6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
PV Power Source Estimation for a Custom-Designed
Growing-Plant Lighting System
Elena Dănilă, Dorin Dumitru Lucache
Department of Energy Utilisation, Electrical Drives and Industrial Automation
Faculty of Electrical Engineering, Technical University Gheorghe Asachi of Iasi
Iasi, Romania
that can damage cells and tissues (so they are largely filtered
by the ozone layer in the stratosphere), and photons at longer
wavelengths don’t carry enough energy to favor
photosynthesis [4]. PAR is expressed in number of photons
per second (not as a direct measure of energy), because the
photosynthetic process occurs when a photon is absorbed by
the plant, regardless of the wavelength of the photon. In other
words, if a number of blue photons is absorbed by the plant,
the process of photosynthesis’ level is equal to the level
produced by the same amount of absorbed red photons. The
total amount of energy emitted per second in the range 400700nm is measured in PAR watts. This is an objective
measure, opposed to lumens that are subjective units. PAR
watts show directly how much energy is available to be used
in plant photosynthesis. "Illumination" for plants is measured
in PAR watts per square meter. There is no specific name for
this unit, but is referred to as "radiation" and noted, for
example, 25 W/m².
Abstract—When solar radiation meets the leaf of a plant,
there occur three optical phenomena: reflection (~10%),
transmission (~27%) and absorption (~63%). In photosynthesis,
the emitted spectrum by a lighting source (natural or electric) is
not entirely used by plants, only the radiation in the range of 400
- 480nm (blue and purple) and 580 - 680nm (red and orange).
The artificial light can stimulate the crop production, in specific
conditions and with a very careful system design, considering
both energetic and biologic aspects. The paper opens a broad
interdisciplinary study direction, also actual - as the management
of food resources in the world becomes a serious problem, and
the deeper integration of technology in agriculture is no longer a
trend but a necessity. The paper assess the use of PV panels to
supply the growing-plant lighting systems placed in protected or
far areas where the grid is absent, as a green power source
alternative.
Keywords— PV panels, photosynthetic active radiation, grow
light, yield photon flux, crop production, solar powered LEDs.
I.
PAR is normally quantified in µmol photons m-2s-1, as a
measure of photosynthetic photon flux density, or PPFD
(parameter used more by biologists). This is sometimes
expressed in Einstein units, for example, µE m-2 s-1, although
this unit is not part of the International System and is
redundant with the mole. Conversion between energy-based
PAR and photon-based PAR depends on the lighting source
spectrum (Table I). PAR can be indirectly calculated based on
the values of direct radiation (Im) and diffuse radiation (Id),
and the values can be expressed in cal/(cm2min), J/(cm2min),
W/m2 [5]:
(2)
PAR = 0.43I m + 0.56 I d
PARAMETERS OF HORTICULTURAL LIGHTING SYSTEMS
Although many design principles apply to both forms of
lighting, general and horticultural, the measurement units used
in general lighting (lumen, lux, lumens per watt and footcandles) are correlated or appreciated by the human eye.
Instead, horticultural lighting systems are best assessed by
parameters showing the use of light or lighting system
capacity to stimulate photosynthesis. These values are,
therefore, related to the absolute number of photons.
As any plant’s absorption spectrum is the spectrum of
radiant energy whose intensity, at each wavelength, is a
measure of the energy amount, of that wavelength, which
passed through chlorophyll [1], it follows that plants obtain all
their energy requirements only from blue and red light [2].
Total solar irradiance describes the radiant energy emitted by
the sun over all wavelengths, on a surface that is normal to the
incident radiation [3], but the plants’ response is based only on
the daily irradiation (the average direct insolation, arriving at
the surface in the absence of clouds, from 6 am to 6 pm).
Yield photon flux (YPF), measured in µmol m-2s-1, is the
parameter that considers not only the photons, but shows how
efficiently the plant uses them (Fig.1).
TABLE I.
η_photon
η_v
(µmol/J* or µmol
(lm/W*)
s−1W*−1)
η_photon
(mol
day−1 W*−1)
3000
(warm white)
269
4.98
0.43
0.0809
4000
277
4.78
0.413
0.208
5800 (daylight) 265
4.56
0.394
0.368
T
(K)
The photosynthetic active radiation (PAR) is defined by
CIE as the total exposure to photons in the solar radiation band
from 400 to 700 nm, in that the energy is absorbed by
photosynthetic pigments. This spectral region corresponds
more or less with the range of visible light to the human eye.
Photons at shorter wavelengths tend to be so "energetically"
CONVERSION FACTORS FOR PHOTOSYNTHETIC PARAMETERS
OF LIGHT SOURCES [6]
W* and J* indicates PAR watts and PAR joules (400–700 nm).
77
η_PAR
(W*/W)
6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
II.
