Advanced materials drive green revolution

The Government's commitment to generate 20% of the UK's energy from renewable sources by 2020 was reinforced recently by Prime Minister David Cameron during his speech at the International Clean Energy Ministerial summit in London.

Describing Britain as at the forefront of the green energy revolution, the Prime Minister said he was proud of the country's 'world leading' energy sectors and maintained that renewables would continue to play a crucial part in its energy mix. In particular, he was keen to point out the UK's ongoing commitment to wind power, a sector which has gained considerable popularity in recent years and prompted the more widespread use of advanced composite materials. This in turn has led to growing requirements for innovative new materials and manufacturing methods.

"The trend in increasing turbine size has created the need for bigger rotor blades to broaden the sweep area and ability to capture more energy," said Philippe Christou, director of technology at Hunstman Advanced Materials. "Rotor blades over 50m are now gaining an increasingly larger market share and 60m blades are set to become more commonplace. The development of 70 to 80m blades has started and the 100m is also being considered for offshore applications."

Christou noted that with larger blades come more complex tooling and composite mould designs, longer production times and higher processing costs. "They also create greater stress on the structural, mechanical and gear components of the turbine," he noted. "As the industry looks to save time and money and improve efficiencies, fabrication processes for composites have come under the spotlight. The formulation of epoxy-based resin systems that can be used to vacuum infuse dry fibres or pre-forms hold significant structural benefits for producing large, complex composite parts with less than 1% void content and controllable resin-to-fibre ratio. This leads to increased strength as well as predictable performance and integrity of the finished part."

Producing higher performance materials, with an emphasis on lowering toxic risk, is a key area for Huntsman, which has more than ten years' experience operating in this market. The company offers a range of Germanischer Lloyd (GL) approved epoxy resin systems under the Araldite brand name, which are specially formulated to meet the stringent processing and performance requirements for wind blade manufacturing.

According to Christou, the systems are designed with enhanced mechanical and processing properties to improve product quality, lower blade weight, deliver high strength and fatigue properties and enhance impact resistance. The low viscosities of these systems are said to facilitate fast infusion processing and reduced production cycles.

The Araldite LY 1568/Aradur 3489 is an example of the company's latest epoxy based resin infusion system, which is designed to offer more control in the manufacturing process. The combination of its low mix viscosity (200 to 300mPas at 25°C), low exothermic reaction and long pot life (850 to 950 minutes) is said to provide benefits for users when infusing very long or thick composite parts.

In addition, the Araldite LY 8615/Aradur 8615 and Araldite LY 8615/XB 5173 are optimised to provide advantages for high temperature composite blade mould production. Both systems can withstand significantly higher temperatures than the curing temperature of the component production process and can be processed from room temperatures up to 40°C. They offer heat resistance up to 200°C and are particularly suitable for the production of blades made of high performance prepregs, which require high post cure cycles.

As well as developing new materials for wind power, Huntsman also offers a range of products for use in solar energy production, including solar panels that produce electricity and thermal panels that convert the power of the sun into heat. The company has worked in co-operation with a number of industry partners to produce a range of new materials and processing technologies for actively temperature controlled photovoltaic (PV) solar cell modules with increased efficiency.

The company's offering includes the Araldite AY 4583/HY 4583 liquid system, which is designed to enable an energy saving encapsulation process at low temperatures and eliminate the critical lamination process with EVA; heat conductive, electrical insulating adhesives that enable improved heat transfer and efficiency; flexible and thermal conductive adhesives which protect the flexibility of solar cells; and the white solder mask Probimer 77 White, which is designed to increase the efficiency of the PV module through higher and multiple reflection.

Huntsman claims the combined technologies have the potential to increase the efficiency of PV modules by more than 50%. In addition, it says that processing can be simplified, costs can be reduced and ageing stability significantly improved.
In March this year, researchers at the Massachusetts Institute of Technology (MIT) also made a significant solar cell breakthrough when they found a way to use metamaterials to absorb a wide range of light with extremely high efficiency.

While most thin materials used to fully capture light are limited to a narrow range of wavelengths and angles of incidence, the material's design utilises a pattern of wedge shaped ridges whose widths are precisely tuned to slow and capture light of a wide range of wavelengths and angles of incidence. According to the researchers, these metamaterials can be extremely thin, saving weight and cost. Their actual structure is etched from alternating layers of metal and an insulating material called a dielectric, whose response to polarised light can be varied by changing an electric field applied to it.

"What we have done is design a multilayer sawtooth structure that can absorb a wide range of frequencies with an efficiency of more than 95%," said postdoctoral researcher Kin Hung Fung. "Previously, such efficiency could only be achieved with materials tuned to a very narrow band of wavelengths. High efficiency absorption has been achieved before, but this design has an extremely wide window for colours of light."

Fung claims the material can be easily fabricated using equipment that is already standard in conventional photovoltaic cell manufacturing. Although the initial work was based on computer simulations, the team is now working on lab experiments to confirm their findings.

Meanwhile in Germany, Bayer MaterialScience has teamed up with the Solar Impulse project in a bid to demonstrate the true potential for pollution free air travel.

The Solar Impulse plane, which has the wingspan of a Boeing 747 and weighs the same as the average car, is powered by some 12,000 solar cells mounted on its wings. It made aviation history in July 2011 by flying through the night on solar power alone and is now gearing up to circumnavigate the globe in 2014.

The solar cells on the upper wing surface and the horizontal stabiliser of the plane generate electricity during the day by powering four electric motors and 3.5m diameter, twin bladed propellers. Surplus energy is stored in 400kg of lithium polymer batteries to allow flight at night, theoretically enabling the single seat plane to stay in the air indefinitely.

As well as nanotube reinforced polymers, Bayer has provided the plane with a number of innovative adhesives, rigid foams and polycarbonate films, including its Makrofol polycarbonate film which has been utilised in the windows of the cockpit. Novel composites have also been used to provide more stability while reducing weight and ensuring better energy efficiency.

According to Bertrand Piccard, co-pilot and initiator of Solar Impulse, Bayer's support has been a significant boost for the project. "With Bayer MaterialScience as an official partner, we have been able to make our airplane even lighter and more efficient. We look forward with great enthusiasm to being able to tap into the company's renowned expertise and innovative materials as the project progresses."

Author
Laura Hopperton

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