EFFECT OF ENERGY MANAGEMENT ON THE ...

29th European Photovoltaic Solar Energy Conference and Exhibition

EFFECT OF ENERGY MANAGEMENT ON THE PERFORMANCE RATIO OF A STANDALONE PHOTOVOLTAIC PLANT.

Amine El Fathi, Amin Bennouna and Abdelkader Outzourhit LPSCM, Department of Physics, Faculty of Sciences Semlalia, Cadi Ayyad University BP 2390, Marrakech Morocco

ABSTRACT: In this paper we will present and discuss the results of monitoring the performance parameters of a modular 7.2 kWp standalone photovoltaic (PV) plant. The plant was installed in Lkaria village (Essaouira, Morocco) and it supplies 16 households via a local isolated mini-grid. The load curve of the village, the reference yield, the final yield and the PR (PR) of the plant were monitored. In addition, the voltage levels, the currents in the different parts of the plant and the grid frequency were followed in order to understand the effect of the grid management on the PR of the plant. The PR of the PV plant was found to vary between 33% and 70 %. The low values of this parameter are observed when the state of charge of the battery bank is high and the energy demand is low. In this case the battery inverter increases the grid frequency and consequently the inverters of the plant reduce their output power which is fed to the local grid. Keywords: Standalone PV Power plant, performance ratio, energy management.

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INTRODUCTION

reported. Particularly, the effect of the energy management and the SOC of the batteries on the PR is investigated and discussed.

The energy demand has been increased over last years, this is due to population and industrial growth. The increase in energy demand requires the use of more fossil fuels which leads to global warming. And to electrify remote areas, a significant investment is required. To solve this problems the attention was drawn to sources with alternative and renewable energies such as solar, wind and hydro. Hybrid renewable energy systems have the advantage of solving the problem of intermittency generated by a single source of renewable energy. Another component which is necessary to increase the reliability of the system is the battery bank (storage system) which becomes less expensive with the use of multiple sources of renewable energy. Several research works have been carried out to perform the architecture of standalone hybrid renewable hybrid systems in the last years [1], there are two basic topologies that describe a hybrid system and the possibilities of interconnections of these components. The first one is DC-coupled system where all generators are coupled on a DC bus and it is also a centralized hybrid system. The other architecture is the AC-coupled system where generators and storage systems can be placed in different locations (distributed systems) and they are feeding the user grid from several points which offer significant advantages over systems based on centralized inverters (DC-coupled system) because they are theoretically unlimited expandability, they offer higher reliability, they can run either in islanding mode or be connected to the grid [2]. The PR in the case of grid connected plants (where all the produced energy is fed to the grid) is only a function of the losses in the system (conversion losses, cable losses...).however, in the case of standalone modular PV plants where an appropriate balance should be established between the produced energy and the load demand, the energy management and the state of charge (SOC) of the batteries have an effect on the PR of the PV plant. This is one of the main objectives of the present work. A stand-alone hybrid PVwind 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 PR of the PV plant are

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DESCRIPTION

2.1 Description of Elkaria Village Elkaria village is located in a coastal area between the cities of Essaouira and Safi, this region is known for its favorable conditions to establish a hybrid renewable energy system due to the interesting levels of irradiance and a satisfactory annual wind speed. 2.2 Description of the PV power plant The PV plant consists of 32 Sunpower SPR-225WHT panels covering 40.6 m², the most relevant characteristics of these panels are shown in table 1. The panels were mounted on galvanized steel supports at an optimum inclination angle of 35°. As shown in Fig.1, the photovoltaic field is divided into four strings of 8 panels each mounted in series, 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 two PV-inverters are then connected to the local isolated grid (2.5 km long). The grid forming unit consists of two Sunny Island SI5040 bidirectional inverters (SMA, Germany) and an 1100 Ah battery bank. The rated power of the SI5040 is 5 kW while the input DC voltage is 48 V. The DC sides of these inverters are connected to the battery bank (Hawker Pb TYS7/2AT 1101 Ah) which consists of a series combination of 24 batteries (2 V each) with a C100 nominal capacity of 1100 Ah. The local grid control is performed by the Sunny Island battery inverter using the droop mode control as will be presented below [2]. According to the energy demand and the sate of charge of the battery and the energy production the battery inverter can acts as both a battery charge controller or as an inverter. The Sunny Boy control plus data logger (SMA, Germany) was used to record the metrological data (wind speed, irradiance, ambient and panel temperatures) acquired using the Sunny Sensor box (SMA, Germany). Other relevant parameters are also recorded in the extent to provide information about the power levels (delivered and consumed), AC/DC voltages and currents in diverse

