Transparent dynamic insulation: a novel system combining ventilation and insulation for sustainable greenhouse applications

Greenhouse horticulture cultivates vegetables, fruits, and flowers in protected glass houses. Approximately 65%-85% of the total energy in greenhouses is used for heating. To reach the climate goals energy reduction is needed. Energy savings cannot only be achieved by improving the insulation value of the greenhouse covers since also the relative humidity of indoor air needs to stay below a critical humidity level to avoid fungus growth. Therefore ventilation is required with an average air change rate (ACH) of 2.5/h. This is often realized by (partly) opening and closing the glass greenhouse cover thereby losing part of the heat. In this paper, we focus on a novel method that improves the thermal insulation of transparent top covers and/or walls while ensuring sufficient ventilation: transparent dynamic insulation (TDI). TDI is based on a concept called Dynamic Insulation, also known as a "breathing wall": infiltrating air flows through the multi-layer insulation panel. While doing this, the incoming air takes up part of the heat and uses this for pre-heating. Thereby it reduces the overall heat transfer value (U-value) and provides pre-warmed, fresh air flow into the greenhouse. The main difference compared to “traditional” dynamic insulation is that its design is transparent for solar irradiation and therefore can be used for greenhouses. In this paper, we show experimental results on a TDI prototype panel using a hot box apparatus with an ACH=2.5/h and prescribed temperature differences between indoor and outdoor of ΔT=10OC and ΔT=20OC. It is shown that the measured overall U-value for the TDI panel is about 19% lower than for double glass and more than a factor 2 lower than single-layered Hortiplus glass. Therefore it is concluded that TDI is a promising approach to reducing energy consumption in greenhouse horticulture while ensuring sufficient air refreshment for moisture control.