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Harnessing Offshore Renewable Energy

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Windmill turbulence in the North Sea. Credit: OTC 29672 (Photo courtesy of Vattenfall).

In recent years, much progress has been made harnessing offshore renewable energy (ORE)—wind, wave, and current—for use in electrical supply. The complete paper is aimed at giving the newcomer to ORE, both with regard to wind and marine-hydrokinetic (MHK) devices, a basic understanding of the subject. The complete paper focuses on physical and technical issues and does not detail financial aspects of such projects. 

Introduction

For any offshore development, especially an ORE project, a specific site investigation is required to qualify environmental, geophysical, metocean-related, and geopolitical issues. Most ORE developments will cover a significant area of ocean or seabed and will require investigation to ensure that marine life, antiquities, unexploded ordinance, and other ocean users will not be put at risk when installation and operation activities are performed.

Obtaining the required permits and approvals from all those potentially affected by an ORE development is a complex and time-consuming operation. All stakeholders connected with a development must be considered because the installation could be in position for 30 or more years.

A summary of the types of wind and MHK devices is presented in Table 1 of the complete paper.

Offshore-Renewable and Power-Conversion Device Requirements. Fig. 1 identifies the components needed for an ORE development to be able to generate clean electricity and feed it into a grid. The term “clean” is used to define the consistency of the power being produced. Electricity coming from a device driven by a force of nature may be subject to fluctuations because wind, wave, and current forces are not always constant. In most parts of the world, strict limits are in place on power variances acceptable to both utilities and end users regarding input into a grid. Therefore, in any power system, the ability must exist to either produce clean power from the source or condition the power to make it acceptable.

Fig. 1—Overview of offshore-renewable development components.

An example of clean power from its source could be that of the classic hydroelectric dam. A somewhat constant head of pressure and flow is applied to a turbine that generates electricity. The pressure and flow rates are somewhat constant. Therefore, the resulting electricity from the system will be somewhat constant. Compare this to a wave-driven vertically oscillating device that must contend with waves of varying heights and period (time between waves). In that case, variance in produced power can be considerable.

Achieving the ability to transform power levels on space-constrained offshore facilities can be challenging. Other issues are appearing as more units are installed. Subsea wet cable connection is an area for further study. The dynamic nature of floating solutions is challenging for power transmission from moving units. Fatigue also is a serious design consideration.

Multiple numbers of generation units are usually required in a wind park or MHK farm. Quantities between 50 and 100 units are common, and space between units is required so that turbulence that could affect the efficiency of the adjacent units is not created.

Wind Devices. Offshore wind resembles that found on land in its bottom-founded or fixed design. A rotor is connected to a turbine in a housing, the nacelle, which sits on top of a tower or pylon. The nacelle can rotate and positions the blades to face into the wind upstream of the tower. Three-blade rotor designs are the most common, a result of function and cost. The blades are usually in front or to windward of the tower so that the blades are not sheltered from the wind as they rotate. The blades can have variable pitch for efficiency in use of available wind. Blades often rotate at approximately 10 to 25 rev/min and can be governed not to exceed certain blade-tip speeds.

Bottom-Founded Wind. Bottom-founded wind devices are attached to the seafloor with a fixed structure, depending on the water depth. Fixed monopile structures are used for depths of the 50- to 60-ft range; jacket structures usually are used for depths ranging from 60 to 300 ft. The structure design is similar to that used in oil or gas platforms in similar water depths.

Floating Wind. The best offshore wind is in water depths beyond the reach of fixed, bottom-founded structures. This can account for 60 to 80% of available wind. The first floating wind park, Equinor’s Hywind Scotland, east of the Aberdeenshire coast is returning output 50 to 60% better than similar fixed wind farms or parks.

Projects in the precommercial range allow the gaining of experience for both manufacturing and installation practices and protocols. In this aspect, multi-unit costs can be estimated; however, cost is not the main focus. Development of processes, logistics, and project management are honed to achieve practical and achievable expense goals.

At the time of writing, prevalent floater hull shapes are similar to those seen in oil and gas, including spar, semisubmersible, tension-leg platform, and barge types. Some concepts are a hybrid of several designs.

Wave Energy Conversion (WEC). Waves have three components of movement: vertical heave, horizontal pitch, and horizontal sway. Numerous WECs in a multitude of designs are being developed and tested. Some are at the precommercial stage, but a few are at the commercial stage, with possible off-the-shelf offerings available.

In general, WEC designs use the vertical or horizontal components of a wave to create movement that can generate power. Some devices use all components and can therefore produce potentially more usable power. WECs can be installed close to shore, where the waves are somewhat turbulent, or further out to sea where the swell is less turbulent. Some WECs float on the water surface, using the movement of waves. Other WECs sit on the seabed and use the increase or decrease in pressure as waves travel overhead, converting that force to flow that runs a turbine.

Ocean Thermal Energy Conversion (OTEC). This approach uses the temperature difference between warm surface water (approximately 25°C) and cold deep-ocean water (approximately 5°C). The approach is currently regarded as suited well to equatorial areas. These systems can be either closed, using a low-boiling-point liquid, or open, using low or negative pressure.

For closed OTEC systems, a medium with a low boiling point, such as ammonia, or a refrigerant such as that used in modern refrigerators, is used. The liquid can be boiled by heat provided by the warm surface water. The resulting vapor can be run through a turbine coupled to a generator to create electricity. Once through the turbine, the vapor is condensed using the cold water from the deep ocean. The resulting refrigerant liquid is pumped back to the heating side and the cycle runs again.

In an open OTEC system, the vapor medium is warm sea water, subjected to a negative pressure and boiling at a lower temperature. The water vapor drives the turbine and is then condensed back to water. This system can generate desalinated water, high-saline-content water, and electricity.

Chemical Reaction. At the time of writing, it is believed that two general types of electricity generation can use salt or high-saline-content water. One is an osmosis process that uses a membrane to attract fresh water to join salt water, with both separated by the membrane. This attraction leads to excess pressure or buildup behind the membrane; this can be converted to flow and used to drive a turbine.

The second type is a seldom-discussed but recently emerging technology that concerns saline-enabled batteries. It may be feasible to generate electricity by applying a strong saline solution to a specific cathode and anode arrangement. A reaction could then take place that generates electricity.

Power Takeoff and Power Export. Concepts of MHK devices are varied in their approach. Power takeoff, the method used to convert kinetic energy to usable mechanical energy, is an area of great interest to the renewable system designer. Power takeoff can be as simple as connecting a shaft to a propeller and then to a rotary generator. Alternatively, power takeoff could involve having the offshore MHK device pump water or fluid into a pressure-containment reservoir equipped with an air-filled bladder. In another scenario, water could be pumped from the offshore device, using the wave or flow MHK resource, to a reservoir on land, such as a lake or lagoon at an elevated position.

Power export concerns the movement of power from the offshore device to a point of use. This can be an electrical cable or conductor. 

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29672, An Overview and Awareness Briefing for Offshore Renewable Energy, Wind, Wave, Flow, Hydrokinetic, and Thermal Converters by Roger Osborne, Ocean Flow International, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed.

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