According to data from the International Renewable Energy Agency (IRENA), over the past years, installed capacity of solar power stations throughout the world have grown by a factor of more than 17 times – from 41.6 gigawatt (GW) to 714 GW. In 2020 alone, a total of 127 GW of new capacity was installed. According to data from the U.S. Consulting company Clean Energy Associates (CEA), world-wide capacity for production of solar panels reached 400 GW by the end of 2021 and capacity in terms of producing new cells for panels now stands at 325 GW.
More sunshine, more technologies!
The rise in demand for solar energy stemming from a drive to diversity sources to create energy and to replace, in part, fossil fuel resources, created several important tasks for those developing panels – raising productivity and effectiveness while at the same time expanding the geography and the different options for using them.
“There are two types of solar panels, the first converts solar energy to heat and the second to electricity. The first type is already widely used to produce hot water from sunlight and the technology is mature as well as commercially well developed. The second type employs photovoltaic (PV) cells and this field is presently undergoing very rapid growth. Thus, during the year 2020, PV cells produced 855 terawatt hours (TWh) of electricity, which is equivalent to 855 billion kWh of electric energy. While this is a stunning number, it corresponds to only 0.5% of the world’s total energy consumption,” said Michael Graetzel, head of the Laboratory of Photonics and Interfaces at the Swiss Federal Institute of Technology.
“In order to meet the engagements of the Paris climate agreement, that is to keep the global warming by greenhouse gases to below 2°C, we need to increase by 2070 the yearly electricity production from sunlight by a staggering factor of 163 to 140,160 TWh. While this is achievable, it requires development of new thin film technologies, such as perovskite solar cells along with the conventional silicon cells which today dominate the market.”
An innovative decision for the use of solar energy able to transform current notions of solar energy could be the use of silicon in tandem with other additional materials able to absorb solar rays.
The main technology for producing most of the photovoltaic solar panels involves the use of cells with a passivative emitter and rear cell (PERC). This provides an efficiency rate for its modules of 10 % to 21 %. Thanks to the technologies of Tunnel Oxide Passivated Contact TOPCON, the efficiency rate of solar modules can approach 25 %. But in order to break out of the framework of rates of 20-25 %, a fundamentally new approach is needed.
“One attractive approach is to use tandem cells that combine for example a silicon cell at the bottom with a perovskite solar cell on top,” Graetzel said. “These multi-junction cells have a higher efficiency than single solar panels made of single semiconductor material. Using tandem cells offers the prospect to reduce further the cost of solar electricity. which is essential to render the solar cells competitive without the need for public subsidies.”
“The best materials to combine with silicon in terms of efficiency are III-V semiconductors, specifically GaAs, in which lab-based cells have demonstrated over 32% efficiency, or metal halide perovskites for which lab based cells have demonstrated 29.8% efficiency,” said Henry Snaith, a Global Energy expert professor in physics in the Clarendon Laboratory at the University of Oxford..
“ III-V semiconductors are still produced via a very expensive slow molecular beam epitaxial growth, which makes them prohibitively expensive. In contrast, metal halide perovskites can be produced very rapidly at low temperature via conventional thin-film manufacturing processes, making them economically very attractive.”
The use of tandem technologies could boost efficiency rate of solar panels to 50 %. But for the moment, development of these technologies is hindered by the disproportionately high costs on introducing their mass production, Graetzel said.
“Single junction solar panels made of a single semiconductor material have attained efficiencies of 29 -30 % in natural sunlight, while higher efficiencies exceeding 50 % have been reached with mulitjunction tandem cells under concentrated sunlight,” he said. “Tracking the sun is mandatory for these high efficiencies cells, but this comes at an extra cost.”
Snaith said a startup has already started working on tandem technologies, but the outcome was for the moment unpredictable.
“As of yet, no tandem cell with perovskite has entered the market, but Oxford PV reported last year that it had completed its factor build out for the first perovskite-on-silicon tandem cell production line, so we should expect this technology to start to become available within a year,” he said.
Another, simpler way to improve the efficiency of solar panels would be the mass introduction of solar tracking technologies, which is akin to a natural mechanism in a sunflower that turns panels towards the sun. A special programme takes account of the location of the panel – its placement and height –, calculates where the sun will be located within any given time period and then turns to find the most advantageous position. This allows for a panel to operate with increased efficiency of 25-30 % and in some areas up to 40-50 % compared to modules with fixed angle, At the moment single axis or two axis trackers are in use.
