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Agrivoltism

Here is an excerpt from my book Grass, Soil, Hope:

What is the best way to utilize sunlight: grow food or to produce fuel?

For millennia, the answer was easy: we used solar energy to grow plants that we could eat. Then in the 1970s the answer became more complicated as fields of photovoltaic panels (PVPs) began popping up around the planet, sometimes on former farmland. This was part of a new push for renewable energy sources, and as the technology has improved over the years so did the scale of solar power projects on land that could otherwise produce food.

In the 1990s, the food vs. fuel debate took a controversial turn when farmers began growing food crops for fuels such as corn-based ethanol, with encouragement in the form of government subsidies. Today the production of biofuels, including massive palm oil plantations, has become big business, often at the expense of hungry people. As a result, the land requirement of the biofuels industry, not to mention its deleterious impact on ecosystems and biodiversity, has become huge—and it keeps growing.

Making the situation even more complicated and controversial is a simple fact: according to scientists, the amount of land needed to replace fossil fuels with biofuels exceeds all the farmland available on the planet. In other words, increased competition between food and fuel for agriculturally productive land means that the stage is set for food shortages and rising conflict as the projected human population on Earth swells to nine billion by 2050.

This food vs. fuel debate has drawn considerable attention recently, with a number of potential solutions being proposed as a way out of what is quickly becoming a serious conundrum. Here are a few, briefly:

  • It’s a definition issue: According to a group of researchers, the trouble is no one can agree on what defines “surplus” land, including idle, marginal, reclaimed, and degraded land. Devising a common language, they said, means we’ll be able to “creatively utilize surplus land” for energy and the environment.
  • It’s a plant issue: Researchers say we’re using the wrong feedstocks for bioenergy production. Native grasses, flowers, and herbs offer the best chance for creating sustainable biofuels instead. Making that dream a reality, however, would require new technology to harvest, process, and convert this plant material, said one report.
  • It’s an ethics issue: Is it moral to produce fuel from food that could otherwise feed hungry people and drive up food prices as a result? Is it right for rich nations to exploit poor ones for their fuel needs? Before we can resolve this conflict, say philosophical types, we need to sort out our ethics first.
  • It’s a free market issue: Allocating which parcel of land should be used for food and which for fuel should only be determined by free market mechanisms, say many in the private sector. And the role of government subsidies and regulations should be minimized, they add.
  • It’s a geopolitical issue: The use of land for food or fuel cannot be separated from wider global struggles for economic security, political dominance, and social justice, say activists and government leaders.
  • It’s a technological issue: The conflict can be solved, say engineers, by improvements in solar technology on the one side, and plant productivity on the other. This includes ongoing research to “improve” photosynthesis, a chemical process considered by some biotech companies to be too inefficient (I kid you not).

Notice that all of these options have one thing in common: they still see it as a choice between food or fuel, silicon or carbon. There is no common ground, no coexistence, no win-win solution.

The food vs. fuel conundrum led French agricultural scientist Dr. Christian Dupraz to ponder a question: could food and fuel production be successfully combined on one plot of land? Specifically, why not build solar panels above a farm field so that electricity and food can be produced at the same time? In addition to resolving the conflict between land uses, he hypothesized solar panels could provide an additional source of income to farmers while sheltering crops from the rising temperatures and destructive hail and rain storms associated with climate change.

“As we need both fuels and food,” he wrote in a scientific paper published in 2010 in the journal Renewable Energy, “any optimization of land use should consider the two types of products simultaneously.”

He said it wasn’t a new idea. It was first proposed in a 1982 paper titled “On the Coexistence of Solar Energy Conversion and Plant Cultivation” by two German scientists. But their idea had never been implemented – until Dr. Dupraz and his colleagues at INRA, France’s agricultural research institution, decided to give it a try.

In the paper Dupraz also coined a new word to describe this system: agrivoltaic. Here’s a photo from the project:

Combining-solar-photovoltaic-panels-and-food-crops-for-optimising-land-use-Towards-new-agrivoltaic-schemesTHREE

To test their hypothesis, Dupraz and his fellow researchers built the first ever agrivoltaic farm, near Montpellier, in southern France. In a 2,000-square-meter test field they planted crops in four adjacent plots—two in full sun (as controls), one under a standard-density array of PVPs (as if the solar panels had been mounted on the ground) and one under a half-density array of PVPs. The panels were constructed at a height of 4 meters (12 feet) to allow workers and farm machinery access to the crops.

