Alan Owen

Tidal current resource assessment

Abstract This paper outlines present thinking on the determination of accessible tidal current resources within channels and other potentially exploitable locations. The fundamental principles behind tides and tidal currents are briefly discussed and the implications of temporal and spatial variations on the evaluation of the resources considered in the context of artificial energy exploitation. The thinking behind the flux approach to resource estimation is presented and an example based on the Pentland Firth is considered. The impact of energy extraction on the flow patterns is considered in both one and two dimensions and the principles required for three-dimensional analyses are presented in a generic form.

Tidal Current Energy: Origins and Challenges

Publisher Summary This chapter introduces the forces that drive the tidal currents and the external variables that modify their behavior, and outlines the challenges faced by engineers in exploiting this worldwide resource. It outlines the underpinning astronomical and meteorological mechanisms that drive tidal currents, and the topographical features that modify and intensify the resulting flows. Tidal currents are not well-behaved, bidirectional laminar flows; they are predictable, but flow behavior at certain points within the cycle may make power extraction very challenging or even impossible. For the commercial generation of power, tidal current devices need to be simple and cost-effective to install, requiring minimal maintenance and able to resist the build-up of biofouling for long periods of time. The exploitation of tidal current power is too attractive to ignore, and the UK has an abundance of resource in this field, but a much more coherent effort is required to make it realistically possible. The UKs position as the global leader in research and development in this field still holds, but only just; historically incoherent and diffuse funding has tended to create an unfocused development effort that has produced a large number of turbine proposals, very few of which have actually been tested at sea. The chapter highlights the range of factors that affect tidal current behavior, the existence of unpredictability within a highly predictable system, and the basic difficulties of cost-effective anchorage and fixing.

Tidal Current Energy

Publisher Summary This chapter introduces the forces that drive the tidal currents and the external variables that modify their behavior, and outlines the challenges faced by engineers in exploiting this worldwide resource. It outlines the underpinning astronomical and meteorological mechanisms that drive tidal currents, and the topographical features that modify and intensify the resulting flows. Tidal currents are not well-behaved, bidirectional laminar flows; they are predictable, but flow behavior at certain points within the cycle may make power extraction very challenging or even impossible. For the commercial generation of power, tidal current devices need to be simple and cost-effective to install, requiring minimal maintenance and able to resist the build-up of biofouling for long periods of time. The exploitation of tidal current power is too attractive to ignore, and the UK has an abundance of resource in this field, but a much more coherent effort is required to make it realistically possible. The UKs position as the global leader in research and development in this field still holds, but only just; historically incoherent and diffuse funding has tended to create an unfocused development effort that has produced a large number of turbine proposals, very few of which have actually been tested at sea. The chapter highlights the range of factors that affect tidal current behavior, the existence of unpredictability within a highly predictable system, and the basic difficulties of cost-effective anchorage and fixing.

Energy extraction implications of structurally significant velocity variation in tidal currents

The magnitude of the UK tidal current resource has been examined by a number of different reports, each attempting to define a figure which is representative of the commercially exploitable energy in the flow. Recent work suggests that the exploitable resource is of the order of one fifth the size of the raw flux passing through a channel. It is not possible to develop an indicative value of the exploitable energy simply from knowledge of the harmonic constants or local tide tables, and over-optimistic resource forecasting has resulted from attempts to do so. Tidal currents are bounded, finite systems with no capacity for energy replenishment from other sources, whereas the atmospheric energy source is, from a wind turbine standpoint, almost infinite, being easily replenished from the upper atmosphere or neighbouring weather systems. The principle sources of tidal flow data to date have been navigational charts and Admiralty tidal stream atlases, but these values are based on surface or near surface measurements of the flow velocities, which can be used to develop a baseline model of the energy, but do not necessarily represent the flow conditions as seen by a turbine fixed in position close to the sea bed. The bathymetry and topography of the geological structures that enclose the tidal current site will heavily influence the nature of the flow, both in terms of velocity and in the generation of metre length scale vortices. Additionally, since tidal currents are driven largely by head difference across a locally narrow channel, meteorological events such as barometric pressure, wind strength and direction, and the situation of a site relative to a large fetch will affect the tidal current flow velocities.

