Deltares Renewable Energy From Water and Subsurface 2010

Renewable Energy from
Water & Subsurface
Discovering the Potential & Considerations for Application

Renewable Energy from Water & Subsurface
PO Box 177
2600 MH Delft
The Netherlands
T+31 (0)88 335 8273
info@deltares.nl
www.deltares.nl

Renewable Energy from
Water & Subsurface
Discovering the Potential & Considerations for Application

Preface

Energy is vital for man and society. However, fossil fuels are becoming
scarcer and more expensive to extract and remain a burden on climate.
Therefore, the policy of the Dutch government – and of many other coun-
tries – is to seek a reduction in energy use and in emissions into the
environment. The Dutch government has set a target to reduce the CO2
emissions by 30% by the year 2020 compared to 1990 and to ensure that
20% of the energy produced is from renewable sources by 2020.

To achieve that goal various developments and innovations are being implemented
to improve energy efficiency, to develop renewable energy technologies and to
reduce CO2 emissions. Many solutions appear to be both energy and water issues.

One example of an underexploited renewable source is water. Of course, hydro-
power is generated at large dams, such as the Hoover Dam in the U.S.A. However,
energy can be produced from water in many other ways, such as: low head hydro
power in rivers, tidal and wave energy, thermal energy from surface and ground-
water and salinity gradient power in coastal areas. Some of these technologies are
ready to be implemented whilst some are still under development but will become
technically feasible within the next few years. Besides the technical challenges, it is
of the utmost importance to create solutions that are friendly to the environment,
acceptable to society and economically feasible.

2 Renewable Energy from Water & Subsurface

This overview document is an attempt to assess the different technologies that
produce energy from water, from an international perspective and – where possible
– with a focus on The Netherlands to create a more detailed understanding. It
comprises a first order approximation of the energy potential, in a manner similar
to the method the oil and gas industry uses for estimating its reserves.

I hope this booklet will create an insight into the different possibilities and consid-
erations for the application of water-based, renewable energy sources, and will
contribute to a faster exploitation. This booklet only covers the most promising
methods and innovations for renewable energy from water. This means that very
early innovations, and small-scale plans or initiatives were not considered, although
it is recognized that the implementation of those may have major local influence.

Enjoy reading this introduction to water as a source of renewable energy and the
related considerations and I hope that governments make their policy and regula-
tions more suitable for exploitation of this resource and that businesses will be
inspired to keep on developing water-based energy solutions.

Marcel Bruggers
Deltares Renewable Energy Team
Deltares, June 2010

Renewable Energy from Water & Subsurface 3

Table of Contents

Preface 2

1 Deltares 7
Our Expertise 8
Coast and Sea 9
Policy and Planning 10
Rivers, Lakes and Groundwater 10
Soil and Subsurface 10
Software 11
Research Facilities 11

2 Energy-related Research at Deltares 13
Water 14
Subsurface 15
Focus on the North Sea 16

3 Fired up by Water 17
Sources 18
Definitions of Energy Resources 21
Estimating the Potential 22


4 Discovering the potential 25
Tidal Energy 26
Wave Energy 32
River Energy 37
Blue Energy 45
Thermal Energy from Urban Surface Water 46
Thermal Energy Storage 49

5 Considerations for Application 53
Renewable energy and the environment 54
Environmental Aspects 57
Life Cycle Analysis 63
Environmental Flows – the Tool to Mitigate Hydropower Impacts 67
Bottlenecks When Innovating 71
Pilot Installations – Crucial Proof of Practice 75
Lessons Learned from a Pilot Tidal Energy Plant 78

6 Acknowledgements 83


Deltares

Deltares is a leading, independent, Dutch-based research institute
and specialist consultancy in matters relating to water, soil and the
subsurface. We apply our advanced expertise worldwide to help people
live safely and sustainably in delta areas, coastal zones and river
basins. To achieve this, we constantly extend our knowledge base via
government research programmes and contract research, forming
consortia with universities and other research institutes, encouraging
innovation, and accelerating the practical implementation of new theo-
retical advances.

At the same time, we continuously develop our own innovative products and
services, integrate them with the advances achieved by other bodies and make
the results publicly available around the world. We advise both the public and
private sector, often as early as the initial phase of a project, using our state-of-
the-art expertise to make sound independent assessments of the physical condi-
tion of delta areas, coastal zones and river basins.

Our Expertise
All over the world, habitable space in deltas and river basins is under increasing
pressure from economic expansion, growing populations, subsidence and the
impacts of climate change. Deltares has the knowledge and resources to tackle


water and subsurface issues worldwide in a new, integrated way we call ‘delta
technology’. This means we never focus exclusively on technological issues.
Our approach invariably takes account of ecological factors and administra-
tive constraints like spatial planning, with all the associated policy agendas,
competing interests, and legal and economic processes. The integrated applica-
tion of our various areas of sophisticated know-how produces solutions that are
more sustainable, better for local people and, often, more economical. We aim
towards the sustainable enhancement of the living environment, with high-grade
technological solutions that have the support of society as a whole, putting into
practice our strategic principle: ‘Enabling Delta Life’.

Coast and Sea
Today’s coastlines are under threat from climate change, rising sea levels and
coastal erosion. To secure them and avert the threat of coastal flooding, it is
vital to understand how coasts and seas function as systems. Deltares has
this understanding of natural processes and applies it to the engineering and
management of coasts. We work hand in hand with nature, pursuing a philos-
ophy of sustainable coastal engineering that involves encouraging the develop-
ment of natural features and using natural dynamics to maintain coastlines
and improve flood protection. Climate change causes more extreme weather
conditions. Deltares studies how this affects the environment, water defences,


coastal engineering projects, energy supplies and transport. Based on inte-
grated coastal management, Deltares supports policies and management for
the coastal zone, targeting the impact of climate change, but also examining
the effects of interventions on water and soil quality. We have integrated what
we know about ecosystems into models and monitoring systems that can be
used to implement European initiatives such as the Water and Marine Strategy
Framework Directives. We help government authorities to tackle pollution and
disaster management more effectively. We develop early warning systems
for the timely identification of threats. We search for solutions that draw on
the potential of the coastal system, that enhance safety in densely-populated
coastal zones and that minimise the ecological impact. Deltares acts as a
specialist consultant during the realisation of projects for coastal engineering,
coastal safety, recreation, energy supply and transport.

Policy and Planning
Around the world, spatial planning now takes increasing account of water and the
subsurface. Deltares supplies the requisite specialist know-how to enable public
authorities to prepare their area development, innovation management and flood
safety policies and plans with these factors in mind. We analyse existing policy
and conduct strategic reviews, scenario studies and integrated studies for the
development of new plans and the elaboration of innovations.

Deltares looks far ahead to recognize the challenges we will face as a society and
to identify the expertise needed to respond to them. Our knowledge and experi-
ence are invaluable in the initial phase of studies and projects, when problems
are being defined and potential solutions examined. Together with our clients
and other research institutes, we work today to confront the major challenges of
tomorrow: the design and management of sustainable and climate-robust deltas,
coastal areas and river basins.

