Existing Wave Energy Converters Assessment Engineering Essay

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Chapter 4

4.0. Introduction

Most current wave energy converters are in the design phase, testing phase or early commercial phase. The leading and more mature technologies abroad as well as local WECs from Mauritian inventors were assessed. The assessment process was made more challenging due to the reluctance of companies to provide more detailed engineering information regarding their WECs in order to protect their Intellectual Property rights.

4.1. Assessment Criteria

The criteria used for assessing the WECs are quite similar to that used by the Electric Power Research Institute (EPRI) in 2004 to assess offshore WECs. Below are listed the assessment criteria classified into different categories and subcategories:

Technical Issues

This category will mainly focus on assessing the design maturity and the functionality of the device. The devices will be analysed on these following issues:

Structural Elements

Power Take Off

Mooring

Survivability/Failure Modes

Performance

Installation and Deployment

Operation and Maintenance

Life Expectancy

Safety

Recovery

Cost

This category will deal with the capital investment needed to purchase the device from the manufacturer. This cost estimate does not include the cost for developing and establishing the required infrastructure (such as mooring, grid connection and power cables) on site, site deployment costs, maintenance and operation costs.

Development Status

This criterion will assess the readiness of the design to be deployed.

Environmental Impact

The environmental impact the device may have during operation or during the construction phase will be assessed.

4.2. Assessment of WECs from Leading Companies

In Table 4.1 are listed the WECs that were assessed.

Device Name

Manufacturer Name

Country

Website

Device Type

Pelamis

Pelamis Wave Power Ltd.

United Kingdom

www.pelamiswave.com

Attenuator

Wave Dragon

Wave Dragon ApS

Denmark

www.wavedragon.net

Overtopping

CETO

Carnegie Wave Energy Limited

Australia

www.carnegiecorp.com

Submerged pressure differential 

Table 4.1 Wave Energy Conversion Devices Assessed.

4.2.1. Assessment of the Pelamis

Figure 4.1 The Pelamis

(Source: Pelamis Wave Power Ltd.)

4.2.1.1. Device Description

Manufacturer's Description: The Pelamis is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints. The wave-induced motion of these joints is resisted by hydraulic rams, which pump high-pressure fluid through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power from all the joints is fed down a single umbilical cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single seabed cable.

4.2.1.2. Specifications

Device Name: Pelamis P1

Structure:

Overall length: 150m

Diameter: 3.5m

Displacement: 700 tonnes (including ballast)

Nose: 5m long, drooped conical

Power take-off: 3 independent power conversion units

Power Conversion Unit:

Power take-off: 4 x hydraulic rams (2 heave, 2 sway)

Ram speed: 0 - 0.1m/s

Power smoothing/storage: High pressure accumulators

Working pressure: 100 - 350 Bar

Power conversion: 2 x variable displacement motors

Generator: 2 x 157kVA / 125kW

Speed: 1500rpm

Power:

Overall power rating: 750kW

Annual output: 2.7GWh

Nominal wave power: 55kW/m

Hydrostatic power limiting: >6 - 7m significant wave height

Generator type: Asynchronous

System voltage: 3-phase, 415/690Vac 50/60Hz

Transformer: 950kVA step up to typ. 11kV or 33kV

Site Mooring:

Depth: >50m

Current: <1 knot

Mooring system: Compliant, slack moored

4.2.1.3. Technical Issues

Structural Elements

The body of the Pelamis is a cylindrical steel structure that can be relatively easily built using standard equipment and construction techniques available at most modern shipyards.

Power Take Off

The Pelamis snake-like structure is made up of 4 cylindrical sections connected by 3 hinged joints. The wave-induced motion causes these joints to power hydraulic rams which pump high pressure fluid through hydraulic motors via smoothing accumulators. The hydraulic motors will drive the electrical generators to produce electricity. The power is fed down an umbilical cable to a junction on the seabed connected to a single sub-sea cable to shore. The umbilical cable is also connected to a transformer found in the nose.

Figure 4.2

(Source: Pelamis Wave Power Ltd.)

Figure 4.3

(Source: Pelamis Wave Power Ltd.)

Mooring

The Pelamis is slack moored and as a result it acts against itself rather than the mooring to absorb power. The Pelamis is slack-moored at 3 points allowing it to turn into wave directions within its moorings constraints. There are 3 mooring weights resting on the seabed. The slack mooring system will add to the survivability skills of the Pelamis.

