- Email: [email protected]
diesel hybrid systems without battery storage for off-grid areas
Renewable Energy 36 (2011) 1780e1787
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental study of electricity generation by Solar PV/diesel hybrid systems without battery storage for off-grid areas D. Yamegueu a, b, Y. Azoumah a, *, X. Py b, N. Zongo a a b
LESEE-2iE, Laboratoire Energie Solaire et Economie d’Energie, Institut International d’Ingénierie de l’Eau et de l’Environnement, 01 BP 594 Ouagadougou 01, Burkina Faso PROMES-CNRS UPR 8521, Laboratoire Procédés Matériaux et Energie Solaire, Université de Perpignan, Rambla de la thermodynamique, Tecnosud, 66100 Perpignan cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2010 Accepted 5 November 2010 Available online 3 December 2010
This paper presents the results of an experimental study of a PV/diesel hybrid system without storage. Experimental results show that the sizing of a PV/diesel hybrid system by taking into account the solar radiation and the load/demand profile of a typical area may lead the diesel generator to operate near its optimal point (70e80% of its nominal power). The present paper shows that for a reliability of a PV/diesel hybrid system, the rated power of the diesel generator should be equal to the peak load. By the way, it has been verified through this study that the functioning of a PV/diesel hybrid system is efficient for higher load and higher solar radiation. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Hybrid system Solar PV Diesel generator Off-grid areas Without storage
1. Introduction In developing countries, a major part of the population doesn’t have access to electricity what inhibits seriously their social, educational and economic development. This is particularly the case in sub-Saharan Africa region where progress in grid extension remains slower than population growth leading to a huge gap between the demand and the supply of electricity [1]. In fact, according to Rabah [2], the population in sub-Saharan Africa region is projected to increase by 124% in the next 20 years while the grid extension has advanced more slowly than in any other major regions. As displayed in Table 1, more than 60% of the sub-Saharan Africa populations are prevented from the benefits linked to electricity access. The situation is more catastrophic in rural areas where less than 12% of their population has access to electricity [3]. In many cases the utility grid extension is quite unconceivable because of population dispersion, rugged terrain or both [4] and mainly because of the high investment costs of transmission and distribution lines. Therefore, in these areas far from any existing grids, the supply of electricity is achieved using stand-alone power systems and broadly by diesel generators (DG). However, the volatile prices of fossil fuels, the very high maintenance cost of DGs coupled with the environmental and climate change concerns make this option unsustainable.
* Corresponding author. Tel.: þ226 50 49 28 64; fax: þ226 50 49 28 01. E-mail address: [email protected] (Y. Azoumah). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.11.011
Fortunately, most of the developing countries and specially those of sub-Saharan Africa region have a high potential in renewable energy resources such as wind, hydropower, biomass and solar; hellip; Renewable energies have a competitive advantage because they provide a long term energy supply based on local available sources and thus could help to reduce dependency on energy imports for countries of sub-Saharan Africa region. In addition, small off-grids stand-alone renewable energy systems provide appropriate technological solutions for the electrification of rural or semi-urban areas where they can be used independently from grid-connection. The West African region is strongly dependent upon fossil fuels while it has a significant solar energy potential ranging from 4 to 6 kWh/m2/day [5] which is today poorly exploited. It is therefore necessary to use this local and renewable energy resource in order to enhance the energy availability and security situation in these countries. However, solar energy presents major advantages but also a major drawback which is its unpredictable nature. This is mainly due to dependence on sunshine hours which are variable. As a matter of fact, the solar flux and the electricity demand time distributions do not match. This leads to extensive use of independent photovoltaic (PV) systems resulting in over-sizing in terms of system reliability and overall over-cost [6]. These are some of reasons why, in case of remote areas, hybrid renewable energy systems (HRES) are often the most cost-effective and reliable way to produce power [7]. The term hybrid renewable energy system (HRES) is used to describe any energy system composed of more than one type of generators. The usual configuration is made of a conventional diesel
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787 Table 1 Electricity access in 2008 [3].
North Africa Sub-Saharan Africa Africa China &East Asia South Asia Developing Asia Middle east Latin America Developing countries Transition economies & OECD World
Population without electricity (million)
National Urban Rural Electrification electrification Electrification rate (%) rate (%) rate (%)
2 587
98.9 28.5
99.6 57.5
98.2 11.5
589 195
40.0 90.2
66.8 96.2
22.7 85.5
614 809
60.2 77.2
88.4 93.5
48.4 67.2
21 34 1453
89.1 92.7 72.0
98.5 98.7 90.0
70.6 70.2 58.4
3
99.8
100.0
99.5
1781
up on the site of International Institute for Water and environmental engineering (2iE) at Kamboinsé (located at 15 km in the north of Ouagadougou, capital of Burkina Faso). This prototype consists of a PV array of 2.85 kWp coupled with a 9.2 kW diesel generator. The major goal of the current study is to present the obtained technical performances of the hybrid facility under various operating conditions. Several fixed load/demand profiles corresponding to different percentages of the diesel generator nominal power have been experimented for different solar radiations and consequently various PV contributions. The obtained raw and calculated performances are presented and discussed in this paper and lead to the perspective of further investigations which will be focused on the system optimization.
2. Experimentions 2.1. Experimental set up
1456
78.2
93.4
63.2
powered generator associated to a renewable energy source such as PV, wind, or even PV/wind. The initial cost of solar or wind energy systems is higher than diesel engine generator of comparable size but the operating and maintenance costs are always significantly lower for renewable systems. Over the last decade, HRES have become viable alternatives for power production because they allow designers to capitalize on the strengths of both conventional and renewable energy sources [8]. Most hybrid diesel/PV systems described in the open literature include battery storage [8e12] to meet the demand when either the demand is a peak load demand or when renewable energy source is not sufficient or available to meet the load. Battery storage also smoothen the mismatch between time of occurrence of peak load and maximum power generated. However batteries present major drawbacks as: additional investment costs and maintenance and extra costs due to periodic replacement induced by their short lifetime [13]. Indeed, the battery lifetime ranges between 3 and 5 years [14e17] and studies made by Muselli et al. [16], and later by Muselli et al. [15] show that the cost of batteries can represent up to 16%e20% of the total life cycle of a hybrid PV/diesel/battery system. Moreover, Notton et al. [18] in a previous work show that the cost of batteries can reach up to 40% of the total system costs for only a PV system. Furthermore, energy losses occur during battery loading and its efficiency decreases due to ageing or bad operating conditions [19]. Current losses are estimated for average overall operating conditions leading to typical value of 20% [20]. Besides, in developing countries like those of sub-Saharan Africa region, there is no policy about recycling of batteries at the end of their lifetime. This particular aspect represents a serious environmental problem. According to all these above reasons, some authors have already studied the case of PV/diesel hybrid system without storage. Ruther et al. [13] studied the possibility of adding a 20 kWp PV installation to a 54 kW existing diesel generator without storage in a typical village of Northern Brazil. Also, Ajan et al. [21] made a technical viability and economy study of a system using a photovoltaic facility to supplement a 150 kW existing diesel generator-based supply in a typical secondary school located in an off-grid area in East Malaysia. These above cases were limited to feasibility studies and then, there is no available experimental result from a real PV/diesel hybrid system without storage in the open literature. Hence to overcome this shortcoming, in this paper, are presented original experiment results of a PV/diesel hybrid system without storage set
The experimental prototype (Fig. 1) is set up at Kamboinsé, located at 15 km far from Ouagadougou (12 220 N and 1310 W) [22] in Burkina Faso. The energy produced by the PV array is proportional to the global irradiation and the cosines effect. The data obtained from the meteorological station of Ouagadougou shows a mean annual solar irradiation of 5.5 kWh/m2 per day and the direct sunshine is over 3000 h per year [23]. The site selected for the experimentations, being at a few kilometers from Ouagadougou is then propitious to implementation and use of PV systems. Hybrid systems can be broadly classified in series, switched hybrid and parallel hybrid configurations [24]. The present prototype has a parallel hybrid configuration (Fig. 1). In this scheme, the PV system is parallel connected to diesel generator by means of a DC-AC converter or inverter. The PV and the diesel generators supply a portion of the load demand directly. The PV system is composed of 15 modules HIT (Heterojunction with Intrinsic Thin layer), each rated at 190 Wp, totalling 2.85 kWp DC at Standard Testing Conditions (irradiance ¼ 1000 W/m2; Solar cell operating temperature ¼ 25 C and AM ¼ 1.5). The PV array covers an area of about 18 m2 with PV modules distributed in a parallel/series configuration of three module strings connected to one inverter rated at 3.82 kW. The diesel generator rated power is 11.5 kVA (9.2 kW) of 1500 rpm. The Table 2 exposes the general specifications of the PV modules, the inverter and the diesel generator as well.
