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How to optimise screen and vent control with lights  

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First published May 2011

Learn what practical changes to make to improve the integration of screen and vent controls with your lighting operation.   

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A practical case study

Because supplementary lights also produce heat they can have a sudden impact on the temperature balance of a greenhouse when they are turned on. 

Get your control strategy right and the ‘waste heat’ from the lights can make a useful contribution to the heating needs of the greenhouse. 

On the other hand, get it wrong and the heat can be a headache which has an adverse effect on both the greenhouse environment and your energy costs. 

It is particularly important to get the interaction between lighting operation and screen and vent control right, so this update looks at the changes the GrowSave team made at Humber VHB’s West End site. 

What are the potential problems to using lights under screens?

What happens when the lights turn on and when the screens open in the morning. 

The graphs in Figure 1 and Figure 2 show what can happen when screen and vent controls are not correctly integrated with lighting operation. 

Graph showing heating, ventilation and greenhouse temperature and vent and screen positions throughout a day where the lights turn on just before midnight, the heat from lights cause greenhouse temperature to rise, then vents open to over 20% to control temperature rise and the screen remains 100% closed even though vents have opened wide.
Figure 1 When the lights turn on

The main issue here is that the vents are open and cold air traps above the fully closed screen. This leads to potential energy waste and problems when the screen opens the following morning. 

Graph showing heating, ventilation and greenhouse temperature and vent and screen positions throughout a day where screen opens as solar radiation levels increase. Cold air trapped above the screen causes a sudden drop in the greenhouse temperature. Heating operation causes temperature control to overshoot - which then needs ventilation to correct.
Figure 2 When the screens open in the morning

Once the screen is opened in the morning, the cold air which has been trapped above it falls into the greenhouse causing the temperature to drop dramatically. Whilst this can be overcome by operating the heating system, unnecessary energy waste occurs because of the rapid change in temperature and the inability of the heating and control systems to accurately restore the required temperature. 

How to correct the problems

What happens when you gap the screens before opening the vents. 

The golden rule for controlling screens is to gap them before you open the vents. The graph in Figure 3 shows the result when this approach was applied on our Focus Nursery. 

Graph showing heating, ventilation and greenhouse temperature and vent and screen positions throughout a day where lights still turn on just before midnight. Again heat from lights causes greenhouse temperature to rise. Now the screen gaps as the first response to the rise in temperature. Vent operation is greatly reduced giving energy savings and more accurate control. Note there is no sudden drop in temperature when the screen opens in the morning. this is because there is no longer cold air trapped above the screen.
Figure 3

The control settings used to achieve this result were to start to gap the screen when the greenhouse temperature was 0.2oC below the vent temperature. The screen gap was then increased until it reaches a maximum of 5% when the greenhouse temperature was 0.5oC above the vent temperature. 

How to improve the effectiveness of greenhouse screens

Learn how to use greenhouse screens to save energy, control humidity and achieve a uniform growing environment. 

Read more about improving the effectiveness of greenhouse screens

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