rus
Issue #7/2023
M. M. Degtereva, Y. Levin, A. E. Degterev, A. A. Bogdanov, I. A. Lamkin, S. A. Tarasov, P. A. Sergeev
Assessment Procedure for the Advantages of LED Phyto-Strip Application in the Industrial Greenhouse Complexes
Assessment Procedure for the Advantages of LED Phyto-Strip Application in the Industrial Greenhouse Complexes
DOI: 10.22184/1993-7296.FRos.2023.17.7.566.578
The efficiency of LED phyto-strip in the field of photosynthetically active radiation has been assessed and compared with the alternative phyto-irradiators used in the industrial greenhouse complexes. The phyto-strip efficiency in the field of photosynthetically active radiation was equal to 42% that was 4.7 times higher than the efficiency of a full-spectrum grow lamp and 4.4 times higher than the efficiency of a fluorescent lamp. The spectral and energy characteristics of the LED phyto-strip have been determined. The average value of the photosynthetic photon flux density applicable for the plant growing process (≈300 µmol / m2 / s) is achieved when radiation is obtained from one meter of phyto-strip at a distance of ≈20 cm from the irradiated area when current is passed through it. Due to its high efficiency, the phyto-strip will improve the growth results of various crops in the autonomous agro-industrial enterprises, and will also reduce the energy costs.
The efficiency of LED phyto-strip in the field of photosynthetically active radiation has been assessed and compared with the alternative phyto-irradiators used in the industrial greenhouse complexes. The phyto-strip efficiency in the field of photosynthetically active radiation was equal to 42% that was 4.7 times higher than the efficiency of a full-spectrum grow lamp and 4.4 times higher than the efficiency of a fluorescent lamp. The spectral and energy characteristics of the LED phyto-strip have been determined. The average value of the photosynthetic photon flux density applicable for the plant growing process (≈300 µmol / m2 / s) is achieved when radiation is obtained from one meter of phyto-strip at a distance of ≈20 cm from the irradiated area when current is passed through it. Due to its high efficiency, the phyto-strip will improve the growth results of various crops in the autonomous agro-industrial enterprises, and will also reduce the energy costs.
Теги: current-voltage characteristic efficiency photosynthetically active radiation photosynthetic photon flux density phyto-strip spectral characteristic watt-ampere characteristic ватт-амперная характеристика вольт-амперная характеристика плотность фотосинтетического фотонного потока спектральная характеристика фитолента фотосинтетически активная радиация эффективность
Assessment Procedure for the Advantages of LED Phyto-Strip Application in the Industrial Greenhouse Complexes
M. M. Degtereva 1, Y. Levin 1, A. E. Degterev 1, A. A. Bogdanov 1, I. A. Lamkin 1, S. A. Tarasov 1, P. A. Sergeev 2
Saint Petersburg Electrotechnical University “LETI”,
Saint-Petersburg, Russia
Svetoyar LLC, Saint-Petersburg, Russia
The efficiency of LED phyto-strip in the field of photosynthetically active radiation has been assessed and compared with the alternative phyto-irradiators used in the industrial greenhouse complexes. The phyto-strip efficiency in the field of photosynthetically active radiation was equal to 42% that was 4.7 times higher than the efficiency of a full-spectrum grow lamp and 4.4 times higher than the efficiency of a fluorescent lamp. The spectral and energy characteristics of the LED phyto-strip have been determined. The average value of the photosynthetic photon flux density applicable for the plant growing process (≈300 µmol / m2 / s) is achieved when radiation is obtained from one meter of phyto-strip at a distance of ≈20 cm from the irradiated area when current is passed through it. Due to its high efficiency, the phyto-strip will improve the growth results of various crops in the autonomous agro-industrial enterprises, and will also reduce the energy costs.
Keywords: phyto-strip, spectral characteristic, watt-ampere characteristic, current-voltage characteristic, efficiency, photosynthetically active radiation, photosynthetic photon flux density
Article received: October 24, 2023
Article accepted: November 07, 2023
Introduction
In the rapid population growth conditions, the food crisis has taken a form affecting the mankind survival and development. The concurrent soil erosion, extreme weather conditions, and environmental destruction further exacerbate its impact on the developmental sustainability of countries. The light strongly influences the biosynthesis and accumulation of various plant secondary metabolites that are rather critical to the crop quality. The spectral light composition has a great influence on the growth and regeneration processes and is one of the main factors in the plant bioproductivity. The LEDs have substantial potential as the phyto-irradiators. Moreover, in the light of active development of the agroindustrial complex, there is a need to study the specifications of industrially produced LED irradiators and evaluate their efficiency for increasing the crop yields and improving the product quality.
