Energy is one of the main challenges of the 21st century, as global energy consumption keeps on increasing while conventional energy sources gradually deplete. Furthermore, global warming is forcing us to ask how much more atmospheric carbon dioxide emissions can be tolerated. The sun could offer a natural solution to all of this - the solar radiation the Earth receives over just a few minutes could supply global energy needs for an entire year.
Scientists all over the world and here at Aalto University are working intensively on the grand challenge to tap the sun's enormous electricity-generation potential on a large scale. It is quite clear that solar power will eventually take the lead position in global energy production - it is only a question of time. Current forecasts predict this will happen by no later than 2060, but it is possible that the solar revolution will take place much earlier. This will partly depend on us - the scientific community - and the discoveries that we make.
It should be noted that solar power is already a multi-billion euro business, with some 80 GW of photovoltaic (PV) capacity already in place. This is equal to the power output of 50 "Olkiluoto 3" new nuclear power reactors operating at full capacity. For example, Germany produced 22 GW of PV on a sunny day this spring, enough to cover 50% of the country's electricity consumption.
The growth of the photovoltaic industry has been faster than anybody could have ever predicted. This is due to recent advances in solar cell technologies and the constantly increasing price of non-renewable energy. The cost per kilowatt hour of electricity produced with residential PV cell systems has already reached so-called grid parity in Germany, which means that the real cost (no subsidy support) of solar electricity for residents is less than the price they would pay for power from the grid. The price of solar electricity inside the grid is already competitive with conventional power sources in many locations, and this general grid parity is approaching us as well. Predictions say that this should happen as soon as 2013 in Southern Europe and by 2020 in the rest of Europe.
In Finland, we should start thinking about the huge potential for the export industry. It is clear that countries that obtain knowledge and experience today will most probably have an edge when solar energy becomes an even bigger business than oil is today.
ALTERNATIVES FOR SILICON TECHNOLOGY?
The core of traditional photovoltaic cells is formed by a semiconductor, a highly purified wafer of crystalline silicon. So far, silicon-based solar cells have been most successful at producing cost-efficient electricity - but there are also promising other technologies, made of more exotic materials and thin films, the price of which has recently become competitive with silicon technology. However, the best alternative low-cost technologies still consist of rare earth materials or at least require a much larger surface area to produce the same amount of electricity as silicon. Accordingly, silicon technology has again increased its global market share in recent years.
Some believe that the main challenger for silicon will be something entirely different: extremely efficient high-cost multi-junction cells, which utilise relatively cheap mirrors/lenses and tracking systems that concentrate sunlight on a small area. This technology is known as concentrated photovoltaics (CPV). Even though it relies on very expensive solar cell technology, CPV is expected to become common in locations like deserts that receive lots of direct sunlight. The main benefit of this method is that it achieves the same power output as competing technologies with much smaller materials consumption and area requirements.
The best technology may not be the ultimate winner, but the possibility to produce massive volumes of modules without too many new investments might play an even more critical role in the future. Most probably, each solar technology will find a niche area and market, and research on each technology will support also the other alternatives. Scientists have in fact already made initial attempts to mix technologies, building cells that apply more than one approach. This may well prove the ultimate way to move forward.
FROM CONVENTIONAL BLUE TO STYLISH BLACK
In addition to cost, consumers and architects also care about the aesthetics of the panels on their rooftops. They are even willing to tolerate a trade-off between appearance and the cost of electricity. Scientists often find this hard to accept, but we cannot ignore the visual aspects and should instead aim at deeper collaboration with designers.
The first colour that comes to mind when people think about solar panels is blue (notice that I am not talking about heat-generating solar collectors that look completely different). The blue colour comes from the panel's anti-reflection coating, which is made of silicon nitride (SiN) of a specific thickness that is proportional to the wavelength of incoming sunlight. The anti-reflective properties are typically optimised for a wavelength of 600 nm, where sunlight has its maximum intensity. From the fabrication point of view, it is rather straightforward to change SiN thickness to produce almost any desired colour. Some companies are already offering pink, yellow, green and red panels. Of course, different colours come with a trade-off in terms of energy production capacity.
Researchers at Aalto are addressing the energy challenge by utilising nanotechnology in the fabrication of solar cells, which can be applied to various materials and photovoltaic technologies. For instance, we are currently working with the novel idea to add specific nanoscale structures onto silicon cells to enable superior performance in light absorption. We have recently demonstrated for the first time that no trade-off between absorbency and energy output needs to exist. This technology enables more efficient solar cells and thereby cheaper kilowatt hours. Moreover, the nanosturctured surface gives these cells a good-looking mat-black finish.
CHEAPER ELECTRICITY THROUGH NANOSCALE ENGINEERING OF DIRTY MATERIALS
One of the most expensive fabrication steps for leading solar panel types is the purification of silicon. The purification step also consumes lots of energy, which naturally increases the price of these panels. Aalto researchers are attempting to get rid of the cleaning procedure simply by making solar cells from more impure materials. The approach that we use requires a deep understanding of the defect and impurity reactions both during cell processing and under final illumination operation. We have recently been able to demonstrate that dirtier material actually results in better electricity output, provided that all processing parameters are well controlled.
Another development that consumers will soon notice is back-contacted cell technology. This means that metal wires will disappear from the visible side of the panel, eliminating shadowing losses and making the panels look more stylish as well. Such technology is, of course, more expensive, but the benefits are expected to outweigh the additional cost.
Before back-contacted cells enter the market more widely, another type of cell, made with so-called wrap-through technology, will become available. Instead of the rather thick straight silver lines that now lie on top of the cells, customers can have any design decorating the front silver pattern. This technology tolerates less pure materials, possibly making dirty silicon a viable option as well.
All of the mentioned trends are good examples of successful innovations, which derive from fundamental research. Yet we are still very far from realising the full potential of each technology, whether based on organics, thin films, silicon or CPV. The fundamental photovoltaic research done at universities is essential to our progress towards the new era of solar energy.
Credit: [Text: Hele Savin, Photo: Sarri Kukkonen]
Source: [Aalto University Magazine, volume 04]
