future ship concepts or designs20, 41, 42. Depending on the application, one option will be better than the other. In this regard, equivalent studies performed on natural gas revealed that, for short distances and small quantities, compressed natural gas (CNG) shipping is cheaper than the liquefied natural gas (LNG) alternative43, 44. Currently, the marine renewable with most installed capacity is offshore wind45. In Europe, the average offshore wind farm is located no further than 110 km from shore and no deeper than 50 m46, and in 2018, the mean installed offshore wind farm had a capacity of 561 MW46. With these data, it seems that compression will make more sense than liquefaction in the short term, since in the field of marine renewables, farms are still close from the shorelines and the quantities of produced hydrogen via electrolysis corresponding to current power capacities of offshore wind farms, would be insufficient to justify liquid transportation. In addition, compression offers less energy losses than liquefaction47. Even though a compressed hydrogen (cH2) ship still does not exist, the technology of CNG ships48 could be adapted to hydrogen. In this sense, 350 bar of pressure seems to be a reasonable one for a hypothetic cH2 scenario. For this reason, the energetic assessment will observe the specific energy needed to produce 1 kg of pure hydrogen at such pressure. This article aims to assess all the existing technologies to point out which one is the most energy efficient in an offshore context, having in mind that hydrogen must increase its density prior to its transportation by ship. For that purpose, first, all suitable technologies to perform electrolysis in a marine context are identified and described; second, an energetic assessment is conducted on hypothetic plants representing each of the appointed solutions, where hydrogen is also compressed to a pressure of 350 bar, which is considered suitable for hypothetic future cH2 carriers. Finally, the results are discussed to determine which technology currently presents the best chance of applicability in an offshore environment and how competitive would be the subsequent produced hydrogen within current markets.

Suitable seawater electrolysis technologies An electrolyzer is an electrochemical device intended to perform the electrolysis reaction49. Such electrochemical reaction aims to separate a chemical compound into two or more elemental species19. The simplest electrolyzer is a single electrolysis cell50.


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ANODE IN

ELECTROLYTE

OUT O2 Cl2 OH-

Seawater OHCl-

CATHODE OUT

IN

H2 Na+

H2O ClNa+

H2

H2O

Sea Temperature

Alkaline O2

OH60-90°C

PEM H2O

O2

H+

H2

60-90°C

Solid Oxide O2

O2-

H2

H2O

700-1000 °C Figure 2-Electrolytes capable of performing the electrolysis reaction for hydrogen production at sea.

As said in the Introduction, electrolysis technologies are classified in the literature regarding the type of electrolyte they bear. In this regard, Figure 2 summarizes the list of electrolytes capable of performing electrolysis for hydrogen production at sea30, which have the following associated technologies: -Direct seawater electrolysis51: in this case, the electrolyte is seawater itself, which is also the feed. -Alkaline electrolysis52: for this type of technology, the electrolyte is a concentrated caustic, either NaOH or KOH, although the latter is more frequent since it features greater conductivity to hydroxyl ions OH-. In this case, pure liquid water is the feed. -Proton Exchange Membrane (PEM) electrolysis53: this type of technology uses a solid polymer as an electrolyte, frequently a perfluorosulfonic one, that exhibits a great conductivity to protons. It uses pure liquid water as feed. -Solid Oxide (SO) electrolysis54: this technology uses a ceramic membrane as an electrolyte, usually yttria-stabilized zirconia (YSZ) that presents high conductivity to O2- ions at high temperatures. This type of electrolysis technology is fed by superheated vapor.

Direct seawater electrolysis There are not many scientific publications citing or describing direct electrolysis of seawater1, 51, 55-57 for hydrogen production, and there is no significant testimony of them being used for commercial 5 ACS Paragon Plus Environment


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hydrogen production. There are mainly three possible electrolysis reactions that can happen when operating with salt water: a) Production of hydrogen and oxygen b) Production of chlorine, hydrogen and alkalis c) Production of hydrogen and hypochlorites The first route follows the mechanisms that are explained in the Low-temperature electrolysis. The two following mechanisms are well known in the chemical industry under the form of the chloralkali process58 and the chlorate production process59. The only difference between direct seawater electrolysis and the mentioned industrial processes is that the industrial ones use saturated and purified brines, containing NaCl or KCl, depending on the desired type of caustic or chlorate60. Both processes, chlor-alkali and chlorate production, share the same redox reactions, defined by the following equations: ― 2 Cl(aq.) →Cl2(g) + 2 e ―

(1)

Cathode:

― 2 H2O(l) + 2 e ― → H2(g) + 2 OH(aq.)

(2)

Overall:

2 NaCl(aq.) + 2 H2O(l)→Cl2(g) + H2(g) + 2 NaOH(aq.)

(3)

Anode:

Note that for KCl(aq.), Eq. (3) changes the NaCl(aq.) for KCl(aq.), and the NaOH(aq.) for KOH(aq.). ― The anode produces a positive electrical field that attracts the OH(aq.) . In the chlorate cell, as a result ― of such attraction, OH(aq.) migrate to the anode as there are no physical barriers between the ― electrodes. Under this configuration, as soon as the OH(aq.) gets in contact with the formed Cl2(g), they react to produce hypochlorites, chlorites or chlorates61. In chlor-alkali cells, in contrast, there ― is a separator that prevents the migration of OH(aq.) to the anode, thus, NaOH(aq.) and Cl2(g) can be 62 obtained as products . As a result, two different electrolytes appear: the catholyte and the anolyte58.

Both aforementioned processes regard the production of H2(g) as a byproduct63, since the other chemicals have more importance in the chemical industry. In addition, typically both electrolysis cells use operating cell voltages around 4.0 V64, which contrast with those of water electrolysis, around 1.8 V19. This means that the voltage efficiency of the cell is less than half for hydrogen production. There are several reasons to consider this process impractical for use in a marine renewable and sustainable context. First, the production of Cl2(g) or chlorates lead to an environmental impact if directly freed to the environment. Second, the much lower concentration of salts in seawater (typically 3.5% in mass) than in the saturated brine (around 25% in mass) offers much lower ion conductivity. This leads to great ohmic losses, which translates into bigger electrolyzers than the ― + + chlor-alkali and chlorate equivalents. Third, the presence of Mg2(aq.) , Ca2(aq.) and SO42(aq.) along with 65 other impurities in seawater lead to scaling , therefore, this type of electrolysis devices requires periodic wash cycles with acid agents or the use of ion exchange resins to prevent scaling58. Fourth, the anodes must be of very expensive materials, normally IrO2 coated Ti, to prevent their corrosion


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and add Ru and Pt as catalysts66. Fifth, this is a very inefficient process regarding the production of H2(g), which holds an efficiency of less than half of the one of water electrolyzers.

Low-temperature electrolysis Both PEM and Alkaline electrolysis are studied together as low-temperature electrolysis technologies due to their similarities. Alkaline electrolysis is a well-known electrolysis technology67. However, not many scientific publications have referenced alkaline electrolysis operating in marine environments while coupled with marine renewable farms30. Alkaline electrolysis is one of the preferred methods of electrolysis for land-based hydrogen production68. The reasons that make this technology one of the most appealing for onshore stationary applications are that there is no need for expensive catalyst and that they use relatively inexpensive materials as electrodes69. But above all, with the correct maintenance, their lifespan can surpass 100,000 hours70. A single alkaline cell is composed of two electrodes, a separator, and a caustic electrolyte19. The cathode, where hydrogen is produced, is usually made of stainless steel. The anode, where oxygen is produced, is usually made of nickel or nickel-based stainless steels to catalyze the Hydrogen Evolution Reaction (HER). The separator is normally a porous diaphragm that prevents oxygen and hydrogen from mixing71. However, in gravity reliant designs, such component is not needed, nor in assemblies that use redox mediators62. The electrolyte can be either NaOH or KOH, as stated in the Introduction. In any case, KOH is usually preferred since it presents better specific ion conductivity19. The redox reactions in alkaline electrolyzers go as follows: Anode:

1 ― 2 OH(aq.) → O2(g) + H2O(l) + 2 e ― 2

(4)

Cathode:

― 2 H2O(l) + 2 e ― → H2(g) + 2 OH(aq.)