The LED-based phyto-irradiators represent a new technology for artificial agricultural lighting. The LEDs have a huge number of advantages for commercial plant growing applications. Their durability and strength make them easier to install and handle than the conventional lighting fixtures such as the high-pressure sodium lamps and fluorescent lamps with the fragile glass envelopes [1]. Another feature of the LEDs is that their chip size is usually much smaller than the plants that allows for a variety of light source designs, both for tissue cultivation in vitro using multiple LEDs, and for illumination of the greenhouse cultures using the large arrays of LEDs depending on the growing scale [2]. Moreover, the small size of LEDs expands the variety of available plant lighting methods, such as irradiation of specific organs and unrestricted targeted irradiation [3]. The location of lighting sources in close proximity to the plants is also possible due to the use of visible LEDs that do not emit a large amount of heat [4]. Thus, in the light of the active agroindustrial complex development, there is a need to study the specifications of industrially produced LED irradiators and evaluate their efficiency for increasing the crop yields and improving the product quality. The purpose of this paper is to evaluate the efficiency of LED phyto-strip with a spectral composition of 15%B + 40%G + 45% R (B - blue, G – green, R – red radiation) in the field of photosynthetically active radiation for growing the high-quality agricultural products under conditions of artificial germination, to make a conclusion on feasible use of the LED phyto-strip under study, as well as to compare with the alternative phyto-irradiators applied in the industrial greenhouse complexes.
Materials and methods
In addition to the conventional radiation sources that include a fluorescent lamp, the full-spectrum LED lamps or LED phyto-strips are also used for the plant growing process. Thus, this paper provides the comparison of the phyto-strip characteristics with the competitors available on the agro-irradiator market.
The Arlight LED phyto-strip FITOLUX-A144–10mm 24V Day4000-Red (14 W/m, IP20, 2835, 5 m) consists of two types of LEDs: the first type is a white SMD LED with a luminophore (a device made in a small housing with a built-in light-emitting crystal that is superficially mounted on a printed circuit board, see Fig. 1a), containing one crystal. The sample has the dimensions of 2.9 × 3.3 × 1.0 mm. The opening voltage is 2.54 V, the operating range of values is 2.56–2.86 V. The second type is a red SMD LED (Fig. 1 b) containing one crystal. The sample has the dimensions of 2.9 × 3.1 × 1.0 mm. The opening voltage is 1.66 V, the operating range of values is 1.68–1.98 V.
The LED phyto-strip (1 meter) consists of 16 sections containing 9 LEDs: 6 white and 3 red ones (Fig. 1 c). The strip sections has serial connection.
For comparison, we used a full spectrum LED grow lamp with a spectral composition of 1%UV + 39%B + 15%G + 38%R + 7%FR (UV - ultraviolet radiation, DC – far red radiation) for growing the ALMGD plants and a fluorescent lamp Camelion 13 W 4200 K with a spectral composition of 1%UV + 23%B + 40%G + 33%R + 3%FR (Fig. 1d, f). For these lamps, the spectral characteristics and densities of the photosynthetic photon flux were measured at various distances at the operating supply voltage.
The following equipment was used for the measurements: Ocean Optics HR4000 spectrometer, with a spectral range from 200 nm to 1050 nm, and a UPRtek PG200N Spectral PAR Meter spectrometer with a spectral range from 350 nm to 800 nm. The Ocean Optics HR4000 spectrometer together with an integrating sphere was applied to measure the spectral and energy characteristics of the phyto-strip and individual LEDs included in its composition to be schematically outlined in Fig. 2. The UPRtek PG200N Spectral PAR Meter spectrometer was used to measure the light quantity and quality in the area of of photosynthetically active radiation.
Performance of experiments
A study of the current-voltage characteristics of various LEDs (Fig. 3) of the same type has demonstrated minor differences that are acceptable and related to the peculiarities of the LED production process.
The differences in the current-voltage characteristics of red LEDs from white ones are caused by the differences in the manufacturing materials. Thus, an AlGaAs solid solution is usually applied for the production of red LEDs that determines the lower operating voltage of red LEDs [5]. The white LEDs are the common white luminiferous LEDs, consisting of a yellow luminophore deposited on a blue crystal made from an InGaN-based solid solution leading to a higher supply voltage.
The LED radiation spectrum is determined by the semiconductor luminescence spectrum specified by its band structure, primarily the bandgap energy [6]. The availability of quantum wells, wires or dots in the active region leads to the fact that the radiation spectrum is determined by their parameters, in particular by the position of size quantization levels [7]. The base material for the short-wave radiation sources (0.25–0.6 μm) is gallium nitride and its solid solutions; for LEDs in the yellow, orange, red and near-IR spectral regions (0.58–0.9 μm) it is arsenide gallium and direct band gap solid solutions in the (Ga, In, Al)P and (Ga, Al)As systems. Only the direct band gap materials are used in the active region [7]. For different ranges, the radiation spectrum range is various since it is determined by the distribution of charge carriers in the band gap.
