Glass Vs Polycarbonate. How to get the best out of daylight in the greenhouse

Different roofing materials provide different performances and to make the right choice one must understand the impact of their characteristics on the energy balance of the greenhouse.

There is a huge variety of materials available when choosing a greenhouse cover.

Different materials provide different performances and to make the right choice one must understand the impact of their characteristics on the energy balance of the greenhouse.

Most growers want to have as much light in their greenhouse as possible. However, this is a vague statement, as there are many factors that determine the right amount of light needed for the growing project.

Getting the maximum light in the greenhouse remains the goal, as the use of natural daylight reduces the cost of any artificial light sources, but there are other factors besides the simple transparency that favor the penetration of light.

Many factors affect the amount of light that reaches plants at the leaf level. Some of these factors include the structural elements of the greenhouse, internal obstacles, the angle of the sun and the directional orientation of the greenhouse, the geographic location and the time of year. For simplicity we see the most relevant:

  • the angle of the sun relative to the earth's surface plays a very important role in the greenhouse DLI readings. This angle varies throughout the day and throughout the year as the earth rotates on itself and around the sun. For example, a greenhouse in Campania (about 40 'north latitude) in July can receive an average of 40-45 mol · m -2 · d -1. The exact same greenhouse in January will receive 10-15 mol m-2 d-1. With this type of variation it is imperative that in the winter months every effort is made to obtain the greatest amount of PAR light available in the greenhouse.
  • there are many plants that need 22 mol · m -2 · d -1 to thrive. One might think that this can be achieved by installing a nice clear glass on the greenhouse. While this sounds logical, it is sadly not that simple. If we measured the light only at 12:00 every day, it could be assumed that glass should be the optimal coverage. Unfortunately (or fortunately for other roofs) we need to measure the light throughout the day from sunrise to sunset. This poses challenges for all flat facing materials.

The angle of incidence of the sun on the surface of the greenhouse water table plays a very important role in the overall transmission of PAR light in the greenhouse. When the sun is at its lowest point in the sky, at sunrise and sunset, the greenhouse slopes reflect more light than they transmit. So while glass can transmit an extra 1% of light when the sun is high in the sky, it will reflect increasing amounts of light as it occupies lower and lower positions in the sky with less efficiency.

The readings clearly indicate that, while at the 90 ° impact angle, the glass and polycarbonate panels exhibit similar light transmission; at low angles of incidence 0.8mm corrugated polycarbonate transmits nearly 50% more light. 

Corrugated polycarbonate refracts light at low angles of incidence, reduces reflection and facilitates greater light penetration into the greenhouse than any other material, so it is able to maintain the same high light transmission, also adding diffusing elements and creating surfaces of light diffusion, the result is a light evenly distributed over the entire crown of the plant, to create an ideal growth environment with a reduced risk of 'burning' the plant due to too intense light beams.

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mastermas

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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.

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