Design and implementation of RELab system to study the solar and wind [630770]

Design and implementation of RELab system to study the solar and wind
energy
P.A. Cotfas, D.T. Cotfas⇑
Transilvania University of Brasov/DEC Department, Brasov, Romania
article info
Article history:
Received 23 March 2016
Received in revised form 27 June 2016
Accepted 28 June 2016Available online 5 July 2016
Keywords:
RELab systemParameter estimationPhotovoltaic cellsThermal collectorWind turbineabstract
It is well known that renewable energy, today more than ever, has undergone an exponential develop-
ment. Thus, it is necessary to create an efficient, compact and relatively cheap system to be used in edu-cation and research, which allows studying these types of energy. The concept, design and
implementation of such a system – RELab are presented in this paper. The RELab system is a very useful
tool for students and engineers to learn about the two types of renewable energy: solar and wind energy.The system has two components: the NI ELVIS platform and the RELab add-on. The RELab add-on is a
modular one and it can be easily redesigned for using the photovoltaic cell, the wind turbine and the solar
thermal collector. The software used is built in the graphical programming language LabVIEW, and it isflexible and open.
/C2112016 Elsevier Ltd. All rights reserved.
1. Introduction
The photovoltaic, solar thermal and wind energy are amongst
the most widespread types of renewable energy. Ref. [1]shows
that from the added renewable capacities in 2012, 39% consists
of wind capacities and 26% of photovoltaic capacities, hovering
on the first two positions. The photovoltaic installed capacity at
the end of 2014 was of 177 GW, of wind installed capacity –
370 GW and of solar thermal glazed and unglazed water capacity
– 406 GW th[2].
The study of the renewable energy was introduced in all coun-
tries in the scholar curriculum on different levels, due to the strong
development and its impact on society. So, it is necessary to
develop new laboratory equipments for the study and understand-
ing of renewable energy. It is useful that these devices allow the
studying of the important parameters and of the factors which
influence the efficiency of the systems that produce renewable
energy. Another important goal of this development is to increase
the interest of the young not only for the renewable energy but
also for measurements and electronic engineering [3].
The efficiency and the other parameters of photovoltaic cells
and panels are affected by the ambient temperature, the levels of
illumination, the wind speed and the inclination angle of the nat-
ural sunlight which falls on the cells or panels. The dc parameters
which influence the output power of the photovoltaic cells andpanels are the series and shunt resistance, the ideality factor of
diode, the photogenerated current and the reverse saturation cur-
rent [4].
The I-Vcharacteristic of the photovoltaic cell or panel is the
most suitable tool to determine the dc parameters and to study
the influence of environmental conditions [5,6] . The short circuit
current, the open circuit voltage and the maximum power can be
very easily determined using this tool. The other parameters of
the photovoltaic cells can be determined using one or more of
the over 35 methods developed by researchers [4].
The generated power is the most important parameter for a
wind turbine. It is strongly dependent on the wind speed, the num-
ber of blades, the shape of the blades, the blade pitch angle and the
circular area of the wind turbine [7–11] . The P-Vcharacteristic of
the wind turbine can be used to study the generated power of
the wind turbine in function of the factors enumerated above.
The performance of the solar thermal collector is given by the
useful thermal power and the efficiency of the collector. These
two parameters can be studied function of a lot of factors such as
the irradiance, the mass flow rate, the fluid used and the type of
the collector – selectivity of the absorber, geometry, inclination,
shape and the parameters of glass [12–14] .
This paper presents RELab, which is a new versatile system for
renewable energy educational laboratories. The RELab system
allows determining and studying the important parameters and
factors which influence the performance of photovoltaic cells,
small solar thermal collectors and mini wind turbines.
http://dx.doi.org/10.1016/j.measurement.2016.06.060
0263-2241/ /C2112016 Elsevier Ltd. All rights reserved.⇑Corresponding author.
