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Performance parameters of a standalone PV plant
Energy Conversion and Management 86 (2014) 490–495
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance parameters of a standalone PV plant Amine El Fathi ⇑, Lahcen Nkhaili, Amin Bennouna, Abdelkader Outzourhit LPSCM, Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco
a r t i c l e
i n f o
Article history: Received 19 February 2014 Accepted 14 May 2014
Keywords: PV plant Performance ratio Droop mode Yields Isolated grid
a b s t r a c t In this work we present a detailed description of a 7.2 kWp photovoltaic power plant installed in the remote rural village Elkaria (province of Essaouira in Morocco). This plant supplies 16 households with electricity through a local grid that was installed for this purpose. The results of monitoring some performance parameters of the plant such as load curve, the yields and the performance ratio are presented and discussed. The performance ratio of the PV plant varied between 33% and 70.2%. The low values of this parameter are mainly attributed to the way the battery inverter manages the energy flow. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The world energy demand is steadily increasing with an average of 1.4% per year from 2010 to 2040 [1]. Consequently, in order to meet this demand and to mitigate the resulting green house gas emissions (GHG) and the resulting global warming, it is necessary to increasingly use renewable energy sources like wind, solar and hydro [2–4]. Particularly, renewable energy sources are a good alternative for remote areas where utility grid extension is either unfeasible or more expensive, and where the cost of fuel increases with the remoteness of the location [5–9]. In order to reduce the effects of the intermittence of the renewable sources on the reliability of the supply and the power quality, it is usually necessary to use storage devices and to combine two or more of these sources. These hybrid renewable energy systems (HRES) offer interesting advantages over single source systems such as reliability of the supply [10,11], reduction of the size of the storage system and increased life time of the battery bank [12]. The architecture of stand-alone hybrid renewable energy systems has improved in the last years to modularly coupled systems where the loads and generators are coupled on the AC bus [5]. Such architecture offers significant advantages over systems based on centralized inverters, because they are expandable, they offer higher reliability, they can run either in islanding mode or be connected to the grid. The coupling of generation technologies on the AC bus through appropriate inverters/converters, also offers the
⇑ Corresponding author. Tel.: +212 673187108 fax: +212 5 24 437410. E-mail addresses: [email protected] (A. El Fathi), [email protected] (L. Nkhaili), [email protected] (A. Bennouna), [email protected] (A. Outzourhit). http://dx.doi.org/10.1016/j.enconman.2014.05.045 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
possibility of placing the generators and storage systems in different locations (distributed systems) [13–16]. Several research works have been carried out on the performance and characteristics of these systems [14,17,18]. However, in the case of PV plants, the performance parameters are usually reported for grid connected plants where all the produced energy is fed to the grid [19–22]. The performance ratio is, in this case, a function of only the losses in the system (conversion losses, cable losses. . .). There are, however, few reports on the performance parameters of standalone modular PV plants where a proper balance should be established between the demand and the production. In this particular case, the energy management can have an effect on the performance ratio of the PV plant. This is one of the main objectives of the present work. A stand-alone hybrid PV-wind plant was designed to supply the Elkaria village in Essaouira (Morocco) with electricity. The results of monitoring the PV plant and some indices such as power consumption; the yields and the performance ratio of the PV plant are reported. Particularly, the effect of the energy management on the performance ratio is investigated and discussed. 2. Description of the PV-power plant The hybrid PV-wind plant is located in a coastal region in the middle of Morocco between the cities of Essaouira and Safi at a latitude of 31.52°N and a longitude of 9.27°W. This region is known for its interesting irradiance levels and an average annual wind speed between 7 and 8 m/s [23]. An overall view of the power plant is given in Fig. 1, which shows the wind turbine, the PV panels mounted on the roof of a house as well as the local grid. Only the results concerning the PV-plant are reported below.
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Fig. 1. Outside view of the power plant.
