DE102017001257A1 - Offshore electrolysis of seawater for hydrogen production - Google Patents

Abstract

One of the biggest problems of renewable energies is the lack of storage capacity to cushion fluctuations. In addition, the demand for sustainably produced hydrogen by the automotive industry is constantly increasing. These two problems are tackled by the invention, a plant for the offshore electrolysis of seawater using the energy of wind farms. The plant prevents the formation of harmful substances (such as chlorine gas), which would occur in conventional salt water electrolysis and is more efficient than the electrolysis of distilled water.
The complete system can also rotate as an option and thus always align perfectly with the ocean currents and exploit them.

Description

problem

One of the biggest problems of renewable energy, such as wind energy, is its strong dependence on weather conditions, which leads to very unequal and inadequate generation of electricity. Wind turbines often produce much more electricity on windy days than is needed. Nor can the relatively small proportion of weather- and thus random energy producers in the total energy production in the case of sudden peaks cushion. However, the higher the feed-in share of the randomly dependent energy sources compared to the controllable sources (power plants), the more difficult and expensive is the correction made by the conventional power plants. There is a lack of electricity storage capacity for a better time distribution of electricity in order to enable the energy transition in terms of grid stability, without the entire wind farm must be disconnected from the grid. [1]

In addition, the transmission of electricity over the normal power grid with grid losses of about 6.4. % connected. [2]

On the other hand, hydrogen as an energy carrier (for example for the automotive industry) is becoming increasingly interesting. For example, a steadily increasing number of German companies, including Audi, are testing carbon-neutral fuels synthesized from hydrogen and carbon dioxide (eg, oxygen methyl ether, OME for short) [3]. Such fuels have a very high energy density, are well storable, transportable and easy to use in conventional engines and for heating buildings.

This raises the question of whether these two problems can be solved by using the power of offshore wind turbines in the North Sea locally (decentralized) from salt water by electrolysis to win hydrogen.

research status

Despite the high relevance of the problem and the closeness to industry, research in this field is even more than manageable. Although there are studies in various areas such as the transport of hydrogen, the Sabatierprozeß and also for hydrogen production, but there are neither exact plans to build an electrolysis cell of this magnitude, nor the energy balance of hydrogen production and the subsequent conversion into electricity, let alone a coherent overall concept for introduction. Accordingly, there is also no research on possible waste products and whether the environment would be affected by the massive use of this technique. It turns out that there is still enormous research potential for us and that we have many starting points.

Results

Our previous research into the realization of a decentralized electrolysis of seawater by means of offshore wind power in the North Sea was based on experiments on optimal electrolysis conditions, the energy accounting for electrolysis, the elimination of waste products and the reuse of hydrogen.

Experiments on the optimal conditions for seawater electrolysis

In order to find out what are the optimal conditions for achieving the highest possible yield of hydrogen with the lowest possible expenditure of energy, electrolysis was carried out under different conditions and the respective hydrogen yield was measured. The influence factors salinity, voltage, current and temperature were investigated. In addition, some basic questions were clarified, such as whether the solution in the electrolysis (strong) heated. For the simulation of seawater, mixtures of distilled water and salts were used. Experiments with sodium chloride solutions took place due to the possible formation of chlorine gas in a closed electrolysis cell and restricted under a trigger. The power source used was a 12V regulated power supply, the electrolysis cell used was a prefabricated cell having graphite electrodes. In order to measure the resulting gas quantities, we have placed top-sealed syringes over the electrodes ( - see list of figures).

Experiment 1: Temperature increase and gas bubble formation

First, an attempt was made to increase the temperature of electrolysis. In addition, gas bubble formation was observed during electrolysis. For the experiment, 100 ml of a 3.1% solution of sodium chloride in water with an initial temperature of 21.5 degrees Celsius was energized (I = 1A, U = 10V). It formed immediately gas bubbles. The temperature rose within the first minute to 22 degrees Celsius and then remained the same for the rest of the time. The trial ended after three minutes. Immediately the gas bubble formation ended.

