In this post I will write from one of my articles published in the Annual Journal of Institute of Engineers of Pakistan - Saudi Arabian Chapter (IEP-SAC) in 2016. This post is regarding some bad practices when installing a solar PV systems or in general any electrical system. I visited a farm in Abqaiq city of Eastren Saudi Arabia.
Introduction
The use of solar energy is growing extensively in the
Kingdom of Saudi Arabia (KSA). Being an oil rich country the dependency of
energy generation on oil is very high. Meanwhile oil also forms the economic
backbone of KSA. With the fall of oil prices the government is encouraging the
use of renewable energy sources like Solar, Wind and Waste to Energy. With the
increasing use of solar photovoltaics (PV) for domestic use the installers tend
to compromise the quality and standards for photovoltaic system installation. This
report is about the mistakes encountered while installing solar power system
and how to avoid them. The best practices for solar power system are published
in the National Electrical Code (NEC) article 690 [1] which ensure highest
performance and safety and can be referred for further details. Another entity
that publishes and enforces the standards for PV installation is the North
American Board of Certified Energy Practitioners (NABCEP) [2].
A case study is presented for a standalone solar power
system installed in a farmhouse in eastern province of Saudi Arabia. The farm
house is situated off the main highway from Riyadh to Dammam at a latitude of
26.184 degrees and longitude 49.49 degree east. The area lies in the
subtropical region and is part of the larger Arabian dessert. To the south of
this site is the Rab-Al-Khali Dessert (the Empty Quarter).
System Attributes
The solar power system under study is a standalone system
with 24 hour battery backup. It is installed on a roof of a small home in a
farm and is used to power up AC electrical load like television, refrigerator
and lights. The installed peak capacity of the system is 1.3 kW. Figure 1
shows the PV array installed on the roof of the home. The system costs
including the installation was about SAR 20,000.
The system is divided into four distinct parts.
1.
1.3 kW Solar Array
2.
450 AH Battery Bank
3.
3000 Watt Inverter System
4.
Charge Controllers
System Design
Physical Layout
The solar power
system has five roof mounted solar panels. The structure of the roof is shown
in Figure 2.
The array is aligned with the tilt of the roof which is 5
degrees and is pointed towards south i.e Array Azimuth angle = 180 degrees.
Electrical Layout
The PV array consists of 5 solar panels. Each solar panel
has a rated power of 260 Watts. The array is divided into two sub-arrays which are
connected to two charge controllers that feed both the solar inverter and
battery bank. Figure 3
shows the block diagram of the system electrical layout.
Figure 3: Block diagram of 1.48 kW off-grid
solar power system
All the major components of this plant are described below.
Solar Panels
The solar panels are from Yingli Solar. The characteristics
of this panel are given in Table 1.
Table
1: Characteristics of Yingli Solar
Panel
Parameter
|
Value
|
|
Rated Power
|
260 Watts
|
|
Open Circuit Voltage
|
38.9 Volts
|
|
Short Circuit Current
|
8.98 Amperes
|
|
Rated Voltage
|
30.9 Volts
|
|
Rated Current
|
8.41 Amperes
|
Charge Controllers
The charge controllers connected to the system are from the
manufacturer Copex. The electrical/thermal characteristics of this charge
controller are given in Table 2.
Table
2: Charge Controller Characteristics
Load Current
|
30 Amperes
|
System Voltage
|
12 Vdc or 24Vdc
|
Max Voltage of Solar Collector
|
47Vdc
|
Protection Class
|
IP32
|
Ambient Temperature Allowed
|
-25 to +50 degree C
|
Thermal Protection
|
85 degree C
|
The purpose of charge controller is to regulate the voltage
of the DC Bus. The DC bus in this case is operating at a voltage of 24 volts.
24 volt bus was selected because the inverter in the system, operates at 24
volts. The charge controller keeps the DC bus at a voltage slightly higher than
the DC Bus voltage i.e. (28 volts approx.). This is to make sure that during
the day, the current flows towards the batteries in order to charge them. The
connections of the charge controller are shown in Figure 3.
