2012 First International Conference on Renewable Energies and Vehicular Technology
Optimal Design of a renewable energy power plant
for an isolated site in Senegal
Abdoulaye Kébé1,2, Sengprasong Phrakonkham1, Ghislain Remy1, Demba Diallo1, Claude Marchand1
1
Laboratoire de Génie Electrique de Paris (LGEP) / SPEE-Labs, CNRS UMR 8507;
SUPELEC; Université Pierre et Marie Curie P6; Université Paris-Sud;
11 rue Joliot Curie, Plateau de Moulon F91192 Gif sur Yvette CEDEX,
demba.diallo@lgep.supelec.fr
2
Université Cheikh Anta Diop, Laboratoire de Traitement de L’Information, Dakar, Sénégal
ABSTRACT
The exploitation of resources of renewable energy has become inescapable today. Indeed, facing the oil cost increase,
the survival of the economies, particularly of the non oil-producing countries requires the evaluation and the exploitation of
their energy potential. The purpose of this paper is to make an analysis of the electrical energy situation in Senegal and to
show the relevance in the implementation of a hybrid power plant using renewable energy resources. The design is done to
minimize the Annualized Cost of System under the constraints related to the components and the availability of the energy
measured with the Loss of Supply Probability.
Index Terms— Isolated site, Optimization, Micro-Grid, Hybridization, Renewable Power Sources
This paper is organized as follows: In the first section,
we assess the electrification rate in Senegal and present the
renewable energy resources. In the second one, the concept
of micro-grid is introduced and the different configurations
are analysed and compared. In the third section, the isolated
site and the corresponding load profile are presented. In the
fourth one, the hybridization of the energy sources and the
optimal design are presented.
1. INTRODUCTION
The sustainable development adopted by several
countries as a modality of any economic and social growth is
today at the crossroads. Indeed, one of the main
consequences of the world energy crisis is the increase of the
fossil resource prices. This has undermined the efforts of
several countries, in particular those in the early stages of
development [1]. In sub-Saharan Africa this energy crisis
has led to an aggravation of the situation of the national
companies of electricity, that were already facing
tremendous difficulties. Indeed, since the middle of the 80s,
these companies have been suffering from a degradation of
their technical and economic performances [2], mainly
because of mismanagement. This is a characteristic of most
of the state-owned companies without a clear roadmap
including the challenges such as the increase of
consumption, preventive maintenance requirements and the
development of renewable energies. Moreover, the absence
of forecast has led to the continuous decrease of the
productivity gains. In fact, combined with the weakness of
the power production capacity, the increase of the interest
rates and the cost of the inputs put a large number of
companies in financial deficit.
As an example, the Senegalese national company of
electricity (SENELEC) is bogged down in a chronic budget
deficit of more than $400 million and a production deficit of
more than 100 MW. In 2010, the Senegalese endured
2950 hours (equivalent to 4 months!) of power cut [3]. The
main reasons are the outdated state of the power plants, the
transportation and distribution network, the lack of fuel and
the failure of the maintenance policy.
2. ELECTRIFICATION IN SENEGAL
2.1. State of Art of electrification in Senegal
The consumption of electricity represents 8.8% of the
total of energy consumption, far behind biomass (43%) and
oil products (47%) [4].The production of electricity is based
on oil products with 600,000 tons (35% of the national
consumption of hydrocarbons). Furthermore, the high
demand of the low voltage clientele grows at a rate above
6% and the capacities of production are insufficient [3]. This
leads to deficits of production due to delays in the
implementation of new basic groups. Figure 1 shows the
evolution of the cover rate in electricity from 2000 till 2006.
Figure 1. Evolution of the electrification rate [4]
978-1-4673-1170-0/12/$31.00 ©2012 IEEE
336
Therefore, the capacity of the offer was reduced by the
decline of the availability of the group (77 to 70% between
2003 and 2006) [5], because of the outdated state of the
equipment and problems of maintenance. So gensets with
prohibitive operating costs were used at an abnormally high
rate.
Despite the recourse to rented gensets, the selling cost of
energy is 91 F CFA/kWh (0.138€) compared to the
production one which is 97 F CFA/kWh (0.14€) [3].
