How Grow Lights Work
Grow lights produce light in the 400 - 700 nm wavelength range to be most effectively be used by plants. Photosynthesis uses the so-called PAR, or photosynthetic active radiation region of the electromagnetic spectrum. Light outside of this range can also be beneficial for plant growth but the peak photosynthetic efficiency falls around the blue and red regions of the range. Blue light is essential for flowering and vegetative stages of plant growth while the red light is more efficient at growth of biomass. Most of the research freely available online at this point in time seems to be centered around the pot industry so it's hard to find too much general information.
The heat from sunlight is what ends up killing plants when they get too much sun rather than the light itself. Keeping that in mind, I want to go overkill with the amount of light I'm producing, so long as I can remove that heat that is created by the modules. I've selected the water cooled method as a way to help easily move this heat from a point inside the greenhouse to outside the structure. I had also considered passive cooling with fans, forced air cooling, and heat pipes but all of those methods had significant downsides. While the water cooling method does have issues, mainly with the amount of maintenance it requires, it is the preferred method for my design goals.
This form of cooling would be costly to make due to the amount of material, whether that's copper or aluminum. It has the benefit of being the simplest maintenance as once it's built, unless an electrical part fails, it won't need ongoing work. Passive cooling fins would also have the side effect of having the heat dissipated near the LED modules. This would need an additional solution to cool the greenhouse as the heat would be able to build up otherwise.
Heat pipes were considered as a passive method of heat control, similar to fins, but have the additional benefit of additional cooling capacity per volume over fins due to the additional surface area and path for heat conduction. They would also allow the heat to potentially be piped outside of the greenhouse rather than letting it build up inside. Heat pipes are very difficult to work with without cracking or pinching, which would ruin the effect, and are quite expensive. This method was quickly crossed off after looking up prices for even small amounts of piping. The economy of scale is not on my side for a project like this, it may be viable in larger quantity though.
Forced Air Cooling
This method of cooling was the only close competitor to the water cooling I ended up deciding on. The upsides to this method include very efficient cooling, cheap to implement, and it needs little maintenance. At the end of the day, once again, it came down to the heat location. Since the air cooling would have to be located inside the greenhouse it would either build up or have to be vented to the outside. While water cooling requires more maintenance the trade off of heat location is worth it to me as it will be easier to create a consistent environment inside the structure.
I laid out and followed the following process to determine how to characterize the LED modules I had.
- Measure voltage and current with single LED module
- Calculate P (electrical power) by multiplying measured voltage and current
- Measure ambient temperature
- Measure average temperature of LED substrate
- Subtract ambient temperature from average temperature of substrate for ΔT
- Divide ΔT by electrical power to find the average Rthermal for the module
Heat output of LED Module
In order to get an accurate estimate of how much heat power is being generated by an LED module. To simplify things I used an aluminum block to minimize the effect of the surface area.
Specific Heat of Aluminum
This means 1 gram of the aluminum material will need to be provided with 0.91 J of heat to raise 1°C.
Heating a Block
To find the amount of heat added to the aluminum block in Joules you can use the formula q=m*c*ΔT where m is the mass of the block, c is the specific heat, and ΔT is the change in temperature. Since Watts is Joules per second, by recording the length of time it takes to arrive at the ΔT we can get an estimate of watts the module is outputting as heat. In order to get an approximate mass of my block I measured out the sides of the block and plugged the resulting volume into a calculator.
70-90°C max junction temperature
Rthermal= (ΔT/P) = [ΔT/(I*V)]
T = Pd*Rth Pd = 30 W * 80% (estimated) = 20 W
Tj - (RJCOB*Pd) 80°C - (RJCOB*24 W)
The design of the greenhouse structure was based on a design found on Thingiverse by Fran Gabriel.
The goal of this project is to create an environment that can effectively keep plants from dying in a house that has a severe lack of windows and natural lighting. The greenhouse will be fully enclosed to keep the plants inside safe from cats and the cats outside safe from the angry pixies flowing through the 90 Watts of life sustaining plant life. The LED modules are powered by mains voltage with the other accessories powered by a 12 Volt power supply.
Manuals and Documentation
LED Module Specifications
- 80-100 LM/W Luminous Flux
- 30-36 V
- 1500-3000 LM
- 30 W
- 4 cm x 5.4 cm
- 380-780 nm
Thermal Glue Specifications
I picked up my thermal conductive glue atAmazon. The small 10oz tube was more than enough for what I needed and will hopefully keep so I can use it on more projects in the future. The manufacturer gives the full specifications in order to use in thermal calculations. One thing to note is that thermal glue, unlike thermal paste, is not meant to come off and is used both for its conductivity as well as its mechanical properties. Since the LED modules are quite light and have a large surface area I have no worries about them falling off so long as the cooling is adequate.
- Thermal Conductivity >1.2 W/mK
- Thermal Impedance <0.06
- Insulation Coefficient >5.1
- Dissipation Coefficient <0.005
Creating a Custom EAGLE LED Module
There's an excellent series on the AutoDesk EAGLE blog, as part of their EAGLE Academy, on how to first start making your own parts. The guide is simple and geared towards a beginner and broken up into three posts:
https://www.efunda.com/formulae/heat_transfer/convection_forced/calc_lamflow_isothermalplate.cfm#calc https://electronics.stackexchange.com/questions/3784/how-do-i-calculate-the-thermal-resistance-of-aluminum-flat-stock https://www.boydcorp.com/aavid.html https://www.instructables.com/How-to-make-a-custom-library-part-in-Eagle-CAD-too/ https://www.quora.com/How-fast-does-aluminum-dissipate-heat
- Soil moisture tracking
- Temperature logging
- Temperature regulation
- IOT implementation
- Timelapse Camera