ANALYSIS ON A GROW LIGHTING SYSTEM FOR A
SPECIFIED CROP
The normal artificial lighting in a greenhouse is designed
in accordance with its natural lighting system, for their use in
parallel (in addition) as much in the daytime. In the same time,
must be also complied the criteria of choosing the light
sources, namely: the spectral quality and how light interacts
with plant’s tissue, plant’s photoperiodicity and the geometry
of space.
In literature there are several recommendations on the
horticultural lighting systems depending on the nature of
culture, but almost all are empirical. No lighting solution
arguments simultaneously by analyzing chamber geometry,
spectral distribution of the light flux, PAR, PPFD and YPF.
Calculation of required PAR for the cultivated crop
The preliminary case study covers a small area of 35cm x
25cm=0.875m2, on which was planted lettuce. It is a long-day
plant, which can be exposed to continuous illumination. The
analysis in [9] shows that subjecting this plant species to
continuous light has no negative effects, just shorten the cycle
of generation and leads to accumulation of large quantities of
dry matter. The light requirement for this type of plant is 4000
lx [10]. It follows that, for the planted area, the required flux
is:
Fig. 1. Relative quantum yield for crop plant photosynthesis [7]
Because red light (or red photons) is used more effectively
to induce reaction of photosynthesis, YPF gives more
importance to red photons and is based on plant sensitivity
curve, between 360 and 760 nm.
Although more than 90% of the blue photons are absorbed,
about 20% of them are captured by the non-active pigments of
the plant and energy is transferred to the collector pigments
(or to the cells of the plant reaction), heat being lost. In other
words, the efficiency of a mole of absorbed blue photons is
20% lower than that of a mole of absorbed red photons. All
these considerations are based on the second law of
photochemistry, Stark-Einstein law [8], which says that the
radiation absorbed not necessarily lead to a photochemical
reaction; however, if the reaction occurs, for each transformed
molecule, a single photon is required.
φ = E ⋅ S = 4000 ⋅ 0.875 = 3500lm
(3)
For this crop, the photosynthetic active radiation at natural
light (medium PAR in summer is 30 mol/m2/day and in winter
is 3 mol/m2/day) is, according to conversion factors from
Table I:
3500lm / 265 = 13.2WattPAR ,
(4)
equivalent to: 13.2 ⋅ 4.56 = 60.2 µmoles / sec , meaning that the
horticultural lighting system has to provide around 3500
lumens and 60.2 µmoles/sec.
Photosynthetic photon flux density (PPFD) measured in
micromoles per square meter per second is another parameter
that characterizes the horticultural lighting systems. This value
represents the total number of photons in the range of active
wavelength for photosynthesis, which falls on a square meter
per second. This is equivalent to illumination.
Evaluation of the grow lighting system
The chosen luminaire is a LED growing module (Fig.2),
with the following characteristics:
• Dimensions: 310x310x32 mm;
• Rated voltage 230 V;
• Total power 14W, so particularly 1 led has 0.062W;
PPF/Watt is a measure of the efficiency of coupling,
expressed in micromoles per Joule. This metric represents the
total number of photons in the range of active wavelength for
photosynthesis, generated by one Joule of electricity.
PPF/delivered Watt is a measure of the effectiveness of the
lighting system, expressed in micromoles reaching the canopy
of plants per Joule. This value represents the total number of
photons in the range of active wavelength for photosynthesis
that reach the plant canopy, generated by one Joule of
electricity. In general, this is the best measure for evaluating
the electrical efficiency of various horticultural lamps.
Regardless of the efficiency of lamps, the flow of photons
must reach the plant canopy, in order to be absorbed and
induces photosynthesis, and/ or photomorphogenesis response.
Fig. 2. Led grow light for irradiating the lettuce crop
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
•
160 red led (630nm wavelength) and 65 blue led (465 nm
wavelength);
• The optical axis is perpendicular on crop, with a hanging
distance of 100 cm.
Using the LED Spectra dedicated software computing, it
was checked whether the luminaire covers the necessary
lighting to the tested crop (Fig.3). For accurate simulation,
suspension height and the angle of incidence were introduced.
Resulted from the simulation that plants on the tested crop
can benefit of:
• A luminous flux of 525.9 lm;
• A radiant flux of 11.78W;
• A photosynthetic photon flux density of 18.41
µmol/m2/sec;
• A photosynthetic active radiation of 57.852 µmol/sec, less
than in the case of growing the same crop at natural light;
• An illumination of 167.4 lx (Fig.4);
• The entire blue spectrum emitted by the artificial source,
with maximum efficiency in the range of 460 – 470 nm.
Fig. 4. Resulted spectrums from simulation
To reach the optimum value of 60.2 µmol/sec – calculated
for the case when the plants are irradiated by the sunlight, are
necessary 155 blue led and 70 red ones of the same power
(Fig.5), meaning that 90 of the red leds must be replaced with
blue ones.
