Photovoltaics: The unstoppable sun

April 25, 1954, was a big day for photovoltaics. On the lawn behind his laboratory, the US physicist Daryl Chapin demonstrated that the sun’s energy could be used to power a radio transmitter and receiver. The New York Times was ecstatic: this invention, the paper wrote, could mark the dawn of a new era – one in which human civilization could make use of almost limitless solar energy.

And so it came to pass. But the presentation that day was the result of a long process. Primitive solar cells had already been invented, but they could not supply enough energy to be very useful. Chapin tried all sorts of materials to improve their efficiency, starting with selenium. But the electrical output was still too low. Then he tried the semiconductor material silicon, which was the basis for the still-nascent transistor technology of the day. This proved to be the answer.

Chapin’s work shows how arduous it was to make photovoltaic (PV) technology what it is today: an almost inexhaustible, inexpensive source of energy that is helping humanity leave the fossil fuel era behind. While Chapin’s solar cell had an efficiency of just a few percent, modern cells achieve 25% and higher. “Efficiency will increase further, but there is a physical limit,” says Arnd Roth, Technical Director at VDE Renewables.

Working towards 50% efficiency

The solar cell of the future is being researched all around the world, and the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg is playing a leading role. Its researchers are working on a cell with 50% efficiency. Using multi-junction cells, they are harvesting light from different parts of the spectrum. They have already neared their target and hit a new world record of 47.6%. “We’re excited by this result, which we achieved only a year after opening our new Center for High Efficiency Solar Cells,” says the department’s head, Frank Dimroth.

Like Chapin in 1954, the Freiburg team is experimenting with different elements, including gallium, indium and arsenide at present. But there are many other materials to try. “There’s a trend toward perovskite, for example,” says Roth. Perovskite solar cells are easy and inexpensive to manufacture. In laboratories, they are achieving record efficiencies, including up to 32.5% in the case of the cell developed by researchers at the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) late last year. So far, however, this has only been accomplished in lab environments.

At least 80% of the panels installed today are made of mono- and polycrystalline silicon. “The efficiency of the solar modules themselves isn’t everything,” says Roth. Instead, the system must be seen as a whole – from the cabling and the inverters to the question of how the equipment ages.

And of course, the best efficiency in the world is no use if aesthetic preferences or regulations pose a barrier to installation. But things are changing here, too: colored solar panels and photovoltaic roof tiles are getting harder and harder to spot. “When PV systems blend more into the overall picture – on listed buildings, for example – acceptance increases, which makes efficiency less important,” says Roth.

Roofs on existing buildings offer potential for even more solar power. Acceptance grows when photovoltaic installations merge into the background like these red solar tiles on the roof on an old building.

| © Fraunhofer ISE / Photo: Sarah de Carvalho (l.), © Fraunhofer ISE / Photo: Thomas Kroyer (r.)

The electric car pioneer Tesla was the first to market solar roof tiles a few years ago. While these versions have yet to arrive in Germany, domestic producers have already delivered: as part of Fraunhofer’s PVHide research project, a brick-red PV installation has been added to the roof of an old gymnasium in Eppingen (southern Germany). Almost unrecognizable as such, the equipment produces at least 90% of the power that would be generated by a traditional solar installation without the colored coating, says Fraunhofer.

No cheaper way to generate electricity

All these developments have contributed to the success of photovoltaics. With systems being installed all over the world, the one-terawatt milestone was hit in 2022. This was the moment the globally installed solar capacity surpassed 1,000 gigawatts – almost three times the total output of the world’s nuclear power stations.

“The annual growth rates in the total photovoltaic capacity installed across the globe have been very high over the last 20 years, and consistently in the double digits,” says Volker Quaschning, a professor of renewable energy systems in Berlin. Indeed, the German Solar Association reports booming demand for new installations, particularly on owner-occupied housing.

The technology is being driven not just by environmental considerations, but also by the unbeatable prices, which fell by 90% between 2010 and 2020 alone. Solar is now one of the cheapest means of electricity generation available, including in Germany. For new PV power plants with outputs of 1 MW or more, Fraunhofer ISE puts the cost of producing electricity at between 3.1 and 5.7 cents per kilowatt-hour, or between 11 and 13 cents for small roof-mounted installations.

According to the German Solar Association, almost 67.4 gigawatts of PV capacity had been installed across Germany by the end of 2022. This supplied 61.9 terawatt-hours, or some 12% of the country’s net power generation. At times on sunny days, PV meets more than two-thirds of our electricity demand.

The success of solar also harbors potential for conflict, however: The current German government is aiming for a total output of 215 gigawatts by 2030 – three times today’s level. By 2040, it is to rise even higher, to 400 gigawatts. The more modules are installed, the more electricity will flow – especially in the middle of the day, when the sun’s rays beat down vertically on the solar panels. “That could cause problems,” says Quaschning: “If all the modules are south-facing, we’ll be drowning in electricity come midday.”