E-mail address: dtcotfas@unitbv.ro (D.T. Cotfas).Measurement 93 (2016) 94–101
Contents lists available at ScienceDirect
Measurement
journal homepage: www. elsevier.com /locate/measurement

In Section 2the idea, the concept and the evolution of the RELab
system are presented. The design of the RELab system modules is
described in Section 3. The software concept and the facilities
implemented are discussed in Section 4. The RELab analysis is pre-
sented in Section 5. The conclusions and the future developments
are presented in Section 6.
2. Concept and evolution
2.1. Concept
The concept, at the beginning of the development, was to build
a compact, flexible and relatively cheap device which allows the
study of the photovoltaic cells’ important parameters. Then, by
using the same structure, of course with a few changes, the system
has to permit the study of other renewable energy types such as
wind energy and solar thermal energy. The concept is based on
the NI ELVIS platform [15] and on the RELab add-on. The flexibility
of the system allows following the evolution of the NI ELVIS plat-
form and it is very easily adaptable to the new version of the latter.
2.2. Evolution
The characterization of photovoltaic cells, from the perspective
of the parameters and the atmospheric factors, is necessary for
researchers and for manufacturers who need to know how they
can improve the efficiency of photovoltaic cells [5]. Also it is very
important for the engineers who implement the photovoltaic sys-
tems and for the students in renewable energy domain. The dc
parameters offer necessary information about the proper or impro-
per behavior of PV systems during the exploitation. The growth of
the series resistance in time shows a degradation of the PV panels,
highlighting the damaging of the contacts, the bus bar oxidization
or the increase in dangling bonds. The decreasing of the shunt
resistance in time can show the occurrence of the hot spots and
a low constant value of the Rshshows the manufacturing flaws.
Also the growth of the reverse saturation current shows a degrada-
tion of the photovoltaic cells or panels [16–21] .
The SolarLab is a device built to cover the majority of these
goals. In the first release, the system consisted of: the NI ELVIS
platform, XY graph with light source, the photovoltaic cell
mounted on the heater support, the electronic circuit for the mea-
surement of the I-Vcharacteristic, the electronic circuit for control-
ling the heater, the stepper motor and the XY graph, Fig. 1 a. All the
electronic circuits for this release are built on the prototyping
board of the NI ELVIS platform. The XY graph with light source isused to vary the level of illumination of the photovoltaic cell by
varying the distance between the light source and the photovoltaic
cell. The irradiance can be varied between 100 W/m
2and 1000 W/
m2which represents 1 sun. The light source is a halogen bulb. The
variation of the photovoltaic cell temperature is ensured using the
heater. The temperature can be varied from room temperature to
85/C176C. The study of the photovoltaic cell behavior in function of
the light inclination angle is made by using the stepper motor.
The incidence angle can be varied from 0 /C176to 90 /C176.
Because the goal was to develop a compact system, the second
release comes naturally with eliminating the XY graph. Another
major improvement was to make all circuits as add-ons which
replace the prototyping board of the NI ELVIS, Fig. 1 b. The variation
of the irradiance is realized by varying the voltage drop on the
bulb.
The last release proposes a modular board, which transformed
it in a versatile and flexible system. The SolarLab board has five
removable modules: light control module (LC), heat control mod-ule (HC), stepper control module (SC), I-Vcharacteristic measure-
ment module (I-VM) and heater module (HM) Fig. 1 c.
The modularity concept of the SolarLab system creates the pre-
requisites to use its modules separately to study other phenomena
as well [22]. Also this concept allows the very easy transition to
study other types of renewable energy, through a few changes.
Thus, through adding some new modules and devices, SolarLab
becomes a complex and useful system to learn the renewable
energy – RELab.
The study of the small wind turbine can be realized by using the
mini wind turbine, the fan and the same board with only one mod-
ule replaced. The I-Vcharacteristic measurement module is
replaced with the wind module, Fig. 2 [23] .
The small wind turbine was chosen to allow the study of as
many parameters as possible. The Horizont wind turbine permitsto change the blades pitch angle, the number of the blade from 1
to 6, the profile and blade material very easily. The fan allows
the variation of the wind speed.
