Study the feasibility of the Rooftop panels on
Schools in the provision of solar energy in Tripoli
Abdalhadi Atia Alateki
Giamal Mashina
School of Engineering and Applied Sciences
Libyan Academy
Tripoli, Libya
alateki.a@gmail.com
The center for solar energy research and studies
Tripoli, Libya
gmashina@yahoo.com
Abstract— There is significant potential for the use of the
solar energy in Libya, which receive abundant amounts of solar
radiation around the year. The schools building of on-grid PV
solar plants can be contribute to the diversification,
independence, ecological and economic sustainability of the
national power supply system.
them, their long axis facing to the southeast and another 40%,
their long axis facing to the southwest.
This paper studied the feasibility of using roofs of government
schools at Tripoli city in the generation of electricity from solar
energy to cover schools load and export excess power to reduce
the loads on the general low voltage electrical network.
SuqAltholatha School was selected to study how to take
advantage of the implementation of the GCPV solar system. In
this work, typical Poly-crystalline modules with capacity of 255
Wp are chosen in the analysis. The horizontal PV system
proposed based on the available roof space was of a 73.4 kW. The
array consists of 288 modules. The PV array is configured in a
way that the string includes 24 modules connected in series with
12 inverters, the estimated specific energy productions is found to
be 1399.09 kWh/kWp per year, the annual performance ratio is
found to be 71.9%, the estimated annual energy production is
found to be 103.05 MWh
Fig 1. Rectangular roof (RR) of SuqAlthlatha School and its orientation
The rest of them (20%), their long axis have completely
southern facing as shown in fig 1. One of rectangular-shaped
roofs (RR) school type is SuqAltholatha School. It is located in
the east of the Tripoli city as shown in Fig 2.
The results showed that it could benefit from the roofs of
schools in the city of Tripoli to produce electricity from solar
energy, thus reducing the load on the public network, especially
in the summer time when the schools are not working.)
Keywords— GCPV, roofs of schools, solar, PV array
I.
INTRODUCTION
The electrical energy is the necessity for the economic
development of Libya and most of the energy produced today
comes from burning fossil fuels, which are limited and caused
pollution. There is significant potential for the use of the
photovoltaic solar energy in city like Tripoli, which receive
plenty, amounts of solar radiation around the year. The roofs of
school buildings in particular could be one of the best options
for the production of electricity from the sun. There are a total
of 380 schools in Tripoli [1], 50% of them can be classified
into two categories based on the building’s roof form, the first
category deals with a rectangular form (RR) and the second
one in the form of (U). There are 110 of (RR) schools, 40% of
Fig 2. SuqAltholatha School
It has been chosen as a case study. Choosing this school (faced
to the NW-SE direction) based on that it produces less energy
compared with the other schools.
II.
SITE CHARACTERISTICS
Tripoli's is the capital and largest city of Libya. It is a
coastal city in the north west of the country. It is located on
latitude 32°52′30″ N. and longitude of 13°11′14″ E. The
climate of this area is typically hot semi-arid climate with long,
hot and extremely dry summers with relatively wet and warm
winters. Climate data for Tripoli city are clearly listed in the
Table 1. [2].
TABLE I.
CLIMATE DATA FOR TRIPOLI
III.
THE AVERAGE YEARLY ENERGY YIELD OF
SUQALTHOLATHA SCHOOL
There are many of different size solar panels available.
Specifications of solar panel are important for sizing the PV
array, these specifications as shown in table II. have been
chosen because of good warranty not high price and not bad
efficiency.
TABLE II.
The daily average solar radiation rate on a horizontal plane of
/d in in coastal region, with an average
about 7.1 (KWh)/
sun duration of more than 3500 hours per year and 5.33 PSH.
The variations of the clearness index values are not far apart,
but in general, as shown in Fig 3. With low values
characterized in winter season and high values in summer time
[3]. The solar data of plant location is assumed to be as of
Tripoli and is adopted from NASA Satellite included in the
database of software Homer.
