What are the factors that LED plant lighting has to consider?
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What are the factors that LED plant lighting has to consider?

Companies assess multiple sources of horticultural lighting, including: light intensity, spectroscopy, light distribution uniformity, energy efficiency, and lamp life. Horticultural lighting systems can convert electrical energy into light for plant growth and development, promoting photosynthesis, while LED-based light sources can provide a tuned spectrum for applications.

However, determining the efficiency or performance of such a solid state lighting (SSL) system is a challenge. There are several factors that affect the overall efficiency of the lighting system. This article discusses how the design of lighting equipment will affect energy efficiency and how energy efficiency in turn affects the overall profitability of a controlled plant growth environment.

In fact, the effectiveness of horticultural luminaires to convert electrical energy into usable light for plant growth is critical to the success of controlled environmental agriculture (CEA). 

Installing the lighting solution within a few inches of the canopy of the crop is a breakthrough in vertical agricultural applications. Properly designed LED solutions deliver higher yields per square foot than poorly designed LED solutions and other lighting technologies such as HPS and fluorescent lamps.

Horticultural related indicators

Plants primarily use photosensitivity with wavelengths between 400 and 700 nm (and therefore, this segment is also commonly referred to as photosynthetically active radiation (PAR). Photosynthetic photon flux (PPF) measures the total amount of PAR produced per second by the illumination system. For measurement, an integrating sphere is used to capture and measure almost all photons emitted by the illumination system. The unit of PPF is the number of moles per square meter of photons per second (μmol/s). 

Photosynthetic photon flux density (PPFD) measures the amount of PAR reaching the plant canopy. PPFD represents the quantum number of light in the visible light wavelength range per unit time area, and the unit is μmol/m2/s. PPFD also indicates the correlation between the number of photons and photosynthesis.

Finally, we discuss photon efficacy. Photon efficacy refers to the efficiency with which a horticultural lighting system converts electrical energy into PAR photons. If the PPF of the light and the input power are both known, the photon effect of the horticultural lighting system can be easily calculated. Given that the unit of PPF is μmol/s, the unit of measurement watt is joules per second (J/s), the number of seconds in the numerator and denominator is eliminated, and the obtained unit is μmol/J. This unit is used to express efficacy. The higher the number, the more effective the lighting system is to convert electrical energy into PAR photons.

Common methods of gardening lighting

Next, we need to understand the nuances of luminaire design and the reasons for energy saving in horticultural lighting systems. The most commonly used horticultural lighting systems in the world are based on high intensity discharge (HID) illumination and high pressure sodium lamps (HPS). High-pressure sodium lamps were not originally designed for planting plants and were designed for light rail and parking garages. However, ready availability and high output levels have led to widespread use in horticulture because they provide very high light intensities, most of which emit light in the 565-700 nm range, an effective band that accelerates photosynthesis.

One disadvantage of using high pressure sodium lamps for horticultural lighting is the generation of large amounts of radiant heat. The surface temperature of the high pressure sodium lamp can reach temperatures above 800 °F (about 430 °C), so there must be sufficient distance between the plant canopy and the high pressure sodium lamp to avoid damage to plant tissue. When the installation height of the luminaire is increased, the inverse square law begins to work, which can reduce the illumination rate. Over time, the energy efficiency of high-pressure sodium lamps increased, and the emergence of double-ended HPS lamps enabled photon efficiencies of 1.7 μmol/J.

Turn to LED

Let's review the process of using LEDs in horticultural lighting. In 2014, the most efficient LED horticultural lighting system was as efficient as the double-ended high pressure sodium lamp. Longer LED life (L70 ≥ 50,000 hours) has led many growers to switch to LEDs compared to high-pressure sodium lamps. However, the cost of LED horticultural lighting systems is relatively high compared to high pressure sodium lamps, limiting the transition to LED lamps.

LED chip manufacturers have improved the performance of existing components over the past few years, enabling them to significantly improve photon performance and continue to improve every year. In fact, LED-based horticultural lighting systems are now capable of achieving 45% greater photon efficiency than double-ended high-pressure sodium lamps. While the efficiency of a single component increases the efficiency of LED horticultural lighting, it is just one variable for LEDs that exceed the technology of high pressure sodium lamps.

LED system heat

There is a common misconception about the heat generated by LED lighting. Many growers believe that LEDs generate less heat than high-pressure sodium lamps, which is true when LED fixtures are driven at lower wattages. If you have a 600W LED luminaire and a 600W double-ended high-pressure sodium lamp, from a macro perspective, they generate the same amount of heat.