(5)

Overall:

1 H2O(l)→ O2(g) + H2(g) 2

(6)

This type of electrolysis technologies operates between 60 °C and 90 °C72. Their architecture is quite simple, and can also operate under pressure, up to 42 bar, which allows energy savings when output hydrogen is needed under compressed state73. Their operating efficiencies pivot around a value of 85% (1.7-1.8 V), referred to the higher heating value of hydrogen, at current densities that typically range from 100 mA/cm2 to 300 mA/cm2, although recently, current densities from 1000 mA/cm2 to 1500 mA/cm2 have been reported for that same efficiency74. The known problems of alkaline technology that restrain its application at sea are the risk of leakage of the corrosive electrolyte and the need for its periodic renewal30. The renewal of the electrolyte is not due to leakage, which is accidental. In contrast, it is due to the reaction of the electrolyte with impurities contained in the input water, also the fouling caused by the accumulation of corrosion products of the electrolyzer components and the loss of electrolyte through product gases19. The need for a specific logistic chain to renew the caustic electrolyte at sea has been a cause for its discarding in marine offshore applications30. In this regard, recent studies reveal that caustic soda



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can be produced from rejected brine sourcing from desalination of seawater75, 76, thus, increasing the chance of applicability of this technology. Other recent studies show that this technology is compliant with NaCl impurities52 in feed water, in the sense that Cl2 is not produced at the anode. This way, the anode does not corrode, although the electrolyte slightly deteriorates through the formation of chlorates, which are not as corrosive as Cl2 51, and as they remain contained within the device, they are less risky for the environment. In + + addition, they are also resilient to the presence of Mg2(aq.) , Ca2(aq.) . This is because their impact on this type of electrolyte is precipitation of Mg(OH)2(s) and Ca(OH)2(s), which can be solved by emptying the electrolysis cell and then washing it with acidic water during the periodic changes of fouled electrolyte. PEM electrolysis technology allows a type of architecture known as zero-gap in the literature19. Such concept is possible thanks to the Membrane-Electrode Assembly (MEA), a single piece that unites all the elements of a single cell in a three-layer sandwich: two porous electrodes with catalyst layers in their inner faces, hot-pressed against a polymer membrane capable of conducting protons77. PEM electrolysis requires very precise machining of the bipolar plates to ensure a homogeneous contact between bipolar plates and the porous electrodes19 to minimize ohmic losses. The required tolerances of the bipolar plates, the precious metal catalysts, the carbon electrodes, and the Nafion® membranes, make this technology expensive78. Despite that, they are very attractive since their electrolyte is maintenance-free throughout their life cycle, which nowadays can go from 25,000 h and in some cases surpass 90,000 h79. PEM electrolyzers can operate under pressure, typically at 30 bar, so that energy is saved in subsequent stages of hydrogen compresion80. Higher pressures, up to 150 bar, have been reported81, however, due to the increased diffusion rate across the membrane, the overall efficiency of the process is penalized due to the drop in the faradaic efficiency 80, and undesired explosive mixtures between the produced hydrogen and oxygen can be achieved, potentially leading to fatal consequences. The faradaic efficiency is defined later at the Energetic Assessment Section. The redox reactions of PEM electrolysis correspond to those of an acidic electrolyte: Anode:

1 + H2O(l)→ O2(g) + 2 H(aq.) + 2 e― 2

(7)

Cathode:

+ 2 H(aq.) + 2 e ― → H2(g)

(8)

Overall:

1 H2O(l)→ O2(g) + H2(g) 2

(9)

In comparison to Alkaline electrolyzers, PEM technology presents a great reduction in size and weight, not only because of the type of architecture but also, because they present greater current densities for the same operating efficiencies. Specifically, they exhibit around 1 A/cm2 for about 85% efficiency (1.7-1.8 V)82, referred to the higher heating value of hydrogen, at temperatures ranging from 60 °C to 80 °C. The expectancy of a cost drop in the following years is another attractive of this technology53. However, PEM electrolyzers suffer from long-term and irreversible damage due to the + + 83 presence of impurities in feed water, such as NaCl(aq.), Mg2(aq.) , Ca2(aq.) . This contrasts with alkaline technology, which can recover from most of the damage suffered by applying cleaning cycles and renewing the electrolyte. The most common type of irreversible damage in PEM cells is blistering 58. 8 ACS Paragon Plus Environment

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This is due to the precipitation of Mg(OH)2(s) and Ca(OH)2(s) in the cathodic side of the MEA, due to the rise in pH, stemming from the chlor-alkali reactions that occur thanks to the presence of NaCl(aq.). Similar to PEM technology, the Anion Exchange Membrane (AEM) technology presents good conductivity to hydroxyl ions84. The reactions that take place in this type of electrolyte are the same as those explained for the alkaline technology. This is possible due to the alkaline membrane polymer, which is often based on quaternary ammonia polysulfone. Nevertheless, this type of membranes still have to deal with durability issues. Therefore, no commercial solutions have been reported so far. Their behavior with impurities in input water is still unknown. The scientific community has high expectations on this technology since it does not require noble metals as catalysts85.

High-temperature electrolysis In this work, the Solid Oxide Electrolysis technology is catalogued as high-temperature electrolysis, due to the high temperatures that it requires. This electrolysis technology uses ceramics with high conductivity to O2 ― at high temperatures as electrolytes. Such ceramics are normally yttriastabilized zirconia. The operation temperatures that enable such conductivities range from 700 °C to 1000 °C, but more often around 800 °C. The redox reactions inside Solid Oxide Electrolysis Cells (SOECs) are the following: Anode:

1 O2(g)― → O2(g) + 2 e ― 2

(10)

Cathode:

H2O(g) + 2 e ― → H2(g) + O2(g)―

(11)

Overall:

1 H2O(g)→ O2(g) + H2(g) 2

(12)

The architecture of SOECs can be either tubular or planar. Tubular architecture shows very good resistance to thermal cycling, however, it is more difficult to benefit from cost reduction under mass manufacturing. For this reason, planar designs are preferred, although they are less robust regarding thermal fatigue19. One advantage of planar designs is that their configuration can greatly reduce their mass and volume. The denomination of every single cell is Single-Repeat Units (SRUs). An SRU can be electrolyte supported or electrode supported. The difference lies in the thickness of the supporting component. Recent designs tend to be anode supported, because such feature allows thinner electrolytes with lower resistance to conduct O2 ― , thus they bear an improvement regarding efficiency. Solid Oxide electrolysis cells typically operate around 500 mA/cm2, and they present efficiencies of around 95% (1.3 V) referred to the Lower Heating Value (LHV) of hydrogen. Their reported durability goes up to 10,000 h at continuous operation86. However, with shutdowns, their durability is shortened due to the induced thermal fatigue. The main problem of this technology in a marine renewable context lies in the unlikeliness of finding an external source of high temperature in the whereabouts of the offshore renewable farm87. Thus, the production of superheated vapor and the heat management of the electrolyzer would be done at the expense of the electricity generated by the farm. Therefore, the overall efficiency of the process, that is thermal and electrical, would be 9 ACS Paragon Plus Environment


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penalized. Pressurized SOECs have been reported in the literature88, however, known commercial solutions operate at ambient or slightly higher pressures89.

