Cover Image
close this bookPhotovoltaic Household Electrification Programs - Best Practices (WB)
close this folderTechnical requirements
View the document(introduction...)
View the documentHardware design
View the documentStandards and specifications
View the documentOther technical considerations
View the documentQuality control
View the documentMaintenance services
View the documentEducating users

Hardware design

7.3 All system components should be of the highest quality. This refers to modules, controllers, lights, wiring, switches, batteries, connectors, and module supports. Design budgets should be treated as guidelines, not as a project's highest priority. High-quality systems will out-perform cheaper ones and help ensure the project's sustainability. An appropriate indicator is the lifecycle cost of the project, which takes into account the cost of warranty and maintenance services and the effect of possible non-payment of fees. For example, in remote sites where service costs are high, it may be more cost-effective to provide an over designed system with high-quality batteries instead of a system designed with tighter performance margins. Where affordability is an issue, users can be offered several system sizes and made aware of the trade-offs between system capabilities and costs. Users can then make an informed choice from a range of system sizes, based on their own requirements and their ability to pay. Systems can be designed to allow for future upgrades. For example, one supplier in Indonesia offers a PV module support structure that can accommodate two modules, although the base system has only one. The controller is capable of handling the power output from two modules, and the distribution panel also allows for adding more outlets and lighting circuits.

7.4 Future household PV initiatives can learn from the successes and problems encountered in similar programs to date and improve procurement specifications and technical infrastructure. Specific technical requirements for solar home system components are discussed below.

7.5 Modules. The PV module is generally the most reliable component of the system. In all four country programs reviewed, module failures or even breakages were rare. Preference should be given to modules that meet JRC 503 or equivalent specifications and to suppliers offering at least ten-year performance warranties.' Countries such as Sri Lanka and India, with local module manufacturers that do not meet such specifications, may have import barriers that limit the availability or raise the cost of modules meeting JRC 503 or equivalent performance specifications. In such cases, if removing the trade barrier is not a feasible option, local suppliers must make sure their products meet specifications and offer enforceable ten-year warranties.

7.6 Crystalline cell modules with 36 cells should be used instead of 32-cell "self-regulating" modules. The benefits from improved performance, particularly in hotter climates, more than outweigh the added cost of the charge controller required for the 36-cell module. All modules currently being installed in Indonesia, Sri Lanka, the Philippines, and the Dominican Republic are the 36-cell type. In the initial stages of PV dissemination in Sri Lanka, 32-cell modules were used to avoid the cost of a controller. The suppliers soon realized that the benefits of using 36-cell modules outweighed the added cost of the controller, and installations now use the 36-cell module.

7.7 Module terminals should include lock washers, nylock nuts, or other devices to assure that the terminals do not loosen during the life of the module. Preferably, only modules with a single-junction box should be used. In the Philippines' household PV program, some modules with two junction boxes were supplied. As a result, the cable had to be stripped to separate the two leads, exposing them to the weather and possible corrosion.

7.8 Module Support Structure. The PV module support structure should be corrosion resistant (galvanized or rustproof steel or aluminum) and electrolytically compatible with materials used in the module frame, fasteners, nuts, and bolts. The design of the module should allow for proper orientation, tilt, and, as has been noted, easy expansion of the system's capacity. Roof mounting may be preferable to ground or pole mounting, since it requires less wiring and reduces the possibility of module shading. The module support should be firmly attached to the roof beams and not loosely attached to the roof tiles. The module or array should not be placed on the roof but kept 10-50 cm above the surface itself, to allow cooler and more efficient operating conditions. If the module is mounted on a pole, the pole should be set firmly in the ground and secured with guy wires to increase rigidity. Pole-mounted modules should be accessible for cleaning but high enough above the ground to discourage tampering.

