Covering materials
- Kshiti Mishra (Deactivated)
- Gert-Jan Swinkels
Typical greenhouse covering materials used in the majority of low- and mid-tech greenhouses are polyethylene-based plastic films, and in most high-tech greenhouses, glass is used.
The greenhouse covering material determines the intensity and spectrum of the sunlight entering the greenhouse - alongside the crop growth and production, this also affects the greenhouse microclimate and hence, the energy use. The spectrum of sunlight ranges from wavelengths of 300 - 2500 nm - including UV (300 - 400 nm), photosynthetically active radiation, or PAR (400 - 700 nm), and near infrared radiation, or NIR (700 - 2500 nm). The PAR component governs photosynthesis and crop growth, while the NIR component provides free heat energy for the greenhouse.
The properties of the covering material which can influence the energy use and microclimate of the greenhouse are:
its characteristic transmission, reflection and absorption for the different components of sunlight, particularly PAR and NIR, which determine the entry of light or the energy input into the greenhouse
its thermal insulation value (or U-value) which is determined by
its characteristic transmission for long-wavelength thermal infrared radiation (≥2500 nm), which is emitted by the warmed up components in the greenhouse, and
its thermal conductivity
The u-value determines the energy losses through the greenhouse to the outside.
its condensation behaviour (whether condensed water on the cover forms droplets or films - which will affect its overall transmission of sunlight)
These properties can be modified in a variety of ways to modify the influence of the cover material on the greenhouse energy use.
Contribution to energy balance and resource use of greenhouse:
Energy Balance:
As the cover determines both the energy input in the greenhouse (through its transmission, reflection, and absorption of PAR/NIR radiation) and the energy losses from the greenhouse (through its transmission, reflection, and absorption of TIR radiation), it has a significant impact on the greenhouse microclimate, and the heating/cooling demand: If energy input from sunlight, particularly NIR, to the greenhouse exceeds the optimal temperature for plant growth (20-35 °C), then energy required for cooling increases. In a temperate winter climate, increased NIR input will reduce the energy required for heating. Moreover, if the energy losses through the cover result in temperature falling below the optimal range, the heating demand increases further.
Another way in which greenhouse covers can impact the energy use is through their PAR transmission - especially in regions receiving lesser light in the winters, if PAR transmission is not maximized, the light needs would have to be met by artificial illumination, which will also increase energy consumption.
In colder climates the cover plays also an important role in dehumidification of the greenhouse. Where a high insulating cover is usually warmer on the inside, condensation will be less and ventilation on humidity will be higher with resulting heat loss. This means that the insulation properties of the material do not represent per definition their performance in a greenhouse.
Water Use:
The impact of the cover on the internal temperature of the greenhouse and the heating/cooling demand of the greenhouse means that it will also affect transpiration - the mechanism used by the crop to cool itself - and consequently the water-use in the greenhouse. Particularly in warm and dry climates, employing evaporative cooling (fogging and pad and fan) to get rid of excessive energy input could further increase the water consumption.
Possible steps towards sustainable, energy-efficient greenhouses
The properties of the cover that improve the energy-efficiency of the greenhouse would differ according to the location of the greenhouse and the climate in the region. These properties can be modified by using different cover materials, or by applying various coatings.
Based on different ‘functions’ that are important based on the location, different properties may be desirable, for example:
Covers with low-emission coatings reduce the radiative losses from the greenhouse to the outer air, reducing the energy requirement for greenhouse heating in temperate climates
Covers with anti-reflective coatings maximize the PAR transmission into the greenhouse, reducing the energy use for artificial lighting in temperature greenhouses with greatly shortened day lengths in winter
Covers with NIR reflecting coatings could reduce the cooling requirements in tropical climates by reducing the input of heating radiation, without without compromising on the PAR transmission and production
It should be noted, however, that except when the property-modifying coatings are temporary, the properties of the cover remain constant across the entire period of its use. For instance, low-emission covers of temperate greenhouses might contribute to increased energy savings in cold periods, but in warmer periods, this will lead to undesirable retention of excess heat. Until materials can be developed that can switch their properties without a significant consumption of energy, a more tunable control on the optical properties and energy use of the greenhouse can be attained through the use of energy screens.
More information about the greenhouse cover can be found on the KasKieswijzer.