Horticultural LED Lighting Part 2: Heat Management, Optics and the Body of LED Fixtures

Lux Light InternationalLED Lighting, Lighting

thermometer

Read the Previous Part:

Part 1: Powering and cooling the fixture

Heat Management with LED Lighting in the Growing Environment

After having a look at how the fixture is cooled down to secure long-term operation without overheating, we have another look at heat management. Heat management refers to the ambient temperature
within a growing environment. First of all: Why is the ambient temperature so important?

Photosynthesis and its reaction rate is, just as every other biochemical reaction, directly related to the environmental temperature. The optimum temperature for enzymes dictates the optimum temperature for
the maximum photosynthesis-rate as well. Only if the demanded minimum of heat for a plant is met, photosynthesis can take place.

For frost-hardy plants, this temperature would be -1°C for example. When the temperature increases by 10°C, the reaction time of the photosynthesis, depending on the plant, can double or even quadruple until the temperature reaches the optimum for the given plant. For most plants, the optimum temperature lies between 20 and 30°C. When the temperature keeps rising after reaching the optimum, the photosynthesis rate begins to fall because of denaturation of the responsible enzymes proteins until it comes to a full stop.

Net photosynthesis in relation to ambient temperature

Net photosynthesis in relation to ambient temperature

 

Water use efficiency in relation to ambient temperature

Water use efficiency in relation to ambient temperature

So, how can one ensure that the temperature in the environment does not exceed the optimum?

The following image shows how it is done with passively cooled LED fixtures.

Heat management with LED grow lights

Heat management in a growing environment with the use of passively cooled LED fixtures

As seen in the image, the heat generated by the LED fixtures can surpass the spaces between the passively cooled systems. By measuring the ambient temperature with the help of an installed sensor within the environment, the ventilation up top can regulate the how much of the warm air has to be dissipated to achieve the optimum warmth for photosynthesis of the cultivation.

With actively cooled fixtures the problem that the ventilation systems suck the air back in, which makes the heat regulation way more difficult. The fans are running to keep the fixture from failing, rather than to regulate the heat-flow and ambient temperature.

Optics and Covers of the LED Fixture

LED fixtures can be put in one of three different categories: Plastic or other artificial material such as silicone, glass and fixtures that use no cover at all.

Plastic and other artificial materials are always the cheapest and easiest way to produce a cover or a protection for the LEDs.
The downside of these materials is that even brand new and clear synthetic materials will always absorb 6 to 8% of the emitted light of the LED – no matter what! (Depending of the quality of e.g. acrylic
glass)

In an environment, which will never be 100% clean (as stated in the part about cooling the fixtures) you will occasionally need to clean your fixture to not lose even more light. It is also very common that
grow lamps get dirty at the bottom over time especially when your crop gets in contact with the cover of the LEDs.

In addition to this, the usage of water alone will not always do the job if you have sticky or resinous materials stuck to the fixture. Some plastics used in the production of LED fixtures are not suitable for cleaners and will go “blind”.
This destroys the cover or optics and you will have to repair it or, if the parts are not removable, buy a completely new LED fixture.

Silicone tends to get yellow over time, especially when it gets heated. This leads to loss of light as well as problems with the fixtures stability of the spectrum.

Fixtures with no cover only use the optics on the LED itself along with some kind of conical reflector. This practice in some cases is better than a plastic cover because you do not lose light due to absorption but it also gives no protection to the optic/LED themselves.

Furthermore, there is no way to clean the fixture without damaging the reflector or lens or at least the risk of doing so in the process.

The usage of glass is the most expensive way to produce covers or optics for LED fixtures but it is also the best way to do it. When using the right glass, which is non-reflective, it is possible to get an absorption rate of 1% and below!

It is easy to clean and suitable for most cleaners without the danger of going blind such as plastics would. The only downside it has is the costs, which leads to a higher price but as well higher quality of the fixture.

Conclusion on optics and covers of LED Fixtures:

Do not buy LEDs with plastic covers, especially not silicone they take away a good amount of light right from the start. Over time those plastics will go blind and are hard to clean without damaging the surface. No covers are better because you do not lose intensity but this makes it almost impossible to clean the LEDs.

Consider spending a little more money on non-reflective glass covers. They are the best option for a long life and constant output and spectrum

 

The Body of the LED Fixture and its Protection


Drops on a leaf

IP Classes give information about protection of water and dust.

This part secures the whole fixture and is about preventing fires, damages to the fixture itself or harm to the life of the operator.
When talking about security within growing environments and the electronics within it is important to look at IP classes. It is important to either separate all electronic devices from areas in which they
can get wet and/or dirty or seal them with a value of IP 54 (resistance against dust and splashing water) at least!

What do these numbers mean?

The first number of the IP Protection indicates the resistance against dust.

Level  Effective against Protection against
0 No protection against contact and ingress of objects
1 >50 mm Any large surface of the body, such as the back of a hand, but no protection against deliberate contact with a body part
2 >12.5 mm Fingers or similar objects
3 >2.5 mm Tools, thick wires, etc.
4 >1 mm Most wires, slender screws, large ants etc.
5 Dust protected Ingress of dust is not entirely prevented, but it must not enter in sufficient quantity to interfere with the satisfactory operation of the equipment.
6 Dust tight No ingress of dust; complete protection against contact (dust tight). A vacuum must be applied. Test duration of up to 8 hours based on air flow.

The second digit indicates resistance against water.

Level Protection against
0 None
1 Dripping water
2 Dripping water when tilted at 15°
3 Spraying water
4 Splashing of water
5 Water jets
6K Powerful water jets with increased pressure
7 Immersion, up to 1 m depth
8 Immersion, 1 m or more depth
9K Powerful high temperature water jets

As a rule of thumb, it can be said: the higher the IP number, the better and resistance against dust and water are necessary.

This secures that no electronic component gets in touch with water or dirt and therefore minimizes the risk of failure, fire or even death of the operator

Read in the following part:

Part 3: LEDs, the Heart and Soul of the Fixture