In one repetition, the same solution was placed under a current of 0.5 amps and under a voltage of 8 volts. It formed immediately gas bubbles. The temperature rose within the first minute to 21.8 degrees Celsius and then remained the same for the rest of the time. The trial ended after three minutes. Immediately the gas bubble formation ended. In a third run, a 4.6% solution was placed under a current of 1 amp and at a voltage of 10 volts. It formed immediately gas bubbles. The temperature rose within the first minute to 22 degrees Celsius and then remained the same for the rest of the time. The trial ended after three minutes. Immediately the gas bubble formation ended.

conclusion:

• • The electrolysis begins and ends synchronously with the flow of current.
• • The temperature only rises at the beginning and only slightly at the electrolysis.
• • Apparently, increasing the voltage and current also increases the hydrogen yield, while the salinity does not matter. But this still has to be checked in further experiments.

Experiment 2: Influence of the temperature on the hydrogen yield

• • Die Wasserstoffgewinnung verläuft proportional zur Zeit. Es wird immer gleich viel Wasserstoff pro Zeiteinheit produziert.
• • Je höher die Temperatur der Lösung, desto höher die Ausbeute an Wasserstoff.

The second experiment was to show how different temperatures of the salt solution to be electrolyzed affect the hydrogen yield. Four 3 % Sodium sulfate solutions with temperatures of 10 ° C, 20 ° C, 30 ° C and 65 ° C were electrolysed at a current of 1 A and a voltage of 12 V. To 3 . 6 . 9 and 12 Minutes, the amount of hydrogen produced was measured in each case and recorded in a diagram ( - see list of figures).

• • Hydrogen production is proportional to time. It always produces the same amount of hydrogen per unit of time.
• • The higher the temperature of the solution, the higher the yield of hydrogen.

It should be noted here that the solution at 65 ° C has cooled to 45 ° C within 12 minutes.

Experiment 3: Influence of the voltage on the hydrogen yield

To investigate the effect of voltage on hydrogen yield, three identical, 3% sodium sulfate solutions were electrolyzed at voltages of 6, 10 and 12 volts at a current of 2 amps and a temperature of 10 ° C (to simulate seawater temperatures). The hydrogen yield was recorded every three minutes for 27 minutes. Note: The amount of hydrogen breaks at the end of the curve for 12V electrolysis because the complete solution is exhausted and the hydrogen is drained. So we used more solutions for the following experiments ( - see list of figures).

conclusion:

• • The higher the voltage, the higher the hydrogen yield, but there is no direct proportionality.

Experiment 4: Influence of the current on the hydrogen yield

Three identical solutions as out of trial 3 were electrolyzed at a voltage of 12 volts and at currents of 0.6, 1, 1.3 and 2 amperes and the hydrogen yield was plotted as follows: Conclusion: It can be seen that the current has a lower effect on the yield than the voltage Has. In addition, the yield is surprisingly at a current of 0.6 A and not at 2 A largest. This seems to be implausible, as according to the Ohm's Law (U = R * I) at increasing current also the voltage and thus the yield of hydrogen increases (see experiment 3 ). Thus, the hydrogen yield is likely to be slightly higher despite the experimental results at higher current and the experiment must be repeated. Also, the measurement had to be stopped at 12 volts after 24 minutes, since the entire salt solution was consumed. So next time we have to fix more solutions ( - see list of figures).

Experiment 5: Influence of the salt content on the hydrogen yield

Two sodium sulfate solutions with sodium sulfate contents of 4.3% and 5.3% were electrolyzed at a current of 2 A and at a voltage of 12 V, and hydrogen production was measured. To 18 Minutes, 10.5 ml of hydrogen had formed during the electrolysis of the 4.3% solution, and 12.3 ml in the case of the 5.3% solution. Conclusion: A higher salt concentration leads to a higher hydrogen yield. This could be due to the salt's higher conductivity of the solution. This could be a fundamentally important finding for the idea of seawater electrolysis since this result presumably suggests that seawater electrolysis is more efficient than the conventional electrolysis of distilled water in the art. Thus, our idea would probably be clearly superior to the state of the art in terms of efficiency - as you can see in the graph.