A better approach would be to distribute power generation more evenly throughout the day, which is where vertically installed PV modules could help. They deliver the most energy in the morning and evening, when the sun is lower in the sky. Ideally, PV systems should be grid-friendly – that is, installed in a way that helps reduce grid costs. East- and west-facing vertical PV panels reach their peak output early and late in the day, when the sun’s rays hit them head-on. This makes them a good complement to south-facing solar modules. They also correlate well with the best prices, says Simon Lahr, a specialist from the vertical PV company Next2Sun in Dillingen, Saarland: “Electricity prices are high in the mornings and evenings, which brings higher returns for our customers.”

Sunrays by the runways

It’s no surprise that vertical solar panels are growing in popularity. More than 13 megawatts worth are already installed across Germany, including at Frankfurt Airport, which has built a “solar fence”. At the southwestern end of its western runway, a demonstration system with 20 panels and a nominal output of 8.4 kilowatts was erected last year. “Vacant green spaces within our runway system are ideal locations for this particular type of facility,” says Marcus Keimling, Head of Network Services at Fraport AG. PV is playing an important role at Frankfurt Airport more generally, as well. At peak output, modules that were installed on the roof of a cargo hangar in 2021 can supply around 1.5 megawatts of power, and more such roofs around the airport are set to follow. The target is to cut emissions by 50,000 tons by 2030.

“The surfaces of the solar fence collect the most energy at the beginning and end of the day. That makes them an ideal complement to our conventional PV modules, which produce their maximum output around midday,” Keimling affirms. “The solar fence is a real achievement for us, as it’s only with this type of technology that we can use the spaces around our runways for photovoltaics. Positioning the panels vertically enables us to preserve all the important vegetation underneath.”

Meanwhile, vertical PV modules lend themselves to agriculture in particular, where they offer remarkable potential according to Simon Lahr. Such panels take up hardly any land and help protect crops from drought and wind erosion. They also allow rain to reach the ground unimpeded. Typically, rows of panels are installed at intervals of eight to twelve meters. Larger gaps are also feasible to allow access to farming machinery.

Vertical PV installations use bifacial modules. Unlike conventional modules, these generate solar power using both sides of the panel, collecting both direct sunlight on the front and indirect light at the back. This also means that bifacial modules have a special design: the rear of the panel consists of transparent film or glass. The brighter the background light, the more power is generated at the back of the panel. “These modules have a considerably higher output in optimal conditions,” says Quaschning. It’s no wonder, then, that they are being installed in Scandinavia, where the abundant light reflected by the snow on the ground in winter produces especially high yields.

Floating solar

In a study published in late 2022, Enervis Energy Advisors concluded that plant design and the influence of the generation profile will become ever more important factors when it comes to adding PV capacity in an economically efficient way. Vertical, bifacial east-west designs in combination with agricultural uses may be an important part of the technology mix. A modernization package adopted by the German government in late March to advance climate protection and accelerate planning approvals could provide a boost to vertically installed PV modules. The study anticipates that solar panels will be installed along highways and railroads, as well. A noise barrier with integrated PV modules was already erected in the Netherlands back in 2018. “That generally makes sense, since highways often already have the necessary infrastructure, such as cable ducts,” says Arnd Roth. Another trend is “floating solar,” where modules are attached to rafts on reservoirs, lakes or even the sea. This adds capacity while minimizing conflicts over land use. The cooling effect of the water is another benefit, since cooler modules deliver higher output. The first such facilities are already being built in Germany, including in Renchen-Maiwald, Baden-Württemberg. The power generated here will largely be used by the neighboring gravel plant. Lakes created by opencast lignite mining, of which there are some 500 in Germany, are also suitable candidates. Fraunhofer researchers calculate that the water bodies suitable for such installations in Germany could potentially add up to 56 gigawatts of capacity.

Floating solar plants have some advantages over their land-based counterparts. For example, there’s no need to compact any land to install the facilities on a stable foundation, or to compete with agriculture, tourism or nature and landscape conservation efforts. But there are downsides, too, warns Roth: “Waves create vibrations that put immense pressure on joints and electrical connections.” He therefore sees the potential for floating solar primarily in countries where land is at a premium.

Strengthening Germany’s PV sector

Before building vertical plants or expensive floating structures, however, there are some 21 million commercial premises whose roofs would lend themselves to solar panel installation. Parking lots offer potential for another 59 gigawatts.

From roofs to fields, to lakes and parking spaces, the sun is playing an even bigger role in our energy mix. The German government’s expansion targets are ambitious. Meeting them will mean strengthening the domestic market, reducing dependencies on other countries and training up experts in the field. The photovoltaic sector’s gradual recovery in Germany is a positive sign. After a dramatic crash in the 2010s, the first German manufacturers have now returned. Meyer-Burger, for example, operates near the one-time solar stronghold of Bitterfeld-Wolfen. Solarwatt produces its modules in Dresden, and Heckert Solar manufactures at plants in Chemnitz and Langenwetzendorf.

Daniel Hautmann is a freelance journalist in Hamburg. He covers technology, energy and the environment.