Moreover, the records of rise in the oil price (25% for the
heavy fuel oil, and 22% for the diesel oil) in 2006 lead to
130 billion F CFA (293 344 957 €) for fuel expenses [5]. All
these factors combined with a penalizing mode of price
setting, dysfunctions in the exploitation and maintenance of
production units (average return on 31%) [5], the recourse to
expensive financing ended in a deficit between 2005 and
2006 of several billion F CFA.
Recently, an audit ordered by authorities in charge of
electrical energy revealed among other reasons that the crisis
in the sector of the electricity results largely from economic
performances registered by Senegal during the last ten years
with a growth of the GDP (Gross Domestic Product) of 5 %
a year which is translated by the growth of the demand of
electricity by 10% a year, and a politic of systematic
electrification to allow the largest number of Senegalese to
access to electricity. In 2000, SENELEC had only 398,000
subscribers and meadows of 3 millions of Senegalese had
access to electricity while today SENELEC has more than
885,000 subscribers and thus more than 7 millions
Senegalese have access to electricity, as shown in Figure 1.
It is also necessary to note that every Senegalese consumes
more electricity today than before. Indeed, 63% of the
current customers of SENELEC are in the group of
consumers of more than 150kWh [3]. From the point of view
of production, the fleet of power stations is largely obsolete,
not reliable, very expensive, unsuitable and insufficient in
terms of capacity. There is a structural deficit of production
of 100 MW with regard to the demand. Therefore,
consumers face frequent power cuts and the industrial
equipment suffers from a continuous degradation [3].
An old equipment of production and transport (12 years
on average), among which 140MW is more than 20 years
old. The 6.6 kV network of Dakar was built in the 1930's
and is more than 80 years old [3].
The production cost for some power plant reaches up to
2.2. Policy of development for the energy sector
The previous analysis clearly indicates that the
government must seriously tackle the energy sector by
defining an ambitious policy.
It should aim at:
- Guaranteeing the energy supply of the country,
- Widening the access of the populations to the modern
services of energy,
- Reducing the vulnerability of the country to the
volatility of the world oil market.
Therefore, an efficient policy for the energy sector
should focus on the development and the exploitation of the
national energy potentialities, in particular renewable
energies (solar, wind, marine, biomass) and the biofuels.
The Senegalese government has defined its policy
concerning the development of energy [5] based on the
following axes:
- Restructure the subsector of electricity with the
introduction of private partners in the capital and the
management of SENELEC,
- Enhance rural electrification,
- Strengthen the conditions of competition in the
subsector of hydrocarbons,
- Intensify the promotion of sedimentary pond,
- Strengthen the sustainable management of ligneous
resources by greater empowerment of local
authorities.
2.3. Renewable Energy Sources
2.3.1. Photovoltaic Energy
Senegal is situated at 12°8-16°41 latitude north and
11°21-17°32 longitude west. The average solar radiation is
5.8 kW/m2/day for an illumination of 1000W/m2 registered
during 3000 hours a year [6], as shown in Figure 2. Several
167 F CFA (0.254€) per kWh while the electricity is sold
projects have been realised or are in progress e.g. installation
between 108 (0.164€) and 120 F CFA/kWh (0.183€) [3].
The non respect of the maintenance program over several
of 11 solar power plants (10–40 kWp) in the islands of
years increased and continued the vicious cycle of the power
Saloum (4000 households), installation of 2648 solar
cuts: the maintenance program has been realized at 59% in
lanterns, electrification of 662 community centres (health
2007 against only 25% in 2010.
337
centres, schools, mosques, churches), and a solar power
network.
plant of 7MW in the southern region, Ziguinchor (on study)
2.3.3. Other Natural Resources
Senegal, because of its favourable geographical position,
[6].
benefits from 500 km of coast. At present, we have no
information on any study on offshore wind, tidal, wave or
any other source of energy potential from the ocean.
However, any site situated on the “Grande Côte” augurs an
interesting potential of hybridisation between solar, wind
and marine energies.
Figure 2. Irradiance map in Senegal
2.3.2. Wind energy
So far, Senegal has no wind farm. However, studies led
by various researchers and organizations reveal an
interesting wind potential along the west coast called
“Grande Côte” in Senegal. In this area, the wind speed
measured at 10 m high ranges from 3.8 to 4.2 m/s, as shown
in Figure 3.