The main difference between LEDs and high-pressure sodium lamps is how much of the PAR energy produced by the two 600Ws is how the heat is dissipated from the fixture. Most of the heat from the high-pressure gas lamp radiates down to the crop canopy, and most of the heat from the LED is generated at the junction of the component and the printed circuit board (PCB). Heat is usually transferred to the PCB or it can be dissipated. Slice and remove by convection upwards.

Therefore, one of the main advantages of LEDs as a horticultural lighting system is that it can be placed near plants while protecting plants from heat radiation. However, if the heat is not effectively removed from the PCB by a suitable thermal management system, the life of the LED assembly will be significantly reduced.

There are two ways to cool a lighting system in a commercial horticultural environment. Passively cooled luminaires use heat sinks to dissipate heat from the board, while actively cooled luminaires rely on fans or water to dissipate heat. Fans used to cool luminaires consume energy and reduce the overall photon efficiency of the luminaire. In addition, if the fan fails during operation of the luminaire, the LEDs on the PCB may overheat and burn out. Even if they do not have a catastrophic failure, the reduced power output will greatly reduce the life of the LED luminaire. This is a very important factor that growers need to consider when comparing horticultural lighting systems.

Spectrum and efficacy

Another important factor affecting the photon efficacy of horticultural lighting systems is the spectrum of luminescence. The most effective wavelengths for horticultural illumination systems are red (660 nm) and blue (450 nm). Traditional LED horticultural lighting technology uses red with a smaller proportion of blue LEDs to achieve the highest photon efficiency.

Although red LEDs have the highest photon efficiency, plants do not grow by themselves at narrowband wavelengths. Therefore, a single red LED does not produce the most efficient spectrum in optimizing plant growth and development.

Harsh and dirty environments, such as greenhouses, can quickly cause an active cooling system to fail, causing the entire lighting system to fail. In addition to improving energy efficiency, passive cooling systems do not require moving parts that are prone to breakage and blockage.

Many horticultural lighting manufacturers claim that their products have a "special spectrum" based on the absorption peaks of chlorophyll a and b. However, they did not mention that these chlorophyll pigments were extracted from plant leaves and measured in vitro. The spectrum of effects of light quality on photosynthesis was proposed by McCree and Inada in the 1970s. Studies have shown a correlation between photosynthesis rate and the spectrum of action of chlorophyll a and b, but they are not the only wavelengths of photosynthesis. Prior to this study, there was a common misconception that chlorophyll primarily absorbed red and blue portions of the visible spectrum, so plant photosynthesis did not use green light.

In the past 30 years, there have been many studies on photosynthesis of plants with high light intensity, indicating that the effect of spectral quality on growth rate is much smaller than that of light intensity. 

Spectral light quality has a great impact on plant development, such as seed germination, stem elongation and flowering, and secondary metabolites and flavonoids that affect the taste, appearance and odor of plants. Therefore, LED manufacturers need to find a balance between using the most efficient LEDs and the LEDs that growers want to promote optimal plant growth and development habits.

Shape factor and beam control

One of the topics we discussed last was related to the shape of the luminaire, beam optics, and light intensity. When considering the overall efficiency of a horticultural lighting system, PPFD and CU need to be considered. However, although the photon efficacy of the luminaire itself is very important for horticultural lighting, the true energy efficiency of the solution will be greatly reduced if the light generated in the actual application is not uniformly and efficiently applied to the crop.

Because each high-pressure sodium lamp has only one light source (360° bulb), it is necessary to rely on the reflector to spread the light evenly over the canopy of the crop. Another advantage of LEDs over high-pressure nano-gas lamps is that LEDs have hundreds of light sources that can be used to create a very uniform band of light through custom beam optics without the need for reflectors. The spectrum of light emitted from horticultural lighting systems (ie, color) has a significant impact on energy efficiency and overall plant growth and development. While red and blue light are more efficient, the broad spectrum achieved with the light engine in the picture can improve culture for more photoreceptors.

When properly designed, this form of flexibility gives a very high CU lighting solution that is very advantageous, with most of the light produced falling on the plant canopy without wasting on the channel or wall. This is crucial for growers when choosing a horticultural lighting system.

The energy efficiency of a horticultural lighting system depends on several factors, not just one factor. Using the right measurement methods, several factors that influence the energy efficiency of a horticultural lighting system will affect the overall profitability of the plant growth room.

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