Climate control & energy > Thermal screen
Moveable (retractable) thermal screen (energy screen)
Due to rising energy prices, greenhouse growers require more radical measures to reduce the energy input in greenhouses. In colder climates, an effective measure is installing a screen that insulates the greenhouse like a curtain or a blanket. This could be a sheet of plastic that is closed day and night, but that increases the humidity and reduces the controllability of the greenhouse. To avoid that, the screen must be a moveable (retractable) thermal screen or energy screen. Basically this screen is closed when it is dark and cold, and is opened when it is light and not so cold. There is a wide range of screen material on the marked, based on, aluminised polyethylene, either knitted, woven, non-woven (e.g. foil), transparent, white, black, etc.
Energy saving
The energy saving depends on the screen material chosen, and how the screen is managed. Some thermal screen materials can save as much as 70% of the energy input, but only during the hours when the screen closed. Over a whole year, the saving is obviously much lower. So-called dual-purpose screens, which are meant for shading in summer and energy saving in winter, can save about 20-30% energy in the hours when closed. It is also important how the screen is managed, namely at what temperature and light levels the screen is closed in the evening and opened in the morning. Greenhouse insulated by a thermal screen can be heated by a smaller boiler. Alternatively, a thermal screen allows expanding the greenhouse without increasing the boiler capacity.
Disadvantages
When a screen is closed it blocks or reduces the light level and hence reduces the plant growth. But even when a screen is opened, it intercepts some natural light, causing yield loss year-round. Closing a screen increases the air humidity, which increases the risk of fungal diseases. This can be mitigated by creating small gaps in the screen to let moisture escapes, while most of the heat is retained. A lot of other things are affected too by the use of a thermal screen. Growers really need to learn how to get an optimal climate under a thermal screen.
Advanced greenhouses
A moveable thermal screen is feasible in advanced greenhouses (glasshouses). A screen is easy to install when a greenhouse is erected, but is hard to retro-fit. Thermal screens are a considerable investment, to be earned back by reduced energy costs. The costs include investment costs (spread out over 5-10 years), running costs, maintenance costs, and yield loss due to reduced natural light. Thermal screens are most feasible in areas with cold winters. When the energy prices rise further, thermal screens become more feasible in milder conditions too.
Further reading
Energy-wise installations for greenhouses (2003)
Thermal screens for energy saving in greenhouses (1) (2003)
Thermal screens for energy saving & climate control (2) (2003)
Thermal screen tested in practice (2005)
Cost & benefit of energy screen in glasshouse in New Zealand (2006)
Energy-wise greenhouse control in winter (2006)
