(178b) Meeting Challenges in a Land Constrained Solar Economy: Co-Production of Energy and Food Using Photovoltaic Systems over Farmland

Authors: 
Miskin, C., Purdue University
Agrawal, R., Purdue University
Li, Y., Purdue University

The
main purpose of this presentation is to demonstrate that contrary to the
popular belief that ample land area is available to install photovoltaic
modules, in an economy mainly supported by solar energy, there will be severe
land availability constraints in many regions of the world—including many
states in the USA.  This will inevitably lead to competition for land between
food and energy [1].
We therefore propose several innovative PV systems that will allow for the
co-production of food and energy from the same unit area of farmland thereby
allowing local energy needs to be met by local supply of photons
. This
allows us to overcome feedstock (land) and transport (long-distance
transmission) limitations for electrical generation that would otherwise be difficult
to overcome in a solar economy.

A
recent study by MacKay showed that the energy consumption per unit area in
densely populated countries such as the UK, Germany, Republic of Korea, and
Japan is approaching the energy generation per unit area of photovoltaic farms [2].
This suggests that a transition to a solar powered economy in these nations
would require deployment of solar on a large fraction of the nation’s total
land area, which will directly compete with land requirements for other needs
(e.g. agriculture). Many other countries, including the United States are also
trending toward this scenario [2].
A recent study by National Renewable Energy Laboratories confirms the large
land area requirements of solar farms [3].

Using
the available data on current U.S. energy consumption and land availability [4]-[5],
we have estimated that as many as 31 states may be unable to meet their needs
without importing energy long distances from other states or offshore. This
scenario arises because much of the open land in the U.S. is devoted to
cropland (21.5%), pasture land (32.3%), and forest land (30.4%). Only 3.6% of
the land falls in the category of miscellaneous/other use land [5].

As
it should remain a priority to preserve our native forest and wildlife areas,
one of the greatest opportunities for increasing the availability of land for
solar energy generation is over farmland. These land areas are often easily
accessed and surround population centers. However, if PV is to be successful
deployed over farmland, novel systems will need to be designed that do not negatively
impact crop production. As most photosynthetic activity requires only the
visible portion of the spectrum (400-750 nm), these wavelengths can be allowed
to past through to plants while the IR portion of the spectrum is harvested as
electricity.  It is also clear that plants can go relatively extended periods
of time without solar irradiance (night) and survive extended periods of
reduced lighting (cloudy days) without significant impact to crop production so
it may be possible to harvest a portion of the visible portion as well.

Figure
1 shows several possible configurations for harvesting solar energy over
farmland. Fig. 1 (a)-(b) are systems that could operate optimally using current
industrially-proven solar absorbers (e.g. silicon). Fig. 1 (c)-(f) shows
systems for which light splitting would result in even higher performance by optimizing
the band gap of the absorber for the portion of the spectrum it is absorbing. Fig.
1(a) is the simplest case in which south-facing PV modules are altered such
that every other cell is left open or replaced with a light diffuser. In this
system crops never experience shading for very long as the shadow will be
constantly moving underneath the panel. Fig. 1(b) is the case in which single-
or dual-axis tracking panels are used. Again, the shadow will move throughout
the day. In the event that crops cannot tolerate extended periods of shade,
these panels can also be patterned and be made twice the size to decrease the
time intervals of shading, though the total time each plant would spend in the
shade over the course of the day would be the same.

Fig.
1(c)-(f) are systems based on industrially available dichroic mirrors that
split the spectrum [6]-[7].
These offer the advantage of concentrating the light for higher efficiencies.
As a downside, diffuse portions of the spectrum may not be harvested efficiently.
In Fig. 1(c), a tracking parabolic trough system is used in which the IR
portion of the spectrum is reflected to a PV cell of appropriate band gap while
the visible light passes through. As an option, a thin bifacial PV cells may be
placed at the back (or a pattern thereof) to harvest a portion of the visible
light and light scattered back from the crops. In Fig. 1(d), the system is
non-tracking for mechanical simplicity and directs much of the IR spectrum to
bifacial PV cells of appropriate band gap. During non-growing seasons, PV
panels could be inserted in place of the transparent support. Fig. 1(e) depicts
a tracking, long-pass system in which the visible light is concentrated and
collected in fiber optic cables and then redistributed to plants underneath (and
potentially other uses such as lighting. Fig. 1(f) depicts a non-tracking,
long-pass system in which visible light is again collected in fiber optic
cables and then redistributed to plants underneath, while the IR passes through
to be collected on PV of appropriate band gap underneath. Promising preliminary
calculations have been completed and are on-going to determine how much power
can be produced and the optimal band gaps for each system.

The
choice of which system to use will depend on how the crops respond to different
shading characteristics and how much light deprivation and which portions of
the spectrum they can tolerate being deprived of. It will also depend on the
relative percentage of diffuse to direct light in a geographic location as the
systems in Fig. 1(a)-(b) are better suited for areas with more diffuse light.
We explore these options and make some general conclusions on system selection.

References

[1]      Solar Projects Sow Tension, Wall
Street Journal, Wall Str. J. (2017) A3.

[2]      D.J.C.
MacKay, Solar energy in the context of energy use, energy transportation and
energy storage., Philos. Trans. A. Math. Phys. Eng. Sci. 371 (2013) 20110431.
doi:10.1098/rsta.2011.0431.

[3]      S.
Ong, C. Campbell, P. Denholm, R. Margolis, G. Heath, Land-Use Requirements for
Solar Power Plants in the United States - Technical Report NREL/TP-6A20-56290,
2013.

[4]      State
Profiles and Energy Estimates, (n.d.). http://www.eia.gov/state/.

[5]      Major
Land Uses, (n.d.). https://www.ers.usda.gov/data-products/major-land-uses/.

[6]      T.
Ulavi, T. Hebrink, J.H. Davidson, Analysis of a hybrid solar window for
building integration, Sol. Energy. 105 (2014) 290–302.
doi:10.1016/j.solener.2014.03.006.

[7]      E.
Gençer, C. Miskin, X. Sun, M.R. Khan, P. Bermel, M.A. Alam, R. Agrawal,
Directing solar photons to sustainably meet food, energy, and water needs, Sci.
Rep. (2017).