The studying of the solar thermal collector can be realized using
a small solar thermal collector which replaces the heater module,
the flow meter, a pump and the same board with only one module
replaced. The I-Vcharacteristic measurement module is replaced
with the solar thermal module, Fig. 3 [23] .
The flexibility of the system permits its adaptability to the new
myDAQ board, developed for the individual work of students. The
mini-RELab can be built using the RELab’s modules, see Fig. 4 . This
new release is dedicated to work outside, in real conditions, but it
can also be used in the laboratory.
3. Design
The modular design of the system presents some advantages:
huge flexibility, the possibility to use the modules separately and
to ensure a good maintenance. From the electronic circuit point
of view, only one module needs to be changed in order to study
another type of renewable energy. The maintenance of the RELab
can also be easily realized due to the modularity. If the system
doesn’t work properly, the module with problems can be detected
and repaired or replaced.
3.1. Design of the I-V characteristic measurement module
The first challenge was to design the electronic circuit to mea-
sure the I-Vcharacteristic of the photovoltaic cell. The current-
voltage curve can be measured using different techniques: the
electronic load, the capacitor, the MOSFET and the potentiometer
technique [24–29] .
Because the I-Vcharacteristic module has to be simple, rela-
tively cheap and accurate, two techniques were tested: capacitor
and MOSFET. These techniques were compared with three devices
dedicated to the measurement of the
I-Vcharacteristic – Autolab,
Agilent and Keithley 2400 which are based on the electronic load
technique. The results obtained with both the capacitor and the
MOSFET confirm the possibility to be used, see [30]. Because the
capacitor technique is faster than the MOSFET technique, the I-V
characteristic module is built using the first. The choice was made
so that the temperature of the photovoltaic cell remains almost
constant during the I-Vcharacteristic measurement.
The electric schemata of the electronic circuit used to measure
theI-Vcharacteristic is made using the NI Multisim software which
also allows making the simulation. All the modules followed the
same path from concept to real electronic circuit.
The I-Vcharacteristic is measured during the charging of the
capacitor, see Fig. 5 . When the solar cell is connected to the capac-
itor, C1, the cell begins charging the capacitors at its full shortP.A. Cotfas, D.T. Cotfas / Measurement 93 (2016) 94–101 95

circuit current. The capacitor continues to charge until it reaches
the cell open circuit voltage. After the sweep is completed using
an electronic relay – a double switch from Analog Devices was
used (here simulated through S1), the capacitor is discharged
across a resistor, R2. The capacitor is pre-charged using the V1 volt-
age source to compensate the voltage drop on the small resistance
added in the circuit, by the contacts, wires and the resistance usedfor measuring the current (here simulated through the R3 poten-
tiometer), in order to have the entire I-Vcharacteristic measured,
seeFig. 5 a. The improved schemata shown in Fig. 5 b. were devel-
oped after the basic simulation for the I-Vmeasurement method.
The S1 switch was replaced with a CMOS single-pole, double-
throw (SPDT) switch, U1, with a very low on resistance (0.41 X)
from Analog Devices. When the U1 switch is On the voltage source
V1 is decoupled using another CMOS switch, U2.
The measurement of the I-Vcharacteristic is made using the 4
wire method to avoid the resistance of wires. The current is mea-
sured using a very precise resistance 0.05 Xwith 0.1% tolerance,
Rsen, and an instrumentation amplifier from Analog Devices to
amplify this signal, U3. The measurement system is simulated here
through the XSC1 osciloscope. The I–Vcharacteristic obtained
through simulation is shown in Fig. 5 c. The oscilloscope is set to
represent the current channel, B, function of the voltage channel,
A. The trigger is fixed at zero level on the A channel. Isc and Voc
of the solar cell are measured using the two cursors. In order to
make the simulation for the circuit the switches and the instru-
mental amplifier were simulated in NI Multism, based on spice
models offered by the manufacturer.