Rivers, Lakes and Groundwater
Deltares’ consultancy work and simulation models are rooted in a clear under-
standing of how water systems work. Our models help public authorities make
vital predictions concerning matters like river levels or the flow patterns and
quality of groundwater and surface water. Since the quality and quantity of
groundwater and surface water are inextricably linked, we produce linked models
in this area and use integrated water management techniques to support policy-
making and management in the area of freshwater reserves. We also apply our
understanding of the interaction between groundwater and surface water to areas
that may seem at first glance completely unrelated, such as energy from seasonal
thermal storage in the subsurface. Local populations need to be protected from
river flooding but river water is also essential to their economic and social well-


Deltares

being. Deltares advises on flood safety measures, water transport, and the use of
groundwater and surface water for drinking, irrigation and cooling, as well as for
nature conservation. Our consultancy services are founded on an advanced knowl-
edge of hydrology, geology, morphology, riverine hydraulic engineering, ecology
and economics. We work hand in hand with public authorities and water boards to
ensure that river valleys are safe, pleasant places in which to live and work.

Soil and Subsurface
The ground beneath our feet is valuable in many different ways. It contains
commodities like sand, gravel and clay. It serves as a firm foundation for infra-
structure and provides extra space for additional functions. And it contains
groundwater, which interacts with the surface water in lakes, rivers, ditches and
streams. Deltares brings together expertise in all these areas to arrive at inno-
vative solutions. For example, we apply our knowledge of geological structures
to expertise in dredging and sand production, or we apply our understanding of
urban groundwater on the one hand and of soil on the other to the creation of
infrastructure. After all, an expert knowledge of geotechnology and foundations is
essential to reduce the risks inherent in construction on and in soft subsurfaces
(like those in The Netherlands).
Furthermore, we also map soil quality risks and advise on remediation in many
places around the world where past industrial activity has resulted in pollution of
the subsurface.

Software
Deltares software gives users rapid access to the latest advances in the area of
water and the subsurface. Out in the field, it generates new research issues and
produces new insights. Together with users and knowledge partners, we engage
in a constant cycle of application and development that results in ever-wider use
of our knowledge through the medium of our software. The integration of data,
software and expert knowledge enhances the range of applications available to
users. For example, Deltares supports decision-making during flood alerts by
producing software that helps authorities predict high water levels, patterns of
flooding following dike failures, and the consequences of measures like evacu-
ations. Likewise, we produce linked models for groundwater and surface water
and, in a major new move, we are working with public authorities and research
institutes to develop a set of National Models for The Netherlands. Our aim is to
provide open architecture software fully compatible with third-party programs.
Under the name Deltares Systems, our software is currently used in more than
60 countries worldwide. It covers our whole sphere of expertise including coastal
waters and estuaries (Delft3D), rivers and urban water management (SOBEK), the
design of diaphragm wall structures (MSheet) and the stability of flood defences
(MStab), as well as an operational forecasting system (FEWS).


Research Facilities
Deltares has its own in-house physical laboratory facilities (including an environ-
mental laboratory, a Delta flume and a GeoCentrifuge). These are used not only
to conduct water and subsurface research for the validation of new models and
software, but also to test designs and scale models for hydraulic and geo-engi-
neering structures or for the biochemical strengthening of the subsurface. They
are also made available to external researchers from around Europe. The wide
range of in-house facilities allows us to study all the facets of ground and water
behaviour. We conduct research not only on the water quality and morphology of
rivers, lakes and coasts, but also on ground and subsurface strength, the effects
of wave loads and currents on structures, and the stability of these structures.
Experiments are often designed to examine multiple physical processes simulta-
neously (for example, both the wave load on a dike and the strength of the dike
in terms of soil mechanics). The extent of our facilities allows us to progress in a
carefully considered way, via a combination of small and large-scale experiments,
towards the practical implementation of our knowledge – building flood defences,
constructing foundations or using bacteria to modify the properties of soils.


2 Energy-related Research
at Deltares

Energy-related
Research at Deltares

Renewable energy will undoubtedly play a major role in the next 20 to
40 years, which creates a great opportunity for science and industry
to get involved in the fast-growing market of research and consulting.
Responding to energy and climate change is a vital social task and one in
which Deltares must be involved.

Alternative renewable energy sources are increasingly being explored and the
development of solar and wind energy is at the forefront of this advance. The
market and the capacity for solar heat has grown exponentially in Europe in the
last 20 years and, in terms of capacity, recently installed wind energy repre-
sents over 30% of all installed electricity capacity in the EU over the last five
years.

In the fields in which Deltares is specialised, there is still a great deal to do. In
some cases, the technologies for generating energy from water and the subsur-
face need further development or still have a low efficiency or are very expen-
sive. These technologies are nevertheless promising, and with further develop-
ment and elaboration, they have – especially in delta areas – great potential for
energy production. Deltares can play a significant role in developing knowledge,
improving technologies, and in estimating the impact of these developments on
the environment.


Water
Hydroelectric power is generally accepted as an effective form of renewable
energy. In Scandinavian countries a large proportion of energy originates from
hydroelectric power (around 98.8 %!) Techniques to extract energy from saline
gradients (i.e. Blue Energy), tidal energy (e.g. the C-Energy project in Borsele),
wave energy (e.g. the Aguçadoura Wave Park, Portugal), and to extract heat
from water (e.g. the Maas Tower in Rotterdam) are less developed, and not yet
frequently used. These technologies offer good opportunities for continuous
research and application.

The technology of extracting heat from water using heat pumps has only
recently begun to advance, and has been found to be very successful in Scheve-
ningen (The Netherlands) where a whole new urban area is heated by seawater.
The large amount of surface water near buildings in The Netherlands facilitates
further development in thermal energy storage and there is therefore a significant
increase in the market for thermal energy pumps.

Subsurface
For some time geothermal energy from land (and geysers) has been produced
in the form of heat and electricity, but the capacity growth in recent years
has been very small. Especially in Southern Europe, where the soil has a high


enthalpy, potential exists for further developments in extracting geothermal
energy. A development which has had strong growth over the past years is
ATES, Aquifer Thermal Energy Storage. ATES systems temporarily store energy
in the form of hot or cold water in an aquifer, for respectively heating or cooling
a building. In The Netherlands ATES is relatively widely used compared to other
European countries. With the knowledge on this subject in The Netherlands and
its widespread application, the possibilities for further development remain
important and necessary.

In The Netherlands 30% of the energy consumption is spent on the heating and
cooling of buildings. With the usage of ATES systems it is possible to reduce
locally the energy demand by 50 to 70%, which can lead to a total overall saving
of 15 to 20% for The Netherlands. To promote this form of renewable energy the
Dutch Ministry of Housing has created an ATES taskforce.

Focus on the North Sea
The government is working on the Spatial Perspective North Sea to provide clarity
on the space for development of the various uses of the North Sea, including
features such as wind energy, oil and gas extraction and storage of gas and CO2.
A major expansion of offshore wind farms and stimulating the production of oil
and gas from small fields are the first steps towards using the North Sea as a
renewable energy source. Other options such as tidal and wave energy, saline
gradient energy and algae for bio-fuels are being considered. The government
will, together with (commercial) market parties and knowledge institutes, develop
and explore different options and provide a clear perspective. This also includes
a multifunctional energy island with large-scale electricity storage in the North
Sea.

The above shows that huge potential exists in delta areas for renewable energy.
What is the role of Deltares in this area? This booklet identifies some of the areas
in which Deltares’ expertise can be deployed.


Fired up by Water
The resource of energy

The first part of this booklet will provide insight into water as a primary
source or carrier of energy. It concerns water directly extracted from
or stored in the environment. It does not cover water used in industrial
processes or secondary sources such as cooling water from industry. The
solutions presented can significantly contribute to the renewable energy
sector in the world.

