(introduction...)

Solar photovoltaic (PV) systems convert sunlight directly into electricity using solid-state physical principles similar to those of transistors and integrated circuits. The electricity produced by PV systems is direct current (DC). Because the electrical power provided by PV panels is indistinguishable from electricity produced by any other source, any electrical device can be powered by PV panels in principle, although it may be necessary to convert the output of the PV panels to other voltages or to alternating current (AC). In practice, it is generally uneconomical to use PV panels for high-load appliances such as electric cooking ranges, air conditioners, and heaters.

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.

Determining the size of the system and its components

Sizing of solar photovoltaic systems is critical: If the system is too small, it will not provide sufficient energy, and the customer will not receive the services desired; if it is too large, the cost will be excessive.

Apart from a general technical understanding of the functioning of PV systems, the designer of the system also needs information that is specific to the site where the PV systems will be installed.

First, it is necessary to estimate the type of appliances the consumer will install and the number of hours the appliances will be operated per day. This information is used to calculate the energy load the PV system will have to meet. If the load is likely to vary from month to month or season to season, then this information must also be available. However, it is often difficult to estimate the daily usage for the appliances before the systems are actually installed because the availability of PV systems itself often brings about a significant change in the energy use of the households.

Second, it is necessary to estimate the amount of sunshine the site receives. Technically, this is measured by the insolation level, which is often stated in units of kilowatt hours per square meter per day (kWh/m2/day). Some measurements made in Tarawa, the capital of Kiribati, indicate a solar insolation of 5.9 kWh/m2/day, but it should be noted that long-term weather patterns and seasonal changes in the length of the day combine to create major changes in insolation from season to season. For this reason, average insolation measurements are not sufficient for designing solar PV systems. If the PV systems are being installed in remote rural areas, it is unlikely that long-term insolation data will be available, so that estimates based on experience in similar locations will have to be used. The insolation level is a major determinant of the "generation coefficient" (see Step 3 under the heading Sizing the System below).

Third, it is necessary to have a sense of the desired reliability of the system. One characteristic that is important in determining the reliability is the ability of the system to meet the consumer's load despite a number of consecutive days with poor insolation levels. Technically, this is termed the days of autonomy. It is common to design PV systems with five days of autonomy. In practice, this specification implies that a fully charged battery will provide for normal appliance use for at least one week of cloudy weather, because the panels do recharge the battery partially even in cloudy weather. Further, if seasonal variations are expected in consumer load, insolation levels, or both, it is also necessary to determine whether the system should be designed to meet all contingencies or whether a failure to provide the adequate power under some conditions may be acceptable. To design the system so that it can provide reliable power under worst-case conditions of insolation and user demands will often require systems three to four times larger and more expensive than smaller systems that provide adequate power most of the time but may impose some restrictions in power use when weather patterns are unfavorable or demand is unusually high.

Sizing the System

Given the uncertainties about the likely consumer load and the insolation levels, in practice, the initial determination of the system size is usually carried out using some simple rules of thumb. Then, the actual performance of the system is monitored, and the system is modified as necessary.

In the Pacific Region, the South Pacific Institute for Renewable Energy, in cooperation with the Pacific Energy Development Programme, has developed a six-step technique for the initial sizing of domestic PV systems. Although quite simple, the technique is based on long-term measurements of actual power delivered by PV panels for charging lead-acid batteries and on the use patterns in existing domestic PV systems. The technique has been in use since 1987 and has reliably provided system sizes that are consistent with more complex design methods. The specific numbers shown below for solar panel performance are generalized for atolls and the "dry sides" of tropical mountainous islands in the Pacific and are appropriate for tropical sites that have little seasonal differences in length of day and a high percentage of partly cloudy days but few periods of extended cloudiness. For use in other climates or on the "wet sides" of mountainous islands, the estimated panel output should be adjusted to compensate for the climatic differences from the parameters used as the calculation base.