By-product recycling

In the electrolysis of seawater, in which mainly sodium chloride is dissolved, but not as in the electrolysis of water, oxygen and hydrogen, but also caustic soda and chlorine gas: 2H ₂ O + 2NaCl → 2NaOH + Cl ₂ + H ₂

1. 1. 2H₂O + 2NaCl → 2NaOH + Cl₂ + H₂ Startreaktion: Meerwasserelektrolyse
2. 2. H₂0 + ClH₂ → HCl + HClO Reaktion von entstandenem Chlor mit Wasser
3. 3. HCl + NaOH → NaCl + H₂O Neutralisationsreaktion
4. 4. 3HClO → 2HCl + HClO₃ Disproportionierung
5. 5. HCl + NaOH → NaCl + H₂O Neutralisationsreaktion
6. 6. 2HClO₃ → H₂ + Cl₂ + 3O₂ Zerfallsreaktion

This happens only in small quantities. However, the caustic soda and the chlorine gas must be removed as cleanly as possible, preferably without the supply of substances. As a solution, we have developed a step-by-step reaction chain to break down the chlorine and brine without chlorate formation:

1. 1. 2H ₂ O + 2NaCl → 2NaOH + Cl ₂ + H ₂ start reaction: Seawater electrolysis
2. 2. H ₂ O + ClH ₂ → HCl + HClO Reaction of formed chlorine with water
3. 3. HCl + NaOH → NaCl + H ₂ O neutralization reaction
4. 4. 3HClO → 2HCl + HClO ₃ disproportionation
5. 5. HCl + NaOH → NaCl + H ₂ O neutralization reaction
6. 6. 2HClO ₃ → H ₂ + Cl ₂ + 3O ₂ decomposition reaction

This results in the following reaction equation: 3H ₂ O + 2NaCl → 2H ₂ + 2NaCl + H ₂ O + O ₂

Using these reactions, which all react independently in this way in the chain, the chlorine gas can be converted step by step into hydrochloric acid, which reacts immediately with the resulting sodium hydroxide solution in the reaction. Thus, the chlorine is again bound in sodium chloride.

Energy balance calculation

$$\text{Reaktionsenthalpie Reaktionskette}\approx 606.670\text{ J/mol}$$

$$\text{Wasserstoffproduction}=\frac{\mathrm{25.200.000.000.000}J\text{/}Jahr}{606670J\text{/}mol}=\mathrm{41538461,54}\text{ mol/Jahr}\approx \mathrm{83,1}\text{ t Wasserstoff/Jahr}$$

As the electrolysis is later to function on an industrial scale, the question arises as to how much energy the entire reaction chain (see above) would need and how much hydrogen a wind turbine could produce per year. This can be realized by calculating the reaction enthalpy and comparing it with the power production of a wind turbine: $$\text{Electricity generation wind turbine}\approx 7000000000\text{Wh / year}=\mathrm{25,200,000,000,000}\text{J / year}$$

$$\text{Reaction enthalpy reaction chain}\approx 606670\text{J / mol}$$

$$\text{Hydrogen Production}=\frac{\mathrm{25,200,000,000,000}J\text{/}JaHr}{606670J\text{/}mOl}=\mathrm{41,538,461.54}\text{mol / year}\approx 83.1\text{t hydrogen / year}$$

$$\text{Effizienz }\left[\text{\%}\right]=100\text{\%}*\frac{\mathrm{11.780.307.692}kJ}{\mathrm{25.200.000.000}kJ}=\mathrm{46,7}\text{\%}$$

From the amount of hydrogen produced, it is possible to calculate how efficient the process of converting electricity into hydrogen and subsequently obtaining electricity from hydrogen is by calculating how much electricity can be obtained from 83.1 tonnes of hydrogen: $$83.1\mathrm{<figure-callout\; id="2"\; label="t\; H"\; filenames="DE102017001257A1\_0008.png"\; state="\{\{state\}\}">t\; </figure-callout>}{H}_{}{}_2\approx \mathrm{11,780,307,692,000}J=3272307692kWH$$

$$\text{efficiency}\left[\text{\%}\right]=100\text{\%}*\frac{11780307692kJ}{25200000000kJ}=46.7\text{\%}$$

It should be noted here that this value is rather optimistic due to unavoidable energy losses in the reactions. Nevertheless, he is probably relatively high and thus to be judged as positive.

