Smart Solar Panels: In-situ monitoring and testing of photovoltaic panels based
on wireless sensor networks
K. Antonakoglou1, K. Andriotis1, P. Papageorgas1, D. Piromalis2, and D.
Tseles2
1.
Department of Electronics, Technological and Educational Institute of
Piraeus
2.
Department of Automation, Technological and Educational Institute of
Piraeus
Corresponding author: P. Papageorgas
Abstract
The paper presents the design methodology for an in-situ solar panel
monitoring and characterisation system based on wireless sensor networks and
web technologies. The system presented provides in-situ performance data for
each solar panel of a solar panel array and allows through a web-based
application the optimization of solar power production. The proposed
characterisation platform is based on high brightness LEDs embedded in each
solar panel in specific geometries and wavelengths for the irradiance of the
panel during idle operational periods. A low-power zigbee wireless sensor
node is also embedded in each panel together with the appropriate electronics
for carrying out the electric isolation from other cells of the PV array and the
measurement procedure, as well as, the wireless communication with a
coordinator node. Current and voltage performance parameters are measured
for each PV panel and are transmitted to the coordinator node. The wireless
sensor coordinator supports both the wireless communication with the nodes
of the PV array and the remote control and reporting to a central station with a
client-server application. The proposed methodology can be an important step
for an Internet of Smart Solar Cells, providing remote monitoring and control
based on low-cost measurement and communication technologies.
Keywords: Wireless sensors networks, Zigbee, Solar cell testing, Photovoltaic
systems, Data acquisition, Internet of things, Smart grid, AMI.
1. Introduction
The electric power grid in most countries is in a large extent old, centralized and
based on non-renewable energy sources as coal. Today, regulatory requirements are
calling for sharp reductions in carbon dioxide and greenhouse gas emissions footprint
from the energy sources utilized, therefore the widespread use of renewable energy
sources is mandatory [1,2].
Grid-tied photovoltaic (PV) Distributed Power Generation Systems (DPGS),
especially roof and ground-mounted, are today becoming very important. In addition
current technology trends toward the smart grid vision as the Autonomous Metering
Infrastructure (AMI), fuses computation, communication and sensing for providing
decentralized energy management. A smart grid would employ real-time, two-way
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communication technologies to allow users to connect directly with power suppliers
[1]. The introduction of wireless sensor technologies in DPGS can realise the vision
of “Internet of Things” projected to distributed electric power management, the
“Internet of DPGS”.
Solar energy is the most abundant renewable energy source and today there is a large
interest for its use globally. The cost reduction of solar cells together with
governmental policies will accelerate the use of grid-tied photovoltaic systems in
commercial, residential and industrial applications. The solar cell manufacturing
technology is continuously improving, however the use of optimization techniques for
solar power production, monitoring and management is at its infancy.
The efficiency of a PV panel is seriously affected by sunlight irradiance blocking
obstacles, dirt accumulated in the solar panel protection glass as well as field-aged
degradation [3-5]. Aging effects of PV cells affects the I-V characteristics, so an insitu measurement system of PV performance characteristic parameters can provide
valuable information for optimized power generation. The PV panels are normally
tested in the production factory once and in standard conditions, with the cost of
dismounting from an installation fixture and testing them to be always prohibitive.
Consequently, each solar panel is usually left unattended during its production life,
thus resulting to sub-optimal electric power generation with considerable cost. On the
other hand, the convergence of informatics and communications with ongoing
advances in microcontrollers and CMOS RF-transceivers are the enabling
technologies for the use of low cost wireless sensor networks for monitoring the PV
panels in the field.
In this paper a design methodology is proposed, that provides the in-situ
characterization of individual PV cells based on wireless sensor networks and the
transfer of data to remote computers with web-based technology. The irradiance
needed for the characterization utilizes an array of high brightness LEDs. The
methodology proposed is more suitable to solar topologies that use individual
inverters connected to each PV panel and to the grid Fig. 1 [6]. This architecture is an
alternative to the string (serial) solar panel connection to a large inverter, with certain
advantages in the cases of shadowing or performance deterioration (due to aging, dirt
etc.) of single solar panels.
Fig. 1 Architecture of Micro Inverters and smart PV panels [6]
2
In the following sections an overview of the sub-system modules developed is given.
First the embedded wireless sensor node and electronic load are presented, followed
by a description of the wireless coordinator and the software platform utilized for
web-publishing and control. Finally, preliminary results and concluding remarks are
presented.
2. System concept for in-situ PV-cell monitoring
In Fig. 2 the overall system concept is given. A three-tire architecture has been
followed for the monitoring and characterisation system, with the first level devoted
to the PV solar panel, the second level to the PV cell array and the third level one to
the communication with the remote monitoring and control computers.
