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close this bookSolar Energy. Lessons from the Pacific Island Experience (World Bank, 1994)
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View the documentForeword
View the documentAbstract
View the documentPreface
View the documentAbbreviations and Acronyms
close this folderExecutive summary
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View the documentCurrent institutional approaches for introducing solar PV systems
View the documentSolar PV and diesel system costs compared
View the documentOrganization of the paper
close this folder1. Introduction
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View the documentThe energy sector in the Pacific Islands
View the documentRural electrification
View the documentFailure of the conventional approach
View the documentPotential of solar photovoltaic systems
close this folder2. Solar energy in the Pacific Islands
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View the documentHistory and prospects
View the documentInstitutional approaches for introducing solar PV systems
View the documentThe Tuvalu experience
View the documentLessons learned
close this folder3. Solar PV and diesel systems compared
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View the documentDesign considerations
View the documentGeneral cost comparison of solar PV and diesel systems
View the documentSolar PV and diesel costs in the pacific islands
View the documentSensitivity analysis and other considerations
View the document4. Conclusions
close this folderAnnex 1. Economic conditions in the Pacific Islands
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View the documentGeneral features
View the documentTuvalu
close this folderAnnex 2 Technical details of solar photovoltaic systems
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View the documentComponents of solar PV systems
View the documentDetermining the size of the system and its components
close this folderAnnex 3 Life-cycle costs of solar PV and diesel systems in rural electrification
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View the documentCost categories
View the documentThree cases
View the documentDesign and costs of solar PV systems
View the documentDetails of cost calculations: solar pv systems
View the documentDesign and costs of diesel systems
View the documentDetails of cost calculations: Diesel systems

Components of solar PV systems

The appliances often used in rural areas—such as lights, TV/VCR, and radios— require relatively small amounts of electrical energy that can be provided by PV systems. PV panels sometimes may be used to power appliances directly, but since most consumers want electric power to be available at all times rather than just when the sun is shining, most PV panels are used to charge storage batteries, which then can provide power to the appliances at any time. PV systems used for rural electrification typically consist of the following components:

· Solar photovoltaic panels
· Storage batteries
· Battery controllers
· Wiring, fuses, and switches
· Appliances.

Solar Photovoltaic Panels

Solar PV panels produce electricity in amounts directly proportional to the amount of sunshine falling on the panel's surface and on the size of the panel (i.e., the area exposed to the sun).

A PV panel is made up of a number of cells. Individual silicon PV cells, no matter how large, produce an output of about 0.5 Volts when exposed to sunlight. In order to generate an output sufficient to charge a 12 Volt (V) battery, many cells (usually 33 to 36) have to be connected in series to form a panel whose output is rated to exceed the voltage of the battery. If a 24 V battery is used, then two PV panels are connected together in series to produce the necessary voltage.

PV panels usually are rated in peak Watt (Wp) output. It is important to realize that this rating is useful mainly for comparing relative sizes of panels; in practice, many factors—such as the type of load connected and the intensity of sunlight—will affect the panel's actual power output in Watts. The peak Watt rating may be considered the effective maximum power that a panel can produce under ideal conditions. Further, since domestic rural electrification PV systems have the appliances connected to a storage battery and not directly to the panels, the peak Watt capacity of the panels has no relationship to the maximum Watts that can be delivered to appliances in a solarpowered home. Thus, a PV system with a 50 Wp panel could be used as the power source for an appliance, such as an electric iron or a film projector, with a power demand of 1,500 Watts.

Solar panels and conventional diesel-powered generators have very different generation characteristics. For example, PV panels may be continuously short circuited without damage, whereas this would destroy a rotary generator by overheating it. Further, a change in the load resistance causes the voltage of PV panels to change without significant changes in the current produced, whereas a change in the load resistance connected to a rotary generator causes significant changes in the current produced but not in the voltage. These technical differences mean that persons familiar with conventional electrical systems but without specialized training in PV technology can often make serious errors in the electrical design or maintenance of PV systems.

Storage Batteries

Electrical storage is usually provided by lead-acid batteries similar to those used in automobiles. However, automobile batteries are designed to produce a high current for a short period to start the engine, whereas consumer appliances typically require a steady current for a long period. Thus, batteries have been specifically designed for solar PV systems, and it is preferable to use them.

Most small PV systems, particularly ones used exclusively for radios and lighting, use 12 V batteries, since both the batteries and the appliances are readily available. Larger systems, such as those intended for refrigerators and video systems, often use 24 V batteries to keep wire size small and to minimize system losses.

Technically, a typical lead-acid battery is made up of a number of physically separate cells connected in series so their cell voltages are added together. All lead-acid batteries are made up of individual 2 V cells. Thus, a 6 V battery has three cells connected in series, a 12 V battery has six cells, and a 24 V battery has 12 cells. Further, to get 12 V, one can connect two 6 V batteries in series; to get 24 V, one can connect two 12 V batteries or four 6 V batteries.

Battery capacity is usually stated in Ampere-hours (Ah), which can be converted into Watthours (the most common measure of electrical energy) by multiplying the Ah value by the battery voltage. Thus, a 100 Ah 12 V battery stores 1,200 Watt hours of electrical energy when fully charged, whereas a 100 Ah 24 V battery stores 2,400 Watt-hours when fully charged.

Note that although it is technically possible to connect many small batteries in parallel to increase total Ah capacity, it is best in practice to use a single battery that is capable of providing the total capacity desired rather than connecting several batteries of smaller capacity in parallel.

Battery Controllers

Batteries can be damaged by consistent overcharging, so an automatic device called a charge controller is usually provided to sense the battery's charge and reduce or switch off the charging current before damage can occur. Small PV systems may not need a charge controller, since the small currents provided by one or two PV panels are not likely to damage good-quality batteries, even though more frequent maintenance may be necessary to replace water lost from the batteries because of mild overcharging.

Batteries also can be damaged by excessive discharging, so an automatic device called a discharge controller, similar in operation to the charge controller, is usually installed. The discharge controller continually senses the battery's charge and disconnects the appliances when the battery's charge falls below a set limit. Small systems in particular need the protection of a discharge controller, since it is easy to discharge the battery excessively by using appliances heavily.

It is common practice to combine the functions of charge and discharge controllers in a single device.

Wiring and Fuses

These components are interconnected with wiring of the same type used in grid-connected homes, although generally a larger-diameter wire is needed because of the lower voltage and higher currents being delivered to the appliances. Fuses or circuit breakers are used to protect the equipment against short circuits.

Appliances

The key reason for installing a solar PV system is lo power appliances. In the domestic settings considered here, these usually are limited to lights, radios, stereos, TVs, VCRs, fans, and refrigeration appliances, although other small appliances such as computers, pumps, or radiotelephones may be connected as well. In general, it is preferable to use appliances specifically designed for use with solar PV systems, because they are energy efficient and can be connected directly to the battery without expensive— and often inefficient—power converters.