Latent vs Sensible Heat for Controlled Environment Agriculture

Proper control of temperature and humidity inside a grow room can be the difference between success and failure. Grow room temperature directly impacts leaf surface temperature, CO2 supplementation, relative humidity, transpiration rates and nutrient uptake – to name a few. Temperature control may appear as simple as setting a programmable thermostat; but of course, it’s more complicated than that.

Overheating forces plants to take up more water and then quickly transpire it, which can cause humidity overload when the lights go off. And a high relative humidity can lead to devastating problems like powdery mildew. During nighttime periods of plant respiration, lights are off, which typically reduces the grow room ambient temperature by 5-10 degrees. The temperature drop and high relative humidity frequently create a wet environment that approaches the dew point.

The leading strategies for controlling temperature and humidity in a grow room include ventilation and dehumidification, or some combination of the two.  Determining which strategy to implement depends on the prevailing type of heat in the room. When plants transpire, stomata open, releasing water vapor through an evaporative process that cools the leaf. Water molecules in the plant absorb heat and are converted to a gas – water vapor. Since there is a phase change during this process, the heat absorbed by the water molecules is defined a latent heat. The other type of heat is called sensible heat, which is heat that is either added or subtracted without a phase change.

For the case of an overheated grow room dominated by sensible heat, a simple method for lowering the temperature is ventilation. Ventilation can be used to lower both temperature and humidity. Passive ventilation techniques have been utilized for thousands of years, so the technology is proven and so are its limitations. The efficacy of ventilation can be subject to local climate conditions and can be challenging during CO2 supplementation.

Dehumidification should be considered when cooling a grow room with excessive latent heat and corresponding high humidity. Latent heat converters (LHC) transform excess water vapor into liquid, which dehumidifies the air and converts the heat of condensation (latent heat) into sensible heat that can be used to heat the environment when needed. Ventilation can be reduced, which lowers heating costs and allows atmospheric CO2 enrichment. In addition to converting “wet” heat to “dry” heat, an LHC can be a good source of clean, readily accessible water. Moisture from inside the grow room can be recycled and used again.

Electric lighting can be the dominant source of heat in a grow room. As we know, LEDs are more efficient than traditional lighting technologies – they convert more electrical energy to light and less to heat. Energy supplied to an LED that isn’t converted to light becomes heat that gets radiated into the air. In contrast, much of the heat generated by an HPS fixture is contained in the light beam in the form of infrared (IR) energy. The IR energy, which is absorbed by the plants, raises the leaf surface temperature and induces higher rates of transpiration. So, HPS lighting can lead to higher amounts of latent heat (via plant transpiration), while LED lighting creates more sensible heat by radiative heating. These differences can influence the strategies used to optimize temperature and humidity in a grow room.

Nutrient Strategies with LED Lighting

In order to produce healthy and vigorous plants, it’s important to understand how nutrient efficacy can be affected by upgrading to LED lighting. Growers are frequently surprised to discover the light intensity and uniformity on their canopies aren’t what they thought. High-pressure sodium (HPS) and ceramic metal halide (CMH) lights degrade in intensity fairly quickly. And in many cases, the intensity and uniformity aren’t up to par even with new bulbs. In contrast, a properly designed and installed LED solution should provide the right light intensity with excellent uniformity across the canopy. Further, the spectral content of LEDs is different from traditional lighting technologies, and the change in spectrum may result in a variance in nutrient uptake by the plant. So how does this impact your nutrient strategy?

Drip irrigation using water-soluble fertilizers, otherwise known as fertigation, is a common practice in controlled environment agriculture (CEA) – see figure below. The primary goal of fertilizing is to augment the nitrogen uptake in the plant, which should enhance fertility and productivity. Since nitrogen is a gas, ammonium nitrate (NH4NO3) is used in powder/crystal form and combined with water to supply nitrogen to the crop.

