1. INTRODUCTION

Grid connected electricity production from wave energy systems is world-wide at an embryonic experimental stage. The low cost of conventional energy and wind energy makes it difficult for wave energy systems to reach a commercial breakthrough without significant support in the R&D phase. Small wave driven buoys for navigation or desalination purposes are , however, commercially available.

Before the establishment of a formal Danish wave energy programme in 1997, only one wave energy system has been investigated thoroughly in Denmark.

The successful completion of a 1 kW pilot test by Danish Wave Power in the North Sea (1) has encouraged further R&D.

The interest in wave energy in Denmark has lead to the establishment of the Wave Energy Association (WEA) in 1998 counting 120 members.

2. DANISH ENERGY POLICY

The fate of future generations is strongly dependent on the natural environment that the present generations are leaving for them. So far, some small industrial nations have made the greatest efforts towards a sustainable energy development. Denmark, with five million inhabitants, was one of the first countries in the world to commit itself to a specified reduction target for CO2-emissions. In the official Danish energy plan from 1990 (Energy 2000, 1990) the target was specified as a CO2-reduction of 20% in year 2005 and 50% in year 2030 as compared to 1988. These target values have been confirmed in the official plan from 1996 (Energy 21, 1996).

The strategy for reducing CO2-emissions from combustion of fossil fuels is based on a combination of energy conservation and a shift to renewable energy sources in the supply system.

The Danish conservation efforts were initiated in the mid-seventies and have resulted in a nearly constant total energy consumption of around 800 PJ per year since then. At the same time the Danish GPN has increased by about 50% illustrating a de-coupling of economic growth from the growth in energy consumption (2).

In relation to alternative energy supply systems, the Danish efforts have been focusing on wind and biomass with additional promotion of solar heating for individual households and district heating systems. The official goal is that 12-14 % of the total Danish energy consumption shall be covered by renewables by year 2005 and around 35 % in year 2030. Today renewables cover a little more than 8% of the total Danish energy consumption.

2.1 Means of implementation

A wide range of means have been applied in order to realise the targets set up in the Danish energy programmes. These have included

· government investment subsidies for wind turbines, solar collectors, biogas plants, heat pumps and insulation of houses;

· favourable tariffs for electricity from wind and biomass and favourable conditions for access to the grid;

· technical support and certification from governmental test stations for wind, solar and biomass technologies;

· government funding of research, devel-opment and demonstration programmes for wind, solar and biomass technologies and installations;

· CO2 tax and energy tax on fossil fuels and electricity based on coal;

· government sponsored local information offices concerned with renewables and energy efficiency.

Due to the Danish tax policy electricity is relatively expensive in Denmark. This policy stimulates energy conservation and enables a special tariff for electricity produced by wind and biomass. This tariff of about 0.57 Danish kroner (DKK) per kWh is almost twice the cost of electricity produced in coal fired plants. It includes refunding a CO2-tax of 0.1 DKK/kWh and adding on environmental credit of 0.17 DKK/kWh. (1 ECU = 7.5 DKK)

2.2 The lesson from Danish wind power history

Since the mid-1970’s, Denmark has been one of the pioneering countries concerning modern use of wind power. In contrast to other countries, the Danish development of modern wind power started in the late seventies from the low capacity end with small wind turbines typically of about 20 kW capacity. During the eighties, the capacity of the typical commercial Danish wind turbine gradually increased to 50 kW, 100 kW and 200 kW. In the mid-nineties the capacity had increased to 600 kW and today Danish wind turbines are commercially available up to 1,5 MW.

More than half of the total installed wind power capacity in the world has been produced in Denmark. Around 8000 people are employed by the Danish Wind Industry which in 1997 exported for more than 1,000 million dollars. These impressive results may be ascribed to the long tradition of wind power in Denmark combined with an active government promotion. An important element of this promotion has been the establishment in the late seventies of a government test and certification station for wind turbines. In the summer of 1998, about 4500 wind turbines are installed in Denmark with a total capacity of about 1200 MW. They cover nearly 7% of the Danish electricity demand. At good sitings, electricity from wind turbines is almost competitive with electricity from coal based production, viz. 0.30 DKK per kWh.

