The Power In and Under the Waves

I live in the UK which is a relatively long thin island (with many smaller satellite islands) in which you are never more than 112km from the sea. The total coastline, at almost 12,500km and even more when the satellite islands are included, would take you over one quarter of the way round the world. For me, this wealth of coastline could offer so much more than blustery cliff top walks and a gateway to fishing and should play a major role in future energy supplies. Currently, the seas do play host to large wind farms but the energy potential under and in the waves is truly huge. A number of pilot plants have been developed around the world with a number of large scale plants in various stages of fruition. The energy sources in the seas can be split into three main types: Tidal, wave and ocean currents which I will discuss in turn. I will also touch on a fourth and special case of osmotic power or blue energy.

    Tidal power is particularly promising in the UK where some of the highest tidal ranges in the world are found. An important and notable project in the pipeline is the Swansea Bay tidal lagoon which will occupy a large part of the bay being encompassed by a 9.5km wall. The concept of such tidal lagoons is simple: As water fills and empties the lagoon depending on the tides it runs past and drives turbines 4 times a day (there are 2 tidal periods per day). This project is largely privately financed but benefits from government incentives which take the form of guaranteed prices for the energy eventually produced (like with nuclear in the UK). Such installations are on truly large scales but they payback with huge outputs with this plant designed to produce 320MW; enough to provide energy for 155,000 homes which is more than the 106,300 estimated households in the whole of Swansea! This lagoon, as a blueprint for future tidal lagoons, rightly aims to add to the coastline offering not only energy but recreation opportunities including a seawall walkway, rockpools, sailing and art installations if the claims are to be believed. The body behind the Swansea Bay project are planning a ‘fleet’ of tidal lagoons around the UK (4 in Wales and 2 in England) at spots particularly well suited due to high tidal ranges and have plans to expand globally. This makes the success of the Swansea Bay project so important for the future of the technology as a proof of concept allowing people to see how we can make the most of our coasts to power significant areas with minimal environmental cost, especially as this is likely to become a tourist attraction in its own right.
     A second type of tidal power plant involves the placement of turbines on the sea bed which has the advantage of being discreet, easily installed and scalable. These act much like underwater wind turbines with the movement of water due to tides flowing over the turbines to generate electricity. The Paimpol-Brehat Tidal Farm in France is currently the worlds largest such installation with an 8MW capacity from the four 22m heigh turbines which sit just below the surface. I mentioned scalable and, as such plants are modular (each module is a turbine), more can be added and hooked up to the grid as needs dictate. Their lack of a visual presence is a plus for many but a key benefit is the ease of installation with minimal disruption to the sea environment. Most construction is carried out on land with the turbines being lowered and secured to a modest platform on the sea bed. 

    Tidal is great but what about those places that have very small tidal ranges and plus tides oscillate between maximums but pass through midpoints where no energy can be generated making them predictable but intermittent. So what about some more constant sources like waves and currents? Capturing the energy from ocean currents relies on much the same approaches as the underwater tidal turbines except that rather than oscillating, the currents tend to flow in the same direction much like a river offering the potential for more constant energy production. However, suitable locations for such plants need to be close enough to land to reduce electricity transport costs while still tapping into the strong currents. One such place might be off the coast of Florida where the Gulf Stream flows close by and is being explored by a crowd funded company Crowd Energy. One area of concern for such plants, depending on this technologies success, is that they do not suck out too much energy from the currents which could affect the rhythm and circulation of the oceans affecting both marine life and the above ground climates.

     Wave energy aims to capture the energy stored in waves (formed by wind in the first place) which are limited to the waters surface requiring floating or just below the surface devices. There is a lot of energy in waves, and there is a plentiful and regular supply of them, however, the technical challenges in harnessing their power are significant. The simplest way of harnessing wave energy is by converting the vertical or horizontal movement into rotational movement to drive a generator in a wave energy converter (WEC) and these simple concepts are explained in this short video with the hinged set-up explained more here. While there are a broad range of possible WEC designs, the principals are generally shared and a steady flow of electricity can be produced by these plants and transported for use on the land. There are, however, two other common applications for WECs, the first in pumping sea water into reservoirs on land and in secondly in water desalination. These couplings aim to solve two separate issues but both maximise the coastal environment. Pumping sea water into reservoirs allows for energy storage through potential energy as discussed here meaning excess wave energy can be stored for when other sources are unavailable or during power surges. The use of WECs in desalination plants means this power hungry process can be carried out sustainably and can provide human usable water for coastal areas or small islands which often suffer from a lack of fresh water. The use in desalination also segways nicely into my last section on osmotic power. 

    Osmotic power (a.k.a blue energy) is a relatively old concept that is slowly emerging from its experimental pupae but could be another useful local energy solution. It is principally a local solution as it requires a unique environment; namely a river estuary where fresh and salt water mix. To tap this energy, a membrane must separate the fresh and salt water. This separation can then be used in a number of ways including pressure-retarded osmosis and the creation of ‘salt batteries’. The pressure approaches rely on the movement of water from fresh water to salt water across a water permeable membrane in a clever chamber system such that this movement changes the pressures of specific chambers (a change in volume=a change in pressure) and these changes can be used to drive a turbine. The salt battery, or reversed electrodialysis, set-up harnesses the chemical energy found in the gradient of ions between the salt and fresh waters to generate a voltage as explained here. This technology is an exciting prospect and is being pursued in the sea embattled Netherlands where the Afsluitdijk dam pilot plant is up and running with plans to massively up-scale. With a theoretical 1MW being derived per 1m3 fresh and salt water, they have hopes for the plant to supply energy for a whopping half a million homes! 

