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Tidal Energy System for Shallow Marine Environments 0

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Contents

1. Introduction

2. Overview of Potential Market0

3. Microgen Technologies ltd Tidal Energy System 0 0

for Shallow Marine Environments (TESSME)

4. The Way Forward

1. Introduction

The ongoing difficulty in producing and finding economical petroleum reserves and the need to address the influence of climate change is moving to the forefront of national policy on energy in many developed economies particularly in Western Europe. It has been widely publicised and acknowledged that this presents a tremendous growth opportunity for renewable energy developments and will eventually lead to a proliferation of renewable energy systems competing with fossil, conventional renewable and nuclear power generation. The UK is in a unique position to exploit the market for offshore tidal power as the resource is abundant offshore UK in a wide range of tidal marine environments. The offshore acreage surrounding the UK presents tremendous opportunities for the development and harnessing of tidal energy. However a number of key criteria are required for these developments to be economically viable:

i. Power output justifies significant capital investment

ii. Facilities are accessible for maintenance in a harsh environment

iii. Reliability and availability of installation has to be much higher than current onshore or above wave technologies

The current configuration of tidal turbine technologies is very much focussed on deepwater environments, though there are some technologies in the market place that are suitable for relatively shallow marine environments. In terms of shallow marine environments for the purpose of this paper we refer to environments where the water depth is 35m or less and particularly where current and future offshore windfarms are sited in areas of potential tidal resource that can be exploited in tandem with Microgen Technologies ltd patented system for tidal power conversion. We call this system TESSME, the acronym derived from the concept being developed to convert tidal energy in shallow marine environments, Tidal Energy System for Shallow Marine Environments.

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2. Overview of Potential Market0

This is a brief overview of the existing technology employed to date primarily in the UKCS by way of example. However these models are applicable to similar marine basin environments in other countries, e.g. Denmark or the Netherlands by way of example.

2.1 Near Shore Tidal Current Concepts (Including wave energy)

No full scale commercial tidal energy scheme exists in the UK where submersible devices are employed on a large scale in significant multiple device arrays. A number of devices have been evaluated at EMEC in Orkney leading to development of full scale single 0

device prototypes but no large scale multiple device commercial system to date.0 0

All of the devices tested to date employ a submerged generator driven by an impeller with efficiencies gained through the mechanical systems connecting the turbine impeller to the final drive system and generator.

2.2 MGTL Concept0

The concept that we have developed has some similarities to the previously cited developments but more conclusively has clear distinct differentiators that we believe make our system more highly competitive in terms of its long term reliability and lower capital cost:

2.2.1 Use of Centrifugal Pumps in a Hydraulic Circuit

A number of concepts that have reached the development stage in the industry utilise high pressure positive displacement pumps, usually coupled to a Pelton wheel type turbine as the final conversion between fluid energy and the generator. It is our view that positive displacement pumps do not offer a good solution to using a hydraulic circuit driven power generation system.

The MGTL concept utilises centrifugal pumps coupled in series to generate high volumetric flowrate coupled with substantial pressure elevation. This allows the siting of a single large power generation turbine within the circuit where the series connected 0turbines drive the fluid within the circuit. Each individual turbine contributes to 0 0

generating a build up of fluid pressure in a closed loop. The reason why this configuration is chosen is to maximise energy conversion from the tidal stream and employ a staged pressure elevation within the circuit to drive a high pressure, high efficiency turbine. The velocities in the tidal stream across the spread of tidal stream turbines is relatively 0 0

uniform and will hence provide a uniform flow through the circuit with each turbine 0 0

elevating the pressure in the circuit to drive a high pressure fluid turbine coupled to the 0

generator.