The main components of the first level embedded in each solar cell comprises a
microcontroller for control and measuring together with the associated electronics and
a zigbee transceiver for wireless communication with the PV array coordinator. The
embedded microcontroller realises a miniature data acquisition system for PV panel
characterisation [7-9]. In the second level a wireless sensor network coordinator
provides gateway services through an Internet connection with an ADSL router to
each individual solar panel. Data from the PV panels are transferred to remote clients
using the Pachube [10] web publishing technology together with the capability of
sending control data to the PV panel embedded microcontroller for monitoring and
“triggering” of the characterization procedure.
LEDs bar
PV panel
LEDs
command
K1
WSN PV
Array
ARRAYAR
Zigbee
Coordinator
Zigbee
Node
Relays,
e-Load
K2
RAY
VOC,ISC
Internet,
Pachube
Remote
computer
Fig. 2 System’s Functional Block Diagram
3
In Fig. 3 the flow of the control and monitoring data for the characterization
procedure of the PV panel is presented. The characterization procedure of the PV cell
uses an array of high-brightness LEDs for the PV panel irradiance. A set of two relays,
K1 and K2, is utilized for the characterization procedure. When K1 is activated
(closed) the PV cell is electrically isolated from the other cells of the PV array, the
LED array is supplied by power from a central power supply unit and the
characterization procedure can be started.
start
LEDs = OFF
K1 =closed
K2 =open
openDISABLE
Char.
command
K1 contact is closed.
PV panel is not
isolated
K2 contact is open.
LOAD is disconnected
from PV
N
Y
K1 = open
K1 contact is open.
PV panel is isolated from
its circuit
TIME DELAY
= t1
LEDs = ON
VOC MEASUREMENT
VVopen
Single Ended,
10bits
2 Bytes
K2 = closed
ISC MEASUREMENT
Vload
Single Ended,
10bits
2 Bytes
SEND VOC
and ISC
Fig. 3: Flow of control data for the PV panel characterization procedure
4
The LED array is activated from the microcontroller through a digital output. This
output is controlled from an embedded timer and generates pulse width modulation
waveforms for the digital control of the LED array lighting level with a dimming
technique.
As is depicted in Fig. 3, the K2 relay controls the measurement of voltage (when is
activated (closed)) or current (when is deactivated (open)). The measurement voltage
is digitized by the 10-bit ADC which is embedded in the microcontroller used. The
characterization parameter measurements are then transferred from the wireless node
which is embedded in the PV panel to the PV array zigbee coordinator.
3. In-situ embedded data-acquisition system for PV testing based on wireless
sensor networks
For PV cell testing a miniature data acquisition platform has been developed [7-9 is
necessary to be used. The selection criteria were a small form-factor data acquisition
with zigbee-based wireless transceiver embedded. The available market choices are
limited and are mainly in a development tool form. Finally, the OEM-type and very
small-factor ZL01-Node [16] was selected to built the proposed PV control system.
This board is based on a reconfigurable System On Chip (SOC) microcontroller and
an embedded wireless sensor network RF modem. The Wireless Data Acquisition
(WDAQ) platform developed is embedded in the PV cell and controls the automatic
in-situ collection of important characterization parameters that affects its electric
power generation performance without the need of panel dismounting. Moreover with
the utilization of WSN and web technologies, the remote monitoring of the PV array
is possible.
For the characterization of the panel an electronic load has been embedded in each PV
panel provide the electronic load for panel characterization, the LED lighting control
and the relays necessary for panel electric isolation and voltage-current measurements.
In addition a microcontroller controls the PV panel operation and allows the wireless
communication with the PV array coordinator. In the following paragraphs details
about these modules are given.
3.1 Embedded control microcontroller and Zigbee Node
The Embedded control microcontroller of the ZL01-Node is based on a
microcontroller from CYPRESS. This microcontroller provides via a 6-hole screw
terminal block a PWM digital output for LED lighting control, two digital ports for
the control of the relays for electric isolation of the panel, the current-voltage
measurement analog input, and a PWM digital output for the control of the electronic
load The ZL01-Node is shown in front and rear views in Fig. 4. The firmware
necessary for the PSoC microcontroller operation has been developed in C-language
within the PSoC Designer firmware development platform from CYPRESS. The
Zigbee Node built is based on a zigbee RF modem available from ATMEL. It
provides the full Zigbee PRO certified stack for wireless communication at a
maximum 3dBm RF output power that is adequate for 20-30 meters range of wireless
communication with the Zigbee coordinator. For wireless communication in
automation and measurements, a number of standards have been evolved as the
TSMP, Wireless HART and ISA100. On the other hand, wireless standards as zigbee
5
and 6LoWPAN prevail in WSN applications. The Zigbee technology is supported by
most of the larger IC manufacturing companies and is in widespread use today for a
large variety of applications. It relies on the IEEE 802.15.4 standard for the physical
and MAC layers and it is considered as a mature choice for WSNs. We have selected
the zigbee standard for the wireless sort range communication between the embedded
PV wireless node and the array coordinator, because of its low cost, maturity and its
widespread use in WSNs.