Figure 4:
Copex Solar Charge Controller (http://copexsolar.com/scd-series/scd-1010/ )
Battery Bank
The battery bank is connected to the system to provide a
backup power for 24 hours. The batteries used in this system are Automobile
duty – Lead Acid batteries from National Battery Company Saudi Arabia. Figure 5
shows one of the batteries connected in the battery bank.
Figure 5: Spark Automotive Battery from
National Batteries Company - Saudi Arabia
The batteries are rated at 12 volts and 150 Amp-hours each.
The battery bank is formed by making a series parallel
connection of six batteries. To attain a DC Bus voltage of 24 volts, three sets
of two series batteries are connected in parallel. Figure 6
shows the battery bank. The bank is placed inside a box with holes to allow air
ventilation. The connection of the
battery bank is shown in Figure 3.
Figure 6: Battery Bank (Dust settled on the
batteries)
Inverter
The inverter used in the system is a general purpose
inverter available in local market. It is a modified sine wave inverter with a
power rating of 3000 watts and DC input voltage of 24 volts. The characteristic
data of this inverter is not available hence it is not mentioned in this
report. Figure 7
shows the power inverter.
Figure 7: 3000 Watt Power Inverter
This inverter produces an output voltage of 330 volts AC.
Inverter System Layout
The inverter is connected with the DC bus formed by the two
charge controllers. This system is shown in Figure 8.
The purpose of inverter system is to collect DC power at maximum rate from
solar array and provide AC power at constant voltage to the load. The maximum
power from the solar panels is ensured by the charge controllers whereas the
inverter provides a constant AC voltage to the load side at 330 volts.
Load on the system
The system is capable to power a load of 1.25 kW peak theoretically.
But at the time of visit following loads were being powered up by the system
Figure 9: All loads connected to the PV system
Table 3: Load Characteristics
No.
|
Load Type
|
Power Rating
|
Quantity
|
1
|
Television
|
70 Watts
|
1
|
2
|
Refrigerator
|
100 Watts
|
1
|
3
|
Energy Saving Lamp – type 1
|
36 Watts
|
3
|
4
|
Energy Saving Lamp – type 2
|
20 Watts
|
1
|
5
|
Miscellaneous Load (mobile charger)
|
2 Watts
|
1
|
Total load
|
300 Watts
|
Observations and Recommendations for Improving the Design – The Don’ts.
Wire Routing:
The routing of wires
must be proper to avoid confusion during inspection. The wires in the system
were improperly routed which caused unwanted stretching at the joints. Secondly
the wires are a potential tripping hazard for person performing cleaning or any
other activity on the roof. Therefore they must be properly secured and routed
along the roof.
Figure 10: Improper routing of wires form PV
Panels
It is recommended to re-route the wires and use proper wire
conduits to secure all the loose wires in place.
Figure 11: Improper wiring of battery bank –
the battery terminals are not covered.
The wires inside the battery bank must be secured to the
bank housing. Loose wires are never preferred especially when the battery
cleaning/maintenance is performed. Loose wire can be easily tangled.
Another observation on the battery bank is the covering of
battery terminals. The terminals must be properly covered to avoid any metal
tool / damp cloth etc. to cause a short
circuit.
Figure 12: Improper wiring of inverter and
charge controller
Figure 10,Figure 11
and Figure 12
show the improper wirings of the system. The wires are not secured in proper
place and are jumbled up. To tackle this issue, the wires must always be
properly attached to the housings and wire conduits must be used to make groups
of wires.
Wire Connections:
The series parallel connections in the PV system have been
done using simple insulation tape. Even-tough the modules are originally
equipped with proper MC-4 connectors. This practice is to be avoided and proper
MC-4 connectors are to be used for making series-parallel connections.
Figure 13:
Insulation tape used for making connections
The problem with insulation tape is that it can easy get
torn apart after long term exposure to changing weather conditions, water and
dust. This may lead to exposed conductors that may get short circuited during
cleaning or accidental contact with metal mounting structure.