We conducted the studies at room temperature at various pump currents of light-emitting structures. The white luminiferous LEDs have peak values in the blue (454 nm) and yellow (575 nm) spectral regions. The peak wavelength of the red LED is 664 nm.
In the aggregate, the radiation spectra of white and red LEDs generate the radiation spectrum of the phyto-strip (Fig. 4). The radiation spectrum of a fluorescent lamp (Fig. 4) is specified by multiple peaks, the highest radiation intensity is observed at the wavelengths of 611 nm, 545 nm and 542 nm. The spectral characteristic of the grow lamp (Fig. 4) contains 2 main emission peaks in the blue (461 nm) and red (638 nm) regions.
Results
As the current increases, a change in the emission wavelength of white and red LEDs is observed. For white LEDs, this change is related to the quantum-dimensional Stark effect (QDSE). The internal piezoelectric fields develop the Stark effect that reduces the internal quantum efficiency. This effect provides for a change in the energy spectrum of atoms, molecules and crystals in an electric field and takes the form of heterostructures with the quantum wells (QWs) in the blue shift of the QW excitonic absorption line. The shift in the luminescence spectrum at low currents is due to the influence of elastic stresses that cause lattice deformation at the heterojunction, and the built-in electric field [8]. Under the built-in electric field influence, the well shape is greatly distorted and a QDSE is occurred. This phenomenon leads to an increase in the distance between the levels resulting in a shift of the luminescence spectrum to the short-wave region (Fig. 5a). As the external voltage increases, influence of the built-in field is compensated and the shift is decreased. The Stark effect is most pronounced in the LEDs based on the InGaN and AlGaN heterostructures [9].
It is well-known that red LEDs show a shift in the peak radiation wavelength: an increase in the current level leads to heating of the active crystal region and a shift of the spectral characteristic to the long-wave region that is related to a temperature change in the band gap [10]. Moreover, we have observed this effect (Fig. 5b).
The most important specification of LEDs is η, namely the efficiency of converting electrical energy into the light energy. The LED efficiency is its coefficient of performance (COP). It is related to the external quantum yield of electroluminescence ηe by the following ratio [7]:
η = ≈ ηe,
where ħω is the photon energy corresponding to the maximum value of the radiation spectrum, U is the applied external voltage.
We have studied dependence of the phyto-strip efficiency on the current (Fig. 6) that show a decrease in the characteristics related to a decrease in the quantum luminescence yield due to the structure heating by the flowing current, as well as to the saturation of radiative recombination and decrease in its efficiency due to the energy level filling in the active region.
The most important indicator for use of a lighting device in the field of plant growing is the quantity of light in the area of photosynthetically active radiation (PAR) provided by it. Let us explain that the photosynthetically active radiation (PAR) is a part of electromagnetic radiation that can be used as an energy source for the photosynthesis process in green plants.
PAR is measured as a photosynthetic photon flux density (PPFD), the unit of measurement is µmol / m2 / s. Although PAR covers the entire visible part of the spectrum, the most efficnet are the blue and red spectrum portions with the secondary peaks in the yellow and orange parts. The ultraviolet (UV) and infrared (IR) radiation are outside the visible range of light and are not included in the PAR range. PPFD is a photosynthetic photon flux density, measured by µmol / m2 / s. PPFD measures the quantity of PAR light (photons) that hits the plant surface every second. For light to be effective for growing essential oil-containing cultures, vegetables and other crops, it must have the PPFD values from 150 to 600 µmol / m2 / s. For comparison, it should be noted that the PPFD of natural solar radiation has a value of 900–1500 µmol / m2 / s when the sun is high.
PPF is a photosynthetic photon flux, measured by µmol / s. PPF measures the total amount of PAR that is produced by a lighting system every second.
The photosynthetic photon flux is calculated by the following formula [11]:
,
where FPAR is the photosynthetic photon flux, µmol/s;
ϕλ – spectral density of the radiation power distribution of the device (in the PAR region), W/nm; λ – wavelength, nm; h – Planck’s constant; c – velocity of light; NA – Avogadro’s number; K – proportionality coefficient.
The efficiency of the emitting device in the PAR region is calculated by the following formula [11]:
ηPAR = ,
where ηPAR is efficiency in the PAR region, µmol / W / s; P – power consumption, W. As the distance increases, PPFD is decreased according to the inverse square law. General PPFD value of the LED phyto-strip (Fig. 7a) is increased as the number of LEDs raised: 6 white and 3 red ones. The average PAR value applicable for the plant growing process (≈300 µmol / m2 / s) and achieved at the operating current was obtained from one meter of phyto-strip at a distance of ≈20 cm (the maximum achievable PPFD value at a given distance) from the irradiated area.