Sources
All the earth’s energy is derived from three natural, primary energy sources.
These are the sun, where solar fusion delivers solar energy; the moon, where the
gravitational force that it causes delivers lunar energy; and the earth, where heat
has been stored in the nucleus since the creation of the planet. Surface water and
subsurface water both directly and indirectly catch a lot of energy and therefore
contain enormous amounts of energy.

The Sun
The largest natural primary source is – of course – the sun. The energy reaches
the earth’s surface by solar radiation, caused by nuclear fusion in the sun. A
part of this energy is converted by plants through photosynthesis into (aquatic)


biomass. The energy content in biomass depends on the amount of carbon or
oil it contains. These substances can be converted in different processes into
thermal, mechanical or electrical energy that we can use. Biomass is not within
the scope of this document.

Solar radiation also heats the surface of the earth. Due to the rotation of the
earth, the surface is not homogeneously heated. Differences in the absorp-
tion capacities lead to an unequal temperature rise of the earth’s surface.
The surface, be it water or land, heats the air above it. Differences in air temper-
ature cause differences in air densities that manifest itself as high and low
pressure areas. This creates the phenomenon of wind and, with the friction of
the wind over water, waves are created. The kinetic energy in waves can be
extracted through a variety of different methods and converted into electrical
energy.

The heating of the surface also includes lakes, seas, and oceans where water evap-
orates. The clouds that contain this water cause precipitation in the higher parts
of the earth’s surface, where they constitute the source of rivers. With free flow
turbines in rivers, or with turbines in hydraulic structures in the rivers, electricity
can be generated. Water also gets desalinated through the process of evapora-
tion. This offers the possibility of generating electricity via ‘Blue Energy’ tech-


niques, such as Reversed Electro Dialysis (RED) and Pressure Retarded Osmosis
(PRO), in or near the mouths of rivers.

The differences between land and water concerning heat capacity and absorption
and emission speed lead to temperature differences between these substances.
Generally, in summer this leads to a water temperature which is lower than the
temperature on land and which in winter is warmer than on land. The temperature
difference represents a thermal energy (difference), which can be extracted using
heat exchangers.

The primary sources of energy
and their conversion to ‘usable’
energy.


Fired up by Water

The radiation of heat on the earth’s surface also leads to vertical differences in
water temperature, thermal stratification. The application of systems, such as
OTEC (Ocean Thermal Energy Conversion), can convert this difference in tempera-
ture – actually, thermal energy – into mechanical energy. Large differences in
temperature, from about 20°C, are sufficient to yield electricity. This situation
hardly ever occurs in The Netherlands, which is why this sort of energy is not
included in the remainder of this report.

The Moon
The second primary source is the moon. Although not an active source of energy,
such as the sun, the presence of the moon creates a gravitational force. This
gravitational force, the rotation of the moon around the earth and the rotation
of the earth on its axis, gives an uneven pull of the moon on the elements of the
earth. Because the ocean covers such a large part of the earth’s surface, gravita-
tional forces within this body continually differ in magnitude and direction. This
creates tidal movement, i.e. tidal currents and fluctuating water levels, which is
best noticeable in the coastal areas. This tidal energy can be converted into elec-
trical energy through the use of (non) free flow turbines.

The Earth’s Core
The earth, the third natural resource, also produces and contains energy. The
earth’s core contains heat that originates from processes during the formation of
the earth and from radioactive decay. By radiation, conduction and flow, a portion
of the heat is transported to and into the crust. In the earth’s crust, at different loca-
tions and depths, aquifers are present that contain hot water. Through deep drilling
in the crust, in the order of several kilometres, this thermal energy is extractable. At
these depths, the thermal energy is so huge that it is feasible to generate electricity
using steam turbines. At places where aquifers are not present, closed systems
with circulating fluid may provide a solution to extract the thermal energy.

Conversion of energy
The presence of energy manifests itself in various forms, such as kinetic energy
(water current/movement), potential energy (water level differences), electro-
chemical energy (saline gradient in the water) and thermal energy (heat or cold)
and they inherently require different methods through which this energy can be
extracted from water. The concentration of energy per unit of water volume, the
method(s) capable of obtaining this energy and the degree of presence in the
particular waters or aquifers being considered roughly determine the potential.


Definitions of Energy Resources
There are several ways to define and to quantify the energy supply. The natural
base supply, or more clearly, the total amount of energy present in the natural
system that – in theory – can be extracted, is the potential energy resource. Despite
all current and future innovations and developments it is not possible to extract
this whole potential. Practical limitations are set by the geometry of the plant
or system, the necessary space for performing maintenance and maintaining
safety and – of course – conversion and frictional losses. The technically extract-
able energy resource is therefore lower than the potential energy resource. The
exploitable supply, however, is even smaller, due to social and economical aspects.
The part of the technical extractable resource that is acceptable from an envi-
ronmental, social and societal point of view, is defined as the socially extractable
energy resource. The other limiting factor is the economic feasibility. Locations
are only exploitable if the investment and operational costs are lower than the
revenues from energy sales. We then speak of the economically extractable energy
resource. The figure below illustrates the relationship between these supplies.

The estimations made for these resources in this report are based on a first
order approach. They do not represent proven resources or reserves. Thorough
quantitative studies with a location-specific approach are necessary to provide
more certainty with the estimates. Since most of the technologies require further
development or are very dependent on location-specific conditions, it is not (yet)
possible to calculate the economically extractable energy resource.

Apart from producing energy, water can also be used as an energy storage
medium. This is especially interesting if economic supply and demand do not
match. This report, however, does not cover this aspect; it is limited to the identi-
fication of opportunities for energy generation from water.

Estimating the Potential
Making quantitative estimates of the extractable supply is a difficult task. The
total extractable potential is huge. However, it is undisputed that it is not feasible
to extract the whole potential energy resource. Especially with innovative solu-
tions, with technologies that are not common or are location-specific, assump-
tions have to be made and criteria have to be set. These are based on ‘engineering
judgment’, i.e. experience and existing knowledge. It is recognised that these
criteria and assumptions determine the outcome of the estimation and with that
the insight into the chance of (commercial) success of the technology.


Fired up by Water

Defenitions of energy resources.


Discovering the potential

Water contains huge amounts of energy in many different forms, sometimes
highly concentrated, but mostly diffused. It can be present in the form of kinetic
energy, thermal energy or chemical energy and the water, containing the energy,
flows through several distinctive cycles. In this chapter six different water/energy
sources are considered, that comprise most of the potential energy present in the
earth’s water.

Tidal Energy
In general with tidal energy, large-scale constructions are required, to
extract the energy from the water. Large constructions, however, can also
be expected to have severe environmental consequences. When exploring
the steps and considerations required to extract energy from this source,
it is necessary to go beyond technical matters. Economic and societal
issues and existing or required legislation are critical in determining the
chances for successful realisation and exploitation of tidal energy.

Methods of extracting tidal energy
Water contains two forms of energy caused by the tides: the potential energy
related to the continuous change of the water level during the tidal cycle (tidal


range energy), and the kinetic energy in the currents caused by the tides (tidal
current energy).

