- Energy Saving
Energy management in protected cropping: the use of screens
In this factsheet, Farm Energy Centre's Chris Plackett, and Steve Adams and Allen Langton from Warwick Horticultural Research International (HRI) outline how efficient thermal screens can save energy in glasshouse production without adversely affecting yield or quality.
- Fixed, perforated polyethylene screens reduce heat loss from the greenhouse but reduce crop yield and quality if kept in place for too long. Their use can only be recommended during the winter months in years when energy prices are high.
- Retractable energy-saving screens are preferable to fixed screens since they can be opened at times of high solar radiation and when humidity is becoming a problem. They typically have a high light transmission, a good insulating effect giving instantaneous energy savings of around 40%, allow the transmission of water vapour, have anti-condensation properties and are virtually non-shrinkable.
- Annual energy savings of around 13% were achieved in a commercial tomato trial using a retractable screen (and conservative settings). Savings were made principally between weeks 41 and 17, and there was no loss of yield.
- In a follow-up trial with sweet pepper retractable screens saved up to 15.5% more heating energy (90 kWh/m) than a temporary screen. The extent of the savings depended very much on the screen deployment strategy, and screen control set-points are suggested.
- Shade screens and blackout screens can also contribute to energy-saving by giving instantaneous energy savings of up to 75%.
- Screens tend to increase the glasshouse humidity and controlled gapping, possibly followed by venting and re-heating, is necessary to control this.
- Screens can cause vertical and horizontal air temperature stratification, and this may result in plants experiencing lower temperatures than those set. The problem can be countered by the use of circulation fans and, in the case of horizontal stratification, side screens and/or additional side heating pipes.
- Using temperature integration (TI), but preferentially heating the glasshouse at night under thermal screens, gives energy savings in the winter months when there is little solar gain.
- Allowing the temperature to fall around one hour before screens are removed, then increasing it again several hours later so as to maintain the average 24 hour temperature (DROP), can save up to 1.5–2% of energy and reduce the need for PGR treatment.