ORKNEY'S ENERGY FUTURE
MARINE TRANSPORT
METHODOLOGY - HYDROGEN
HYDROGEN FERRIES
Hydrogen (H₂) has been identified by Orkney research groups as an energy vector capable of decarbonising the marine transport sector. Projects BIG HIT and HySeas III are in progress where green hydrogen is produced via electrolysis using electricity from renewable sources. The hydrogen is stored at pressure and can then be used in a fuel cell which has large enough capacity to power the drive train of a ferry, hence providing a zero-carbon fuel. Only the inter-island fleet have so far been considered by research groups investigating hydrogen ferries. This section will present the calculated hydrogen and electricity demands for a fleet of hydrogen ferries.
INTER-ISLAND FERRIES
The electricity for the electrolysis process required to produce the mass of hydrogen required to fuel the inter-island fleet was calculated and the ferries were assumed to be running using hydrogen fuel cells and a propulsive drive train as in the HySeas III project [1].
The required mass of hydrogen for each ferry was calculated by following the approach presented by Gray et al. [2] and the current daily marine gas oil fuel (MGO) consumption [3]. For all 9 inter-island ferries, total daily fuel consumption is 19,771 litres.
The energy content of the daily fuel consumption was found using Equation 1, where MGO energy density is 35.98 MJ/litre [2].
(1)
The energy used by burning MGO in the combustion engine was found using the engine efficiency, ηMGO, assumed to be 40% efficient [2].
(2)
The energy required for propulsion through a hydrogen fuel cell was found using the fuel cell efficiency, ηH₂, assumed to be 52% efficient [4].
(3)
The mass of hydrogen required daily was then calculated using Equation 4, where the specific energy of hydrogen at 200 bar is 119.98 MJ/kg [5]. 200 bar is the pressure at which BIG HIT has been producing hydrogen at for research and development projects.
(4)
The calculated daily mass of hydrogen required for the full fleet is 4,561 kg. This shows a 23% difference to the result of 5,744 kg found by Aquatera’s Low Carbon Ferries Feasibility Study [4] for the same ferry fleet. However, in this report the values used for combustion engine efficiency and the pressure of the hydrogen are not presented. Therefore, a result in the same order of magnitude provides adequate validation of the result. A breakdown of the daily mass of hydrogen for the 9 ferries is shown in Figure 1.
Figure 1: Mass of hydrogen required (kg) for inter-island ferry fleet on a daily basis.
Equation 5 [4] was used to calculate the electricity requirement for the electrolyser to produce the required hydrogen.
(5)
Where HHV is the Higher Heating Value of hydrogen, equal to 39.39 kWh/kg [4] and η is the electrolyser efficiency, equal to 59%; the efficiency of the 0.5 MW ITM electrolyser currently situated on the island of Eday [5]. The daily electricity requirement was calculated to be 304.5 MWh for the electrolyser to provide the inter-island fleet with 4,561 kg of hydrogen every day. This gives an annual electricity consumption of 111.1 GWh.
There are currently two electrolysers on Orkney, in Shapinsay and Eday, with a total capacity of 1.5 MW, capable of producing around 31 kg of hydrogen per hour combined. To produce the required mass of hydrogen, more electrolysers would be needed and their locations considered in relation to the overnight ferry ports.
A simple analysis based on that presented by Aarskog et al. [6] was undertaken to determine if the existing ferries could feasibly carry the mass of hydrogen and tanks holding it. The tanks used were TitanXL and Magnum3, used in a study by Aarskog et al. [7], which hold 150 kg and 27.8 kg of hydrogen at 200 bar respectively and weigh 2,400 kg and 342 kg respectively. If the combined hydrogen and tank weighed over 80% of the deadweight tonnage (DWT), it was considered infeasible that the ferry could carry the required mass of hydrogen. The results of the analysis show that it would be possible to retrofit the full inter-island fleet as hydrogen powered ferries based on this criterion (Figure 2).
Figure 2: Comparison between mass of hydrogen and tanks required per ferry and its deadweight tonnage. The results show that the mass of hydrogen and tanks is less than 80% of the deadweight tonnage in all cases.
MAINLAND FERRIES
Similarly to the inter-island ferries, the mainland ferries were assumed to run using hydrogen fuel cells and the mass of hydrogen was found for each of the ferries using the current MGO consumption. The technical details of the balance of the ferry and location of fuel tanks are beyond the scope of this project.
The mass of hydrogen required per trip for each of the mainland ferries was calculated based on a methodology presented by Wood ([8]; Table 1). This method is the same as the process described for the inter-island fleet; with two differences:
-
A 20% fuel reserve was incorporated.
-
Hydrogen requirement found per trip rather than per day because not all ferries travel every day.
The MV Hjaltland has different fuel consumption found for the North and South bound journeys [9].
Table 1: Mass of hydrogen required per trip to fuel Orkney’s mainland ferries.
Using the current ferry timetables [10] [11] [12] a daily demand profile was built up and using Equation 5 the annual consumption of electricity to produce the hydrogen for all ferries was calculated to be 685.1 GWh, which is greater than 4 times Orkney’s current annual electricity demand.
An analysis [6] was carried out to determine if each of the mainland ferries could feasibly carry the hydrogen and the tanks required per trip. The feasibility was based on 80% of the DWT as with the inter-island fleet. The results show that all mainland ferries could feasibly be retrofitted to hydrogen ferries, based on the weight criteria analysis (Table 2).
