Use of Adapted Energy Screens in Tomato Production with Higher Water Vapour Transmission

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1 Use of Adapted Energy Screens in Tomato Production with Higher Water Vapour Transmission Hugo F. Plaisier Ludvig Svensson Marconiweg LV Hellevoetsluis The Netherlands Keywords: screens, energy saving, glasshouses, tomato, air humidity Abstract Energy saving screens contribute a lot to the reduction of the energy use in glasshouses. Most modern glasshouses are already equipped with one or more energy screens. However, some growers still hesitate to use them, because they are afraid for the negative influence of these screens on the growing conditions for the crop. This is especially true for tomato growers, who dislike increases in air humidity due to the use of screens. In 2003 Svensson introduced a range of energy screens with a higher water vapour transmission by means of small, evenly distributed openings. The results, both in small- as well as large scale trials, show that these screens have little or no effect on the air humidity, while keeping the energy saving value at a relatively high level. It is expected that the use of energy screens among tomato growers will be increased considerably and consequently the energy use will be reduced. INTRODUCTION It is no discussion that the glasshouse industry is one of the biggest consumers of fuel and natural gas. That s why energy saving measures are important, both for the profitability of individual growers as for the image of the glasshouse industry as a whole. Energy screens are used for many years for saving energy. There are many types, both aluminised as well as transparent. The amounts of energy that can be saved are quite big: in closed position up to %, on an annual basis between 20 and 50%. Most growers have already adopted energy screens into their greenhouses. However, there are still growers that are hesitant to invest in screens. This is especially true for tomato growers. They are afraid that screens increase the air humidity to too high levels, leading to softer plants and more fungal diseases (Botritis). Gapping of the screen in order to reduce air humidity is not always an option as it may create horizontal temperature differences. In order to remove the objection of tomato growers Svensson started a development for more water vapour transmitting screens without loosing much of its energy saving value. In this article we give you the details of this development as well as the obtained results. MATERIALS AND METHODS As most vegetable growers prefer a highly light transparent screen we based our development on SLS 10 ULTRA PLUS. This transparent screen combines a high light transmission with a good energy saving effect. Light transmission is 88% for direct and 81% for diffuse light (measured with an integrated sphere) and energy saving 43% (Svensson internal measurement). This screen is made of 4 mm wide PE strips, knitted together with fine polyester yarns. Due to this structure it transmits a certain amount of water vapour. The increase in water vapour content was realised by replacing at regular distances some of the 4 mm strips by 2 mm strips, leaving two 1 mm small openings in the knit structure. In order to determine the optimum balance four different types were developed: Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS

2 - SLS 10 ULTRA PLUS 1:5 (to be mentioned 1:5) - SLS 10 ULTRA PLUS 1:10 (to be mentioned 1:10) - SLS 10 ULTRA PLUS 1:15 (to be mentioned 1:15) - SLS 10 ULTRA PLUS 1:20 (to be mentioned 1:20) where each fifth, tenth, fifteenth or twentieth strip was replaced by a half strip, respectively. Firstly the behaviour of the above types was determined on a small scale in a trial glasshouses of 10 m² at Svensson in Sweden. This took place in the spring of The sides of these trial houses were well insulated with 10 cm insulation plates to exclude gable effects. In order to simulate a well-transpiring, mature tomato crop wet blankets were placed vertically, which could suck water from a reservoir freely. The amount of water, absorbed from the reservoir, the energy use and the increase in air humidity due to the screen were monitored during several nights. Inside temperature was kept on 17 ºC, outside temperature was around 0 ºC. Heat consumption was measured by monitoring the exact gas use per trial glass house Secondly the effect of the screens in big, commercial tomato greenhouses was determined in the winter season of 2003/2004, both by interviewing the growers as well as by analysing the heating pipe temperature, air temperature and humidity readings from the climate computers. Out of the pipe temperatures and inside temperatures the energy use is calculated, using following forms: Radiative heat from heating pipes (per m pipe): 4*PD/1000*π*((PT/100) 4 - (AT/100) 4 ) Convective heat from heating pipes (per m pipe): 7.5* π *PD/1000*(PT-AT) PD: Pipe diameter in mm PT: Pipe temperature in ºK AT: Air temperature in ºK RESULTS AND DISCUSSION Small Scale Test The results of the tests, carried out in the trial houses in Sweden, are shown in table 1. The results clearly show the effect of the application of half strips in all prototypes. The increase in air humidity with the standard screen was 6%, with the four prototypes only 1 or 2%. As a result more water is evaporated. In this test the energy saving decreased from 43% with the standard screen to 34 37% with the prototypes. Condensation on the screen hardly took place, in contrast with the standard screen, which got wet under the test conditions. Commercial Glasshouse Experiences In the large scale tomato glasshouses results differed from the small scale trials, both for the air humidity and for the energy saving, see table 2. The highest increase in air humidity with the standard screen (SLS 10 ULTRA PLUS) was 11%, while with the prototypes the increase was 0 to 6%. Energy saving values of the most open types was somewhat lower in the large scale glasshouses than in the small scale test. For both 1:10 and 1:15 we have added graphs, showing the effect on the air humidity as well as on the heating pipe temperature. All growers, working with the new prototypes, indicated that the screens remained virtually free from any condense droplets, unlike the standard SLS 10 ULTRA PLUS some tomato growers use. This is highly appreciated, as it takes care for a dry crop (no dripping) and it keeps the screen free from algae and dust. Another advantage of the new prototypes the growers found out is that it is possible to lower the air humidity, if necessary. This is realised by simultaneously closing the screen and opening the vents. The openings in the screen allow for enough water vapour drainage. In case that there is no screen opening vents create drop of cold air, which can be harmful for the crop. 584

