New materials for renewable energy：The power of being made very small
Big improvements in the production of energy, especially from renewable sources, are expected over the coming years. Safer nuclear-power stations, highly efficient solar cells and the ability to extract more energy from the wind and the sea are among the things promised. But important breakthroughs will be needed for these advances to happen, mostly because they require extraordinary new materials.
The way researchers will construct these materials is now becoming clear. They will engineer them at the nanoscale, where things are measured in billionths of a metre. At such a small size materials can have unique properties. And sometimes these properties can be used to provide desirable features, especially when substances are formed into a composite structure that combines a number of abilities. A series of recent developments shows how great that potential might be.
Grand designs
Researchers have already become much better at understanding how the structure of new nano-engineered materials will behave, although the process remains largely one of trial and error because different samples have to be repeatedly manufactured and tested. Michael Demkowicz of the Massachusetts Institute of Technology is developing a model that he hopes will address the problem from a different direction: specifying a set of desired properties and then trying to predict the nanostructures needed to deliver them.
Dr Demkowicz is working with a team based at the Los Alamos National Laboratory, one of a number of groups being funded under a new $777m five-year programme by the American government to accelerate research into energy technologies. The material Dr Demkowicz is looking for will be good at resisting damage from radiation. It could be used instead of stainless steel to line a nuclear reactor, which would extend the reactor’s working life and allow it to be operated more efficiently by burning a higher percentage of nuclear fuel. At present, says Dr Demkowicz, reactors burn only around 1% of their fuel, so even a modest increase in fuel burn would leave less radioactive waste.
The reason why the linings of nuclear reactors degrade is that metals can become brittle and weak when they are exposed to radiation. This weakness is caused by defects forming in their crystal-lattice structure, which in turn are caused by high-energy particles such as neutrons bumping into individual atoms and knocking them out of place. When these displaced atoms collide with other atoms, the damage spreads. The result is holes, or “vacancies”, and “interstitials”, where additional atoms have squeezed into the structure.
Dr Demkowicz says it is possible to design nanocomposites with a structure that resists radiation damage. This is because they can be made to exhibit a sort of healing effect in the areas between their different layers. The thinner these layers are, the more important these interfaces become because they make up more of the total volume of the material. Depending on how the nanocomposites are constructed, both the vacancies and the interstitials get trapped at the interfaces. This means there is a greater chance of their meeting one another, allowing an extra atom to fill a hole and restore the crystal structure. In some conditions the effect can appear to show no radiation damage at all, he adds.
The ideal nanocomposite would not only resist radiation damage. It would also not itself become radioactive by absorbing neutrons. Dr Demkowicz has used his modelling techniques to come up with some candidates; iron-based ones for fission reactors and tungsten-based ones for those that may one day use nuclear fusion. It could still take years before such materials are approved for use, but the modelling methods will greatly speed up the process.
Across the spectrum
Nano-engineered materials will also play an important role in a more efficient generation of solar cells, according to an exhibition by researchers at Imperial College, London, called “A Quantum of Sol”, which opened this week at the Royal Society Summer Science Exhibition, also in London. Again, the desired effects are obtained by using combinations of material produced at extremely small sizes. In this case, they are used to make “multi-junction” solar cells, in which each layer captures energy from a particular colour in the spectrum of sunlight. Overall, this is more efficient than a conventional solar cell which converts energy from only part of the spectrum.
Whereas conventional solar cells might turn 20% or so of the energy in sunlight into electricity, multi-junction solar cells already have an efficiency of just over 40% and within a decade that could reach 50%, predicts Ned Ekins-Daukes, a researcher at Imperial. Until nano-engineering costs come down with economies of scale, multi-junction solar cells will remain expensive. The researchers expect that electricity-generation costs can still be cut in the meantime by using mirrors to concentrate sunlight on the cells.
Through the glass
Solar cells could also be incorporated into the structure of buildings, including windows. Researchers at the Fraunhofer Institute for Mechanics of Materials are looking for suitable transparent materials to make them. They too are using computer models to explore atomic structures and then to simulate how electrons will behave in them. With the right combination of conductive and transparent material, says Wolfgang K?rner, from the German institute, it should be possible to produce completely see-through electronics.
The nanostructure of composites can also provide great mechanical strength in relatively light materials. Composites such as fibreglass and carbon fibre bonded in a plastic resin are already widely used to replace metal in making, for instance, cars and aircraft. But by controlling the direction and the tension of the fibres during their construction it is possible to produce a morphing composite, which adjusts its shape under certain conditions. The change can be instigated by an external control or it can be automatic, for instance in response to variations in heat, pressure or velocity.
These morphing composites could be used to produce more efficient turbine blades in wind and tidal generators, a seminar at Bristol University’s Advanced Composites Centre for Innovation and Science was told this week. A bistable composite capable of altering its aerodynamic profile rapidly when wind and current conditions changed would help to remove unwanted stresses in the blades. That would increase the efficiency of the blades and extend the working life of the generator systems they power, says Stephen Hallett, a member of the Bristol team. Morphing composites would mean, for instance, that tidal generators could be made smaller and would last longer, which would make them more viable commercially. In this way many tiny changes in the science of materials could generate a big future for renewable energy.