Fig 3. Tripoli global horizontal radiation
In order to choose the optimum placement of solar modules on
the rooftop of the school, it is important to consider and
determine the free space of the shadows because of the
surrounding obstacles, the structure of the roof, and due to the
self-shadowing of the solar modules themselves during all
hours of the day and throughout the year. The available roof
free area of shadows for SuqAltholatha School and the number
of solar modules can be installed on its roof is estimated using
the Skechup program. It is found that a 464
free roof top
area and the required numbers of modules are obtained using
the module area as follows:
THE MODULE DATA
The average yearly energy yield can be determined by using
the following formula [4]:
=
*
*
*
*
*
= rated output power of the array under standard
test conditions. The factor
including derating due to
manufacturers output tolerance (
, derating due to
dirt (
), and derating due to Temperature
). In SuqAltholata School, there are 288
(
modules and its PSH equal 5.33, the actual DC energy output
from the solar array will be equal the derated output power
from all modules multiplying the PSH. The DC energy will be
reduced again due to the losses (3%) in the cable connecting to
the inverter. By assuming that the inverter efficiency (
) is
95%, the DC energy will be further reduced due to conversion
of DC to AC. The AC energy will be reduced through the
cable connecting to the grid. By assuming that the AC cables
losses (
) is 1%. The AC energy delivered to the grid
will be:
=255*0.873*0.95*0.95*288*5.33*0.97*0.95*0.99
=281.35KWh.
The energy yield of the solar arrays over a typical year for
SuqAltholatha School will be:
= 365 * 281.35 =102693.9 kWh/y
The specific energy yielded will be equal
SY=
= 102693/ 73.4= 1399.09 kWh/Wp-y
The ideal energy can be calculated by using the following
formula:
=
x
Where
The sunny time, 6 hours during the normal day, start from
10:00 am to 4:00 pm. The possible plant capacity is 73.4 KWp.
= yearly average daily irradiation, in KWh/
for the
specified tilt angle. Therefore the yearly PSH would be:
=5.33*x 365= 1945.45 h/y.
The ideal energy from the system per year would be:
=73.4x 1945.45 = 142796.03 KWh/y.
summer holiday period (June, July and August) is at the lowest
levels. At this time, most of the electrical energy produced by
The performance ratio is a reflection of the system losses.
PR=
=102693/142796.03 = 0.719= 71.9%
Based on: manufacturer, dirt, DC and temperature losses
for Tripoli, the inverter of GCPV system is chosen to be large
enough to handle the total amount of Watts at one time, it is
rated at 84 KW [6]. When the temperature is high as in Tripoli
during summer time, it is important that the maximum power
(MPP) and voltage at Maximum Power ( ) of the array is not
falling below the minimum operating voltage of the inverter.
The maximum effective cell temperature will be 73°C.
According to the module selected, the
voltage would be
= 48 x - 0.431=-20.69V
The
Fig 4. The scheme in the Homer tool (a) without and (b) with battery
PV system will be transferred to the grid, which is support the
local electricity network at its annually rush time. In addition,
input Values as shown in Fig 5. , tell Homer that Load reach to
63.75KW.
at 73°C will be
= 30.8 -20.69 = 10.11 V.
If the maximum voltage drops in the DC cables of 3% then
the effective minimum MPP voltage input at the inverter for
each module in PV system will be:
=0.97 * 10.11 = 9.81 V
If the minimum voltage window for an inverter is 180V and
by taking in the account, safety margin of 10%, then the
minimum inverter voltage should be equal 198V.The minimum
number of modules in a string is
=198 / 9.81= 20.18 ≈ 21modules.
During the winter season, the temperature could reach 0°C
with the open circuit voltage ( ) of 38.19 V and a voltage coefficient of (-0.431V /°C). Solar panel' specifications).
Therefore, the
would be increased by
= (0-25) * (-0.431) = 10.78 V
The
at 0°C would be =30.8+10.775= 41.58V
If the maximum voltage allowed by the inverter is 1000V.
The maximum number of modules connected in series in each
string will be:
=1000 / 41.58 = 24.05 ≈ 24modules.
Then the number of inverters will be 12 inverters of size 7 KW.
IV.
IMPLEMENTATION OF THE HOMER MODEL
The grid-connected system was modeled using Homer
software as shown in Fig 4.a. A GCPV system Composed of
73.4 kW of PV and 84 kW inverter, grid utility and primary
load of school. On the schematic, it is could be noted that it is
possible to add a backup battery bank for critical loads in case
of off utility as shown in Fig. 4.b, the arrow connects PV
system to the DC bus and shows the direction of the energy
flow to the inverter. The load profile and the roof area is
essential for the designing process, the average load of typical
school in Tripoli has been inputted as AC load in the load
profile table. Homer showed that the school load in the
Fig 5. Load inputs
In the fact it is impossible to operate all loads together from 8:
am to 4: pm for school but there is a load factor of about 13.2%
of the Load does not operate during all this period of time with
annual average of energy consumption for this school was 363
kWh/d.