When upgrading to LED lighting with different spectrum, intensity and uniformity profiles, fertigation strategies may need to be modified. We know that as light intensity (PPFD) increases, transpiration typically increases. Higher transpiration rates will pull more water out of the fertigation solution – leaving behind a relatively high concentration of mineral ions from the ammonium nitrate (nitrogen salts) in the media or root zone. This can make it difficult for the plant to take in water and nutrients, thereby creating nutrient imbalances.

The best way to ensure the ideal fertilizer concentration is to measure the pH and electrical conductivity (EC) of the nutrient solution and in the growing media. Since mineral ions in the fertilizer solution conduct electricity, measuring the EC is a perfect way to determine if the right amount of nutrients are being used to meet the needs of the plants without over fertilizing. It’s important to measure both the nutrient solution and the soil/media near the root zone to determine the salinity level of each. An elevated EC reading means a high concentration of mineral ions (fertilizer). If the EC reading is too high in the nutrient solution, the solution should be diluted with additional water. If the EC is too high in the soil/media, it is best to perform a leaching operation to bring it back into balance. The figure below shows a typical meter that measures pH and EC.

Likewise, measuring pH of the soil/media is an indicator of salt build up. Ammonium-based fertilizers will acidify soil, meaning the pH will increase. For most commonly grown hydroponic crops, an optimal pH range is between 5.5 and 6.5.

There are a number of factors that determine the ideal EC value for a grow operation including, crop species, light intensity, temperature and humidity.  However, some general guidelines for CEA grown cannabis are shown in the chart below. From the chart, the electrical conductivity, and hence the nutrient concentration, increases with increasing maturity of the plant. An EC meter measures electrical conductivity in units of millisiemens per centimeter(mS/cm).

An incorrect nutrient concentration during fertigation can cause plant leaves to discolor, which is the result of a toxic effect commonly referred to as “fertilizer burn.” In the extreme, over fertilizing leading to a salt build up at the root zone can pull water out of the plant and cause it to wilt and die. To ensure an optimal harvest, the nutrient concentration may need to be adjusted to accommodate changes in light spectrum, intensity and uniformity from a lighting upgrade.

How is CO2 supplementation affected by converting to LED Lighting?

Growers working in controlled environments strive to optimize the variables in their control to produce the highest yield and best quality products. These key variables include temperature, humidity, nutrients, light, and CO2. This article examines how switching to LED technology from older lighting technologies such as high-pressure sodium (HPS) could impact CO2 supplementation.

First, some background on the critical role of CO2 in plant development. During the Calvin cycle of photosynthesis, the plant enzyme rubisco enables carbon fixation, which ultimately results in CO2 and water being converted into simple sugars (carbohydrates). The chemical reaction involving rubisco is temperature-dependent, so to optimize photosynthesis we need to understand the interplay between light, temperature and CO2 concentration.

Looking at each variable independently in figure 1, we see that increasing light, CO2 and temperature (leaf surface temperature) increases photosynthesis. There are clearly diminishing returns in the photosynthetic rate at very high levels of light and CO2.

Interestingly, increasing temperature beyond an ideal point actually decreases the photosynthetic rate. This is attributable to the temperature dependence of the rubisco reaction – see figure 2. Without supplementing, the concentration of CO2 in ambient air is roughly 300 parts per million (PPM). Under these ambient conditions, the ideal leaf temperature is about 25°C.

If we add CO2 to the environment, we can generate higher rates of photosynthesis at higher leaf surface temperatures – see figure 3.

So the key to CO2 supplementation is to achieve the concentration where you begin to experience minimal additional photosynthetic production by adding more CO2. That ideal concentration will depend on the light intensity and the leaf surface temperature – see figure 4.

The grower will ensure the leaf surface temperature is within a narrow window to achieve the required vapor pressure deficit (VPD). An optimized VPD enables maximal transpiration and photosynthesis.