ZL01-Node
front view
rear view
Fig. 4: ZL01-Node – The embedded data-acquisition
wireless node for PV panel characterisation
3.2 Electronic load and electric isolation electronics
The performance characterization of the PV panel is normally performed through I-V
curve measurement in standard conditions (Irradiance of 1000 W/m2 @ 25ºC). The
methodologies followed for fast field testing of PV panels are normally based on one
of three techniques [8]. In the first technique a variable resistance is used, the second
uses a capacitor charging methodology while the third one relies on an electronic load
digitally controlled.
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Fig. 5 Electric isolation relay for PV performance monitoring
We have selected the third technique for panel characterization with the development
of an electronic load. In the realization described the performance parameters Isc Voc
are measured at specific irradiance conditions provided by an array of HighBrightness LEDs. The I-V characteristic of a PV panel depends on Temperature and
irradiance so sensors are used for their measurement. Temperature and humidity
measurements of the PV panel are based on a smart sensor (SHT11) manufactured by
the Sensirion Co. and under the control of the PV embedded microcontroller. A
photodiode is also used for the measurement of the irradiance level.
As already referenced previously, an electromechanical relay is used (named K1 in
Fig. 3), that provides the electric isolation of the PV panel before the characterization
procedure (Fig. 5) [11]. The electronic load developed uses a voltage regulation
technique for measuring the Isc current of the PV panel [7,12]. A second relay is used
(named K2) for selecting the Isc or Voc measurement with the 10-bit ADC which is
embedded in the control microcontroller. When Voc measurement is selected, the
output voltage of the PV panel is attenuated with a voltage divider and directed to the
ADC input. For the Isc measurement the electronic load depicted in Fig. 6 is utilised.
The WDAQ microcontroller provides a reference voltage (VREF) that drives the U1A
comparator. Thus the voltage reference sets through the MOSFET the IDS and thus the
current supplied by the solar panel. This voltage reference is build with a PWM
digital port of the embedded microcontroller and an RC filter. The circuit of Fig. 6
follows a voltage regulation scheme where a shunt resistor R1 is in series with the Q1
IRFP150N MOSFET. The voltage sensed from the R1 is directed to the comparator
and the ADC input of the microcontroller. The current sensed is Isensed=IDS=VREF/R1.
The procedure that is followed starts with a VDS close to VOC and is continuously
reduced in predetermined steps until the sensed voltage from the ADC is
approximately constant. When the above referenced condition is met the sensed
current Isensed is considered as ISC. The ADC resolution determines the minimum
error for the current and voltage measurements.
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Fig. 6: Schematic diagram of the Electronic load used for Isc measurement
4. Open Software /Hardware platform for the web-based remote monitoring
and characterisation of PV panels
The distributed power management of the electric power produced from small
renewable source generators demands the use of new technologies based on the fusion
of communication and computer technologies. A number of commercial solutions
exist for monitoring DPGS [13,14]. These software solutions cannot be easily adapted
to the rapid evolution of wireless sensing standards for measurement and webpublishing technologies. On the other hand, open software solutions can be easily
modified and adapted to the technology trends enabling further research in the
associated scientific fields.
For the aforementioned reasons, we have selected Pachube platform [10] for the webpublishing and management from a remote computer. Pachube is an open source
platform, suitable for the internet of things envisagement, for transmitting and
receiving real-time sensor, energy and environment data from objects, devices and
buildings, through the internet using “feeds” (data format with frequently updated
content). Using the Pachube API (Application Programming Interface) one can input
data to monitor and visualize them in graphs simply by updating a feed, or use a feed's
output to control remote devices and environments with the use of “triggers:. The
Pachube API uses the REST architecture, that is, it supports HTTP requests like GET,
POST and PUT, making the client interface simple and bandwidth efficient. The data
formats supported for client requests, are the Extended Environments Markup
Language (EEML), JSON, comma-separated values (CSV), RSS and Atom.
There are two ways of sending data as input to Pachube, the automatic and the manual
one. With the automatic mode, servicing requests from Pachube are allowed every 15
minutes or whenever another client requests them, with the use of Arduino or the
EEML library, as the Processing language. In the later case, Pachube reads the EEML
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document that includes the sensor data and meta-data. In the manual mode the data
are uploaded from the client to Pachube. The default rate limit for calls (access or post
new data) to Pachube's API is 50 requests every 3 minutes. Pachube can return data to
the client when previously set conditions are met, all of which are stated in a “trigger”.
