High tide - making the most of tidal power24 March 2005
Long considered but rarely realised, innovative techniques to harness the energy of the tides are now set to usher in a whole new era of hydro power generation
CANUTE’S memorable demonstration reminds us all that the power of the tides waits for no man, not even the kings of men. Nonetheless, mankind has been waiting since even before then to develop a system that will effectively utilise tidal energy. Although the use of tidal energy is close to 1000 years old – given accounts of tide mills along the coasts of France, Spain, and the UK – it has been given scant regard as a resource that might be practical to harvest.
However, a breakthrough in tidal technology has been developed by a research and development company based in the UK’s Cornwall. Unlike previous techniques, the new system relies on both the tidal stream and the natural rise and fall of the water with the ebb and flow of the tide to generate electricity and, crucially, obviates the frequently perceived need for barrages and dams.
Development and technology
The new design is the brainchild of engineers at Hi-Spec Research and Development, based in Fowey in the southwest of the UK, a small business focusing on renewable energy development which has brought together a number of existing technologies and
combined those with a new patented system. The idea behind the project initially came when company founders Pat Cooke and Fred Hill observed large tankers rising and falling on the tide. While these observations were recorded in the late 1980s, it wasn’t until late January this year that patents were filed for the concept that relies on using both the kinetic energy in tidal flow and the potential energy created by tidal height changes.
The so-called Hydro Electricity Generator (OHEG) would be sited offshore and comprises a series of four channels created from three rows of chambers housing chamber turbines and two outer walls. The overall dimensions of the power plant would be about 2km by 1.5km, making its construction a sizable undertaking. These channels direct the flow of the tidal stream through turbines, placed between the rows of chambers and the outer walls in a series of banks, with four banks per channel. But, in a fundamental difference with other systems, within the chambers groups of energy accumulators are fixed which create power from the rise and fall of the tide. The tide changes in height from high to low and low to high four times a day and after every change in tide height, potential energy is stored by the energy accumulators. Along the chambers, turbines, which remain empty while the tide is flowing in, are opened at high tide allowing the water to enter and begin generating electricity.
Each accumulator has a large hexagonal piston, effectively a float that is also a weight and this rises from its natural buoyancy or descends under gravity, depending on the height of the water’s surface. Hydraulic power is generated from the movement of this cylinder and a hydraulic motor is driven, which subsequently turns a generator.
In addition, the accumulator takes in extra water ballast of perhaps 1000 tonnes for the downstroke. Turbines mounted in the accumulator are driven when the water ballast is entering or discharging. The chambers also generate when they are flooded or discharged, which takes place at high or low tide. The plant is designed to boost the natural tidal stream by diverting the flow through the channels and this flow is also increased when chambers are filling and discharging.
With their enhanced flow rates created by the channels, the tidal turbines create power while the tide is flooding or ebbing, approximately 16 hours a day. In addition to the tidal stream energy, the chambers and energy accumulators create power for the remaining eight hours and when the systems are combined in the OHEG plant, power is generated continually.
The preliminary designs for OHEG plant see its turbines transferring energy from more than 6M tonnes of water every six and a half hours and in doing so should generate some 200MW. Hi-Spec managing director, Pat Cooke, says: ‘An ideal location would be the Bristol Channel, due to its high tide heights, strong tidal flow rates and a flat sea bed of the right depth. Whilst some concerns have previously been expressed regarding the possible erection of a barrage or dam in the channel, which might perhaps then have an adverse effect on the immediate environment, it should be stressed that the OHEG is neither a barrage nor a dam.’
‘We have brought together a number of existing technologies to create the patented OHEG system,’ adds Cooke, ‘when combined with our own energy accumulator invention, this provides a unique method for generating electricity from the sea, 24 hours a day, seven days a week.’
The plant would also make a suitable foundation for around 48 offshore wind turbines, with an additional 30MW possible from the associated wind farm. Although the OHEG system is inevitably a more reliable energy source than wind, it is estimated that the combined system may deliver upwards of 20% of the Duchy’s energy requirements.
In another fundamental advantage, the technology is very similar to that of large dams and consequently the system might easily be expected to have a lifespan of perhaps 50 or even 100 years, giving more than a fair return on the investment required, says Hi-Spec spokesman Stephen Gyles. Nonetheless, very little work has so far been put into the development of tidal energy systems, with the relatively mature wind sector attracting much more investment. Gyles argues that this may be a false economy with the comparatively short lifespan of a wind turbine, particularly given that the typical wind farm will deliver a load-factor of only around 30%. In contrast, Hi-Spec estimates the load factor for the OHEG as closer to 75% - 80%.