Energetic Assessment General considerations Energetic The energetic study is conducted on hypothetic large-scale plants that operate under steady state conditions, even though electrolysis plants coupled with renewables are expected to be unsteady. The targeted rated power of the plant will be enclosed in a 500-700 MW margin, matching current projections of near future offshore wind farms46. In each hypothetic plant, the efficiencies of the different components have been chosen so that they comply with typical values shown in the literature. Specifically, electrolyzer faradaic efficiencies and cell voltages are chosen on a case-bycase basis. Pumps and compressors, in this work bear isentropic efficiencies of 95% and 75%, respectively. Heat exchangers, HE, which are considered adiabatic, present a minimum temperature difference of 5 °C between the cold and hot streams. The number of compression stages with intercooling for the cathodic outlet stream should imply a comprmise between simplicity and energy efficiency of the plant. In that regard, three stages are considered a good balance between energy efficiency and complexity of the plant. Additional stages are added whenever the pressure ratio 𝑟𝑝 in the compression stages exceeds the value of seven, i.e., 𝑟𝑝 > 7. The equation that relates 𝑟𝑝 and the number of compression stages 𝑁 is the following:

𝑟𝑝 =

1 𝑁

( ) 𝑝𝑜𝑢𝑡 𝑝𝑖𝑛

(13)

There, 𝑝𝑖𝑛 and 𝑝𝑜𝑢𝑡 are the pressure at the inlet and at the outlet of the set of compressors, respectively. To remove all crossover gas sourced from the anodic side of the cells, passive autocatalytic recombiners are used. This provides an effective means for purifying the hydrogen stream, through the reaction of the crossover anodic gas with hydrogen90. The plant is considered to be 10 m above the sea level. From that point onward all the plant is considered to be at the same height, with ideal flow, without pressure drops. Therefore, kinetic and potential terms of the total enthalpies are neglected. The power consumption estimation is assessed by appliying the first principle of Thermodynamics on every device: 𝑄+𝑊=

|

𝑑𝐸 𝑑𝑡

+ Σ𝐻𝑜𝑢𝑡 ― Σ𝐻𝑖𝑛

(14)

𝐶𝑉

where 𝑄 is the heat transferred to the device, which is zero for adiabatic devices, 𝑊 is the power supplied to the device and

|

𝑑𝐸 𝑑𝑡 𝐶𝑉

is the energy variation rate inside de control volume under study,


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which is zero at steady conditions. Σ𝐻𝑜𝑢𝑡 ― Σ𝐻𝑖𝑛 is the enthalpy rate variation between outlets and intlets to the device. Eq. (14) applied to pumps and compressors leads to the following expression: (15)

𝑊𝑝𝑢𝑚𝑝, 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 = 𝐻𝑜𝑢𝑡 ― 𝐻𝑖𝑛 where 𝐻𝑜𝑢𝑡 is calculated by assuming the isentropic efficiency 𝜂𝑖𝑠. For Electrolyzers Eq. (14) takes the form:

(16)

𝑄𝑒𝑙𝑒𝑐 + 𝑊𝑒𝑙𝑒𝑐 = Σ𝐻𝑜𝑢𝑡 ― 𝐻𝑖𝑛

where 𝑄𝑒𝑙𝑒𝑐 is the heat supplied to the electrolyzer and 𝑊𝑒𝑙𝑒𝑐 is the electric power supplied to it, which is obtained by: 𝑊𝑒𝑙𝑒𝑐 = 𝐼·𝑉𝑒𝑙𝑒𝑐 = 𝐼·(𝑛𝑐𝑒𝑙𝑙·𝑉𝑐𝑒𝑙𝑙)

(17)

being 𝑛𝑐𝑒𝑙𝑙 the number of single cells that compose the electrolyzer and 𝑉𝑐𝑒𝑙𝑙 stands for the cell voltage. Eq. (16) also considers the chemical transformations that result from the electrolysis. As heat exchangers are considered adiabatic and without pressure drops, Eq. (14) results in: (18)

Σ𝐻𝑜𝑢𝑡 = Σ𝐻𝑖𝑛

The enthalpy rate at each point 𝑖 in the gaseous streams is calculated using the ideal mixture model, which takes into consideration the composition of such streams:

(

)

𝐻𝑖 = 𝑚𝑖 𝑥𝑚𝐻2𝑂 ℎ𝐻2𝑂𝑖 + 𝑥𝑚𝐻2 ℎ𝐻2𝑖 + 𝑥𝑚𝑂2 ℎ𝑂2𝑖 𝑖

𝑖

𝑖

(19)

where 𝑥𝑚𝑗𝑖 is the mass fraction of the component 𝑗 at the point 𝑖; ℎ𝑗𝑖 is the enthalpy of the species 𝑗 at 𝑇 and 𝑝 conditions at the point 𝑖. The enthalpy is calculated by using the standard enthalpy of formation 91 at 298.15 K and the increment of enthalpy that results from the variation of the conditions from 298.15 K and 100 kPa, to the corresponding 𝑇 and 𝑝. This last enthalpy increment is calculated by using real gas properties using NIST REFRPOP® 39: ℎ𝐹(𝑇,𝑝) = Δℎ𝑜𝐹298.15 𝐾 + [ℎ(𝑇,𝑝) ― ℎ𝑜(298.15 𝐾)]

(20)

From an energetic perspective, seawater and brine are regarded as pure water with a correction that represents the decrease in enthalpy due to the dissolved salts in water, Δℎ𝑠𝑎𝑙𝑡. Such parameter is calculated using the salinity parameter 𝑇𝐷𝑆 which corresponds to the one of standard seawater92. 𝑇𝐷𝑆 stands for total dissolved solids, and it relates the mass of dissolved solids per unit mass of dissolution. Δℎ𝑠𝑎𝑙𝑡 is obtained using the formulation from the reference93 without the pressure term, which is already considered with the increment of enthalpy sourcing from the variation of the conditions from 298.15 K and 100 kPa, to the 𝑇 and 𝑝 at the point 𝑖, [ℎ(𝑇,𝑝) ― ℎ𝑜(298.15 𝐾)]. Therefore the expression that applies for saline streams results in: ℎ𝐹(𝑇,𝑝) = Δℎ𝑜𝐹298.15 𝐾 + [ℎ(𝑇,𝑝) ― ℎ𝑜(298.15 𝐾)] + Δℎ𝑠𝑎𝑙𝑡

(21)

The efficiency of the three types of electrolysis will be appraised by the specific energy needed to produce hydrogen at 350 bar, by using seawater at 5 °C. 11 ACS Paragon Plus Environment