2.2.2 Use of High Pressure Turbine

A number of renewable energy concepts using a hydraulic circuit of some description to drive a power generation turbine do not employ the use of a high pressure turbine, the 0 0

system final power output device employed is a Pelton wheel turbine due to the intermittent pulsing of the devices in the circuit. In the TESSME device fluid pressure is 0 0

developed in a smooth continuous output to aid energy conservation and at the same time provide a smooth generation output with higher voltages than generated by existing tidal devices. The smooth delivery of fluid power in this system eliminates a number of issues 0

when using vertical risers and retrofitting to existing structures like with turbine monopiles in that it does not add to the cyclical fatigues that these structures suffer from when grouted foundations are employed. This is a key advantage of this system over wave conversion technologies when used in combination with existing monopile structures.

2.3 Combination with Wind Power

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As illustrated in the figure above the COmbined Wind And Tidal (COWAT) system concept developed by Microgen Technologies ltd employs a series of tidal stream devices linked in series to develop hydrodynamic pressure sufficient to drive a power generation turbine in a flow loop. The hydroelectric turbine is a compact device mounted on the wind turbine support structure and this provides a number of distinct advantages that will be described in the following sections.

3. Microgen Technologies ltd Tidal Energy System for Shallow 0

Marine Environments (TESSME)

3.1 Overview

In 2008 MGTL identified a concept that filled a gap in the technology applied in tidal power generation. The concept basically involved linking series connected tidal stream 0

turbines transferring tidal flow into fluid energy and utilise this to drive a power 0

generation turbine. The power generation turbine being driven from the discharge of the final tidal turbine stage with the pressure elevated to its highest level. This obviously in 0

hindsight appears to be a simple concept. However to make this a viable industrial power generation concept required much further refinement and employment of engineering experience. This experience parallels experiences in oil and gas sector in fluid energy 0

conversion technology coupled to knowledge of subsea engineering and intervention 0

expertise is required. The system referred to as Tidal Energy System for Shallow Marine Environments (TESSME) also has the distinct advantage in that it can easily be integrated with offshore wind turbine infrastructure to take advantage of available tidal energy at the site if in sufficient abundance.

3.2 Principles of Operation

The TESSME system in essence is a collection of series connected tidal stream turbines 0

in a connected flow loop with motive force coming from a directly coupled impeller 0 0

driven by the tidal currents in the body of water in which it is immersed. The discharged pressure and flow is converted to electrical energy in a high pressure turbine connected to the discharge of the final turbine section. Where this offers advantage over a system with 0

an impeller directly coupled to the generator is in cost and system access and reliability.

3.2.1 Flow Loop Design

The flow loop design and operation is critical to the success of this concept. Key factors that need to be addressed in the flow loop are:

1. Avoidance of entrained solids.

2. Avoidance of corrosive fluids.

3. Avoidance of environmentally harmful substances.

4. Flexibility and corrosion resistance

5. Ability to contain high pressures.

All of the above requirements can be satisfied by this concept using existing materials and readily available technology typically employed in the subsea sector of the oil and gas industry.

The use of a self contained flow loop also allows for the turbine generator itself to be mounted at sufficient height above water surface therefore access to electrical systems and switchgear is in a totally dry environment. This is particularly relevant in minimising the subsea cable network that would be present in multiple submerged devices where the generator is directly coupled to the tidal turbine impeller.

Installation procedures in the construction phase are not dissimilar to those employed in other submerged devices the main difference being that in the combined wind and tidal configuration the power generator is mounted above the waves and the electrical connection and switchgearing is less complex and technically challenging than in a subsea environment. Other alternative scenarios can be utilising a single larger structure with multiple generators connected to a large array of submerged tidal turbines over a wide area. This structure could also serve a transformer station for a nearby windfarm as an example.

3.2.2. Energy Conservation

The key relationship in a tidal flow system is that the power available is by converting the linear energy flow from the tidal stream in to rotational energy of the impeller.

This is the same for practically all systems whether the impeller is connected directly to a generator or as in our case a pump. In our case the tidal energy available to the pump is the same as that available to an electrical generator driven by a similarly sized impeller.

