Dr Thomas Mestl, DNV
May 26, 2011 | 9 Comments
In the next 10 years or so, technology trends onshore and offshore are expected to change significantly. Capacity onshore is typically in the range of 2.3-3 MW, constrained by the size (width, height, and length) that can be transported by truck. It's unlikely that future onshore turbines will become significantly larger, but they will, however, be smarter, and may be tailored for each specific location.
Today's offshore turbines are basically slightly modified onshore versions. Towards 2020, two different trends will emerge in offshore wind power: turbines will increase in size and they will be installed further from shore. In 2009, the average distance from shore for European wind farms was 12.8 km, but the planned UK Dogger Bank site is 125-195 km offshore.
Optimum control is key to smooth and safe running of a wind turbine and the latest systems can measure loads in each of the blades, and are used to smooth out loads in turbulent wind conditions. The same measurements can be used to calculate fatigue effects and can shut down the turbine if damage is critical. The data can also allow the operating strategy to be altered to maximise energy yield.
Operating individual turbines in wind farms with different operating rules requires smart sensor technology and complex control algorithms. Improved data transfer capabilities and decision support systems will enable centrally located centres to optimise operations.
In addition, wind turbines use blades that can be twisted (pitched) to suit the wind speed and to maintain the desired output. Faster responses could be obtained if the blade itself were to twist when the loads on it increase. This can be achieved by designing the turbine blades with some degree of sweep (like a curved sword).
Another solution is to orient fibres in the blade so that they twist slightly whenever bent. It is also possible to design 'smart blades' with active controls, i.e. blades that change their aerodynamic properties according to the measured loads.
More sophisticated blade designs are anticipated within the next decade. The challenge will be to prove that new designs, with limited field history, function satisfactorily over their operating life. Key aspects will be robustness and fatigue strength.
Fixed and Floating Foundations
At present, offshore turbines are limited to shallow water (20-30 metres), and most use a single, tubular, monopole foundation. For deeper waters, various 'jacket' structures that comprise several footings and are similar to (but smaller than) offshore oil and gas installations, will be developed. Optimised platform types and better understanding of loads and foundation design, will increase the viable water depths to about 50 metres.
Towards the end of the next decade, floating platforms will be used for wind turbines, enabling them to operate in almost unlimited water depths and where winds are best. Prototypes are being tested and several concepts are on the drawing board. One challenge is the need for complex dynamic cables to enable connection to the grid.
As floating wind turbines require deep water for connecting/mating of the nacelle to the support structure, they will require new installation methods. However, several areas with high wind energy potential, including the US and Asia, have shallow water close to shore, and therefore the mating operation will either have to occur at site, requiring complex and costly offshore operations, or a completely different installation method must be developed.
One new concept is to transport fully assembled turbines horizontally, by barge, from the fabrication yard to the offshore site. Once on site, the barge tilts 90 degrees through a ballasting operation, to release the turbine in a vertical position. Provided that challenges related to the up-ending and release of turbines are resolved, horizontal installation will be commercially available by 2020.
Most turbines use a gearbox to increase the generator speed, but they are prone to failure and increase the mass of the turbine. A number of manufacturers have replaced gearboxes with a single, large-diameter generator which, combined with a converter, can be connected to the grid but this is not without its own challenges. Both the cost and weight of this option currently exceed the more conventional gearbox designs. However, this approach is very promising, particularly if permanent magnets become cheaper and more powerful, lighter materials are introduced, and converters become more versatile. Direct drive options will become cost-competitive towards 2020, and are likely to become dominant.
Within a decade, many countries, especially in the EU, will experience significant challenges in managing variable output from wind and solar plants. Managing the uncertainty of renewables will be a key issue in the future design and operation of power systems. A long-term sustainable solution calls for inter-regional transmission highways (or supergrids), intraday markets, demand response, harmonised grid codes, and bulk and distributed energy storage.
Meeting EU 2020 renewable energy targets will entail more than 50 GW of wind power in and around the North Sea region. The winds there are highly correlated, in which moving wind fronts simultaneously hit large areas, leading to steep ramps of several gigawatt-hours.
Transmission enables wide area balancing through cross-border power exchange, greater market pools, and subsequent smoothing. However, transmission projects can take up to 10-15 years to complete due to long concession times and frequent public opposition. Electricity markets today are designed for dispatchable generation under relatively low load uncertainty. Going from the present 'day-ahead' to 'two-hour ahead' forecasting could cut uncertainty by 50 percent, a market evolution which is ongoing in many countries.
Another important measure is grid code requirements for variable renewables. Conventional power plants above a certain size are obliged to provide the full range of ancillary services, including frequency response, up/down regulation, voltage and reactive power regulation, and fault ride through. Although these services are essential for system stability, smaller power plants, such as wind and solar, are exempt from providing many of them. An increasing amount of renewable energy will thus challenge the power system.
Many variable-output plants are now obliged to provide voltage and reactive power regulation. Down-regulation of wind power to avoid overproduction is also being used. However, up-regulation of wind or solar will require continuous operation below available capacity. This measure is not in use today, but might be implemented in systems with a very high share of wind and solar power by 2020.
