Grow Lights and Automation

Fig 1.  Chlorophyll.

The Vertical Farm (TVF) is an implementation of Controlled Environment Agriculture (CEA), employing technology that enables the manipulation of the environment to meet a crop's optimum growing conditions.  Greenhouses, aquaculture, hydroponics and aquaponics are examples of CEA, where temperature, lighting, heating, pH, humidity and nutrient analysis are all tightly controlled.  Every plant has a natural tolerance range for each of these variables, but it is light that plants depend on most, keeping all other variables within their extrema.  It is also one the more intense areas of research in CEA.  For commercial operations and home growers alike, lighting is the key to a consistent yield - the determining factor to retaining customers and remaining in business. 

To understand the importance of light, we need a quick biology and physics lesson.  Inside each leaf of a plant are hundreds of thousands of microscopic organelles called chloroplast, whose function, among other things, is to conduct photosynthesis.  The green pigment found in a plant leaf is called chlorophyll and it is the job of chlorophyll to capture photons and convert them into energy-storage molecules.  During photosynthesis, the chlorophyll use photons to break down carbon dioxide \(CO_2\), which contains one carbon and two oxygen and water \(H_2O\), two hydrogen and one oxygen.  Carbon dioxide and water are broken down to produce sugar and other plant products like cellulose \((C_{6}H_{10}O_{5})n\), in the process releasing the oxygen in the carbon dioxide molecule back into the atmosphere.  Animals then eat the plants and combine the carbons in the plant sugars with two oxygen atoms we breathe, producing the carbon dioxide \(CO_{2}\) we exhale as waste and adenosine triphosphate (ATP), the chemical energy our bodies use.  The cycle repeats.

If we could keep the amount of light consistent 365 days a year, we would produce a consistent crop, in quality and quantity.  Remember, a commercial greenhouse operation expecting to sell 1500 heads of lettuce a week needs to sell 1500 heads of lettuce, not 1000 and not 2000; excess production is just as much a problem as a lack of production.  If you produce too much you probably don't have enough transplants ready and will have unused space in your grow beds.

Mother nature is rarely cooperative in providing the same intensity of light on a daily basis, so it was only natural for indoor farmers to employ artificial lighting.  If we want to increase the efficiency of lighting for plant growth we start with the observation that chlorophyll is green.  Obvious, but important, because it means chlorophyll does not absorb the entire light spectrum and artificial lighting drawing power to produce green wavelengths is wasting money.

Continuing our research, we find there are two important forms of chlorophyll, chlorophyll a and chlorophyll b, and each absorbs distinct wavelengths of the visible electromagnetic spectrum.  Fig. 2, shows the peak absorption wavelengths.

Fig. 2 - The EM absorption spectrum of chlorophyll.  Image Source: Wikipedia.

You'll notice they absorb mostly in the darker blue and red spectrum.  This is critical because it means the majority of the visible light is useless in the process of photosynthesis.  It also means those nice bright white lights are drawing electricity to output wavelengths that are simply wasted in the growing process, reducing efficiency and increasing costs. 

The parameters for determining optimal lighting are:
  1. The type of plant being grown - intensity
  2. The stage of growth - wavelength
  3. The photoperiod required by the plant - time
The vegetative phase requires the blue spectra while fruiting requires the red, or red-orange spectra and typically requires a higher intensity. Seedlings require less intensity and are planted much closer together (an average of 2 inches apart).  This allows one grow light to be used for a large number of seedlings and is therefore more economically viable.  As the plants grow, the single grow light can no longer provide the necessary intensity for the same number of plants, forcing growers to either buy more lights or rely on natural light alone, thus losing control of the schedule.

Not all lights are created equally.  Incandescent bulbs are inexpensive, but have very poor intensity and waste the most electricity in the form of heat.  Fluorescents also lack enough intensity for larger plants or plants in the fruiting stage, but they can be used for plants not requiring too much light, such as herbs.  Newer fluorescents are making big strides in heat loss and increased intensity, but obviously cost more.  High Intensity Discharge (HIDs) lamps are on the high end, both in cost and quality.  They are very bright and are more efficient, but they still output a lot of thermal radiation.  Metal halide bulbs have a blue tint to them and based on your new knowledge of wavelengths you know these are good for the growth stage; they also output a lot of lumens.  While metal halides are good for growth, high pressure sodium lights hit the spectrum for fruiting/flowering with a red-orange tint.  To get the most out of artificial lighting it has become a common practice to put the grow lights very close to the plants to achieve a higher intensity, however, this is dangerous with lights that output too much heat. 