Table 1 Electrical parameters of the PV panels measured at standard test conditions (STC: irradiance of 1000 W/m2, air mass 1.5 and cell temperature 25 °C). Peak power Rated voltage Rated current Open circuit voltage Short circuit current
Pmax Vmp Imp Voc Isc
225 W 41.0 V 5.49 A 48.5 V 5.87 A
2.1. The plant configuration The PV field is composed of 32 Sunpower SPR-225-WHT panels. Each panel consists of 72 single crystalline silicon cells with back contact technology. Their nominal power is 225 Wp and their efficiency is 18.1%. The other relevant characteristics of the panels are summarized in Table 1. The panels were mounted on galvanized steel supports at an optimum inclination angle of 35° for this location. This optimal value is usually with ±10° of the latitude of the site. The photovoltaic array is divided into four strings of 8 panels each mounted in series. As shown in Fig. 2, each two independent strings are connected in parallel to a PV-inverter (Sunny Boy 3800, SMA, Germany) with a rated power of 3.8 kW. The compatibility of this configuration with the inverters was checked by the Sunny Design software (SMA, Germany). The two inverters are then connected to the local isolated grid than runs through the village (about 2.5 km long) powered by the grid forming unit (Fig. 2). The later consists of two Sunny Island SI5040 bidirectional inverters (SMA, Germany)
and a 1100 Ah battery bank. The rated input DC voltage of the SI5040 is 48 V while its rated power is 5 kW. The DC sides of these inverters are connected to the battery bank which consists of a series combination of 24 batteries (2 V each) with a C100 nominal capacity of 1100 Ah suggesting that each battery can deliver a nominal current of 11 A during 100 h before the end of discharge voltage is reached. The local grid control is performed by the Sunny Island battery inverter (BI) using the droop mode control as will be described below [24,25]. The BI acts as both a battery charge controller and an inverter. The other relevant characteristics of the inverters are summarized in Table 2. 2.2. Data acquisition and monitoring A Sunny Boy control plus data logger (SMA, Germany) was used to record the data provided by the various instruments. The recorded parameters provide information about the power levels (fed to the grid and consumed by the households), AC/DC currents and voltages in various parts of the plant, local grid frequency
Table 2 Some characteristic of the inverters.
Rated power Nominal output voltage Nominal output current Maximum efficiency
Sunny Island (BI)
Sunny boy 3800
5000 W 230 V 21 A 95.6%
3800 W 230 V 16.5 A 95%
Fig. 2. The block diagram of the PV-power plant (the arrows indicate the direction of energy flow).
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Fig. 3. Inside view of the power plant.
as well as the metrological data (wind speed, irradiance, ambient and panel temperatures). The metrological data are acquired using the Sunny Sensor box (SMA, Germany). The data is recorded with a five minutes time step and saved on a daily basis. The data logger communicates with the various instruments through an RS485 cable and interface. An inside view of the power plant is illustrated in Fig. 3. 3. Results and discussions 3.1. Irradiance and output power Fig. 4 shows the evolution of the active power output of one of the two PV inverters and the in-plane irradiance measured on the surface of the panels for a sunny day (noted D1). It can be seen that the produced power which is effectively fed to the local grid follows the changes in the irradiance values. Fig. 5 shows the PV-inverter output power and the incident irradiance for a cloudy day (noted D2). As in the previous case, the power follows the instantaneous changes in the irradiance. This is not the case for the data in Fig. 6 acquired for another sunny day (D3). In this case, a sharp drop is observed in the inverter output power around
Fig. 5. Inverter power output and in plane irradiance for a sunny day (D2).
Fig. 6. Inverter power output and in plane irradiance for a sunny day (D3).
Fig. 4. Inverter power output and in plane irradiance for a sunny day (D1).
11h30 which is flowed by some power fluctuations despite the high and smooth irradiance. A similar trend is observed in Fig. 7 for another cloudy day (D4) where a drop in the output power is observed in this case after 14 h. These figures represent the four different trends that were observed for this system.
A. El Fathi et al. / Energy Conversion and Management 86 (2014) 490–495
493
Fig. 7. Inverter power output and in plane irradiance for a cloudy day (D4). Fig. 9. Typical measured load curve of the village.