Page 12 of 33

Mass balance Mass balance is done by prefixing the output of pure hydrogen at 350 bar to 1 kg/s. Purities of the gaseous streams throughout the plants have been considered. The composition of the gaseous streams that leave the electrolyzer has been calculated by assuming that they leave saturated with water and by estimating the crossover of the gaseous outputs of the electrolyzer. The crossover is estimated with faradaic efficiencies by assuming that the only source of faradaic efficiency loss is crossover. The composition of the hydrogen stream after the autocatalytic recombiners has been calculated by assuming that all oxygen present in such stream reacts with hydrogen, forming water. The water produced in such process adds up to the already existing water in such stream. After each intercooling step in the streams of oxygen and hydrogen, part of the water present is condensed and evacuated. Finally, hydrogen leaves the plants at 350 bar and 25 °C saturated with water vapor. Faradaic efficiency 𝜂𝐹 is defined by the ratio of the actual molar flow of the pure gaseous output that comes out of the electrolysis cell and its theoretic counterpart: 𝜂𝐹 =

𝑛 𝑛𝑡ℎ

(22)

The theoretic molar flow rate is defined by the Faraday law: 𝑛𝑡ℎ =

𝐼 𝑧𝐹

(23)

where 𝐼 is the current, 𝑧 is the ratio of exchanged electrons per mole of the gaseous output in the redox reaction (4 for oxygen and 2 for hydrogen), and 𝐹 is the Faraday constant 96485.3399 C/mo le ― . The following equations define the molar flow rates in the cathode output, where hydrogen is the main component: 𝐼 𝜂 2𝐹 𝐹𝑐𝑎𝑡

(24)

𝐼 (1 ― 𝜂𝐹𝑎𝑛) 4𝐹

(25)

𝑛𝐻2𝑐𝑎𝑡 = 𝑛𝑂2𝑐𝑎𝑡 = 𝑛𝐻2𝑂𝑐𝑎𝑡 =

𝑝𝑠𝑎𝑡(𝑇) 𝑝 ― 𝑝𝑠𝑎𝑡(𝑇)

(𝑛𝐻2𝑐𝑎𝑡 + 𝑛𝑂2𝑐𝑎𝑡)

(26)

where 𝑝𝑠𝑎𝑡(𝑇) is the saturation pressure of water and 𝑝 is the pressure of the stream at that point. The following equations define the molar flow rates in the anode output when oxygen is produced:


Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝑛𝐻2𝑎𝑛 =

𝐼 (1 ― 𝜂𝐹𝑐𝑎𝑡) 2𝐹

(27)

𝐼 𝜂 4𝐹 𝐹𝑎𝑛

(28)

𝑛𝑂2𝑎𝑛 = 𝑛𝐻2𝑂𝑎𝑛 =

𝑝𝑠𝑎𝑡(𝑇)

(𝑛𝐻2𝑎𝑛 + 𝑛𝑂2𝑎𝑛)

𝑝 ― 𝑝𝑠𝑎𝑡(𝑇)

(29)

where 𝑝𝑠𝑎𝑡(𝑇) is the saturation pressure of water and 𝑝 is the pressure of the stream at that point. The volumetric composition of each species for the gaseous streams that leave the electrolyzer can be calculated using the following expression: 𝑥𝑖 =

𝑛𝑖 𝑛𝐻2 + 𝑛𝑂2 + 𝑛𝐻2𝑂

(30)

The gravimetric composition of each species can be calculated with: 𝑥𝑚𝑖 =

𝑀𝑖 𝑀𝑚

𝑥𝑖

(31)

where 𝑀𝑖 is the molar mass of the species 𝑖, and 𝑀𝑚 is the molar mass equivalent of the mixture, which is calculated as follows: 𝑀𝑚 = Σ(𝑥𝑖𝑀𝑖)

(32)

Once the mass flow rates of the gaseous streams that leave the electrolyzer are calculated, the feed of distilled water into the electrolyzer is calculated by summing the mass flow rates of the gaseous outputs. 𝑇𝐷𝑆 is the parameter that sets the mass flow rate balance between seawater, brine and distilled water at the distiller. The equation that defines such balance is the following: 𝑚𝑠𝑤𝑇𝐷𝑆𝑠𝑤 = 𝑚𝑏𝑟𝑇𝐷𝑆𝑏𝑟

(33)

where the 𝑠𝑤 and 𝑏𝑟 subscripts stand for seawater and brine, respectively. The value of 𝑇𝐷𝑆𝑠𝑤 corresponds to the one of standard seawater 35.165 g/kg92, whereas the value of 𝑇𝐷𝑆𝑏𝑟 is set to 120 g/kg, which is considered a reasonable value for rejected concentrated brine, that does not involve any precipitation of solids in the corresponding device94.

Direct seawater electrolysis For the energetic analysis, similar to other electrolysis technologies, the salinity of liquid output of the electrolyzer has been set to a value of 120 g/kg. However, the proportion of dissolved solids ― + + varies in the liquid output. This is because Cl(aq.) , Mg2(aq.) and Ca2(aq.) are depleted in the output, while 13 ACS Paragon Plus Environment


― OH(aq.) concentration rises. For this reason, Eq. (33) does not apply for direct electrolysis of seawater. Different assumptions are made for this study. Such assumptions are based on references52, 56, and are the following:

a) The preferred evolution reaction in the anode is the one Cl2(g) formation: Cl2(g) is formed ― ― until dissolved Cl(aq.) is completely depleted. The corresponding amount of OH(aq.) is formed in accordance with Eqs. (1)-(3). ― + + b) The increase in OH(aq.) concentration leads to Mg2(aq.) and Ca2(aq.) precipitation, according to the respective solubility products of Mg(OH)2 and Ca(OH)2. Due to the high concentration ― of OH(aq.) the equilibrium is disregarded, and total precipitation of Mg(OH)2 and Ca(OH)2 is assumed. c) Then the oxygen evolution reaction in the anode is produced, concentrating the remaining solutes until the liquid output reaches a salinity of 120 g/kg. The production of hypochlorite is disregarded because, from an energetic point of view, the resulting energy balance can be considered equivalent to the actual one. Regarding mass balances, these assumptions must be treated as an aproximation, since actual equilibrium reactions are unconfirmed until the corresponding laboratory tests are conducted. Figure 3 shows the simplified diagram of the direct electrolysis plant of seawater. First seawater is taken from the sea surface (1) and then is pumped to the plant 10 m above. Next, it is introduced into the electrolyzer (2). The anodic stream that leaves the electrolyzer (3) is rich in O2 but also carries some Cl2, very little water vapor, and some traces of H2. The cathodic stream leaves the electrolyzer (6) rich in H2, carrying some water vapor, some O2 and small traces of Cl2. The rejected caustic brine (referred as brine* in Figure 3) leaves the electrolyzer (4) with a pH>14, sweeping all the precipitated Mg(OH)2 and Ca(OH)2. The cathodic stream is then conducted to the autocatalytic recombiner, where all O2 and Cl2 react with H2 to form water and HCl(g) (7), respectively. Then such stream is bubbled into the rejected brine, in a HCl remover, to remove all the HCl(g). This reduces slightly the pH of the rejected caustic brine (brine* in Figure 3), in any case, the pH of caustic brine will remain over 14 (5). The cathodic stream leaves the HCl remover with just H2 and moisture, and enters the set of compressors (8), where, in the different intercooling stages, part of the water content is condensed.


Page 14 of 33

COND4

HE4

5 Brine*

8 COND2

HCl remover

HE3

4 Brine*

Auotocatalytic recombiner

6

H2

HC1

𝑊̇��

9

H2

H2

HE1

𝑄̇��

COND1

10

HC2

𝑊̇��

7

11

𝑄̇��

HE2

12

HC3

𝑊̇��

13

𝑄̇��

COND3

14

HC4

𝑊̇��

15

𝑄̇��

3 O2

𝑊̇��

SW pump

1

𝑊̇��

Seawater

Anode 𝑄̇��

2

VS

Cathode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16

Page 15 of 33

Figure 3: Simplified diagram of the direct electrolysis plant of seawater. COND is condensate; HC is hydrogen compressor; HE is heat exchanger; SW is seawater; VS is voltage source; Brine* is caustic brine.