3 All growers indicated that they could use the prototypes for many hours until late in the season, longer than colleagues working with the standard SLS 10 ULTRA PLUS. This longer use compensated for the lower energy saving value of the prototypes. Discussion The reason that the results between the two experiences differed is most probably due to the fact that in big greenhouses there is more air movement. This results into more water vapour drainage for the prototypes than in the small scale tests and apparently lowers the energy saving effect with 1:5 and 1:10. Quite notable is that the porous structure of the prototypes has a much bigger effect on the water vapour transmission than on the heat transmission. Fortunately, because it eliminates the increase in air humidity by the screen, still keeping its energy saving effect on quite a high level. Based on the presented results and on growers experiences we predict that prototype 1:15 will be the most chosen one, as this prototype offers the best combination between a good energy saving value and a humidity neutral behaviour. CONCLUSION Our conclusion is that we have succeeded in developing a screen solution for tomato growers who were hesitant to use an energy screen up to now. For the glasshouse industry this means a big extra energy saving potential. A conservative calculation is that around 1000 ha of tomato glasshouses in North Western Europe did not yet invest in energy screens and now could be convinced to start investing in this technology. The total resulting energy saving per year could reach m³ of natural gas, equal to litres of fuel. Tables Table 1. The effect of several transparent energy saving screens in small 10 m² big trial greenhouses. Type Increase in RH Evaporated water, L/night Energy savings Condensation on screen? Percentage openings No screen 0% :5 1% 7 34% No 5,0% 1:10 2% 5 36% No 2,5% 1:15 2% 4 36% Some traces 1,6% 1:20 2% 3 37% Wet spots 1,3% Standard* 6% 2 43% Wet all over 0,0% *The standard is SLS 10 ULTRA PLUS without half strips. Table 2. The effect of several transparent energy saving screens in large scale tomato greenhouses. Type Increase in RH Energy savings Condensation on screen? Percentage openings 1:5-0% 30% No 5,0% 1:10-0% 33% No 2,5% 1:15 3% 37% Some traces 1,6% 1:20 6% 40% Wet spots 1,3% Standard* 11% 43% Wet all over 0,0% *The standard is SLS 10 ULTRA PLUS without half strips. 585

4 Figurese Relative air humidity, % With screen SLS 10 Ultra Plus 1:10 Without screen 18:00 20:00 22:00 0:00 2:00 4:00 6:00 8:00 10:00 Time, h Fig. 1. Air humidity in commercial tomato glasshouse with and without SLS 10 Ultra Plus 1:10. Date: 15/16 feb 2004, location Steenbergen, The Netherlands. Temperature, ºC Pipe temp with SLS 10 Ultra Plus 1:10 Pipe temp without screen Inside air temp :00 20:00 22:00 0:00 2:00 4:00 6:00 8:00 10:00 Time, h Fig. 2. Heating pipe temperature and inside air temperature in commercial tomato glasshouse with and without SLS 10 Ultra Plus 1:10. Date 15/16 feb 2004, location Steenbergen, The Netherlands. 586

5 90 85 Relative air humidity, % With SLS 10 Ultra Plus 1:15 Without screen 15:00 17:00 19:00 21:00 23:00 1:00 3:00 5:00 7:00 9:00 Time, h Fig. 3. Air humidity in commercial tomato glasshouse with and without SLS 10 Ultra Plus 1:15. Date: 19/20 feb 2004, location Monster, The Netherlands. 90 Temperature, ºC Pipe temp with SLS 10 Ultra Plus 1:15 Pipe temp without screen Inside air temp :00 17:00 19:00 21:00 23:00 1:00 3:00 5:00 7:00 9:00 time, h Fig. 4. Heating pipe temperature and inside air temperature in commercial tomato glasshouse with and without SLS 10 Ultra Plus 1:15. Dat 19/20 feb 2004, location Monster, The Netherlands. 587

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