Figure 4.4

(Source: Pelamis Wave Power Ltd.)

Survivability/Failure Modes

The Pelamis is a narrow and flexible structure, giving it good survivability characteristics. It is also slack-moored which should decrease the stress on the mooring system in case of extreme conditions.

Performance

The Pelamis is rated at 750kW. Depending on the wave resource at the site location, output is typically 25-40% of rated power over the course of the year. The Pelamis is designed to generate power optimally in high frequency conditions (low periods) with maximum motion between its tubular sections. It is therefore be less suited to long period wave conditions.

For the wave climate at Blue Bay, with a wave height of 3.06m and a period of 9s, approximately 292kW of power will be produced for most of the time from a rated 750kW device, and the wave climate of Riambel, with a wave height of 2.14m and a period of 8.6s, approximately 147kW of power will be produced. These represent annual productions of 2,558MWh and 1,288MWh of electricity respectively.

Table 4.2 Pelamis Power Table

(Source: Pelamis Wave Power Ltd.)

Installation and Deployment

For installation at a site, Pelamis machines are towed or components are shipped by land or sea, for local assembly. There is no need for divers for the main installation work. However, there are sometimes installation work that require diver intervention during the initial installation, and during the quayside commissioning process.

Operation and Maintenance

Once Pelamis machines have been installed on site, operation of the machines is handed over to shore control.

For its maintenance, only modest facilities are required:

A quayside or pontoon that the Pelamis can be moored to

A shed storage

A small crane for lifting modular components out of the machine should they require replacement.

For maintenance purposes, the Pelamis is towed back to sheltered water for all maintenance actions to be carried out in safety on a quayside. No divers are needed for operation and maintenance purposes.

Life Expectancy

Pelamis is designed to last the life of the project, which is normally in the order of 20 years according to the manufacturer.

Safety

Project boundary markers, as stipulated by the governing navigational authority, will need to be installed prior to Pelamis associated equipment in order to delineate the area to be avoided by marine traffic. Usually cardinal marker buoys are the standard method to mark out the boundaries of offshore renewable projects. The sites are delineated with navigational markers with lights and radar reflectors. Wave Farm sites are obviously not located in shipping lanes. In the unlikely event that the Pelamis gets loose, the Pelamis has a GPS (Global Positioning System) that is monitored 24/7 so any apparent 'out of position' event would be immediately investigated.

Recovery

The Pelamis needs to be towed back to sheltered water for maintenance. The system is designed to avoid the use of human intervention. In both connection and disconnection, the only manual intervention is the connection of slack synthetic ropes on the surface prior to controlling the latching and unlatching mechanisms by remote Wi-Fi link from the tow vessel. Pelamis has successfully performed these operations in waves of over 2m significant height and is currently engineering developments to stretch the limiting weather criteria for recovery to 3.5m significant wave height.

4.2.1.4. Cost

The cost of a single device is reported to be at $2 to $3 million USD (2004 Prices) (ERPI 2004). Assuming an initial cost of $3million USD, the cost is $4000/kW. Taking into account inflation rates, 2011 prices should correspond to approximately $4,700/kW.

Calculating the electricity generation cost per kWh for the Pelamis

The total electricity generation cost of a project per kWh = per kWh construction cost + per kWh production cost

Assumptions:

Useful life = 20 years

Capacity Factor is assumed 0.40

Per kWh production cost is assumed zero

Calculations:

Per kWh construction cost = _______________Device Cost_________________

(kW Rating x Useful Life x Capacity Factor x 8,760)

= _______3,500,000____

(750 x 20 x 0.40 x 8,760)

= $0.067/kWh (2011 prices)

Per kWh production costs = ______________Production Cost_______________

(kW Rating x Useful Life x Capacity Factor x 8,760)

= assumed zero

The total electricity generation cost of the Pelamis per kWh = $0.067/kWh (2011 prices).

4.2.1.5. Development Status

The Pelamis is a promising and relatively mature technology which is one of the few WECs currently in the commercial stage. The first 'next generation' Pelamis P2 machine was constructed in 2010, currently in the first stages of deployment to be tested at the European Marine Energy Centre (EMEC) in 2011.

