INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

A Micro PV Power Plant ― Practical Case Study S. Saraiva, R. Melício, J.P.S. Catalão, J.C.O. Matias, C. Cabrita UBI – Universidade da Beira Interior e-mail contact: catalao@ubi.pt

Conference Topic – CT9 – Energy Abstract This paper focuses on supplying the technical data of a micro photovoltaic (PV) power plant located in Covilhã, Portugal. This is an area of the Portugal with an excellent solar resource. It is followed by a description of the plant and the equipment that make up the main system. The PV system is fully described, and the main results regarding energy production and economic return are duly presented.

Key Words: photovoltaic power plant; energy production; economic return 1. Introduction A photovoltaic (PV) system directly converts solar energy into electric energy. The main device of a PV system is the solar cell. Cells may be grouped to form arrays and panels. A PV array may be either a panel or a set of panels connected in series or parallel to form large PV systems. Power-electronic converters, battery and charge controller are usually required to process the electricity from the PV device [1]. All PV panels use silicon as the base material, mainly as monocrystalline or multicrystalline cells, but more recently also in amorphous form. At the front of the panel electrical contact is made by a metallic grid; at the back, contact usually covers the whole surface. An anti-reflective coating is applied to the front surface. The modules in a PV array are usually first connected in series to obtain the desired voltage; the individual strings are then connected in parallel to allow the system to produce more current. They are then protected by encapsulation between glass and a tough metal, plastic or fibreglass back. This is held together by a stainless steel or aluminium frame to form a module. Modules may be connected in series or parallel to increase the voltage and current, and thus achieve the required solar array characteristics that will match the load. Typical module size is 50Wp and produces direct current electricity at 12V (for battery charging for example) [1,2]. Usually a PV panels has a duration of 20 years and the maintenance is generally small, needing only to keep the panels relatively clean and make sure trees don't begin to overshadow them [3]. The electrical energy produced by a PV system depends on its properties and on the incoming solar radiation [4], known as irradiation. The energy produced by a PV system during the day, which wasn’t consumed by loads, is stored in batteries. Stored energy can be used at night or during the days with bad weather conditions. Batteries in PV systems are often charged/discharged; therefore, they must meet stronger requirements than car batteries. There are many solar battery types available in the market. The most often used are classic Pb acid batteries produced especially for PV systems, where deep discharge is required. Other battery types, such as NiCd or NiMH are rarely used, unless in portable devices. Hermetical batteries often consist of electrolyte in gel form. Such batteries do not require maintenance. Typical solar system batteries lifetime spans from three to five years, depending heavily on charging/discharging cycles, temperature and other parameters. The more often the battery is charged/discharged the shorter the lifetime will be.

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

Power-electronic converters are usually required to process the electricity from the PV device. Power-electronic converters may be used to regulate the voltage and current at the load, to control the power flow in grid-connected systems and for implementing the maximum power point tracking (MPPT) on PV systems [6]. The typical configuration of a PV system [1] is shown in Fig. 1.

Figure 1 – Typical configuration of a PV system [1]

The total cost of a PV system can be divided in: 1) investment costs; 2) maintenance costs; 3) replacement costs. Investment costs are the costs for the initial purchase, transportation and installation of equipment, accounting for 70 to 75% of the system cost over its useful life (between 15 and 20 years) [5]. The PV system maintenance costs occur during the lifespan of the system, representing between 3 to 5% of the total system cost. Replacement costs occur when the batteries reach the end of their useful life, representing between 20 to 27% of the total system cost throughout their life [5]. The installing cost of a PV system is relatively high, but the maintenance cost is very low. To meet the installing costs, funding mechanisms can be sought, such as credit or rental. 2. Case study This paper focuses on supplying the technical data of a micro PV power plant located in Covilhã, Portugal. This is an area in Portugal with an excellent solar resource. The micro PV power plant has its legal framework in the micro energy production program, applicable to small power plants, designated by micro energy production, whose legal status is established by Decree-Law No. 363/2007 of November 2. The micro PV power plant registry is dated of July 2009, and has an installed rated power of 3.68 kW, under the subsidized tariff. The installation site is interconnected to the EDP grid, which is assigned a delivery point of consumption and also a delivery point code of the producer, with a single-phase electrical contracted power of 10.35 kVA. The PV arrays of the micro PV power plant are shown in Fig. 2. The energy delivered to the grid by the micro PV power plant is purchased by EDP at 0.6175 €/kWh. The micro PV power plant integrates PV modules of the 72 PV cells, 24 PV modules, and the installed rated power is the 3.68 kW. These modules are interconnected in four arrays. The modules are connected in series to get a voltage increase and in parallel to get a current increase, which in turn are connected to the inverter. The PV arrays are mounted on the front of the residence. The power center, the inverter, the other balance of system components, the control and data acquisition system are located outside, near the PV panels.