Triggers send HTTP POST requests to a URL chosen from the client through the
Pachube API or created from the web interface. All the needed information is located
inside the request's body. To extract the desired data, a script has to deal with this
request. The minimum interval between sending out two different notifications is 300
seconds.
Fig. 7 The pachube “ecosystem” [10]
For the wireless coordinator of the PV panel array and communication with the
remote computer the Arduino platform was selected [15]. Arduino is an open-source
electronics prototyping platform that utilizes the Atmel Atmega328 microcontroller
for the Arduino Duemilanove platform selected. The microcontroller is programmed
by uploading code using the Arduino environment. The Arduino's features are
extended by simply connecting its “shields” (stacked boards) on top of it. There is a
variety of shields available enabling the easy adaptation of the Arduino platform to
the needs of the specific application. An Xbee Shield is used in the described
application that allows the deployment of a Wireless sensor network with the Arduino
board to play the role of the host node that communicates wirelessly over the Zigbee
protocol with the wireless sensor nodes that are embedded in the PV cells. In addition
an Ethernet Shield is used that allows the board to connect to the internet and perform
gateway services.
Arduino can communicate with Pachube over Ethernet or USB connection utilizing
existing open software libraries. In the platform utilised the Ethernet connection was
established using the Etherned Shield, that incorporates the WIZnet WS100 module.
With this Shield, Arduino communicates directly (through a router) with Pachube as a
9
Fig. 8 Photo of the Arduino-based PV array coordinator
web-server using the for the software developed. The Pachube API supports also
DHCP, to establish a network connection without the need of hard-coding the IP
address. In the current implementation the Arduino platform is used as a Web-server
for receiving calibration commands and triggers from a remote client and transmitting
performance measurements at predetermined intervals.
Current alternatives to this paper’s suggested methodology are closed source solutions
and products for monitoring PV systems. Enecsys’ [13] micro-inverters are installed
separately in each solar module and send data wirelessly to the internet via a gateway
providing real-time information to the user, such as error notifications, but without
further interaction (like error handling with use of triggers). Sunny’s technology [14]
for PV monitoring, the Sunny WebBox, is a device that can log data and interface up
to 50 Sunny Boy inverters with free access to the Sunny Portal. The communication
with the inverters is via cable (RS485 or Ethernet).
The free and open source alternative of “Pachube” in combination with Arduino
resulted to a low-cost platform that supports two-way wireless communication with
the PV panels and web communication with remote computers. The number of panels
supported is limitless, while the use of geographical data provides a natural way of
managing the distributed power generation sources.
5. Preliminary PV characterisation measurements
Preliminary measurements of performance parameters of a PV panel are presented in
Fig. 9 and 10. The measurements were extracted in the laboratory for two different
10
2
W
1,8
1,6
57400 lux
1,4
27400 lux
1,2
1
0,8
0,6
0,4
0,2
0
0
2
4
6
8
10
12
14
16
18
20
V
Fig. 9 PV panel P-V measurements for different irradiance conditions
mA
160
140
57400 lux
120
100
80
60
40
20
0
0
2
4
6
8
10
12
14
16
18
20
V
Fig. 10 PV panel I-V measurements
illumination conditions provided. The two different curves reveal an expected
behaviour for different irradiations. A unique electronic identity was reserved for each
solar panel which was added with the performance measurements for identification of
the panel in the PV panel array.
6. Conclusions and Future Work
The methodology demonstrated in this article fuses computation and communication
technologies for real-time monitoring of PV panel operational status. The use of
wireless sensor network technology provides low complexity and cost communication
with a coordinator, while new web-based publishing technologies simplify the design,
11
maintenance and operation of large networks of distributed generators based on PV
arrays.
As a future work an infrared illumination setup will be tested thoroughly. A number
of illumination geometries will be tested for dirt detection in order to recognize with
accuracy these situations and provide valuable data for the supporting maintenance
services. In addition advanced management technologies will be realized for real-time
control of the PV panel operation. In the future each panel in a PV array should have
embedded a platform similar to the proposed one, for characterisation and two-way
communication with remote computers. Panels with efficiency changes due to aging
or other effects can be identified through in-situ measurements of performance
characteristics thus enabling the vision of “Internet of DPGS” for optimum use of
renewable energy sources.
Acknowledgements
This research has been supported through the Operational Program "Education and
Lifelong Learning" and is co-financed by the European Union (European Social
Fund) and Greek national funds.
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