Fig. 1. Schematic diagram of the experimental set up.
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
the hourly fuel consumption (l/h) for supplying a given load during 1 h. According to Skarstein and Uhlen [20], the hourly fuel consumption can be approximated as follows:
PV modules Type Rated capacity (W) Maximum power voltage (V) Maximum power current (A) Open circuit voltage (V) Short circuit current (A) Module efficiency (%) Dimensions: L l D (mm mm mm) Weight (kg)
HIP-190BA3 190 54.8 3.47 67.5 3.75 16.1 1319 894 35 14
Inverter Type Maximum input power (W) Maximum input voltage (V) Maximum input current (A) Maximum output power (W) Nominal power (W) Nominal voltage (V) Nominal frequency (Hz) Maximum efficiency (%) Dimensions: L l D (mm mm mm) Weight (kg)
Sunny boy 3300 3820 500 20 3600 3300 220e240 50/60 95.2 450 352 230 41
Diesel generator Type Nominal Power (kWe/kVA) Velocity (rpm) Voltage (V) Maximum current (A) Dimensions: L l h (mm mm mm) Weight (kg)
T 12 K 9.2/11.5 1500 400/230 17 1405 715 1053 438
2.2. Experimental matrix In order to enhance the development of PV/diesel hybrid systems in off-grid areas, an extensive study must be carried out to check their feasibility, their efficiency and technical competitiveness. An important driving element of any power generating system is the load which is highly specific to the concerned application [25]. A very wide range of load profiles should be actually considered with respect to the local needs, cultural behavior, fuel and solar availability. This means that any general study over such a system should be done using first a set of hypothetic loads allowing the analysis of its performances and limitations. The consideration of typical loads in agreement with particular situations has to be done further in order to optimize the system. Therefore, in the present investigation, the load/demand profiles are considered as constants for the simulation of the hybrid system. Four load profiles have been considered (3.7 kW, 5.7 kW, 7.6 kW and 9.7 kW) corresponding to 40%, 62%, 82% and 105% of the diesel generator nominal power. The demand profiles are experimentally simulated by two resistive bank loads of 4 kW each with different switches allowing, by combination, to reach the desired load. Mix of interdependent key parameters such as: power generated by PV array (depending of solar radiation), power generated by diesel generator and specific/hourly consumption of diesel generator to meet the predefined load have been studied.
qðtÞ ¼ a:PðtÞ þ b:Pr
(1)
Where a(l/kW) and b(l/kW) are constants for a typical DG, Pt(kW) is the power generated by DG and Pr(kW), the rated/nominal power of the DG. As shown in Fig. 2, the relation (1) has been validated with the current fuel consumption data of a typical diesel generator of 9.2 kW. In fact, basing on these data, a similar linear model was fitted for the total fuel consumption as a function of the output power. The model coefficients obtained for these data are a ¼ 0:2476 l=kWh and b ¼ 0:07391 l=kWh with a coefficient of determination (R2) of 0.9895. Fig. 2 also shows the specific fuel consumption of a typical diesel generator of 9.2 kW as a function of its output power. It can be noted that when the DG operates near 20% of its rated power, its specific consumption is too high (about 0.64 l/kWh) corresponding to a low efficiency of the generator. The minimum specific fuel consumption of the DG is 0.324 l/kWh and corresponds to about 80% of its rated power meaning an important fuel saving; thus a good efficiency may be obtained if the DG operates close to this load fraction. This result corresponds to what stipulated by authors as Ashari et al. [12] and El-Hefnawi [26] saying that DGs have typically a maximum fuel efficiency of about 0.33 l/kW when run above 80% of its rated power (for the first authors) and when the optimum diesel generator operating range is 70e89% of its rated power (for the second authors). Later, the behavior of the diesel generator in the hybrid system is thoroughly analyzed. 3.2. Inverter characteristics Connection between the PV system and the diesel generator is done by the inverter which converts DC power generated by PV array to AC power. Inverter technology is therefore a key technology to have reliable and safe grid interconnection operation of PV system [27]. There are many types of inverters available today in the market and the parameter generally used to evaluate their functioning is the efficiency defined as the ratio of DC power
3.5
0.7 q(t) = 0.2476 P(t) + 0.6791 R² = 0.9895
3.0
0.6
2.5
0.5
2.0
0.4
1.5
0.3
1.0
0.2
0.5
0.1
3. Results and discussion 3.1. Diesel generator characteristics The characteristics of the pure diesel generator system were studied. A diesel generator (DG) is characterized by its efficiency and its fuel consumption (hourly and specific fuel consumptions). The specific fuel consumption (l/kWh) is defined as the fuel consumption required to produce 1 kWh of energy and it is equal to
0.0
0.0 0.0
2.0
4.0
6.0
8.0
10.0
Generated power (kW) Fig. 2. Fuel consumption of the diesel generator versus its output power.
Specific consumption (l/kWh)
Table 2 Equipment specifications.
Hourly consumption (l/h)
1782
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
1783
0.42
6
100
Generated PV power (kW) Generated DG power (kW)
90
Specific consumption (l/kWh)
80
0.41
0.39
60
4
Power (kW)
Inverter efficiency (%)
0.4
70
50 40
0.38
3
0.37
30
0.36
20
Specific consumption (l/kWh)
5
2 0.35
10 0.34
0
1
0
500
1000
1500
2000
2500 0.33
Ppv-DC (W) 0
Fig. 3. Inverter efficiency as a function of PV DC power.
generated by the PV array (PpvDC) and the AC power obtained by the inverter (PAC):
PAC PpvDC
Hours Fig. 5. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 62% of its nominal power.
(2)
Fig. 3 displays the efficiency of a typical single-phase inverter (sunny boy 3300) used for this study. One can notice that, for the low DC input power, up to almost 70% of the incoming energy to the inverter could be lost. However, for high DC power, the inverter efficiency could reach 95% meaning that only 5% of the incoming energy is lost. For the single-phase inverters the losses in inverter
can be about 8e20% of the total energy generated [26]. But the results obtained in this present work show that these losses can reach 70% for very low DC power coming from PV generator not close to the nominal power of inverter. This input DC power depends on the availability of the solar radiation and the load demand. One should therefore pay close attention to these parameters when sizing the system for optimizing the functioning of inverters.
0.37
8 4
Generated PV power (kW) Generated DG power (kW) Specific consumption (l/kWh)
0.52
7
0.44 0.42 0.4 0.38 1
0.35
Power (kW)
0.46
0.36
6
Specific consumption (l/kWh)
3
Power (kW)
0.36
0.5 0.48
2
Generated PV power (kW) Generated DG power (kW) Specific consumption (l/kWh)
5 0.35 4 0.34 3 0.34 2
Specific consumption (l/kWh)
hinv ¼
0.32
0.33
0.36 0.34 0.32
0
Hours Fig. 4. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 40% of its nominal power.
1
0.33
0
0.32
Hours Fig. 6. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 82% of its nominal power.
