Find the optimum point between the needs of the greenhouse structure and the photovoltaic generator

The identification of the optimum point between the cultivation needs, the structural costs and the production needs of the photovoltaic generator is determined by a set of factors which, if well managed, can positively influence the photosynthesis process of plants.

Over the years we have created several installations that have allowed us to calibrate the right amount of light needed, from the various vegetable crops to fruit trees and identify a series of solutions in terms of alternating light and shadow and the relative incidence on the surface and find the right balance in the alternation of light and shadow in their natural horizontal movement.

The identification of the optimum point is therefore determined by a complex of factors which, if well managed, can positively influence the photosynthesis process.

In some photovoltaic greenhouse structures already built, we were able to verify the equilibrium point by making sure that the photovoltaic generator consisting of shading panels, can in some cases reach up to 60-70% of the total useful surface, consisting of the south-facing slopes, with the creation of favorable cultivation environments and not in competition with the shadow created by the photovoltaic generator. Indeed, for some crops this has proved to be an improving and functional growth factor with significant increases in both quality and quantity depending on the crop species used. 

In warm climates around the world, vegetables and cut flowers are grown under shade cloths to reduce heat and light intensity, resulting in better quality and higher yields

Shade fabric is a weatherproof woven or knitted fabric available in densities 12% to 90%. The density represents the percentage of light blocked by the fabric; for example, a 47 percent shade cloth blocks 47 percent of the light.
Most vegetables should be grown under 30-50 percent shade. Shade sheets with a density greater than 50 percent are generally used for shade-loving plants or as a windbreak.

In the case of a photovoltaic greenhouse, designed according to precise criteria, the shading element is the photovoltaic module which behaves in a totally different way compared to a sheet, but this can turn into a real and powerful crop advantage.

Let's see how

  • The photovoltaic module is placed high enough above the plants, let's say at no less than 5-6m, we provide sufficient ventilation, even by convection, so that the heat does not accumulate under these shading elements.
  • The photovoltaic modules are arranged so that the shadow is never persistent, but has a motion, to allow an alternating light radiation in the shadow so that it becomes a perfect stimulus for photosynthesis.
  • It is a good idea to combine structural solutions for natural ventilation, through controlled opening windows, forced convection of the external air and forced convection of the treated air for both heating and cooling.

Suggested publications

Estimation of the agricultural sustainability of photovoltaic greenhouses in Europe

Summary
The integration of photovoltaic (PV) energy in greenhouses has raised concerns about the agricultural sustainability of this specific agrosystem in terms of crop planning and management, due to the shading of the photovoltaic panels placed on the canopy.

The photovoltaic greenhouse (PVG) can be classified according to the photovoltaic coverage ratio (PVR), that is the ratio between the projected area of the photovoltaic panels on the ground and the total area of the greenhouse. In this article, we estimated the yield of 14 greenhouse horticultural and flower crops within four commercial types of PVG common in southern Europe, with a PVR between 25 and 100%.

The aim of the work is to identify the types of PVG suitable for the cultivation of the considered species, based on the best compromise between photovoltaic shading and agricultural production.

The daily light integral (DLI) was used to compare the light scenarios within the PVGs with the light requirements of the crops and estimate the potential yield.

  • The structures with a PVR of 25% are compatible with the cultivation of all the species considered, including those very demanding in terms of light (tomato, cucumber, sweet pepper), with a negligible or limited reduction in the estimated yield.
  • Medium light species (such as asparagus) with an optimal DLI below 17 mol / m2d1 and low light crops can be grown inside PVG with a PVR up to 60%.
  • Only flower species that require little light with an optimal DLI of less than 10 mol / m2d1, such as the poinsettia, the kalanchoe and the dracaena, are compatible within the PVG with a PVR up to 100%.

Innovative cultivation systems must be considered to overcome the most penalizing lighting scenarios of high PVR PVGs, also implementing additional LED lighting. This work helps to identify the types of sustainable PVG for the chosen species and alternative crop management in terms of transplant period and precision farming techniques, aimed at increasing the productivity and adaptability of crops within the PVG agrosystems.

To find out more, go to the Publication European Journal of Agronomy - Agricultural sustainability estimation of the European photovoltaic greenhouses

Although light is the most important element for the implementation of photosynthesis, it can also damage the plant. This problem involves the damage and degradation of the D1-polypeptide reaction center. The degradation of this very important protein appears to be a direct consequence of the photosystem chemistry which involves highly oxidizing radicals and toxic oxygen species.

The frequency of this damage is relatively low under normal conditions, but it becomes a significant problem for the plant as light intensity increases, especially when combined with other environmental stressors.

To find out more, go to the Publication Trends in biochemical sciences "Too much of a good thing: light can be bad for photosynthesis"

To deepen the topic

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Calculate the DLI (Daily light integral) of the greenhouse you have in mind. Starting from the Italian geographical area concerned and knowing the limiting factors, in terms of the reduction of light radiation determined by the structure and material chosen as roofing, you can calculate the DLI of your project.

Energy green house photovoltaic rooftop solar
Naturae Plena

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