Figure 4. Location of the site
3. THE SELECTED ISOLATED SITE
3.1. Presentation of the site
The selected site, MBoro sur Mer, is a village located on
“Grande Côte” at 5km from the municipality of MBoro,
Figure 4. It consists of about fifty concessions distributed on
approximately 500 m long. The main activities are fishing
and truck farming.
Figure 3. Wind map in Senegal
3.2. Energy resources
Currently, several wind farm projects are being
considered and planned e.g. 50MW in the northern region
(Saint Louis), 125 MW, 10.2 MW, 10.2 MW, 40 to 60 MW
on the west coast respectively at Taïba NDiaye, Kayar,
Potou and MBoro [7].
Concerning the site of Kayar, the project aims at
installing a dozen of 850 kW wind turbines 71 to 74m high
[8]. The production will be fed into the interconnected
3.2.1. Wind energy
Figure 5 presents the daily wind speed (in m/s) at the site
of MBoro in 2003. Regarding the data collected, wind speed
fluctuates in a low range of values corresponding to a
maximum wind speed of 30 km/h.
338
Figure 5. Wind speed measured at 20m at MBoro every
Figure 7. September, daily radiation in Dakar
10min
3.2. Load Profile
Previous measures made at the same site at 15 m high
To determine an estimation of the required power to feed
the village, we have determined the number of electrical
devices and their rated power. The results are illustrated in
Table 1:
between 1998 and 1999 [9] gave an average wind speed
equal to 4.2 m/s, as shown in Figure 6. Results show a
globally stable average wind speed that makes available low
speed wind turbine for such site.
Table 1. Power estimation
Type of device Rated output (W) Number Total power (W)
TV set
60
44
2640
Lamp
15
257
3855
Radio set
5
45
225
DVD
25
20
500
Taking into account the habits of the populations and
their rhythm of life, we have determined the following 4
ranges of operation:
- Range 1 (from 1 am to 6 am): no professional
activity. Only the street lighting is switched on,
P
= 765W
- Range 2 (from 6 am to 2 pm): economic and school
activities. Only some devices (radio sets for the
elders for example) work, P = 225W
- Range 3 (from 2 pm to 8 pm): between lunch break
and the end of the afternoon. Children are back home
and watch television for example, P = 2640W
- Range 4 (from 8 pm to 1 am): all families are
gathered at home; it’s the highest period of energy
consumption. The power is estimated at P=7,220W.
Figure 6. Average variation of the wind speed at 15 and 30
m high at MBoro
3.2.2 Solar energy potential
By observing the irradiance map of Senegal in Figure 2,
we notice that the irradiation is rather homogeneous on the
“Grande Côte” from Dakar to St-Louis. We can thus
consider that the irradiation observed in MBoro is equivalent
to that of Dakar. A value of 5.4 kWh/m2 can be considered.
In Figure 7, we can observe the daily irradiation measured in
Dakar in September 2010.
The energy consumption behavior is summarized in
Figure 8 depicted as a load profile.
339
energy flows through an AC bus. AC/AC converters should
be inserted to enable stable coupling of the components. If a
battery is used as storage device, a bidirectional AC/DC
static converter is required. It may also feed DC loads
through a DC bus [16]. This configuration is better suitable
for islands and villages including several points of
generation without a centralised connection.
Figure 8. Load power profile
4. MICRO-GRID
4.1. Definition and Concept
Figure 9. Micro-grid DC configuration
For the AC-DC configuration depicted in Figure 11, the
energy flows through DC and AC buses. If a battery is used
as a storage device, a bidirectional AC/DC static converter is
required. DC loads can be fed through the master AC/DC
static converter or directly from the DC bus. On the AC bus,
AC generating components may be connected directly or
through AC/AC converters to enable stable coupling of the
components [17]. This configuration is suitable for small
systems with a fairly constant load.
The concept of Micro-grid has appeared as a new
paradigm of the well-known classical power grid [10].
Hybrid Power Systems combine the advantages of
conventional and renewable power conversion systems [10].