The authors appreciate that the error of:
57.85µmol / s − 60.2 µmol / s
* 100 = −3.9%
ε=
60.2µmol / s
(5)
is acceptable for this analysis, taking into account that the
wavelength in the simulation software is a range, not a precise
value.
Fig. 5. Determination of the optimal number of LED light necessary
equivalence
There can be also calculated the yield photon flux, using
the conversion factor for led luminaires of PPF/YPF=1.12
[11]:
18.41µmol ⋅ m −2 s −1
YPF =
⋅ 0.875m 2 = 14.38µmol / sec (6)
1.12
Results and analysis regarding plants’ growth
To estimate the gain get by applying the growing lighting,
parallel with the test crop, it was planted a control crop, raised
in daylight, under the same conditions of temperature and
humidity. The lettuce on the test crop was measured daily and
watered according to soil (dynamic biometric is shown in
Fig.6). The temperature inside the growing room varied
between 19.50C and 24.20C.
After one month, the gain in plants' medium height was of
183% and the estimated dry mass gain result about 242%
when compared with the daylight-exposed control crop. For
this
gain,
the
energy
consumption
was
14W*24h*27days=9.072kWh, so energy expenses of about 1
Euro.
Fig. 3. PAR and PPFD calculation for the chosen luminaire, with 65 blue led
and 160 red ones, of the same active power
79
6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
splashing water, dust, temperature range in which it has to
work at full power.
6. The reliability, life cycle and price.
There are three types of sources for LEDs’ supply:
a) CV&CC – is a source of constant voltage during startup,
after that it provides constant current to drive LEDs. It has the
advantage of not requiring driver integrated circuit, has high
efficiency (lumen / watt), low price and can be used in all
types of LED lighting systems, making them more flexible.
b) CC – is a source of constant current. The output voltage
can be determined by the total voltage drop across the LEDs
(the forward voltage). It is used to supply directly a series of
LEDs. It has the disadvantage of the appearance of unbalanced
currents in the branches that reduce the lifetime of the LEDs in
a string failure. LEDs do not require the same number of
milliamps when powered at the same voltage. For this reason,
constant current sources are required to maximize the lifetime,
particularly in the case of high-power LEDs.
Fig. 6. The graph of the plant’s height evolution in the reference period
III.
THE GREEN SUPPLY OF GROW LIGHTING SYSTEM
c) CV – is a source providing constant voltage, which must
be regulated within voltage tolerance specification. For this
reason, and for having an AC input (90~264Vac), the source
operates always in series with a DC/DC LED driver. It
presents some advantages over the first two: independent
channels for balanced currents, higher lifecycle and do not
require sorting LEDs by voltage. Instead, the price of these
sources is higher, their circuit is more complicated (therefore
have a larger volume) and have a lower efficiency.
For an efficient growth process, the horticultural lighting
system can be integrated into a fully automated system (like
KNX [13-Schneider]), which commands the on/off powering,
cooling or heating the greenhouse, water pumps, irrigation
tapes, the micro sprinklers, lighting when the solar radiation
falls below an acceptable limit value. Also, the energy
consumption could be directed as a response to growth
conditions and biological stage of the plant (remote
management of energy demand). All this should finally lead to
minimization of greenhouse’s dependence on the public
energy supply grid once with the integration of a renewable
energy source.
The horticultural LED module, which comprises
combinations that emit in the F12, F33, F64 and F75 growth
spectra or monochrome red / blue, has electrostatic discharge
(ESD) protection with Zener diode (Fig. 8) and can be
connected in series or in parallel with the power supply
modules. It requires radiator for heat dissipation. Integrated
ESD grids from silicon are devices that protect analog and
digital signal lines, the space occupied on the printed circuit
being therefore small.
Lighting is responsible for a significant weight of
electricity consumption, regardless the activity domain:
industrial - 10%, residential - 40%, commercial - 25%...50%,
public lighting - 100%. A current trend is to integrate
management devices which allow the optimization of the
energy consumption by managing lighting control according
to: hour, daytime, imposed limited operation, movement or
presence of personnel, light level, report of natural light. In
addition to this, a fully autonomous chamber/construction can
be built by using a renewable energy source to power the
lighting system (and, if appropriate, other specific consumers).