Regarding mass balance calculations, the total available chloride in seawater to form Cl2 is 𝑚2·[Cl ― ] where 𝑚2 is the mass flow at the point 2, and [Cl ― ] is the chloride concentration, which is taken from the standard seawater composition92. The variation of concentration of different solutes in water, in terms of balance of energy, have been regarded as the total variation of 𝑇𝐷𝑆 by using the formulation from the reference93. It is known that this introduces a small error with negligible impact in the energy balances due to Δℎ𝑠𝑎𝑙𝑡 ≪ (Δℎ𝑜𝐹298.15 𝐾 + [ℎ(𝑇,𝑝) ― ℎ𝑜(298.15 𝐾)]). The rest is calculated in compliance with the General Considerations. 15 ACS Paragon Plus Environment


The cell voltage of this technology has been assumed to be of 4.5 V, matching reported values of direct seawater electrolysis56 and chlor-alkali process58. Regarding faradaic efficiencies, values of 𝜂𝐹 = 0.99 have been assumed. In Figure 3, it can be seen that there are four stages of compression with intercooling. The reason to have more than three compression stages is because, according to Eq. (13), with 𝑁 = 3, 𝑟𝑝 > 7. The additional compression stage improves the energy efficiency of the process.

Low-temperature electrolysis In contrast to direct electrolysis of seawater, the feed of the low-temperature electrolyzers must be pure water. In this regard, the separation of seawater into fresh water and brine is considered in the plant. Even though reverse osmosis might be more appealing than distillation from an energetic point of view, distillation methods produce water of higher purity than reverse osmosis, in addition, they do not necessarily require pre-treatment95. In this sense, distillation is preferred in plant designs. Distillation can be attained at ambient pressure and a temperature around 100 °C, or at lower temperatures at partial vacuum pressures96. In an offshore renewable context, where no hightemperature heat sources are easily found, the only practical way of producing heat is by consuming electricity generated by the renewable farm. The simplest mechanism to produce heat for distillation using electricity is ohmic heating97. However, vapor-recompression mechanisms are chosen, because they offer energy savings under steady-state operation of distillers98. Their working principle consists in using the vapor from the distillation as the only heat source for the distillation. To achieve this, the produced vapor must be compressed to raise its condensing temperature over the one of boiling seawater. This way, if the system is correctly designed, at steady state, the only electrical consumer for the production of distilled water is the vapor compressor. Therefore, this type of mechanism is the chosen one for the operation of the plant, whereas ohmic heating is left as a backup for transient states. Figure 4 shows a simplified diagram of the low-temperature electrolysis plant that applies to PEM and alkaline technologies. First seawater is taken from the sea (1), and then is pumped to the plant 10 m above the sea surface. Second, seawater is split into different streams (2) for heat recovery from the plant outputs. Then, such streams are merged in a mixing chamber and introduced into de distiller (11), where the outputs are brine (12) and water vapor (14) at the boiling temperature of the brine according to the formulation from the reference93. Then, the water vapor is compressed and condensed (15) in the distiller. The saturated liquid water is cooled in a heat recovery device (16) to fit the operating temperature of the electrolyzer and then it is pumped to raise its pressure to the operating one of the electrolyzer (17). There, anodic (19) and cathodic (21) streams leave the electrolyzer. The cathodic stream is purified by means of an autocatalytic recombiner, where all crossover O2 present in the stream reacts with H2 to form water (22). Then the stream is cooled down, allowing some water to condense. Then, the stream is compressed with three compression stages with intercooling, with the corresponding water condensations, until the stream reaches 350 bar of pressure (28). Afterward, it is cooled down in a heat recovery device (29) before it leaves the installation.


Page 16 of 33

21

VS

18 𝑊̇��

FW pump

W SC1 19

15 16

9

10

Mixing chamber 7

8

COND3

HE3 27 𝑊̇��

5 𝑊̇��

Brine 13

1

20

29

SW pump

O2

H2

2

4

3

6

17

HR4

HR3

COND5

HR2

COND4

HR1

28

HC3

11

12

COND2

14

𝑊̇�� 𝑊̇��

26

HC2

25

HE2

𝑄̇��

24

HC1

SC1

COND1

23

HE1

𝑄̇��

22

H2

O2

Anode

𝑄̇��

𝑊̇��


Cathode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝑄̇��

Page 17 of 33

Figure 4: Simplified diagram of the low-temperature electrolysis plant. COND is condensate; FW is fresh water; HC is hydrogen compressor; HE is heat exchanger; HR is heat recovery; SC is steam compressor; SW is seawater; VS is voltage source.

As it was discussed in the list of suitable electrolysis mechanisms, one of the advantages of the two technologies of low-temperature electrolysis, is that they are capable of working under pressure. The operating pressure of 30 bar is chosen since it is the most common pressure among commercial pressurized electrolyzers. The cell voltage of the electrolysis cell has been set to 1.75 V and the anodic and cathodic faradaic efficiencies have been set to 0.99, which comply with industry standards19. 17 ACS Paragon Plus Environment


The calculation regarding the mass balances are done in accordance with the General Considerations. Both faradaic efficiencies are set to 0.99, given that under 30 bar of operation the crossover rate is increased, and such values seem to comply with other reported values of pressurized electrolyzers73, 81.

High-temperature electrolysis The input of high-temperature electrolyzers must be superheated water vapor. In this regard, the production of steam from seawater is considered in this plant. Given the marine renewable setting, the existence of an external high-temperature heat source is unlikely. For this reason, the chosen method to produce heat is ohmic heating. In any case, as high efficiency is fostered, heat recovery is used whenever possible within the plant. This means that whenever a hot stream with cooling needs, with a temperature of at least 5 °C above a cold counterpart with heating needs, a heat exchanger is placed. This can be checked in Figure 5 below the boiler, where different sources of heat that originate from the plant are used, specifically, the intercoolers of the first three compression stages 𝐻𝐸1, 𝐻𝐸2, and 𝐻𝐸3 of the H2 stream and the heat delivered by the electrolyzer. With such heat links, at the steady operating conditions of the plant, there is no need to perform ohmic heating at the boiler. In Figure 5, it can be seen that there are four stages of compression with intercooling. Similar to the case of direct seawater electrolysis, the reason to have more than three compression stages is that according to Eq. (13), with 𝑁 = 3, 𝑟𝑝 > 7, hence the four compression stages. Similar to the other plants, the determination of the mass flow rates has been achieved by fixing the output of pure hydrogen to 1 kg/s at 350 bar. The crossover of the gaseous streams of H2 and O2 have been calculated taking into consideration the faradaic efficiencies of 𝜂𝐹 = 0.99. However, the conversion of steam to H2 and O2 in the high-temperature electrolyzer is not complete 99. This is reflected by a factor named steam conversion factor, 𝑆𝐶. An 𝑆𝐶 = 1 results in acute concentration polarization, thus, a source of energy loss99. In this regard, 𝑆𝐶 = 0.6 has been chosen, which complies with the literature19. The result of having some remnant of H2O in the cathode is that there will be some H2O crossover from the cathode to the anode. The estimation of steam crossover is calculated by using of the Graham law of effusion, which relates the molar flow rate of different gaseous species through a pore with the molar masses of such species. According to such law, the flow rates are inversely proportional to the square root of the molar masses of the different species. As hydrogen crossover coincides with the direction of the steam crossover, hydrogen is used as a reference. Therefore, in the anode output, the molar flow rate of water is estimated with:


Page 18 of 33

17


13

VS

H2

14

𝑊̇��

Mixing chamber

COND2

10

9

𝑄̇�� 𝑄̇��

15

𝑄̇�� 𝑄̇��

COND1

𝑊̇��

8 HR3

HR2

COND4

5 O2

4

𝑊̇��

16

Seawater

1

SW pump

11

27

2

H2

Brine

3

𝑊̇��

HC4

26

HR1

COND3

25

HE3

𝑄̇��

24

6

7

𝑊̇��

HC3

23

HE2

𝑄̇��

22

HC2

21

HE1

𝑄̇��

20

HC1

19

12

𝑊̇��

O2

HR3

Anode

18

Cathode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


𝑄̇��

Page 19 of 33

Figure 5: Simplified diagram of the high-temperature electrolysis plant. COND is condensate; HC is hydrogen compressor; HE is heat exchanger; HR is heat recovery; SC is steam compressor; SW is seawater; VS is voltage source.

(

𝑛(𝐻2𝑂) 𝑜𝑢𝑡 = 𝑛(𝐻2𝑂)𝑖𝑛·(1 ― 𝑆𝐶)· 1 ― 𝑎𝑛

𝑛𝐻2𝑎𝑛

·

𝑀 𝐻2

𝑛𝐻2𝑐𝑎𝑡 𝑀𝐻2𝑂


)

(34)


Page 20 of 33

In the cathode output, the molar flow rate of water is estimated with: 𝑛(𝐻2𝑂)𝑜𝑢𝑡 = 𝑛(𝐻2𝑂)𝑖𝑛·(1 ― 𝑆𝐶)· 𝑐𝑎𝑡

(

𝑛𝐻2𝑎𝑛

·

𝑀 𝐻2

𝑛𝐻2𝑐𝑎𝑡 𝑀𝐻2𝑂

)

(35)

Results Direct seawater electrolysis Following the diagram shown in Figure 3, Figure 6 (a) depicts the mass ratios of the cathodic stream compounds, referenced to the output of pure hydrogen. There, it can be checked that leaving the electrolyzer the stream of hydrogen carries impurities. After the purification and compression steps, in 16, the volumetric purity of hydrogen is above 99.99%, being water vapor the only impurity. Important results in an environmental sense, are the calculated mass ratio of the rejected brine that is returned to the sea and its 𝑝𝐻 value, which are 2.14 kg/kgH2 and 𝑝𝐻 = 14.33, respectively. The alkalizing power of such stream is even higher than what the pH value suggests, due to the presence of swept Mg(OH)2(s) and Ca(OH)2(s), because once diluted in seawater, such solids dissolve. Out of the 2.14 kg/kgH2that compose the rejected brine, 36.5 g account for Mg(OH)2(s) and 9.0 g for Ca (OH)2(s). From an environmental perspective, another important fact is the calculated Cl2(g) sent to the atmosphere along with the anodic stream, which in total is 0.23 kg/kgH2. Figure 7 (a) shows the results of the energy balance of the direct seawater electrolysis plant. There, it can be seen that more than 97% of the plant consumption accounts for the electrolyzer. The main Balance of Plant (BOP) consumers are the four compressors denoted by HC1 to HC4, being the total energy needed for compression 12.47 MJ/kgH2. In this electrolysis plant, the heat delivered by the electrolyzer, operating at 15.9 °C, is 290.0 MJ/ kgH2. Under the studied conditions, such heat is generated at 15.9 °C rendering it unusable for cogeneration.


Page 21 of 33

(a)

mass of species per mass of output hydrogen (kg/kgH2)

1.5

H2

O2

Cl2

HCl

H2O

1

0.5

0 6

7

8

9

10

11

12

13

14

15

16

Points of cathodic stream of Figure 3

(b) mass of species per mass of output hydrogen (kg/kgH2)

1.5

H2

O2

H2O

1

0.5

0 21

22

23

24

25

26

27

28

29


(c) 5

mass of species per mass of output hydrogen (kg/kgH2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.5 4 3.5

H2 H2O

3 2.5

O2

2 1.5 1 0.5 0 17

18

19

20

21

22

23

24

25

26

27


Figure 6: Mass ratio results of the cathodic stream of the different hypothetic plants, shown in Figures 3-5, referenced to the output of pure hydrogen. (a) Direct seawater electrolysis, (b) low-temperature electrolysis and (c) high-temperature electrolysis.



(a) HC3 3.05

HC4 3.27

HC2 3.02

HC1 3.13

12.47 BOP Electrolyzer 439.55

(b)

5.37 BOP

HC3 1.72

HC2 1.60 HC1 1.55

Electrolyzer 169.23

SC 0.47 FWp 0.03

(c) HC3 18.92 BOP

HC2 5.32

4.14 HC4

Electrolyzer 135.38

4.14 HC1 5.33

Figure 7: Energy balance of the different hypothetic plants, shown in Figures 3-5. (a) Direct seawater electrolysis, (b) lowtemperature electrolysis and (c) high-temperature electrolysis. All the figures are expressed in 𝑀𝐽/𝑘𝑔𝐻2. BOP stands for Balance of Plant, HC for hydrogen compressor, FWp for Fresh Water Pump and SC for Steam Compressor.

Low-temperature electrolysis Following the diagram shown in Figure 4, Figure 6 (b) summarizes the mass ratios of the cathodic stream compounds, referenced to the output of pure hydrogen. There, it can be checked that after leaving the electrolyzer at 21, the stream holds some impurities. After the purification and compression steps, in 29, the volumetric purity of hydrogen is above 99.99%, being water vapor the only impurity. Figure 7 (b) shows the power consumption of the different consumers of the plant. There, it can be noted that almost 97% of the plant consumption accounts for the electrolyzer. In this case, the total energy for compressing the hydrogen is reduced to 4.87 MJ/kgH2. Such figure proves the benefits of having the electrolyzer operating under pressure. Under this scheme, the heat delivered by the electrolyzer is 26.5 MJ/kgH2, when operating at 76.3 °C.

High-temperature electrolysis Following the diagram shown in Figure 5, Figure 6 (c) shows the results of the calculations of the cathodic stream. There, it can be noted that the stream holds an important amount of water after leaving the electrolyzer in 17. This is mainly due to the steam conversion factor 𝑆𝐶 = 60%. After 22 ACS Paragon Plus Environment

Page 22 of 33

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the purification and compression steps, in 27, the volumetric purity of hydrogen is above 98.79%, being water vapor the only impurity. The reason why under this scheme there is more water content in the output hydrogen is due to its higher temperature, of 80.4 °C. Figure 7 (c) shows the power consumption of the different consumers of the plant. There, it can be seen that almost 88% of the plant consumption accounts for the electrolyzer. In this case, the total energy for compressing the hydrogen is 18.93 MJ/kgH2. The higher energy needs regarding compression are due to the high water content in the cathodic stream, as can be seen in Figure 7 (c). Under this scheme, the heat delivered by the electrolyzer is 8.8 MJ/kgH2, when operating at 757.2 °C. Part of such heat is conveyed to the boiler. Thanks to that, no additional ohmic heating is needed to boil seawater at steady state.