In costs table horizontal PV system size of 73.4 KW with
lifetime of 25 years, capital cost of PV panels was 38,168 $ ,
Operation & Maintenance 300 $per year , 77 % derating
factor , 15.5 % efficiency and it can operate in hot weather
where the ambient temperature reach to
C . Values tell
Homer that the inverters of 84 KW with efficiency of, 96 %,
cost 8400$ with lifetime of 15 years, that means inverters cost
16800$ for 25 years of the system lifetime where they are
replaced after 15 years of operation..
V.
RESULTS ANALYSIS
The results show that the horizontal PV system installed on
RR of SuqAltholatha School facing NE-SW can produce
103.05 MWh/y as shown in Fig 7.
For this system, as shown in Fig 6. , it can use an inverter
with capacity of 84 KW with maximum output of 58.1 KW and
capacity factor of 14%, with losses of 4.3 MWh/y during 4384
hours.
Fig 8. Energy purchased/sold between PV system and grid
Figure 6. Inverter output
Fig 7. shows that the seasonal peak sales to the grid during
summer time when schools' load at the lowest level which is
one of the advantages of using this system.
In this study 3 KW load considered as a critical load, this
load need number of batters can be calculated as following: In
case of choosing a deep cycle battery of 105Ah and 12V DC
and cost 233 $ [6] the cost can be found as below:
= 233*1.33= 310 LD for each battery.
Now to cover this critical load of 3KW for at least 3.5
hours in the blackout periods, it is necessary to determine the
suitable number of batteries. Where the inverter stop at 25% of
battery discharging for battery safety then each battery can
deliver only:
= 105* 0.75= 78Ah
The Batteries that is needed to install
= (3.5 h *3000) / 12*78) ≈ 12
Figure 7. Annually electric production from horizontal PV system on RR faced
NE-SW
The results of Homer program as shown in table 3. idecats
that SuqAltholtha School will produce annually an amount of
emission approximately 57346 Kg/y due to burning fossil fuel
to generate electrical energy, which is caused pollution [5].
Using horizontal PV system of 73.4 KW, can save the
environment from amount of 5070.5 Kg /y of emission (The
percentage saved about 8.8 %) as shown in Table III.
TABLE III.
THE ANNUALLY AMOUNT OF EMISSION PRODUCED (FOSSIL
FUEL) AND SAVED (PV SYSTEM)
Although using the PV system, still some gases produced due
to the consumption from grid’s fuel. Saved emissions means
saved money. Where each barrel of oil can produce about
600Kg of
[5], then the saved money over 25 years due to
using GCPV system in SuqAltholata School, with assumption
that the barrel cost of 20$, will be 4225.4 $.
From the simulation results, it can be pointed out that annually
school purchase energy of 47.31 MWh from the grid to operate
its load and PV system sell energy of 61.66 MWh/y to the grid.
For insuring a continuance for energy supply, backup system is
needed to be installed.
Then the cost of 12 batteries for each 5 years will be 3720
LD.
VI. CONCLUSION
Tripoli is the largest consumer of electrical energy in Libya
is suffering from a shortage of electricity and forcing the
distributor to practice regular load shedding. Therefore,
decision-makers aimed to compensate for the lack of electricity
through the introduction of solar energy technologies.
The solar potential of the country is very good. The schools
building of on-grid PV solar plants can be extremely contribute
to the diversification, independence, ecological and economic
sustainability of the national power supply system.
A proposed PV system were planned to meet a part of the
school's entire electric load and inject some of their high
energy production to the low voltage electricity network, the
systems were sized and simulated using SketchUp and
HOMER programs.
In our case (RR-rectangular school's rooftop), the system is
composed of 73.4 KW of PV and 84 kW converters with
average school's load consumption of 88695 KWh/y. The
production of PV system was 103045 KWh/y, which is, covers
all school's demand of electrical energy all over the year,
producing additional energy transferred to the public network
The results showed that it could benefit from the roofs of
schools in the city of Tripoli to produce electricity from solar
energy, thus reducing the load on the public network,
especially in the summer time when the schools are not
working.
[4]
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