4.2.1.6. Environmental Impact

The Pelamis is one of the most environmentally friendly forms of electricity generation in terms of emissions during their operational lifetime. Initial life cycle analyses that have been carried out for Pelamis, taking into consideration energy usage in manufacture of the machine and its components as well as energy usage through its operational and decommissioning phase, indicates that a Pelamis machine operating in a good wave resource (40kW/m annual average wave energy level) will have an energy payback period of less than 20 months with a life cycle emission of approximately 25g/kWhr (Pelamis Wave Power Ltd., 2011). Under these conditions a Pelamis machine will offset the production of approximately 2,000 tonnes of CO2 from a conventional combined cycle gas power station each year.

4.2.1.7. Conclusion

This is currently considered as the most advanced and mature wave energy technology. The survivability skills of the Pelamis make it extremely attractive in terms of launching it in rough seas, the shape of the Pelamis shoul allow it to survice even the most extreme conditions.

4.2.2. Assessment of the Wave Dragon

Figure 4.5 The Wave Dragon

(Source: Wave Dragon ApS)

4.2.2.1. Device Description

Manufacturer's Description: The Wave Dragon is a floating slack-moored wave energy converter of the overtopping type. It basically consists of two wave reflectors focusing the waves towards a ramp. Behind the ramp there is a large reservoir where the water that runs up the ramp is collected and temporarily stored. The water leaves the reservoir through hydro turbines that utilise the head between the level of the reservoir and the sea level. This results in a three-step energy conversion:

Overtopping (absorption)

Storage (reservoir)

Power-take-off (low-head hydro turbines).

Figures 4.5 and 4.6 show the Wave Dragon and its operating principles.

Figure 4.6 Operating Principles of the Wave Dragon

(Source: Wave Dragon ApS)

The main components that make up a Wave Dragon are:

Main body with a doubly curved (elliptical + circular) ramp.

Two wave reflectors in steel and/or reinforced concrete.

Mooring system.

Propeller turbines with permanent magnet generators.

4.2.2.2. Specifications

The physical dimensions of a Wave Dragon are optimised to the wave climate at the deployment site as shown in Table 4.3.

Table 4.3 Wave Dragon Dimensions According to Wave Climate

(Source: Wave Dragon ApS)

4.2.2.3. Technical Issues

Structural Elements

The main body or platform is one large floating reservoir made of a combination of reinforced concrete and steel plates. The Wave Dragon needs to be large and heavy to ensure stability of the structure, in order to obtain the desired weight, water ballast is added. The water reservoir is located on the top of the Wave Dragon's main body.

To maximise water overtopping efficiency a combination of wave reflectors are attached to the main body are two wave reflectors also made of a combination of reinforced concrete and steel.

Power Take Off

The Wave Dragon has only one kind of moving parts: the turbines. This is an advantage the Wave Dragon possesses, the lack of moving parts will mean less chance of failure. Propeller turbines are used in the Wave Dragon. As the Wave Dragon's turbines will rotate with a variable and low speed, permanent magnet generators are used. In this way no gear-box is needed, thereby reducing both losses in power and maintenance costs significantly.

Mooring

The Wave Dragon is a floating slack-moored WEC. The device needs to be slack moored as its floating height is adjustable. The Wave Dragon is constructed with open-air chambers where a pressurized air system makes the floating height of the Wave Dragon adjustable. This is used to adjust to varying wave heights as overtopping efficiency depends on choosing the right ramp height. Figure 4.7 and 4.8 show the mooring configuration of the Wave Dragon.

Figure 4.7 Side View of the Mooring System of the Wave Dragon

(Source: Wave Dragon ApS)

Figure 4.8 Top View of the Mooring System of the Wave Dragon

(Source: Wave Dragon ApS)

Survivability/Failure Modes

The Wave Dragon is essentially a large floating platform. Being freely floating, there is little concern that the device will fail under extreme conditions. The worst case would be that the mooring system fails and the device breaks free and starts drifting. Backup mooring lines should be installed to prevent the device from drifting away in case of failure.

The Wave Dragon has multiple Kaplan Turbines running in parallel; the device will most likely have a very high reliability rating. Even in case that one turbine fails, the device will continue to produce power. Critical elements from a survivability perspective are the moorings system and the structural integrity of the body.