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

Figure 2 – The PV arrays of the micro PV power plant installed in Covilhã

3. Evaluating PV components

3.1 PV modules

PV modules correspond to the energy generating portion of the system and currently constitute roughly half of the system cost. Table 1 summarizes the data for the silicon monocrystalline PV modules Chaori CRM-180 [7].

Table 1 - PV module data for Chaori CRM-180

Parameter PV module Rated power Pmax 180 W

Tolerance +2 % Voltage at Pmax (Vmp) 35 V Current at Pmax (Imp) 5.15 A

Open circuit voltage (Voc) 43 V Short circuit current (Isc) 5.58 A Maximum system voltage 1000 V

Nominal efficiency 14.1 % Temperature coefficient of Voc -0.146 %/ºK Temperature coefficient of Isc 4.7 %/ºK

Temperature coefficient of Pmax -0.39 %/ºK NOCT 45 ºC

Solar cells 72 Cell dimensions 125×125 mm

Module dimensions 1580×808×46 mm Weight 15 kg

Construction Front cover: high transmission 3,2mm±0,1mm tempered, ≥91 % anti-reflective coated glassRear: white polyester; encapsulany

EVA

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

The I-V characteristics of the Chaori CRM-180 module are shown in Fig. 3, while the P-V characteristics are shown in Fig. 4.

0 10 20 30 40 500

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Figure 3 – The I-V characteristics for the Chaori CRM-180 module

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Figure 4 – The P-V characteristics for the Chaori CRM-180 module

3.2 PV Inverter

The PV arrays generate direct current (DC). Since most household appliances use alternating current (AC), an inverter is used to convert the DC voltage to AC voltage, matching the frequency and voltage of the local electrical grid. The inverters for PV applications include control functions to optimize the power output, which is referred to as maximum power point tracking (MPPT). The PV inverter used in the micro PV power plant is a SMA Sunny Boy. The PV inverter [8] used in the micro PV power plant installed in Covilhã, Portugal, is shown in Fig. 5. The PV inverter is located in the structure of the PV arrays, under the PV modules, as shown in Fig. 6. Table 2 summarizes the data for the inverter SMA Sunny Boy [8].

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

Figure 5 – The PV inverter used in the micro PV power plant [8]

Figure 6 – The PV inverter is located in the structure of the PV arrays, under of the PV modules

Table 2 - PV inverter SMA Sunny Boy

Parameter PV inverter Max. DC power 3900 W Max. DC voltage 500 V

MPP voltage range 200 V – 400 V Max. input current/per string 20 A/16 A

Number of MPP trackers/strings per MPP tracker 1/3

AC nominal power 3680 W Max. AC apparent power 3680 W

Max. output current 16 A Nominal AC voltage; range 220, 230, 240 V; 180 V – 265 V AC grid frequency; range 50, 60 Hz; ± 4.5 Hz

Power factor (cos ϕ) 1 Phase conductors / connection phases 1/1

Max. efficiency / Euro-eta 95.6 %/94.7 %

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

3.3 Energy metering The energy injected in the grid by the micro PV power plant is accounted for by a three phase meter equipped with GSM modem for telemetry measurement. The energy meter used in the micro PV power plant is an A1800Alpha meter. The energy meter [9] used in the micro PV power plant installed in Covilhã, Portugal, is shown in Fig. 7.

Figure 7 – The energy meter [9]

3.4 Charge controller and MPPT

The charge controller’s primary function is to protect the battery bank from overcharging. It does this by monitoring the battery bank. When the bank is fully charged, the controller interrupts the flow of electrical energy from the PV power plant. Batteries are expensive and to maximize their life span the charge controller avoids their overcharging or undercharging. The charge controller with maximum power point tracking (MPPT) incorporated, used in the micro PV power plant, is an AKA controller. The charge controller [10] used in the micro PV power plant installed in Covilhã, Portugal, is shown in Fig. 8.