1784
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
10 9
Generated PV power (kW) Generated DG power (kW) Specific consumption (l/kWh)
0.40
1000
7
0.39
900
7
0.37
6
0.36
5
0.35
4
0.34
3
0.33
2
0.32
1
0.31
800 5
600
4
500 3
400 300
2
1 0
700
0.30
Generated DG Power (kW) Generated PV Power (kW) Generated Total Power (kW) Load (kW) Global solar radiation (W/m²)
0
Global solar radiation (W/m²)
0.38
Power (kW)
8
Specific consumption (l/kWh)
Power (kW)
6
200 100 0
Hours
Hours
Fig. 7. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 105% of its nominal power.
Fig. 9. Generated total power by PV/diesel hybrid system for meeting a load representing 62% of the diesel generator nominal power.
3.3. Behavior of hybrid system under different loads In this section, the behavior of a typical hybrid PV/diesel system without storage whose characteristics is presented in the previous sections has been studied. The contribution of PV and diesel generators, specific consumption of the diesel generator for a given load (L) profile have been studied. As mentioned previously, we consider four constant load profiles (3.7 kW, 5.7 kW, 7.6 kW and 9.7 kW) corresponding respectively to 40%, 62%, 82% and 105% of the diesel generator nominal power. The behaviors of the PV and diesel generators in the hybrid system are interdependent. Indeed, a given load is supplied as well by the diesel generator as by the PV generator. It can be observed
4.5
that for loads in the order mentioned above, the specific fuel consumption of the DG ranges respectively from: 0.45e0.51 l/kWh (Fig. 4), 0.35e0.41 l/kWh (Fig. 5), 0.33e0.36 l/kWh (Fig. 6) and 0.33e0.34 l/kWh (Fig. 7). These results show that for small loads (less than 62% of the rated power of the DG), the contribution of PV generator (PAC/L) which ranges between 4 and 38% for a load values of 3.7 kW and 5.7 kW does not enable an optimal functioning of PV generator. In fact, as mentioned by Ashari et al. [12] and also verified by results obtained in the present investigation, the maximum
9
1100
1000
8
600
900
4
7
500
2.5 300 2 1.5
200
1 0.5
700
Power (kW)
Power (kW)
400
Global solar radiation (W/m²)
6
3
5
600
4
500
400 3
Global solar radiation (W/m²)
800
3.5
300 2
Generated DG Power (kW) Generated PV Power (kW) Generated Total power (kW) Load (kW) Global solar radiation (W/m²)
100
200 1
100
0
0
0
Hours Fig. 8. Generated total power by PV/diesel hybrid system for meeting a load representing 40% of the DG nominal power.
0
Hours
Fig. 10. Generated total power by PV/diesel hybrid system for meeting a load representing 82% of the diesel generator nominal power.
900
10
90
10 (Pt-L)/Pt 9
80 Ppv/Pac-max
800
8 8
7
700
Power (kW)
500 5 400 4
Global solar radiation (W/m²)
600
(P t-L)/P t(%)
60
7
6
70
6 50 5 40 4
Ppv/P ac-max (%)
9
30 3 20
2 1
10
0
0
300 3
200
Hours
2
1
Generated DG power (kW) Generated PV power (kW) Generated Total power (kW) Load (kW) Global solar radiation (W/m²)
Fig. 13. Gap in energy production for a load representing 62% of the diesel generator nominal power.
100
0
0
Hours
Fig. 11. Generated total power by PV/diesel hybrid system for meeting a load representing 105% of the diesel generator nominal power.
fuel efficiency of a diesel generator is about 0.33 l/kW, but for these two rated loads (3.7 and 5.7 kW), the specific fuel consumptions are far from this optimal value. This is due to the fact that these loads are already far from the optimal functioning point of the DG and
12
therefore the PV generator contributes rather to decrease the performance of the engine. However, for high loads (more than 82% of the rated power of the DG), the contribution of PV array (which ranges from 1.5% to 33% for a load value of 7.6 kW and from 2.5% to 22% for a load value of 9.7 kW) does not affect considerably the performance of the engine. As a matter of fact, for these loads, the values of specific fuel consumption vary between 0.33 and 0.36 l/ kWh which is close to the optimal functioning point of the DG. As the specific fuel consumption and then the operation cost of diesel generator increases considerably when the hybrid system operates below the load values representing less than 62% of the rated power of the DG, the design of the system must be done so that the diesel generator operates near its optimal point (70e80% of its nominal power).
60
5
100
(Pt-L)/Pt
(Pt-L)/Pt 10
Ppv/Pac max
Ppv/Pac max
4
90
50 80
3
30
4
20
(P t-L)/P t (%)
6
2 60
1
50
40
0
Ppv/P ac-max (%)
70
40
Ppv/P ac-max (%)
(P t-L) / Pt (%)
8
30
-1
2
20
10 -2
10
0
0
Hours Fig. 12. Gap in energy production for a load representing 40% of the diesel generator nominal power.
-3
0
Hours Fig. 14. Gap in energy production for a load representing 82% of the diesel generator nominal power.
1786
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
90
5 (Pt-L)/Pt 4
80
Ppv/Pac-max
3
70 60
1 50 0 40 -1 30
-2
Ppv/P ac-max (%)
(P t-L)/Pt (%)
2
20
-3 -4
10
-5
0
Hours Fig. 15. Gap in energy production for a load representing 105% of the diesel generator nominal power.
We also analyzed in this section particularly the influence of the solar radiation and load profile on the PV array production. From Fig. 8 to Fig. 11, it can be observed that the power supplied by the PV array through the inverter depends not only on the solar radiation but also on the load. In fact, one can see that for the close values of solar radiation, the production of PV generator is more important for the higher loads than for the lowers. This means that for an optimum production of the PV array, the ideal would be that the maximum solar radiation moments meet the peak load moments during a day. We also look closely if the total power generated (Pt) by PV and DG generators meets the load at any time of the day. For the lower loads 3.7 kW (Fig. 8) and 5.7 kW (Fig. 9), representing respectively 40 and 62% of the rated power of the DG, the total generated power meets at any time the demand. But for the higher loads (7.6 kW (see Fig. 10) and 9.7 kW (see Fig. 11) representing
60 105 % of diesel generator nominal Power 82% of diesel generator nominal Power
50
62% of diesel generator nominal Power 40 % of diesel generator nominal Power
Ppv/L (%)
40
30
20
10
respectively 82 and 105% of the rated power of the DG), it can be noted that at certain moments of the day, the demand is not met by the total power generated. For a better estimation of this situation which mentioned above, we have studied the parameter ðPtPLÞ t permits to exactly see the gap between the total energy production and the load. One can observe that for the lower loads, there is an excess of energy varying between 4 and 11% for a load value of 3.7 kW (Fig. 12) and between 1 and 10% for a load value of 5.7 kW (Fig. 13). However for the load values more than 82% of the rated power of DG (Figs. 14 and 15), one can observe, at certain moments of the day, a deficit in energy production. This is more considerably in the case of a load value of 9.7 kW (105% of rated power of DG) where we have a deficit within 0.9e3% (see Fig. 15) and this practically during all the day. In what follows, we have looked if the PV system feeds in real time to the mini-grid all the power produced. Hence, the parameter PPV PACmax has been defined, where PPV is the power feeds in the grid and PACmax the total power generated by the PV array. Through this parameter (see Fig. 16), one can confirm that the contribution of PV array depends also on the load/demand. Indeed, for high loads the power feeds by PV array in the mini-grid is more important. Towards these results, one can say that the sizing of a hybrid system must be done so that the nominal power of DG is equal to the peak load. Also, only the DG should run when the load does not exceed a certain threshold (20% of its nominal power) because as stipulated by Hochmuth [9], running the diesel generator with a low capacity factor increases fuel consumption and wear, and then the operation and maintenance costs. The use of dummy loads (for pumping or heating water) could also be a solution to absorb the excess output power and keep the engine loading high. The use of multiple generator sets that can be run in parallel is one of the solutions which may contribute to enhance the overall system efficiency. 3.4. Penetration level in a PV/diesel hybrid system In a PV/diesel hybrid system, the PV penetration level is defined as the ratio of maximum PV array power (kWp) to the rated power of the diesel generator (kW). But such as defined, this ratio doesn’t give clear information about the behavior of the system. Therefore, the ratio of the real output power of the PV system on the maximum load PLPV is a best indication about the contribution of the PV array for a given load. The Fig. 16 shows the ratioPLPV as function of the solar radiation for different loads. It can be observed that for lower loads (3.7 kW and 5.7 kW, corresponding to 40 and 62% of the DG nominal power), the ratio PLPV ranges respectively from 2 to 49% and from 2 to 33% (for a solar radiation value varying between 150 and 950 W/m2). However for higher loads (7.6 kW and 9.7 kW, corresponding to 82 and 105% of the DG nominal power), it is noticed that the ratio PLPV varies respectively from 3 to 26%, and from 2 to 21% for the same values of solar radiation. Fig. 16 gives also a good indication about possible contribution levels of PV array for which PV/diesel hybrid system may be relevant, depending on load and solar radiation. Indeed, it is noticed here a high PV contribution for lower loads but this is not a synonym of a good performance because as mentioned in a previous section the best performance of PV array is obtained for higher loads. Also, this high PV penetration for lower loads does not favor an optimal functioning of the diesel generator.