The controlled environment agriculture industry is experiencing a shift in lighting technology. LEDs are rapidly displacing HPS, metal halide and fluorescent technologies. LED lighting has some unique characteristics that must be taken into account when upgrading a grow facility from older lighting technologies. LED lights typically have very little infrared energy in the beam, which reduces the leaf surface temperature. With a lower leaf temperature, the grower may choose to either decrease the relative humidity or increase the heat in the room in order to maintain the necessary VPD. And this decision is likely to influence the ideal set-point for CO2 concentration. For example, if the leaf temperature is lower, the CO2 concentration should be lowered to prevent working in the “dark-limited” phase as shown in figure 4. Conversely, if the grow facility can return the leaf surface temperature to its previous level, then an adjustment is CO2 would not be required.

Although the ideal CO2 concentration is a function of the plant species, light intensity and, leaf surface temperature, figure 5 presents typical CO2 values for the various phases of cannabis development.

Maximizing biomass production in a commercial grow facility requires a deep understanding of the critical control parameters. While the benefits of LED grow lights are obvious, growers need to consider how the new technology impacts the canopy – especially as it relates to CO2 supplementation.   

Avoid this common mistake when comparing LEDs to HPS

So, you’ve decided to take the plunge and upgrade your grow room to LEDs. Simply unplug the HPS lights and install the LED lights. Better yields and lower electric bills – you are going to be a hero. Well, you might be a hero if you take into account how the switch to LEDs impacts your leaf surface temperature and the corresponding vapor pressure deficit (VPD).

It is well-established in plant biology that leaf surface temperature must be kept within a specific window to optimize primary metabolism (photosynthesis), as well as production of secondary metabolites. The relationship between leaf surface temperature and photosynthesis is shown in the figure below. The figure consists of data from a variety of plant species.

But leaf temperature is only part of the story. The critical factor is the interplay between leaf temperature and the relative humidity in the grow room. Those two factors (temperature and humidity) determine the vapor pressure deficit (VPD), which, in turn, determines transpiration efficacy and ultimately photosynthetic rates. An example of the relationship between temperature, humidity and VPD is illustrated in the chart below. To optimize production yield, the VPD must remain in the “sweet spot” identified in the green boxes.

High pressure sodium (HPS) lighting, has long been the workhorse in many indoor grow facilities. HPS emits in a broad portion of the electromagnetic spectrum that includes infrared (IR) energy – otherwise known as heat. IR energy from HPS heats the canopy and increases the leaf surface temperature. LED grow lights typically have only a small fraction of their emission in the IR portion of the spectrum, so they do not increase leaf surface temperature like HPS. In fact, it is typical to see a 5°-10° decrease in leaf surface temperature by changing the lighting from HPS to LED. If no other action is taken, the decrease in leaf temperature may throw the VPD out of its sweet spot – thereby decreasing transpiration and photosynthesis. This will most certainly not make you a hero in the grow room.  

So how do you ensure you are still in the proper VPD range after installing LED lights? Follow the steps below:

  1. Understand your baseline. Measure the leaf surface temperature and relative humidity while you’re still using HPS. Although humidity is easily measured, measuring leaf surface temperature requires specialized equipment such as a forward-looking infrared camera. Here’s one IR camera that will do the job: tequipment.net/fliri7.html. Don’t assume the leaf surface temperature is the same as the ambient air; this is rarely the case. Once you’ve taken the measurements, the VPD can be determined.
  2. Repeat step #1 after switching to LED.
  3. Determine if your VPD is still in the optimal range. If it isn’t, you should:
    1. Increase the ambient air temperature to raise the leaf temperature to the target temperature that satisfies the VPD requirement.
    2. Modify the relative humidity in the room to bring the VPD into the ideal range. 

One reason LED grow lights are so efficient is that they don’t produce excess heat in the light beam like older technologies (including HPS). However, to fully achieve all the benefits of LED technology, growers must understand how the lower heat content will affect their plants and take the proper steps to achieve optimal production.

A Lighting Facts Label for Horticulture

In 2009, the Department of Energy (DOE) created the LED Lighting Facts program to help manufacturers, utilities, and consumers in the early days of LED lighting, when products entered the market with little or no verified information on product performance. The voluntary DOE LED Lighting Facts effort paved the way for the mandatory Federal Trade Commission (FTC) label required for most commercial and residential lighting products (including incandescent, compact fluorescent, and LED light bulbs), which was introduced in 2010. This initiative had a significant impact in advancing the adoption of LED technology in general lighting.