We measured the photosynthetic photon flux of the lighting device (Fig.7b) using an integrating sphere. The internal part of the hollow sphere is white (fluoroplastic) that provides high reflective and scattering properties. In accordance with the lighting requirements of GOST R 57671-2017, the efficiency of devices in the PAR region must be at least 2.0 µmol / J for the devices intended to illuminate the plants from above. As it is shown by Fig. 7c, the efficiency of phyto-strip under study in the PAR region at an operating current is 2.62 µmol / J that is 31% higher than the normal value.
The light absorption by the photosystems is promoted by the photosynthetic pigments. The oxygen photosynthesizers including the plants such as algae and cyanobacteria, use the chlorophyll pigments. There are several different types of chlorophyll, while chlorophyll a and chlorophyll b being the most common types for the ground vegetation [12]. Both pigments have absorption peaks in the red and blue spectral regions, while chlorophyll a having the peaks at 430 and 662 nm, and chlorophyll b having the peaks at 453 and 642 nm (the location of these peaks is displaced depending on the cell properties where they are located).
Algae and cyanobacteria contain additional types of chlorophyll pigments such as chlorophylls c, d and f [13]. Anoxygenic photosynthesizers such as purple bacteria, green sulfur bacteria, heliobacteria and filamentous anoxygenic phototrophs, used the alternative but related pigments called “bacteriochlorophylls” [14]. In addition to the pigments that capture primary light, the photosynthesizers apply “the antenna pigments”, such as carotenoids, to capture higher energy photons and transmit them to the photosystem. Figure 8 shows the absorption spectra of various chlorophylls and other light-catching pigments, as well as the radiation spectrum of phyto-strip, fluorescent lamps and LED lamps. The main PAR share falls on the red and green ranges: LED phyto-strip has a following PPFD ratio: 15% blue (400–500 nm) + 40% green (500–600 nm) + 45% red (600–700 nm).
The spectrum shape influence on the efficiency of irradiation device in the PAR region was assessed by superimposing the plant pigments such as chlorophyll a, chlorophyll b and carotenoids, on the absorption spectrum (Fig. 8). The best overlap between the radiation spectra of devices and the absorption spectra of photochemical pigments, with due regard to the efficiency of converting electrical power into the optical power, is observed for the LED emitters and is equal to 42.2%, in contrast to 8.9% for a grow lamp and 9.6% for a fluorescent lamp.
Discussion of results
The fluorescent lighting, combined with the incandescent lamps, promotes the vegetative plant growth, color, appearance and product quality, similar to the results obtained with the solar radiation. However, there is a decrease in the growth rate and biomass accumulation. The white LEDs combined with the red LEDs promote carbohydrate storage and improve the energy efficiency level compared to the fluorescent lamps during the germination stage and subsequent cultivation stages [15].
We have found that the average value of the photosynthetic photon flux density required for the efficient plant growth (≈300 µmol / m2 / s) is achieved with an operating current of one meter of phyto-strip at a distance of ≈20 cm from the irradiated area. The spectral composition of LED phyto-strip with a ratio in the PAR region of 15% blue, 40% green and 45% red light is not considered optimal for growth stimulation of green agricultural crops. This phenomenon is consistent with the results presented in the papers [16, 17], the authors of which have shown that special selection of LEDs emitting the light at certain wavelengths makes is possible to provide sufficient energy to stimulate the photosynthetic pigments and greater accumulation of dry mass. To maximize the growth stimulation of such plants, a spectrum with a higher content of blue and red light that constitutes up to 80% of the total PPFD, is more preferable.
It is well-known that the application of LEDs, compared to the conventional devices such as the high-pressure sodium lamps or fluorescent lamps, provides higher energy efficiency. The impact effectiveness of the phyto-strip radiation on plants evaluated in this paper, significantly exceeds the impact effectiveness of alternative radiation sources and is equal to 42% being 4.7 times higher than the efficiency of a full-spectrum grow lamp in the PAR region and 4.4 times higher than the efficiency of a fluorescent lamp.
Conclusion
The LED phyto-strip with a spectral composition of 15% blue, 40% green and 45% red light has high efficiency, dimming possibility (adjustment of the light brightness) and a radiation spectrum corresponding to the absorption spectra of plant pigments. Such properties make the device most preferable for use in the agroindustrial enterprises to increase the agricultural productivity. In this case, the installation simplicity and safety of the lighting device should also be considered. The high efficiency of phyto-strip will improve the growth quality of various agricultural crops in the autonomous agroindustrial enterprises. During the further research, it is planned to study the energy cost reduction methods when using the LED phyto-strips in the industrial greenhouse complexes.