The main future achievement of Danish Wind Energy will be the installation of 750 MW offshore wind energy by year 2008.

It is hoped that the broad public support for wind power can be transferred to the area of wave power.

3. BIRTH OF THE WAVE ENERGY PROGRAMME

The Danish wave energy programme is a result of a political agreement in 1996 to develop and promote new renewable sources of energy and storage options. The agreement includes a financial support of 100 mio. DKK for four issues

· wave energy;

· seasonal storage of solar heating;

· hydrogen technology and systems;

· organisation of privately owned offshore wind farms.

A Danish Wave Energy Association was formed in the spring of 1997 and has approximately 120 members. The association arranges meetings, disseminates information for the members and other parties interested in wave energy.

During the first two-year period, 1998 – 2000, the Danish Wave Energy Programme has an initial budget of 20 mio. DKK (2.7 MECU). Based on an evaluation of the obtained results, a second two-year programme may be funded by additional 20 million DKK. The programme is administrated by the Danish Energy Agency.

To advise on appropriate testing and research, the Danish Energy Agency has established an Advisory Panel of experts representing the hydraulic and maritime institutes in Denmark, the Folkecenter for Renewable Energy, the University of Aalborg, the Technical University of Denmark and the Wave Energy Association.

Different Systems

OWC systems:

Pico Plant Azores

Wagen Islay

AWS Off-Shore systems:

Pelamis off-shore system:

Wave energy resource in the North Sea:

4. PROGRAMME 1998 - 2000

4.1 Objective

To investigate future optimal wave energy solutions the Danish Wave Energy Programme aims at a broad development of different wave power concepts. Prototype development has been postponed until comparable model test results on different systems have been analyse

This calls for a programme structure able to deal with wave power systems at different stages of development.

The ultimate aim of the Wave Energy Programme is to develop relevant expertise and technologies, enabling installation of MW to GW wave energy plants.

4.2 Method of approach

A successful development requires an understanding and documentation of the amount of power that different types of wave power converters can utilise and of the reliability and survivability of the systems.

The Danish Wave Energy Programme aims at stimulating:

· the R&D environment for wave energy;

· a multidisciplinary approach;

· co-operation between developers;

· openness regarding results;

· selection of the most promising types of devices from continuous R&D;

· involvement of expertise from related industry, whenever possible;

· utilisation of know-how and technology developed in previous and parallel wave energy programmes.

The economic frames for different topics included in the programme are indicated in Table 1. The advisory activities are administrated by the Wave Energy Association and the Advisory Panel, respectively.

Table 1. Funding for topics 1998 - 2000 in Mio. DKK

1. Wave Energy Association
Model building of new concepts	2
Construction of test site	2.5
Dissemination of results	1
1. Total	5.5
2. Advisory panel
R&D and testing of concepts	12.5
Supporting projects	1
Dissemination of results	1
2. Total	14.5

5. DEVELOPMENT STAGES

A typical development of an idea for a wave energy converter is illustrated in Figure 1. After each step, the project can be evaluated technically before the next step of development may be undertaken. The initial wave programme years 1998 to 2000 exclude prototype testing, and support research up to the level of “Further R&D” at step 3. Based on results obtained at Step 3, the reliability and efficiency of the wave energy converter can be evaluated.

Idea

Application to Wave Energy Association (WEA)

ßÝ

Concept evaluation by WEA

Step 1

New Ideas

Model construction (up to 50.000 DKK)

Simple experiments & visualisation

Presentation of results

Application for intermediate testing

ßÝ

Evaluation by the Advisory Panel

Step 2

Intermediate Phase Projects

ßÝ

Application for further R&D

ßÝ

Step 3

Further R&D

(Optimisation)

ßÝ

Evaluation by the Advisory Panel

Prototype development

Demonstration

Commercial products

Figure 1. Development stages.

The Wave Energy Association and the Advisory Panel co-operate as projects after Step 1 must apply to the Energy Agency for further project support. At this stage the project will be evaluated by the Advisory Panel.

The technological clarification will gradually be obtained, as several projects reach a development corresponding to Step 3.

