Wave Power Conundrums: Marine Renewable Energy for beginners

I’m very excited to welcome the participants of ‘One planet – one ocean’, a Massive Open Online Course. From watching earlier Q&A sessions, I can see that you come from all around the world, and from all walks of life. For those of you who would like to dig deeper into Topic 10.5, ‘Sustainable Energy from the Ocean’, I have put together this fact sheet (liberally sprinkled with opinion). I used the questions and comments you left next to this lecture to decide what to include. I will be a guest speaker on the Q&A session on Marine Renewables this Thursday. I am hoping that this blog post will spark the curiosity of those who were still deciding whether to listen in, and inspire thoughtful questions from those who already know they will be joining us.

This article covers the following topics: making the case for marine renewables, a list of the main features of the different types of marine renewables, a technology status update, a discussion of the current challenges, and some thoughts on how marine renewables could fit into a future low carbon energy mix.

Why Ocean renewables?

Why should we consider getting energy from the ocean when we can get energy elsewhere cheaper; when there are other things we could be doing with that ocean space, and so many other concerns to do with the oceans? Here are the reasons that matter.

Demand for low-carbon, locally sourced energy

Our civilisation is built on the foundation of an energy-hungry economy. The status quo cannot continue; problems will arise long before rising demand drains the last fossil fuel reserves. If we were to burn all our coal reserves, the changes in climate would throw our world into chaos. Collectively, we need to ensure that the carbon in the ground does not end up in the atmosphere. It is also in our interests to prevent localised energy scarcity, as this has a tendency to be politically destabilising. For these reasons we need to decarbonise our economy. While I do not believe that the solution is as simple as exchanging fossil fuels for renewables, I do think renewables need to be a big part of the solution. Unless we crack fusion, we will need a lot more renewables in the decades to come.

Building a green economy

Some countries are blessed with a large ocean energy resource; while others have locations that make fossil fuels more expensive. It would be economically and politically advantageous to such countries to harvest marine energy. Furthermore, the prize of building an industry based around a future-proof technology is very attractive.

An insurance policy

Marine energy is currently expensive compared to renewables that have been developed for decades. Stuff always gets cheaper when you learn how to make it better, and when you make lots of it. This is known as ‘technology learning’. Marine renewables have not yet experienced enough technology learning to know for sure whether they will be competitive when they are mature technologies. Developing marine renewables now acts as an insurance policy, so that we have more options for decarbonisation in the future.

Limits to the alternatives

One of David Mackay’s biggest concerns about renewables was the vast land area required to replace our fossil fuel needs, and concerns that this would compete with food production. With rising population and sea levels, the land area per person available for renewable energy production is decreasing. In the ocean, there is less competition for space. Ironically, those most concerned about loosing space to marine renewables, fishermen, will benefit in the long run. One sure way to avert the looming tragedy of the commons due to overfishing is to impose reserves where fish can regenerate.

Technology comparison

Seaweed biomass

This is often missing from lists of marine renewables, but it is technically possible to grow seaweed as a fuel. It is not currently being exploited, as it is presently more expensive to grow plants (biomass) in the sea than on land. As nutrient-rich seaweeds are used as food and animal fodder, there is some concern that biomass production could displace food production. However, seaweed is not currently being exploited intensively, so it should be possible to grow fuel in the sea without having a significant impact on food security.

The challenges are to reduce production costs, and to develop more efficient alternatives to burning unprocessed seaweeds, such as first creating liquid or gas biofuels.

Offshore wind

With over 12GW operational installed capacity, offshore wind is the most mature marine renewable. It is not always classed as a marine renewable, as the energy captured comes from the wind rather than the ocean. However, it makes use of sea rather than land area, interacts with marine ecosystems, and experiences operational challenges due to the marine environment. Hence people working in wind energy consider offshore wind to be a marine renewable. Wind energy is a concentrated form of solar energy, as the driving thermal gradients are caused by the sun.

Offshore wind is currently more expensive than onshore wind, as there are several new challenges that need to be solved: limited weather windows for doing repairs, and the costs of installation and repair. Marine installation vessels are more expensive than their land equivalents. There is the cost of the additional under water structure, which attracts wave loads and acts as a lever arm, turning forces on the top of the turbine into turning moments on the foundations. Despite these disadvantages, engineering multinationals are investing development effort into offshore wind because of its huge potential. The resource is large, so this represents a large future income stream. Wind energy can be made cheaper with economies of scale: huge turbines, installed in vast arrays. On land, industrial scale wind farms will be limited by competition with other land uses. So in the future, offshore wind may be cheaper than onshore wind. There are also some technical advantages of offshore wind, such as the higher wind speeds, and the ease of transporting large structures.

Wave energy

Like wind energy, wave energy is a form of concentrated solar energy. This is because waves are caused by the wind blowing over long expanses of ocean. The waves travel thousands of kilometres in the direction of the winds that created them, until the energy is dissipated by bottom friction or wave breaking.

