PV E LECTRIFICATION IN O FF -G RID A REAS

C ONTENTS 1.Introduction 2.Stand-alone PV systems PV generator Power conditioning Energy storage: batteries 3.Design of stand-alone PV systems Optimal orientation and tilt angles Energy generated by a PV system Sizing of a stand-alone PV system

1. I NTRODUCTION Energy is one of the most important available resources Access to energy is not universal. 1,300 million people (18% of global population) lack access to electricity 2,700 million people (40% of global population) rely on burning biomass for cooking Photovoltaics offer major advantages for rural electrification Modularity: efficiency and cost does not depend on system size. Autonomy: sunlight is widely available. Low maintenance: PV components are highly reliable and neither fuel nor supplies are needed on a regular basis. Non pollutant: clean technology whose operation has no impact on the environment.

2. S TAND -A LONE PV S YSTEMS Two main categories of PV systems: Grid-connected PV systems Stand-alone PV systems

2. S TAND -A LONE PV S YSTEMS Most common configurations for stand-alone PV systems: DC bus. AC bus.

2.1. PV G ENERATOR The PV generator converts light into electricity. Characterized by its current-voltage curve (IV curve). The IV curve is affected by the environmental conditions. The maximum power point is the pair of current and voltage values that maximizes energy generation.

2.2. P OWER C ONDITIONING The power conditioning elements adjust the energy delivered by the PV generator to voltage and current levels suitable for the rest of the system. Charge regulator. DC/DC converter. Regulates the charge and discharge of the battery. Inverter. DC/AC converter. Bidirectional inverter. Charges and discharges the battery in an AC bus system. Power conditioning elements must be chosen according to: Configuration of the system. Maximum power of the system. Voltage and current levels.

2.3. E NERGY S TORAGE : B ATTERIES Batteries provide stored surplus electricity for use during night or when irradiation levels are low. Wide use of lead acid-batteries in stand-alone PV systems. Automotive batteries of Starting, Lightening and Ignition (SLI) batteries. Advantages: low cost, widely available, locally produced and easy to recycle. Drawbacks: short lifetime. Deep cycle batteries. Advantages: long lifetime. Drawbacks: high cost and low availability in developing countries. Batteries are characterized by their state of charge (SOC): Indicates energy available on the battery. To avoid damage, the SOC must never be lower than a value specified by the manufacturer.

3. D ESIGN OF S TAND -A LONE PV S YSTEMS A stand-alone system must assure that the loads receive enough electricity throughout the year. Worst month. Design process: Determine the optimal surface. Estimate energy generation. Determine the size of the PV system.

3.1. O PTIMAL O RIENTATION AND T ILT A NGLES Orientation (α): angle between the projection of line perpendicular to the generator and the South. Optimal value: facing South in the Northern Hemisphere and North in the Southern Hemisphere. Tilt (β): angle between the PV generator and the horizontal. Optimal value: complementary angle of Sun’s elevation at noon during the central day of the worst month. Sun’s elevation at noon: φ: latitude δ: solar declination

3.2. E NERGY G ENERATED BY A PV S YSTEM Energy generated by a PV system (E PV ): G t (α,β): solar irradiation over PV generator. P nG : nominal power of the PV generator at Standard Test Conditions (STC). PR: performance ratio. Measure of the overall efficiency of the PV system. Typical values: between 0,6 and 0,7. L SH : shading losses. Irradiation losses due to nearby obstacles casting shadows over the PV generator. Typical values: between 0,2 and 0,25.

3. S IZING OF A S TAND -A LONE PV S YSTEMS The PV generator and the batteries must be given appropriate values: The PV generator nominal power must assure that the loads receive enough electricity during the least sunny month. The battery capacity must assure that the loads remain powered when the levels of solar irradiation are low or zero (full cast days and at night). The minimum recommended SOC of the battery must be taken into account.