The observed drop in the PV-inverter output power, despite the high levels of irradiance, is due to the way the battery inverter manages the local grid. As we will see below, the PV inverters are instructed (through changes in the grid frequency) to reduce their output power when the state of charge (SOC) of the battery is high and the demand from the households is low during daytime. 3.2. Energy flow and battery state of charge Fig. 8 shows the power flow of the bidirectional (battery) inverter (BI) and the state of charge (SOC) of the battery. During the day, the power flow is negative indicating that the BI is charging the battery bank from the local grid (power produced by the PV array). As a consequence, the SOC of the battery increases until it reaches a maximum of 90%. During the night the energy flow from the battery inverter is positive indicating that the BI is feeding the local grid with energy from the battery bank; as a consequence the SOC of the battery decreases. This is also the case when the produced energy is not enough to satisfy the demand (cloudy days, low irradiance levels. . .). It can also be seen in Fig. 8 that the SOC of the battery continues to decrease even after midnight showing that some households are equipped with devices that work even after midnight (refrigerators. . .).
although some smaller peaks are observed around 8 h and around noon. It should be noted that the peak power consumption has steadily increased since the installation of the system. It increased from 1400 W to about 4000 W. This suggests that the households have acquired new electrical appliances that consume more energy (mainly TVs and refrigerators). 3.4. Energy management of the plant The energy management of the plant is performed by the Sunny Island battery inverter (BI) using the droop mode control [26]. In this mode, the BI varies the grid frequency depending on its active power P. This is illustrated in Fig. 10 which presents the power fed to the grid by one of the PV-inverters (Fig. 10a), the village load curve (Fig. 10b), the battery bank SOC (Fig. 10c) and the voltage frequency (Fig. 10d) for the same day. As it can be seen in this figure, the battery inverter increases the grid frequency if more
3.3. Village load curve The measured daily load curve of the village is shown in Fig. 9. Most of the consumption occurs after 18 h (lights and TVs are on)
Fig. 8. Evolution of the state of charge (SOC) of the battery bank and of the power of the battery inverter.
Fig. 10. (a) Injected power, (b) village consumption, (c) SOC of the battery bank and (d) voltage frequency (Hz) for the same day.
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renewable energy is available than the load and the batteries are fully charged (Fig. 10d). This occurred between 13 h and 16 h for this day. As a consequence, the PV inverters reduce their output power in response to this increased frequency (Fig. 10a). The bidirectional inverter decreases the grid frequency if less renewable energy is available than the load and the batteries are not fully charged. In response to this situation the inverters raise their output power as can be clearly see in Fig. 10a after 14 h and in Figs. 7 and 8 around 16 h. This is also responsible for the fluctuations in the output power of the PV inverters despite the smooth variations of the irradiation as can be clearly seen for example in Fig. 7 between 14 h and 16 h.
