Quantifying energy saving by screens – the role of humidity transport

The application of new energy saving cultivation techniques in Dutch greenhouses heavily relies on using one or more movable screens. Screens affect greenhouse climate by thermal radiation, air and humidity exchanges between the lower and upper part of the greenhouse compartment separated by screens. Thus, screens affect the sensible and latent heat exchange and therefore the insulation value of the greenhouse, they have an important role in energy saving. The objectives of this project are to quantify screen properties by standardised measurements: thermal properties (transmissivity and emissivity for thermal infrared radiation), air transport properties (sensible heat losses) and humidity transport characteristics (latent heat losses). All energy flows around screen materials are analysed, the overall potential energy saving is then calculated dynamically yearround. The project aims to quantify the energy loss through dry and wet screens materials with different properties with special emphasis on humidity transport and the consequences for humidity control by different control strategies. Screen property measurement and energy calculation results are given for four materials. It is shown that a single screen typically saves ~25-30% of energy compared to no screen in a tomato crop. A screen with optimised properties having no/low air and humidity permeability and low TIR transmissivity and emissivity, can even save ~40%. An increasing humidity setpoint from 80 to 90% RH saves 40% more energy, using screens more intensively at the beginning and end of the day by increasing the closure setpoint from 0 to 50 W m-2 saves 10-15%. Double/ triple screens add 25-40% energy saving compared to a single screen of the same material. Results further show that highest energy savings can be reached with a triple screen with optimised properties, with high RH setpoints (90%), more screen hours (up to 100 W m-2) and a mechanical dehumidification for humidity control.