Lighting Facts Label for Commercial & Residential Lighting

Today we are seeing a similar effort for grow lights.  Development of a lighting facts label for horticulture is being spearheaded by university researchers that believe there is a need for clarity and consistency in communicating to growers the performance metrics for horticultural lighting products. The objective is to create a label that is easy to read and understand and would aid in the comparison of products from different manufacturers. Researchers have proposed a horticulture lighting facts label that they hope will one day become an industry standard. Information reported on the label is intended to come from measurements taken at certified independent test labs.

Proposed Lighting Facts Label for Horticulture

Key information listed on the label:

 

  • Light Output

Output within the photosynthetic active radiation (PAR) spectrum of 400-700nm is listed at the fixture’s nominal input power, which is also on the label.

  • Spectral Power Distribution & Intensity

Many growers have a preference for the spectral composition of the light they need to optimize crop yield and health. The label provides a graph of the normalized photon flux vs. wavelength. It also quantitatively breaks down the light into its spectral components of red, green, blue, UV, far red and infrared. These spectral buckets are reported in units of intensity (umols/m2/sec) with the fixture mounted 2-feet (61cm) over the canopy.

  • Uniformity

The proposed label displays a graph of the intensity of the light as a function of distance from the center of light (also at the 2-foot mounting height). This information provides insight into the uniformity of intensity on the canopy from a single fixture.

  • Efficiency

Understanding how efficiently a grow light converts electrical input power to light output obviously has a major impact on the operating costs of a grow operation. The proposed label displays PAR efficiency in umols/joule.

  • Color Quality

Although terms such as lumens and color rendering index (CRI) are not important to plants, they can be important to people working in a controlled growing environment. Of particular import, the proposed label lists the light fixture’s CRI, which indicates how accurately humans can see colors reflected from objects they are viewing. There are many grow lights on the market that consist primarily of blue and red LEDs, and it can be very difficult for people to determine plant health under those lights. The closer the CRI is to 100, the more accurately the plant’s colors (and health) can be determined by workers.

 

The proposed label is currently under review, and the team that created it is collecting feedback from industry stakeholders. Presently there is no requirement for approval of the label from the Department of Energy, so it is currently a voluntary standard. The hope is that once growers see the label on a few lighting products, they will demand its use from all lighting manufacturers. While benefits of the label to growers is clear, a major benefit to serious horticulture lighting manufacturers is as a weapon against low-quality products that overpromise, underdeliver and slow the adoption of new technologies.

What are DLC listed LED Grow Lights? And why does it matter?

LED lighting for controlled environment agriculture (CEA) continues to mature as new standards and regulations are implemented across the industry. And the emergence of the Design Lights Consortium (DLC) horticulture lighting program is an important step forward.

Design Lights Consortium is a non-profit organization with a mission of advancing the adoption of energy efficient lighting. The organization historically focused on general lighting applications such as retail lighting and street lighting. But very recently DLC started accepting submissions of LED grow lights for qualification testing and inclusion in its Horticulture Lighting Qualified Products List (QPL).

For growers, buying DLC listed products provides peace-of-mind that the lights meet rigorous performance, safety and reliability standards. Further, utilities are likely to begin mandating DLC listed lights in order to qualify for energy efficiency rebates.

A few of the critical performance and reliability requirements to achieve DLC listing include:

  • Efficacy > 1.9 umol/J between 400-700nm
  • Long-term performance: Q90 > 36,000 hours photon flux maintenance
  • Driver Lifetime: > 50,000 hours
  • Warranty: > 5 years

A comprehensive overview of DLC horticulture lighting requirements is here: DLC Requirements

In addition to performance and reliability, DLC requires products to be certified by a relevant safety certification body in the United States or Canada. Underwriter’s Laboratories (UL), for example, has defined UL 8800 for the review and safety certification of horticulture lighting products.