    I hope I’ve illustrated some of the incredibly promising ways we can harness the power of the seas in ways that are inventive but often elegantly simple, sustainable and effective on large scales. Future energy demands require a mixed energy policy and with almost 170 countries having over 100km of coastline, the potential of the untapped energy in these waters is huge. So the next time you look out over a brooding, tumultuous sea (or watch a video of one online) reflect on the power of and under the waves and how it could power your next cup of tea.

Renewable Energy Storage

Green energy technologies are often criticised on the grounds that they cannot provide a steady supply of energy, nor can they provide energy when the wind stops blowing or the sun stops shining. The primary issue here is with energy storage to overcome this intermittency. One high-tech solution in the development pipeline is the development of large and efficient chemical batteries – something being worked on by Tesla. There are however a number of drawbacks with such technologies including inefficiencies as the energy can often decay or leak over time, very high cost and their components have their own environmental problems with their extraction and disposal. Despite these issues, when these, and other, issues are cracked the potential of these batteries is great. There are, however, a number of ‘low-tech’ options which I would like to discuss which could indeed be cleaner, cheaper and often a little more off the wall than a chemical battery (literally in the case of the Tesla Powerwall).

    Many low-tech energy storage options make use of gravity allowing energy to be stored as potential energy which has the advantage of not depreciating over time. A common principal is to use excess electrical energy to raise a mass which then can then be ‘dropped’ in a controlled manner when needed to drive a motor or generator and thus regenerate electrical energy. Check out the ‘gravity light’ for a very small scale example of this idea which is summarised in three steps below.
    1) Excess electrical energy used to raise a mass.

    2) The energy input is now stored as potential energy.
    3) Dropping the mass back down converts potential energy back into electrical.
A good option for the mass, especially in wet and hilly places, is to use water, pumping it up hill into a reservoir before releasing it down through turbines. This has the advantage that dams or reservoirs can be pretty big meaning more mass and therefore are able to hold significant amounts of potential energy. Other possibilities include lifting actual solid weights however these are only really appropriate for smaller scale operations such as for individual houses. This is because you need very large masses to store and generate significant amounts of energy. However, it is not too hard to imagine a house that has an array of solar panels able to use some of the energy on a sunny day to crank up a heavy block of concrete (or other potential waste material like scrap metal) before dropping it down through a high resistance hydraulic turbine to power the lights and TV at night before raising it back up the next day.

    The concept is relatively straight forward and there are as many ways of implementing it as the imagination allows and one example in particular has caught mine and others attention. A left-field suggestion for using this concept has been cooked up which looks to reuse existing resources and infrastructure in America, namely trains and train tracks, and will work particularly well in the drier parts. The idea involves using excess energy from wind and solar plants to drive electric motors on heavily loaded trains to move them up hill. When needed, the trains rumble back down hill and now turn the electric motors the other way so acting as generators to produce electricity. This system has some advantages over vertical lifting systems of solid weights, which generally need unrealistically tall lifts to operate at such levels, as the trains on tracks act more like water being able to run along slopes rather than vertically and therefore are scalable (just strap on more loaded carriages). 

Beacons of hope?

    Another really exciting, but slightly less low-tech technology being developed for energy storage I wanted to include here is high-temperature solar thermal. The concept here is the use solar energy to directly heat a medium such as water, salt or oil and the release of heat can be used to drive steam turbines to produce electrical energy. Some of the most interesting examples of these include so called 'power towers'. Here, thousands of mirrors track the sun and concentrate its energy on a central tower where salt is heated to extremely high temperatures (over 500°C) such that it melts. This hot molten salt is stored in insulated tanks to minimise heat loss and then this heat can drive a steam turbine when power is required and especially at night. The first working commercial example of this, an 11MW plant that uses water rather than salt, was set up in the hot, sunny dry south of Spain. Morocco, another hot and dry country just across the straight of Gibraltar, has also been investing (or rather been invested in by the EU) heavily in solar and was even touted as a potential base for providing solar for west Europe (investment, not charity!). They have recently finished the first stage on a plant that aims to produce 580MW with up to 8 hours energy storage using molten salt as the storage medium but with a different design to the power towers. Despite solar energy being in plentiful supply in these locations, water is not which is a problem as a lot is required not least in cleaning the mirrors (although a dry cleaning method is being developed) to keep them shiny and effective! 

    As I hope you can see, there are alternate options out there to help make more sustainable energy sources more reliable and our slow progress should not be blamed completely on a lack of energy storage systems, it is still largely a lack of will and investment. A range of smaller scale low-tech options exist which means poorer regions can benefit from green(er) energy despite not being able to afford the expensive batteries when they arrive. Our future energy solutions will require imagination and policies that match and optimise the local conditions – one size does not fit all.