5.1 Step 1, New ideas

New concept ideas are evaluated by the Wave Energy Association (WEA), and inventors may obtain funding up to 50,000 DKK (6,600 ECU) for building a model of the system. The WEA promotes open and constructive evaluation, exchange of ideas, co-ordination of expertise and co-operation among developers.

5.2 Sheltered open sea “Playpen”

The Wave Energy Programme provides economic support for appropriate initial testing of the new ideas. This testing can either take place in indoor test facilities or in the open sea.

The establishment of a test site in open sea has been planned and 2.0 mil. DKK have been allocated for this purpose.

The establishment of such a test site has been a key issue for the WEA. The WEA has pointed out a test site located at Nissum Bredning in north - west Jutland. The location of the site is near the Folkecenter for Renewable Energy. The Centre will provide facilities for model construction and improvements, assistance with testing and dissemination of results.

At the test site, inventors will be able to gain practical experience with their models of wave power converters.

The wave conditions at the test site have been investigated based on a numerical wind-wave model and wind data from the area (3). The yearly distribution of significant heights Hs and interval of zero crossing periods Tz is indicated in Table 3.

The number of test periods per year with waves within an interval of 5 cm - 25 cm and with a duration of 6 and 12 hours (day and night) has been calculated to be 177 and 100, respectively. These conditions are expected to meet the demands for open sea testing at a model scale of 1:10 or 1:20. Several projects can be tested side by side at the location.

Table 3. Wave Conditions at the planned test site at Nissum Bredning, 5 meter depth

Interval of significant Wave Heights Hs m.	Interval of zero crossing periods Tz sec.	%
0-.1	0 - 1.5	33
0.1 - 0.2	0.75 - 1.75	19
0.2 - 0.3	1.00 - 2.00	18
0.3 - 0.4	1.50 - 2.25	15
0.4 - 0.5	1,75- 2.50	10
0.5 - 0.6	2.00 - 2.50	4
0.6 - 0.7	2.00 - 2.50	1
0.7 - 0.8	2.00 - 2.75	1
0.8 - 0.9	2.25- 2.75	-
0.9 - 1.0	2.50	-

Similar test conditions can be achieved in the existing indoor facilities at the Danish universities, hydraulic and maritime institutes. In the wave basins testing will be limited only by ongoing research and commercial testing.

5.3 Intermediate testing, Step 2

After successful initial testing and demonstration of the function of the wave energy converter the inventor can apply for further testing. At this stage the inventor may co-operate with a company or an institute undertaking development of wave power systems and apply for funding for further R&D up to 1 mio. DKK (133 kECU). The intermediate testing must be conducted at an authorised test facility. The experiments shall focus on device performance and survivability.

5.4 Optimisation, Step 3

Based on the results obtained at Step 2, a project might benefit from further research and optimisation. In this case the project team may apply for further R&D funding. In order to optimise the outcome of each project the Advisory Panel has adapted a policy of supporting short project phases with a duration of typically half a year. After each phase, results will be evaluated and new steps considered.

6. GUIDELINES FOR TESTING AND REPORTING

In order to compare experimental results based on different wave energy systems tested at different test facilities, the Advisory Panel has prepared guidelines for testing and reporting.

Indoor testing may be carried out at three main facilities in Denmark: Danish Hydraulic Institute (DHI), Danish Maritime Institute (DMI) and Aalborg University Centre (AUC). The wave flumes and basins available are as follows:

· Test Facilities at DHI are:

Basins:

30mx20m, 3m deep, 3D waves, wind & current;

30mx30m, .80m deep, waves & current;

32mx30 m, .45m deep, waves;

30mx30 m, .75m deep, waves;

68mx30 m, .45m deep, waves.

Flumes:

28mx0.74m, 1.2m deep, for waves;

35mx5.50m, 0.8m deep, for waves & current.

· Test facilities at DMI are:

Flume:

220mx12m, 5.4m deep, waves.

· Test facilities at AUC are:

Basins:

17mx10m, 1.5m deep 3D waves & current;

20mx12m, 1.0m deep 3D waves.

Flumes:

25mx1.5m, 1.5m deep, for waves & current;

20mx1.5m, 1.0m deep, for waves.

