Magenta Solar Panels in Sweden Grow Broccoli, but 25 Days Slower

Two transparency levels tested

Researchers compared broccoli in an uncovered plot with two groups grown under modules with nominal transparency of 50% and 70%. They measured environmental variables, yield, nutritional composition and even photosynthetic response.

Radiation use efficiency, the ratio of aboveground dry biomass to photosynthetically active radiation received, was 4.5 times higher than in full sun for the 50% panels and 2.8 times higher for the 70% panels. In other words, a similar harvest was obtained with less light, though with the 25-day delay. According to Ma Lu, quoted in the researchers' statement, the plants behave similarly in both cases. The results are limited to the test conditions and need validating over more seasons and in other settings.

The electricity trade-off

The panels use cadmium telluride cells, which form opaque bands on the glass. The denser the bands, the more light they capture for electricity, but the less reaches the plants.

Ramesh Rayudu, an electrical systems expert at Victoria University of Wellington, says the growing period yields an estimated 224.7 megawatt-hours per hectare. That is about 100 MWh per hectare less than the conventional reference. To offset the gap, an extra 1.2 to 3.2 tonnes of broccoli per hectare would be needed, depending on electricity and crop prices.

Ma Lu sees nearer-term uses in community gardens or greenhouse roofs. The team is already testing magenta, red and blue panels under controlled conditions, so it can separate the effect of the light's spectral characteristics from other variables.