There is a variety of materials when it comes to choosing the cover for your commercial greenhouse.
Different materials provide different properties, and understanding the impact of those properties on the energetic balance of the greenhouse is vital for making the right choice.
Most growers want to get as much light into their greenhouses as possible. However, this is a vague statement as there are many factors involved in determining light measurements.
Getting maximum light into the greenhouse is the goal, as using natural daylight cuts down on the cost of artificial light sources, but there are more factors than just clarity that contribute to light penetration.
Photosynthetic Active Radiation(PAR):
The intensity of the portion of the light spectrum between 400 nm and 700 nm.
Photosynthetic Photon Flux Density(PPFD):
A measurement unit for the amount of photons that reach the plant leaf over a 1 square meter area every second, and is indicated in micromoles or (μmol) per square meter (m-2)) per second (s-1), or: μmol·m-2·s-1 of PAR.
Daily Light Integral(DLI):
The light measurement most growers are relying on is Daily Light Integral (DLI) which indicates the daily amount of PAR accumulated over the course of a 24 hour (1 day) period.
This sounds simple enough, but there are many factors that affect the amount of light reaching the plants down at the leaf level. Some of these factors include the greenhouse structural members, internal obstructions, angle of the sun, and directional orientation of the greenhouse, geographic location and time of year. To simplify things we will focus strictly on the factors involved in creating optimal sunlight penetration of the greenhouse covering.
The angle of the sun to the earth’s surface plays a very large part in greenhouse DLI readings. This angle varies throughout the day and throughout the year as the earth rotates around itself and around the sun. For example, a greenhouse in Pennsylvania, USA, 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 kind of variance it is imperative that in the winter months we make every effort to get as much of the available PAR light into the greenhouse.
There are many plants that need as much as 22 mol·m-2·d-1 to thrive and flourish. One might think that this can be achieved by installing clear glass on the greenhouse and calling it a day. While this sounds logical, it unfortunately, isn’t this simple. If we were only measuring light at 12:00 noon every day, it could be assumed that glass would be the optimal covering. Unfortunately, ( or fortunately for other coverings) we need to measure light over the course of the entire day from sunrise to sunset. This poses challenges for all flat covering materials.
Pennsylvania is located around 40' north latitude, the same latitudes which cross the world's busiest greenhouse areas such as Spain, Italy, Greece, Turkey, Southern Russia, Kazakhstan, Central China, North California and Colorado.
The angle of incidence of the sun to the greenhouse canopy surface plays a very large part of the overall PAR light transmission into the greenhouse. When the sun is at its lowest point in the sky at sunrise and sunset, flat greenhouse canopies reflect more light than they transmit. So while glass may transmit an extra 1% of light while the sun is high in the sky, it will reflect l increasing amounts of light as it occupies lower and lower positions in the sky.
This is where corrugated polycarbonate is able to increase its average light transmittance into the greenhouse.
This graph, based on data from Wageningen UR light lab (The Netherlands), compares 0.8mm corrugated polycarbonate to standard 4mm greenhouse glass, the two most common long term greenhouse covering materials.
Readings clearly indicate that, while at direct impact angle the glass and the polycarbonate panels show similar light transmittance; at low angles of incidence the 0.8mm corrugated polycarbonate transmits almost 50% more light.
The reason for this is that the corrugation profile is able to capture the sunlight at lower angles and redirect it inwards as opposed to reflecting it outwards.
At latitudes of 45º-55 º this advantage is even more significant during the 5 critical months of winter in the Northern Hemisphere between November and March, as the sun shines most of the day at a 15º-25º angle with the horizon.
Corrugated polycarbonate refracts light at low angles of incidence, reduces reflection, and facilitates more light penetration into the greenhouse than any other material.