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parts of the plant, another important parameter which is also recorded is the local grid frequency, this later helps to bring out the grid control as will be showed below. The data is recorded 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.2.

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The energy management is one of the most important research issues carried out over last years, in the case of standalone hybrid systems the control strategies must be robust and reliable in the extent to ensure an efficient supply of energy to the user grid. D. Giaouris et al [3] employs generic and flexible decision making models to develop complex power/hydrogen management strategies and investigate their performance on systems integrating multiple power generation and storage devices. A.T.D. Perera et al [4] present the dispatch strategy in their research work to design a standalone hybrid energy systems minimizing initial investment, life cycle cost and pollutant emission. Authors of [5-7] have presented in their works research the droop mode control, which is used in the case of Elkaria village to perform the energy management of the HRES. A brief overview is presented below. 3.1 Droop mode control In this control strategy for AC-Coupled hybrid system, the grid frequency and voltage were determined by the battery inverter and it’s requires communication between inverters (the inverters use the AC-grid frequency for communication). The concept of the droop mode control is based on active power frequency-statics (P/f) and reactive power/voltage-statics (Q/v) [O ]. If the available renewable energy exceeds the load demand and the batteries are fully charged, so the battery inverter increases the net frequency, in this case the PV inverters reduce their output power. If the load demand exceeds the available renewable energy and the batteries are not fully charged, then the frequency is reduced, accordingly the PV inverters increase their output power.

Table I: Relevant parameters of the PV panels Measured at Standard Test Conditions (STC).

Solar cells m Efficiency Peak power Rated voltage Rated current Open circuit voltage Short circuit current

ENERGY MANAGEMENT

72 SunPower all back-contact monocrystalline 18.1% 225 W 41 V 5.49 A 48.5 V 5.87 A

4 THE PERFORMANCE PARAMETERS OF THE PLANT The performance of a PV power plant is useaully evaluated on the basis of some parameters such as the final yield (YF), the reference yield (YR), the PR as defined by the IEC Standard 61724 [8]. These parameters are evaluated on daily, monthly of yearly basis. The final yield is defined as the annual, monthly or daily net AC energy output of the system divided by the peak power of the installed PV array at standard test conditions (STC) of 1000 W/m² solar irradiance and 25 °C cell temperature. YR is the total in-plane solar insolation Ht (kWh/m2) divided by the array reference irradiance of 1 kW/m2, thus, the YR is the number of peak sun-hours. The PR is defined as the final yield divided by the reference yield and it can inform on the total losses in the system.

Fig.1 The block diagram of the PV-power plant

5 RESULTS AND DISCUSSIONS 5.1 Battery state of charge and energy flow The energy flow in the PV power plant depends on the available renewable energy affected by the meteorological conditions and the load demand. Fig. 3 shows the state of charge (SOC) of the battery bank and the power flow of the bidirectional inverter. When the power flow is negative, the BI is charging the battery bank from the produced PV energy, therefore the SOC of the battery increases. During the night the BI is feeding the local grid with energy from the battery bank, the SOC of the battery

Fig.2 :Inside view of the PV power plant.

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decreases, in this case the power flow from the battery inverter is positive. The maximum value of the SOC reaches for this day is 95% between 13h and 18h.

