ORKNEY'S ENERGY FUTURE
ELECTRICITY GENERATION
ORKNEY'S ELECTRICITY GENERATION AND GRID
Orkney’s renewable electricity generation is made up of onshore wind, photovoltaics, wave, tidal and hydro. Most of the installed capacity is from onshore wind and it consistently accounts for over 98% of the electricity production (Figure 1; [1]). Data from the UK government [1] show that wave and tidal are the only other technologies with large installed generation capacity (13.5 MW). However, the capacity is attributed to experimental devices which are usually temporary instalments and do not export electricity to the electricity grid. As such, electricity generated through wave and tidal does not correlate with the installed capacity. Due to the dominance of onshore wind, this was the only source of renewable energy considered in this project.
Figure 1: Electricity generation contribution for each technology on Orkney in 2018 [1].
As described in About Orkney, Orkney’s electricity grid is connected to mainland Scotland through two 20 MW (33 kV) cables which ensure that the Flotta gas turbine and diesel generator of Kirkwall power station are rarely used. As there is currently greater than 50 MW of installed onshore wind capacity but an export limit of 40 MW, often the generation of onshore wind is curtailed in times of low electricity demand. The curtailment keeps the network infrastructure (Figure 2) within its safe operating zone. The Active Network Management was introduced to manage curtailment, but there is still 30-50% curtailment of wind turbines across each year.
Due to the curtailment problem a new 220 MW transmission cable to the UK and inter-island upgrades have been proposed (Figure 3). However, a condition of new cable is that 135 MW of new generation must be approved in Orkney by December 2021 [2] to ensure that the new cable is not underutilised and represents value for money for the consumer.
Figure 2: Orkney's electricity network [6].
Figure 3: Proposed upgrades to Orkney's electricity network [3].
GENERATION AND FUTURE INFRASTRUCTURE SCENARIOS
A primary aim of this project was to calculate surplus electricity (the difference between the electricity demand and the electricity that could be generated) based on current and future renewable generation. The first step to meet this aim was to simulate current and future electricity generation for a typical year in Orkney. Additionally, this allowed:
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The curtailment problem to be better understood.
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Supply and demand profiles to be balanced when adding new electrical loads.
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Energy storage options to be fully explored.
Four scenarios were created based on the electricity generation and infrastructure that could be in place by 2030 when Orkney aims to have decarbonised (Figure 4). The scenarios consider the current onshore wind capacity and the planned wind farms that are at differing stages in the approval process. The situation with and without the new transmission cable is also considered.
Figure 4: Transmission and generation scenarios created for the project.
Figure 5: Map showing the locations of the proposed onshore wind farms and the new transmission cable [7].
The total capacity of the 8 proposed new wind farms is expected to be 107 MW (Figure 5). To meet the condition for the new transmission cable an additional 28 MW of capacity would be required. However, for this project, only the currently 8 proposed wind farms are being considered. Within the 107 MW there are 35.6 MW from 4 wind farms which have been approved and 71.4 MW from 4 wind farms which are in the early stages of applying for approval. It should be noted, however, that there is uncertainty in the true final generation capacities given the stage of the projects. It was assumed that all the proposed wind farms would be incorporated into Orkney’s electricity network and not connected to ‘off-grid’ industries. A 25-year typical turbine lifespan has been assumed in the calculations.
CALCULATING WIND ELECTRICITY GENERATION PROFILES
To calculate the electricity generation profiles, two methods were used. The first was a self-built Microsoft Excel tool. A benefit of this was that there was an understanding of the inputs and formulae that were being used in the calculations. It also meant that the results were ‘live’ and analysis could be linked to the generation profiles therefore updating automatically allowing greater flexibility and time savings. When using software, results are often required to be exported after each simulation and can be limiting.
The second modelling method was using HOMER, a microgrid simulator. HOMER was used to validate the results that were found in Excel. As HOMER is a well validated simulation tool [4] it gave confidence in the results from Excel. A comparison between results from both methods is presented in Wind Profiles and Generation.
To calculate the power generated by each turbine hourly average wind values were required. The dataset used was for Kirkwall Airport weather station between 2014 and 2018 [5], sourced from the Centre for Environmental Data Analysis (CEDA). Figure 6 shows the hourly wind speeds at the weather station in 2018. The height of the anemometer used at the weather station was 26 m above ground level which was at an elevation of 18 m.
Figure 6: Hourly wind speed at Kirkwall weather station in 2018.
In both methods, the hourly power generated was calculated for each year from 2014 to 2018 using each year’s respective wind data and a profile for a representative year was constructed by averaging the power every hour across the 5 years.
MICROSOFT EXCEL TOOL
Hourly wind data from the weather station anemometer were corrected for turbine height in the Excel tool using Equation 1:
To calculate the power generated each hour by the turbines, Equation 2 was used:
The C values were taken from power curves supplied by the wind turbine manufacturers as they are unique to each wind turbine (Figure 7). An hourly time step was used to give suitable detail on electricity generation and which could also be used for supply and demand matching.