With tidal range energy, constructions are needed to convert the potential energy
into kinetic energy. This is always done by creating a water level difference, the
head, by restricting the water flow in or out of a storage basin (often an estuary).
Dams or (natural) barriers usually form the boundary of these storage basins.
The turbines that extract the potential energy are placed in the barrier that
encloses the lagoon. The head over the basin boundary or dam creates a current
through the installed turbines, which generates electricity. The storage basin
can be a natural part of the water system, such as an estuary or an inland sea
arm, or can be a man-made lagoon. In the first case ‘simply’ a barrier has to be
constructed, while in the second case a lagoon must be created. These lagoons
can be connected to the coast or detached at sea, depending on local conditions.

Energy can be extracted whenever there is a head over the basin boundary, either
when the outside water level is higher than inside, or vice versa. However, the flow
characteristics of the basin determine if it is possible to generate electricity by
filling or emptying the basin, or in both situations. Two-directional generation
requires special turbines, which are more expensive than one-way turbines, but
generate more energy.


Artist’s impression of a power
plant for Tidal range energy.
The higher the head that is created, the higher the potential for electricity produc-
tion. In addition, closing constructions are required to minimize water flowing in
or out, in order to optimize the head over the tidal cycle. The timing of the opening
and closing of the closing constructions in relation to the conditions of tides and
wind is crucial to achieve a high production efficiency. Extra financial benefit can
be obtained when the turbines are also used for pumping the water up at night
when the energy tariff is low, and extracting the energy again when prices are
high. This is done at several hydropower facilities in mountainous areas to create
a larger head at low costs. However, the utilization of the daily tariff fluctuation is,
generally speaking, not efficient in tidal basins or lagoons since the water surface
is often too large, and the water level increase by pumping is negligible. Tidal
energy generally depends on large surface areas with a relatively small head,
whereas mountainous hydropower depends on relatively small surface areas with
large heads.

The other way of extracting energy from tides is to use the tidal current energy
directly. Energy in currents can be harvested by free-flow driven turbines. Tidal
currents are variable, both in velocity and direction. The energy production
depends on the water velocity, direction and the type of turbine that is used. When
turbines are used that rotate in a plane perpendicular to the flow direction, like
windmills in the wind, a construction is needed to adjust their orientation to the


Tidal Energy

direction of flow. Generally, this type of turbine will be placed in barriers, under
bridges or in tidal flow channels where flow directions are more or less constant.
A two-directional-flow turbine can then generate electricity during ebb and flood
tides. Turbines with a vertical axis (the Darrieus type) are not dependent on the
direction of the flow.

Potential energy resource
In most parts of the world the tidal cycle describes a semidiurnal pattern with
a period of 12 hours and 25 minutes, exactly half a tidal lunar day. Thus twice
a day there is a high tide and a low tide. The potential energy resource is deter-
mined by the volume of water that is moved during this tidal cycle. For tidal
range energy and for tidal current energy this is the same base supply. The esti-
mation of the potential energy resource is based on the following approximations
and assumptions:
- the vertical cross section of the sea or ocean, parallel to the coast
- the average flow velocity through the cross section, twice a day
For The Netherlands, the cross section measures about 250 kilometers in length,
22 meters in depth and the average flow velocity is about 1 m/s. With this it is
estimated that the total potential energy resource along the Dutch coast is about
85 PJ (24 TWh), independent of the presence of lagoons or estuaries.

Artist’s impression of devices for
Tidal current energy.


Technically extractable resource
No more than a fraction of this potential can be extracted. Tidal energy extrac-
tion systems, based on tidal ranges, can only be considered at a few locations,
depending on local conditions such as tidal ranges, current velocities, the presence
of ‘natural’ basins or the possibilities to create storage basins. To determine the
technically extractable resource, only estuaries and sea arms are considered to be
potential sites. The water level difference between high and low water – together
with the basin surface area determine how much potential energy is available.
Some estuaries in the United Kingdom have tidal ranges up to 14 meters, but
unfortunately along the Dutch coast the tidal range is limited from approximately
1 m at the Northwest to about 4 m at the Southwest.

The Dutch water surface that can be put under the influence of the tides is esti-
mated at about 1900 km2. Taking into account the estimations on effective-
ness, location specific tidal ranges, friction and conversion losses the technically
extractable tidal range energy resource is calculated at 11 PJ per year.

The exploitation of tidal current energy is less bound by the presence of reser-
voirs; however, most of the interesting tidal currents, i.e. flow velocities exceeding
1 m/s, are located within estuaries. Other interesting locations lie between islands
near the coast. Free-flow turbines convert maximally 30 – 40% of the potential
energy into electricity. A rough estimation of the technically extractable tidal free
flow energy along the Dutch coast amounts to 5 PJ.

Social feasibility
The acceptance of large constructions by the public may be a difficult issue.
Although the generation of electricity from tides will be received positively by the
public, the interference with other functions in the estuary or sea may become
problematic. Closing off parts of an estuary may have enormous consequences
for fishing, ecology, the environment, navigation, recreation etc. In addition,
these projects require large investments and complicated connections to existing
infrastructure. The best opportunities are found where tidal energy extraction can
be integrated in existing infrastructural constructions such as dams, barriers and
bridges. Another opportunity is to create underwater installations; an additional
advantage of this is that the best locations from an efficiency point of view, i.e.
high current velocities, can be found in deeper gullies and in openings of dams. If
the water depth is sufficient to avoid interference with navigation and sediment
transport, good opportunities can be found here. One of the ever-present require-
ments is that turbines are designed such that fish mortality is avoided.

Viewed from an environmental perspective, the creation of tidal lagoons may
cause many ecological concerns. Estuaries play an important role in the ecology


Tidal Energy

and health of the connected coastal sea. They serve as nurseries for fish and they
feed, with their high productivity, the coastal sea. Therefore, building a dam or
otherwise changing the water circulation pattern may affect many aspects of
the local ecosystem. Thus, it is necessary to prevent the algal population from
expanding and to prevent deterioration of soil populations and shellfish produc-
tion grounds due to changes in sedimentation patterns and to prevent interfer-
ence with migration routes for fish and mammals and feeding grounds for bird
populations.

In particular, the closure of an estuary may affect the morphological processes
in the area. These processes take many years to reach equilibrium, if ever. In the
short term, navigation channels may be affected leading to increased dredging
volumes. In the longer term, morphological changes may result in the (dis)
appearance of tidal flats, possibly affecting the stability of dikes and thereby
affecting the safety of the land behind them. Morphological changes may also
affect salinity gradients in the estuary, which in turn may have a serious impact
on the ecology.

Finally, the closure of an estuary has an impact on the landscape and seascape.
This is difficult to avoid unless underwater installations are used. Since empti-
ness and unobstructed views are two of the major qualities of the coast for the
public, large-scale constructions will not be accepted easily. Large-scale interfer-
ence with estuaries requires very careful study into the many and widespread
consequences.

Technical challenges
A proper selection of both location and technology, in line with other functions, is
a first step, and determines the issues that need to be solved. Turbine technology
for tidal range energy is well developed, although designs avoiding fish mortality
remain a challenge. Free-flow turbines are still under development; however, some
are in a (pre-)commercial phase. The main challenges that are faced now are to
combine these technologies with structures that also serve other purposes and
to reduce the environmental impact. Besides this, the technologies have to be
optimized in order to extend the range of flows where high conversion efficiencies
can be achieved. In addition, maintenance is an important issue. In the first place,
turbines must be protected against debris, rubbish, and larger animals (the latter
must also be protected against the turbines) to ensure an uninterrupted operation.
In addition, the salty marine climate is particularly harsh, reducing the lifetime of
the machines and necessitating high-quality housing and installation. If cleaning
is frequently required, downtime will accumulate and the production will be lower.
The less regular maintenance and repair are required, the better. Downtime may
accumulate substantially over the lifetime of the machines (20 – 40 years).


