'Plant temperature-based climate control easier than first thought'

Cucumber grower Pleun Struijk had plant temperature sensors installed in all of his glasshouse compartments. His aim was to prevent the ambient temperature rising too high in summer and to save energy in spring and winter. The plant temperature now determines 50% of his climate control strategy.

Virtual temperature
The Clima 500 allows you to control the climate based on a ‘virtual temperature’, which is a user-defined ratio between a number of sensor units and/or a plant temperature sensor. Pleun now uses this virtual temperature instead of the measurement of just one sensor unit. Pleun Struijk: ‘After installing the PT sensors, my climate control strategy was based for 20% on the plant temperature and for 80% on the glasshouse air temperature. Despite the moderate temperatures last summer, I clearly noticed that my control strategy enabled the computer to much better anticipate excess plant temperatures.’ Measuring the plant temperature results in healthier crops, since it allows growers to avoid accelerated leaf aging caused by excess plant temperatures. ‘I programmed my Clima 500, so it would prevent the plant temperature from exceeding 30ºC. As a result, if there was a risk of the plant temperature rising above the pre-set limit, both the air vents and fogging system were implemented sooner. Under this strategy, the screen was also used more frequently’. 

More sunlight
Traditional climate control based on light increase starts from the premise that plant metabolism increases when there is more sunlight. More light usually means more photosynthesis. However, it also means an increase in the plant temperature, and with it a rise in respiration and breakdown of sugars. Plant growth, therefore, not only relies on the production of sugars, but also the breakdown of sugars, which is called the ‘net photosynthesis’. If you allow for a higher breakdown of sugars when the plant temperature increases in your control strategy, a higher net photosynthesis can be achieved. It should be noted, however, that this also requires a closer look at the light increase settings.

Generating heat according to need
The last few years have seen a sharp decline in the use of the minimum pipe temperature. More and more growers are now generating heat according to their needs. This has the drawback that the plant temperature is not taken into account. If the plant temperature rises as a result of sunlight, but the surrounding temperature remains the same, a minimum pipe temperature will be applied, while this is not necessary for the plants. In other words, a waste of energy! ‘If the plant temperature increases, my Clima 500 automatically lowers the minimum pipe temperature, so heat is only generated according to my needs. This works both ways, of course, because a higher pipe temperature will be applied when the plants cool down. It remains to be seen what the net energy savings will be,’ said Pleun Struijk.

Other features
Both the Clima 500 and the MultiMa allow the plant temperature to be used in climate control. The MultiMa lets you set different plant temperature ratios in heating, ventilation and screen control. Although the Clima 500 lets you set one ratio, it can be enabled or disabled for a particular control element. In addition to the temperature, the Vapor Pressure Deficit (VPD) plays an important role in plant health and development. The VPD indicates the plants’ transpiration potential. Both the MultiMa and the Clima 500 include a VPD readout.

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