E Available = E Turbine + Losses0

Tidal energy available = E Available

Energy transferred to fluid by turbine = E Turbine0 0

The energy at the turbine is converted to head/pressure and flow. Because the flow loop 0

is confined the flow is constant for all components in the system, i.e. each turbine 0produces the same flowrate and this in turn is the flowrate through the power generation turbine section. Energy increase across the system at each stage is created in the form of system pressure, each turbine systematically generates an increase in pressure and this 0

pressure is dissipated in driving the high pressure turbine stage.

Overall equation

Fluid Power = ∑n (Pressure elevation x flowrate)n – Frictional flow losses

This is the power available to drive the turbine section of the hydroelectric generator.0

Other losses that occur in the system are through the conversion of fluid to mechanical power in rotating the turbine and further conversion to electrical energy.

At this point the view may be formed that this system is vastly inefficient compared to competing technologies. However the losses are minimised and offset compared to competing systems by:

• Low inertia of the tidal turbines employed0

• The ability to drive the electrical power generation turbine at high speed

• Transmission losses reduced through

o Reduced electrical cable network from generator to transformer

o Higher generator voltage reduced transformer losses.

Furthermore where the generator is mounted above surface on a wind turbine support or other suitable structure it allows for minimal head losses due to the elevation head losses at turbine intake being offset by the higher net positive suction head pressure at the first turbine unit intake and therefore no significant hydraulic loss penalties from mounting the generator and main electrical equipment above water surface.0

Figure 3.2.1 Configuration of wind turbine monopile and COWAT hydroelectric generator above splash zone

3.3. Reliability

The reduction in the number of generators located below the waves significantly increases the inherent system reliability. The system concept also reduces the capital expenditure dramatically as there can be complete elimination of subsea connectors if the generators are located above the sea level. The removal of a number of expensive generators mounted subsea and replacement of these with much less expensive and lower complexity fluid turbines leads to increased reliability due to reduce number of component links that 0

can fail. Additionally the elimination of subsea mounted generation equipment leads to vastly reduced capital expenditure.

The system in a new build windfarm offers considerable combination advantages with the wind turbine infrastructure and gives the obvious advantage that during periods of no wind the windfarm still provides considerable output via the tidal power generation component.

3.3.1.Offshore Access0

This system because it is designed to capture tidal energy from relatively shallow marine environments is fully accessible by divers either in air or saturation. However the system is also designed to be fully ROV maintainable for most activities.

3.3.2 Generator Access0

This is where the system has a significant advantage over the competition in tidal stream renewables. Because TESSME operates in a flow loop configuration the generator can be 0

mounted on a platform above the waves. The generator could also be mounted subsea

itself on the seabed along with the tidal stream turbines and this is a highly viable 0

alternative where the system is located where it could be a shipping hazard or visual impact has to be completely eliminated. In essence this system offers a level of flexibility not offered by other tidal and wave power generation systems in development or 0

deployment and currently at advanced prototype. Where the TESSME system could have application using an example in the UK is in the East Irish Sea. The Liverpool Bay and Morecambe Gas fields are already a significant infrastructure in this area and the tidal energy is significant, in essence this system could be integrated with the existing oil and gas facilities in this area both providing power for oil and gas extraction as well as transmission of power to either a land fall power connection to grid on Lancashire or North Wales coast lines. A number of small platforms could be used to house generators and subsea control equipment for control and power capture from a large number of TESSME tidal stream turbines mounted on the sea bed.

However for the purposes of describing the COWAT system the focus is on the combination of access to current wind turbine infrastructure designs. Access is achieved by boarding the wind turbine structure using conventional means for personnel, e.g. boat transfers or in rough weather via the helicopter winching platform as is employed today. The generator and turbine can be built in a modular unit and can be retrieved easily by a conventional small jack up unit with suitably rated crane and replaced by a new unit.

3.3.3 Subsea System Access

Access to the submerged impeller pump system is obviously more difficult as these are located subsea. The final design will incorporate a complete ROV retrieval and replacement system to allow removal in a slack tide and also replacement. Diver intervention may be required for minimal tasks that could not be performed by an ROV.

