As most renewable energy systems are located outdoors, sometimes in harsh conditions ranging from the extreme cold of Inner Mongolia, Scandinavia, Canada and North Dakota to the scorching hot deserts of the United States and Australia, critical attention must be paid to the suitability and robustness of equipment in such hostile environments.
Mitigating the risk of potential failures caused by extreme temperatures (differential thermal expansion, brittle materials and potential cracks, highly viscous oils, etc.) early in the development phase can help to reduce maintenance costs in the long term.
This has been the main driver for OWI-Lab in performing temperature testing in the climatic chamber on multiple turbine components and systems associated with wind energy projects.
While the vast majority of wind turbines are located in moderate climates, a rising number of wind turbine assets are being installed in challenging and remote locations, such as offshore, in subarctic locations, in the mountains or in deserts. Since these locations often have good, stable winds and are sparsely populated, they are ideal locations for installing large numbers of wind turbines. For such applications, it is necessary to develop appropriate and dedicated specifications for turbines and components.
The current international standards (IEC, GL) for wind turbine design do not fully take into account all the challenges encountered in cold-climate (CC) use. For standard wind turbines the operational temperature limit is -10 °C, and the survival temperature (standstill) is -20 °C.
Cold weather packages have been developed and marketed to extend the operational and standstill temperature range of wind turbines in cold climates and subarctic locations at limits of -30 °C in operation and -40 °C in standstill. However, cold weather packages also lead to additional ‘parasitic’ power consumption needed for the heating of turbine components. To maximise overall performance and profitability, these solutions need to be optimised in terms of levelized cost of electricity and risk reduction.
Even for regions with moderate climatic conditions, temperatures may unexpectedly exceed the wind turbine operational and survival limits during a cold snap, as happened, for example, in February 2011 when the ERCOT (The Electric Reliability Council of Texas – Texas grid operator) reported that 16% of its wind turbine assets had failed. 709 MW was caused by icing and 1,237 MW by turbine limits being exceeded, resulting in production stops and therefore production losses. So even for standard wind turbines, thoroughly understanding and optimising the turbine for cold conditions may pay off.
For cold climates, of course, the stakes are high. Last year’s ‘polar vortex’ cold wave in the US and Canada showed that long periods of low temperatures (down to -40 °C in some cases) can occur and must be taken into account for certain cold-climate locations. In the case of last year’s polar vortex, 1,000 MW of wind power outages due to extremely low temperatures were reported.
It makes sense for equipment to be capable of operating in extremely cold temperatures, as production yield is greater in winter due to high average wind speeds and greater air density. With regard to the latter positive effect of cold-climate production sites, E.ON’s Wind Turbine Technology and Operations Factbook mentions, for example, that 11% more power is produced at -10 °C than at +20 °C with similar wind speeds. Firstly, production losses caused by shutting down wind turbines for longer periods in (extreme) cold weather situations should be avoided. Secondly, the repowering time following such events needs to be as short as possible in order to minimise production losses. Thirdly, the ‘parasitic power’ used for heating during cold temperature survival needs to be optimised in order to ensure an efficient cold-climate wind turbine.
In some cold-climate wind turbines, the components – such as the nacelle drivetrain, yaw and pitch system, rotor, slip ring, batteries, etc. – are heated using 200 kW to 300 kW of parasitic power at conditions below -20 °C for survival. Currently, state-of-the-art wind turbines tend to perform better. For example, components can survive lower temperatures without additional heating. OWI-Lab states that there is still potential to perform better by using new materials and oils to, for example, extend the low-temperature operational and survival limit. Lowering parasitic power consumption and shortening cold-start times by using new control strategies to increase the efficiency and reenergising time of the turbine are the way forward here.
Another issue that needs to be addressed is downtime losses due to maintenance and repair. These are more expensive for cold-climate wind turbines than for regular onshore variants. Just as we see higher maintenance costs in the offshore market, the same applies to the cold-climate market as access time is limited and repair costs in winter are more expensive. In addition, special tools are needed to access wind turbines in winter. Therefore, OEMs and component suppliers tend to perform climatic chamber tests in order to check and validate if their products are capable of operating in and surviving extreme temperature events. They strive to deliver robust and reliable products in order to eliminate expensive repair work in winter. Climatic chamber testing tends to be valuable for proving performance under aggregated climatic conditions and shortening the product’s time-to-market, especially since field tests are expensive and very time-consuming.
To help you understand why climatic chamber testing could be beneficial in the product development cycle for wind turbines and components, we provide below an overview of some of the equipment tested in the laboratory and some of the issues that need to be examined during the design phase with regard to cold-climate wind turbines.
Mechanical and hydraulic components: gearbox, pitch and yaw system, hoists
Wind turbines are equipped with several large and small mechanical and hydraulic components, such as gearboxes, bearings, and yaw and pitch systems that can suffer from long exposure to cold weather. When the temperature drops, the viscosity of lubricants and hydraulic fluids increases, causing the oil to stiffen and making it unable to lubricate the gearbox and bearings sufficiently.
One critical component in the wind turbine is the gearbox. Special attention must be paid to this component in cold climates as damage to the gears can occur in seconds after the start of operation if the oil is too thick to freely circulate. Highly viscous oil also puts extra pressure on the oil pumping equipment and reduces the efficiency of the drivetrain. Plastics and steels are also affected by low temperatures. They become brittle, which could lead to cracks and leakage. For these reasons, most manufacturers provide cold-climate version (CCV) gearboxes with special lubricants, steel alloys, (additional) enhanced heating systems, etc. to reduce the risks caused by low-temperature operation. Seals, cushions and other rubber parts also need to be checked as they tend to lose flexibility at low temperatures. This may not necessarily result in part failure but can cause a general decline in performance.
