Tidal Power

Harnessing Energy from Water Currents

Victor M. Lyatkher

• Publisher: Wiley-Scrivener
• Year: 2014
• Print ISBN: 9781118720912
• eBook ISBN: 9781118721032
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Energia

Offers a unique and highly technical approach to tidal power and how it can be harnessed efficiently and cost-effectively, with less impact on the environment than traditional power plants

With the demand for energy outstripping conventional sources from fossil fuels, new sources of energy must be found. Tidal power is a potentially rich source of renewable energy. Even though power plants that run on hydropower have been around a long time, new types, such as those run without dams, have not been as heavily researched and presented to the scientific community. This book is a step in that direction—suggesting more cost-effective and less environmentally intrusive methods for creating power sources from rivers, the tides, and other sources of water.

Presenting a detailed discussion of the costs, risks, and challenges of building power plants that run on hydropower, Tidal Power: Harnessing Energy From Water Currents:

• Covers the technical aspects of tidal power plants without dams, or "Marine Hydro Kinetic (MHK)" power plants
• Covers tidal barrages with reservoirs and how they can be utilized to generate renewable energy
• Details the regulations of hydro power plants that do not use dams
• Gives real examples of turbines that have been built using these new technologies

As the global supply of conventional energy sources, such as fossil fuels, dwindles and becomes more and more expensive, unconventional and renewable sources of energy, such as power generation from water sources, becomes more important—making Tidal Power an incredibly valuable text for engineers, students, and researchers working in the field.

Chapter 1

Marine Hydro Kinetic- MHK

Traditional tidal power plants (TPP-tidal barrages) result in a complete separation of a sea from a gulf in order to create one or several power producing reservoirs [1]. Such systems have a number of fundamental flaws, which hamper the development of this energy industry.

• All projects require the building of dams and other hydro-technical construction, which cut off the TPP basin from the sea. The existence of this kind of front noticeably changes the ecological situation in the basin.
• The existence of a head front (dams and head construction) defines an often unfavorable system of finance where it is necessary to completely finish the building (and completely pay for all required work) and only after will the station begin to generate energy and return capital investments.
• The heads at TPP are not large, the traditional hydropower equipment is expensive, and production is relatively small. The cost of TPP building and head front constructions designed for gale oceanic wave and heavy ice load is high. This establishes high capital investments per unit of the installed capacity and a relatively high primary cost of energy.
• The power of TPP is changed in accordance with the tidal water regime with may be at zero many times per day.

A new MHK design approach has no such shortcomings and is based on the use of free tidal currents at maximum water speed areas. The tidal power plants proposed, consist of a floating or fixed hydraulic unit, on the surface (fixed to the bottom surface) or underwater (in areas with heavy ice), that converts the energy of tidal currents into electrical energy or into gaseous or liquid hydrogen in the event that direct delivery of electricity is not possible or is not economical. For the use of tidal energy, it is not necessary to completely block the mouth of the basin in order to create the structural works. The maximum possible power and maximum possible energy output is obtained under a specific hydraulic resistance introduced by turbines, and due to the economic feasibility, is also obtained by structures that funnel the water flow at the entrance to the MHK reservoir (at the narrowest part) [2].

The energy potential of sea tidal flows is usually estimated by the maximum potential energy of a body of water that rises in the gulf during high tide over the minimum low tide water level. The actual fact is that in order to obtain high tide energy, it is necessary to create an obstacle in the tidal flow path. The obstacle may be created by building power plants and dams that obstruct the strait between the reservoir and the sea, as is the case at the existing tidal barrages in France, Canada, Russia and China.

The obstacle may also be in the form of hydropower units of one sort or another, positioned in a strait without a dam or in the presence of embankments, which narrow the strait to the optimal size. Such design, proposed by the author in 1985, is implemented in the Myongyang Channel in Korea, and uses helicoidal orthogonal turbines. Other experts in different countries independently proposed this same solution for additional projects around this time.

In any case, a question arises about the maximum power and output, which can be obtained under given conditions. Obviously, for very large applied resistance (a tidal barrage), water expenditure will be minimal and, despite the maximum pressure, the power will be close to zero. With the absence of resistance, a pressure drop will be zero and, despite the maximum flow rate, power will be also zero. It was shown that there is an optimal resistance that corresponds to the maximum power and possibly another resistance that corresponds to the maximum energy output [3].

Let z₀(t) and z(t) denote the water levels in the sea and the MHK reservoir, respectively, Ω – reservoir water surface area, Ω_p – cross-sectional area of MHK water channel, A - height of tide with period T, and ξ - coefficient of resistance of the MHK water tract. Consider first the case when the reservoir connects with the sea only through one strait.

In this case, the flow through the strait (Q(t)) is determined by the movement of the water level in the pool:

(1.1)

The pressure loss in the strait is proportional to the current flow through the power tract:

(1.2)

The energy balance equation, in the zero-dimensional approximation for the periodic tide, takes the following form by the calculations below:

(1.3)

(1.4)

where

(1.5)

The water level z(t), in the reservoir and the sea, is measured in fractions of the initial amplitude equal to half the tide height (A). Time is measured in fractions of the period T. Thus, all the variables in (1.4) are dimensionless. Height of tide in the sea is the same.

Equation (1.4) can be written in explicit form with respect to the time derivative of the water level in the pool:

(1.6)

(1.7)

(1.8)

Equation (1.6) was solved numerically under conditions as α → 0 and t = 0.

(1.9)

The calculations were performed before the solution z(t) assumed periodic regime. Usually this has taken place, beginning no later than the fifth oscillation. Steady-state oscillations in the reservoir can be approximated as:

(1.10)

Traditional design tidal barrage power at a fixed water cross-sectional channel area is:

(1.11)

Here, η is the total efficiency of traditional design generators is usually:

(1.12)

Expression (1.11) reduces to:

(1.13)

(1.14)

(1.15)

(1.16)

Maximum power, within a tidal cycle, occurs when cos(4πt − β) = −1 and is expressed below:

(1.17)

where

(1.18)

MHK parameter calculations results (1.13, 1.17) are in table 1.1:

Table 1.1

Maximum MHK power, with fixed reservoir area, height, and tidal period, corresponding to the maximum coefficient C, takes place at α = 0.03 ÷ 0.04 and is approximately 7.27 (fig. 1.1).

Figure 1.1 A relative amplitude (z_m) of fluctuating water levels in the MHK reservoir and relative maximum power of the MHK (C), depending on a.

A graph of MHK reservoir water level fluctuations is shown in fig. 1.2:

Figure 1.2 The reservoir water level z(t), for MHK at maximum power.

Relative height of inflow in the pool and phase angle are shown on fig.1.3:

Figure 1.3 Amplitude-phase characteristics of tides in the free-flow MHK reservoir.

Maxim...

Table of contents

• Cover
• Half Title page
• Title page
• Copyright page
• Preface
• Chapter 1: Marine Hydro Kinetic- MHK
• Chapter 2: Rivers (Channels) Power Plants without a Dam
• Chapter 3: Low-Speed Hydro-Kinetic Turbines
• Chapter 4: Large Power Hydro Turbines
• Chapter 5: Examples of Turbines Produced
• Chapter 6: Water Current Power-Looking to the Future
• Subject Index