0 0
200
400
600
800
1000
Global solar radiation (W/m²) Fig. 16. Contribution of PV generator for a given load versus solar radiation.
4. Conclusion All the results presented in this paper have been obtained taking four different daily constant loads to simulate the behavior
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
of a typical PV/diesel hybrid system without storage. Experimental results show that the contribution of PV generator for a given load affects the performance of the diesel generator. As a matter of fact, when the hybrid system operates under small loads (less than 62% of the diesel generator rated power), the PV generator contributes rather to decrease the performance of diesel generator. A specific fuel consumption ranging from 0.35 to 0.51 l/kWh is obtained for loads varying from 40 to 62% of the diesel generator rated power. This fuel consumption shows a bad performance for the diesel generator. However, it has been shown that for higher loads (82 and 105% of the DG rated power), the specific fuel consumption of DG ranges from 0.33 to 0.36 l/kWh which is a good performance. Therefore the design of PV/diesel hybrid system must be done so that the diesel generator operates near its optimal point (70e80% of its nominal power). At this level, the use of dummy loads (for pumping or heating water) to absorb the excess output power and multiple diesel generators running in parallel could be a solution to enhance the system efficiency. It has been also observed a deficit in energy production for load higher than 100% of rated power of diesel generator which could mean that for a reliability of a PV/diesel hybrid system, the rated power of diesel generator should be equal to the peak load. It has been verified through this study that the functioning of a PV/diesel hybrid system is efficient for higher loads and higher solar radiations. This study being conducted with constant loads, it will be of a great interest for future investigation to study experimentally the behavior of the system under variable loads. Acknowledgments This work has been supported by the International Institute for Water and Environmental Engineering (2iE) of Ouagadougou (Burkina Faso) and the National Center of Scientific Research (CNRS) of France through the program “doctorate grant for national engineers of developing countries” (BDI-PED). References [1] Nfaha EM, Ngundamb JM, Tchinda R. Modelling of solar/diesel/battery hybrid power systems for far-north Cameroon. Renewable Energy 2007;32:832e44. [2] Rabah KVO. Integrated solar energy systems for rural electrification in Kenya. Renewable Energy 2005;30:23e42. [3] World energy outlook-WEO. The Electricity Access Database, http://www.iea. org/weo/electricity.asp; 2009. [4] Barley CD. Optimal dispatch strategy in remote hybrid power systems. Solar Energy 1996;58(4e6):165e79. [5] Deutsche Gesellschaft fur Technische Zusammenarbeit-GTZ. Regional reports on renewable energies. 30 Country Analyses on Potentials and Markets in
[6] [7]
[8] [9] [10]
[11]
[12]
[13]
[14]
[15]
[16] [17] [18]
[19] [20] [21]
[22]
[23]
[24] [25]
[26] [27]
1787
West Africa (17), East Africa (5) and Central Asia (8). Frankfurt/Eschborn. Energy policy framework papers; 2009. Section “Energy and transport”. Elhadidy MA, Shaahid SM. Parametric study of hybrid (windþsolarþdiesel) power generating systems. Renewable Energy 2000;21(2):129e39. Notton G, Muselli M, Louche A. Autonomous hybrid photovoltaic power plant using a back-up generator: a case study in a Mediterranean Island. Renewable Energy 1996;7(4):371e91. Deshmukha MK, Deshmukh SS. Modeling of hybrid renewable energy systems. Renew Sustain Energ Rev 2008;12:235e49. Hochmuth GCS. A combined optimisation concept for the design and operation strategy of hybrid-PV energy systems. Solar Energy 1997;61(2):77e87. Shaahid SM, El-Amink I. Techno-economic evaluation of off-grid hybrid photovoltaic-dieselebattery power systems for rural electrification in Saudi Arabia-a way forward for sustainable development. Renew Sustain Energ Rev 2009;13:625e33. Papadopoulos DP, Maltas EZ. Design operation and economic analysis of autonomous hybrid pvediesel power systems including battery storage. J Electr Electron Eng Aust 2010;61(1):3e10. Ashari M, Nayar CV. An optimum dispatch strategy using set points for a photovoltaic (pv)edieselebattery hybrid power system. Solar Energy 1999;66(1):1e9. Ruther R., Martins D., Bazzo E. Hyrid Diesel/Photovoltaic systems without storage for isolated mini-grids sin Northem Brazil. In: Proceeding of the 28th IEEE Photovoltaic Specialists Conference. Anchorage, USA 2000:1567e70. Koutroulis E, Kolokotsa D, Potirakis, Kalaitzakis K. Methodology for optimal sizing of stand-alone photovoltaic/wind-generator systems using genetic algorithms. Solar Energy 2006;80:1072e88. Muselli M, Notton G, Poggi P, Louche A. PV-hybrid power systems sizing incorporating battery storage: an analysis via simulation calculations. Renewable Energy 2000;20:1e7. Musseli M, Notton G, Louche A. Design of hybrid-photovoltaic power generator, with optimization of energy management. Solar Energy 1999;65(3):143e57. Lopez RD, Agustin JLB. Design and control strategies of PV-Diesel systems using genetic algorithms Rodolfo. Solar Energy 2005;79:33e46. Notton G, Musseli M, Poggi P, Louche A. Autonomous photovoltaic systems influences of some parameters on the sizing: simulation time step, input and output power profil. Renewable Energy 1996;1(4):353e69. Yang H, Lu L, Zhou W. A novel optimization sizing model for hybrid solarwind power generation system. Solar Energy 2007;2007(81):76e84. Skarstein O, Uhlen K. Design considerations with respect to long-term diesel saving in wind/diesel plants. Wind Eng 1989;13:72e87. Ajan CW, Ahmed SS, Ahmad HB, Taha F, Zin AABM. On the policy of photovoltaic and diesel generation mix for an off-grid site: East Malaysian perspectives. Solar Energy 2003;74:453e67. Borrmann S, Kunkel D, Weigel R, Minikin A, Deshler T, Wilson JC, et al. Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility. Atmos Chem Phys 2010;10:5573e92. Azoumah Y, Ramdé EW, Tapsoba G, Thiam S. Siting guidelines for concentrating solar power plants in the Sahel:Case study of Burkina Faso. Solar Energy 2010;84:1545e53. Nayar CV. Recent developments in decentralised mini-grid diesel power systems in Australia. Appl Energy 1995;52:229e42. Elhadidy MA, Shaahid SM. Promoting applications of hybrid (windþphotovoltaic þdieselþbattery) power systems in hot regions. Renewable Energy 2003;29:517e28. El-Hefnawi SH. Photovoltaic dieselegenerator hybrid power system sizing. Renewable Energy 1998;13(1):33e40. Elthawi MA, Zhao Z. Grid-connected photovoltaic power systems: technical and potential problems-a review. Renew Sustain Energ Rev 2010;14:112e29.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental study of electricity generation by Solar PV/diesel hybrid systems without battery storage for off-grid areas D. Yamegueu a, b, Y. Azoumah a, *, X. Py b, N. Zongo a a b
LESEE-2iE, Laboratoire Energie Solaire et Economie d’Energie, Institut International d’Ingénierie de l’Eau et de l’Environnement, 01 BP 594 Ouagadougou 01, Burkina Faso PROMES-CNRS UPR 8521, Laboratoire Procédés Matériaux et Energie Solaire, Université de Perpignan, Rambla de la thermodynamique, Tecnosud, 66100 Perpignan cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 August 2010 Accepted 5 November 2010 Available online 3 December 2010
This paper presents the results of an experimental study of a PV/diesel hybrid system without storage. Experimental results show that the sizing of a PV/diesel hybrid system by taking into account the solar radiation and the load/demand profile of a typical area may lead the diesel generator to operate near its optimal point (70e80% of its nominal power). The present paper shows that for a reliability of a PV/diesel hybrid system, the rated power of the diesel generator should be equal to the peak load. By the way, it has been verified through this study that the functioning of a PV/diesel hybrid system is efficient for higher load and higher solar radiation. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Hybrid system Solar PV Diesel generator Off-grid areas Without storage
1. Introduction In developing countries, a major part of the population doesn’t have access to electricity what inhibits seriously their social, educational and economic development. This is particularly the case in sub-Saharan Africa region where progress in grid extension remains slower than population growth leading to a huge gap between the demand and the supply of electricity [1]. In fact, according to Rabah [2], the population in sub-Saharan Africa region is projected to increase by 124% in the next 20 years while the grid extension has advanced more slowly than in any other major regions. As displayed in Table 1, more than 60% of the sub-Saharan Africa populations are prevented from the benefits linked to electricity access. The situation is more catastrophic in rural areas where less than 12% of their population has access to electricity [3]. In many cases the utility grid extension is quite unconceivable because of population dispersion, rugged terrain or both [4] and mainly because of the high investment costs of transmission and distribution lines. Therefore, in these areas far from any existing grids, the supply of electricity is achieved using stand-alone power systems and broadly by diesel generators (DG). However, the volatile prices of fossil fuels, the very high maintenance cost of DGs coupled with the environmental and climate change concerns make this option unsustainable.