Discussion This work studies different marine electrolysis plants relying on diverse technologies operating at steady state. Figure 8 shows, for each hypothetic plant studied, the specific energy needed to produce one kg of H2, divided into electrolyzer and BOP, the heat delivered, and the operation temperature of the electrolyzer. Starting with the specific energy required, the direct electrolysis of seawater bears 452.0 MJ/kgH2, the low-temperature electrolysis holds 174.6 MJ/kgH2, and the high-temperature electrolysis of seawater presents 154.3 MJ/kgH2. In this regard, low-temperature and high-temperature electrolysis plants have very similar specific energy requirements to produce hydrogen at 350 bar, being the difference of only 13%. Regarding the heat delivered to the marine environment, direct electrolysis of seawater presents the highest value, with 303.2 MJ/kgH2, followed by low-temperature electrolysis with 32.0 MJ/kgH2. High-temperature electrolysis exhibits the lowest value in this regard, 6.7 MJ/kgH2. The reason to have such a small value stems from the fact that the heat sources within the plant present very high temperatures, making it easy to use them for cogeneration within the plant, mostly for the production of superheated steam. Otherwise, the heat needs for the production of superheated steam would have required ohmic heating. This would have made an impact as well on the BOP specific energy of the high-temperature electrolysis plant, increasing the total specific energy to produce hydrogen at 350 bar. Regarding the calculated purity of the output hydrogen, all three hypothetic plants have water vapor as the only impurity. This is explained by the assumptions made regarding the complete removal of crossover gasses, which were the only impurities considered apart from water. Other sources of impurities were not assessed in this work. In this sense, such results should be treated with care. The operation temperature of the electrolyzer is important since the electrolyzer represents the main source of heat produced within the plants. Such value is important energetically speaking, because the higher the temperature, the easier it is to produce electric power with a thermal cycle. In this regard, the 6.7 MJ/kgH2 of the high-temperature electrolysis cell, could be used in a Rankine cycle to produce about 2.7 MJ/kgH2 with steam turbines, if a cycle efficiency of 0.4 were considered. 23 ACS Paragon Plus Environment


500.00

800

Heat delivered

450.00

700

BOP 400.00

600

Electrolyzer 350.00 Temperature

500

300.00 250.00

400

200.00

300

150.00 200 100.00 100

50.00 0.00

0 Direct electrolysis of seawater

Low-temperature electrolysis

High-temperature electrolysis

Figure 8: Comparison of results plants shown in Figures 3-5: Heat delivered to the marine environment, specific energy for hydrogen production at 350 bar and operation temperature of the electrolyzers. BOP stands for Balance of Plant.

Direct electrolysis of seawater should be discarded for large-scale marine green hydrogen production, because it is very inefficient and very harmful to the environment, due to the strong alkalization of the sea caused by rejected caustic brine, and the production of highly toxic Cl2. The only two viable options capable of producing H2 at sea are the low-temperature and the hightemperature electrolysis technologies. In this sense, high-temperature electrolysis is the best technology regarding efficiency when operating at steady state conditions, even when no external high-temperature heat sources are used. However, when coupled with renewable energies such as offshore wind, actual conditions are often far from steady. This means that plant dynamics should be considered to have a better judgment on the matter. Under this perspective, high-temperature electrolysis could be less practical, especially due to current cold start times of this technology, which are currently measured in hours19. Nevertheless, slow dynamics of high-temperature electrolysis could be solved by maintaining the plant always at operating temperature even at times when the farm does not generate enough power for hydrogen production. Therefore, part of the produced hydrogen would have to be consumed for this purpose. For obvious reasons, this would raise the specific energy to produce H2, but it would maintain the lifetime of the high-temperature technology at sea, as it would reduce thermal fatigue. Another problem of high-temperature technologies regarding the power input variation is that the steam conversion factor would be variable, which has a great impact on the specific energy of hydrogen production. This means that this technology would be penalized in a marine renewable context.


Temperature (°C)

Specific energy for hydrogen production at 350 bar (MJ/kgH2)

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Low-temperature electrolysis has much quicker dynamics and does not have the problem of the water conversion factor. The only issue they might have regarding safety is that when operating at partial loads, i.e. low current densities, the hydrogen concentration in the anodic stream rises, leading to potential explosive mixtures100. In any case, this problem is applicable to all electrolysis technologies, including the high-temperature one. On one hand, even though further research is needed, low-temperature electrolysis, namely PEM or alkaline technologies, seems to be fitter for most scenarios of green hydrogen production at sea, when coupled with marine renewable energies. On the other hand, High-temperature electrolysis could be the best solution for cases where high-temperature heat sources are available, such as geo-thermal energy, to solve the problems associated with thermal dynamics. Currently, marine offshore renewable farms are connected to the shore by means of submarine power cables that link the offshore substation to its onshore counterpart, which at the same time is connected to the grid101. The hypothetic hydrogen infrastructure of this study ideally aims to replace the described link to generate an off-grid marine renewable farm concept. At present, the LCOE of marine renewables is still very high for making large-scale green hydrogen production at sea profitable without state aid. In this regard, one of the lowest reported LCOE of offshore wind is around 90 $/MWh34. This means that without considering both the investment costs and operational costs of the hydrogen production plant, and also ignoring the associated costs of the hydrogen logistic chain, the base cost of marine green hydrogen, using low-temperature electrolysis technologies, is estimated around 4.25 $/kg. Such a figure is well above the 2020 target set by the DOE of 2.30 $/kg102. Perhaps in the mid-term, marine green hydrogen produced under this scheme could become competitive with other production methods of hydrogen, provided that costs related to marine renewables drop. This is perfectly possible since the costs of offshore wind have kept a downtrend in the last decade, and show signs of maintaining that trend in the following years103. In any case, hydrogen could benefit from grid curtailment, which is one problem derived from gridconnected offshore wind farms104. In some grid connection agreements, part of the output power is curtailed at certain times, irrevocably causing energy loss. If the curtailed power were to be connected to a hydrogen production plant, like the low-temperature type of this study, cheap hydrogen could be produced with power that otherwise would be lost. In fact, equivalent installations onshore that employ marine renewables have been successful at producing low-cost green hydrogen by using curtailed electricity105. This is possible by connecting such curtailed electricity to low-temperature electrolyzers that feature quick responses. The downside of this exploitation model is that they have limited applicability and can only produce small amounts of hydrogen directly related to the power curtailment of the farm.

Conclusions An energetic assessment for offshore production of pressurized hydrogen at 350 bar, using electricity sourcing from marine renewable energies has been performed on three different electrolysis technologies: direct seawater electrolysis, low-temperature electrolysis, and hightemperature electrolysis. Results have revealed that direct seawater electrolysis is completely unfit for green hydrogen production at sea, mainly because of the very high specific energy needed to produce hydrogen. 25 ACS Paragon Plus Environment


Specifically, it requires about 160% more energy than the low-temperature electrolysis, but also due to the environmental impact that it implies. For most cases in marine renewable scenarios, when plant dynamics come into play, lowtemperature technologies seem to be the most practical. This is because low-temperature electrolysis adapts better to the power variation inherent to most renewables. In this regard, the specific energy to produce hydrogen at 350 bar with respect to high-temperature electrolysis in marine offshore conditions at steady state is only a 13% higher. Regarding the specific energy consumption, the best technology operating at steady state conditions is the high-temperature electrolysis. Such technology could be unrivaled whenever an external heat source of high temperature was available nearby. Large-scale production of marine green hydrogen is not currently profitable because its resulting price without state aid is not competitive with other production methods. This is mainly due to the high costs of electricity sourcing from marine renewables. Such costs are expected to decrease in the following years, perhaps to the point where this source of green hydrogen becomes competitive. For grid-connected offshore renewable farms, small-scale production of marine green hydrogen could be competitive at present, since in most cases curtailed electricity often benefits from better prices.