Wiring/Instrument Labels:
There are no wiring labels on any of the wires. This can be
a serious problem if any modification/upgrade or inspection is required on
site. To overcome this issue it is recommended to use proper water/weatherproof
wiring labels marking all the DC Wires, Strings, Modules, Inverter, Charge
Controller and Batteries inside the bank.
Cleaning of modules is necessary and there are different
hazards involved if proper precautionary measures are not taken. e.g. the
modules are located very near the edge of the roof giving rise to fall hazard.
There must be a support rail near the edge to avoid any accidental fall.
DC Disconnect Switch
In the present design, there is no DC disconnect switch in
the system. The DC disconnect is required to disconnect the solar panels from
the system for reasons that may include
1.
Maintenance of Inverter
System
2.
Emergency Shut-down
Apart from a DC disconnect switch, there is no other way to
stop the power flow from solar panels to the system. The solar panels will
produce power as long as there is light. Therefore in order to carry out any
maintenance activity on the inverter system or the battery bank the solar
panels are to be disconnected otherwise there exists a shock hazard for the
maintenance personnel.
AC circuit Breaker
In the present system design, there is no AC Circuit breaker
to interrupt the system in case of a fault such as a short circuit on load
side. This is regarded as the biggest hazard because in case of a short circuit
there is nothing to stop the current from flowing to the loads except for the
inverter itself with its built-in short-circuit protection. A short circuit can
easily damage the inverter in that case.
Battery Fuse and battery DC disconnect switch
A battery fuse and a DC disconnect switch are required to
protect the battery bank and disconnect it from the DC bus respectively. The purpose
of battery fuse is to protect the battery bank from providing too much current
in case of a short circuit on DC Bus. A short circuit on DC bus will case a
very high current in the orders of hundreds of amperes to flow from the battery
bank. This will cause a high discharge rate on the battery and can render the
battery bank useless if its “state of charge (SOC)” goes below a minimum
threshold.
Copper DC-Bus Bars for Battery Bank
The battery connections are made using wires jumping from
one terminal to other terminal in a haphazard way. The terminal connectors are
pointing inwards and are covering the battery caps hence creating a hindrance
for opening the caps. To avoid this the battery connections are to be made
using copper bus bars.
Wire Color Codes
Proper color codes for the wires must always be used. In the
present design, the wires for making the connections on the PV array are all
grey colored hence making the positive and negative wires indistinguishable.
Moreover this color coding needs to be consistent in the whole system.
AC wires should have different colors than DC Wires.
Placement and Orientation of Inverter
The inverter in the current design is placed in an incorrect
orientation. The ventilation openings of the inverter are pointing upward hence
dust / water can easily penetrate inside the inverter.
Secondly the indicator LEDs on the inverter are all pointing
upward and hence they are not easily observable for an inspector. The
indicators should be clearly visible so as to fulfil their purpose which is to provide
information about the inverter status.
The sockets are also pointing upwards hence they are
vulnerable to water ingress and dust settlement as is seen in Figure 14.
Figure 14: Inverter ventilation ducts,
indicators, sockets and switch.
Cleaning the Modules:
The modules are to be cleaned regularly. The power produced
by the modules is inversely related to the amount of dirt accumulated on the
module surface. It is recommended to clean the modules at-least once per week
or as determined by weather conditions. The modules are to be cleaned using
water with light brushing to avoid scratches. A dry brush must never be used to
remove the dust from the panels. Once per month, the modules can be cleaned
using water with detergent. It must be made sure that all the wires are
properly secured and no conductor is exposed so that there is no short circuit
in case of their contact with water. Figure 15
shows the solar panels before and after the cleaning was performed. The modules
in this case were not cleaned for 6 months hence the battery bank was never
properly charged leading to the degradation of all the batteries.
Figure 15: Dirty Modules (Before cleaning
(Left), after cleaning (Right).
