Hydrogen Production

This project focuses on the production of green hydrogen. The production of cheap green hydrogen is considered one of the breakthroughs in the effort towards zero emissions.

Electrolysis

Assuming sea water has undergone the stage of desalination (out of this project’s scope), the high purity water enters the PEM electrolyser at the anode electrode. High purity water is decomposed based on the electrochemical reaction with the utilization of Ir/RuOx catalysts:

It is worth mentioning that due to corrosion issues the anode electrode contains no carbon products. Also, the anode’s current collectors and diffusion layers are made from Ti. The electrons at the anode create a circuit with the cathode and as a result current is generated.

The hydrogen produced from the anode electrode crosses the membrane in the middle of PEM and goes to the cathode electrode. The membrane is approximately 20-300 μm thick and this the reason why PEM has significant advantages over other relevant configurations. The operating temperature of the PEM electrolyser is 50-80, with the cathode electrode to have slightly larger operating temperature range than the anode electrode. At the cathode electrode, the following electrochemical reaction occurs:

The cathode electrode contains Pt catalysts and can support the existence of carbon materials since it does not suffer from corrosion issues. The cathode’s collectors are the same with the ones of the anode. Taking into consideration the 2 reactions that happened, the water electrolysis is achieved and based on the total reaction:

All in all, the high dynamic range operation, that PEM electrolyser provides, makes it suitable to match with renewable energy sources such as offshore wind, in our case.

PEM has several advantages concerning its operational characteristics along with its efficiency and design. Due to its design (Proton Exchange Membrane), it has high proton conductivity resulting in higher current densities and reduction of ohmic losses. As a result, the cost of electrolysis is significantly reduced. Moreover, it is compatible with different power inputs (10-100%) because of the low gas crossover rate of its membrane and its rapid response to the power inputs without facing inertia phenomena.

The advantages mentioned make PEM dominate over Alkaline electrolysers, which also exist at the market [1].

Figure 1: PEM Electrolysis

Selection of Electrolyzer

The production of hydrogen is directly reliable to the rated capacity of the Electrolyzer. Having set the demand for the ferries to 11712 kg/day, the same capacity plus a 100% backup capacity (based on the results of Homer Pro) is needed with electrolyzers. The group decided to select an existing model of electrolyzer in order to explore the options that we have today in the market.

To select the right electrolyzer four criteria were set, the cost, efficiency, energy consumption and number of electrolyzers needed. Because of lack of information on the prices for models from different manufactures , a specific manufacture was selected, NEL, based on a press release of the company that they were aiming to reduce the prices of its electrolyzers by 75%[3] something that was used in the financial analysis as well. After examining the models of NEL, as illustrated in Figure 1 it was determined that model M2000 is the best for our project. The main reason behind the decision was that despite the cost and energy consumption, is preferable to have more electrolyzers with less capacity than a big one in order for them to work more time on their rated capacity.

Figure 2: Radial diagram for the selection of electrolyzer

The main specifications for the M2000 are given from NEL showing in Table 1.

Table 1: Specifications of Nel's M2000

Using the specifications of the M2000 model the average consumption was calculate using eq.1. With the average consumption calculate the electrolysis efficiency was determined using eq.2 assuming a 33.33 kWh energy content per kg of hydrogen.

References

[1] M. Carmo, D. L. Fritz, J. Mergel και D. Stolten, «A comprehensive review on PEM water electrolysis,» International Journal of Hydrogen Energy, vol. 38, pp. 4901-4934, 2013.

[2] Amin Mohammadi, Mehdi Mehrpooya, A comprehensive review on coupling different types of electrolyzer to renewable energy sources, Energy, Volume 158, 2018,Pages 632-655,ISSN 0360-5442, https://doi.org/10.1016/j.energy.2018.06.073.(https://www.sciencedirect.com/science/article/pii/S0360544218311381)

[3] Website article, Nel to slash cost of electrolyzers, https://www.rechargenews.com/transition/nel-to-slash-cost-of-electrolysers-by-75-with-green-hydrogen-at-same-price-as-fossil-h2-by-2025/2-1-949219

[4]Moriarty P, Honnery D. Intermittent renewable energy: the only future source of hydrogen? Int J Hydrogen Energy 2007;32(12):1616e24.

[5] Acar C, Dincer I. Comparative assessment of hydrogen production methods from renewable and non-renewable sources. Int J Hydrogen Energy 2014;39(1):1e12.