They appear as necessity rises to feed isolated sites with
electrical energy. The first solution is to connect the area to
the national power utility [11]. Unfortunately because of
geographical constraints or due to the weakness of the grid
or because of non-profitability, this solution has very often
been abandoned. The alternative has been to produce, store
and distribute electrical energy locally. Therefore, a small
(compared to the national one by size and power) grid has
been built [12]. A connection to the national grid could be
considered but is not mandatory. The power could range
from 1 to 100kW. The advantages lie in the possibilities to
design and tune the network to minimise the cost or/and to
fulfil environmental constraints for example [13-14]. The
main disadvantage is the absence of backup in case of power
cuts if there is no connection to the main grid. However, the
solution is still attractive as it boosts the use of locally
available renewable energy sources.
Figure 10. Micro-grid AC configuration
4.2. Micro-Grid Configurations
There are essentially 3 types of micro grid
configurations: DC, AC and AC-DC coupled [15-16].
For the DC coupled configuration depicted in Figure 9,
all the components are connected to a DC bus. Rectifiers are
required to connect AC generators. AC loads are connected
to the DC bus through inverters [16]. The storage device is
usually a battery, controlled and protected from overcharge
and discharge by a charge controller. This configuration is
best suited for small systems with a fairly constant load.
In AC configuration depicted in Figure 10, the electrical
Figure 11. Micro-grid DC-AC configuration
340
Tables 2, 3 and 4 sum up the main advantages and
inconvenients of the three configurations:
Table 2. DC Configuration analysis
Advantages
‐
‐
‐
‐
we have focused on a Micro-grid AC configuration only.
5. APPLICATION TO THE ISOLATED SITE
Inconvenients
‐
Direct use of energy
Few losses
Few equipments
Easy expansion
5.1. Simulation models
Required to increase
the capacity of the
inverter or to add a
second inverter in
case of point brought
up what provokes a
low weak
Higher cost of DC
equipment
No available backup
for AC in case of
breakdown
‐
‐
The hybrid system simulated consists of photovoltaic
panels, wind turbines and battery storage. The load is
connected to these sources through a 220V AC grid, as
shown in Figure 12. The produced energy excess is used to
charge the batteries, depending on their state of charge
(SOC) [18]. Whenever the load demand is higher than the
energy produced, the batteries will supply the difference.
Table 3. AC Configuration analysis
Advantages
‐
‐
‐
Inconvenients
‐
Possibility of
increasing the voltage
with a passive
component (the
transformer)
Cheap equipment of
connection
Possibility to use the
frequency as means
of control
‐
Losses due to
multiple conversions
Requirements to
synchronize all the
sources
Figure 12. Diagram of the hybrid system
The simulation has been be conducted under the Matlab
R2011b/Simulink environment for a period of 24 hours
using average data recorded in Senegal, as described in
Section 3. The Genetic Algorithm (GA), integrated in the
Global Optimization Toolbox of Matlab/Simulink, has been
used as the optimization tool. Mainly because the MI-LXPM
algorithm have been implemented in GA function of Matlab
(above version R2011b). So, we are now able to design
system using Integer value of the variables. Integer
programming has involved several modifications of the basic
GA algorithm: Special creation, crossover, and mutation
functions enforce variables to be integers [19].
Table 4. AC-DC Configuration analysis
Advantages
‐
‐
‐
‐
Efficiency
Possibility to
connect AC load
directly
Less requirement
for the inverter
Allow to feed loads
raised from an
important AC
source
Inconvenients
‐
Less efficient
consumption of fuel in
case of partial load
Precisely, the genetic algorithm attempts to minimize a
penalty function, not the fitness function. The penalty
function includes a term for infeasibility [20]. This penalty
function is combined with binary tournament selection to
select individuals for subsequent generations. The penalty
function value of a member of a population is:
- If the member is feasible, the penalty function is the
fitness function, as depicted in Figure 13.
- If the member is infeasible, the penalty function is
the maximum fitness function among feasible
members of the population, plus a sum of the
constraint violations of the (infeasible) point.
From a practical point of view, there is no ‘best
configuration’. For each application, a thorough analysis has
to be conducted and it takes into account all the various
aspects from the accessibility of the site, environmental
aspects, safety concerns and technological choices.
In the next section, for the application on an isolated site,
341
each, leading to 2000h of time computation to evaluate all
cases. Using GA, the calculations takes only 30 minutes on
an Intel(R) Core(TM) i7 CPU Q720 at 1.60GHz with 8,00
Go of Memory.
