ORKNEY'S ENERGY FUTURE
MARINE TRANSPORT
METHODOLOGY - BATTERIES
BATTERY FERRIES
Battery ferries use the electrical power stored in batteries to provide propulsive power to electric motors to turn the turbine for the ferry. If the electricity used to charge the batteries comes from entirely renewable sources, the battery powered technology can be considered emissions free at the point of use – a clear advantage over fossil fuels. Analysis was carried out to assess the feasibility of having a battery powered inter-island and mainland ferry fleets on Orkney and calculate the resulting hourly, daily and annual electricity demand.
The configuration of a battery ferry is shown by the single line diagram in Figure 1, where the grid is shown to charge the battery packs when docked onshore which provide electrical power to the thrusters to propel the ferry throughout the journey.
Figure 1: Single line diagram of the MF Ampere’s battery system [1].
STATE OF THE ART
There are a number of battery ferries in operation globally, with the most pioneering ferries primarily in Scandinavia. The e-Ferry Ellen operates in Denmark and is considered a leading example of a battery ferry due to its extensive range and carrying capacity (Figure 2).
Specifications of three ferries formed a basis for modelling Orkney’s ferries and demonstrate the development stage for the most innovative battery powered ferries (Table 1).
Figure 2: The e-Ferry Ellen; a pioneering electric car ferry operating in Denmark [2].
Table 1: Specifications of 3 battery ferries operating in Scandinavia: the e-Ferry Ellen, MF Ampere and Tycho Brahe.
CHARGING DEMAND
The charging demand was calculated based on the distance travelled by a ferry at a given discharge rate. The discharge rate of a battery ferry is the rate at which electricity is discharged from the batteries to power the motors to propel the ferry over its journey. Discharge rates were based on specifications of existing ferries operating in Scandinavia (Table 1) and assigned to Orkney’s ferries depending on gross tonnage of the vessel. The discharge rate for these ferries is variable, for example the e-Ferry Ellen’s discharge rate can vary from 35-43 kWh/km [3]. To quantify the maximum electricity demand, the worst-case discharge rate was used in the calculations.
All inter-island ferries and MV Pentland Venture were modelled using 43 kWh/km, MV Alfred used 36 kWh/km and MV Hamnavoe, MV Hjaltland and MS Hildasay used 235 kWh/km. The charging demand for any given trip between two harbours was found using Equation 1:
(1)
The maximum charging demand for each ferry, based on the longest trip distance along its route, was used to calculate the required battery capacity for each ferry (Equation 2; Figure 3). It is multiplied by 3 so that the ferry is not at risk of running out of charge during a trip.
(2)
It has been assumed that each Orkney ferry would be replaced by a battery ferry of the same model used to assign its discharge rate.
The most suitable battery type are lithium nickel cobalt manganese oxide (Li-NCM) batteries (lithium-ion), the same type used in the e-Ferry Ellen that have been developed for lightweight maritime applications [3]. These batteries have an energy density of approximately 197 Wh/kg [5], relatively high for a lithium-ion battery, and have an effective response time. The weight of the pack is 8 kg/kWh [6]; the battery pack weight would vary between ferries depending on the battery capacity requirement.
An analysis was carried out based on a methodology presented by Aarskog et al. [7] to determine if the modelled ferries would have the capacity to carry the batteries required for power on the Orkney routes. The steps taken were:
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The weight of each ferry’s required battery capacity was found using the pack weight of 8 kg/kWh and the battery capacity calculated using Equation 2.
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80% of each ferry’s deadweight tonnage (DWT) was calculated and set as a cut-off point.
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If the weight of batteries exceeded the 80% cut-off point, it was considered infeasible to power the ferry by this means.
It was found for the largest mainland ferries, the MV Hjaltland and MS Hildasay, the required number of batteries would be too heavy for a ferry to carry and these two ferries were ruled out as being able to be decarbonised using batteries, hence they do not appear on Figure 3. This is significant as these ferries contribute over 27% to the fuel use of the marine transport sector [8] [9]. It is considered feasible that all 12 other ferries can carry the required batteries based on their DWT and can be powered using batteries.