* Corresponding author. Tel.: þ226 50 49 28 64; fax: þ226 50 49 28 01. E-mail address: [email protected] (Y. Azoumah). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.11.011
Fortunately, most of the developing countries and specially those of sub-Saharan Africa region have a high potential in renewable energy resources such as wind, hydropower, biomass and solar; hellip; Renewable energies have a competitive advantage because they provide a long term energy supply based on local available sources and thus could help to reduce dependency on energy imports for countries of sub-Saharan Africa region. In addition, small off-grids stand-alone renewable energy systems provide appropriate technological solutions for the electrification of rural or semi-urban areas where they can be used independently from grid-connection. The West African region is strongly dependent upon fossil fuels while it has a significant solar energy potential ranging from 4 to 6 kWh/m2/day [5] which is today poorly exploited. It is therefore necessary to use this local and renewable energy resource in order to enhance the energy availability and security situation in these countries. However, solar energy presents major advantages but also a major drawback which is its unpredictable nature. This is mainly due to dependence on sunshine hours which are variable. As a matter of fact, the solar flux and the electricity demand time distributions do not match. This leads to extensive use of independent photovoltaic (PV) systems resulting in over-sizing in terms of system reliability and overall over-cost [6]. These are some of reasons why, in case of remote areas, hybrid renewable energy systems (HRES) are often the most cost-effective and reliable way to produce power [7]. The term hybrid renewable energy system (HRES) is used to describe any energy system composed of more than one type of generators. The usual configuration is made of a conventional diesel
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787 Table 1 Electricity access in 2008 [3].
North Africa Sub-Saharan Africa Africa China &East Asia South Asia Developing Asia Middle east Latin America Developing countries Transition economies & OECD World
Population without electricity (million)
National Urban Rural Electrification electrification Electrification rate (%) rate (%) rate (%)
2 587
98.9 28.5
99.6 57.5
98.2 11.5
589 195
40.0 90.2
66.8 96.2
22.7 85.5
614 809
60.2 77.2
88.4 93.5
48.4 67.2
21 34 1453
89.1 92.7 72.0
98.5 98.7 90.0
70.6 70.2 58.4
3
99.8
100.0
99.5
1781
up on the site of International Institute for Water and environmental engineering (2iE) at Kamboinsé (located at 15 km in the north of Ouagadougou, capital of Burkina Faso). This prototype consists of a PV array of 2.85 kWp coupled with a 9.2 kW diesel generator. The major goal of the current study is to present the obtained technical performances of the hybrid facility under various operating conditions. Several fixed load/demand profiles corresponding to different percentages of the diesel generator nominal power have been experimented for different solar radiations and consequently various PV contributions. The obtained raw and calculated performances are presented and discussed in this paper and lead to the perspective of further investigations which will be focused on the system optimization.
2. Experimentions 2.1. Experimental set up
1456
78.2
93.4
63.2
powered generator associated to a renewable energy source such as PV, wind, or even PV/wind. The initial cost of solar or wind energy systems is higher than diesel engine generator of comparable size but the operating and maintenance costs are always significantly lower for renewable systems. Over the last decade, HRES have become viable alternatives for power production because they allow designers to capitalize on the strengths of both conventional and renewable energy sources [8]. Most hybrid diesel/PV systems described in the open literature include battery storage [8e12] to meet the demand when either the demand is a peak load demand or when renewable energy source is not sufficient or available to meet the load. Battery storage also smoothen the mismatch between time of occurrence of peak load and maximum power generated. However batteries present major drawbacks as: additional investment costs and maintenance and extra costs due to periodic replacement induced by their short lifetime [13]. Indeed, the battery lifetime ranges between 3 and 5 years [14e17] and studies made by Muselli et al. [16], and later by Muselli et al. [15] show that the cost of batteries can represent up to 16%e20% of the total life cycle of a hybrid PV/diesel/battery system. Moreover, Notton et al. [18] in a previous work show that the cost of batteries can reach up to 40% of the total system costs for only a PV system. Furthermore, energy losses occur during battery loading and its efficiency decreases due to ageing or bad operating conditions [19]. Current losses are estimated for average overall operating conditions leading to typical value of 20% [20]. Besides, in developing countries like those of sub-Saharan Africa region, there is no policy about recycling of batteries at the end of their lifetime. This particular aspect represents a serious environmental problem. According to all these above reasons, some authors have already studied the case of PV/diesel hybrid system without storage. Ruther et al. [13] studied the possibility of adding a 20 kWp PV installation to a 54 kW existing diesel generator without storage in a typical village of Northern Brazil. Also, Ajan et al. [21] made a technical viability and economy study of a system using a photovoltaic facility to supplement a 150 kW existing diesel generator-based supply in a typical secondary school located in an off-grid area in East Malaysia. These above cases were limited to feasibility studies and then, there is no available experimental result from a real PV/diesel hybrid system without storage in the open literature. Hence to overcome this shortcoming, in this paper, are presented original experiment results of a PV/diesel hybrid system without storage set
The experimental prototype (Fig. 1) is set up at Kamboinsé, located at 15 km far from Ouagadougou (12 220 N and 1310 W) [22] in Burkina Faso. The energy produced by the PV array is proportional to the global irradiation and the cosines effect. The data obtained from the meteorological station of Ouagadougou shows a mean annual solar irradiation of 5.5 kWh/m2 per day and the direct sunshine is over 3000 h per year [23]. The site selected for the experimentations, being at a few kilometers from Ouagadougou is then propitious to implementation and use of PV systems. Hybrid systems can be broadly classified in series, switched hybrid and parallel hybrid configurations [24]. The present prototype has a parallel hybrid configuration (Fig. 1). In this scheme, the PV system is parallel connected to diesel generator by means of a DC-AC converter or inverter. The PV and the diesel generators supply a portion of the load demand directly. The PV system is composed of 15 modules HIT (Heterojunction with Intrinsic Thin layer), each rated at 190 Wp, totalling 2.85 kWp DC at Standard Testing Conditions (irradiance ¼ 1000 W/m2; Solar cell operating temperature ¼ 25 C and AM ¼ 1.5). The PV array covers an area of about 18 m2 with PV modules distributed in a parallel/series configuration of three module strings connected to one inverter rated at 3.82 kW. The diesel generator rated power is 11.5 kVA (9.2 kW) of 1500 rpm. The Table 2 exposes the general specifications of the PV modules, the inverter and the diesel generator as well.
