Water: an overview of how it can produce electricity

Water or at least the movement of water can be exploited to produce electricity in a variety of ways:

 

Hydropower
Hydropower produces one fifth of the worlds energy needs and in emerging economies up to a third of electricity generated comes from Hydropower.

 

Large-scale Hydro is typically taken to mean more than 20 MW of grid-connected generating capacity and is usually associated with a dam and a storage reservoir. In the UK the majority of large schemes are located in Scotland and were built during the 1950s. Due to environmental constraints it is unlikely that we will see any further large scale Hydropower sites developed in the UK . That said a number of schemes have already been repowered or are in the process of being repowered, where we will see the actual MW output increase, sometimes by as much as 20%.

 

Small-scale Hydro, recognised as being schemes of less than 20 MW now offer a greater opportunity for providing a reliable, flexible, and cost-competitive power source with minimal environmental impacts. These small-scale schemes are making an increasing contribution towards new renewable energy installations in many regions of the world, especially in rural or remote regions where other conventional sources of power are less readily available. Small scale schemes can be associated with a dam and storage reservoir or can be located in a moving stream

 

Wave
Ocean waves are caused by winds as they blow across the surface of the sea. The energy that waves contain can be harnessed and used to produce electricity. Due to the direction of the prevailing winds and the size of the Atlantic Ocean , the UK has one of the largest wave energy resources in the world.

 

Just as electrical power is not stored in transmission lines but flows from the generator to the user, so wave energy is not static but flows in the direction of wave propagation. If the energy flowing past a particular point in the ocean or arriving at a shoreline is not captured there then it is lost. Fortunately for the wave energy industry if one wave is lost there will be another one along soon bearing more energy. Because the energy is flowing we can consider the amount of power in kW contained in each linear metre of wave front.

 

Off the West coast of Scotland the winter availability may be four times the summer average. In our part of the world this can be considered an advantage because our energy demand in the cold season is so much higher than that in the summer months. This is not necessarily true worldwide. The Atlantic seaboard of the British Isles has one of the best wave energy climates on the planet with 60-70kW/m in deep water off the Western Isles falling to 15-20kW at the shoreline as the effects of bottom friction and wave breaking take their toll. With the land mass of Scotland offering shelter to the south west the available power falls as we move east along the northern coast of Scotland but is still a remarkably attractive 25-50kW/m (dependent on water depth) by the time we reach the waters of Orcadia. The reality is that the power is there, the challenge is to harness it.

 

The Challenge for Wave Power

For more than two centuries inventors have been filing patents for systems to capture power from the waves and yet we still do not have a wide application of wave energy devices as power generators. So what is the problem? Actually there is no conceptual problem. We can extract power using articulated rafts, nodding ducks, compressible floating bags, tethered buoys, bottom standing oscillating water columns, over-spilling systems, submerged pressure chambers etc etc. Similarly there are no insurmountable technical problems. Whilst there is much engineering difficulty the wave energy community has solutions to just about every aspect of the technology. The reality is that the only long term problem is making the technology work at a cost of power which a consumer is willing to pay. In the long term fossil fuel generation will become more expensive and wave generated power will fall in cost, but until that time the development of wave power is hampered by the need to introduce a fledgling technology into a commercial market dominated by subsidised low cost fossil fuel and nuclear generation.

 

Twenty years ago the wind industry suffered similar problems but largely through the far sighted long term support to manufacturers offered by the Danish government that nation was able to develop an industry which with the premiums offered for green power can now compete on a commercial footing. The wave energy industry is now in a similar stage of development to the wind industry in the 1980's with privately funded prototype devices under development with public support and some public money. There will be failures on the way, that is the nature of technical development, but with sustained public support to create conditions where new energy sources can be introduced to the market there is every expectation that wave power will mature to make a major contribution to our energy needs.

 

The Technical Challenge

The technical challenge in Wave energy is driven by the commercial challenge. Notwithstanding political considerations the success of wave energy in relation to other energy supply technologies will ultimately be determined by the price at which it can deliver power to the market. The cost of producing wave generated electricity is comprised primarily of the capital expenditure in building and installing the device and connecting to the electricity grid. Capital expenditure typically accounts for more than 90% of the cost of producing wave power. This is in marked contrast to fossil fuel plant where the input fuel is a high proportion of cost. A successful wave energy device will therefore have a minimum capital expenditure and a maximum electrical output. This rather obvious fact creates a dilemma for the designer of wave energy plant. The device structure has to survive the worst that the sea can throw at it; but only just. Looking at it simplistically if we over-design a wave energy machine by a factor of two it will cost twice as much and the price of power will double. We will then have a very reliable wave power device that no-one will want to buy. We thus have to go through a development stage where we build prototype units which, as far as we can tell with the available information, will survive fabled storms and which may not be economic generators but will give us the information on loads and performance to enable the next design to be closer to the limit.

 

The long term future of bulk wave energy generation lies in exploiting the offshore resource and as engineers we have to produce optimised designs for:

As a general rule proponents of wave energy are trying to do everything that engineers have for years been trying to avoid. We are looking to place our structures permanently in areas of high wave activity so that whilst a super tanker might seek shelter we will seek the storm. Whilst a Cruise liner might fit stabilisers for passenger comport we are more often than not relying on a high response amplitude to some form of motion in order to extract power. Whilst ship and offshore jackets are designed to shed wave forces we are, at least in small to moderate waves, trying to interact with them. It is not surprising therefore that in pushing the design envelope of marine structures we are having to develop and extend our analytic tools. These tools then need testing and calibrating against field data; which takes us back to prototype devices and testing.

 

There is a debate within the wave energy industry as to how to best to develop a wave energy device. There are many schools of thought. Some advocate that everything can be learnt in wave tanks and that there is no need to go to sea until all problems are solved to a high degree of confidence. Others prefer a progressive increase in scale from small tank models, to larger models which can be tested in sheltered waters and thence to the full size. Others believe that the time cost of the progressive approach is unacceptable and that if a device is worthwhile the best way to develop is to build the first unit at the full scale so that real data become immediately available and the route to bulk generation is thereby shortened. There is no doubt that this is could be true but it is equally true that the risk of failure increases with the latter approach. There is also a debate within the industry as to whether research into shoreline generation has any merit or whether all our efforts should be focussed offshore.

 

Applications of Wave Power
The long term goal for the wave energy industry is to be bulk suppliers of power feeding national grids from offshore wave farms. This will happen in the fullness of time. Bulk electricity generation is not however the only application of wave power. In concert with solar power, wave powered buoys are already used for powering marine buoys. They have also been proposed as pumps for low pressure transmission of water, for producers of high pressure water for desalination and as sea calming devices for coastal protection.

 

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