The MV Hamnavoe’s large battery capacity is due to the gross tonnage of the vessel - it carries 600 passengers and 95 cars - hence the assigned discharge rate of 235 kWh/km. Eleven ferries are assumed to be replaced, whereas the MV Hamnavoe is assumed to be retrofitted similarly to the Danish Tycho Brahe ferry [10]. This is due to the size of the vessel itself and the fact it was built in 2002, making it relatively young compared to the rest of the fleet [4].
Figure 3: Battery capacity of the battery powered Orkney ferries compared to the maximum trip distance for each route.
CHARGING POINTS
A charging point of the MF Ampere is shown in Figure 4 and similar equipment would be installed at each of Orkney’s harbours where a charging point is required. Ferry routes were allocated charging points (Figure 5) and charging times and locations were based upon the current Orkney Ferries [11], NorthLink Ferries [12], Pentland Ferries [13] and John O’Groats Ferries [14] timetables. There are 14 charging points for the 12 ferries as a result of the Northern Isles services requiring multiple charges along the route and the MV Hamnavoe requires to be charged at both ends of the journey – therefore there is an additional charging point at Scrabster on mainland Scotland. The ferries were modelled to charge at specific charging points along the routes and the time of charge was when the ferry arrived at that location according to the current timetable.
Figure 4: Charging point in Norway for the MF Ampere – similar to what is required for a battery powered ferry fleet on Orkney [15].
CHARGING THE FERRIES
It was assumed all ferries have a charging rate of 60 kWh/min [3], except the MV Hamnavoe which has a charging rate of 130 kWh/min [4], reflecting values found in the literature for vessels of similar size. These charging rates allow the ferries to keep to current timetables, but considering the limitations of Orkney’s electricity network (one of the reasons behind curtailment) it is unlikely these charging rates are feasible at present as it would impose a strain on the local grid [15]. An assumption made throughout this project is an upgrade of the grid [16] [17]; ensuring these charging requirements are feasible could be part of the upgrade.
A consideration is that when electricity demand is greater than the electricity generated locally and electricity is imported from the National Grid, it may not be guaranteed to be generated solely from renewable sources. Onshore batteries could be charged up when electricity supply is greater than demand so the ferries could then be charged when generation is low, but this would increase the overall battery capacity required.
Aquatera’s feasibility study [15] outlined limiting factors for battery ferries on Orkney and identified the grid as a main constraint in being able to provide the required charge and a clear requirement for onshore storage.
The ferries are modelled to be charged to the required extent for each journey, and therefore charge prior to the first journey of the day rather than charging overnight. There are practical reasons for this as charging the ferries overnight would require a security presence and a technical operator. This would be an extra expense to the ferry companies, hence the drive to have daytime charging.
Figure 5: Map of Orkney’s Inter-island and Mainland ferries and the designated charging points along the routes [18].
ELECTRICITY DEMAND PROFILES
Using the journey charging requirements alongside the time the ferries needed to be charged, found from the current timetables, hourly and daily electricity demand profiles for a full year were built. There is seasonal variation in the demand profile due to extra services in the summer (Figure 6). Using the hourly profiles, the total annual electricity consumption for 12 battery ferries (9 inter-island and 3 mainland) was calculated to be 35.8 GWh – 16.8 GWh for the 9 inter-island ferries and 19.0 GWh for the 3 mainland ferries. The full demand profiles for a year on an hourly and daily basis are available to be downloaded.
Figure 6: Daily electricity demand profile for Orkney’s battery powered mainland and inter-island ferries.
In conclusion, battery ferries were found to be a feasible option to decarbonise 12 Orkney ferries with the assumption that the electricity grid is upgraded. This may come at significant financial cost due to a substantial investment in infrastructure and new ferries; this analysis is presented here.
