Estimation of Thermal Performance and Heat Loss in Plastic Greenhouses with and without Thermal Curtains
Greenhouses are important for stable food production, but require large amounts of energy to maintain their microclimate in regions with harsh climates. This study assessed the internal thermal insulation performance of thermal curtains in double-layered plastic greenhouses in Korea in winter using cover surface temperature changes and heat transfer coefficients (U values). The thermal curtain performance increased as the temperatures of the inner cover surface increased and the outer cover
... the outer cover surface decreased. The outer cover surface temperature with thermal curtains was almost uniformly 1.9 • C lower than that without thermal curtains, whereas the inner cover surface temperature was higher, demonstrating the warming effect of thermal curtain use. Under a constant indoor and outdoor air temperature difference, the daily average heating energy consumption was directly proportional to the U value. The U values were 2.76 W m −2 • C −1 with thermal curtains and 3.85 W m −2 • C −1 without thermal curtains. In double-layered plastic greenhouses that were covered with 0.1-mm-thick polyethylene, incorporating thermal curtains at night resulted in energy savings of about 28.7%, which was related to the decrease in U values. Installing and using thermal curtains at night in winter is a highly economical method for heating savings. These results can be used to promote energy savings in greenhouses in harsh climates. Energies 2018, 11, 578 2 of 11 Arinze et al.  reported that double-layered greenhouses inflated with air with a fan had U values of 2.1 W m −2 • C −1 in the closed shutter position and 4.4 W m −2 • C −1 in the open shutter position, showing that the closed shutter position offered better insulation. Papadakis et al.  found that single-and double-polyethylene-covered greenhouses without thermal curtains had U values of 6.0-8.0 W m −2 • C −1 and 4.2-6.0 W m −2 • C −1 , respectively, showing that a double-layered cover offered better insulation. Meanwhile, the Japan Greenhouse Horticulture Association  reported that double-layered greenhouses with and without thermal curtains had U values of 3.1-3.2 W m −2 • C −1 and 3.8 W m −2 • C −1 , respectively. Moreover, Seginer et al.  and Nijskens et al.  reported the U values of double-layered greenhouses without thermal curtains to be 6.9-7.3 W m −2 • C −1 and 4.8-6.4 W m −2 • C −1 , respectively. These results indicate that U values are not always suitable, and can be influenced by the type and condition of the material, convection, and radiation heat transfer process, greenhouse type, and the use of thermal curtains  . Therefore, although a number of researchers have experimentally studied the utility of thermal curtains in reducing heat loss in greenhouses, there is a need to analyze differences in the thermal insulating effect of greenhouses between those with and without thermal curtains according to various environmental conditions (e.g., indoor and outdoor air temperature difference, etc.) and greenhouse cladding material conditions  . Moreover, to the best of our knowledge, few studies have attempted to analyze the cover surface temperature and air temperature, the heat insulating effect (i.e., U values), and energy consumption with and without thermal curtains in single-span plastic greenhouses that are covered with double-layered polyethylene, which are used widely in Korea. Plastic greenhouse use has expanded rapidly worldwide, including in Europe, North Africa, South America, and China  . Of the 44,177 ha of single-span plastic greenhouse used to grow vegetables in Korea in 2014, 13,704 ha were double-layered greenhouses, and only 5669 ha had thermal curtains ; therefore, more research is required in order to support the expansion of thermal curtain use. The objectives of this study were to quantify the effect of thermal performance on energy conservation in double-layered plastic greenhouses with and without thermal curtains. To this end, we measured the air and cover surface temperatures of greenhouses with and without thermal curtains, and analyzed the relationship between the insulating effects of experimental greenhouses and cover surface temperatures. In addition, we assessed the overall heat transfer coefficients (U values) and the energy consumption of greenhouses with and without thermal curtains.