Fig. 1. Schematic diagram of the experimental set up.
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
the hourly fuel consumption (l/h) for supplying a given load during 1 h. According to Skarstein and Uhlen [20], the hourly fuel consumption can be approximated as follows:
PV modules Type Rated capacity (W) Maximum power voltage (V) Maximum power current (A) Open circuit voltage (V) Short circuit current (A) Module efficiency (%) Dimensions: L l D (mm mm mm) Weight (kg)
HIP-190BA3 190 54.8 3.47 67.5 3.75 16.1 1319 894 35 14
Inverter Type Maximum input power (W) Maximum input voltage (V) Maximum input current (A) Maximum output power (W) Nominal power (W) Nominal voltage (V) Nominal frequency (Hz) Maximum efficiency (%) Dimensions: L l D (mm mm mm) Weight (kg)
Sunny boy 3300 3820 500 20 3600 3300 220e240 50/60 95.2 450 352 230 41
Diesel generator Type Nominal Power (kWe/kVA) Velocity (rpm) Voltage (V) Maximum current (A) Dimensions: L l h (mm mm mm) Weight (kg)
T 12 K 9.2/11.5 1500 400/230 17 1405 715 1053 438
2.2. Experimental matrix In order to enhance the development of PV/diesel hybrid systems in off-grid areas, an extensive study must be carried out to check their feasibility, their efficiency and technical competitiveness. An important driving element of any power generating system is the load which is highly specific to the concerned application [25]. A very wide range of load profiles should be actually considered with respect to the local needs, cultural behavior, fuel and solar availability. This means that any general study over such a system should be done using first a set of hypothetic loads allowing the analysis of its performances and limitations. The consideration of typical loads in agreement with particular situations has to be done further in order to optimize the system. Therefore, in the present investigation, the load/demand profiles are considered as constants for the simulation of the hybrid system. Four load profiles have been considered (3.7 kW, 5.7 kW, 7.6 kW and 9.7 kW) corresponding to 40%, 62%, 82% and 105% of the diesel generator nominal power. The demand profiles are experimentally simulated by two resistive bank loads of 4 kW each with different switches allowing, by combination, to reach the desired load. Mix of interdependent key parameters such as: power generated by PV array (depending of solar radiation), power generated by diesel generator and specific/hourly consumption of diesel generator to meet the predefined load have been studied.
qðtÞ ¼ a:PðtÞ þ b:Pr
(1)
Where a(l/kW) and b(l/kW) are constants for a typical DG, Pt(kW) is the power generated by DG and Pr(kW), the rated/nominal power of the DG. As shown in Fig. 2, the relation (1) has been validated with the current fuel consumption data of a typical diesel generator of 9.2 kW. In fact, basing on these data, a similar linear model was fitted for the total fuel consumption as a function of the output power. The model coefficients obtained for these data are a ¼ 0:2476 l=kWh and b ¼ 0:07391 l=kWh with a coefficient of determination (R2) of 0.9895. Fig. 2 also shows the specific fuel consumption of a typical diesel generator of 9.2 kW as a function of its output power. It can be noted that when the DG operates near 20% of its rated power, its specific consumption is too high (about 0.64 l/kWh) corresponding to a low efficiency of the generator. The minimum specific fuel consumption of the DG is 0.324 l/kWh and corresponds to about 80% of its rated power meaning an important fuel saving; thus a good efficiency may be obtained if the DG operates close to this load fraction. This result corresponds to what stipulated by authors as Ashari et al. [12] and El-Hefnawi [26] saying that DGs have typically a maximum fuel efficiency of about 0.33 l/kW when run above 80% of its rated power (for the first authors) and when the optimum diesel generator operating range is 70e89% of its rated power (for the second authors). Later, the behavior of the diesel generator in the hybrid system is thoroughly analyzed. 3.2. Inverter characteristics Connection between the PV system and the diesel generator is done by the inverter which converts DC power generated by PV array to AC power. Inverter technology is therefore a key technology to have reliable and safe grid interconnection operation of PV system [27]. There are many types of inverters available today in the market and the parameter generally used to evaluate their functioning is the efficiency defined as the ratio of DC power
3.5
0.7 q(t) = 0.2476 P(t) + 0.6791 R² = 0.9895
3.0
0.6
2.5
0.5
2.0
0.4
1.5
0.3
1.0
0.2
0.5
0.1
3. Results and discussion 3.1. Diesel generator characteristics The characteristics of the pure diesel generator system were studied. A diesel generator (DG) is characterized by its efficiency and its fuel consumption (hourly and specific fuel consumptions). The specific fuel consumption (l/kWh) is defined as the fuel consumption required to produce 1 kWh of energy and it is equal to
0.0
0.0 0.0
2.0
4.0
6.0
8.0
10.0
Generated power (kW) Fig. 2. Fuel consumption of the diesel generator versus its output power.
Specific consumption (l/kWh)
Table 2 Equipment specifications.
Hourly consumption (l/h)
1782
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
1783
0.42
6
100
Generated PV power (kW) Generated DG power (kW)
90
Specific consumption (l/kWh)
80
0.41
0.39
60
4
Power (kW)
Inverter efficiency (%)
0.4
70
50 40
0.38
3
0.37
30
0.36
20
Specific consumption (l/kWh)
5
2 0.35
10 0.34
0
1
0
500
1000
1500
2000
2500 0.33
Ppv-DC (W) 0
Fig. 3. Inverter efficiency as a function of PV DC power.
generated by the PV array (PpvDC) and the AC power obtained by the inverter (PAC):
PAC PpvDC
Hours Fig. 5. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 62% of its nominal power.
(2)
Fig. 3 displays the efficiency of a typical single-phase inverter (sunny boy 3300) used for this study. One can notice that, for the low DC input power, up to almost 70% of the incoming energy to the inverter could be lost. However, for high DC power, the inverter efficiency could reach 95% meaning that only 5% of the incoming energy is lost. For the single-phase inverters the losses in inverter
can be about 8e20% of the total energy generated [26]. But the results obtained in this present work show that these losses can reach 70% for very low DC power coming from PV generator not close to the nominal power of inverter. This input DC power depends on the availability of the solar radiation and the load demand. One should therefore pay close attention to these parameters when sizing the system for optimizing the functioning of inverters.
0.37
8 4
Generated PV power (kW) Generated DG power (kW) Specific consumption (l/kWh)
0.52
7
0.44 0.42 0.4 0.38 1
0.35
Power (kW)
0.46
0.36
6
Specific consumption (l/kWh)
3
Power (kW)
0.36
0.5 0.48
2
Generated PV power (kW) Generated DG power (kW) Specific consumption (l/kWh)
5 0.35 4 0.34 3 0.34 2
Specific consumption (l/kWh)
hinv ¼
0.32
0.33
0.36 0.34 0.32
0
Hours Fig. 4. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 40% of its nominal power.
1
0.33
0
0.32
Hours Fig. 6. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 82% of its nominal power.
1784
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
10 9
Generated PV power (kW) Generated DG power (kW) Specific consumption (l/kWh)
0.40
1000
7
0.39
900
7
0.37
6
0.36
5
0.35
4
0.34
3
0.33
2
0.32
1
0.31
800 5
600
4
500 3
400 300
2
1 0
700
0.30
Generated DG Power (kW) Generated PV Power (kW) Generated Total Power (kW) Load (kW) Global solar radiation (W/m²)
0
Global solar radiation (W/m²)
0.38
Power (kW)
8
Specific consumption (l/kWh)
Power (kW)
6
200 100 0
Hours
Hours
Fig. 7. Diesel generator (DG) specific fuel consumption in the hybrid system for a load representing 105% of its nominal power.