(1)
(2)
p
Table 1: Nomenclature for the equations used to calculate the power generation from wind turbines.
There were no losses factored into the model as this project is a feasibility study and small losses were not considered to be a priority for calculating the electricity generation. The losses could be through equipment such as electrical inverters and rectifiers, substation transformers and network cables.
The results were checked by reviewing the wind turbine capacities against the known capacities to ensure an accurate output from the calculations.
Figure 7: Power curve and power coefficient for the most populous wind turbine on Orkney, an Enercon E-44 900.
HOMER
Orkney’s current hourly electricity demand profile was imported into HOMER. The method of how this was calculated can be found here. A converter was necessary as some of the wind turbines on Orkney operate in direct current (DC) and so the electricity needs to be converted to alternating current (AC) to be connected to the grid. The converter can act as an inverter or rectifier depending on what is required. The efficiency of both these functions was set to 95%. As there was no converter used in the Excel tool this difference was considered when comparing the results. The size of the converter was set to be ‘optimised’ which allows HOMER to size the converter according to the electrical load and ensure it is always met. A 40 MW generator was used in the model to act as the two 20 MW mainland cables to determine whether the demand could be satisfied. This was used due to the inability to constrain the ‘Grid’ component in HOMER to 40 MW.
Figure 8: An example of a schematic taken from HOMER for a model used to calulate generation profiles.
A limitation of HOMER is that it only allows the use of 10 wind turbines models. Consequently, the actual wind turbines installed on Orkney had to be condensed. The turbine data used were checked to ensure that the same specification was used in HOMER and Excel for correcting the wind speed and calculating the power generation. The wind turbines used in each scenario and each tool can be found in Available Downloads.
WIND PROFILES AND GENERATION
The power generation was calculated in hourly increments and extrapolated out into weekly, monthly and annual generation. The electricity generation is greater over the winter months due to higher wind speeds and decreases in the summer months. However, due to Orkney’s exposed location, the electricity generation remains significant throughout the year (Figure 9).
Figure 9: Average generation profiles from all wind turbines in each scenario from Excel.
A comparison between the annual electricity generation results from Excel and HOMER was conducted to validate the electricity generation calculated in the self-built Excel tool. The comparison shows that the differences are below 10% and become smaller with more generation (Figure 10). This gave confidence in the Excel tool and it was used in the rest of the project. The main reason for the difference in electricity generation between the two tools is the difference in the turbines used due to HOMER’s 10 turbine limit.
Figure 10: Comparison of the yearly generation between the HOMER and Excel tools.
A benefit of the comparison exercise was working through each tool more carefully and gaining a better understanding of any differences in the results between each. Inaccuracies were found with the self-made tool which included overcalculations in the power output compared to the manufactures data. This was due to errors in the coefficient of power and specifications of the turbine such as the rotor diameter. By working through each turbine individually all the inaccuracies were found and rectified accordingly which gave more robust results.
AVAILABLE DOWNLOADS
The wind turbines used in each scenario and each tool can be found here.
The spreadsheet with the current generation calculations for 2014 wind speeds is availble here.
REFERENCES
[1] UK Government, "Regional renewable statistics," 2018. [Online]. Available: https://www.gov.uk/government/statistics/regional-renewable-statistics. [Accessed March 2021].
[2] The Orkney News, "Ofgem Approves New Orkney Subsea Cable Subject to Windfarm Developments," 16 September 2019. [Online]. Available: https://theorkneynews.scot/2019/09/16/ofgem-approves-new-orkney-subsea-cable-subject-to-windfarm-developments/. [Accessed 7 April 2021].
[3] Scottish and Southern Electricity Network, "Orkney Transmission Connection and Infrastructure Project," 2018. [Online]. Available: https://www.ssen-transmission.co.uk/media/2955/orkney-september-2018-consultation-booklet1.pdf. [Accessed 2 May 2021].
[4] J. L. Bernal-Agustín and R. Dufo-López, "Simulation and optimization of stand-alone hybrid renewable energy systems," Renewable and Sustainable Energy Reviews, vol. 13, no. 8, pp. 2111-2118, 2009.
[5] Centre for Environmental Data Analysis, "CEDA Archive," 2018-2014. [Online]. Available: https://data.ceda.ac.uk/badc/ukmo-midas-open/data/uk-mean-wind-obs/dataset-version-201908/. [Accessed February 2020].
[6] Scottish & Southern Electricity Networks, "Active Network Management," [Online]. Available: https://www.ssen.co.uk/ANM. [Accessed 7 April 2020].
[7] Google Maps, [Online]. Available: https://www.google.com/maps/@58.9477199,-3.5905151,9.25z?hl=en. [Accessed 5 May 2021].