Black Shade Cloth Versus Reflective Screens
Few factors can affect plant development as much as temperature fluctuation, particularly in the heat of summer. Monitoring and controlling both air and leaf temperature can drastically affect a plant’s productivity and quality.
Leaf and air temperature can differ greatly, by as much as 10°F to 30°F. Proper greenhouse shading, however, can help keep leaf temperatures in the most productive range.
As greenhouses transmit sunlight, they also trap heat, raising the internal temperature of the structure. Both plants and workers become stressed, and reducing the heat load becomes imperative. But what is the best material to accomplish that goal?
Recent studies conducted by Dr. Daniel H. Willits at North Carolina State University (NCSU) help shed light on the impact greenhouse shading selection has on plants.
These studies found that common black, knitted shade cloth provided little to no thermal improvement in the greenhouse. Installing reflective materials for external shading, in this case the Svensson FLS, resulted in lower greenhouse temperatures and net radiation.
The studies reported a 30 percent decrease in greenhouse heat gain for reflective materials compared to black shade cloth at the 60 percent shading level. This translated to significant reductions in both leaf temperature and greenhouse temperature, which increases a greenhouse’s productivity potential overall.
As early as 1983, researchers speculated that shade cloths should reduce temperature in direct proportion to shade value.
Studies by Willits and Matthew M. Peet, an assistant professor of aerospace engineering at the Illinois Institute of Technology, in 2000 and Willits in 2001 found that shade cloths reduced energy gains less than the shade factor would suggest. Field tests indicate little or no temperature reduction can be expected from 30 percent black shade. The best estimates suggest black shade cloths are about 40 to 50 percent effective at reducing energy gain inside a greenhouse.
To compare the cooling characteristics of the two different types of shading, Willits and his team conducted computer model testing, physical model testing and full-scale testing on a single span polyethylene film covered greenhouse.
Computer modeling included radiation properties of the under and upper sides of the shade/poly layer and also transmissivity testing.
During the physical modeling, researchers mounted different shade cloths on wooden frames, then monitored and collected data on each configuration for four to six days. For these tests, researchers oriented the greenhouse shade frames south, and then tilted them to minimize the incidence angle at solar noon. Each was equipped with direct current fans at the upper end of the frames, and thermocouples were added to the air intake and exhaust to measure energy rise across the simulated fan-cooled house.
For the full-scale testing, researchers covered two 22 by 40 foot double-polyethylene covered Quonset-style greenhouses in the various cloths.
They tested two cloths from Ludvig Svensson, both rated at 60 percent shade. OLS 60 featured aluminum facing upward with white facing down. The second, FLS 60, featured white facing upward with black facing down.
Each four to six day testing cycle compared one of the Svensson cloths on one greenhouse with typical 60 percent black shade cloth on the other greenhouse. The researchers then switched the cloths to the other frame to eliminate any differences between the two frames. Each test greenhouse contained 208 tomato plants.
Researchers measured dry and wet bulb temperatures in two locations at both the entrance and exhaust end of each greenhouse, as well as dry bulb temperature and relative humidity at 12 locations along the length of the house, solar radiation at the top of the house and leaf temperature on the underside of each canopy. An external weather station recorded outside weather conditions.
They calculated the percentage difference in temperature rise from inlet to exit, energy rise and leaf canopy temperature for each reflective shade, using the 60 percent black shade cloth as the standard.
Black vs. White Results
Researchers found that 60 percent shades, both the FLS and OLS, reduced temperature rise by approximately 24 percent, energy gain by approximately 36 percent and leaf temperature by nearly 1°C.
They concluded that the cooling efficiency of external shade cloth is related to its temperature. In addition, reducing cloth temperature, either by water evaporation or other means, increases cooling efficiency.
Cooling efficiency is a term that could be used to express the amount of heat that accompanies light useful for growth (PAR light). This means that by replacing an absorbing (black or dark colored) shade with a reflective shade, you could:
• select the same shade and realize less heat gain,
• select a lower level of shading and benefit from higher PAR light transmission
Where heat stress is a challenge, choose a more efficient shade cloth to improve cooling. Where heavy shade has been necessary, choose a more efficient shade cloth with higher light transmission.
While black shades block light, they allow heat to radiate onto plants. External screens like Svensson’s FLS are made with highly reflective aluminum foil laminates or white films to reflect heat and light. They protect plants from stressing under too much sun, yet allow some light to penetrate by shading in a more efficient way.
Reflective external shades are not made to retain heat during the night. Instead, the screens provide daytime shade to cool greenhouse temperatures.