As large amounts of renewable energy will be connected to grids in many parts of the world, a significant increase in transmission capacity is essential. Supergrids, defined as wide-area transmission networks, connecting large geographical areas into a single, unified system, enable trading of high volumes of electricity over long distances and the leverage of production variability. In Europe, supergrids could enable the transmission of offshore wind in the North Sea, along with solar energy in the Sahara, to load centres in mainland Europe. In the US, the three non-synchronous areas can be more closely interconnected to improve security of supply and to facilitate integration of renewables.
The drivers for such grids are bulk transmission of power from production sites to load centres, large-scale integration of variable renewables, and lower use of peak power plants. Supergrids will use technologies such as high voltage direct current (HVDC), ultra high voltage alternating current (UHVAC), high temperature low sag conductors (HTLS), and flexible alternating current transmission system units (FACTS).
Operating the Supergrids
The successful operation of supergrids will involve stakeholders from different states and countries to a substantially greater extent than at present. A single operator should be responsible and will ensure increased utilisation of generation assets on, for example, a US or European level. This can be achieved by interconnections enabling levelling of loads due to different consumption patterns and time zones. However, an overlaying supergrid introduces potential dangers in terms of single outages that could affect power systems on an unprecedented scale. Therefore, this calls for the development of robust system controls, including the increased use of load-shedding, real-time monitoring and self-restoration.
By 2020, large offshore wind farm projects may be sited hundreds of kilometres from shore and produce power in the range of several gigawatts. Transmitting the output back to shore will require HVDC. The challenges of developing offshore HVDC grids towards 2020 are substantial, and related to AC/DC converter technology, development of meshed DC networks and DC circuit breakers, offshore substations, and subsea developments. Interconnection of countries with different regulatory regimes will also be a major obstacle to overcome.
In order to accommodate the EU 2020 renewable target, up to 40 GW of offshore wind power could be installed in the North Sea, requiring grid investments in the order of €11-28 billion.
The US Department of Energy estimates an offshore wind potential for the United States of 54 GW within 2030. In the US, the Atlantic Wind Connection project is one of the first steps towards interconnecting multiple offshore wind power plants using an offshore transmission grid.
In addition to technological challenges, regulatory issues such as operation of the offshore grid, cost coverage, and market coupling are major obstacles for interregional offshore grid developments.
Converters are used in HVDC systems, either to rectify or to invert the current. Voltage source converters (VSC) have been commercially available since 1997, but suffer twice the energy losses and carry a fifth of the capacity of traditional line commutated converters (LCC).On the other hand, their compact design means that VSCs are feasible for offshore platforms. Unlike LCCs, VSCs can be connected to weak or passive AC networks (low short circuit capacity) such as wind power plants and offshore oil and gas installations (loads). In addition, VSCs provide voltage control and black start capabilities.
VSC technology will pave the way for multi-terminal DC networks, enabling interconnection of wind parks to shore and trading links between countries. Within 2020, VSC energy losses will be comparable with those of LCCs (about 0.5 percent at both terminals).
Two-terminal (point-to-point) HVDC connections have been installed in many parts of the world, connecting asynchronous systems - systems with different frequency levels - and being used for bulk power transmission. An integrated offshore grid will require further development of multi-terminal HVDC (MTDC) technology.
MTDC will reduce the necessary number of converter stations, and therefore platform space offshore, and reduce subsequent energy losses. However, a MTDC system is very sensitive to DC faults; without a DC circuit breaker, the entire MTDC system would be shut down to clear a fault. Towards 2020, some smaller offshore MTDC networks, without DC breakers, will arise on a national level. Inter-regional offshore MTDC networks will not emerge until after 2020 due to the lack of inter-regional frameworks and long lead times.
In order to provide a comparable level of redundancy and reliability to today's AC networks, MTDC networks will require DC breakers capable of clearing DC faults within milliseconds. The intrinsic nature of AC results in fairly simple circuit breakers, breaking the current when it is close to zero. In order to be able to clear a DC fault, a DC circuit breaker must be able to break full power as there is no natural zero current crossing. DC breakers for HVDC are currently not commercially available. DC breaker prototypes for 2000A DC currents and 500 kV DC voltage have been successfully tested on an existing MTDC scheme. Solid-state, hybrid circuit breakers and forced commutation are possible solutions for DC breakers. For the DC breakers to add value, stringent requirements demonstrating high reliability of the breaker itself must be met.
Exploration of deep and ultra-deep water, oil, and gas fields, as well as government requirements for lower emissions through power supply from shore, will result in more electrical equipment being placed subsea. Voltage levels will typically increase from a couple of kilovolts to tens of kilovolts, and power capacities in the range of tens of megawatts. A major advantage of subsea power supply is reduction or elimination of topside equipment.
When locating land based equipment on seabed, one solution is air or gas filled modules with atmospheric pressure bringing along weight challenges, while another is modules filled with fluid challenging electric insulation and power electronics.
Towards 2020, numerous substations, of increasing complexity and size, will be located offshore and will contain large AC/DC converters, DC and AC bus bars, and high voltage equipment, including transformers. Today, AC/DC converters have been placed on offshore oil and gas platforms, and some have been constructed for offshore wind.
However, with existing capacity not exceeding 400 MW, they are far below the capacity necessary for large-scale wind integration (1-2 GW). Scaling up from today's rather simple, seabed-fixed constructions, to large structures, both floating and seabed-fixed, with 2 GW capacity requires further reductions in equipment size, while costs must also be kept at a reasonable level.
Dr Thomas Mestl is a project manager at DNV. The DNV Research and Innovation unit is headed by Elisabeth Harstad.
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