Costs and yield
Recovery of tidal range energy requires large investments, which is uneconomic
for smaller heads. Consequently, the designed lifetimes of constructions need
to be long (~100 years). Economic calculations are rather arbitrary since large-
scale structures such as dams or barriers will not be built for the single purpose
of energy extraction, but will provide other functions such as protection against
floods, traffic connection etc as well. Nevertheless, there are some estimates of
the unit production price of barrage systems in the UK and The Netherlands,
which amount to 0.075 and 0.15 €/KWh, compared to 0.04 €/KWh for fossil fuels.
Since tidal current energy is still in a pre-commercial phase, cost price estimation
cannot generally be done.

Spatial planning issues
As previously stated, with the exception of tidal currents, extracting tidal energy
comes down to large-scale projects and its realisation is therefore a national
matter, with regards to investments and acceptance, as well as with regards to
spatial planning issues. Careful designs, evaluations, impact assessments, cost
calculations and acceptance trajectories are part of these developments, as is
complying with spatial planning legislation.

Wave Energy
The seas and oceans are used in several ways. Besides the natural inhab-
itants of the aquatic ecosystems, the seas and oceans are used for naviga-
tion, defence, oil and gas recovery and – of course – fishing. The nearshore
locations are also perfect for recreation and tourism. In addition, the sea
wind delivers a cool breeze on the beach – especially welcome during
warm weather. We already extract (wind) energy from this cool breeze.
In the Dutch part of the North Sea the first wind park was installed just off
the coast at Castricum. The wind waves on the North Sea are not yet used
for energy generation. This is because with the current state of the art,
commercial exploitation has not yet been proven.

On the North Sea, the waves are relatively small compared to the waves on the
ocean. This is caused by the small fetch on the North Sea, the length over which
wave generation can take place. The average energy content in the North Sea
supply is around 10 kW per meter of wave at thirty kilometres off the coast.
That may be five to eight times less than elsewhere, but each 10 meters of wave
still offer as much power as a car engine of 140 hp. The advantage that comes
with this is that the wear and tear on wave energy facilities in the North Sea is
supposedly much lower than in the oceans. The North Sea seems to be an inter-


Wave Energy

esting testing ground for developing new techniques. Depending on the cost of
energy of these systems and the development of (fossil fuel) energy prices, it
will become clear whether commercial exploitation is possible in the near future.
Some pilot installations are already installed in several places in the world, such
as in Scotland (Pelamis, WaveGen, Oyster1), Ireland (WaveBob) and Australia
(Oceanlinx). Some systems are designed specifically for nearshore application,
such as Oyster1, Oceanlinx and Wavegen. Generally these are located in areas
just before the waves break. The driving principle is an oscillating movement in or
of a chamber, caused by the fluctuating water level of the wave. An advantage of
the nearshore installations could be the reduction of wave loading on the coast. In
the Mutriku project in Spain wave energy convertors are integrated in the break-
water system.

Methods of extracting wave energy
Hundreds of concepts are currently under development. Roughly they can be
divided into the following four principles of operation:

1. Point absorbers and surface attenuators. As a result of a passing wave a
buoyant body at, or below the surface, makes an up-and-down, a back-and-
forth or a rotary motion. One or more of these movements can be converted
into electricity.

Artist’s impression of floating
devices for Wave energy.


2. Overtopping. The upper part of a wave is channelled to an elevated reservoir.
The kinetic energy of the upper part of the wave is converted into potential
energy. This water flows through a turbine back to the seawater level and
generates energy.
3. Oscillating water columns. Waves enter an enclosed structure pushing or
sucking the air in an air chamber, which in turn drives an air turbine. The prin-
ciple is similar to a blow hole.

Artist’s impression of air
chamber devices for
Wave energy.

4. Hydrodynamic lift. Especially in the upper layers of the water, the water parti-
cles in a wave move in an orbital motion. Through a combination of vertical
and/or horizontal blades the wave energy can be converted into a rotary or
oscillating motion, which can be converted to electrical energy via conven-
tional transmissions.

Just like we build offshore wind farms, we can also build offshore wave power
farms, possibly near to wind farms developed in the future. These combinations
would offer great synergetic benefits. The waves on seas and oceans are caused
by friction with the wind. The longer the fetch, the more energy is transferred into
the waves, resulting in higher waves and longer wave periods. Depending on the
height and the period waves contain energy that is to some extent exploitable.
Analysis of the waves in the Dutch North Sea leads to the conclusion that the


Wave Energy

annual average significant wave height (H1/3) is 1 to 1,5 meters. The significant
wave height is defined as the average height of the highest one-third of all wave
heights in a wave field (sea state). The corresponding average wave period TH1/3
gives the average wave periods of the highest third of the waves and appears to
be 5.8 s. Based on these data, the average supply of wave energy per meter corre-
sponds to a wave power 10 kW/m, at about 30 km from the coast. The potential
energy supply is determined by integrating the energy flux to the coast over the
whole coastline, and over all occurring wave heights, wave periods and wave direc-
tions. This integration gives a potential energy resource of approximately 54 PJ
(15 TWh). This means an average instantaneous power of 1700 MW, which is
enough for about 900,000 households per year.

Technically extractable resource
The advantage of wave energy over other forms of renewable energy is that the
energy is constantly present and that it is more predictable and constant than
wind or solar energy. The disadvantage is the extreme loading conditions it has to
withstand and environmental conditions such as corrosion and fouling. Although
this type of energy has much attention and many wave energy systems are
developed, there is still a lack of pre-commercial plants, of knowledge about the
effects or yields and of evidence about the most appropriate wave concept in rela-
tion to efficiency and survivability. Research on the application of wave energy
facilities in parks, such as is the case with wind turbines, is also still necessary.
Because of the diffuse nature of this energy, it is indeed necessary to build parks
so that several installations benefit from the infrastructure built and make use of
the reserved space. The technically extractable energy supply is, considering the
above, an extremely difficult matter. The aspects which – in any case – should be
addressed to make a quantitative estimate on this resource are:
– conversion efficiency of kinetic energy from the wave into mechanical energy
in the plant
– the efficiency of conversion from mechanical to electrical energy
– the installation density and the yield of the park (group effect)
Given a multitude of comments and a hypothetical application of wave energy
plants on a large scale, the technically extractable resource of 3 TWh (10 PJ) per
year seems realistic.

Socially acceptable extractable resource
The North Sea is not available exclusively to extract energy. As already mentioned,
a large number of functions lay spatial claims on specific parts of the seas and
oceans. Especially the locations near the shore are full of activity (navigation,
fishing, etc) and combining multiple functions is not only desirable, but often
necessary. Using the existing areas already assigned to wind turbines for wave
energy will provide both economic and social benefits. In the light of these claims


on space, the possibilities of combining functions and other limitations, it is esti-
mated that roughly 50% of the locations along the Dutch coast assigned to wind
power can be made suitable for the exploitation of wave energy. Given the current
understanding of expected returns, the socially acceptable extractable resource
for wave energy is approximately 5.5 PJ (1 to 1.5 TWh) per year.