The largest flume is available at DMI which is able to generate long-crested waves. At AUC and DHI both wave flumes and basins for 3D (short-crested) waves are available.

6.1 Survival testing

The guidelines for evaluation of survival involve testing in a number of severe sea states representing the North Sea conditions. The combinations of significant wave heights and spectrum peak periods Tpeak are shown in Table 3 and the combinations of sea states for evaluation of fatigue are shown in Table 4. The recommended spectrum peak period is based on the present knowledge of the prevailing sea conditions in the north sea.

Table 3. Survival conditions for 50 metre water depth.

Hs (m.)	Tpeak (sec.)
9	11,5	12,8	14,1
10	12,1	13,4	14,1
11	12,9	14,2	15,4

Table 4. Conditions for fatigue testing

Hs (m.)	Tpeak (sec.)
1		5,6
2		7
3		8,4
4		9,8
5		11.2

6.2 Performance testing

For experimental investigation of energy production the basic test matrix is thesame as for fatique conditions Table 4. This matrix corresponds to typical combinations of significant wave heights and peak spectrum periods in the North Sea. In order to test the devise sensitivity to spectral shape, long- compared to short-crested waves and variation in peak spectral periods, the following additional test series are recommended:

1. Basic test in long crested waves as indicated in Table 4. PM spectrum;

2. Basic test in short crested waves as indicated in Table 4. PM spectrum;

3. Test using JONSWAP spectrum shape, test for Hs: 2m, 3m; (towards 70% of the annual wave energy in the north sea is contained in wave conditions with significant wave heights of 1m and 2 m.)

4. Test for sensitivity to wave period for Hs = 2m, test Tp: 6.3sec., 7.7sec., 8.5sec.

Measurement of the sensitivity of the con-verter to spectrum shape and wave periods can be conducted in either short or long crested waves, depending on the nature of the wave power converter.

6.3 Reporting

In order to be able to obtain an overview of the results obtained from the testing, a standard reporting framework has been prepared.

The main information is a summary of the efficiencies measured in recommended sea states. The report provides information on the device geometry and structure and the layout of the wave power plant.

Based on these data, a summary of the results may be presented as shown in Table 5.

Table 5 Presentation of device performance

Location:
	Weibull parameters
	Mean Power Level kW/m
Data for one Wave Energy Converter
	Length of installation
	Available mean wave power
	Absorbed mean power
	Capture efficiency
	Mechanical efficiency
	Generator capacity per device
	Hydraulic and electrical efficiency
	Produced mean electricity
	Conversion efficiency per device
Data for a large wave power plant
	Number of devices
	Distance between converters
	Total length of installation
	Directional factor on incoming power
	Available power
	Transmission efficiency
	Availability
	Factor on directional capture efficiency
	Annual energy output
	Total conversion plant efficiency

6.4 Economic evaluation

The Advisory Panel has suggested that at this stage of development economy should not be used as a guideline for device selection. Economic considerations can be introduced at a later stage of the development. The main task at the present stage is to develop and select the types of wave energy converters that can work at sea and reliable produce power.

7. STATUS JULY 1998.

A summery of the projects funded by the Danish Energy Agency during the first six months of the programme is given in this section. At the programme start, two calls for proposals was announced: The first in March, 1998, the second in October 1998. After the first call the projects described in section 7.1 and 7.2 has been funded.

7.1 New Concepts

Seven new concepts have been selected for Phase 1 testing. These projects have been selected based on project ideas evaluated by the Wave Energy Association’s and have been funded by the Danish Energy Agency.

7.1 Intermediate phase projects

· Wave Plane

Principle: Waves role up over a beach and get caught between funnel shaped guide vanes. Water stored in the funnels flow into a horizontal pipe in a spiral shaped flow. This flow will be converted into mechanical energy by a special turbine.

Model testing has been undertaken at DMI, with the objective to measure the performance of the device with a hydraulic load on the flow.

· Wave Dragon

Principle: Waves are captured between long floating concrete reflectors that will focus the wave into a floating reservoir. From the reservoir, the water flows back into the sea through water turbines.

Model testing aimed at investigating the loads and motions between the floating reflectors and the reservoir. Tests will be carried out at AUC.