3.3.4 Tidal current Direction Variation0

As for any situation involving renewable energy devices the local geographical conditions need to be taken in to account, e.g. effect of sea bed features and tidal current direction and speed variation to name a few examples. With regards to tidal current variation we have a number of methods readily available for practical application to address this that are unique to our system and potentially of use by competing technologies, hence we do not wish to disclose this at this point suffice to say whatever the type of tidal direction variation that could be encountered by our technology we have a number of solutions to 0 0

accommodate this challenge practically and effectively if the site location requires the tidal stream devices to yaw for optimal safe tidal stream direction changes.0

3.3.5 Adverse Flow Patterns

A key issue in siting a tidal stream device at an offshore windfarm is that the wind turbine monopile structures will themselves create wakes and these can propagate and dissipate the tidal stream energy. The COWAT system will have the sea bed array modelled for these patterns using CFD techniques to site the tidal stream turbines optimally. The largest challenge is in windfarms with square pitch arrays. For new build installations employing a square pitch array the wind turbines and tidal turbines will be sited in an optimal configuration for wind and tidal combined power output. In a retrofit system there will be suboptimal tidal stream turbine siting but this can be offset by the significant base load addition to the overall power output of the site.

Where wind turbines are sited in a linear or curved line array pattern this is envisaged as presenting no issues for the siting strategy for the tidal stream turbines. An additional

advantage of the system is that it can operate with some tidal stream turbines acting unidirectionally, i.e. will only engage the final drive when the tide is in a favourable direction and will disengage where the tidal direction will lead to turbulence created by upstream structures in its path.

3.4 Performance Considerations Compared to Competing 0

Technologies.

3.4.1. Direct coupled impeller to generator0

A number of systems close to commercialisation or at advanced stage of development employ this concept and it does have advantages, however we believe the advantages of the TESSME system outweighs this.

Advantages

• Low power losses from tidal stream to electrical power output.

Disadvantages compared to TESSME system

• Every impeller is coupled to a generator, excessive cost compared to TESSME 0fluid to power turbine combination

• Generators have to be mounted to subsea. Ingress protection standard raises capital costs.

• Generator speed constrained by impeller speed via gear box. TESSME generator section can operate at much higher speeds and voltage output, reducing transformer losses.

• Multiple subsea cable connections in a network are required. TESSME can completely avoid subsea cable jointing/connectors when generator is surface mounted and vastly minimise these with generator mounted subsea.

3.4.2 Indirect Coupling Between Impeller and Generator0

The TESSME concept falls in to this category, and whilst we are not aware of any technologies that are near commercialisation we did encounter a number of patents for systems attempting to achieve a similar outcome but missing some of the fundamental fluid machine principles our system applies correctly. All we can state at this stage 0 0

without speculating on systems that bear no potential for commercialisation is that our TESSME technology employs proven engineering principles and can apply these to existing available technology in a novel and efficient manner, the COWAT application being a prime example of this

4 Strategic Technology Plan0

In the UK with imminent energy deficiency and ever increasing costs of carbon emissions coupled to fluctuating oil prices the case for subsea tidal energy development can be very 0 0

robust. However the cost involved in developing and installing subsea systems whether it be renewables or traditional oil and gas industry applications is high. The TESSME/COWAT system offers significant inherent cost advantages in its design simplicity in that:

i. The use of fluid power turbines in place of generators driven by tidal stream turbine 0

reduces the number of highly expensive subsea mounted generators.

ii.The use of a hydraulic flow loop allows the main generator to be mounted either 0

seabed or above wave with little penalty on power loss.

iii.This system offers a number of advantages in cost of installations compared to other technologies.

The key advantage of this system is that it can be a significant enhancement to the numerous offshore windfarms already constructed or in planing phase around the coasts of the UK and other European nations offering favourable tidal environments but not sufficient to justify a tidal array in place of a windfarm.

The next key step for the TESSME system is to move to a prototype test in a marine environment. The test system would be suited to installation and testing at EMEC similar to other concepts that have been tested at EMEC as it is expected to operate effectively in 0

similar marine tidal environments.