Techno-economic assessment of solar panel integration onto a wave energy converter

Abstract

A national grid based on renewable energy needs a diverse portfolio of technologies to reliably produce power for all variations in weather. Wave energy offers a new breed of renewable technology which is both predictable and energy dense. However, despite decades of funding and development, wave energy is yet to reach the commercialisation stages due to high testing costs and lack of convergence of design.

One way to achieve commercial success is to first target kilowatt-scale niche demands which are less concerned with the price of energy and instead desire consistent, year-round renewable power. Niche demands thus offer a stepping stone for wave energy converter developers to prove their technology, make revenue, and scale up to megawatt grid-tied devices. The main challenge to providing consistent power is that the available wave energy resource observes a seasonal variation peaking in the winter and lulling in the summer, especially in northern latitudes.

The smoothing of seasonal energy variations is critical for small-scale wave devices so that consumers do not have to rely on fossil fuel backups during summer months. The impact of seasonal variations can be reduced by oversizing the wave energy converter so that the minimum is satisfactory for the consumer. An alternative is to use large-capacity inter-seasonal energy storage. Whilst both solutions are plausible, they may not make economic sense. Instead, utilising the available deck area of a wave energy converter with solar panels will offer an alternate source of energy which is out of phase with the winter peaks observed in the wave energy resource.

This thesis explores the novel idea of combining wave and solar renewable technologies into one floating device, a Solar-Wave Energy Converter. It uses the Mocean Energy hinged raft BlueX wave energy converter as its example. A literature review shows that whilst this topic has been introduced before, the models to predict performance are basic and there are no published examples of experimentation at scale or at sea.

This work completes two sets of experiments to examine how the dynamic offshore environment of the BlueX wave energy converter influences the yield of solar panels on its deck area. In the first set of experiments flexible and rigid solar panels are compared before a storm damages the test kit after 16 days at sea. Despite the short iii deployment period, the panels were covered in bird guano which could not be washed by the waves. This would have a significant impact on the yield and survivability of the panels. Methods to deter birds should be considered if the device is deployed in locations where birds are likely to reside.

The second experiment uses lessons from the first and applies flexible panels to the hull as they offer greater protection from the elements. The panels power a continuous hotel load of the wave energy converter, allowing the wave power produced to go directly to the consumer. The panels located on the front of the device were regularly washed by the waves and had deposits of marine growth on the surface. The panels located on top of the nacelle were in good condition as birds were not as prevalent at this deployment. Despite the marine growth, the panels powered the hotel demand for 96% of the 122 days of testing. It was found that the efficiency of the maximum power point tracking converter is sensitive to the motions of the wave energy converter, this is described by a linear relationship with significant wave height.

A numerical model is produced which takes into account the motions and heading of the wave energy converter as well as the curvature of the panels. This model is compared to the experimental results where a good agreement is found, with an average error over the 122 testing period of 1.1% and a worst-case error in March of 4.22%, increasing confidence in the results. The model is then used in sensitivity studies to obtain a performance ratio for various headings and hull diameters for deployments in the North Sea and the Caribbean Sea.

A six-kilowatt solar array is modelled to assess the benefits for deployments in the North Sea, Mediterranean and Canary Islands. The increase in the magnitude of the continuous available power, the amount of power which can be reliably provided to a consumer, is assessed for each location where increases between 13% and 35% are found, depending on the location. Additionally, the increase in annual energy production was found to be between 6% and 24%. The standard deviation of the monthly energy shows how variable the resource is. Assessing this before and after the addition of the solar array shows how it smooths the device output. It was found that the standard deviation can reduce by 43%. However, the addition of the solar array can also increase the standard deviation of the monthly energy when the wave and solar resources are not perfectly out of phase. This occurs for deployments near the Canary Islands, where the 6kW array increased the standard deviation of the monthly energy by 17%.

Finally, the costs of the experiments are used to obtain the cost of a full-scale array showing that the benefits reaped from hybridising the system far outweigh the costs. The cost metrics of the BlueX are confidential and thus, the levelised cost of energy scaling factor, the amount the original wave energy converter levelised cost of energy is scaled by, for varying wave device costs is found. For deployments in the north sea, the levelised cost of energy reduces up to 4.5% when the wave device costs more than 1.466 million GBP. For other locations, the reduction can be up to 17.5%.

This work shows that the addition of a solar array onto a wave energy converter can be beneficial by increasing the continuously available power, smoothing the monthly energy variations and reducing the levelised cost of energy. Further work suggests the exploration of deployable arrays to increase the capacity of the solar array beyond the available deck area, to improve the benefits further.