Wind & Wave Synergy

Introduction

As a result of the landmark agreement achieved at the United Nations Paris Climate Summit (COP21), the cosignatory countries agreed upon the aim of reducing greenhouse emissions in order to achieve a level that will restrict global average temperature not to rise above 2 ℃, and if possible 1.5 ℃. This has represented a significant consensus declaring that it is high time to accomplish renewable targets. This project, by combining wind and wave energy resources, aims to provide the industry with an innovative approach [1].

Why mix of Wind and Wave

The primary advantage of the wind and wave synergy is that it can provide a much steadier power output, therefore, raising the reserved capacity of the national grid, which makes the system more stable and not so dependent on non-dispatchable energy systems. The possibility to have a steady power output does not have any economic advantages for the developer at present, as the output would not be stable enough to qualify for the Capacity Market, but it could have the potential to reduce the subsidy level required to bridge the gap between the day-ahead sales price and the Contracts for Difference (CfD) strike price. It would also help the national grid to be consistent and not highly reliant on storage technologies, such as batteries, or Short-Term Operating Reserve (STOR) to guarantee the grid stability and power quality.

So, it is clear that steadying the output is both beneficial and desirable, but what is the best way to achieve this balance?

Ultimately, the optimum mix of wind and wave could be found by plotting the estimated hourly power output form the wave energy collectors integrated into the platform, and the surrounding wind turbines. However, hourly wind and wave data in deep offshore sites is not available for that majority of sites. One site where this data has been recorded, but is not publicly available is on the West coast of Ireland. A study was carried out into the optimum mix of wind and waves for this site, and t was found that a ratio of 20% wave energy to 80% wind energy (average annual outputs) was found to provide a good balance of grid stability and output [2]. Clearly a similar study would be required at the site selected for this project – however this provides us with a good starting point and, as such, one of the main aims of this project is to achieve this 80:20 ratio.

Variability Reduction

Figure 1. Wave & Wind Combination [3]

As previously mentioned, the reduciton of variablity of power output is one of the main benefits of combining wind and wave. In non-ocean sites, such as the North Sea wind and wave are complementary energy sources; this means that the wave energy output can be maintained when wind drops, as waves often continue to travel 3-4 hours after the wind has begun to subside [4].

As demonstrated in Figure 1, courtesy of Wavestar, the power provided by the system is much more stable than the one provided individually by wind or wave. Peaks and falls are avoided by combining these two systems. Another advantage associated with this is the increased predictability of the combined resources, as waves have less variability and are more predictable than wind, therefore decreasing the uncertainty characteristic of this kind of renewable energy projects, which ultimately will enable to attract more investors [5].

In addition, in ocean sites such as the one selected for this project, waves normally travel long distances with minimal losses. Considering this, it can be concluded that it is possible to harvest the energy of ocean swells coming from distant storms while being in a temporary windless area – the wind and ocean swell are effectively decoupled. Consequently, the wave energy factor would increase [6].

Figure 2 illustrates the equivalent uniform power capturing area per unit of lateral frontal length (UPCAULFL) from waves and wind independently. It can be seen that there are 2 regions in the graph, when the wind speed is less than 18 knots (9.26 m/s), the UPCAULFL is higher in the wind than in wave, but from 9.26 m/s wind speed, is greater the one coming from waves. Considering the location of our project has a wind speed average of 9.6-11 m/s, it is highly recommended to install energy converters as the UPCAULFL for the wave converters are higher than the wind converters.

Figure 2. Wind & Wave Energy Capture Comparison [2]

As far as economic aspects are concerned, integrating and combining offshore wind and wave energy devices on the very same structure, such us the PSP Platform, reduces the capital cost. Apart from the structure, mooring lines, export cable, substation, O&M facilities and accommodation are also shared, hence, contributing to the cost saving [6]. Another important factor to be highlighted is the fact that there are higher levels of subsidy for wave energy than for wind because of its lower level of development and the need for its promotion. As a result of the Electricity Market Reform, a fixed price of £305/MW is guaranteed when exporting to the national grid energy coming from wave energy collectors [7].

Another asset behind the decision to integrate wave energy devices in this concept is to raise wave energy awareness. This technology needs to be researched in depth in order to be commercialised in the next decades.