Current Status of Tidal Power Generation

This is the first part of the first portion of a multi-part series featuring papers on OE-related topics by students from the 2.65 class. The series is intended to raise interest and awareness of ocean energy research at MIT and in the world. This first section will be on Tidal Power generation and is written by James Modisette, a graduate student in course 16. His sources and the full-text with citations are available upon request.

James Modisette

Overview
Tidal power remains a mostly forgotten source of renewable energy. Historical record indicate that tidal power was first used in the middle ages to power tidal mills on the shores of present-day England, France, and Spain. By 1824 it was being used for pumping part of London’s water supply. With the Industrial Revolution and development of cheap power from fossil-fired power plants, tidal power was not able to compete and quickly disappeared. Although it still may not be able to compete with fossil fuels, tidal power represents an additional renewable energy source that can provide clean sustainable energy. La Rance, the exemplary tidal facility, has been reliable. It has been providing 240 megawatts for the past 40 years. After an era of cheap fossil fuels, there has been significant recent development of tidal stream generators around the world, and once again the future of tidal power is looking bright.

Introduction
Essentially all major technical obstacles for large-scale tidal barrage power generation projects have been resolved. A number of small and experimental tidal power plants have been constructed around the world to demonstrate the feasibility of the technology. These facilities mostly generate less than ten megawatts, but there is a facility producing 240 megawatts. The most remarkable aspects of tidal power as an energy source are that it is renewable, emits zero pollution during operation, has no fuel costs, and does not suffer from the unpredictability of wind and solar power production.

Why then has this resource been left unexploited? Because like all large power production techniques tidal power runs into NIMBY attitudes and requires large initial capital investments. The tidal industry, which once consisted entirely of low head potential turbines, has been forced to revamp itself and return in the form of tidal stream generators. These generators are small (on the order of a single megawatt),cheap, easily dispatchable, and have been deployed in small scale tests by those who hope they will soon be expanded into large-scale power producing installations at economically viable costs. The history of tidal stream generators is short, yet in an age of increased attention to climate change they present the possibility of contributing significantly to energy production in some regions. It is estimated that the potential for tidal stream generation in the U.K. is in the range of ten gigawatts, which would meet about a quarter of the U.K.’s electricity demand.

Design Basics
There are two general categories for tidal power generators. Those that rely on geographical
features and barrages to create low-head potential energy, and those that use the kinetic energy in fast moving tidal currents.

Single-Basin
The single-basin scheme relies on low head potential energy. It is the simplest solution to generating power from tidal flow and consists of a barrage which is built across the entry to an estuarie. Embedded within the barrages are both sluices and turbines.

In single-effect schemes, figure 1(a), the sluice gates are opened during a rising tide to
allow the basin to fill. At high tide the sluice gates are closed. Then there is a short waiting period while a potential head – a small one, between the basin level and the ocean develops. The turbines then use the head to generate power just as any low head hydro-plant would. This process is termed “ebb generation,” as power is only produced on the ebb flow of the tide.

