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A scientific review of wave power resources and the implications of the spatial and temporal variability
The depletion of traditional finite energy resources and their negative environmental impacts have driven exploration of renewable energy sources globally. Governments around the world are working toward reducing fossil fuel dependence as their continued consumption constitutes significantly to the rise of CO2 in the atmosphere, and global climate change.
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Over recent decades wind, solar and bio energy technologies have matured significantly. With 40% of the UK’s electricity (20% wind, 12% bio and 6% solar) supplied from renewables in the third quarter of 2019 . This transition to renewables was responsible for a 37% reduction in emissions within the UK electricity sector in 2017 .
The marine renewable resource in the UK offers the potential to reduce carbon emissions by a further 4MT (million tonnes) of C02 per year by 2040, if exploited . Tidal stream energy is currently at the forefront of the UK’s marine energy renewables, with the world’s first tidal stream array project, MeyGen generating over 8GWh (gigs watt hours) of energy to the grid to date . However, the ocean offers another huge resource of relatively untapped high-density energy. Driven indirectly by the sun’s energy, ocean waves or wind waves are generated when wind blows over the ocean’s surface. These waves can travel huge distances from their generating source with little depletion of energy as swell and offer a considerable resource for energy exploitation.
Although discussed in patents since the late 18th century, wave energy convertor technology (WEC) is not yet commercially
viable [7,11]. However, Investments into the research and development of the marine energy sector in the UK, estimated at £450 million to date, have pushed growth in the industry . There are currently 23 UK based wave developers and wave energy test centres located in Scotland, Wales and Cornwall. All dedicated to exploring different WEC technology in onshore and offshore locations .
1.1 Southwest UK research
Specifically focusing on the wave energy research facilities in the South west region, the first area in the UK to become a designated marine energy park in 2012 . There are three sites dedicated to testing WEC arrays. The Wave hub is a 20MW (mega-watt) grid connected site in North Cornwall. Located 10 miles offshore, the site can facilitate up to four different WEC’s connected to the grid generating power [3,7]. Supported by the faBTest nursery site in Falmouth and the South Wales Demonstration Zone, for pre commercial array demonstration. Ongoing research at the facilities are working toward accelerating growth in the industry in the direction of a commercially viable product.
1.2 Advantages and challenges
There are significant advantages to using wave power over other types of renewable resources such as wind and solar;
- Wind waves offer the highest energy density (2-3kW/m2) far greater than other renewable sources such as wind (0.4-0.6kW/m2) and solar (0.2kW/m2) .
- Waves can travel across oceans from their origin with little energy loss, energy only begins to significantly diminish as waves interact with the seabed when wavelength is less than 1/5 of the depth and processes such as friction occur [1,7,].
- Although still a stochastic energy source like wind energy, wave power is enormous, and its predictive capacity far exceeds wind .
- Wave energy convertors have the capacity to generate power for 90% of the time far exceeding that of wind and solar (20-30%) .
- Demand and resource correlation are good, with 37% of the global population living along the 90km of coastline .
- Wave energy converters (WEC’s) can be placed in multiple locations from shoreline to deep waters, expanding the availability of the resource .
Far from a new technology and with modern research interest growing over recent decades harnessing energy from waves still presents its challenges.
2. Wave energy resource
Several papers have been published using high quality model, satellite altimetry and buoy data to give a theoretical view of the available wave power worldwide [2,8,9]. Estimates by  of theoretical net wave power are in the region of 3TW (terra-watt) globally highlight the potential of the resource. Europe one of the most abundant areas of the world has a deep-water resource of 381GW (giga-watt), of which 120GW is estimated to descend on the UK shores . The Scilly Isles are ideally placed for the exploration of a potential wave power resource. Located in open ocean and exposed to swells from the North Atlantic, Bay of Biscay and the English Channel . Marine renewables could offer the islands an opportunity to generate electricity locally.
3. Spatial and temporal variability
The intermittency of renewables raises concerns when considering their supply to the electricity grid. Wave power is a stochastic form of renewable energy that varies spatially and temporally. In order to consider suitable sites for generating energy for harnessing wave power, knowledge of the available wave climate and power estimation are necessary . Evaluating the variability in the wave energy resource is a key factor to define. Studies by [5,6,10,12] all found that wave power varies with time, space and wave direction. With highest mean energy resources found throughout winter months and lowest in the summer months [5,6,10,12]. Highlighting the importance of considering monthly, seasonal and yearly variability in wave power resources. Longer term processes such as the North Atlantic Oscillation (NAO) can also impact variability in wave power. , discussed how the NAO can significantly reduce the strength of the westerlies in the North Atlantic. Any reductions in wind would mean a corresponding reduction in wave height (Hs) and increase resource variability.
Sites with less variability in energy flux, where the resource is steady and wave energy moderate are found to be more favourable [7,10]. Most WEC prototypes have a max working efficiency for a particular range of wave periods (T) and wave heights (Hs). Where efficiency decreases as wave conditions become more variable [1,7]. Another factor is extreme waves, more prevalent in conditions of higher variability . Studies by [5,10,12] all suggest potential sites best for wave energy harnessing are where variability is low.
Mean directionality of power at offshore locations (less so with nearshore) is also found to be important when considering WEC type and placement . Research by  found at the Scillies buoy 77.2% of sea states arrived from the west (wave direction 225-315). Suggesting that the Scilly Isles western coast may provide the sites most suitable for wave power exploration. The same study found that along the Cornish coastline spatial variation in wave power was dependant on offshore wave direction  and exploiting sites with varying directional wave power with multiple arrays of WEC reduced the time of zero power output .
Due to the lack of long-term buoy data at multiple locations, most studies into wave power rely on hydrodynamic wave model data [3,5,10,12]. Research indicates that in order to establish wave climates at least 10 years data is needed . Studies by [3,5,10,12] all use the parameters wave height Hs and wave period Tp, taken from various hydrodynamic wave models that provide hindcast data over periods of 10 years or more to compute power. Studies [3,10] use the deep water equation to compute power as output locations considered were 50m depth+. Although low frequency waves will interact with the sea bed, the deep water equation wont introduce significant errors.
Deep water power equation;
Where P is power in W/m, is density of water, Hs is significant wave height and Te is the energy period. Because the energy period Te is rarely specified previous studies [10,14] assumed Te= Tp and  assumed a more conservative Te= 0.9Tp. Although these assumptions necessary they do introduce uncertainty in wave power estimates. However, using this assumption along with Hs the total wave energy resource was assessed in studies [10,14] at multiple grid points and then used to compute the power average. Mean directionality (also taken from the model) and the power average were then used to predict previous sea states and estimate the spatial and temporal variability in the resource. Buoy data close to the study site in all studies [3,5,10,12] is used to validate model data used.
Using the research discussed and applying similar methods. A study of the spatio-temporal variation of the Scilly Isles wave energy resource will be carried out. Using a combination of Copernicus wave model data and buoy data. A hindcast of sea-sate variables from 2009-2018 will be made to assess the wave energy resource at grid points around the Islands. Average wave energy will be computed using the deep water wave equation as all grid point outputs are 50m or above depth.
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