It was found that it is not feasible for the two largest mainland ferries to be powered by batteries, due to the weight of the required battery capacity in comparison to the DWT. The possibility of using hydrogen fuel for the inter-island and mainland ferry fleets has also been investigated. This may provide a suitable alternative for the largest mainland ferries.
ENVIRONMENTAL IMPACT OF LITHIUM-ION BATTERIES
Battery technology has advanced significantly to now be regarded as key in decarbonising the transport sector. If electricity is generated from renewable sources, battery powered vehicles can be considered as having zero direct emissions, but the procurement of raw materials and manufacturing are not emission free and have associated environmental impacts.
From the analysis of Orkney’s ferry fleet, lithium-ion (Li-ion) batteries have been identified as the most suitable type to use for the 12 proposed battery ferries. As a result, 80.1 MWh of Li-ion battery capacity is required to build the new ferries. The specific battery identified for use is lithium nickel cobalt manganese oxide, or Li-NCM. A review of the potential environmental and social impacts resulting from the manufacture of batteries such as those that would be deployed for the Orkney fleet has been carried out.
There are a range of studies that have analysed the cradle-to-grave emissions of Li-NCM batteries as well as studies that go further to look at recycling the batteries. Melin [19] provides an in-depth study into over 100 research papers about life-cycle emissions of Li-ion batteries and finds a range of results from 39-196 kg CO2 emissions per kWh of capacity. Each of the research papers analyse the energy requirements to produce 1 kWh of a cell, although some suggest caution when scaling values up to a given application.
The varied findings of the literature review were used to calculate the CO2 emissions associated with the 80.1 MWh of battery capacity required for Orkney’s ferries (Figure 7). This shows the variation from study to study in associated emissions of the production process.
Figure 7: Range of life-cycle CO2 emissions of integrating 80.1 MWh of Li-ion batteries into the Orkney ferries based on literature studies [19] [5] [20].
The 12 ferries that have been proposed to be converted to battery ferries use 8.85 million litres of marine gas oil (MGO) annually [8] Combustion of MGO emits 2.738 kg of CO2 per litre [21] resulting in annual total emissions of 24 kilo tonnes of CO2. Introducing battery ferries reduces this significantly and even using the highest value found in the literature (Figure 1) results in a net reduction in emissions of over 200,000 tonnes of CO2 up to 2030.
Dai et al. [5] found that 752 litres of water is required per kWh of battery production. This is due to the mining process that is used to procure the lithium carbonate for the Li-ion batteries [22]. The majority of the world’s reserves of lithium carbonate are in the South American salt flats in Bolivia, Argentina and Chile where it is mined. Using the value of 752 litres per kWh, the water consumption to procure 80.1 MWh of battery capacity is over 4.4 billion litres. This is 0.77% of Bolivia’s annual renewable water resource [23]. Depleting this resource can affect farming and the lives of local communities [24].
Lithium-ion batteries have a lifespan of 8-10 years [25] and the demand for them is expected grow exponentially from 100 GWh of production in 2017 to 800 GWh in 2027 [24]. Lithium is a finite resource and there is a risk that with the growth in the technology there could be supply and demand challenges. Dodson et al. [26] present that lithium could have 100-500 years until depletion of known reserves based on the current rate of extraction. However, manganese and cobalt have fewer; 5-50 and 50-100 years, respectively.
There are also social impacts associated with the production of Li-NCM batteries due to the mining of cobalt found in the cell. Seventy percent of cobalt is mined in the Democratic Republic of the Congo [19], where it is extracted from the ground using old fashioned techniques; a result of poverty and lack of investment in the region and the practices being unable to improve. Often conditions are unsafe, lack safety equipment, workers are poorly paid or not paid at all and children are often forced to mine [27]. This brings up human rights issues, with high chances of accidents, unlawful employment and potentially child slavery.
AVAILABLE DOWNLOADS
The spreadsheet with the battery ferry specifications is available here.
The spreadsheet with the hourly demand calculations is available here.
The spreadsheet with the daily demand calculations is available here.
REFERENCES
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