An obvious possible improvement to the single-effect mode is to generate power on the
flood tide as well as the ebb tide using double-effect mode, figure 1(b). In this case, the
sluice gates are again closed at high tide, a head is developed, and the ebb power generation
occurs. As the ebb tide begins to weaken and the head shrinks again, the turbines are shut down and the sluice gates are once again opened. By low tide the basin level is equal to the ocean level and the sluice gates can be close. The double-effect turbines then generate power during the flood tide. The double-effect mode’s primary improvement is that it provides power for almost twice the time as single-effect mode. Unfortunately, there are some limiting side effects to double-effect mode. Toward the end of each ebb and flood cycle the sluice gates must be opened to allow for the necessary head to be built up for the next period of power generation after there is a change in flow direction. This leads to “waste” water flow during each cycle. The waste water combined with the generally lower heads for each flow direction and the inefficiencies due to two-way turbines, means that there is not a dramatic increase in power generation. In addition, double-effect generation is about ten percent more expensive per unit of electricity than single-effect generation. Although there is an added cost and complexity with double-effect power generation, it could very easily be worthwhile to help integrate tidal power into an existing grid by providing more of the base-load power the grid demands. Both single-effect and double-effect power generation systems can be advanced with the addition of pumping to augment the tidal flow. In single-effect mode, for instance, water could be pumped to increase the basin level above high-tide level when the basin and sea levels were nearly equal, that pumped water could be used later for power generation at a greater head level. Under very low heads, the turbines would operate in pumping mode to overfill the basin and then in subsequent power generation the head would be higher and more power would be taken out than was put into the system. This would be like a pumped storage system with an efficiency greater than one. Pumping could be used to play a role in creating consistent power generation by maintaining a more uniform head (preferably at the design head level for the turbines), particularly at facilities that have significant fluctuations in neap-to-spring tidal ranges. Pumping could also be used to delay power production to align it with peak grid power demand. Pumping adds a great deal of flexibility to a single-basin scheme.

Dual-Basin
Even with double-effect mode and pumping, single-basin schemes will always suffer from an inability to perfectly provide constant base power to the grid. A relatively simple solution, first proposed by Decoeur is the double-basin scheme which is shown in figure 2(a). For the double basin system, there are two basins separated by barrages from the ocean and each other. Each basin would be connected to the ocean by a set of sluices and one basin would serve as the high basin and the other as the low basin. In Decoeur’s design the turbines would be within the barrage separating the two basins and would generate power constantly and always in the direction from high to low. In order to maintain the highest head, the smaller of the two basins would be deemed
the high basin as shown in figure 2 (a). Shortly after high tide, when the high basin is at sea level, the sluices would be closed. The high basin would then be emptied through the turbines into the low basin, which would be closed to the ocean until it was at the same level as the ocean. The lower basin’s sluices would then be opened until low tide. In this process constant power is generated and it is possible to plan ahead for peak demand by delaying the use of the high basin level and developing a higher than normal head. Using this method, the double-basin scheme can be used for base load or parts of peak demand, making it a more adaptable tidal scheme.

Unfortunately, linked double-basin schemes typically have higher capital costs than single-basin schemes per kilowatt hour. They require more barrages to be built and more sluiceways to maintain the basin level. They also have more specific costal configuration needs which may not always be feasible. Finallly, their total energy production would be lower than that of a single-basin scheme using either of the basins. The benefit of constant power generation must be offset by the added costs and complexities of the double-basin scheme. In assessing the tidal power future in the USSR in 1990 Bernstein found that “employment of double basin schemes, and even improved ones in the recent design of the Severn TPP, has been rejected because the benefit-cost ratio proved to be less than one.” With the limitations of the above double-basin scheme, it may make more sense to pair the geographical features of the double basin as two single basins as shown in figure 2 (b). The paired single basins, or paired-basin system, would provide the flexibility to either provide maximum energy production with both basins in ebb flow production, or operate one facility in the ebb flow and the other in flood flow. Thus, the two paired basins can be operated to maintain a more uniform production based on needs. In the most basic implementation, uniform production would never be perfect as there would always be periods at low and high tides when the head would not be sufficient for power generation, but with combined pumping the paired-basin scheme could be used to provide a constant base power load.

There is one particularly interesting geographical site for a double-basin scheme on the southern coast of Argentina. The Valdez Peninsula’s configuration has two gulfs on either side of it, the San Jose Gulf and the Nuevo Gulf. The unique feature of this peninsula is that although there is only a six kilometer isthmus separating the gulfs there is almost a half-period of tidal difference between them. This geographical feature could be used as either a paired-basin scheme, where both basins would operate at the more efficient ebb flow, or as a linked-basin scheme with a channel dredged through the isthmus.

Be sure to keep following this series, the next portion of this paper will feature kinetic energy turbines.