Fig. 9. Generated total power by PV/diesel hybrid system for meeting a load representing 62% of the diesel generator nominal power.
3.3. Behavior of hybrid system under different loads In this section, the behavior of a typical hybrid PV/diesel system without storage whose characteristics is presented in the previous sections has been studied. The contribution of PV and diesel generators, specific consumption of the diesel generator for a given load (L) profile have been studied. As mentioned previously, we consider four constant load profiles (3.7 kW, 5.7 kW, 7.6 kW and 9.7 kW) corresponding respectively to 40%, 62%, 82% and 105% of the diesel generator nominal power. The behaviors of the PV and diesel generators in the hybrid system are interdependent. Indeed, a given load is supplied as well by the diesel generator as by the PV generator. It can be observed
4.5
that for loads in the order mentioned above, the specific fuel consumption of the DG ranges respectively from: 0.45e0.51 l/kWh (Fig. 4), 0.35e0.41 l/kWh (Fig. 5), 0.33e0.36 l/kWh (Fig. 6) and 0.33e0.34 l/kWh (Fig. 7). These results show that for small loads (less than 62% of the rated power of the DG), the contribution of PV generator (PAC/L) which ranges between 4 and 38% for a load values of 3.7 kW and 5.7 kW does not enable an optimal functioning of PV generator. In fact, as mentioned by Ashari et al. [12] and also verified by results obtained in the present investigation, the maximum
9
1100
1000
8
600
900
4
7
500
2.5 300 2 1.5
200
1 0.5
700
Power (kW)
Power (kW)
400
Global solar radiation (W/m²)
6
3
5
600
4
500
400 3
Global solar radiation (W/m²)
800
3.5
300 2
Generated DG Power (kW) Generated PV Power (kW) Generated Total power (kW) Load (kW) Global solar radiation (W/m²)
100
200 1
100
0
0
0
Hours Fig. 8. Generated total power by PV/diesel hybrid system for meeting a load representing 40% of the DG nominal power.
0
Hours
Fig. 10. Generated total power by PV/diesel hybrid system for meeting a load representing 82% of the diesel generator nominal power.
900
10
90
10 (Pt-L)/Pt 9
80 Ppv/Pac-max
800
8 8
7
700
Power (kW)
500 5 400 4
Global solar radiation (W/m²)
600
(P t-L)/P t(%)
60
7
6
70
6 50 5 40 4
Ppv/P ac-max (%)
9
30 3 20
2 1
10
0
0
300 3
200
Hours
2
1
Generated DG power (kW) Generated PV power (kW) Generated Total power (kW) Load (kW) Global solar radiation (W/m²)
Fig. 13. Gap in energy production for a load representing 62% of the diesel generator nominal power.
100
0
0
Hours
Fig. 11. Generated total power by PV/diesel hybrid system for meeting a load representing 105% of the diesel generator nominal power.
fuel efficiency of a diesel generator is about 0.33 l/kW, but for these two rated loads (3.7 and 5.7 kW), the specific fuel consumptions are far from this optimal value. This is due to the fact that these loads are already far from the optimal functioning point of the DG and
12
therefore the PV generator contributes rather to decrease the performance of the engine. However, for high loads (more than 82% of the rated power of the DG), the contribution of PV array (which ranges from 1.5% to 33% for a load value of 7.6 kW and from 2.5% to 22% for a load value of 9.7 kW) does not affect considerably the performance of the engine. As a matter of fact, for these loads, the values of specific fuel consumption vary between 0.33 and 0.36 l/ kWh which is close to the optimal functioning point of the DG. As the specific fuel consumption and then the operation cost of diesel generator increases considerably when the hybrid system operates below the load values representing less than 62% of the rated power of the DG, the design of the system must be done so that the diesel generator operates near its optimal point (70e80% of its nominal power).
60
5
100
(Pt-L)/Pt
(Pt-L)/Pt 10
Ppv/Pac max
Ppv/Pac max
4
90
50 80
3
30
4
20
(P t-L)/P t (%)
6
2 60
1
50
40
0
Ppv/P ac-max (%)
70
40
Ppv/P ac-max (%)
(P t-L) / Pt (%)
8
30
-1
2
20
10 -2
10
0
0
Hours Fig. 12. Gap in energy production for a load representing 40% of the diesel generator nominal power.
-3
0
Hours Fig. 14. Gap in energy production for a load representing 82% of the diesel generator nominal power.
1786
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
90
5 (Pt-L)/Pt 4
80
Ppv/Pac-max
3
70 60
1 50 0 40 -1 30
-2
Ppv/P ac-max (%)
(P t-L)/Pt (%)
2
20
-3 -4
10
-5
0
Hours Fig. 15. Gap in energy production for a load representing 105% of the diesel generator nominal power.
We also analyzed in this section particularly the influence of the solar radiation and load profile on the PV array production. From Fig. 8 to Fig. 11, it can be observed that the power supplied by the PV array through the inverter depends not only on the solar radiation but also on the load. In fact, one can see that for the close values of solar radiation, the production of PV generator is more important for the higher loads than for the lowers. This means that for an optimum production of the PV array, the ideal would be that the maximum solar radiation moments meet the peak load moments during a day. We also look closely if the total power generated (Pt) by PV and DG generators meets the load at any time of the day. For the lower loads 3.7 kW (Fig. 8) and 5.7 kW (Fig. 9), representing respectively 40 and 62% of the rated power of the DG, the total generated power meets at any time the demand. But for the higher loads (7.6 kW (see Fig. 10) and 9.7 kW (see Fig. 11) representing
60 105 % of diesel generator nominal Power 82% of diesel generator nominal Power
50
62% of diesel generator nominal Power 40 % of diesel generator nominal Power
Ppv/L (%)
40
30
20
10
respectively 82 and 105% of the rated power of the DG), it can be noted that at certain moments of the day, the demand is not met by the total power generated. For a better estimation of this situation which mentioned above, we have studied the parameter ðPtPLÞ t permits to exactly see the gap between the total energy production and the load. One can observe that for the lower loads, there is an excess of energy varying between 4 and 11% for a load value of 3.7 kW (Fig. 12) and between 1 and 10% for a load value of 5.7 kW (Fig. 13). However for the load values more than 82% of the rated power of DG (Figs. 14 and 15), one can observe, at certain moments of the day, a deficit in energy production. This is more considerably in the case of a load value of 9.7 kW (105% of rated power of DG) where we have a deficit within 0.9e3% (see Fig. 15) and this practically during all the day. In what follows, we have looked if the PV system feeds in real time to the mini-grid all the power produced. Hence, the parameter PPV PACmax has been defined, where PPV is the power feeds in the grid and PACmax the total power generated by the PV array. Through this parameter (see Fig. 16), one can confirm that the contribution of PV array depends also on the load/demand. Indeed, for high loads the power feeds by PV array in the mini-grid is more important. Towards these results, one can say that the sizing of a hybrid system must be done so that the nominal power of DG is equal to the peak load. Also, only the DG should run when the load does not exceed a certain threshold (20% of its nominal power) because as stipulated by Hochmuth [9], running the diesel generator with a low capacity factor increases fuel consumption and wear, and then the operation and maintenance costs. The use of dummy loads (for pumping or heating water) could also be a solution to absorb the excess output power and keep the engine loading high. The use of multiple generator sets that can be run in parallel is one of the solutions which may contribute to enhance the overall system efficiency. 3.4. Penetration level in a PV/diesel hybrid system In a PV/diesel hybrid system, the PV penetration level is defined as the ratio of maximum PV array power (kWp) to the rated power of the diesel generator (kW). But such as defined, this ratio doesn’t give clear information about the behavior of the system. Therefore, the ratio of the real output power of the PV system on the maximum load PLPV is a best indication about the contribution of the PV array for a given load. The Fig. 16 shows the ratioPLPV as function of the solar radiation for different loads. It can be observed that for lower loads (3.7 kW and 5.7 kW, corresponding to 40 and 62% of the DG nominal power), the ratio PLPV ranges respectively from 2 to 49% and from 2 to 33% (for a solar radiation value varying between 150 and 950 W/m2). However for higher loads (7.6 kW and 9.7 kW, corresponding to 82 and 105% of the DG nominal power), it is noticed that the ratio PLPV varies respectively from 3 to 26%, and from 2 to 21% for the same values of solar radiation. Fig. 16 gives also a good indication about possible contribution levels of PV array for which PV/diesel hybrid system may be relevant, depending on load and solar radiation. Indeed, it is noticed here a high PV contribution for lower loads but this is not a synonym of a good performance because as mentioned in a previous section the best performance of PV array is obtained for higher loads. Also, this high PV penetration for lower loads does not favor an optimal functioning of the diesel generator.