5.3. Results
Table 5 shows the various solutions obtained by the
optimization:
Figure 13. Evolution of the penalty during generations
Table 5. Optimization Results
5.2. Optimization problem
The optimization of the system is determined for a period
of 20 years. We have been considering that the load profile
and the potential of sources will remain unchanged on the
studied period. The costs are converted to the prices
practiced in Senegal and expressed in euro [25].
The main parameters used for the sizing of the
components are detailed in the lines:
- The meteorological data (speeds of wind) were
measured on the site of MBoro at a height of 20m,
whereas the solar radiation data results from the site
of Dakar. The radiation can be considered as
equivalent to that of the MBoro because of the
relatively close geographical positions of both sites.
- The photovoltaic panels under consideration are of
the SP 130 [21] and BP 250 type [22], with 130Wp,
24 V and 50Wp, 12V respectively.
- The wind turbine is of the WH3-G2 type with a rated
output of 1kW, for a beam of 2m and a mast height
of 11m [23].
- The battery has a capacity of 75Ah for a voltage of
12V [24]. The battery charger works with a DC Bus
of 12V and the inverter uses a 48V DC input.
Nwt
Npvp1
0
0
1
4
2
3
Npvp2
Nbatp
0
4
4
3
4
4
Ninv
8
8
8
ACS
($)
ΔP
(kW)
SOC
(%)
38,404
41,989
51,845
0.77
0.09
10.78
42.00
50.84
54.68
Figure 14. Evolution of the batteries SOC, the wind turbine
and load power (kW)
Analyses of the results lead to several conclusions:
- The initial SOC is important in the design of the
system. In order to reduce the maintenance, batteries
are used between 40 and 90% of the rated energy
storage, as presented in Figure 14. The initial SOC
value was set to 50% and not 40% to allow short
power consumption unforeseen in the morning.
- To avoid intermittent power sources to let the system
under-supply, batteries have been also oversized
using final SOC > initial SOC as a criteria. More
SOC margin can then be preferred in order to avoid a
loss of power supply when having a higher power
demand in the evening.
- Solution number 1, shown in Table 5, has the lowest
ACS but cannot satisfy the SOC constraint (final
SOC >= 50%).
- Solution number 2 ranked 2nd but meets the
constraint set
- Solution number 3 is most expensive while meeting
the SOC constraint. However, it shows that even a
1kW wind turbine is not sufficiently cheap compared
The system has been optimized with one objective
function, namely the annualized cost of system (ACS) of the
hybrid system subjected to the batteries characteristics and
zero loss of power supply probability (LPSP) [25-28]. Five
variables are considered here for the optimization as can be
seen in Table 5. The GA configuration used a Population
Size of 128, an Elite Count of 6, a Crossover Fraction of 0.8
and a Generations number limited to 50. Variables are
evaluating in a restricted domain: LB = [0 0 0 0 0], UB =
[10 20 20 200 10].
This integer problem could also have been solved using
Design Of Experiment (DOE) instead of GA. However, due
to the high number of cases to evaluate initially, around 8
million evaluations with a computation time of around 1s
342
[13]
to photovoltaic panels. Indeed, the first acceptable
solution including a wind turbine is around 10k$
more expensive.
[14] E. Ortjohann, M. Lingemann, O. Omari,A. Schmelter, N. Hmasic,
A. Mohd, W. Sinsukthavorn, D. Morton, "Modular Architecture for
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and Motion Control Conference (EPE-PEMC 2008), 2008.
6. CONCLUSION
In this paper, we have first presented an analysis of the
electrical energy situation in Senegal. Then a hybrid
renewable energy system composed of renewable energy
sources i.e. wind turbines, PV and batteries as store for an
isolated site in MBoro, Senegal has been designed. The
designed system was optimized with the objective to
minimize the annualized cost of system (ACS) under
constraints of components and with zero Loss of Power
Supply Probability.
If the cheapest solution is selected while satisfying the
load demand and not violating the constraints, the solution
with an ACS of $41,989 is the most viable economically.
However, if a wind turbine is considered alongside PV to
build a hybrid system, a 3rd solution is preferable but on the
expense of more budget.
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