Also important for the course of a reaction is entropy. Together with the enthalpy, the entropy is calculated in the Gibbs-Helmholtz equation: ΔG = HΔ - T * SΔ. If this is calculated for the reaction chain (→ ΔG = 606.67-278.15 * 677 = -187.701), a negative result is obtained. This means that the reaction is exergonic and thus voluntary (the temperature in Kelvin is here chosen to correspond to the temperature of the seawater of 5oC). This is an important result for a functioning electrolysis, since otherwise the reaction chain would have to be forced by means of energy.

Also note the Gibbs-Helmholtz equation for the pure electrolysis of water, as it also takes place (the concentration of salts in the water is relatively low): $$\mathrm{\Delta }G=\mathrm{\Delta }H-T*\mathrm{\Delta }S=572-278.15*327=-\mathrm{90.383.1}\to \text{exergonically}$$

This reaction is also exergonic, since the result is in the negative range.

Further use of hydrogen

• • Durch Verbrennung kann der Wasserstoff wieder in Strom umgewandelt werden, um die Endverbraucher zu versorgen.
• • Durch eine Reaktion kann der Wasserstoff mit Kohlenstoffdioxid reagieren (Sabatier-Prozess). Dabei entstehen Methan und Wasser (CO₂+ 4H₂ → CH₄ + 2H₂O). Das Methan kann in das Erdgasnetz eingespeist werden und so zur Beheizung von Haushalten beitragen. Da die Reaktion exotherm ist, kann die entstehende Wärme zusätzlich in Strom umgewandelt werden. Der Wasserstoff lässt sich ohne Umwandlung in Methan in das Erdgasnetz einspeisen, aufgrund seiner Kapazität ist das Erdgasnetz sehr gut für die Aufnahme von Wasserstoff aus regenerativer elektrischer Energie geeignet. [4]
• • • Autos können in Zukunft mit dem Wasserstoff fahren, oder der Wasserstoff kann mithilfe von Prozessen wie dem Sabatier-Prozess in synthetische, CO₂-neutrale und energiedichte Treibstoffe für bestehende Verbrennungsmotoren umgewandelt werden.

The generated hydrogen can be used in a variety of ways, as mentioned in the beginning of the formulation of the problems:

• • Combustion allows the hydrogen to be converted back into electricity to supply the end user.
• • Through a reaction, the hydrogen can react with carbon dioxide (Sabatier process). This produces methane and water (CO ₂ + 4H ₂ → CH ₄ + 2H ₂ O). The methane can be fed into the natural gas grid and thus contribute to the heating of households. Since the reaction is exothermic, the resulting heat can be additionally converted into electricity. The hydrogen can be fed into the natural gas grid without conversion into methane, because of its capacity, the natural gas network is very well suited for the absorption of hydrogen from renewable electrical energy. [4]
• • Cars can use hydrogen in the future, or the hydrogen can be converted into synthetic, CO ₂ -neutral and energy-dense fuels for existing combustion engines using processes such as the Sabatier process.

Effects of our results on the question

Our results show very clearly that the concept of seawater electrolysis for hydrogen production is definitely conclusive. First of all, it is more concrete that this can actually be implemented, since the waste products produced can be degraded in an environmentally friendly manner. Also, the electrolysis already works with the salt content contained in the sea with sufficient efficiency. The efficiency in general is so great that the implementation of the concept is quite attractive. To sum up, our results have proven that further study of the topic is very worthwhile and should definitely be continued.