The criteria for choosing sources to power a LED-based
lighting system are:
1. The value of appropriate power level, including the
safety margin;
2. The topology of lighting system;
3. The type of supplying source: of constant current (CC)
or of constant voltage (CV), on which there are integrated
additional drivers1 (LDD-L or LDD-H) to get a more accurate
constant current level;
4. The type of application – if it requests power factor
correction (PFC) and flyback controller;
5. The mounting location of LEDs power supply (site type,
indoor/outdoor) and the environmental conditions: dry/wet/
1
Fig. 7. (a) ESD protection circuits using multiple pn junctions or Zener
diodes. (b) ESD protection incorporated in Si submount [12]
It is ascertained that the LED grow lighting module used in
this study has the powering components integrated. In case
they were external, it will be necessary to take into account the
power dissipated in the driver and the power indicated for one
or several associated LED modules [13].
2
blue (22.8%), green (0.3%), red (76.8%), far-red (0.1%)
3
blue (11%), green (7.7%), red (81%), far-red (0.3%)
4
blue (49.5%), green (0.5%), red (49.9%), far-red (0.1%)
5
blue (74.5%), green (0.6%), red (24.8%), far-red (0.1%)
encapsulated module type DC-DC buck converters for LED driving purposes
80
6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
The power consumption in the case study presented in
section II (for continuous lighting – 24 hours/day, for one
growing stage of 27 days and for a cultivated crop of 0.875m2)
is of 9.072kW – absorbed from the public network. But not
always the location of a greenhouse allows the access to
electricity network, as the location depends first on taking
maximum amounts of sunlight, at least 6 hours a day of direct
sun. A method of increasing the degree of autonomy in
powering a greenhouse is installing solar panels to collect in
the daytime the solar radiation and during the night to power
the lighting for growth. The basic principles of designing a
solar greenhouse set that it must have glazing oriented to
receive maximum solar heat in the winter, must be built from
heat storing materials, and must have large amounts of
insulation where there is little or no direct sunlight [14].
IV.
The artificial light can stimulate plant’s growth without
causing organic or morphological changes, as the sources emit
only in the red and blue region of the light spectrum - the most
effective radiations in photosynthesis. A proper design of a
growing system, considering biological and energetic aspects,
can increase the capacity of sustaining vegetable and fruit
consumption and lead to the reduction of energy intensity of
the horticultural sector.
The coverage of the greenhouses with solar panels appears
as a modern energy-saving measure, designed to ensure a
certain degree of energy self-efficiency of the greenhouse.
Unfortunately, the measure has serious limitations, bringing
with it a series of disadvantages like the higher costs, the need
of a natural light valve or cooling the panels. Even there are
used transparent solar panels, their natural light transmission
coefficient is only of 20%, which is not sufficient for plant
growth in the daytime.
To provide the input parameter for the LED-based growing
sources, there can be adopted a solar PV system, of latest
technology with transparent photovoltaic glass, all-aluminum,
2.4m x 3.5m, generating 600kWh/year of renewable electricity
[15]. The price is of 6300 €/piece, without including
foundation ground works, greenhouse assembly, storage units,
electronic timers and controls for driving LED on lighting
cycles and electrical connections. Assuming that the
greenhouse operates in off-grid mode, it results that the PV
system must provide all the energy demand for lighting the
cultivated crop (hypothetically being of 500 m2, by
extrapolating the analysis in the previous section) during the
night. In this case, for powering the lighting for growing one
batch (27 days, approximately 1 month) are needed on average
2592 kWh (see Table II). It must be taken into account that the
amount of supplied energy depends on geographic location, as
the amount of solar radiation also varies. To ensure the yearly
energy demand calculated at 31104 kWh, the greenhouse must
be covered with 435.45 m2 of solar panels, i.e. 52 pieces, tune
of 327600€. Including the installation and electricity storage
costs, the investment in an autonomous PV power source
become extremely high and difficult to be recovered in due
time.
TABLE II.
Lately, photosynthesis efficiency calculation has aroused
the interest of specialists to determine the energy efficiency of
crops for bio-fuels. Ensuring the independence of the
greenhouse by recycling their waste (biomass) is not only an
energy management solution, but an environmental-friendly
one because, as specified above, in the particular case of
lettuce, continuous exposure to light radiation increases the
production of dry mass.
REFERENCES
[1]
[2]
[3]
[4]
ENERGY DEMAND FOR THE LED GROW LIGHTING SYSTEM IN
CASE OF EXCLUSIVE NIGHT OPERATION
[5]
Month
January
February
March
April
May
June
July
August
September
October
November
December
1 year
1 month
Darkness
[hours/day]
14
13
12
11
10
9
10
11
12
13
14
15
144hours
12 hours
CONCLUSIONS
Energy demand [Wh]
Cultivated
Extrapolation for a
crop
passive greenhouse (500
2
(0.875m )
m2)
5292
3024000
4914
2808000
4536
2592000
4158
2376000
3780
2160000
3402
1944000
3780
2160000
4158
2376000
4536
2592000
4914
2808000
5292
3024000
5670
3240000
54432Wh
31104000Wh=31104 kWh
4536Wh
2592kWh
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
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