Table 2: Comparison between mass of hydrogen and tanks required per ferry and its deadweight tonnage. The results show that the mass of hydrogen and tanks is less than 80% of the deadweight tonnage in all cases.
In conclusion, the annual electricity demand to produce the hydrogen for the 9 inter-island ferries is 111.1 GWh, far greater than the 16.8 GWh annual electricity required for charging 9 battery powered inter-island ferries. Hydrogen is less suited to the inter-island fleet due to this electricity requirement and the lack of existing hydrogen ferries slows down possible progress, making it overall a less attractive option.
For the mainland ferries, the annual electricity demand to produce the calculated hydrogen required for all 5 mainland ferries is 685.1 GWh. This is a significant demand and makes it a less attractive option for the mainland ferries where batteries are feasible. However, for the two largest mainland ferries, which travel between Aberdeen, Kirkwall and Lerwick, batteries have not been deemed feasible and hydrogen could be considered for those.
The electricity consumption of the 12 battery ferries that were considered to be feasible was found to be 35.8 GWh and the electricity consumption to produce the hydrogen for the same 12 ferries is over 11 times greater; 396.2 GWh. For this reason, battery powered ferries are the preferred option.
MARINE TRANSPORT PROPOSAL
From the analysis of the feasibility and demand of battery and hydrogen ferries on Orkney, the following is concluded as the optimal solution to decarbonise the ferries:
-
9 inter-island ferries and 3 mainland ferries are battery powered. These 12 battery ferries annual electricity consumption is 35.8 GWh.
-
11 of the ferries assumed to be wholly replaced – based on existing battery ferries – and the MV Hamnavoe retrofitted.
-
The two largest mainland ferries could be decarbonised using hydrogen as a fuel, with an annual hydrogen demand of 5,992 tonnes. The annual electricity required to produce this hydrogen through electrolysis is high – 400 GWh – and this may only be generated in certain future scenarios. Therefore, this will not be taken forward as a solution.
-
The details of retrofitting the ferries to be fuelled by hydrogen have not been considered by this project and as such there may be outstanding practical challenges.
AVAILABLE DOWNLOADS
The spreadshet for the mainland ferries demand calculations is available here.
The spreadshet for the inter-island ferries demand calculations is available here.
REFERENCES
[1] HySeas III, "The Project," 2021. [Online]. Available: https://www.hyseas3.eu/the-project/. [Accessed 9 April 2021].
[2] N. Gray, S. McDonagh, R. O'Shea, B. Smyth and J. D. Murphy, "Decarbonising ships, planes and trucks: An analysis of suitable low-carbon fuels for the maritime, aviation and haulage sectors," Advances in Applied Energy, vol. 1, 2021.
[3] Aquatera, "Orkney-Wide Energy Audit 2014: Energy Sources and Uses," Orkney Renewable Energy Forum, 2015.
[4] D. J. Goldmeer, "Power to Gas: Hydrogen for Power Generation," February 2019. [Online]. Available: https://www.ge.com/content/dam/gepower/global/en_US/documents/fuel-flexibility/GEA33861%20Power%20to%20Gas%20-%20Hydrogen%20for%20Power%20Generation.pdf. [Accessed 5 February 2021].
[5] European Marine Energy Centre (EMEC), "Hydrogen Production Plant," EMEC, 2021. [Online]. Available: http://www.emec.org.uk/facilities/hydrogen/. [Accessed 10 April 2021].
[6] F. G. Aarskog and J. Danebergs, "Estimation of Energy Demand in the Norwegian High-Speed Passenger Ferry Sector Towards 2030," Institute for Energy Technology, 2020.
[7] F. G. Aarskog, J. Danebergs, T. Stromgren and O. Ulleberg, "Energy and cost analysis of a hydrogen driven high speed passenger ferry," International Shipbuilding Progress, vol. 67, no. 1, pp. 97-123, 2020.
[8] Wood plc; Point and Sandwick Trust, "Scottish Western Isles Ferry Transport using Hydrogen," 12 May 2019. [Online]. Available: http://www.pointandsandwick.co.uk/wp-content/uploads/2019/07/Scottish-Western-Isles-Ferry-Transport-using-Hydrogen-Feasibility-Report.pdf. [Accessed 31 March 2021].
[9] A. J. Baird, "Policy Implications of CO2 Emissions for Island Ferry Services," Transport Research Institute, Edinburgh Napier University, 2012.
[10] NorthLink Ferries, "Booking Info - Timetable," 2021. [Online]. Available: https://www.northlinkferries.co.uk/booking-info/timetables/. [Accessed 10 April 2021].
[11] Pentland Ferries, "Timetable," 2021. [Online]. Available: https://pentlandferries.co.uk/timetable-2/. [Accessed 14 April 2021].
[12] John O'Groats Ferries, "Timetable," 2021. [Online]. Available: https://www.jogferry.co.uk/Ferry.aspx. [Accessed 14 April 2021].
[13] Department for Business, Energy and Industrial Strategy (BEIS), "Regional and local authority electricity consumption statistics," 22 December 2020. [Online]. Available: https://www.gov.uk/government/statistical-data-sets/regional-and-local-authority-electricity-consumption-statistics. [Accessed 2 February 2021].