3.5. Performance parameters of the plant From the recorded data, the performance parameters of the PV plant, specified by the IEC standard 61724 [27], can be evaluated. The reference yield (YR) is defined as the total in-plane solar irradiance Ht (kWh/m2) divided by the array reference irradiance of 1 kW/m2. This parameter corresponds to the average number of peak sun hours and defines then the solar radiation resource available for the PV plant. The array yield YA (kWh/kWp or in hours), on the other hand, is defined as the daily, monthly or annual energy output of the PV array divided by the peak power of the installed PV array at standard test conditions (STC). This parameter represents the number of hours that the PV array would need to operate at its rated power to provide the measured (DC) energy. The final yield YF (kWh/kWp or in hours) is defined as the net annual, monthly or daily energy output of the system (ac in our case) divided by the peak power of the installed PV array at standard test conditions (STC). The array yield and the final yield are convenient parameters to compare the energy produced by PV arrays and PV plants of different sizes since the energy produced is normalized with respect to the system size. Finally, the performance ratio (PR) is the ratio of the final yield (YF) and the reference yield (YR). It can be interpreted as the actual produced (fed to the local grid in our case) PV energy divided by the expected theoretical production. This parameter can inform on the overall losses in the system which can be subdivided into two types: (1) the array capture losses (LC = YR YF), due to the PV array losses (temperature, reflection. . .); and (2) the system losses (LS = YA YF), due to the inverter and cable losses. Based on the definitions given above, the parameters for the specific days presented above are summarized in Table 3. The values of the reference yield are in good agreement with the widely accepted values for the Essaouira region for these specific days [28]. The total energy fed to the local grid is the highest for day D1 for which all the produced power is used to charge the batteries and to supply the households. The maximum performance ratio (PR), which is on the order of 70.2%, is obtained for day D1, for which the injected energy and the final yield are the highest, despite the fact that the reference yield of this day is lower than that of the sunny day (D3). In addition, the losses for this day are the lowest as it can be seen in Table 3. On the other hand, the PR of the cloudy day D2 is higher than that of the cloudy day D4 despite the fact that the reference yield (number of peak sun Table 3 Injected energy, yields and loss parameters for the specific days. E (kW h) YR (h) YA (h) YF (h) LS (h) LC (h) LC + LS (h) PR (%) D1 D2 D3 D4
(Fig. (Fig. (Fig. (Fig.
5) 6) 7) 8)
30.26 16.28 15.94 24.90
5.98 3.39 6.60 6.30
4.98 5.64 10.98 10.38
4.2 2.27 2.21 3.45
0.78 3.37 8.77 6.93
1 1.18 4.39 2.85
1.78 4.55 13.16 9.78
70.2 66.7 33.5 55.4
Fig. 11. Measured efficiency curve of the PV inverter.
hours), the injected energy and the final yield are lower for this day. The minimum PR of about 33.5% is observed for the sunny day D3, despite the fact that the reference yield for this day is the highest compared with the other specific three days. This is because the energy injected into the grid and consequently the final yield is the lowest for this day. The lower values of the PR for days D3 and D4 are mainly due to the sharp drop in the power fed to the grid (Figs. 6 and 7) as a result of the way the energy flow from the various inverters is managed by the bidirectional inverter. This drop started when the SOC of the battery has almost reached its maximum value (Fig. 10a and c). At this stage, the BI increased the frequency of the AC voltage (Fig. 10d) and the PV inverter started to reduce its AC power output as a consequence (Fig. 10a). As we can see in Table 3, the lowest value of the losses, described by the parameter (LC + LS) is observed for day D1 which explains the highest value of the PR for this day. Day D3, on the other hand, has the highest losses (LC + LS) which explains the lower value of the PR for this day. These losses represent the energy that is cut out by the control strategy and which is not used by the system. Fig. 11 shows the measured efficiency of one of the PV inverters as a function of the inverter power. The analysis of the results shows that a maximum efficiency of 95% is reached when the output power is greater than 400 W. If the inverters operate below this value, the efficiency drops sharply and can highly affect the PR of the plant especially in cloudy days and low irradiance levels. From these observations, we conclude that for this local grid control mode, the PR of the plant depends not only on the losses of the system (array, inverts, wires. . .) but also on the state of charge of the battery bank (SOC) and the energy demand of the households. 4. Conclusions We have described a PV-plant designed to supply a remote rural village with electricity. The performance parameters of the plant were evaluated from the monitored data. It is observed that the yields and consequently the performance ratio of the plants depend strongly on the energy demand as well as on the state of charge of the battery. This a consequence of the method used to control the energy flow in the local grid. The performance ratio of the plant varied between 33.5% and 70.2%. The lower limit (33.5%) is relatively small compared to values usually reported for grid-connected PV plants (70% to 75%). Low energy demand during the day and the high state of charge of the battery bank
A. El Fathi et al. / Energy Conversion and Management 86 (2014) 490–495
are mainly responsible for the low values of the performance ratio. On the other hand the upper limit (70.2%) is very close to those usually reported for grid-connected PV plants where only system losses are responsible for the drop in the performance ratio. This parameter will be further improved by using all the available PV energy through the use of a dump load such as water pump.