Whether this supply will be extracted depends on political choices and extensive
collaboration and is therefore not easy to determine. However, as sites for wind
energy can simultaneously be used for wave energy, the economic feasibility will
increase greatly, because a great deal of infrastructure for maintenance and elec-
tricity networks has already been built.

Extracting wave energy is not easy. Besides the above aspects, there are tech-
nical limitations. The movement of the water particles must be converted by a
system into electrical energy. The problem is that every moving object in the
water creates its own waves. The yield will therefore usually be relatively low.
It is estimated that the maximum proportion of energy from waves that can be
captured is approximately 40-50%. In addition, there is the conversion efficiency
of mechanical energy into electrical energy, which is between 80% and 90%.

A construction for extracting wave energy will be most effective for a limited range
of wave periods. The plant also has to be well adjusted to the most common range
of periods of waves. Preferably, a location must be chosen with a large supply of
wave energy, which would typically be at sites with high swells and swell waves.
If techniques require a dominant wave direction, it is also important that they are
placed in locations with a relatively constant main wave direction.

In addition, the construction must be built very robustly, so that the heaviest
storms cannot cause any damage. Because these situations do not match the
optimal range, energy production under such circumstances is not possible. More-
over, the generated energy has to be transported to the mainland. This requires
additional installations such as transformer stations and grid connection points.
This makes this type of energy extraction relatively costly compared with the
amounts of energy that can be gained.

In conclusion, it may be said that in the short term, energy from waves can only
be extracted in an economically feasible way at coastlines by open oceans. At
these locations a relatively constant supply of long swell waves is present.



River Energy
Water currents have always been used as a source of energy. Tradition-
ally, energy was extracted in the form of mechanical energy by water
mills, and in modern times, energy is extracted in the form of electricity.
The ubiquity of this source is appealing. Flowing rivers transport massive
amounts of water and thereby offer a great potential energy resource.
In this chapter, the focus is on small-scale techniques, such as low head
hydropower and free flow energy, suitable for various locations. In most
rivers, flowing water has to provide different services simultaneously,
such as for navigation, irrigation, recreation, drinking water and as an
industrial commodity. Energy extraction must comply with these other
services. This chapter explore the steps and considerations required to
extract energy from rivers. This goes well beyond technical matters since
economic and societal issues as well as existing or required legislation
also determine the opportunity for successful exploitation.

Methods to extract river energy
The presence of energy in rivers is in essence diffuse, though at hydraulic struc-
tures in rivers or at rapids it is concentrated and the energy is extractable using
existing techniques. Mainly two types of energy extraction are applicable in
rivers; first the installations that extract the potential energy (head difference)
and second, the types that extract the kinetic energy (water currents). Both work
best when the maximum volume of water is channelled through the construction.
The constructions needed, however, may not interfere with the other functions.
Therefore, most promising opportunities are located where electricity generation
can be combined with some other function(s).

Potential energy extracting methods enforce the water flow though a turbine,
where the potential energy changes into kinetic energy, which is converted by
a turbine into electricity. Turbines used are of various types such as the Kaplan,
Cross flow, Archimedes (screw), Bulb and the water wheel. The prevailing flows
and heads at the location are important selection parameters for the turbine type
to be used. Especially in cases of variable flow – the head is relatively constant
in most cases and has therefore few consequences – careful design is required
to create high efficiency over an extended range of flows. In order to obtain
the highest amount of energy, (hydraulic) losses must be avoided as much as
possible. Much care must given to the optimal design of inflow and outflow struc-
tures, though construction costs may rise considerably.


plant for River energy.

Kinetic energy extraction methods, also known as free flow turbines, do not require
a head difference, but do require a minimum velocity to generate electricity. The
various types of turbines can be categorized as follows:
– turbines rotating on a plane perpendicular to the flow direction, like a wind-
mill;
– turbines rotating on a plane in the direction of the flow, like traditional water
wheels;
– turbines with a vertical axis (the Darrieus type).
Sometimes the flow velocity is increased by a venturi construction, located in
front of the turbine. Behind the turbine a construction is needed to mix the flow
smoothly with the surrounding flow to avoid turbulence losses. Many variations
exist in turbine and construction to turn the water flow into a rotation that drives
the electricity generator. Some rotate in one direction only, while others support
two directions of rotation. Depending on the type of turbine energy extraction
is possible with uni-directional flows (rivers), bi-directional flows (tidal rivers) or
varying flow directions (broad rivers with non-consistent flow patterns).

Between the location where rivers enter (e.g. geographical border) and where they
leave (e.g. the ocean) the river bed level difference causes the water to discharge.
When the bed level difference is larger than strictly necessary for discharging the


River Energy

water, this surplus height difference can be utilized to extract potential energy.
The total potential energy resource, so the energy included for discharging the
water, is calculated for the rivers in The Netherlands. As a rule of thumb, a river
discharging an average 500 m³/s over a level difference of 50 m represents a
potential power of ca 250 MW, i.e. an energy content of ca 2150 GWh per year. For
comparison, an average annual discharge of 500 m³/s represents a river some-
what larger than the Meuse (250 – 400 m³/s), but much smaller than the Rhine,
which has an average annual discharge of 2200 m³/s. The bed level difference
for the Rhine is only about 10 m and for the Meuse approximately 45 m. The
combined annual flow of rivers in The Netherlands amounts to almost 3000 m³/s.
This provides the potential energy resource of 3 TWh (11 PJ) per year. It must be
noted that a part of this energy is still needed to discharge the water from the
location of entry to the location of exit.

Technically extractable potential
Only a small part of this potential at a limited number of locations can be
extracted. This is due to the requirements of the turbines in terms of head,
velocity, requirements by other functions and – as mentioned – to discharge the
water to the ocean. It is also due to the fact that river discharges vary over time.
When discharge is low, energy generation is very limited. However, with high river
discharges no obstructions, including turbines, can be allowed in the river flows
to prevent risks of flooding.

When a river is used for navigation or irrigation, hydraulic structures are often
already in place to maintain safe water depths and to provide a substantial irri-
gation resource. At these hydraulic structures potential energy is extractable
under regular conditions. In practice, substantial energy generation is limited to
situations with a head of at least 1 m and a minimum average flow of 25 m³/s.
Currently, electricity production in Dutch rivers is about 100 GWh, and given the
possibilities for expansion of hydropower the technically extractable potential is
estimated to be 300GWh.

Social feasibility
Energy can only be produced when not interfering with the other functions of a river.
A number of other potential obstacles must also be considered. An important envi-
ronmental issue is fish passage. Most turbines are not fish friendly and any selec-
tion of a technology should take this into account. Improvements on this point may
very well decrease the efficiency of electricity generation or require complicated,
expensive constructions. Connecting the generators to the grid is another issue,
both from a landscape point of view as well as economically. The public resistance
to overhead power lines in an open and unspoilt landscape might be severe, while
underground connections might be very expensive and less economically feasible.


A careful selection of both location and technology, in line with other river
functions, is a first step, and determines other issues that need to be solved.
Hindrance to other river services and environmental issues such as fish passage
can be solved in many ways, and are highly determined by local circumstances.
Turbine technology is well developed. The challenges are mainly how to combine
this technology into structures used for other purposes, such as sluices, weirs and
bridges and how to reduce any environmental impact, and with that increasing
economic feasibility. An existing turbine challenge is extending the range of flows
where high conversion efficiencies are realised. Maintenance is also an important
issue. In the first place, turbines must be protected against debris and rubbish
to ensure uninterrupted activity as much as possible. If cleaning is required
frequently, downtime will accumulate, lowering the production. The lesser regular
maintenance and repair that is required, the better. Downtime may accumulate
substantially over the lifetime of the machines (20 – 40 years).