0 0
200
400
600
800
1000
Global solar radiation (W/m²) Fig. 16. Contribution of PV generator for a given load versus solar radiation.
4. Conclusion All the results presented in this paper have been obtained taking four different daily constant loads to simulate the behavior
D. Yamegueu et al. / Renewable Energy 36 (2011) 1780e1787
of a typical PV/diesel hybrid system without storage. Experimental results show that the contribution of PV generator for a given load affects the performance of the diesel generator. As a matter of fact, when the hybrid system operates under small loads (less than 62% of the diesel generator rated power), the PV generator contributes rather to decrease the performance of diesel generator. A specific fuel consumption ranging from 0.35 to 0.51 l/kWh is obtained for loads varying from 40 to 62% of the diesel generator rated power. This fuel consumption shows a bad performance for the diesel generator. However, it has been shown that for higher loads (82 and 105% of the DG rated power), the specific fuel consumption of DG ranges from 0.33 to 0.36 l/kWh which is a good performance. Therefore the design of PV/diesel hybrid system must be done so that the diesel generator operates near its optimal point (70e80% of its nominal power). At this level, the use of dummy loads (for pumping or heating water) to absorb the excess output power and multiple diesel generators running in parallel could be a solution to enhance the system efficiency. It has been also observed a deficit in energy production for load higher than 100% of rated power of diesel generator which could mean that for a reliability of a PV/diesel hybrid system, the rated power of diesel generator should be equal to the peak load. It has been verified through this study that the functioning of a PV/diesel hybrid system is efficient for higher loads and higher solar radiations. This study being conducted with constant loads, it will be of a great interest for future investigation to study experimentally the behavior of the system under variable loads. Acknowledgments This work has been supported by the International Institute for Water and Environmental Engineering (2iE) of Ouagadougou (Burkina Faso) and the National Center of Scientific Research (CNRS) of France through the program “doctorate grant for national engineers of developing countries” (BDI-PED). References [1] Nfaha EM, Ngundamb JM, Tchinda R. Modelling of solar/diesel/battery hybrid power systems for far-north Cameroon. Renewable Energy 2007;32:832e44. [2] Rabah KVO. Integrated solar energy systems for rural electrification in Kenya. Renewable Energy 2005;30:23e42. [3] World energy outlook-WEO. The Electricity Access Database, http://www.iea. org/weo/electricity.asp; 2009. [4] Barley CD. Optimal dispatch strategy in remote hybrid power systems. Solar Energy 1996;58(4e6):165e79. [5] Deutsche Gesellschaft fur Technische Zusammenarbeit-GTZ. Regional reports on renewable energies. 30 Country Analyses on Potentials and Markets in
[6] [7]
[8] [9] [10]
[11]
[12]
[13]
[14]
[15]
[16] [17] [18]
[19] [20] [21]
[22]
[23]
[24] [25]
[26] [27]
1787
West Africa (17), East Africa (5) and Central Asia (8). Frankfurt/Eschborn. Energy policy framework papers; 2009. Section “Energy and transport”. Elhadidy MA, Shaahid SM. Parametric study of hybrid (windþsolarþdiesel) power generating systems. Renewable Energy 2000;21(2):129e39. Notton G, Muselli M, Louche A. Autonomous hybrid photovoltaic power plant using a back-up generator: a case study in a Mediterranean Island. Renewable Energy 1996;7(4):371e91. Deshmukha MK, Deshmukh SS. Modeling of hybrid renewable energy systems. Renew Sustain Energ Rev 2008;12:235e49. Hochmuth GCS. A combined optimisation concept for the design and operation strategy of hybrid-PV energy systems. Solar Energy 1997;61(2):77e87. Shaahid SM, El-Amink I. Techno-economic evaluation of off-grid hybrid photovoltaic-dieselebattery power systems for rural electrification in Saudi Arabia-a way forward for sustainable development. Renew Sustain Energ Rev 2009;13:625e33. Papadopoulos DP, Maltas EZ. Design operation and economic analysis of autonomous hybrid pvediesel power systems including battery storage. J Electr Electron Eng Aust 2010;61(1):3e10. Ashari M, Nayar CV. An optimum dispatch strategy using set points for a photovoltaic (pv)edieselebattery hybrid power system. Solar Energy 1999;66(1):1e9. Ruther R., Martins D., Bazzo E. Hyrid Diesel/Photovoltaic systems without storage for isolated mini-grids sin Northem Brazil. In: Proceeding of the 28th IEEE Photovoltaic Specialists Conference. Anchorage, USA 2000:1567e70. Koutroulis E, Kolokotsa D, Potirakis, Kalaitzakis K. Methodology for optimal sizing of stand-alone photovoltaic/wind-generator systems using genetic algorithms. Solar Energy 2006;80:1072e88. Muselli M, Notton G, Poggi P, Louche A. PV-hybrid power systems sizing incorporating battery storage: an analysis via simulation calculations. Renewable Energy 2000;20:1e7. Musseli M, Notton G, Louche A. Design of hybrid-photovoltaic power generator, with optimization of energy management. Solar Energy 1999;65(3):143e57. Lopez RD, Agustin JLB. Design and control strategies of PV-Diesel systems using genetic algorithms Rodolfo. Solar Energy 2005;79:33e46. Notton G, Musseli M, Poggi P, Louche A. Autonomous photovoltaic systems influences of some parameters on the sizing: simulation time step, input and output power profil. Renewable Energy 1996;1(4):353e69. Yang H, Lu L, Zhou W. A novel optimization sizing model for hybrid solarwind power generation system. Solar Energy 2007;2007(81):76e84. Skarstein O, Uhlen K. Design considerations with respect to long-term diesel saving in wind/diesel plants. Wind Eng 1989;13:72e87. Ajan CW, Ahmed SS, Ahmad HB, Taha F, Zin AABM. On the policy of photovoltaic and diesel generation mix for an off-grid site: East Malaysian perspectives. Solar Energy 2003;74:453e67. Borrmann S, Kunkel D, Weigel R, Minikin A, Deshler T, Wilson JC, et al. Aerosols in the tropical and subtropical UT/LS: in-situ measurements of submicron particle abundance and volatility. Atmos Chem Phys 2010;10:5573e92. Azoumah Y, Ramdé EW, Tapsoba G, Thiam S. Siting guidelines for concentrating solar power plants in the Sahel:Case study of Burkina Faso. Solar Energy 2010;84:1545e53. Nayar CV. Recent developments in decentralised mini-grid diesel power systems in Australia. Appl Energy 1995;52:229e42. Elhadidy MA, Shaahid SM. Promoting applications of hybrid (windþphotovoltaic þdieselþbattery) power systems in hot regions. Renewable Energy 2003;29:517e28. El-Hefnawi SH. Photovoltaic dieselegenerator hybrid power system sizing. Renewable Energy 1998;13(1):33e40. Elthawi MA, Zhao Z. Grid-connected photovoltaic power systems: technical and potential problems-a review. Renew Sustain Energ Rev 2010;14:112e29.