AUTHORS
Mariya M. Degtereva –postgraduate student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0001-6797-0595
Yevgeniy Levin –postgraduate student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0009-0000-3811-487X
Alexander E. Degterev –postgraduate student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0002-6151-6567
Alexander A. Bogdanov – master student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0009-0004-2540-4228
Ivan A. Lamkin – Cand. of Tech. Sciences, Associate Professor of the Department of Photonics, St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0002-3680-7725
Sergey A. Tarasov – Doctor of Technical Sciences, Head of the Department of Photonics, St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0002-6321-0019
Pavel A. Sergeev – Engineer, General Director of OOO Svetoyar, St. Petersburg, Russia.
CONTRIBUTION OF THE AUTHORS
Degtereva Mariya Mikhailovna – measurement of phytotape parameters in the field of photosynthetically active radiation, editing of the article, analysis of literature; Levin Yevgeniy – measurement of spectral and energy characteristics of phytolent, editing of the article, analysis of literature; Degterev Alexander Eduardovich – measurement of the electrical characteristics of LEDs, editing the article; Bogdanov Alexander Alexandrovich – electrical and colorimetric characteristics of the phytotape, editing the article; Lamkin Ivan Anatolyevich – scientific work management, work planning, article editing; Tarasov Sergey Anatolyevich – problem setting and scientific research management; Sergeev Pavel Andreevich – setting the task of providing samples for research.
CONFLICT OF INTEREST
The authors state that they have no conflict of interest. The results of the research were discussed and reflected in the manuscript, which is a joint work.
M. M. Degtereva 1, Y. Levin 1, A. E. Degterev 1, A. A. Bogdanov 1, I. A. Lamkin 1, S. A. Tarasov 1, P. A. Sergeev 2
Saint Petersburg Electrotechnical University “LETI”,
Saint-Petersburg, Russia
Svetoyar LLC, Saint-Petersburg, Russia
The efficiency of LED phyto-strip in the field of photosynthetically active radiation has been assessed and compared with the alternative phyto-irradiators used in the industrial greenhouse complexes. The phyto-strip efficiency in the field of photosynthetically active radiation was equal to 42% that was 4.7 times higher than the efficiency of a full-spectrum grow lamp and 4.4 times higher than the efficiency of a fluorescent lamp. The spectral and energy characteristics of the LED phyto-strip have been determined. The average value of the photosynthetic photon flux density applicable for the plant growing process (≈300 µmol / m2 / s) is achieved when radiation is obtained from one meter of phyto-strip at a distance of ≈20 cm from the irradiated area when current is passed through it. Due to its high efficiency, the phyto-strip will improve the growth results of various crops in the autonomous agro-industrial enterprises, and will also reduce the energy costs.
Keywords: phyto-strip, spectral characteristic, watt-ampere characteristic, current-voltage characteristic, efficiency, photosynthetically active radiation, photosynthetic photon flux density
Article received: October 24, 2023
Article accepted: November 07, 2023
Introduction
In the rapid population growth conditions, the food crisis has taken a form affecting the mankind survival and development. The concurrent soil erosion, extreme weather conditions, and environmental destruction further exacerbate its impact on the developmental sustainability of countries. The light strongly influences the biosynthesis and accumulation of various plant secondary metabolites that are rather critical to the crop quality. The spectral light composition has a great influence on the growth and regeneration processes and is one of the main factors in the plant bioproductivity. The LEDs have substantial potential as the phyto-irradiators. Moreover, in the light of active development of the agroindustrial complex, there is a need to study the specifications of industrially produced LED irradiators and evaluate their efficiency for increasing the crop yields and improving the product quality.
The LED-based phyto-irradiators represent a new technology for artificial agricultural lighting. The LEDs have a huge number of advantages for commercial plant growing applications. Their durability and strength make them easier to install and handle than the conventional lighting fixtures such as the high-pressure sodium lamps and fluorescent lamps with the fragile glass envelopes [1]. Another feature of the LEDs is that their chip size is usually much smaller than the plants that allows for a variety of light source designs, both for tissue cultivation in vitro using multiple LEDs, and for illumination of the greenhouse cultures using the large arrays of LEDs depending on the growing scale [2]. Moreover, the small size of LEDs expands the variety of available plant lighting methods, such as irradiation of specific organs and unrestricted targeted irradiation [3]. The location of lighting sources in close proximity to the plants is also possible due to the use of visible LEDs that do not emit a large amount of heat [4]. Thus, in the light of the active agroindustrial complex development, there is a need to study the specifications of industrially produced LED irradiators and evaluate their efficiency for increasing the crop yields and improving the product quality. The purpose of this paper is to evaluate the efficiency of LED phyto-strip with a spectral composition of 15%B + 40%G + 45% R (B - blue, G – green, R – red radiation) in the field of photosynthetically active radiation for growing the high-quality agricultural products under conditions of artificial germination, to make a conclusion on feasible use of the LED phyto-strip under study, as well as to compare with the alternative phyto-irradiators applied in the industrial greenhouse complexes.
