Energy saving: the use of thermal screens in glasshouses

1. Experiments over three seasons demonstrated that drawing a Tyvek thermal screen over a tomato crop at high had not adverse affect on yield when compared with the productivity of crops in compartments having identical screens that were not drawn at night (unscreened).
2. The Tyvek screen reduced energy consumption at night by 60% when drawn over the crop, and reduced it by 39% over the whole period of its use.
3. Relative humidity at night was generally 4 to 5% higher under the Tyvek screen than in the unscreened compartments. The average relative humidity was no more than 85% in January and February and experiments at Efford EHS have since shown that average levels of 90% or more would be required in those months to produce a detectable reduction in yield.
4. In the light of the Efford results, it is not surprising that the use of a computer programme to limit the increase in relative humidity above 80% had little beneficial effect on yield. Such a programme might still be of value to ensure that the relative humidity did not rise above 90% for prolonged periods.
5. In conjunction with the other evidence from investigations at IHR Littlehampton, it is concluded that light loss by day from the parked screen is the main cause of loss of yield in comparison with the productivity of crops in houses without a screen fitted. Further work might concentrate on developing systems of screening that produce even less light loss by day and screen materials that have even better insulation properties.
6. The use of “anti-condensate” polythene as a mobile thermal screen did reduce yield in April and May, mainly because it generated high relative humidities both by day (c90% in January) and by night (>85% in January and February). This is consistent with observations on the effects of high humidity at Efford EHS. In addition, water condensed on the polythene and fell on the crop when the screen was moved.
7. Energy saving by the polythene screen was more difficult to assess. It was estimated to be between 23 and 29% at night and about 35% by day. Overall, using the screen by day and by night reduced energy consumption by about 22% to 26%, and 9 to 16% when used only at night.
8. At present, the best of polythene is as a cheap fixed screen but further research and development may enable other strategies to be developed.

To develop control strategies for use with thermal screens that will achieve the economic optimisation of crop production in insulated greenhouses.