70% in the case of Elkaria village PV power plant. In the second case (fig. 6) the PR decreased to a very low value of 33%.

5.2 Village load curve A typical daily electrical power consumption of the village is illustrated in Fig. 4. I can be seen that the demand is low during the day while the peak consumption occurs at night (between 18h and 23h).

Fig.5. PV-Inverter output power and in plane irradiance for the day D1

Fig.3. Evolution of the state of charge (SOC) of the battery bank and the power of the bidirectional inverter

Fig.6. PV-Inverter output power and in plane irradiance the day D2

In order to shade more light of this behavior, the state of the charge of the battery and the grid frequency are reported in figure Figs. 7 and 8 for both days. For the first case (D1), the grid frequency is constant (50 Hz), the state of charge increased from a minimum of 22% to maximum of 90% and then decreases again. In the second case however (day D2), the frequency is lower than 50 HZ when the battery is not fully charged and then around 12:00 pm, the state of charge of the battery reached 95% and the frequency jumped to 51.5 Hz, thus signaling to the PV inverters to reduce their output power accordingly despite the availability of irradiance at this period. This explains why the injected power doesn’t follow the irradiance shape (Fig. 6) and the PR of the plant is lower for this day since a large part of the available solar energy is not exploited.

Fig.4. Power consumption of Elkaria village. 5.3 Effect of the SOC on the PR The injected power in the local grid and the available irradiance are the most important elements to understand the effect of the energy management and the SOC on the PR. Figs. 5 and 6 show the injected power and the irradiance for two different days (D1 and D2). The injected power follows the irradiance in the first case (Fig.5) while in the second case, and despite the availability of solar energy between 12h and 18h, the injected power doesn’t follow the irradiance. In fact a drop in the power fed to the local grid is observed after 12:00 pm In the first case, the PR (calculated based on the definitions given above) reached its maximum value of

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[5] K. De Brabandere, B.Bolsens, J. Van den Keybus, A. Woyte, J. Driesen and R.Belmans K.U.Leuven, 2004 35th Annual IEEE Power Electronics Specialists Conference. [6] Chia-Tse Lee, Chia-Chi Chu, Po-Tai Cheng, A New Droop Control Method for the Autonomous Operation of Distributed Energy Resource Interface Converters. [7] Panahandeh Bahram, Bard Jochen, Outzourhit Abdelkader, Zejli Driss, Int J Hydrogen Energy 2011;36:4185–97. [8] Britich Standard. Photovoltaic system performance monitoring-Guidelines for measurement, data exhange and analysis, BS EN 61724;1998. Fig.7. The state of charge of the battery bank and the frequency of the AC grid for the day D1

Fig.7. The state of charge of the battery bank and the frequency of the AC grid for the day D2

Conclusion In this paper a standalone PV power plant installed to supply a remote area by electricity is described. From the recorded data it’s showed that the energy management and the state of charge of the battery bank affect the PR of the PV power plant where a part of the energy can be wasted despite the availability of irradiance unlike the grid connected PV power plants. Due to this method used to manage the energy flow, the lower PR reached is 33%, and the maximum PR reached in the case of Elkaria village is 70%. References [1] Kenfack J, Neirac FP, Tatietse TT, Mayer D, Fogue M, Lejeunec A, Renewable Energy 2009;34:2259–63. [2] Panahandeh Bahram, Bard Jochen, Outzourhit Abdelkader, Zejli Driss. IntJ Hydrogen Energy 2011;36:4185–97. [3] Damian Giaouris, Athanasios I. Papadopoulos, Chrysovalantou Ziogou, Dimitris Ipsakis, Spyros Voutetakis, Simira Papadopoulou, Panos Seferlis, Fotis Stergiopoulos , Costas Elmasides, Energy 61 (2013) 621635 [4] A.T.D. Perera, R.A. Attalage*, K.K.C.K. Perera, V.P.C. Dassanayake, Energy 54 (2013) 220-230.

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