New forms of lighting are being engineered for the express purposes of emitting just the right wavelengths plants need and increasing efficiency by drawing only the energy needed to produce them and preventing the loss of energy to thermal radiation.  LEDs (light emitting diodes) have been around for years, but are relatively new in application to agriculture while OLEDs (organic light emitting diodes) hold the potential to accurately target specific wavelengths.  Another exciting aspect of OLEDs is that they can be "printed" on thin pieces of plastic, allowing for uniquely shaped lights.  Imagine a hollow cylinder that produces light only on the inside, and that can enclose an entire plant.

The production of light is not the only area for improvement.  A second area of research is the high tech plastics and glasses used in greenhouse windows and a key component in a Vertical Farm.  Glazing on the glass can lead to absorption of the wrong spectrum and reduce the overall intensity, both highly undesirable characteristics.  There is active research into windows that allow for higher emissivity, one-way emissivity, heat retention, increased tensile strength, lighter weight, improved weathering and made from recycled plastics.

Cornell University's CEA program has done some important research in the amount of light a plant needs.   They have developed an algorithm to meet optimal lighting with what they call the Daily Light Integral.  By using a combination of artificial lighting on cloudy days, natural light and active shading on sunny days, they can hit a target DLI over the course of three days and therefore keep a consistent crop rotation schedule. For commercial operations and self sufficient families who depended on their aquaponics systems, consistency is key.  Exceeding the photoperiod for a plant is a waste of money and can cause physiological symptoms.  Plants producing fruits (or flowers) need about 16 hours a day of solid light, while non-fruiting plants are around 10 to 12 hours.  Don't forget: plants need their "sleep" too.

Fig 3.  Using a LDR input and a relay to control grow lights.

Automating aquaponic grow lights for systems utilizing a combination of daytime and artificial lighting can be done using a light dependent resistor (LDR) and a 120V @15A relay, as seen in Fig 3.  In this case, the LDR output is monitored until the resistance produced meets a threshold you define through calibration (indicating a decreasing natural light intensity), triggering the relay and turning on the grow light(s).  This setup is much more adaptive than a fixed timer, allowing for changing weather patterns and lengths of the day throughout the year.  A more complex version can replace the LDR with a photometer to more accurately analyze the natural light for the specific wavelengths you need.  An extension of this project is to use a second photoresistor to monitor the grow light and alert you if the light goes out unexpectedly and is covered in depth in the upcoming book, Automating Aquaponics with Arduino.  While this addresses the photoperiod, it says nothing of the wavelengths necessary for each stage of plant growth.  Some home aquaponics and hydroponics growers are experimenting with using matrices of off-the-shelf red and blue LEDs as artificial lighting.

To evaluate the electrical costs of artificial lighting per month:
  1. Add up the combined wattage of all grow lights
  2. Divide (1) by 1000 to get total kW (kilowatts)
  3. Multiply (2) by the kW/hour rate your your utility company charges
  4. Multiply (3) by the number of hours your lights are on in one month
As an example, lets say you have one, indoor, 4 x 4 foot grow bed.  A 400W HID lamp could cover this area with the right light intensity.  The average winter rate of my electric company is 9.66 cents per kWh and assuming I am running my grow lights for 16 hours per day for 31 days, I have 496 hours per month.

\((400W) * (\frac{1E-3kW}{W}) * (\frac{$.0966}{kWh}) * (\frac{496h}{month}) = $47.9/month\)

Compared to conventional agriculture, which is entirely dependent on oil, aquaponics runs off of electricity and therefore can be powered by alternative energies such as wind, photovoltaic and hydro-electric.  For those who run their systems "on the grid" and then lose power when the grid goes down, it is very simple to incorporate a backup diesel/gas electric generator.  There are no backups for conventional agriculture.

According to Cornell University's CEA, "...the KEY to acquiring and retaining long-term customers for a commercial production facility involves ONE simple concept:  Produce a consistent, high-quality crop at a predictable and steady rate 365 days per year....More than any other environmental variable, the total sum of light received by the plants in a 24-hour period determines the rate that the plants will grow and thus the amount of produce available for sale."

The success of the Vertical Farm will inevitably be evaluated on its commercial returns and rightly so.  For those employing aquaponics at home (kudos to you), you can take advantage of this biology and physics knowledge to enhance your own systems for consistent output and also evaluate any physiological symptoms of your plants for evidence of poor lighting conditions.

If you would like to learn more about automating your aquaponics system, sign-up for our new book Automating Aquaponics with Arduino.

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