Performance

According to the manufacturer, one Wave Dragon unit will produce electricity corresponding to the wave climate present:

Wave Climate [kW/m]

Annual Power Production [GWh]

24

12

36

20

48

35

60

43

72

52

Table 4.4 Annual Power Production According to Wave Climate

(Source: Wave Dragon ApS)

The above information has been plotted in Figure 4.9 to be able to obtain an approximate power production estimation for local wave climates. With wave climates of 46.6kW/m at Blue Bay, expected annual power production from a rated 11MW unit will be approximately 32GWh. The wave climate of 21.7kW/m at Riambel will produce approximately 10GWh (if we consider the plot as a linear line in Figure 4.7) from a rated unit of 4MW.

Figure 4.9 Annual Power Production According to Wave Climate

Installation and Deployment

The installation may be tricky as it is an extremely large device. The deployment should be done in calm sea conditions. Multiple tugs will be required for towing operations.

Operation and Maintenance

Most of the operation and maintenance activities should be relatively easily carried out on the device itself as the Wave Dragon is a large and stable platform. Access to the platform can be done by boats or even helicopters during extreme weather.

Life Expectancy

Most wave energy devices have a life expectancy of at least 20 years. The most critical component that has the highest risk of failure is the mooring system.

Safety

Cardinal marker buoys are the standard method to mark out the boundaries of offshore renewable projects. It will be deployed out of the way of major shipping lanes. In case the Wave Dragon breaks free from its restraints, the device should be equipped with a GPS so that rapid intervention may be carried out before any more damage occurs.

Recovery

The main structure and the two wave reflectors will need to be disassembled offshore then towed separately into a nearby port/shipyard.

4.2.2.4. Cost

The cost for a single 4MW unit is estimated to be in the range of $10 - $12 million USD (2004 prices) (ERPI 2004). This is just the device cost. Mooring and electrical interconnection are not included. Assuming a cost of $12million USD, the price per kW is $3000 per kW (2004 prices). Taking into account inflation rates, this should correspond to approximately $3,500/kW in 2011 prices.

According to the manufacturer, in a wave climate of 24kW/m, an electricity generation cost of €0.052/kWh is expected and in a 36kW/m wave climate the corresponding cost of energy will be €0.04/kWh. Assuming an exchange rate of 1 Euro is equal to $1.4 USDs, the expected generation costs in USDs corresponds to $0.073/kWh and $0.056/kWh respectively in 2011 prices.

Calculating the electricity generation cost per kWh for the Wave Dragon of a 4MW Device

The total electricity generation cost of a project per kWh = per kWh construction cost + per kWh production cost

Assumptions:

Useful life = 20 years

Capacity Factor = 0.34

per kWh production cost is assumed zero

Calculations:

Per kWh construction cost = _______________Device Cost_________________

(kW Rating x Useful Life x Capacity Factor x 8,760)

= _______14,000,000____

(4000 x 20 x 0.34 x 8,760)

= $0.059/kWh (2011 prices)

Per kWh production costs = ______________Production Cost_______________

(kW Rating x Useful Life x Capacity Factor x 8,760)

= assumed zero

The total electricity generation cost of a 4MW Wave Dragon per kWh = $0.059/kWh (2011 prices)

4.2.2.5. Development Status

The Wave Dragon is still in the demonstration phase with a 7MW demonstrator project in Wales currently delayed due to the recent financial crisis.

4.2.2.6. Environmental Impact

The Wave Dragon has relatively little environmental impact. The biggest impact the Wave Dragon has on the environment is through its mooring system installation and the underwater cables deployment. The effects are negligible. Marine life may also be physically harmed as they go through the turbines.

4.2.2.7. Conclusion

The main issue with the Wave Dragon is that it is a low efficiency device as most of the energy contained in the waves is dissipated as they climb the ramp to finally go through turbines with only the potential energy gained converted to kinetic energy. The survivability of the Wave Dragon should be of little concern if the mooring is well overdesigned as well as being equipped with backup cables in case of failure.

4.2.3. Assessment of the CETO

Figure 4.10 A CETO unit operating offshore at Fremantle, Western Australia

(Source: Carnegie Wave Energy Ltd.)

4.2.3.1. Device Description

Manufacturer's Description: Unlike other wave energy systems currently under development around the world, the CETO wave power converter is the first unit to be fully-submerged and to produce high pressure water from the power of waves. By delivering high pressure water ashore, the technology allows either zero-emission electricity to be produced (similar to hydroelectricity) or zero-emission freshwater (utilising standard reverse osmosis desalination technology). The system can also be used for co-production of zero-emission electricity and freshwater. It also means that there is no need for undersea grids or high voltage transmission nor costly marine qualified plants.