While the wave itself travels in one direction, the water particles oscillate, moving in circular orbits. They move both up & down (potential energy) and back & forth (kinetic energy). As the particles reach a crest or trough, they slow to a halt. All their kinetic energy is exchanged for potential energy, like a pendulum at its highest point. As waves pass into shallow waters, the particle orbits become elliptical. Even though what we notice is the up & down motion, shallow water waves have more side to side motion. Some wave energy devices work by bobbing up & down, capturing the potential energy of the wave. This works best in deep water. In shallow waters, devices that extract energy from the back & forth motion of the waves are more economic. This is why we see such different looking devices for shallow (e.g. Oyster) and deep waters (e.g. Ceto).

Of course, for deep water devices, there is ultimately no reason why we should only attempt to capture half the energy. At the moment the many devices that work by bobbing up and down reflects the need for a step by step development approach. Reliability is a big problem for any new technology, and even more so for anything that is going to be put somewhere that is difficult to access for maintenance, so there is a strong drive to limit system complexity until the teething problems have been worked through.

Ocean current

Ocean currents are caused by a mean flow of water in the ocean. There are various things that cause this flow, including the earth’s rotation, wind, waves, and salinity and temperature differences.

The description just given of the circular particle orbits in ocean waves was a simplification: in fact, each particle slightly overshoots its starting position (see anmation), resulting in a mean drift in the direction the wave is travelling.

The currents arising from these various sources are concentrated in narrow bands, rather than averaging out in the surrounding waters (see video below). They can be visualised as conveyor belts: water flows to fill the place of water that has recently vacated.

Ocean currents do not fluctuate in direction, but their positions do meander in a chaotic fashion. Obstructions often cause vortices to break off from the main current, and these vortices tend to get dragged along with the current, so that there can be localised conditions where the current appears to change direction. Ocean currents are three dimensional disturbances: it is possible to have a surface current in one direction, overlaying a deep ocean current in another direction.

Tidal streams

Tidal streams are distinct from ocean currents. They are caused by a difference in tidal level, and this in turn is caused by the gravitational attraction of celestial bodies, principally the sun and moon. The flow is driven by a difference in head (water height), so happens all the way down to the seabed, and across the whole expanse of the ocean. When tidal flow is obstructed by land masses, it is concentrated and diverted, which can result in high velocities. These tidal streams change direction with the tides, and using harmonic analysis, can be predicted years in advance. The available resource is less than that of wave energy. However, the resource is very localised, so the incentive for development is more about energy security than building a global industry.

Tidal barrage

Tidal barrage technology makes direct use of difference in tidal head. In some parts of the world, there is a very large tidal range due to local amplification. Tidal barrage is to tidal stream what hydropower dams are to run of river turbines: they can capture a greater proportion of the available energy, but they require construction of an expensive impoundment with a lifespan that greatly exceeds the timeline over which the project’s economic viability will be determined.

Recently there has been a distinction made between tidal barrages and tidal lagoons. The driving design aim of tidal barrages is to minimise the length of barrage with respect to the volume of impounded water. Barrage sites include bays and river mouths. A major difficulty in obtaining planning permission for barrages is the disruption to shipping, and the large areas of coastline that are impacted. The driving design aim of tidal lagoons is to minimise the disruption to shipping, and the amount of coastline impacted by the lower tidal range. They tend to impound the water with more wall than coastline, resulting in a greater cost of energy. Nevertheless, it is telling that plans for the Swansea Bay tidal lagoon have progressed much further than those for a Severn barrage, which would produce cheaper electricity. https://en.wikipedia.org/wiki/Tide

Salinity gradients

Osmosis is driven by a difference in concentration of solutions that are in contact via a semipermeable membrane. It is possible to generate power from osmosis between salty sea water and fresh river water. Water will migrate across a membrane in order to balance out a difference in salinity. An example of a semipermeable membrane is the skin of a slug, which is why salt is lethal for these creatures.

The salinity difference causes an osmotic pressure: this is the pressure you need to put on the salty water to stop fresh water leaking through the membrane. If the two bodies of water are in walled off areas, the exchange of water will result in a head (height) difference, which can be used to generate electricity. The energy has been gained at the expense of increasing the disorder of the system: instead of some fresh water and some sea water, you get less useful brackish water and some energy.

Ocean Thermal Energy Conversion (OTEC)

A temperature gradient can be made to do work. It requires a working fluid that is a liquid when in contact with the cooler temperature, and becomes a gas when in contact with the higher temperature. To get temperature differences high enough requires water from the ocean surface and the deep ocean off the continental shelf. This technique is currently an order of magnitude too expensive. Challenges include the energy demand in pumping the deep water up to the surface, the low efficiency due to the small temperature difference, and biofouling, which can degrade efficient heat transfer.