Costs and yield
Cost calculations for economic feasibility should include investment costs
including construction costs and maintenance costs. Both are difficult to estimate
due to the combination with other constructions and location specific conditions.
Investment costs for turbines range from ca. 1000 – 5000 €/kW installed power,
not including construction costs. Maintenance costs might be substantial if the
turbine is sensitive to debris or substantial growth of mussels, or other biofouling.
Then the choice has to be made between regular cleaning or accepting lowered
efficiencies. For each location and set of (environmental) conditions the downtime
has to be assessed. This factor determines to a large extent the cost price of the
energy delivered. An estimation of the cost price for energy production in the
Dutch rivers amounts to ca 0.09 €/kWh, compared to 0.04 €/kWh for fossil fuels.

The fact that installations to extract energy from rivers always interact with other
river functions may complicate spatial planning. In addition, connecting to the
grid may require an enhancement of the capacity of power lines, and of control
technology. Governments at various levels will probably be involved. In addition,
political decisions must be taken with respect to the importance of ‘river energy’
compared to other functions. A careful design project, involving stakeholders and
government, is required, but public acceptance will support the process due to
sympathy for fossil-free energy generation.



Blue Energy
Blue Energy is the name for energy generated from saline gradients (i.e. the
difference in salinity concentration between two solutions). Energy from
saline gradients in water is one of the most promising concepts of renewable
energy. The global potential is enormous and will prove especially valuable
in densely populated delta areas where rivers flow into a sea or ocean.

Suitable locations for Blue Energy power plants are those marked by big saline
gradients. Interesting saline gradients occur at sites where freshwater and salt-
water meet. The most striking example of one such location is the mouth of a
river into a sea or ocean. At these sites large quantities of fresh and saltwater
are usually guaranteed and provide the necessary flows for a reliable operation
of a Blue Energy power plant. Unfortunately, due to e.g. tidal movements and
currents and fluctuating river flows, the river water and sea or ocean water mix,
thereby reducing the potential. Therefore, sites with a physical separation of the
salt and freshwater, as in the case of a lock, dike or dam near the mouth of a river,
that prevent this mixing and maintain the concentration difference are even more
interesting for the realization of a Blue Energy power plant.

Methods of extracting saline gradient energy
Two main techniques are known to generate electricity from fresh and saltwater:
Pressure Retarded Osmosis (PRO) and Reverse Electro Dialysis (RED). Both tech-
niques require membranes that initially separate the fresh and saltwater and let
the water from the two water sources ‘mix’ under controlled conditions, producing
vast amounts of electricity.

The PRO technique is based on the principle of osmosis. The PRO technique uses
a semi-permeable membrane between the two liquids that can only be passed
by water molecules and therefore prevents the salt ions from migrating. The
principle of PRO through which energy is gained from water is named osmosis.
Osmotic phenomena occur when fluids with different concentrations of dissolved
substances are brought into contact with each other. In the same way as diffusion,
they strive for equality in concentrations, but the semi-permeable membrane
allows only the water molecules to migrate. The osmotic pressure drives water
molecules through the membrane from the freshwater to the saltwater side,
thereby diluting the saltwater and balancing the concentrations. If freshwater is
available in abundance, this process can lead to a water level rise on the saltwater
side of up to approximately 250 m, depending on local conditions. By limiting the
water to rise up to the maximum, the water pressure builds, which can then be
used to set a turbine in motion. In practice, about half of the 25 bar pressure built


up is used for driving a turbine since at this point the pressure-flow optimum is
reached and with that an optimum for energy generation is achieved.

The RED technique is based on reversed electro-dialysis. In the RED method-
ology, there are again two compartments, this time separated by ion-selective
membranes, which can only be passed by salt ions. The chemical potential differ-
ence creates a transport of ions through the membrane from the saltwater side to
the freshwater side. Here the salt ions (parts of salt molecules) migrate instead of
the water molecules through a membrane. The migration of ions, that all have an
electrical charge, create a potential which can be transferred to an electrical current
when linked to a cathode and anode. An energy cell consists of a cathode and anode
permeable membrane, providing a migration route for the positive and negative
ions. The cells can be stacked as regular batteries, and the energy output equals
the product of the number of cells and the output per cell. The total energy yield
per cubic meter from fresh and saltwater is comparable to that of the PRO method.

The power output – and therefore the annual average energy production – depends
on the installation and the present resistance, such as required for treatment,
and losses on the membrane pumps and possibly turbines. Pressure Retarded
Osmosis (PRO) as well as Reverse Electro Dialysis (RED) state that a realistic
production is around 0.7 MW per m³/s.

plant for Blue Energy.


Blue Energy

Both methods have already been known about for decades, but so far high costs
and low technological and energy yields have prevented these techniques from
becoming a realistic substitute for fossil fuels. However, technological improve-
ments, anticipated economies of scale and rising awareness of the energy
problem, together with the increasing costs of fossil fuels, mean that Blue Energy
is slowly becoming an interesting and viable alternative.

The total potential for Blue Energy in The Netherlands is very large. The combined
average annual flows of the rivers in The Netherlands deliver almost 90,000 billion
litres of freshwater into the North Sea. That means that every second approxi-
mately 3,000 m³ of freshwater flows into the sea unused. The osmotic pressure
difference between freshwater and saltwater is about 25 bars. This means that
every cubic meter of freshwater – enough for the presence of saltwater – represents
2.52 MJ of energy. This corresponds to 0.7 kWh. The total potential energy resource
is thus 220 PJ per year (60 TWh). Converted into instantaneous power this is 7000
MW. As an indication, the approximately 7.2 million Dutch households consume a
total of between 400 and 500 PJ annually. If a full utilization of this potential was
possible, this source would provide half of the required energy supply.

Technically extractable potential
Despite all the technological possibilities and innovations – now and in the near
future – it is not possible to extract the whole potential. The causes lie in the
efficiency of conversion processes, the practical limitations of the geometry of
the plant, the necessary space for performing maintenance and safety, and – of
course – the friction losses within the plant. The technically extractable supply is
therefore lower than the potential energy supply.

In the remainder of this section it is assumed that the PRO method has similar
losses to the RED method. The losses are partly due to the efficiency of the stack,
which is about 40%. In addition, a further reduction of the estimated poten-
tial has to be applied, because of differences between actual and average river
discharge. It is very likely that a PRO or RED system cannot adapt to all discharge
fluctuations that occur in reality. Where there is an actual discharge at a produc-
tion location which is below the average discharge, it is clear that the production
is lower than the production based on average discharge. If the actual discharge
exceeds the average discharge, the production then depends on the installation
for whether it can adapt sufficiently. It is obvious that this is not always the case
and that both situations lead to an additional reduction in production.

The estimated annual number of hours that a RED or PRO system actually oper-
ates for is 6,500 hours, due to variability in freshwater discharge. Furthermore, it


is assumed that the PRO method has similar losses to the RED method. For RED,
these losses are partly due to the efficiency of the stack, which is about 40%.
This means that the annual technically extractable resource in The Netherlands
is estimated at 65 PJ (20 TWh). Converted to instantaneous power this would be
equal to 2000 MW.