The main issue, they knew going in, was the effect of shade created by the PVPs on plant productivity. The researchers assumed productivity would decline, though there was scant data in the scientific literature to consult. That’s why they built two different shade combinations, full- vs. half-density, so that they could compare the effects to each other and to the control plots in full sun.

“Basically, solar panels and crops will compete for radiation,” Dupraz wrote in the paper, “and possibly for other resources such as water, as solar panels may reduce the available water quantity for crops due to increased runoff or shelter effects.”

By the same token, shade can improve the productivity of crops in a warming world. Water availability limits many crop productions, he wrote, and shade will reduce transpiration needs and possibly increase water efficiency.

As the experiment progressed, it became clear to the researchers that a compromise needed to be struck between maximizing the amount of electricity produced by the solar panels and maintaining the productive capacity of the farm. It was the Goldilocks Principle at work: too much shade hurt the crops, too little hurt electricity generation. Everything had to be just right. Could this balance be achieved? Variables the researchers identified included:

  • The proper angle or tilt of the PVPs
  • The proper spacing between solar panels
  • Making adjustments for localized conditions (such as latitude)
  • Choosing between fixed panels or panels on trackers (cost is a factor)
  • The proper height of the PVP array
  • Engineering issues involved with the construction of the structure that holds the PVPs in place (must be durable)

By the end of three growing seasons they had their answer: yes, balance was possible. But not quite for the reason they expected.

Not surprisingly, the crops under the full-density PVP shading lost nearly 50 percent of their productivity compared to similar crops in the full-sun plots. However, the crops under the half-density shading were not only as productive as the control plots; in a few cases they were even more productive!

The reason for this surprising outcome, according to Dr. Hélène Marrou, who studied lettuce in the plots, was the compensating ability of plants to adapt to lower light conditions. She reported that lettuce plants adjusted to decreased levels of radiation by (1) an increase in the total plant leaf area, and (2) an increase in total leaf area arrangement in order to harvest light more efficiently.

She also had good news to report on the water front. “We showed in this experiment that shading irrigated vegetable crops with PVPs panels allowed a saving of 14-29% of evapotranspired water, depending on the level of shade created and the crop grown,” she wrote in a 2013 paper (one of three).In the context of global warming and water shortage, she said, reducing water demand by shading plants could represent a big advantage in the near future.

Dupraz noted that while commercially available solar panels operate at 15 percent efficiency, the intrinsic efficiency of photosynthesis is “quite low” at roughly 3 percent (which is why companies are trying to “improve” it). This makes PVP systems more attractive to landowners than farms from a solar radiation perspective, especially at big scales. Combined, however, silicon and carbon systems can be very efficient, he wrote.

Being good scientists, Dupraz and Co. were careful to say that more research was needed, including questions about rain redistribution under the panels, wind effects on the crops, soil temperature changes, the effect of dust from farming on PVP efficiency, and the validity of the results for various latitudes, and a special focus on plants that have a demonstrated ability to compensate for reduced light conditions

However, their early results were very hopeful.

“As a conclusion,” Dr. Marrou wrote, “this study suggests that little adaptation in cropping practices should be required to switch from an open cropping to an agrivoltaic cropping system and attention should be mostly focused on mitigating light reduction and on plant selection.”

To this end, Dr. Dupraz wrote me recently to say the next step in their research is to evaluate the advantages of using mobile solar panels mounted on trackers. This would allow them to adjust the radiation levels for crops to meet their physiological needs. It will also allow the panels to be tilted to a vertical position during rainfall events, giving the water a chance to fall uniformly on the crops.

I’ll wager this is the ticket.

Combining-solar-photovoltaic-panels-and-food-crops-for-optimising-land-use-Towards-new-agrivoltaic-schemes

To buy Grass, Soil, Hope: http://www.chelseagreen.com/bookstore/item/grass_soil_hope

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