A study of U value of greenhouse films with thermal screens using the hot box method

A laboratory guarded hot box was used to measure the overall heat transfer coefficient (U value) of two types of plastic film (IR 200 μm, 8% thermicity and IR 100 μm, 35%, thermicity), with or without thermal screens (sealed aluminet 50% shade and sealed aluminet 100% shade). Tests were performed in the hot box at different wind velocities and temperature differences. It was found that the overall heat transfer coefficient U increases with the increase in temperature difference. For a given air temperature difference, the U value increases as the wind velocity increases, as expected. There was only a small difference between the U values of the two types of plastic films that were tested. It was found that using a thermal screen would reduce the U value by about 50-60% (depending on type of thermal screen), an energy saving of about 50-60%. Although there was a large difference between the shading percentages of the two thermal screens, the difference between the U values of the combined plastic film and screen was relatively small. Possible reasons for this result are discussed.