When the oil is highly viscous, internal friction reduces the power transmission capacity of the gearbox and thus negatively impacts efficiency. Consequently, the cold-start time needs to be as short as possible while ensuring that the safe gearbox oil temperature is reached quickly enough to begin full-load operation. In most cases a cold-start time (also known as time-to-grid time) needs to be taken into account in order to reach a minimum component and oil temperature before full-load production can be applied. In many cases the turbine will first idle (with or without reinforced heaters) and then produce at partial load before working at full load.
As OWI-Lab stated in its presentation at EWEA, newly designed wind turbine gearboxes have cold-climate modifications to handle the risks encountered during cold-start scenarios, and gearbox systems including pumps, filter, valves, tubes and cooling systems are tested in large climatic chambers. (For details, see Extreme cold start-up validation of a wind turbine gearbox by the use of a large climatic test chamber.)
In order to encourage research in this area, OWI-Lab purchased a 2 MW research gearbox. In combination with its cold-start test bench for gearbox system testing, OWI-Lab is well equipped to accept the challenge and support and perform testing and design validation, and to build up deeper knowledge together with the wind industry to further optimise cold-climate wind turbines.
CASE STORY – wind turbine gearbox and cold start
A cold-start event is actually a rather unusual load case, but the impact can be high if it occurs and the machinery is not fitted appropriately. Lessons Learned from Winter Storm Issues, published by NERC (North American Electric Reliability Corporation), addresses the need for a full-system approach instead of only performing sub-component and component tests for cold temperatures. In the NERC case study, 15,000 MWh of lost energy production was related to cold and stormy weather as a chain of events resulted in the failure of the majority of turbines in a 200 MW wind farm.
Surprisingly, the turbines where shut down due to high gearbox oil temperatures. Snow and ice had accumulated on the nacelle-mounted radiators, and, due to shut down for repair, stationary oil in the radiator became viscous. When the turbines were returned to service, the bypass valve (which operated due to high differential pressure across the cooling system) caused hot oil to return the gearbox, resulting in a high gearbox oil temperature fault. Apart from highlighting the level of robustness required during extreme cold, this case study shows that it is also important to validate a proper cold-start sequence. It also makes sense to perform a frost/ defrosting tests for example on the cooling equipment when covered with ice and snow and check the effect on other equipment (like for example the gearbox). Physical design verification becomes important here, as simulating the effects described above is very complex and time-consuming. Moreover, when models are used, they need to be verified via physical tests.
A wind turbine’s pitch and yaw system can also be affected by cold temperatures, which means special attention must be paid to the behaviour of viscous hydraulic oils and the seals in order to prevent leakage. Due to their proximity to the outside environment, these systems are more vulnerable to extreme cold than the other electromechanical components in a wind turbine.
Wind turbine heating systems cannot operate during power outages (grid-falls). Restarting a unit after a power outage at cold temperatures is one of the most critical events for any wind turbine subsystem, with the ensuing risks of component failure, expensive repairs and prolonged periods of unavailability.
In electrical pitch systems, batteries are a weak point as they can be affected by cold weather and the windings of the pitch motor can suffer from thermal shock. When starting up in cold conditions, damage can occur due to the sudden increase in heat and the resulting differential thermal expansion in the cold machine, ultimately leading to failure or a decrease in lifetime.
In order to cope with such challenges in the design phase, GL published a specific technical note on cold-climate tests for pitch systems as part of the Certification of Wind Turbines for Extreme Temperatures. This document states that all parts of the pitch system (including the accumulator, pitch drive/cylinder, valves, controller, pipes and cables, and pitch gearboxes) must be tested in a climatic chamber to ensure operability under extreme conditions.
With regard to operational safety, special attention must be paid to the safety brake system, among other things. Mechanical hoists used for service cages located in the turbine tower also need to be modified for safe cold weather operation, as the cold causes cables to become brittle. In most cases no heating is installed in the tower section, making this component vulnerable to cold temperatures in wintertime. Special attention needs to be paid to this equipment to ensure nacelle accessibility under all conditions for maintenance and to ensure that work can be reliably performed via the service cage.
OWI-Lab’s large climatic test chamber:
OWI-Lab’s test facility is located at one of the break-bulk terminals in the port of Antwerp. Housing a 10m x 8m x 7m (LxHxW) climatic chamber and the ability to handle machinery of up to more than 150 tonnes in size on the quay directly from ship, train or lorry is a real asset when serving international customers. The laboratory also houses a test bench for the cold-start testing of rotating wind turbine machinery, as well as an energy supply for functional testing of electrical equipment (50Hz, 60Hz, different powers, voltages and currents). Temperature testing ranging from -60 °C to +60 °C is possible, as is humidity and IR radiation testing.
Also, the testing can be monitored remotely by the use of an IP camera, thermal camera (FLIR) and dedicated data acquisition tools and analysis software (NI). The large testing space makes it possible to test multiple systems simultaneously under the same test conditions. Simultaneous testing can be useful for benchmarking the results of changes, modifications and different versions of machinery under the same climatic conditions.
Press release; Image: OWI-Lab