The type of lamp that has dominated as a grow light so far is high-pressure sodium lamps, abbreviated HPS or SON-T. These are relatively inexpensive lamps with a good light spectrum and good light output. There has long been talk of replacing them with LED fixtures that have even higher light output, but they have had high prices and there has been some uncertainty about the quality and durability of the fixtures. Another issue has been the climate for plant growth under LED. In particular, the fact that LED has less radiant heat, with the risk of lower leaf temperature and too high humidity. Now we see that more people are switching to LED both in new construction and replacement in existing facilities.
At Mære we have gradually switched to LED over the last 6-7 years. The first time was in 2018 when we installed LED lights between the tomato rows, then as hybrid lights with 50% LED and 50% HPS in the potting department, and now most recently in November 2023, two of the three rows of lights at the top of the tomato department were switched to LED, while a number of HPS were retained.
As mentioned, the light output and efficiency are much higher with LEDs. In plant cultivation, we measure light in a spectrum where photosynthesis is efficient and we call it photosynthetically active radiation PAR (400-700 nm). We use the unit of measurement micromoles (µmol) and divide it per watt (µmol/W), to say something about the efficiency of the light source in relation to the power consumption. HPS lamps are not bad 1.7-1.9 µmol/W, but LEDs are better 3.0-3.5 µmol/W.
Our first LED installation was so-called intermediate light. There were two rows of lights down in the tomato plants about 1.2-1.5 m above the ground. 180 luminaires of 100W each are installed on half the area (18 kW in total). We have registered 15-20% increased yield on that side. Partly due to the extra energy, but also due to the placement down in the plants.
The ceiling light in the tomato department was replaced in November 2023, when 2/3 of the light was replaced with LED. There were previously 263 kW of HPS installed. After replacement, the power was 166 kW with approximately the same light level of 450-470 µmol/m2. The savings in electricity consumption for 4200 hours will be just over 400,000 kWh. We do not have data from the entire season here yet.
It is a challenge to keep the temperature and humidity at the right level. Some of the gains can be lost by having to add more heat via the heat pipes. We also learned that the old HPS lamps are important to have when it is cold outside so that the plants do not get cold at the top.
More information
Here are some good considerations about light quality https://www.youtube.com/watch?v=kUpEQ4kU148