One cubic meter of river water and one cubic meter of seawater provide a theoret-
ical capacity of about 1.7 megawatts, and when there is endless saltwater avail-
able, as much as 2.5 megawatts per cubic meter of freshwater can be generated.
In practice, approximately one megawatt maximum is achievable. This value is
still dependent on local conditions, such as salt concentrations in different rivers,
seas and oceans, temperature and environmental factors. The Rhine is the most
‘energetic’ river of Europe. The technically extractable potential of the Rhine and
Meuse is estimated at 2.4 gigawatts and the economic potential at 1.5 gigawatts
– enough to power four million households.

Socially acceptable energy resource
In reality, the extractable supply is even less since environmental arguments and
social interests have a limiting influence on the number and size of exploitation
locations. In particular, navigation (to maintain the required water depth), agricul-
ture (irrigation), the production of drinking water and providing cattle with fresh-
water, all reduce the socially acceptable extractable resource. Therefore, what is
available is difficult to quantify. Based on engineering insight, only one third of
the river flow seems available for power generation. The available cumulative flow
is thereby determined at 900 to 1,000 m³/s. The socially acceptable extractable
resource lies at just over 22 PJ per year; this is equivalent to 6 TWh annually.
Converted into instantaneous power this is about 700 MW, which is still about
5% of the total annual electricity demand in The Netherlands. The last limiting
factor is economic feasibility. Locations are only exploitable if the investment and
operational costs are lower than the – possibly subsidized – revenues from energy
sales. It is expected that within five or ten years Blue Energy power plants will be
in operation and working commercially. The magnitude of the energy production
depends on the chosen location.

A membrane costs about five dollars and generates about five watts per square
meter. It is expected that if a large commercial market for Blue Energy membranes
forms, the price will go down significantly. With regard to yields, current findings
show a large negative impact on the energy yield of the membranes when the
saline gradient drops slightly. Robustness remains a critical aspect when trying
to commercialize Blue Energy. The thickness of membranes has recently been
reduced from 0.6 to 0.2 mm. However, when installing a Blue Energy plant of 300


Blue Energy

MW about 60 million membranes are needed, creating a volume of approximately
1,200 shipping containers.

The source water may contain many impurities. These range from floating mate-
rials such as algae and sediment to dissolved substances at the molecular level,
such as salts and nutrients. The impurities reduce the efficiency of the membranes
which therefore need to be cleaned regularly, which is quite costly. Something
that certainly requires further study is ‘biofouling’ – the encrustation and attach-
ment of organic material, which can result from being clogged.

For both RED and PRO, laboratory-scale experiments have demonstrated that
the technology works (proof of concept). Both techniques have now reached the
stage of scaling up to small-scale pilot plants under realistic field conditions: for
example, there is a pilot plant with PRO in Norway and in The Netherlands with
RED. A major pilot demonstration would be the necessary next step, and after
that an operational plant, both of which still lie in the future.

Research indicates that RED is best suited for The Netherlands because of the slightly
turbid rivers. PRO works well in highly concentrated salt streams (such as brine in
the salt industry), and with relatively clear water. The reason for this is that with
PRO huge amounts of (polluted) water need to pass through the PRO membrane,
while only with RED the ions have to pass the membranes. Whether this also means
that the PRO method has less future depends also on other factors, such as the
complexity of the installation and developments in the PRO and RED or membranes.

Not all challenges are physically related to the actual power plant. The integration
and positioning of a plant in the water system lead to interesting questions. Blue
Energy requires the supply of fresh and saltwater and the discharge of brackish
water. The inlets for fresh and saltwater should be positioned in such a way
that short circuit currents between the inlet and outlet will not occur. In addi-
tion, the positioning of the inlets must be positioned at those locations where the
maximum saline gradient is achieved.

In addition, the water intake and outlet points change the flow pattern of the river
locally, which can lead to the deposition and erosion of sediment. To maintain
the current functionalities of the river, such as shipping, dredging might become
necessary, or a well-proportioned design should be used. The use of freshwater in
the power plant will limit the quantity remaining available for other functions and
uses. How much water can be used is a political choice: how much water should
be reserved for navigation, agriculture, environment or energy? Possibly already
existing hydraulic structures can play a role in dealing with this potential problem.


Thermal Energy
from Urban Surface Water
The focus on using renewable energy sources is an important effort in
reducing the ongoing environmental impact of conventional energy
generation. One renewable option for heating and cooling of buildings
is the use of surface water coupled with underground thermal energy
storage (TES), instead of traditional heating systems that often rely on
gas combustion. Urban surface waters have a large potential for heating
and cooling buildings located in the same neighbourhood, regardless
whether it is occupied by households or industries. In The Netherlands,
the surface water bodies coupled to TES are usually lakes with a water
depth of 20-40 m. However in Dutch urban areas these depths are not
met. Urban water is subject to special micrometeorological charac-
teristics such as low wind speeds and relative high air temperatures.
The effect is known as the heat island effect. TES is under development
in many countries e.g. The Netherlands, USA, Germany, and China.
Although the technical feasibility of TES has been proved, a detailed
study of the different components, the combination with the usage of
urban water and the assessment of the effects on ecology are still neces-
sary. A feasibility study has been carried out for a new urban develop-
ment ‘De Draai’ in Heerhugowaard, The Netherlands.

Method of extracting energy from urban surface water
There are four important stages when deploying this system, which are: the
extraction from the source (the urban water), the extraction and the storage of
the thermal energy and the distribution to the final users. In order to extract
the thermal energy from the water an exchange system must be established. A
heat exchanger is usually installed to do this. For thermal energy storage the
reader is referred to Section 4.6. The figure below schematizes the four stages
as a chain, but in reality, loops can be created. For instance, after distribution of
thermal energy to the user, the heat or cold might be recaptured and returned to
the source. Another option could be that the heat or cold is directly distributed,
often using water as a medium, for heating or cooling spaces without the usage
of a heat exchanger.

The locations of the extraction, storage and the users are critical to the efficiency
of the system. When distributing thermal energy over longer distances large
losses can occur, as is generally the case with thermal energy systems. There-
fore the source, i.e. the water body, must be relatively close to the final users.


Thermal Energy from Urban Surface Water

Artist’s impression of a system
for using Thermal energy from
surface water.

Luckily, within larger cities it is common to find ponds, canals, natural existing
urban waters and sometimes lakes within close range. It is obvious that the size
of the surface water area and the depth of the water bodies determine the thermal
energy exchange with the atmosphere and the total storage capacity. It is worth
mentioning that the stored thermal energy usually needs to go to boilers before
going to the user. This applies especially for the provision of hot water. Depending
on the user’s demand and the state of the thermal energy system, storage can
be skipped and a direct connection between the heat exchanger and user can be
established.

Urban waters are readily available and are considered a renewable source of
energy. As an example of the potential of urban water as an energy source it has
been computed that the pond located in the ‘Paleiskwartier’ in Den Bosch, The
Netherlands, which is 1,000 m², could provide 4,355 [GJ] of heat per year. This
amount of energy is sufficient to provide thermal energy (heat) to 132 house-
holds. Another example is ‘De Draai’ in The Netherlands, where 207,000 m² of
water area could provide sufficient heat for 2,816 households.

Social feasibility
As long as the urban water bodies preserve their ecological and visual value,


Deltares Renewable Energy From Water and Subsurface 2010

Deltares Renewable Energy From Water and Subsurface 2010

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