The system-sizing technique is illustrated below for design of a typical 12 Volt system that is used to power three lights. It is assumed that there is (a) one light of 12 Watts that will be used 6 hours a night; (b) another light of 10 Watts that will be used 4 hours per night; and (c) a third light of 10 Watts that will be used 5 hours per night. It is also assumed that the system will be powered by PV panels rated at 47 Wp and that the only available batteries are rated at either 50 Ah or 100 Ah capacity at 12 V. A second example is shown in Box 1. Note that the design parameters (number of panels and battery capacity) derived by this technique represent the minimum acceptable values; in practice, the systems may be oversized based on other considerations such as anticipated load growth.

Step 1: Estimate the Total Daily Appliance Energy Requirements. For each appliance. compute the Watt-hours per day expected to be consumed. This is done by multiplying the Watt power demand of the appliance by the average number of hours per day the appliance will be using electricity.

For example, for the three lights described above, the total appliance load per day, on average, will he as follows:

First light: 12 Watts x 6 hours = 72 Watt-hours
Second light: 10 Watts x 4 hours = 40 Watt-hours
Third light: 10 Watts x 5 hours = 50 Watt-hours
TOTAL LOAD 162 Watt-hours per day.

Step 2: Estimate the Total Energy per Day that Must Be Delivered by the PV Panels. Like conventional AC systems, solar PV systems suffer losses (the battery never delivers as much energy as goes into it, wires and connections lose energy, controllers use energy, and so on). Hence, it is necessary to take account of these losses in determining the energy that must be delivered by the panels. Losses are likely to be in the 10 to 30 percent range, with the lower value applicable to newer systems using high-quality batteries. However, as systems age, the internal of their batteries falls, and losses may exceed 30 percent. Hence, a conservative but reasonable estimate of the system losses is 30 percent. This is taken into account in calculating the total energy that the panels must deliver by multiplying the total appliance load (from Step 1) by 1.3.

For example, the three lights above are expected to require 162 Watt-hours per day (Step D, so the panels must be capable of delivering 162 x 1.3 = 210.6 Watt-hours per day to cover the losses and still have sufficient power for the lights.

Step 3: Estimate the Energy per Day Produced by One Panel. The energy produced by a panel depends on many factors, with its capacity, measured in peak Watts (Wp), and the insolation level having the greatest effect. To obtain the energy produced by a solar PV panel in a particular site, it is necessary to calculate a "generation coefficient," measured in Watt-hours/day per rated Wp of the panel. This generation coefficient is site-specific and is based on measurements of actual battery charging by a PV panel at the site or on experience at similar sites. The generation coefficient summarizes all the factors related to energy production by a PV panel except the Wp rating. The generation coefficient is multiplied by the Wp rating to determine the Watt-hours/day energy output produced by a particular panel. For the typical small Pacific island environment, the generation coefficient has been estimated as 3.43.

For example, in an installation with 47 Wp rating panels, the estimated energy output from a panel for battery charging will be 47 x 3.43 = 161.21 Watt-hours per day of energy.

Step 4: Estimate the Minimum Number of Panels Needed. The number of panels needed is determined by dividing the total Watt-hours/day requirement (result of Step 2) by the Watt-hours/day output of one panel (result of Step 3), and rounding up the requirement to the nearest integer.

For example, the total energy that the panels must deliver is 210.6 Watt-hours/day (Step 2), while one 47 Wp panel is estimated to produce 161.21 Watt-hours/day (Step 3). Thus, the number of panels required is 210.6/161.21 = 1.31 panels. This is rounded up to two panels, which can deliver 161.21 x 2 = 322.42 Watt-hours/day. Thus, with two panels there will be a potential excess capacity of 322.42 - 210.6 = 111.82 Watthours/day.

Step 5: Estimate the Ampere Hours (Ah) per Day that Must Be Delivered by the Battery. Since Amperes equals Watts divided by Volts, the Ampere-hours per day equal Watthours per day divided by the battery voltage. Note that the battery delivers power directly to the appliances, so only small losses will occur (less than 5 percent), mostly in the connecting wires. Therefore, the value of Watt-hours per day which is used here is the actual requirement of the appliances (Step 1), not the amount that includes the 30 percent system losses (Step 2).