7.9 Charge and Load Controller. The charge and load controller prevents system overload or overcharging. In the past, some programs have not paid adequate attention to the controller. Unsuccessful attempts were made in Sri Lanka to operate solar home systems without a controller or with only a simple charge indicator (a simple controller that provides a low-voltage disconnect (LVD) and charge indication has since been added). Poor-quality controllers have also caused problems. In the Philippines, for example, problems caused by low-quality, locally manufactured controllers were resolved when the controllers were replaced with high-quality, imported components. To operate reliably, the controller design should include:

· A low-voltage disconnect (LVD);

· A high-voltage disconnect (HVD) which should be temperature-compensated if wide variations in temperature are expected in the battery compartment. Temperature compensation is especially important if sealed lead-acid batteries are used;

· System safeguards to protect against reverse polarity connections in the DC circuits (reverse energy flows through the PV module(s) short circuits in the input or output terminals) and lightning-induced surges or over-voltage transients; and

· A case or covering that shuts out insects, moisture, and extremes of temperature.

To enhance the solar home system's maintainability and usability, the controller should:

· Indicate the battery charge level with a simple LED display and/or inexpensive expanded scale analog meter. Three indicators are recommended: green for a fully charged battery, yellow for a dangerously low charge level (pending disconnect), and red for a "dead" or discharged battery;

· Be capable of supporting added modules to increase the system's capacity;

· Be capable of supporting more and bigger terminal strips so that additional circuits and larger wire sizes can be added as needed (this is necessary to ensure that new appliances are properly installed); and

· Have a fail-safe mechanism that shuts the system down in case of an emergency and allows the user to restart the unit.

7.10 Field surveys suggest that most controllers have some (but not all) of these characteristics. However, even highly sophisticated controllers must be tested and proven in the field. A sophisticated new controller that was used in the Pansiyagma project in Sri Lanka created problems which led to user dissatisfaction, poor cost recovery, and serious skepticism regarding PV programs elsewhere in the country.

7.11 Battery. The battery that is most often used in solar home systems is a lead-acid battery of the type used in automobiles, sized to operate for up to three cloudy days. Automotive batteries are often used because they are relatively inexpensive and available locally. Ideally, solar home systems should use deep-cycle lead-acid batteries, which have thicker plates and more electrolyte reserves than automotive batteries and allow for deep discharge without seriously reducing the life of or damaging the battery. In a well-designed solar home system, such batteries can last over five years. However, deep-cycle batteries are not usually made locally in developing countries and high duties often increase the price of importing such batteries.

7.12 In Indonesia, a good-quality automotive battery in a well-designed system can last three years or longer, if the battery is well maintained and rarely subjected to deep discharges; the average daily discharge should be limited to about 20 percent of battery capacity. For example, lightly stressed automotive batteries in the Sukatani, Indonesia PV project had exceptionally long life -- 75% of the batteries last nearly five years (Panggabean, 1993). Field investigations in Sri Lanka reveal that poor-quality materials and questionable workmanship have led to random cell failures in some locally-made automotive batteries and shortened battery life to about two years. Solanka has introduced an extended warranty program that guarantees the performance of the entire system (including the battery and light bulbs) for two years for a fee of Rs 500 ($ 10).

7.13 Battery Mounting. Batteries are sometimes left exposed on the ground and accessible to children. The potential dangers (burns from battery acids, shorts, and explosions) highlight the need for a well-designed battery enclosure to maximize safety and minimize maintenance. Such enclosures are being introduced in Indonesia and the Pacific Islands. Made of injection-molded plastic or fiberglass, the enclosure contains the battery, charge controller, charge indicator, and switches. The electronic elements are isolated from the battery, and the battery enclosure has vents to disperse gases and channels to divert any acid overflows. There is no exposed wiring and the battery can be checked and filled easily.

7.14 Lamps, Ballasts, and Fixtures. The principal reason most householders acquire a solar home system is that it provides brighter, safer, cleaner, and more convenient lighting than kerosene lamps. Users tend to have more "lighting points" after they acquire a system. Field observations show that the additional lights increase satisfaction with and acceptance of the solar home systems. In many cases, householders have subsequently added additional lights to their system while maintaining the overall level of energy consumption (systems with 12 lights have been observed in Indonesia).