[14]
[15]
[16]
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Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance parameters of a standalone PV plant Amine El Fathi ⇑, Lahcen Nkhaili, Amin Bennouna, Abdelkader Outzourhit LPSCM, Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco
a r t i c l e
i n f o
Article history: Received 19 February 2014 Accepted 14 May 2014
Keywords: PV plant Performance ratio Droop mode Yields Isolated grid
a b s t r a c t In this work we present a detailed description of a 7.2 kWp photovoltaic power plant installed in the remote rural village Elkaria (province of Essaouira in Morocco). This plant supplies 16 households with electricity through a local grid that was installed for this purpose. The results of monitoring some performance parameters of the plant such as load curve, the yields and the performance ratio are presented and discussed. The performance ratio of the PV plant varied between 33% and 70.2%. The low values of this parameter are mainly attributed to the way the battery inverter manages the energy flow. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The world energy demand is steadily increasing with an average of 1.4% per year from 2010 to 2040 [1]. Consequently, in order to meet this demand and to mitigate the resulting green house gas emissions (GHG) and the resulting global warming, it is necessary to increasingly use renewable energy sources like wind, solar and hydro [2–4]. Particularly, renewable energy sources are a good alternative for remote areas where utility grid extension is either unfeasible or more expensive, and where the cost of fuel increases with the remoteness of the location [5–9]. In order to reduce the effects of the intermittence of the renewable sources on the reliability of the supply and the power quality, it is usually necessary to use storage devices and to combine two or more of these sources. These hybrid renewable energy systems (HRES) offer interesting advantages over single source systems such as reliability of the supply [10,11], reduction of the size of the storage system and increased life time of the battery bank [12]. The architecture of stand-alone hybrid renewable energy systems has improved in the last years to modularly coupled systems where the loads and generators are coupled on the AC bus [5]. Such architecture offers significant advantages over systems based on centralized inverters, because they are expandable, they offer higher reliability, they can run either in islanding mode or be connected to the grid. The coupling of generation technologies on the AC bus through appropriate inverters/converters, also offers the
⇑ Corresponding author. Tel.: +212 673187108 fax: +212 5 24 437410. E-mail addresses: [email protected] (A. El Fathi), [email protected] (L. Nkhaili), [email protected] (A. Bennouna), [email protected] (A. Outzourhit). http://dx.doi.org/10.1016/j.enconman.2014.05.045 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.
possibility of placing the generators and storage systems in different locations (distributed systems) [13–16]. Several research works have been carried out on the performance and characteristics of these systems [14,17,18]. However, in the case of PV plants, the performance parameters are usually reported for grid connected plants where all the produced energy is fed to the grid [19–22]. The performance ratio is, in this case, a function of only the losses in the system (conversion losses, cable losses. . .). There are, however, few reports on the performance parameters of standalone modular PV plants where a proper balance should be established between the demand and the production. In this particular case, the energy management can have an effect on the performance ratio of the PV plant. This is one of the main objectives of the present work. A stand-alone hybrid PV-wind plant was designed to supply the Elkaria village in Essaouira (Morocco) with electricity. The results of monitoring the PV plant and some indices such as power consumption; the yields and the performance ratio of the PV plant are reported. Particularly, the effect of the energy management on the performance ratio is investigated and discussed. 2. Description of the PV-power plant The hybrid PV-wind plant is located in a coastal region in the middle of Morocco between the cities of Essaouira and Safi at a latitude of 31.52°N and a longitude of 9.27°W. This region is known for its interesting irradiance levels and an average annual wind speed between 7 and 8 m/s [23]. An overall view of the power plant is given in Fig. 1, which shows the wind turbine, the PV panels mounted on the roof of a house as well as the local grid. Only the results concerning the PV-plant are reported below.
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A. El Fathi et al. / Energy Conversion and Management 86 (2014) 490–495
Fig. 1. Outside view of the power plant.
