The Protected Cultivation Conundrums

We have at our APV facility at Parbhani, an elevated Shadehouse, with clear height below modules maintained at 3.75m. There is a shading cloth approx 1 m below this. In our conceptual research over the design of this structure, we saw that there are layers of thinking and confusion, in India - over the correct use of protected cultivation methods.

The first confusion is between - do you need a polyhouse or a shadehouse? A polyhouse costs you approx 3X that of a shadehouse. Per acre costs of 35-40 lakhs were told to us in our market studies, while a shadehouse can be erected in much lesser.

A shadehouse is a porous structure, that just shades the sun. It is porous to air all through, and only performs the important task of cutting of excess sunlight , of which we have too much in India.

But it does cut out wind circulation too - the way one would feel less air when one is sleeping inside a mosquito net, small breezes of air are filtered out by the shadehouse.

So now - you have one factor - cutting off sunlight, that will reduce temperature, and one factor - reducing air circulation - that will increase temperature. Which of these will win? It depends on the shadenet construction. There are open weave and closed weave shadenets.

Open weave shadenets are designed to improve circulation and not trap heat. They are better for hot climates. Closed weave shadenets are designed to trap heat - better for colder climates.

The bottomline being - just a single degree of freedom - a shade cloth, created the above decisions.

What then would happen in a polyhouse?

Polyhouses have polythene impermeable membranes, and hence air circulation is fully cut off where the membrane exists. There then are 2 possible ways of creating ventilation -

Passive Ventilation - by having air vents at top - in a saw-tooth shape(see image). This would NEED the polyhouse to get hotter than air outside for the inside air to rise and disperse.

Polyhouses with Open ventilation gaps at top

Active Ventilation - using Fan and Pads. Using this - you can create forced ventilation, and atleast achieve same temperatures as outside. If you want lesser temperatures than outside, then water has to be involved - for evaporative cooling. This is done using evaporative screens - and the latent heat of evaporation provides colder air than outside for the polyhouse.

NOW - add a third degree of freedom to both shadehouses and polyhouses - Using Foggers!

Foggers can be installed over all the beds, and result in again - evaporative cooling as they spray out a mist that instantly evaporates.

There are different thoughts out there - on frequency of fogging needed for effective cooling. But the calculation is a simple one - Rate of fogging(m3/hr) X efficiency factor(say 0.75) X Latent heat of evaporation (2265 kJoules/kg) X 1000 kg/m3 is the rate at which heat is removed from system.

Specific Heat of Air(0.71 kJ/kg/C) X (weight of air inside shadehouse) X ( Delta T) where Delta T is the temperature difference achieved by fogging.

This basic energy balance can give you amount of fogging needed for achieving a modest temperature difference.

Generally the rate of fogging needs to be in the range of 2 mm - 3 mm per sq metre per hour.

A Shadenet below a polyhouse roof

Now add one more degree of freedom to the polyhouse operation - Polyhouses also have a “mezzanine” shadenet that can be retracted to prevent more sunlight from reaching the plants. Since this shadenet is already within the polyhouse - any blockage of light is automatically converted to heat energy. In my opinion having this OUTSIDE the polyhouse above the roof would be more efficient.

The overall bottom-line of the above is - we need some great tech products in this space which can make sense of the above degrees of freedom, and give the farmer what he needs without having him become a thermodynamics and fluid mechanics semi expert!