Since the horticulture program is new for DLC, there are not currently any qualified lights on the horticulture QPL. However, Thrive Agritech is already in the process of having its LED grow light fixtures qualified by DLC, and anticipates having some of the first lights listed. To learn more about DLC, visit: www.designlights.org.

Congratulations are in order for our industry!

LED Lighting cut carbon dioxide emissions by 570 million tons in 2017

We all know that LEDs are efficient and can help save energy by displacing older, less efficient lighting technologies. But how much energy is really being saved? 162 coal-fired power plants worth of energy, according to IHS Markit.

The efficiency of LEDs is essentially what makes them environmentally friendly,” comments Jamie Fox, principal analyst, lighting & LEDs group. “Therefore, LED conversion is unlike other measures, which require people to reduce consumption or make lifestyle changes.”

We think Fox is 100% right. LED lighting has the opportunity to save massive amounts of energy, not because LEDs are efficient, but because they can be BETTER at providing light and happen to be efficient.

Growers select LED lighting because the technology has a better spectrum, can provide better uniformity and ultimately can help produce amazing plants with higher yield at a lower operational cost. The rest of the world benefits from reduced contribution to climate change and improved sustainability of our food and plant production industries.

Beyond this, not accounted for in the energy and CO2 savings estimates are the longer lifespan of LEDs. There are many pollutants created by producing, shipping and disposing of lighting products. By lasting 2-5x longer than conventional lighting, many of these pollutants are avoided. Even betters, LEDs don’t include heavy metals like mercury, making them more environmentally friendly at end-of-life.

We are proud to be part of an industry that is saving hundreds of millions of tons of CO2 from entering our atmosphere. We have a long way to go as a planet, but it is a great feeling to know that the path for our industry is Win-Win, saving energy and getting better light.

Thrive Agritech Boost LED Illuminates Basil farm in West Seneca, New York

Weber Greenhouse installed supplemental LED lighting from Thrive Agritech to optimize basil production, especially in the low light periods of the year typical of upstate new york. The energy efficient full spectrum white lighting not only increases plant growth but also improves canopy consistency and plant morphology.

 

Increased basil sales result in a quick return on investment, especially considering the low operational costs of the Boost LED light bar. Installation was easy and economical, with each fixture connected end-to-end and mounted directly to the existing greenhouse truss system. Compared to other LED systems considered, ease of installation was a major benefit, especially considering each light row is operated and controlled with a single plug.

“Weber Greenhouse is a leader in advanced Controlled Environment Agriculture,” says Brian Bennett, CEO and Founder, Thrive Agritech. “We are thrilled to work with a company that represents the future of sustainable indoor plant production and support their efforts to achieve increased crop production while being environmentally sustainable.”

 

Transitioning to LED grow lighting

LED grow lighting has many advantages over high-pressure sodium, metal halide, and fluorescents – they use less energy, emit less heat, reduce water usage and have spectra that are optimized for plant growth.  Another great thing about LEDs is their reliability.  Instead of re-lamping every year, many LEDs have lifespans of 10 years or longer.  But as more growers transition to LED grow lighting, more questions and concerns arise.

 

For decades cannabis growers have been honing their grow “recipes” on their own and when the light technology “ingredient” changes so do the other variables like nutrient uptake, temperature, humidity requirements and so on.  Implementing LEDs into an already dialed in grow can be challenging so let’s look at the first step to make this a successful transition:

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How much HVAC do I need to cool LED Lights? 

We hear this question all the time, especially in the summer.

Our rule of thumb – it takes one watt of air conditioning to cool three watts of lighting.

For a more precise calculation, there are a few things that we need to know about heat

  • A BTU, or British Thermal Unit, is the traditional unit for heat or heat removal
  • A ton of refrigerant is 12,000 BTUs.
  • One watt of energy is equal to 3.412 BTUs

When you turn on a light some of the energy is turned into photons and some of the energy is turned into heat. In the case of LEDs, about 40% of the energy is turned into light and 60% is turned into heat.

Unfortunately…. you need to cool all of the energy, even the energy emitted as light. (more…)