Table 1 Electrical parameters of the PV panels measured at standard test conditions (STC: irradiance of 1000 W/m2, air mass 1.5 and cell temperature 25 °C). Peak power Rated voltage Rated current Open circuit voltage Short circuit current
Pmax Vmp Imp Voc Isc
225 W 41.0 V 5.49 A 48.5 V 5.87 A
2.1. The plant configuration The PV field is composed of 32 Sunpower SPR-225-WHT panels. Each panel consists of 72 single crystalline silicon cells with back contact technology. Their nominal power is 225 Wp and their efficiency is 18.1%. The other relevant characteristics of the panels are summarized in Table 1. The panels were mounted on galvanized steel supports at an optimum inclination angle of 35° for this location. This optimal value is usually with ±10° of the latitude of the site. The photovoltaic array is divided into four strings of 8 panels each mounted in series. As shown in Fig. 2, each two independent strings are connected in parallel to a PV-inverter (Sunny Boy 3800, SMA, Germany) with a rated power of 3.8 kW. The compatibility of this configuration with the inverters was checked by the Sunny Design software (SMA, Germany). The two inverters are then connected to the local isolated grid than runs through the village (about 2.5 km long) powered by the grid forming unit (Fig. 2). The later consists of two Sunny Island SI5040 bidirectional inverters (SMA, Germany)
and a 1100 Ah battery bank. The rated input DC voltage of the SI5040 is 48 V while its rated power is 5 kW. The DC sides of these inverters are connected to the battery bank which consists of a series combination of 24 batteries (2 V each) with a C100 nominal capacity of 1100 Ah suggesting that each battery can deliver a nominal current of 11 A during 100 h before the end of discharge voltage is reached. The local grid control is performed by the Sunny Island battery inverter (BI) using the droop mode control as will be described below [24,25]. The BI acts as both a battery charge controller and an inverter. The other relevant characteristics of the inverters are summarized in Table 2. 2.2. Data acquisition and monitoring A Sunny Boy control plus data logger (SMA, Germany) was used to record the data provided by the various instruments. The recorded parameters provide information about the power levels (fed to the grid and consumed by the households), AC/DC currents and voltages in various parts of the plant, local grid frequency
Table 2 Some characteristic of the inverters.
Rated power Nominal output voltage Nominal output current Maximum efficiency
Sunny Island (BI)
Sunny boy 3800
5000 W 230 V 21 A 95.6%
3800 W 230 V 16.5 A 95%
Fig. 2. The block diagram of the PV-power plant (the arrows indicate the direction of energy flow).
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A. El Fathi et al. / Energy Conversion and Management 86 (2014) 490–495
Fig. 3. Inside view of the power plant.
as well as the metrological data (wind speed, irradiance, ambient and panel temperatures). The metrological data are acquired using the Sunny Sensor box (SMA, Germany). The data is recorded with a five minutes time step and saved on a daily basis. The data logger communicates with the various instruments through an RS485 cable and interface. An inside view of the power plant is illustrated in Fig. 3. 3. Results and discussions 3.1. Irradiance and output power Fig. 4 shows the evolution of the active power output of one of the two PV inverters and the in-plane irradiance measured on the surface of the panels for a sunny day (noted D1). It can be seen that the produced power which is effectively fed to the local grid follows the changes in the irradiance values. Fig. 5 shows the PV-inverter output power and the incident irradiance for a cloudy day (noted D2). As in the previous case, the power follows the instantaneous changes in the irradiance. This is not the case for the data in Fig. 6 acquired for another sunny day (D3). In this case, a sharp drop is observed in the inverter output power around
Fig. 5. Inverter power output and in plane irradiance for a sunny day (D2).
Fig. 6. Inverter power output and in plane irradiance for a sunny day (D3).
Fig. 4. Inverter power output and in plane irradiance for a sunny day (D1).
11h30 which is flowed by some power fluctuations despite the high and smooth irradiance. A similar trend is observed in Fig. 7 for another cloudy day (D4) where a drop in the output power is observed in this case after 14 h. These figures represent the four different trends that were observed for this system.