Snaith said three axis trackers are being developed.
“In order to extend the power generation to earlier in the morning and later in the evening, two or three-axis solar tracking can be used, or simply mounting the modules with alternating east west orientation on fixed axis,” he said. “The latter configuration actually results in some of the highest power output per square kilometre.”
Cold and clear – a wonderful day
The popularity of solar batteries is leading gradually to the expansion of their frontiers for use. Even just a few years ago, the general feeling was that solar power stations were confined to sunny countries with a temperate climate, The standard base temperature for the operation of solar panels was considered 25 C. But now technologies are being introduced rapidly to enable them to be used in extreme weather conditions in the cold Arctic or in hot deserts.
“Solar panels can work in any conditions, there are no moving parts and solar farms are designed to survive severe weather. However, the amount of power generated is directly proportional to how much sun light is present – both diffuse and direct – and clearly on a stormy cloudy day, there is lower brightness,” Snaith said.
“All solar panels lose some efficiency as the temperature increases, and they are rated with a temperature coefficient, which states the % efficiency loss per 10 C temperature increase (standard reference temperature is 25 C). Therefore, periods of extreme heat will lead to lower overall efficiency, but these are usually accompanied by bright sunshine, so the power output from the solar farm will be high. This is easy to account for, but different technologies have different temperature coefficients, ranging from -0.4% for the worst to -0.25% for the best,” he added.
“Low temperature is generally good for solar panels, they operate much more efficiently and they are all stressed tested via thermal cycling from -40 to +85C, so extreme cold should not be a problem. “bi-facial” solar panels, which allow reflected sun light to be absorbed on the rear side, will also generate power and warm up when the front is covered in snow. This has the benefit of melting the snow in contact with the panel enough, for it to slide off and ‘self-clear,’” Snaith said.
Added Gratezel: “Solar panels are widely used in northern latitudes. They do sustain wide variations in temperatures as also present in space. They need to be well encapsulated to prevent ingression of water which upon freezing would damage the cells… There are certain types of solar panels, such as dye sensitized solar cells, that work particularly well in ambient light lower intensity than the full sun.”
Hydrogen to the rescue
Solving the problem of short-term fluctuations in generating power during overcast or rainy days is possible by introducing a universal system of electricity storage.
And in the Arctic, there is yet another natural phenomenon – the polar day, followed in turn by the polar night. In those conditions there is simply no modern storage system that will cope. But help could be on the way with modern hydrogen technologies.
“Another issue that will emerge during the wide deployment of solar panels is that the power grid does not have the capacity to adapt to large fluctuations in electricity input due to the large diurnal and seasonal variations of solar power,” Graetzel said. “One way to address this problem is to produce the electricity in a decentralised fashion and convert it to solar fuels. Thus hydrogen, a key clean energy vector of the future could be produced by combining solar electricity with an electrochemical cell. “
Added Snaith: “Solar panels will generate a lot of power in the polar summer, and clearly no power in the polar winter. In this scenario, they should be combined with the generation for green hydrogen via the electrolysis of water, which can then be burnt in a conventional (but suitably adapted) gas power station in the winter months, or used to power a fuel cell.”
Not an inch of land to spare
One of the disadvantages of solar power stations – as compared to, for instance, a nuclear power station – is that they require large swathes of unoccupied land. If we were to secure all our power from solar panels with an efficiency rate of 20 %, from 1 % to 2 % of the world’s surface would be required. That is equivalent to the area now covered by roads – considerably less land than is used for agriculture, which is approacing 50 %. And new technologies in making solar panels will lower the amount of square area that is required.
“Moving to higher and higher efficiencies is very important for minimising the land use required for PV. With technologies like tandem or ‘triple junction’ cells, over the next two decades we expect to have modules close to 40% efficiency,” Snaith said.
“This is twice today’s average module efficiency and will hence immediately halve the required land use. Furthermore, deploying three axis tracking, or arguably densely packed east/west facing module arrays, will further increase the power density and hence reduce the required land use.”
The use of new technologies in developing solar panels will therefore result not only in their cheaper and more efficient use, but will also improve environmental conditions on Earth.
“Making use of land already used by buildings, roads and other man-made objects, is also a key strategy to minimising any negative environmental impact of land use,” Snaith said. Furthermore, the dual-use of land for farming and power generation in ‘agrivoltaics’ is also a progressive means to minimise our negative footprint on planet Earth.”