3.2. Design of the heater control and heater modules
The temperature is a very important factor which influences the
efficiency of the photovoltaic cells or panels. The heat control mod-
ule was designed to vary the temperature of the photovoltaic cell.
The temperature control is realized using a PID control algorithm.
The heater module consists of: the heater, which is made as a
resistor by spiral shaped on the back of the PCB, the photovoltaic
cell support, the analog temperature sensor and the irradiance sen-
sor. The temperature sensor has a precision in temperature mea-
surement by ±0.5 /C176C which ensures a good accuracy for our
purpose. The irradiance sensor is a sensor with high-resolution
conversion of light intensity to frequency without external compo-
nents, which consists of a configurable silicon photodiode and a
current-to-frequency converter on single monolithic CMOS inte-
grated circuits [31].
This module is mounted on the bipolar stepper motor shaft and
it has a micro contact to initialize the rotation. The stepper motor is
Fig. 1. The evolution of the SolarLab: (a) first release; (b) second release; (c) final release.
Fig. 2. Wind part of the RELab.
Fig. 3. Solar thermal collector of the RELab.96 P.A. Cotfas, D.T. Cotfas / Measurement 93 (2016) 94–101

used for changing the incidence angle of the irradiance to solar cell
surface.
3.3. Design of the other modules
The irradiance which hits the photovoltaic cells or panels can
vary due to atmospheric conditions, the seasons or the sun move-
ment on the sky.
The light module is designed to vary the level of illumination
between 100 and 1000 W/m2. The light control module is made
on the same principle as the heater control module.
The stepper module is designed to control the rotation of the
heater module so that the angle between the light and the photo-
voltaic cell can be modified. The stepper motor is controlled with a
stepper controller from Allegro MicroSystem. The electronic circuit
allows controlling the steeper motor in different work regimes: full
step, half step, quarter step and sixteenth step. This advantage is
useful if the stepper motor has the high step angle from 7.5 /C176to
15/C176or the stepper module is used to teach the students or the engi-
neers to control the stepper motor.
The wind module is built to measure the P-Vcharacteristic, the
voltage and the current generated by the wind turbine. This mod-
ule works as an electronic load, based on MOSFET. The wind mod-
ule has the same pin connection as the I-Vcharacteristic module in
order to be easily replaced.
The thermal collector module is designed to measure the inlet
and outlet temperature from the flat plate solar thermal collector
and the flow of water through the system. The thermal collector
module has the same pin connection as the I-Vcharacteristic mod-
ule so that the replacement would be made easily.
The main board assures the connection between these modules
and the NI ELVIS platform. This board offers the possibilities to
connect to other facilities for the development of other applica-
tions by users. One example of these possibilities is to use the NI
ELVIS Impedance Analyzer to measure the solar cell impedance
at different frequencies. The impedance spectroscopy technique
can be used in order to analyze the AC behavior of the solar cell.
The frequency can be varied from low levels (1 Hz) to high levels
(35 kHz). In this case the I-VM module must be removed and then
the solar cell terminal has to be connected to the two terminals of
the impedance analyzer.
The transition from the RELab to the mini-RELab is realized to
replace the main connection board. The new board keeps most ofthe facilities of the RELab main board connection. The modules
developed for RELab can be used in the same manner for the
mini-RELab, see Fig. 4 . The stepper control module is the only mod-
ule which is not used. This is due to the fact that this system is ded-
icated to measurements in natural conditions without a power
supply. In this case the variation of the angle between the incident
light and the photovoltaic cell is performed manually. The compar-
ison between the I-Vcharacteristics measured when the photo-
voltaic cell is illuminated with halogen bulbs and natural
sunlight can be made using the mini-RELab, and thus a study can
be made on the influence of the spectrum on the photovoltaic cells
response.