Figures 4.11 and 4.12 show the principle of operation of the CETO device.

Figure 4.11 CETO Power Schematic

(Source: Carnegie Wave Energy Ltd.)

Figure 4.12 CETO Fresh Water Schematic

(Source: Carnegie Wave Energy Ltd.)

4.2.3.2. Specifications

Device Name: CETO III

Buoyant actuator(BA)

The BA is the energy collection system of the CETO technology. It is a spherical structure 7m in diameter and 5m high It is manufactured primarily from steel and rubber and weighs around 25 tonnes. The buoyancy is provided by both internal fixed and variable buoyancy. In operation, the BA will typically sit one to two metres below the ocean surface. The BA contains a proprietary system within it to reduce energy in very high wave energy climates

The CETO device operates in waters of depth >25m.

Figure 4.13 shows a more detailed view of the CETO III device.

Figure 4.13 CETO III

(Source: Carnegie Wave Energy Ltd.)

4.2.3.3. Technical Issues

Structural Elements

CETO units are manufactured from steel, rubber and hypalon® materials. These materials are all suited for use in marine environments.

Power Take Off

The BA pumps water onshore that goes through Pelton turbines to produce electricity.

Mooring

The CETO units are permanently anchored to the sea floor to allow for better resistance to wave motion thus capturing more energy compared to slack moored WECs. The construction of the foundation on the sea bed may be a bit tricky as it involves high cost and challenging civil engineering works.

Survivability/Failure Modes

The CETO technology should have no issues with survivability as they are completely submerged. It should survive even the most extreme conditions.

Performance

The manufacturer confirmed that the prototype successfully produced electricity as well as desalinated water.

Installation and Deployment

The construction of the foundation may involve local site considerations such as ocean floor properties. Each site may require customisation of the mooring system. The device will probably be towed to the site and with controlled buoyancy the device will position itself at the right depth so that divers may attach the device to the foundation.situate a

Operation and Maintenance

Divers would most probably be needed to undertake any maintenance work.

Life Expectancy

Most wave energy devices are expected to last at least 20 years.

Safety

Marine safety markers will be located around the devices.

Recovery

The device must first be detached from its mooring system then brought into port by towing.

4.2.3.4. Cost

There are currently no cost estimates for the cost of a single device.

4.2.3.5. Development Status

Prototypes of 1/3rd scale have been tested with success in 2006. The commercial scale CETO III unit (third generation CETO) was unveiled in October 2010 and its testing in the waters off Garden Island in Western Australia. The company is currently undertaking the deployment of a single autonomous unit to be followed by a 2MW plant and a further expansion to a nominal 15MW. The company is also carrying out investigations for attractive sites to implement the CETO technology, Mauritius being one of them.

4.2.3.6. Environmental Impact

The CETO device will not have any visual impact and it should even attract marine life although the construction of the CETO device may have implications on marine life.

4.2.3.7. Conclusion

The CETO technology is certainly a very attractive technology as it addresses two of the world's major issues: clean electricity production and potable water production. The survivability of the device should not be a concerned as it is a completely submerged device that should be protected and easily survive rough conditions. Although the construction of the foundation will have implications on marine life, the CETO technology remains an extremely environment friendly energy source.

4.3.6. Conclusion

Wave energy is still currently an immature technology, without a clear consensus on which are eventually likely to prove the successful devices. Additionally, in many countries there is a high cost associated with obtaining licences, gaining permits and carrying out environmental impact assessments, which small companies usually find difficult to meet unless they benefit from economic incentives. The protection of Intellectual Property for commercial reasons is also preventing the rapid progress of wave energy as research and ideas are mostly kept secret by most companies. Moreover, once deployed in free energy markets, wave energy has to compete with established renewable energy technologies that have benefited from decades of research and billions of dollars of cumulative investment. Table 4.5 shows a summary of costs involved for WECs assessed and it can be deduced that wave energy still remains an expensive way to produce electricity although it is expected that the costs will decrease significantly in the future.

Technology

Electricity Generation Cost [$/kWh]

Capital Cost per kW [$/kW]

Pelamis

$0.067

$4,700

Wave Dragon

$0.059

$3,500

CETO

N/A

N/A

Table 4.5 Summary of Costs Involved for WECs assessed.

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