Splitting The Solar Spectrum Could Produce Power, Food, & Water From The Same Land

Instead of producing solar power on one site, and purifying water on another, and growing food on yet another piece of land, it might be possible to do all three on the same site, which would enable true local sustainability.

That’s the gist of a new research paper from a Purdue University research team, which makes the case for splitting the solar spectrum between the three elements of the food/energy/water nexus according to their needs. With the right set of equipment, the same piece of land could enable not only that photosynthetically active radiation hits the crop leaves for growth, and the right spectrum is directed at solar photovoltaic cells for electricity, but the longer wavelengths could be captured as thermal energy for water purification as well.

With a rapidly growing global population that is outpacing our ability to produce enough food, water, and energy to sustain itself, we’re approaching what the authors call a “Full Earth” that could see more than 10 billion inhabitants in the next century, and the concepts presented in the paper could very well form the basis for next generation land use, especially in the developing world. The paper, titled “Directing solar photons to sustainably meet food, energy, and water needs,” proposes that by adopting the new “solar spectrum unbundling FEW [food, water, energy] systems,” the same amount of end products (food crops, electricity, purified water) could be harvested in much less land than if produced separately.

“The grand challenge before us is to sustainably meet humanity’s FEW needs on a Full Earth using scarcer resources. The sun is the key energy source that can sustainably meet humanity’s FEW needs now and in the future. In light of this, we have developed novel solar spectrum unbundling FEW systems (SUFEWS), which meet local FEW needs for any foreseeable future, while reducing the overall environmental impact of meeting these needs.”

The authors proposed three variations on the solar photon unbundling system, each of which has a different mechanism for harnessing the full spectrum of sunlight for producing crops, electricity, and clean water.

Illustration of the solar photons unbundling concept for solar photons through three alternative arrangements: (A) A parabolic trough with a reflective surface that transmits full intensity F (scenario A) for plant growth and reflects E&W. The solar cell absorbs E and transmits W for heat collection to purify water by multistage flash or multieffect distillation. (B) A heliostat with the same reflective surface as in A, but W is reflected off of a hyperbolic mirror to a heat cavity before E is incident on solar cells. (C) For plants that can thrive in a reduced intensity of F (scenario B), the same arrangement as A, but a thin bifacial solar cell is used under the reflective surface to harness a portion of F for electricity generation and any reflected light from underneath the trough. (Figure credit: Ryan Ellis, Purdue University).

“A major feature is the ability to produce food, energy and water resources locally without interfering with agricultural production, which will be increasingly important, as expected population growth will require increased dedication of land resources to agriculture. At the same time, the local generation of electricity will allow the use of microgrids in villages and provide a new paradigm for electricity generation and distribution. Local generation of power and clean water also is expected to reduce the long-distance transmission losses inherent in any power supply grid system.” – Rakesh Agrawal, Purdue University’s Winthrop E. Stone Distinguished Professor in the School of Chemical Engineering

Although this is a concept, not a working model or prototype, it’s too early to tell if this would be feasible with today’s technology at current costs, but it certainly shows a more integrated approach to solving the food/energy/water nexus at the local level. If the same piece of agricultural land could also generate electricity and energy for water purification, it could go a long way toward self-sufficiency, especially in regions where all three resources are scarce. The full paper, which was authored by Emre Gençer, Caleb Miskin, Xingshu Sun, M. Ryyan Khan, Peter Bermel, M. Ashraf Alam, and Rakesh Agrawal, is available online at the journal Scientific Reports.

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