Renewable energy is at the forefront of the energy policy of all major economies. The European Commission has proposed raising the renewable energy target to 40% by 2030, calling for further development of wind, water, and solar power. In Europe, for the first time in 2020, the share of renewable energies in electricity production exceeded that of fossil fuels.
Wind generators comprise three elements:
- Foundation and Tower – for rigid positioning, and to elevate the Turbine
- Nacelle – for housing the Yaw System, the Drive Chain, and Electrical Cabinets
- Rotor – contains the Rotor Hub and the Blades
Turbine blade design
The length of the blade is crucial to improve the efficiency of the wind turbine. The larger the blade, the higher the power production capacity. As a result, wind turbine blade length has increased from 60m in 2010 to 107m in 2020.
The weight of the blade also increases with an increasing blade length and that can reduce the optimal efficiency of the turbine. Polymer composites, reinforced with glass, carbon, and aramid fibers, play a key role in reducing the weight and increasing the strength and efficiency of the turbine. Glass fiber composites represent the largest market share, largely combining liquid epoxy resin with E-glass or boron-free E-CR glass. Polyester and vinyl ester resins are also sometimes used.
The adhesion of resin systems is important as it defines the mechanical properties of composite components. Epoxy resins have high strength to weight ratio, dimensional stability, and high adhesion properties compared to polyester and vinyl ester resins. The shelf-life of epoxy resins is several years as compared while the shelf-life of polyester resins is six months only. Epoxy resins offer excellent moisture resistance properties when reinforced with fiber.
Carbon fiber composites are increasingly being specified for the spar or structural elements of wind blades longer than 45m, they provide higher stiffness and lower density than glass fiber composites. This allows a thinner blade profile while producing stiffer, lighter blades. A 100m blade made entirely out of glass fiber composites can weigh up to 50 tonnes, while carbon fiber composites can achieve a 20 to 30% weight saving.
The high cost of carbon fibers has led to the use of hybrid fiber reinforcement. E-glass/carbon and E-glass/aramid are cost-competitive and improve mechanical properties. Basalt fiber, in combination with carbon fiber, is also emerging as an interesting hybrid option.
Manufacturers report some challenges in replacing glass fiber with carbon fiber. The latter has a relatively low damage tolerance, and its compressive strength is greatly affected by fiber alignment. Molders also encounter greater difficulty in achieving fiber wet-out during vacuum infusion; given this, manufacturers have tended to use more expensive pre-preg products.
Constructions made of polyurethane are now coming to the fore. Because recycling existing commonly used thermoset composites is a major challenge, research activities at the US National Renewable Energy Laboratory (NREL) and elsewhere are ongoing into recyclable thermoplastic composites. Such designs also enable thermal joining and shaping, which is a lighter and potentially more reliable manufacturing process.
In August 2020, a project for the use of advanced polymers in the renewable energy chain was implemented by Goldwind, claimed to be the world’s largest wind turbine manufacturer, which has used Covestro’s new patented direct infusion machine to make 64.2m long polyurethane turbine blades. The patent covers:
- an accurate vacuum process
- important casting volumes and resin infusion
- accurate dosage and variable and controlled output of the pressure in the mold.
- In line with the digital innovations of Industry 4.0, real-time data management ensures a high-quality direct infusion process.
The polyurethane resin developed by Covestro is said to be less viscous than the classic epoxy resin used in the lamination of composite materials. This allows for faster infusion and efficient polymerization: it allows almost complete polymerization in less than four hours. Also, pre-curing can be even shorter, saving valuable time in preparing the laminate and increasing production capacity.
Solar energy
According to the IEA, 114.9GW of new solar power installations happened globally in 2019, and solar energy constituted around 3% of global electricity demand in 2019. But it forecasts that the share could reach 16% by 2050.
Solar technology installations are set to become lighter and more flexible, with developments including floating solar farms, BIPV solar technology, solar skins and fabrics, and photovoltaic solar noise barriers (PVNB). Innovative residential solar technologies are in development, such as perovskite solar cells, which could soon be used to create solar paint.
Polymers for photovoltaic panels
Photovoltaic cells consist of two layers of semiconductor material, such as silicon, connected to each other by two metal electrodes. The cells are sandwiched between two glass plates.
Although this technology allows for very good energy yields, it is still expensive because of the cost of the materials and, above all, because their manufacture requires a lot of energy. It is estimated that a photovoltaic module of this type must operate for at least three years just to compensate for the energy required to manufacture it.
Solar panel manufacturers have been looking at replacing the glass with polymers. Polymer-based organic photovoltaic cells are opening up new prospects for solar energy, particularly in transport.
Initially, they focused their efforts on ethylene-vinyl acetate (EVA) to encapsulate the cells and on polyvinylidene fluoride (PVDF) to design the protective films for the panels. Other polymers used for encapsulation include polyvinyl butyral (PVB), polydimethylsiloxane (PDMS), ionomer, thermoplastic polyurethane (TPU), and polyolefin. In recent years, polymethylmethacrylate (PMMA) has played an increasingly important role; it is highly transparent and allows light to be concentrated on tiny cells. Thanks to PMMA, the size of the silicon cells has been reduced by a factor of three. polymers are also used as UV and anti-reflective coatings.
There are many advantages to using polymers rather than glass to design the substrates (the parts that encapsulate the cells). Solar panels of this type are particularly light and capable of floating on water, which is an opportunity to optimize surfaces such as artificial water bodies (irrigation ponds, drinking water reservoirs, aquaculture ponds, quarry lakes, etc.). There are floating panel systems composed of modular floats made of HDPE (High-Density Polyethylene), a material that is extremely resistant to UV rays, corrosion, and strong winds.
This mature technology allows good yields and has opened the way to so-called thin-film technologies which make it possible to do away with silicon. Thin-film technology has enabled polymers to make a dramatic entry into the world of solar panels. Both lighter and cheaper, they are set to be used more and more frequently.
Creating them involves using a laser to deposit a few microns of other semiconductor materials, such as telluride and cadmium alloys, on an inexpensive, flexible, or rigid substrate generally made of plastic. This technology has opened up a new field of applications for certain polymers, such as polyimides or fluorinated polymers, which are the only ones capable of withstanding the temperatures of laser processing. Its efficiency remains low, but it is more than sufficient to power small, low-powered electrical appliances. It has long been used in small calculators, for example.
Research is now focusing on 100% “organic” photovoltaics. In this case, the cells are made up of two semiconducting polymers. One, polythiophene, is used as an electron donor, and the other, fullerene, as an acceptor. These cells are encapsulated between two thin layers of metal that act as electrodes, a protective plastic film, and placed on a substrate also made of polymers.
For the time being, their energy performance has not yet reached the level of their silicon cousins. Their stability is not yet perfect, and they are still quite expensive. One of the challenges involves simplifying the production process by using 3D printers to manufacture them.
Due to their high stability and optical transparency in the conducting state, conductive polymers such as poly (3,4-ethylene dioxythiophene) or PEDOT are also expected to grow rapidly in the solar energy industry in the coming years.
References
1 Innovative and Recyclable Thermoplastic Wind Turbine Blades: NREL
NREL/FS-5400-73790 • July 2019
2 International Energy Agency (IEA) World Energy Outlook 2019
3 Polymers for Battery Applications—Active Materials, Membranes, and Binders
Adrian Saal, Tino Hagemann, Ulrich S Schubert
First published: 27 September 2020
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