7.15 Efficient lights (CFL or tube lights) are usually preferable to incandescent lights. However, fluorescent lamps require well-designed ballasts that ensure an operating life of the tubes of more than two years and that do not interfere with radio or television reception. In some programs, such as Bolivia's, fluorescent light ballasts have been the most problematic component of the system.

7.16 Low-watt (1-2 W) incandescent lights may be preferable in cases where low-level area lighting is needed for short periods (less than 1/2 hour per day) or intermittently (such as in bathrooms or secondary bedrooms). A choice may also need to be made between supplying high-efficiency fluorescent lights (tube lights with preheating elements or CFLs) or locally available low-efficiency tube lights. While the high efficiency lights can offer lower lifetime costs, users may become dissatisfied if they are not available locally.

7.17 Fixtures with reflectors are recommended to increase the effectiveness of the lights. Fixtures that use diffusers must be sealed against insects, since the "useful light" output and efficiency of the fixtures with diffusers can be drastically reduced by a buildup of dirt and insects inside.

7.18 Wiring, Switches, and Outlets. Solar home systems should have high-quality switches and outlets (preferably rated for DC operation). In Sri Lanka, standard AC surface-mounted wall switches were used along with two-prong AC wall outlets. If the lights and appliances draw little current, these AC switches and outlets are satisfactory substitutes for DC-rated components.

7.19 Undersized wiring is sometimes used in solar home systems, particularly when additional light fixtures are installed. This practice leads to energy losses and unacceptable voltage drops and should be strongly discouraged. All wiring should be stranded copper, sized to keep voltage drops to less than 5 percent between battery and the load. Since DC electricity has a polarity that must be maintained, insulation color conventions or labeled wires should be used (red for positive, black for negative, green or bare wire for the grounding conductor). The four country field surveys uncovered several twisted wire or spring-clip wire connections. These do not provide a good electrical connection and should not be used. Soldered or crimped connections or screwed terminal block connectors are the best choices. Soldered wire connections require a non-corrosive rather than an acid-flux solder, especially in regions where corrosion can be a serious problem. As in any electrified home, the wiring should be neatly and securely attached to the walls, either on the surface, in conduits, or buried inside the walls.

7.20 Solar home systems should also have a distribution panel that allows users to connect additional loads simply and safely, utilizing the circuit protection and LVD features of the controller. In Sri Lanka, the panel is a simple terminal strip or similar connection point that allows secure and reliable connection of multiple loads. The terminal strip is located at the regulator, in a separate distribution box, or in the battery box. The distribution panel must always be used if additional circuits are to be installed. Direct battery connections must not be made.

7.21 Other Appliances. Any appliance that operates on electricity can be powered by a solar home system, if the system has sufficient capacity. In the four countries surveyed, the most widely used appliances (other than lights) were black-and-white, 15-inch televisions and radio/cassette players. Users in the Dominican Republic expressed a desire to operate other appliances, such as refrigerators, irons, and VCRs. However, most of the users do not have the capacity to pay for larger systems and the availability of DC-compatible appliances such as color televisions and fans is limited. These users will have to save to buy larger systems. Appliances with high energy requirements, such as irons and air conditioners, cannot be used with small systems, since converting DC to AC electricity involves additional costs and energy losses. Large 1-kWp systems supply 100 kWh/month to customers of Southern California Edison. This is the equivalent of supply grid-quality AC power and is sufficient to operate most of the appliances found in a typical American home.

7.22 Inverters. Customers requiring AC electricity will need an inverter to convert the DC electricity from PV systems. It is very unusual for users with systems smaller than 100 Wp to require inverters (none was observed during ASTAE's field investigations). In any case, the cost of an inverter and associated energy losses recommend against its use in small systems. Several types of inverters are available: square wave (the least expensive and least efficient), modified square wave, and pure sine wave (the most expensive and most efficient). If needed, the inverter with lowest life-cycle cost should be selected. Small inverters dedicated to the specific loads that require AC power should be used rather than a centralized inverter for all loads.