Stay in control

“How do I control my energy/shade screen?” That’s a question I frequently get from growers that have recently installed a new system.

Energy screens or curtains, as they are frequently called, are becoming more popular as growers see the advantages of reduce fuel bills during the winter and lower indoor temperature during the summer. These systems are being installed in all types of greenhouses including hoophouses. Multiple screens are now quite common in gutter-connected houses as they provide more options for temperature and light control.

Deciding when to open and close the screen can be done several ways. The simplest is manual control with a switch that turns the gear drive motor. The disadvantage is that someone has to be around to make the decision on when to activate it. As with other control devices such as heaters or vents, this decision may not always be made at the best time.

These decisions can also be made by controls that are based on time of day, light level, greenhouse temperature, humidity level or all of these. Controllers have been developed that integrate some or all of these functions.

A clock is incorporated to open and close an energy screen base on sunrise and sunset. It is adjusted daily to account for the lengthening or shortening of the daylength. A photo sensor is included to activate the system when the light level inside the greenhouse reaches a preset level either too low during the winter or too high during the summer. Temperature and humidity sensors may be included to provide control when preset levels are reached.

A weather station is needed to provide outdoor light level, temperature, humidity, wind and rain or snow conditions.

Based on the above, the grower is required to enter the following parameters into the controller:

Start time – when in the morning will the screen system start operation

Stop time – when in the afternoon will the screen system stop operation

Light level – at what light intensity will the screen be activated either to open or close

Maximum/minimum temperature – what are the limits of temperature desired

Maximum/minimum humidity – what are the limits of humidity desired

Gap positions – this sets the percent open or close position under different conditions such as early on a winter morning to reduce thermal shock or during the summer to provide better shading. The gap steps can be controlled in some systems.

Time delay – under changeable weather conditions this avoids frequent opening and closing

Wind speed – to reduce heat loss, the screen can be closed early under high wind speed conditions

Snow detection – this can open the screen and turn on the heat when it starts to snow.

In multiple screen systems, settings for each screen have to be made. For example, in a three screen system, the lowest may have a blackout material with 99 percent sun shading and 75 percent energy savings. This is will provide daylength control. The middle layer may be an energy/shade material with 65 percent shading and 62 percent energy savings that is used during the summer to reduce temperature. The top screen may be a clear material that provides 19 percent shade and 47 percent energy savings and can be closed on cloudy winter days when there is insufficient light. These can provide about 88 percent energy savings as compared to a single glass or corrugated polycarbonate cover on a winter night with all screens closed. Each can be controlled independently depending on the parameters set.

Energy screens and controllers are eligible for USDA NRCS funding in some states.