4. Software
The software is realized in the graphical language program
LabVIEW because it can be successfully used for the simple or
complex applications in the field of virtual instrumentation and
laboratory measurements [32]. It is built as an open project
because it is useful for users to add new components if they need
to develop other experiments. The application interface for open-
ing the lab works has the same structure with the Instrument
Launcher of the NI ELVIS platform. This is made because the NI
ELVIS users are familiarized with this type of interface and the
structure is very user friendly.
The RELab software has three components presented in three
tabs, see Fig. 6 :
/C15the theory tab – in this tab the theoretical aspects are presented
in a comprehensive manner.
/C15the control and measurement tab – in this tab the user can
make the settings, see the parameters on the numerical indica-tors or on the graphs, save the data or import the old data to be
processed and create the report at the end of the lab, see Fig. 6 a.
/C15the calculate tab – in this tab the users can calculate the impor-
tant parameters of the photovoltaic cell, of the wind turbine or
of the solar thermal collector using different methods, see
Fig. 6 b.
5. Discussion
The RELab system allows making twenty-one lab experiments
to characterize the different photovoltaic cells as monocrystalline
Fig. 4. Mini RELab.P.A. Cotfas, D.T. Cotfas / Measurement 93 (2016) 94–101 97

silicon, polycrystalline silicon, amorphous silicon, gallium
arsenide, organic and multijunction solar cell, six lab experiments
to characterize the small wind turbine and five lab experiments to
characterize the small flat plate solar thermal collector [23].
The I-Vcharacteristics measured for a monocrystalline Si solar
cell with the RELab system and with Zahner device were comparedfor validation. The matching between the I-Vcharacteristics, mea-
sured with the RELab and with Zahner at 1000 W/m2irradiance
and 25 /C176C temperature, is shown in Fig. 7 . The zoom of the I-Vchar-
acteristic around the maximum power point is also presented.
Because the matching is very good, the RELab system can be used
to measure and determine the parameters of solar cells.
Fig. 5. The I–Vcharacteristic simulation in Multisim (a) the simulation of the I-Vcharacteristic measuring circuit; (b) the electronic circuits; (c) I-Vcharacteristic.98 P.A. Cotfas, D.T. Cotfas / Measurement 93 (2016) 94–101

The possibility to determine the important parameters in real
time is one of the advantages of the RELab system in comparison
with the other characterization systems of solar cells, wind turbine
or solar collector.
The most of the 34 methods which allow the determining the dc
parameters of solar cells [4]are implemented in the software of the
RELab system and the methods can be compared. The users can
discover the advantages and the disadvantages of the methods.
For example, if the ideality factor of diode, m, is considered equal
with 1, which is the theoretical value in some of the methods, by
using the Cotfas method [4]the users can study the influence of
mon the accuracy of the series and shunt resistance of the solar
cell, Fig. 8 . The series resistance can be calculated using the follow-
ing equation:
Rs¼DVk
Imaxð1Ț
where kcan be ideal or meas, Imaxis the current for the maximum
power point, the DVideal represents the voltage difference at Imax
between the real I-Vcharacteristic and ideal characteristic for
m= 1 and the DVmeas represents the voltage difference at Imax
between the real I-Vcharacteristic and characteristic for real m.
The conclusion using this analysis is that the methods which use
m= 1 have to be carefully used.The RELab system has a module which allows studying the pho-
tovoltaic cells (two) when they are connected in series or in paral-
lel, using two jumpers, Fig. 9 . The concept allows measuring the I-V
characteristic for one photovoltaic cell or two in series or two in
parallel, through the positioning the jumpers. This facility can be
used to study what happens when one photovoltaic cell is partially
or completely shaded, also.
Another facility offered by RELab system is the possibility to
study what happens with the parameters of the photovoltaic cells
under the ageing process. The temperature of the photovoltaic cell
can be maintained at the desired value (ex. 85 /C176C) by using the PID
algorithm for accelerating ageing.
The determination of the ac parameters of solar cells and pho-
tovoltaic panels has become very important in previous years.