For example, the appliances are estimated to require 162 Watt-hours/day (Step 1), and assume that a 12 Volt battery is used. Therefore, the battery must deliver 162/12 = 13.5 Ah/day.

Step 6: Estimate the Minimum Battery Capacity Needed. So that the system can meet the load even during cloudy periods, the battery size has to be larger than the daily requirement. The battery size is determined by multiplying the daily requirement (Step 5) by the specified days of autonomy.

For example, with a specification of five days of autonomy, the 12 V battery must have a capacity of at least 13.5 (Step 5) x 5 = 67.5 Ah. If only 50 Ah and 100 Ah batteries are assumed to be available at the site, then the 100 Ah capacity battery will have to be selected, which will give an excess capacity of 32.5 Ah/day, equivalent to 32.5 x 12 = 390 Watt-hours/day.

Thus, for a household to use three lights for 4 to 6 hours per day, the system will require, at least, two 47 Wp panels and one 100 Ah 12V battery. However, there is a surplus of both battery and panel capacity. The excess panel capacity (111.82 Watt-hours/day) is less than the excess battery capacity (390 Watt-hours/day), so the excess capacity of the system is 111.82 Watthours/day. This implies that, in principle, another appliance, such as a radio rated at 10 Watts, could be operated 11 hours per day in addition to the lights without overloading the system.

Box 1 The following steps illustrate the calculations to determine the minimum number of solar PV panels and battery capacity needed to meet the needs of a rural consumer who has a limited number of appliances. In this example, we assume that the consumer has (a) one 150 Watt TV/VCR combination operated 2 hours per day; (b) one 12 Watt light operated 6 hours per day; (c) two 10 Watt lights operated 4 hours per day; (d) one 60 Watt refrigerator whose compressor runs 10 hours per day; and (e) one 10 Watt radio/cassette player operated 8 hours a day. Further, assume that only 55 Wp panels are available and that the system is to be operated at 24 Volts.

Step 1: Estimate the total appliance energy requirement Watt-hours

			(Wh)/day
TV/VCR	150 Watts x 2 hours/day	=300	Wh/day
Large light	12 Watts x 6 hours/day	=72	Wh/day
Smaller lights (2)	10 Watts x 4 hours/day	=80	Wh/day
Refrigerator	60 Watts x 10 hours/day	=600	Wh/day
Radio/cassette player	10 Watts x 8 hours/day	=80	Wh/day
TOTAL		=1,132	Wh/day

Step 2: Estimate total energy per day to be delivered by the PV panels

Total appliance load (from Step 1)	=1,132	Wh/day
Compensation for losses (30% of load)	=339.6	Wh/day
Total energy needed	=1,471.6	Wh/day

Step 3: Estimate the energy per day produced by one panel

Panel capacity (Wp)	=55
Generation coefficient (Win/day per Wp)	=3.43
Energy supply (capacity x generation coefficient)	=188.65	Wh/day

Step 4: Estimate the minimum number of panels needed

Total energy needed (from Step 2)	=1,471.6	Wh/day
Energy supplied by one panel (from Step 3)	=188.65	Wh/day
Panels needed (1,471.6/188.65)	=7.8
Panels needed, rounded up	=8

Step 5: Estimate the Ampere-hours/day (Ah) to be delivered by the battery

Total appliance load (from Step 1)	=1,132	Wh/day
Battery voltage	=24
Ah needed per day (1,132/12) at 24 V	=47.17	Ah/day

Step 6: Estimate the minimum battery capacity needed

Ah needed per day (from Step 5)	=47.17	Ah/day
Specified days of autonomy	=5
Battery size needed (47.17 x 5)	=235.85	Ah at 24 V

This battery capacity can be achieved by connecting two 12 V batteries or four 6 V batteries in series.