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Fig. 7. Inverter power output and in plane irradiance for a cloudy day (D4). Fig. 9. Typical measured load curve of the village.
The observed drop in the PV-inverter output power, despite the high levels of irradiance, is due to the way the battery inverter manages the local grid. As we will see below, the PV inverters are instructed (through changes in the grid frequency) to reduce their output power when the state of charge (SOC) of the battery is high and the demand from the households is low during daytime. 3.2. Energy flow and battery state of charge Fig. 8 shows the power flow of the bidirectional (battery) inverter (BI) and the state of charge (SOC) of the battery. During the day, the power flow is negative indicating that the BI is charging the battery bank from the local grid (power produced by the PV array). As a consequence, the SOC of the battery increases until it reaches a maximum of 90%. During the night the energy flow from the battery inverter is positive indicating that the BI is feeding the local grid with energy from the battery bank; as a consequence the SOC of the battery decreases. This is also the case when the produced energy is not enough to satisfy the demand (cloudy days, low irradiance levels. . .). It can also be seen in Fig. 8 that the SOC of the battery continues to decrease even after midnight showing that some households are equipped with devices that work even after midnight (refrigerators. . .).
although some smaller peaks are observed around 8 h and around noon. It should be noted that the peak power consumption has steadily increased since the installation of the system. It increased from 1400 W to about 4000 W. This suggests that the households have acquired new electrical appliances that consume more energy (mainly TVs and refrigerators). 3.4. Energy management of the plant The energy management of the plant is performed by the Sunny Island battery inverter (BI) using the droop mode control [26]. In this mode, the BI varies the grid frequency depending on its active power P. This is illustrated in Fig. 10 which presents the power fed to the grid by one of the PV-inverters (Fig. 10a), the village load curve (Fig. 10b), the battery bank SOC (Fig. 10c) and the voltage frequency (Fig. 10d) for the same day. As it can be seen in this figure, the battery inverter increases the grid frequency if more
3.3. Village load curve The measured daily load curve of the village is shown in Fig. 9. Most of the consumption occurs after 18 h (lights and TVs are on)
Fig. 8. Evolution of the state of charge (SOC) of the battery bank and of the power of the battery inverter.
Fig. 10. (a) Injected power, (b) village consumption, (c) SOC of the battery bank and (d) voltage frequency (Hz) for the same day.
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renewable energy is available than the load and the batteries are fully charged (Fig. 10d). This occurred between 13 h and 16 h for this day. As a consequence, the PV inverters reduce their output power in response to this increased frequency (Fig. 10a). The bidirectional inverter decreases the grid frequency if less renewable energy is available than the load and the batteries are not fully charged. In response to this situation the inverters raise their output power as can be clearly see in Fig. 10a after 14 h and in Figs. 7 and 8 around 16 h. This is also responsible for the fluctuations in the output power of the PV inverters despite the smooth variations of the irradiation as can be clearly seen for example in Fig. 7 between 14 h and 16 h.
