ORKNEY'S ENERGY FUTURE
ENERGY STORAGE
WHY STORAGE IS REQUIRED
After decarbonising the sectors, the maximum load (48 MW – 17th January 17:00) is now larger than the maximum import capacity from the currently installed mainland cables (40 MW) in Scenarios 1, 2a and 2b. The load is over 40 MW for a total of 156 hours over the full year. In a worst-case scenario when electricity generation from wind is low and the demand is high, this could lead to a power shortage. In this situation, to avoid burning fossil fuels in Orkney’s gas turbine or diesel generator, there needs to be energy storage capable of supporting the grid in these times. Scenario 3a and 3b were not considered due to the ability to import limit of the new 220 MW transmission cable never being less than the peak demand.
STORAGE TECHNOLOGY USED
The energy storage solution selected for this was lithium-ion batteries due to:
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Ability to charge or discharge instantaneously.
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High energy density: ~197 kWh/m² [1].
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High round trip efficiency: up to 95%. 90% used due to the climate in Orkney [2] [3].
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Technology is well understood and simple to simulated.
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Technology has already been used in Orkney – 2 MW battery in Kirkwall [4].
There are disadvantages associated with batteries such as the environmental impact from the use and mining of rare earth metals which is discussed here.
MODELLING THE ENERGY STORAGE
As the electricity storage is only required when the import limit is 40 MW and the demand has been increased due to decarbonisation, the scenarios that were simulated for energy storage were 1, 2a and 2b.
To simulate the worst-case scenarios, a model was created in HOMER Pro which would propose the fewest number of batteries that were needed when using the wind data from 2014 to 2018. The batteries used in the model were 1 MWh lithium-ion batteries. A condition was set that the batteries would never discharge to lower than 20% capacity to ensure a factor of safety and avoid a power deficit. To simulate the situation where the electricity is needed 40 MW was subtracted from the load to act as the imported electricity from the grid. Any electrical load over the 40 MW then must be met by electricity being produced by the onshore wind turbines or from the batteries.
Figure 1: Schematic of the model created in HOMER.
Table 1: Specification of the batteries used for energy storage.
RESULTS
The greatest battery storage capacity would be required in Scenario 1. In this case, battery storage of 4 MWh is required due to the smaller turbine capacity compared to Scenarios 2a and 2b. The differing result between different years are due to the variability in the wind and the minimal times than the demand is above 40 MW. If more sectors are decarbonised than proposed by this project and electricity demand was greater, then it is expected that the number of batteries and frequency that they are required would also increase.
Table 2: Number of batteries needed for each scenario in different years.
Figure 2 shows that when the total wind generation is low and the decarbonised electricity load is at a peak above 40 MW, the batteries discharge. The power that the battery discharges is added to the 40 MW import to meet the power demand for that hour. The batteries are then charged when the wind generation next exceeds the magnitude of the decarbonised electricity load.
Figure 2: Example of when the energy storage batteries are required.
CONCLUSIONS
This study has shown that it is possible to facilitate decarbonisation of the chosen sectors without building a new cable by utilising batteries for energy storage. This would be a simple and less expensive solution although there would be lost income from wasted electricity, especially in Scenario 2b.
As more sectors additional to this project are decarbonised the peak demand will increase and there would be a need for more storage. This could become expensive and cost analysis would need to be conducted to understand whether it is economical to use battery storage or if building the new transmission cable would be a better option. Additionally, the use of batteries would not solve the issue of curtailment unless all the generated electricity was stored and then used or exported to the UK when there is capacity for it.
Alternatively, the batteries in electric vehicles could be used as energy storage if ‘vehicle to grid’ functionality was installed. This would allow the energy stored in the batteries of electric vehicles to discharge as electricity when required by the grid in Orkney.
REFERENCES
[1] Q. Dai, J. C. Kelly, L. Gaines and M. Wang, "Life Cycle Analysis of Lithium-Ion Batteries for Automotive Applications," Batteries, vol. 5, no. 2, 2019.
[2] C. Iclodean, B. Varga, N. Burnete, D. Cimerdean and B. Jurchiș, "Comparison of Different Battery Types for Electric Vehicles," in IOP: Materials Science and Engneering, Orlando, 2017.
[3] "Battery Aging in an Electric Vehicle (EV)," Battery University, 22 August 2020. [Online]. Available: https://batteryuniversity.com/learn/article/bu_1003a_battery_aging_in_an_electric_vehicle_ev. [Accessed 12 April 2021].
[4] Orkney Renewable Energy Forum, "Storage," 2021. [Online]. Available: https://www.oref.co.uk/storage/. [Accessed 1 May 2021].