· Wave mill (horizontal axis)

Principle: The converter has the shape of two horizontal cylinders rotating in opposite directions. The cross section of the cylinder is shaped as a water wheel with L-shaped vanes. The weight of the water on one side provides a downward force and air trapped in the opposite site provides an upward force; these forces will turn the cylinder.

Model testing at DHI has been planned in order to measure the energy absorption.

· Wave mill (vertical axis)

The project comprises a vertical tower with flexible wings mounted just below the surface of the sea. The lower end of the tower is mounted to a reference plate providing sufficient resistance to the vertical motion of the device. As waves pass by, the wings will rotate in a horizontal plane and turbines mounted at the damping plate will be activated by the flow and will convert the absorbed energy into electricity.

· Optimisation of ramp geometry for wave energy utilisation

The project relates to both fixed and floating structures using the wave run up principle for wave energy conversion, such as the Norwegian TAPCHAN and the two Danish projects: The Wave Plane and The Wave Dragon. The project will investigate how the vertical and horizontal cross section geometry of the ramp and the draught and freeboard of the ramp will influence the conversion efficiency in relation to the incoming waves.

7.2 Further R&D

· Point absorber survival testing.

Principle: A float is connected to a suction cup anchor with a flexible polyester rope. Between the rope and the float, a hydraulic actuator pumping fluid is inserted into a high pressure hydraulic accumulator. The return stroke is provided by hydraulic fluid from a low pressure accumulator. As waves activate the float, a pressure difference between the high and low pressure accumulator builds up. This pressure difference drives a hydraulic motor and generator. The limited stroke length calls for a mechanical end stop able to absorb the energy of the floats in conditions with large waves. The proposed end stop is based on rubber fenders. The end-stop is activated by a guide rod inserted between the actuator and the rope. Survival tests have in June 1998 been carried out at DMI in scale 1:25, and a design basis for the system established.

· Swan DK3

Principle: The project is a co-operation between a Danish company and Commander Y. Masuda. The project aims at further optimisation of the Backward Bend Duct Buoy, the BBDB system (4). To enable comparison with previous tests carried out in China, a geometrically identical model has been built. The project will investigate if additional ballast tanks can improve the bandwidth of high efficiency. Tests are ongoing at DHI.

7.4 Supporting projects and dissemination

A number of projects aiming at dissemination of results and wider understanding of problems related to wave energy have been supported. During the first half year of the programme these projects are:

· mapping of the wave conditions in the Danish part of the North Sea;

· film about the wave energy programme;

· information folder on wave energy systems;

· exhibition of wave energy systems.

8. FUTURE PERSPECTIVES

Based on the results achieved during the first two year period of the programme, a continuation will be considered. It is expected that there will be a demand for continued R&D support for wave power development.

In the development of wind energy systems, the establishment of test facilities helped to guide the development towards realistic commercially reliable products.

In the next phase of the wave energy programme, some wave energy converters will hopefully be ready for construction as prototypes. At this stage it is expected, that the establishment of an open sea test site for prototypes will be required to help the provision of practical experience. Such an open sea prototype test site might possibly be combined with offshore wind farms.

Results ultimo 2001

9. ACKNOWLEDGEMENT

The preparation of the paper has been sponsored by the Danish Energy Agency. The guidelines for standardisation of testing and measurements have been made by Danish Hydraulic Institute, Danish Maritime Institute and Department of Civil Engineering, Aalborg University.

REFERENCES

(1) Hanstholm Phase 2B, Offshore Wave Energy Test 1994 - 1996, Danish Wave Power, November 1996.

(2) Energy for Sustainable Development Niels I. Meyer, Department of Buildings and Energy, Technical University of Denmark. Proceedings of International Conference on Human Resources and Future Generations in Islands and small States, Malta, November 1997

3) Wave conditions at Hellingsø Teglværk, Nissum Bredning, Februar 1998, Danish Hydraulic Institute. (In Danish)

(4) Y. Masuda et al. Regarding BBDB Wave Power Generating Plant, Proceedings of The Second European Wave Power Conference, November 1995, Lisbon, Portugal.

ABSTRACT