Materials and methods
In addition to the conventional radiation sources that include a fluorescent lamp, the full-spectrum LED lamps or LED phyto-strips are also used for the plant growing process. Thus, this paper provides the comparison of the phyto-strip characteristics with the competitors available on the agro-irradiator market.
The Arlight LED phyto-strip FITOLUX-A144–10mm 24V Day4000-Red (14 W/m, IP20, 2835, 5 m) consists of two types of LEDs: the first type is a white SMD LED with a luminophore (a device made in a small housing with a built-in light-emitting crystal that is superficially mounted on a printed circuit board, see Fig. 1a), containing one crystal. The sample has the dimensions of 2.9 × 3.3 × 1.0 mm. The opening voltage is 2.54 V, the operating range of values is 2.56–2.86 V. The second type is a red SMD LED (Fig. 1 b) containing one crystal. The sample has the dimensions of 2.9 × 3.1 × 1.0 mm. The opening voltage is 1.66 V, the operating range of values is 1.68–1.98 V.
The LED phyto-strip (1 meter) consists of 16 sections containing 9 LEDs: 6 white and 3 red ones (Fig. 1 c). The strip sections has serial connection.
For comparison, we used a full spectrum LED grow lamp with a spectral composition of 1%UV + 39%B + 15%G + 38%R + 7%FR (UV - ultraviolet radiation, DC – far red radiation) for growing the ALMGD plants and a fluorescent lamp Camelion 13 W 4200 K with a spectral composition of 1%UV + 23%B + 40%G + 33%R + 3%FR (Fig. 1d, f). For these lamps, the spectral characteristics and densities of the photosynthetic photon flux were measured at various distances at the operating supply voltage.
The following equipment was used for the measurements: Ocean Optics HR4000 spectrometer, with a spectral range from 200 nm to 1050 nm, and a UPRtek PG200N Spectral PAR Meter spectrometer with a spectral range from 350 nm to 800 nm. The Ocean Optics HR4000 spectrometer together with an integrating sphere was applied to measure the spectral and energy characteristics of the phyto-strip and individual LEDs included in its composition to be schematically outlined in Fig. 2. The UPRtek PG200N Spectral PAR Meter spectrometer was used to measure the light quantity and quality in the area of of photosynthetically active radiation.
Performance of experiments
A study of the current-voltage characteristics of various LEDs (Fig. 3) of the same type has demonstrated minor differences that are acceptable and related to the peculiarities of the LED production process.
The differences in the current-voltage characteristics of red LEDs from white ones are caused by the differences in the manufacturing materials. Thus, an AlGaAs solid solution is usually applied for the production of red LEDs that determines the lower operating voltage of red LEDs [5]. The white LEDs are the common white luminiferous LEDs, consisting of a yellow luminophore deposited on a blue crystal made from an InGaN-based solid solution leading to a higher supply voltage.
The LED radiation spectrum is determined by the semiconductor luminescence spectrum specified by its band structure, primarily the bandgap energy [6]. The availability of quantum wells, wires or dots in the active region leads to the fact that the radiation spectrum is determined by their parameters, in particular by the position of size quantization levels [7]. The base material for the short-wave radiation sources (0.25–0.6 μm) is gallium nitride and its solid solutions; for LEDs in the yellow, orange, red and near-IR spectral regions (0.58–0.9 μm) it is arsenide gallium and direct band gap solid solutions in the (Ga, In, Al)P and (Ga, Al)As systems. Only the direct band gap materials are used in the active region [7]. For different ranges, the radiation spectrum range is various since it is determined by the distribution of charge carriers in the band gap.
We conducted the studies at room temperature at various pump currents of light-emitting structures. The white luminiferous LEDs have peak values in the blue (454 nm) and yellow (575 nm) spectral regions. The peak wavelength of the red LED is 664 nm.
In the aggregate, the radiation spectra of white and red LEDs generate the radiation spectrum of the phyto-strip (Fig. 4). The radiation spectrum of a fluorescent lamp (Fig. 4) is specified by multiple peaks, the highest radiation intensity is observed at the wavelengths of 611 nm, 545 nm and 542 nm. The spectral characteristic of the grow lamp (Fig. 4) contains 2 main emission peaks in the blue (461 nm) and red (638 nm) regions.
Results
As the current increases, a change in the emission wavelength of white and red LEDs is observed. For white LEDs, this change is related to the quantum-dimensional Stark effect (QDSE). The internal piezoelectric fields develop the Stark effect that reduces the internal quantum efficiency. This effect provides for a change in the energy spectrum of atoms, molecules and crystals in an electric field and takes the form of heterostructures with the quantum wells (QWs) in the blue shift of the QW excitonic absorption line. The shift in the luminescence spectrum at low currents is due to the influence of elastic stresses that cause lattice deformation at the heterojunction, and the built-in electric field [8]. Under the built-in electric field influence, the well shape is greatly distorted and a QDSE is occurred. This phenomenon leads to an increase in the distance between the levels resulting in a shift of the luminescence spectrum to the short-wave region (Fig. 5a). As the external voltage increases, influence of the built-in field is compensated and the shift is decreased. The Stark effect is most pronounced in the LEDs based on the InGaN and AlGaN heterostructures [9].