Community Sustainability Plans to enable change towards sustainable practice - a Scottish case study

There is a need for a global shift towards a low-carbon society and this requires action at the local level. The aim of the study into sustainable community development is to clarify the role of a community as a whole rather than the role of individuals within that community. Mixed methods of literature review and empirical study have been used to translate the knowledge and experience with urban sustainable development into empirical knowledge of a rural, market town setting. The exemplary case study is the Scottish market town Huntly. The main findings of the study show the need for three ingredients for a Community Sustainability Plan for a town such as Huntly. Community involvement is crucial in developing such plans in a rural setting. Further research in more remote settings, e.g. coastal and island communities, is needed to allow broader conclusions on rural sustainable development.

Turbulence: Characteristics and its implications in tidal current energy device testing

The need for a tidal energy conversion system is enumerated in addition to the possible challenges in its installation and operations, stemming from the interactions of the energy device with its surrounding. The highly idealised case of isotropic, homogenous and stationary turbulence is discussed and its deviation from real flows, as observed in a tidal estuary is presented. Also, the principle of dimensional analysis and the concept of similarity are introduced, as tools to access the effect of scaling in a tidal current energy device testing. A better understanding of the structure of turbulence in any tidal site allows for the right choice of dimensionless numbers that are relevant to the system. Turbulence characteristic differ from site to site and, introducing an energy device into the environment modifies the velocity and pressure distribution of the site. The dimensionless numbers essential for the tidal current energy conversion process are clearly defined.

Chapter 16 – Tidal Current Energy: Origins and Challenges

This chapter introduces the forces that drive the tidal currents, the external variables that modify their behaviour and outlines the challenges faced by engineers in exploiting this worldwide resource. With the transfer of corporate ownership of the leading UK devices in recent years it can no longer be argued that the position of the United Kingdom as the global leader in research and development in this field still holds. The race for the largest device instead of simple, reliable, robustness has failed the economic and business development test; tidal devices are too complex and, as yet, show no clear investment benefit. In addition, understanding of the potentially damaging coherent turbulence that exists within the flow is still very poor, and very few robust and cost-effective installation methodologies have been developed. Both of these issues are fundamental to the business case and should have been developed alongside the turbines, but have been treated as a design afterthought. This chapter discusses the range of factors that affect tidal current behaviour, the existence of unpredictability within a highly predictable system and the basic difficulties of cost-effective anchorage and fixing.

The Transition to Future Energy

This chapter examines the societal changes required to enable the transition to future energy systems. The emphasis of this change is usually placed on technology; preferably the invention of a silver bullet that will allow us to consume at our present rate without damaging our environment. Unfortunately the truth is, at present, that we have a finite biosphere within which to operate and that the solution requires billions of individual human inputs to overcome the problems created by billions of individual human inputs over the past centuries. Given that our present global consumption requires two Earths to service our needs, and that some overdeveloped countries are closer to four whilst developing countries consume around half, then the overdeveloped nations must consume fewer resources. This will take a huge leap in political leadership.

Chapter 29 – Energy Resources in Developing Nations

Because of the multidimensional challenges of energy provision in developing nations, energy resources assessment in these regions need to be approached not only through sustainability but also with equity and dignity in mind. A brief introduction on the concept and context of sustainable energy resources for the future is given in the first section. This chapter then specifically addresses the issues surrounding sustainable energy resources in developing nations through a number of important resource streams, for example: • natural resource (e.g. hydro, solar, marine) – what is the energy availability? • human resource – what is the indigenous skills base and its potential capacity? • technological resource – what technology is appropriate for the need? • capital-infrastructure – what is the business capability and what infrastructure exists? The last section then discusses the implications of exploiting sustainable energy resources in these nations.

Modeling tidal turbines

Performance prediction models are valuable tools for use in the development of tidal turbines. A model that can accurately predict the performance of a device can be used to optimize the design parameters more rapidly and at a much lower cost than carrying out the design studies using scale model tests. Tidal turbine models are principally dependent on the aerodynamic models developed for wind turbines, but the use of aerodynamic models fails to take into account the differences between wind and tidal turbines, one of which is that tidal turbines operate in a fluid with a free surface. The effect of this difference in boundary conditions on the flow is considered by analysis of the change in far field pressure distribution caused by the free surface. This analysis shows that the change in the far field pressure distribution will affect the turbine power coefficient.