Figure 8 – The charge controller with maximum power point tracking incorporated [10]

3.5 Batteries

The batteries bank lifetime depends on charge/discharge cycle rates numbers. The deeper the battery bank is discharged the shorter the lifetime will be. The most important battery parameter is battery capacity, which is measured in Ah. Battery capacity depends on discharging current; the higher the discharging current the lower the capacity, and vice versa. The charging characteristics are recommended and prescribed by different standards.

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

The solar batteries prices are higher than the prices of classic car batteries, yet their advantages are longer lifetime and lower discharging rates. Consequently, the maintenance costs of the photovoltaic systems are lower. The batteries used in the micro PV power plant are of the type PVX-1040T. The PVX-1040T battery is a common 12 volt renewable energy battery for solar energy storage. The battery [11] used in the micro PV power plant installed in Covilhã, Portugal, is shown in Fig. 9.

Figure 9 – The battery [11]

3.6 Electrical drawing

The electrical drawing of the micro PV power plant installed in Covilhã, Portugal, is shown in Fig. 10.

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House loadsBatteries

PV modules

NSurge arrestor

Figure 10 – The electrical drawing of the micro PV power plant

4. Monitoring of the micro PV power plant Monitoring and control of PV systems is essential for a reliable performance and maximum yield. The energy generated by the PV power plant from January to October 2010 is shown in Fig. 11.

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

Figure 11 – The energy generated by the PV power plant from January to October 2010

In sum, given that the PV power plant had its beginning on January 19, 2010, the data presented relate only to the first nine months of production, specifically, between 19 January and 19 October 2010. The total cumulative production is 6518 kWh, corresponding to a total of almost € 4025. The total number of sun hours was 3488 for a total of 273 days of production. Comparing with the estimated values by the own brand, it is estimated that the annual production is 7666 kWh. Table 3 summarizes the real data for the micro PV power plant.

Table 3- Real data for the micro PV power plant

Total accumulated energy 6518 kWh Total days of service 273 Total hours of service 3488

Average monthly of energy generated 724 kWh Average daily energy generated 23.88 kWh Average hours of service daily 12.78

Return of accumulated production 4024,87 € Return of the monthly production 447,07 €

Return daily production 14,75 € The photovoltaic installation represents an investment of around € 23,000. So we can say that on average there will be a return of € 5,000 a year, knowing that in the first five years of operation the rate of sale of kWh will remain unchanged. Hence, we can conclude that the initial investment will be amortized over five years, which, in the current economic climate, represents an investment considered safe, made available to any small investor. References (1) Technical information online [Online].Available: http://practicalaction.org/practicalanswers/product_info.php?products_id=194. (2) Photovoltaics [Online].Available: http://en.wikipedia.org/wiki/Photovoltaics. (3) Solar electricity [Online].Available: http://www.energysavingtrust.org.uk/Generate-your-own-energy/Solar-electricity. (4) Eltawil, M. A.; Zhao, Z.: ''Grid-connected photovoltaic power systems: Technical and potential problems—A review'' Renewable and Sustainable Energy Reviews, Vol. 14 (2010), pp. 112-129.

INTERNATIONAL CONFERENCE ON ENGINEERING UBI2011 - 28-30 Nov 2011 – University of Beira Interior – Covilhã, Portugal

(5) Batteries [Online].Available: http://www.pvresources.com/en/battery.php. (6) Villalva, M. G.; Gazoli, J. R.; Filho, E. R.: ''Comprehensive approach to modeling and simulation of photovoltaic arrays'' IEEE Transactions on Power Electronics, Vol. 24 n º 5 (2009), pp. 1198-1208. (7) Posharp [Online].Available: http://www.posharp.com/crm-180s-mono-solar-panel-from-chaori-solar-energy_p1952094443d.aspx. (8) SMA Solar Technology [Online].Available: http://www.sma.de/en/products/solar-inverters/sunny-boy/sunny-boy-3300-3800.html. (9) A1800Alpha meter [Online].Available: http://www.elstermetering.com/en/925.html. (10) Charge controller [Online].Available: http://homepower.com/basics/solar/. (11) PVX-1040T Solar Battery [Online].Available: http://www.sunxtender.com/solarbattery.php?id=8