It is well-known that red LEDs show a shift in the peak radiation wavelength: an increase in the current level leads to heating of the active crystal region and a shift of the spectral characteristic to the long-wave region that is related to a temperature change in the band gap [10]. Moreover, we have observed this effect (Fig. 5b).
The most important specification of LEDs is η, namely the efficiency of converting electrical energy into the light energy. The LED efficiency is its coefficient of performance (COP). It is related to the external quantum yield of electroluminescence ηe by the following ratio [7]:
η = ≈ ηe,
where ħω is the photon energy corresponding to the maximum value of the radiation spectrum, U is the applied external voltage.
We have studied dependence of the phyto-strip efficiency on the current (Fig. 6) that show a decrease in the characteristics related to a decrease in the quantum luminescence yield due to the structure heating by the flowing current, as well as to the saturation of radiative recombination and decrease in its efficiency due to the energy level filling in the active region.
The most important indicator for use of a lighting device in the field of plant growing is the quantity of light in the area of photosynthetically active radiation (PAR) provided by it. Let us explain that the photosynthetically active radiation (PAR) is a part of electromagnetic radiation that can be used as an energy source for the photosynthesis process in green plants.
PAR is measured as a photosynthetic photon flux density (PPFD), the unit of measurement is µmol / m2 / s. Although PAR covers the entire visible part of the spectrum, the most efficnet are the blue and red spectrum portions with the secondary peaks in the yellow and orange parts. The ultraviolet (UV) and infrared (IR) radiation are outside the visible range of light and are not included in the PAR range. PPFD is a photosynthetic photon flux density, measured by µmol / m2 / s. PPFD measures the quantity of PAR light (photons) that hits the plant surface every second. For light to be effective for growing essential oil-containing cultures, vegetables and other crops, it must have the PPFD values from 150 to 600 µmol / m2 / s. For comparison, it should be noted that the PPFD of natural solar radiation has a value of 900–1500 µmol / m2 / s when the sun is high.
PPF is a photosynthetic photon flux, measured by µmol / s. PPF measures the total amount of PAR that is produced by a lighting system every second.
The photosynthetic photon flux is calculated by the following formula [11]:
,
where FPAR is the photosynthetic photon flux, µmol/s;
ϕλ – spectral density of the radiation power distribution of the device (in the PAR region), W/nm; λ – wavelength, nm; h – Planck’s constant; c – velocity of light; NA – Avogadro’s number; K – proportionality coefficient.
The efficiency of the emitting device in the PAR region is calculated by the following formula [11]:
ηPAR = ,
where ηPAR is efficiency in the PAR region, µmol / W / s; P – power consumption, W. As the distance increases, PPFD is decreased according to the inverse square law. General PPFD value of the LED phyto-strip (Fig. 7a) is increased as the number of LEDs raised: 6 white and 3 red ones. The average PAR value applicable for the plant growing process (≈300 µmol / m2 / s) and achieved at the operating current was obtained from one meter of phyto-strip at a distance of ≈20 cm (the maximum achievable PPFD value at a given distance) from the irradiated area.
We measured the photosynthetic photon flux of the lighting device (Fig.7b) using an integrating sphere. The internal part of the hollow sphere is white (fluoroplastic) that provides high reflective and scattering properties. In accordance with the lighting requirements of GOST R 57671-2017, the efficiency of devices in the PAR region must be at least 2.0 µmol / J for the devices intended to illuminate the plants from above. As it is shown by Fig. 7c, the efficiency of phyto-strip under study in the PAR region at an operating current is 2.62 µmol / J that is 31% higher than the normal value.
The light absorption by the photosystems is promoted by the photosynthetic pigments. The oxygen photosynthesizers including the plants such as algae and cyanobacteria, use the chlorophyll pigments. There are several different types of chlorophyll, while chlorophyll a and chlorophyll b being the most common types for the ground vegetation [12]. Both pigments have absorption peaks in the red and blue spectral regions, while chlorophyll a having the peaks at 430 and 662 nm, and chlorophyll b having the peaks at 453 and 642 nm (the location of these peaks is displaced depending on the cell properties where they are located).