3.5. Performance parameters of the plant From the recorded data, the performance parameters of the PV plant, specified by the IEC standard 61724 [27], can be evaluated. The reference yield (YR) is defined as the total in-plane solar irradiance Ht (kWh/m2) divided by the array reference irradiance of 1 kW/m2. This parameter corresponds to the average number of peak sun hours and defines then the solar radiation resource available for the PV plant. The array yield YA (kWh/kWp or in hours), on the other hand, is defined as the daily, monthly or annual energy output of the PV array divided by the peak power of the installed PV array at standard test conditions (STC). This parameter represents the number of hours that the PV array would need to operate at its rated power to provide the measured (DC) energy. The final yield YF (kWh/kWp or in hours) is defined as the net annual, monthly or daily energy output of the system (ac in our case) divided by the peak power of the installed PV array at standard test conditions (STC). The array yield and the final yield are convenient parameters to compare the energy produced by PV arrays and PV plants of different sizes since the energy produced is normalized with respect to the system size. Finally, the performance ratio (PR) is the ratio of the final yield (YF) and the reference yield (YR). It can be interpreted as the actual produced (fed to the local grid in our case) PV energy divided by the expected theoretical production. This parameter can inform on the overall losses in the system which can be subdivided into two types: (1) the array capture losses (LC = YR YF), due to the PV array losses (temperature, reflection. . .); and (2) the system losses (LS = YA YF), due to the inverter and cable losses. Based on the definitions given above, the parameters for the specific days presented above are summarized in Table 3. The values of the reference yield are in good agreement with the widely accepted values for the Essaouira region for these specific days [28]. The total energy fed to the local grid is the highest for day D1 for which all the produced power is used to charge the batteries and to supply the households. The maximum performance ratio (PR), which is on the order of 70.2%, is obtained for day D1, for which the injected energy and the final yield are the highest, despite the fact that the reference yield of this day is lower than that of the sunny day (D3). In addition, the losses for this day are the lowest as it can be seen in Table 3. On the other hand, the PR of the cloudy day D2 is higher than that of the cloudy day D4 despite the fact that the reference yield (number of peak sun Table 3 Injected energy, yields and loss parameters for the specific days. E (kW h) YR (h) YA (h) YF (h) LS (h) LC (h) LC + LS (h) PR (%) D1 D2 D3 D4
(Fig. (Fig. (Fig. (Fig.
5) 6) 7) 8)
30.26 16.28 15.94 24.90
5.98 3.39 6.60 6.30
4.98 5.64 10.98 10.38
4.2 2.27 2.21 3.45
0.78 3.37 8.77 6.93
1 1.18 4.39 2.85
1.78 4.55 13.16 9.78
70.2 66.7 33.5 55.4
Fig. 11. Measured efficiency curve of the PV inverter.
hours), the injected energy and the final yield are lower for this day. The minimum PR of about 33.5% is observed for the sunny day D3, despite the fact that the reference yield for this day is the highest compared with the other specific three days. This is because the energy injected into the grid and consequently the final yield is the lowest for this day. The lower values of the PR for days D3 and D4 are mainly due to the sharp drop in the power fed to the grid (Figs. 6 and 7) as a result of the way the energy flow from the various inverters is managed by the bidirectional inverter. This drop started when the SOC of the battery has almost reached its maximum value (Fig. 10a and c). At this stage, the BI increased the frequency of the AC voltage (Fig. 10d) and the PV inverter started to reduce its AC power output as a consequence (Fig. 10a). As we can see in Table 3, the lowest value of the losses, described by the parameter (LC + LS) is observed for day D1 which explains the highest value of the PR for this day. Day D3, on the other hand, has the highest losses (LC + LS) which explains the lower value of the PR for this day. These losses represent the energy that is cut out by the control strategy and which is not used by the system. Fig. 11 shows the measured efficiency of one of the PV inverters as a function of the inverter power. The analysis of the results shows that a maximum efficiency of 95% is reached when the output power is greater than 400 W. If the inverters operate below this value, the efficiency drops sharply and can highly affect the PR of the plant especially in cloudy days and low irradiance levels. From these observations, we conclude that for this local grid control mode, the PR of the plant depends not only on the losses of the system (array, inverts, wires. . .) but also on the state of charge of the battery bank (SOC) and the energy demand of the households. 4. Conclusions We have described a PV-plant designed to supply a remote rural village with electricity. The performance parameters of the plant were evaluated from the monitored data. It is observed that the yields and consequently the performance ratio of the plants depend strongly on the energy demand as well as on the state of charge of the battery. This a consequence of the method used to control the energy flow in the local grid. The performance ratio of the plant varied between 33.5% and 70.2%. The lower limit (33.5%) is relatively small compared to values usually reported for grid-connected PV plants (70% to 75%). Low energy demand during the day and the high state of charge of the battery bank
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are mainly responsible for the low values of the performance ratio. On the other hand the upper limit (70.2%) is very close to those usually reported for grid-connected PV plants where only system losses are responsible for the drop in the performance ratio. This parameter will be further improved by using all the available PV energy through the use of a dump load such as water pump.
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