The ac parameters of the solar cell can be determined by using
the Impedance Spectroscopy application. The comparison between
Nyquist diagrams obtained with RELab and Zahner are presented
inFig. 10 . A very good matching can be observed. The RELab sys-
tem, which is much cheaper than other devices used for impedance
spectroscopy, can be successfully used to determine the ac param-
eter of the solar cells.
The RELab system allows to study the variation of the power
and maximum power generated by the small wind turbinefunction of the wind speed which is measured with hot wire
Fig. 6. The RELab software interface (a) the measuring interface; (b) the calculating interface.
Fig. 7. The I-Vcharacteristics measured with RELab and Zahner.P.A. Cotfas, D.T. Cotfas / Measurement 93 (2016) 94–101 99

anemometer connected to PC, the number of blades (between 1
and 6), the inclination angle of the blades, the shape of the blades
and the blades material [23]. The experimental labs for small solar
collectors are: the determination of the solar collector efficiency at
1000 W/m2irradiance (the collector is positioned in the same place
where the photovoltaic cell was previously mounted, and the irra-
diance is known through the voltage applied on the bulb); the vari-
ation of the solar collector efficiency function of the irradiance
levels, the inclination angle, the flow rate and determination of
the glass or plastic transmission coefficient: the collector efficiency
is determined with glass (plastic) and without glass (plastic); the
ratio between the two efficiencies gives the value of the transmis-
sion coefficient [23]. The system can be relatively easily adapted to
study a small real solar collector. In this case, the collector illumi-
nation must be achieved by another source, which can be the nat-
ural sunlight.
The software for the RELab system is very flexible. The same
version of the software can be used for both systems RELab and
mini-RELab. The software recognizes automatically the acquisition
system used, NI ELVIS or myDAQ. If the myDAQ board is recog-
nized, it appears in the ‘‘Device Name In” control and then, by
using the ‘‘Manual” control, the mini-RELab system can be used
to study the photovoltaic cell and the small wind turbine in natural
environment.
By using the Web Publishing Tool from LabVIEW facilities, the
RELab system can be used as a remote system. This is another
important feature of the system, because it allows the system to
be used by users from outside the lab, via internet, thus covering
the new concept of didactic labs domain [33].
6. Conclusion
The RELab and mini-RELab systems are original, compact and
relatively cheap, built based on a graphical system design concept
using the following steps: Design, Prototyping and Deployment.
These systems are successfully used in some universities from
Denmark [34], Romania, Greece and India.
By using the RELab or mini-RELab, the users can acquire theo-
retical knowledge, measure and characterize the different types
ΔV ideal
ΔV meas
Fig. 8. The influence of the ideality factor on the I-Vcharacteristic of the monocrystalline Si solar cell.
jumpers
Fig. 9. Parallel or series photovoltaic cells module.
0 200 400 600 800-400-350-300-250-200-150-100-500
Zahne r
RELabXs[ohm]
Rs[ohm]
Fig. 10. The Nyquist diagrams comparison.100 P.A. Cotfas, D.T. Cotfas / Measurement 93 (2016) 94–101

of the photovoltaic cells, the small wind turbines and the small
solar collectors and their behavior in different environment
conditions.
Many lab experiments are developed allowing the study of the
parameters of the three components of renewable energy by com-
bining the theory with the practical applications. The lab experi-
ments are for both educational and research levels.
The accessibility of the RELab system is huge due to the fact that
the system can be used in both situations – hands-on and remote.
The users’ creativity and initiative are developed because the
system allows adding new applications created by them, through
comparing the theoretical results with the experimental ones
and respectively by the new methods and technologies applied
to the system realization.
The study of the energy storage is very important for the renew-
able energy. The energy storage part of the RELab system will be
developed in the future. This part consists of studying the energy
storage in the battery, the super cap and the hydro storage system.
Acknowledgement
This work was supported by a grant of the Romanian National
Authority for Scientific Research and Innovation, CNCS – UEFISCDI,
project number PN-II-RU-TE-2014-4-1083.
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