outlook

• • Sollte man die Elektrolyse wirklich im Meer durchführen, oder an Land in Sammelstellen für den Strom? Denn sonst müsste man Anlagen unter jedem Windrad errichten, die Stürmen, Korrosion durch Meerwasser und weiteren extremen Belastungen standhalten müssten.
• • Wie verhält sich die Elektrolyse unter den real am Windrad abfallenden Spannung und Stromstärke, da diese erheblich größer als es in einem Schülerexperiment normalerweise darstellbar sind?
• • Welches Material ist für die benötigten Elektroden am besten geeignet? Die Elektroden müssten bei geringer Abnutzung eine effiziente Elektrolyse ermöglichen, sowie sich nicht in umweltschädliche Stoffe auflösen.
• • Der Aufbau der Elektrolysezelle ist unklar, möglicherweise wäre ein Kammersystem oder Membranen sinnvoll.
• • Es gibt einen Konflikt bei der Wassertemperatur, während für die Elektrolyse eine erhöhte Temperatur über der natürlichen Temperatur des Meerwassers der Ostsee optimal wäre, ist eine stark erhöhte Temperatur für das Ökosystem Meer potentiell sehr schädlich. Es muss hier also ein futer Mittelwert gefunden werden.
• • Uns ist die physikalische Löslichkeit von Gasen, insbesondere von Wasserstoff und Sauerstoff, nicht entgangen. Jedoch hatten wir bisher keine Gelegenheit, deren Einfluss experimentell oder rechnerisch zu bestimmen.
• • Während der Elektrolyse insbesondere bei den Zerfallsreaktionen, entsteht ein gewisses Risiko, da einige Stoffe und Prozesse ein Explosionsrisiko in sich tragen und vor einer möglichen Zulassung zum tatsächlichen Einsatz müssen jegliche Zweifel an der Sicherheit ausgeräumt werden.
• • Die Gewährleistung das kein umwelt- und gesundheitsschädliches Chlorgas austritt muss ebenso gegeben sein, wie die Sicherheit, dass die Reaktion zum Abbau von diesem immer wie gewollt abläuft.
• • Eine Übersäuerung des Meeres muss auf alle Fälle verhindert werden, daher muss auch hier die Sicherheit der Abbaureaktion gewährleistet sein
• • Der Zustand des verwendeten Meerwassers ist ebenfalls fraglich, denn neben dem erwünschten Salzen enthält das Meerwasser weitere unerwünschte Bestandteile, wie Algen und Müll, welche die Elektrolyse negativ beeinflussen könnten. Ihr Einfluss muss genauer untersucht werden und eventuelle Lösungen wie Filter müssen gefunden werden.
• • Die genaue Effizienz der Anlage ist sehr schwer zu bestimmen, da es viele Faktoren gibt, die das Ergebnis beeinflussen, Hierbei sollte auch über mögliche Verbesserungen wie Katalysatoren nachgedacht werden.
• • Die Frage nach dem Transport des Wasserstoffs ist ebenso ungeklärt, eine Einspeisung ins Erdgasnetz ist ebenso möglich, wie der Transport über ein eigenes Piplinesystem, hierbei stellt sich die Frage ob der Wasserstoff verflüssigt oder gasförmig transportiert werden sollte.
• • Die weitere Nutzung des Wasserstoffs wäre ein weiteres Forschungsfeld von großem Interesse.
• • Für wasserarme Regionen bietet sich die Möglichkeit aus dem Wasserstoff Trinkwasser zu gewinnen.

Our concept still lacks concrete plans for a possible large-scale implementation. In addition, it is still unclear how this concept can be implemented offshore in an environmentally friendly and cost-saving manner. The following questions are still open and we will try to answer them in the future:

• • Should the electrolysis really be carried out in the sea, or on land in collection points for the electricity? Because otherwise you would have to build facilities under each wind turbine, the storms, corrosion by sea water and other extreme loads would have to withstand.
• • How does the electrolysis behave below the real voltage and current dropping at the wind turbine, since these are much larger than can normally be seen in a student experiment?
• • Which material is best for the required electrodes? The electrodes would have to allow for low wear an efficient electrolysis, as well as not dissolve in environmentally harmful substances.
• • The structure of the electrolysis cell is unclear, possibly a chamber system or membranes would make sense.
• • There is a conflict in the water temperature, whereas for the electrolysis an elevated temperature above the natural temperature of the seawater of the Baltic Sea would be optimal, a high elevated temperature for the marine ecosystem is potentially very harmful. It must be found here so a herd average.
• • We have not missed the physical solubility of gases, especially of hydrogen and oxygen. However, so far we have had no opportunity to determine their influence experimentally or by calculation.
• • During electrolysis, especially in the case of decomposition reactions, there is a certain risk, as some substances and processes carry a risk of explosion, and prior to possible approval for actual use, any doubts about safety must be eliminated.
• • The guarantee that no chlorine gas harmful to the environment and health must be given, as well as the certainty that the reaction to dismantle this always runs as intended.
• • Acidification of the sea must be prevented in any case, therefore, the safety of the degradation reaction must be guaranteed here
• • The state of the seawater used is also questionable, because in addition to the desired salts, the seawater contains other undesirable components, such as algae and garbage, which could adversely affect the electrolysis. Their influence needs to be investigated more closely and possible solutions such as filters have to be found.
• • The exact efficiency of the plant is very difficult to determine, as there are many factors that influence the outcome, and consideration should be given to possible improvements such as catalysts.
• • The question of the transport of hydrogen is just as unclear, a feed into the natural gas network is just as possible as the transport via its own Piplinesystem, this raises the question of whether the hydrogen should be liquefied or gaseous transported.
• • Further use of hydrogen would be another field of research of great interest.
• • For water-scarce regions, it is possible to extract hydrogen from drinking water.

These are all questions that have to be answered and answered before you can equip all offshore wind turbines in the North Sea with this system. Thus, these questions are likely to shape our further research.

To answer these questions and to get closer to realizing our idea, we have contacted some industrial companies.

We also want to carry out many more electrolyses with the help of a newly acquired sea salt mixture in order to be able to answer the above-mentioned questions at least partially using as many realistic data as possible. We also aim for a patent and / or utility model application to protect our idea. So our project is far from finished and we will continue to work on it.

Acknowledgments

We thank Dr. Ostertag and Mr. Bernstein for the care and guidance of the Jugend forscht-AG, as well as Dr. Munioz for their technical support. In addition, special thanks go to Linde for the contact production.

literature

1. [1] unknown. Wind power dissolves in the air - consumers still pay. Technical report, rationality, 2016.
2. [2] Thorsten Zoerner. Development of grid losses in Germany. Technical report, Proteus Solutions, 2013.
3. [3] Annika Grah. Synthetic fuels hope for a phased-out model. Technical report, Mirror, 2016.
4. [4] M. Eng. Jens Hüttenrauch and Dipl.-Ing. (FH) Gert Müller-Syring. Admixture of hydrogen to natural gas. GAT Newasletter, 2010.
5. [5] Joachim Strähle and Eberhard Schweda. Introduction to the inorganic chemistry internship (including quantitative analysis). Hirzel S. Verlag, 2005.
6. [6] Arnold Fr. Holleman and Egon Wiberg. Textbook of Inorganic Chemistry. deGruyter, 1995.
7. [7] IRENA. Summary perspectives of renewable energies: Germany. Technical report, Irena, 2015.
8. [8] Working Group Renewable Energy Statistics. Development of renewable energies in Germany in the first half of 2016. Technical report, Federal Environment Agency, 2016.
9. [9] Prof. dr. Bruno Burger. Electricity generation in Germany in 2016. Technical report, Frauenhofer Institute for Solar Energy Systems, 2017.

Claims (1)

The following components of the abovementioned research claims are claimed: 1. Installations that serve for the electrolysis of seawater and work with an identical or similar structure as described in this work and / or based on the same chemical principles and their technical implementation as described 2. Concepts for the storage of wind energy by electrolysis of seawater directly at the power producers (wind farms) 3. Concepts that provide for a reduction in waste products (such as chlorine gas) resulting from the electrolysis of sea water or chemically comparable substances, as described in 3.2

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