29.04.21 Innovations: the energy transition’s sunny future Author: Hans-Joachim Ziegler • Reading time: 3 min.

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Wind provides twice as much of Germany’s energy as solar. Innovative ideas could increase solar’s share. We present five of them.

1. Organic photovoltaics (OPV)

OPV has the potential to revolutionize the solar industry. The modules consist of light, flexible, wafer-thin layers that are easy to manufacture and conserve resources. Completely new applications are possible, such as transparent solar cells for windows that convert infrared light into electricity. The only hurdle is OPV’s modest efficiency. Polycrystalline modules achieve 15% to 20% efficiency, monocrystalline modules 20% to 22%. By contrast, OPV’s efficiency has long barely exceeded 10%. This is changing, at least in the lab: certain carbon compounds are already achieving efficiencies above 20%. Researchers are aiming for 30%. When that happens: let the revolution begin.

2. Go vertical

Sun-drenched building façades are just as suitable for solar panels as roofs. In fact, the potential is huge. The Leibniz Institute of Ecological Urban Development and the Fraunhofer Institute for Solar Energy Systems estimate that Germany’s façades have twice as much suitable surface area—12,000 square kilometers in all—than its rooftops. The ratio is probably similar in other countries. The necessary technology—printed and semi-transparent modules in numerous colors and shapes (triangular or rectangular), even modules that can be attached to curved surfaces—is readily available, placing almost no limits on applicability. Façade-mounted solar modules can also help heat or cool buildings. A system developed at ETH Zurich, for example, uses movable panels that let in just as much sun or provide just as much shade as is needed. Moreover, on clear summer days the movable solar panels produce 50% more electricity than static panels.

3. Up on the roof

As the asphalt jungle expands and urban green space contracts, cities and municipalities worldwide are increasingly embracing green roofs and even including them in their development plans. But could rooftop gardens crowd out rooftop solar panels? The German Green Building Association is convinced that the two can coexist. Here’s why: as temperatures rise, solar modules’ output declines because of greater electrical resistance. In fact, above 25 degrees Centigrade, their output falls by 0.5% for each degree of temperature increase. Surrounding plants, however, would provide a cooler environment and thus increase solar modules’ efficiency. The difference plants make is huge. On a hot summer day, the temperature on a bare flat roof can reach 60 to 80 degrees Centigrade, whereas the temperature in a rooftop garden tends to stay around 35 degrees.

4. Just scratching the surface

About 73,000 solar modules float on the surface of an 18-hectare quarry pond outside the Dutch city of Zwolle. With a capacity of roughly 27 MW, Europe’s largest floating solar farm produces enough electricity for 7,200 households. The potential of such facilities is enormous. The Fraunhofer Institute for Solar Energy Systems has calculated that Germany has artificial lakes suitable for 2.74 GW of floating solar capacity, enough to power well over 2 million households. Similarly, the Denver-based National Renewable Energy Laboratory (NREL) estimates that if artificial reservoirs worldwide were used for floating solar power, these installations could produce more than 10,000 terawatt-hours of electricity annually, enough to meet almost half of the world’s electricity needs. NREL researchers admit that their scenario doesn’t address questions of economic and environmental feasibility.

5. PPAs

In the past, renewables developers were able to finance their projects because the output would be sold at guaranteed, above-market prices. In Germany and other countries, amendments to renewables legislation is bringing these days to an end. Project developers therefore need a new source of financial security for their up-front investments. The answer is a power-purchase agreement (PPA). Under a PPA, a counterparty agrees to buy a percentage of the output of a future renewables asset—usually a wind or solar farm—for a set period (typically 10 or 15 years) at an agreed-on price. PPAs enable companies to access green power without owning their own assets and help developers fund projects without subsidies. For example, a PPA funded the development of Germany’s largest solar farm, a 187 MW facility located in Weesow-Willmersdorf in Brandenburg that entered service in November 2020. In fact, corporations worldwide concluded almost 24 GW of PPAs in 2020, with Amazon alone entering into PPAs for a total of 5.1 GW of renewables capacity.


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