Algae and cyanobacteria contain additional types of chlorophyll pigments such as chlorophylls c, d and f [13]. Anoxygenic photosynthesizers such as purple bacteria, green sulfur bacteria, heliobacteria and filamentous anoxygenic phototrophs, used the alternative but related pigments called “bacteriochlorophylls” [14]. In addition to the pigments that capture primary light, the photosynthesizers apply “the antenna pigments”, such as carotenoids, to capture higher energy photons and transmit them to the photosystem. Figure 8 shows the absorption spectra of various chlorophylls and other light-catching pigments, as well as the radiation spectrum of phyto-strip, fluorescent lamps and LED lamps. The main PAR share falls on the red and green ranges: LED phyto-strip has a following PPFD ratio: 15% blue (400–500 nm) + 40% green (500–600 nm) + 45% red (600–700 nm).
The spectrum shape influence on the efficiency of irradiation device in the PAR region was assessed by superimposing the plant pigments such as chlorophyll a, chlorophyll b and carotenoids, on the absorption spectrum (Fig. 8). The best overlap between the radiation spectra of devices and the absorption spectra of photochemical pigments, with due regard to the efficiency of converting electrical power into the optical power, is observed for the LED emitters and is equal to 42.2%, in contrast to 8.9% for a grow lamp and 9.6% for a fluorescent lamp.
Discussion of results
The fluorescent lighting, combined with the incandescent lamps, promotes the vegetative plant growth, color, appearance and product quality, similar to the results obtained with the solar radiation. However, there is a decrease in the growth rate and biomass accumulation. The white LEDs combined with the red LEDs promote carbohydrate storage and improve the energy efficiency level compared to the fluorescent lamps during the germination stage and subsequent cultivation stages [15].
We have found that the average value of the photosynthetic photon flux density required for the efficient plant growth (≈300 µmol / m2 / s) is achieved with an operating current of one meter of phyto-strip at a distance of ≈20 cm from the irradiated area. The spectral composition of LED phyto-strip with a ratio in the PAR region of 15% blue, 40% green and 45% red light is not considered optimal for growth stimulation of green agricultural crops. This phenomenon is consistent with the results presented in the papers [16, 17], the authors of which have shown that special selection of LEDs emitting the light at certain wavelengths makes is possible to provide sufficient energy to stimulate the photosynthetic pigments and greater accumulation of dry mass. To maximize the growth stimulation of such plants, a spectrum with a higher content of blue and red light that constitutes up to 80% of the total PPFD, is more preferable.
It is well-known that the application of LEDs, compared to the conventional devices such as the high-pressure sodium lamps or fluorescent lamps, provides higher energy efficiency. The impact effectiveness of the phyto-strip radiation on plants evaluated in this paper, significantly exceeds the impact effectiveness of alternative radiation sources and is equal to 42% being 4.7 times higher than the efficiency of a full-spectrum grow lamp in the PAR region and 4.4 times higher than the efficiency of a fluorescent lamp.
Conclusion
The LED phyto-strip with a spectral composition of 15% blue, 40% green and 45% red light has high efficiency, dimming possibility (adjustment of the light brightness) and a radiation spectrum corresponding to the absorption spectra of plant pigments. Such properties make the device most preferable for use in the agroindustrial enterprises to increase the agricultural productivity. In this case, the installation simplicity and safety of the lighting device should also be considered. The high efficiency of phyto-strip will improve the growth quality of various agricultural crops in the autonomous agroindustrial enterprises. During the further research, it is planned to study the energy cost reduction methods when using the LED phyto-strips in the industrial greenhouse complexes.
AUTHORS
Mariya M. Degtereva –postgraduate student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0001-6797-0595
Yevgeniy Levin –postgraduate student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0009-0000-3811-487X
Alexander E. Degterev –postgraduate student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0002-6151-6567
Alexander A. Bogdanov – master student of St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0009-0004-2540-4228
Ivan A. Lamkin – Cand. of Tech. Sciences, Associate Professor of the Department of Photonics, St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0002-3680-7725
Sergey A. Tarasov – Doctor of Technical Sciences, Head of the Department of Photonics, St. Petersburg Electrotechnical University “LETI”, St. Petersburg, Russia.
ORCID: 0000-0002-6321-0019
Pavel A. Sergeev – Engineer, General Director of OOO Svetoyar, St. Petersburg, Russia.
CONTRIBUTION OF THE AUTHORS
Degtereva Mariya Mikhailovna – measurement of phytotape parameters in the field of photosynthetically active radiation, editing of the article, analysis of literature; Levin Yevgeniy – measurement of spectral and energy characteristics of phytolent, editing of the article, analysis of literature; Degterev Alexander Eduardovich – measurement of the electrical characteristics of LEDs, editing the article; Bogdanov Alexander Alexandrovich – electrical and colorimetric characteristics of the phytotape, editing the article; Lamkin Ivan Anatolyevich – scientific work management, work planning, article editing; Tarasov Sergey Anatolyevich – problem setting and scientific research management; Sergeev Pavel Andreevich – setting the task of providing samples for research.
CONFLICT OF INTEREST
The authors state that they have no conflict of interest. The results of the research were discussed and reflected in the manuscript, which is a joint work.
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