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Light sum control

If the artificial light is to be used optimally, it is important that the greenhouse's climate is optimal. Both temperature and CO2 concentration must be so that the artificial light can be utilized. It is therefore important that the CO2 concentration in the greenhouse is high enough when the artificial light is on. Depending on the price of both electricity and CO2, you can even consider replacing some of the artificial light with a higher CO2 concentration. In theory, it will work, but to my knowledge it has not been investigated further. Even in winter, we have periods of clear weather and more than average light. During these periods, we have the opportunity to utilize an actual light sum control.

Origin of the light

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When saving on artificial light, it is important to assess how much the artificial light actually means for plant production. The figure shows for 5 days how the light at plant height (dark orange) is distributed as part from the artificial light (bright green) and part from the natural light (bright orange).


The green column is the light outside the greenhouse.


With data around December 1st, it is the light of artificial light that is crucial. A similar figure from September will show that natural light means most things and that artificial light may not be so necessary at all.

Light sum and weather forecarst

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If the plants get a lot of light on some days, you can save on light in other dark days. In practice, it has been shown that in the transitions from winter to spring and late summer to winter, you can save up to 20% of artificial lighting costs. Both the Senmatic and Priva climate computers have built-in functions for light sum control. The full effect of control by light sum is achieved when you combine the control with both weather forecasts and the variation in electricity prices. The figure shows how such a control can work when using the HortiAdvice program InfoGrow. Data from 2 houses with artificial lights have been shown. The first column shows the desired light sum and the subsequent columns show the light obtained in the last 5 days. The light is calculated at plant height based on the system's knowledge of curtains, curtain strategy, covering materials, installed artificial light, etc.

 For example, you can see that in House 1 you want a light sum of 7 moles of light per m2. It has achieved 6.3 mol/m2 and is thus slightly behind. The yellow row shows how much light the weather forecast for the nursery's position expects in the coming days. Below you can see how many hours the artificial light must be turned on to reach the desired light sum. The idea is that the climate manager changes the lighting strategy daily, so that you achieve the desired light sum by turning on the artificial light for the specified number of hours when the electricity price is lowest and it fits into the gardener's work and the needs of the plants.

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