Finance and progress
It had been hoped that a prospective £1M (US$187M) grant from the UK’s Department of Trade & Industry (DTI) under its Technology Programme, together with possible support from independent commercial organisations, would help turn the concept into reality with a scale prototype. However, the DTI turned down the application and while the group will reapply, it is now looking to the private sector to develop this project further. To date Cooke has been solely funding the technology through his personal finances, but Hi-Spec is now in the process of approaching major generators such as PowerGen or organisations such as The Carbon Trust in order to attract finance.
A small team is currently producing a comprehensive report on the mechanical and economic viability of the project along with detailed designs. This is expected to be completed by the end of the year and will also deliver specific costings for the construction phase and the cost per kWh for generation that are not currently available. Nonetheless, Gyles estimates that while the construction costs are expected to be somewhat higher than those of a conventional gas-fired generator – currently around US$1.3M per installed MW – the operating costs are not affected by volatile gas prices and there are considerable advantages to the environment too.
The feasibility study is being produced in parallel with work on a scale model prototype, but its development will ultimately depend on attracting sufficient finance. Given the funds, it is possible that development of an 800m x 400m prototype system might be underway by the end of the year that will generate a respectable 12MW.
Hi-Spec is currently in talks with concrete companies regarding the necessary materials for the development, but is also very interested in the possibility of forging alliances with other tidal energy developers. Indeed, while Hi-Spec is keen to keep the development of this technology within the UK, the group would be prepared to forego this in favour of alliances with overseas groups if necessary.
Policy issues and the future
Despite Hi-Spec’s setback with its DTI grant application, the UK has made much of fostering the emerging tidal technologies industry in recent months. Last August, the secretary of state for trade and industry, Patricia Hewitt, announced finance of up to US$93.5M for the marine energy sector as the Marine Research Development Fund. This fund is designed to help bridge the gap between R&D and commercial deployment for wave and tidal generation where, surrounded on all sides by water, the UK has taken the worldwide lead. The DTI, together with the Scottish Executive, Carbon Trust and other key stakeholders, is currently developing the details of the funding mechanism, but the announcement has already prompted a significant response from industry.
The European Marine Energy Centre (EMEC) in Orkney off the Scottish coast, which is set to get a US$1.9M grant to help it expand its research facilities and establish a test centre, has a number of international tidal energy companies already indicating willingness to use these facilities when they are developed. EMEC is a purpose-built open sea test facility which has four network-connected test berths several km offshore among some of the most powerful waves in Europe. EMEC has now joined forces with three other UK organisations in order to pursue tidal energy technologies and establish a research, development, test, and certification base. The partners have formed the UK Centre for Marine Renewable Energy and include the University of Edinburgh, the New and Renewable Energy Centre (NREC), and the Robert Gordon University in Aberdeen. NREC, located at Blyth in Northumberland, is a unique facility in which larger models of tidal and wave devices can be fully tested before full-scale versions are deployed in the open sea. The marine generators are connected at full voltage to the electricity network to prove their overall performance. The connection to the UK research base is via the SuperGen Marine Energy Consortium at the University of Edinburgh. The Centre for Marine Renewable Energy therefore combines a long research pedigree in marine energy conversion at the universities with facilities for intermediate and full-scale testing and aims to provide a coherent approach to new projects as well as training and development.
Energy Minister Stephen Timms said of the alliance: ‘Encouragement is important but not sufficient to capture those real opportunities. This partnership offers a real forward facing group that can facilitate the development of a sustainable marine renewable energy industry.’
Alongside the Marine Research Development Fund, the UK government is funding other work on tidal energy. A recent example comes from Bristol-based Marine Current Turbines which has just won a US$7.2M DTI grant to develop its latest design turbines.
The US$15M SeaGen project is expected to generate 1MW of electricity using a twin rotor sub sea turbine. This follows successful trials of the 300kW Seaflow turbine that has been under testing since mid-2003 off the coast of Lynmouth, North Devon. The new device is expected to be connected to the grid in 2006.
However, despite the promise that is evident in tidal energy technologies, concerns have been raised that such projects could change underwater environments and adversely affect marine life. In addition, over-stating the power capacity available might lead to a damaging credibility crash. Nonetheless, Douglas-Westwood Ltd’s World Wave and Tidal Database, which contains project information on every identified wave and tidal energy project worldwide both in operation and planned, currently has information on 100 projects, representing 615MW of capacity and US$1.4B of investment. These figures are certain to rise given the current projections and the Hi-Spec OHEG could be expected to be among their number.
The author is David Appleyard, a freelance journalist specialising in the energy and process sectors Email: [email protected]