Acknowledgements The authors acknowledge the Spanish Ministry of Economy and Competitiveness and European Regional Development Funds through the Research Project ENE2017-86711-C3-2-R (DECARBOPIME). This work has been partially funded as well by Cátedra Empresa SoermarUniversidad Politécnica de Madrid through the Multiannual plan of doctoral grants.

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(78) Friedrich, K. A.; Lettenmeier, P.; Stiber, S.; Ansar, A. S.; Wang, L.; Gago, A. S. In Cost-Effective PEM Electrolysis: The Quest to Achieve Superior Efficiencies with Reduced Investment, 2018; The Electrochemical Society: pp 15-15. (79) Bertuccioli, L.; Chan, A.; Hart, D.; Lehner, F.; Madden, B.; Standen, E., Development of water electrolysis in the European Union. Fuel Cells and Hydrogen Joint Undertaking 2014, 83. (80) d'Amore-Domenech, R.; Santiago, O.; Villalba-Herreros, A.; Mora, E.; Leo, T. J. In Energetic assessment of high-pressure PEM electrolyzers for the production of hydrogen at 900 bar for Hydrogen Refueling Stations, Iberconappice, Huesca, APPICE, Ed. APPICE: Huesca, 2017; pp 175-178. https://appice.es/Congresos/H2-Iberconappice2017.pdf (81) Millet, P.; Grigoriev, S. A.; Porembskiy, V. I., Development and characterisation of a pressurized PEM bi-stack electrolyser. International Journal of Energy Research 2013, 37 (5), 449-456, DOI 10.1002/er.2916. (82) Siracusano, S.; Van Dijk, N.; Backhouse, R.; Merlo, L.; Baglio, V.; Aricò, A. S., Degradation issues of PEM electrolysis MEAs. Renewable Energy 2018, 123, 52-57, DOI 10.1016/j.renene.2018.02.024. (83) Agersted, K. D5.1 Report on expected marinised hydrogen generator performance; www.h2ocean-project.eu, 2014; p 157. (84) Vincent, I.; Bessarabov, D., Low cost hydrogen production by anion exchange membrane electrolysis: A review. Renewable and Sustainable Energy Reviews 2018, 81, 1690-1704, DOI 10.1016/j.rser.2017.05.258. (85) Vincent, I.; Kruger, A.; Bessarabov, D., Hydrogen Production by water Electrolysis with an Ultrathin Anion-exchange membrane (AEM). Int. J. Electrochem. Sci 2018, 13, 11347-11358, DOI 10.20964/2018.12.84. (86) Schefold, J.; Brisse, A.; Tietz, F., Nine thousand hours of operation of a solid oxide cell in steam electrolysis mode. Journal of The Electrochemical Society 2011, 159 (2), A137-A144, DOI 10.1149/2.076202jes. (87) Abanades, J.; Torregrosa, J. P. In MAESTRALE: The Implementation of Blue Energy in the Mediterranean Sea, 2018; American Society of Mechanical Engineers: pp V010T09A018V010T09A018, DOI 10.1115/OMAE2018-77593. (88) Jensen, S. H.; Sun, X.; Ebbesen, S. D.; Knibbe, R.; Mogensen, M., Hydrogen and synthetic fuel production using pressurized solid oxide electrolysis cells. International Journal of Hydrogen Energy 2010, 35 (18), 9544-9549, DOI 10.1016/j.ijhydene.2010.06.065. (89) Sunfire-HyLink e-Brochure. https://www.sunfire.de/files/sunfire/images/content/Produkte_Technologie/factsheets/SunfireHyLink_FactSheet.pdf (accessed March 22, 2019). (90) Arnould, F.; Bachellerie, E.; Auglaire, M.; Boeck, B. d.; Braillard, O.; Eckardt, B.; Ferroni, F.; Moffett, R.; Van Goethem, G., State of the art on hydrogen passive auto-catalytic recombiner (european union Parsoar project). 2001. (91) Chase, M. W., NIST—JANAF Thermochemical Tables (Journal of Physical and Chemical Reference Data Monograph No. 9). American Institute of Physics 1998. (92) Millero, F. J.; Feistel, R.; Wright, D. G.; McDougall, T. J., The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale. Deep Sea Research Part I: Oceanographic Research Papers 2008, 55 (1), 50-72, DOI 10.1016/j.dsr.2007.10.001. (93) Nayar, K. G.; Sharqawy, M. H.; Banchik, L. D.; Lienhard V, J. H., Thermophysical properties of seawater: A review and new correlations that include pressure dependence. Desalination 2016, 390, 1-24, DOI 10.1016/j.desal.2016.02.024. (94) Morillo, J.; Usero, J.; Rosado, D.; El Bakouri, H.; Riaza, A.; Bernaola, F.-J., Comparative study of brine management technologies for desalination plants. Desalination 2014, 336, 32-49, DOI 10.1016/j.desal.2013.12.038. 31 ACS Paragon Plus Environment


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Rafael d’Amore-Domenech is Naval Architect and Marine Engineer. He received his M.S. in Marine and Offshore Engineering from the Technical University of Madrid (UPM). He is a member of the Research Group of Fuel Cells, Hydrogen Technologies and Reciprocating Engines (PICOHIMA), located at the Faculty of Naval Architecture and Marine Engineering (ETSIN) in UPM. In such Faculty, he is a Ph.D. Candidate and Teaching Assistant as well. There, Rafael works on the development of seawater electrolysis technologies, in addition to the implementation of fuel cell systems in marine 32 ACS Paragon Plus Environment

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crafts. Since 2017, he represents UPM in the biannual expert meetings organized by "IEA-Hydrogen Task 39", on the implementation of hydrogen in marine applications. In 2017, he received the Graduate Paper Honor Prize from the Society of Naval Architects and Marine Engineers (SNAME) for co-authoring the technical paper entitled “Towable TLP solution for Offshore Wind”.

Teresa J. Leo is Full Professor at the Technical University of Madrid (UPM). She leads the Research Group of Fuel Cells, Hydrogen Technologies and Reciprocating Engines (PICOHIMA). PICOHIMA collaborates with several universities and research centers to improve fuel cells regarding weight, cost, volume, power density, durability, and efficiency; in addition to the research on the production of hydrogen from marine renewable energies. She is a member of the Technological Platform of Hydrogen and Fuel Cells (PEHPC), the Spanish Maritime Platform (PTME), and the Spanish Hydrogen Association (AEH2). She participates in the IEA Task 39: Hydrogen in Maritime Transport, representing the UPM. She has published a number of research papers and presented many works in national and international conferences and co-owns Intellectual and Industrial Property of software for design and behavior simulation of fuel cells. In her academic activity, she has taught in two Spanish universities subjects as Thermodynamics, Thermal Engineering, Hydrogen, and Fuel Cell Technology, and has coordinated or participated in many competitive Education Innovation Projects.

H2 cH2

For Table of Contents Use Only: Study of production of compressed hydrogen at sea, using electrolysis coupled with marine renewable energies.


Sustainable Hydrogen Production from Offshore ... - ACS Publications

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