For more than 35 years, the National Rural Electric Cooperative Association (NRECA) has been helping partners in Latin America and Asia develop rural electrification programs.

During this time, NRECA programs sponsored by the U.S. Agency of International Development, the World Bank and others have provided electric service connections to more than 30 million rural residents-a number roughly equal to the population served in the United States by rural electric cooperatives.

Rural electric program needs vary from country to country. Some countries, such as Costa Rica, have well-established rural electric institutions. In countries such as Peru and Bolivia, about 50% of rural households do not have electricity. The challenge is to find cost-effective and environmentally benign ways to expand electrification coverage.

Profiles of electric consumption also vary widely between countries. In much of rural Latin America, the average consumption is close to 40 kWh to 50 kWh per month. With rural domestic energy consumption so low, conventional rural electrification projects have been successful where productive uses of electricity have been sufficiently high to carry the load of these projects. Many rural electrification projects have not been so fortunate, as revenues can fall short of expenses because of low plant factors. The rural electric systems employed need to be carefully selected to match the community's projected demand and ability to pay for the service.

Through the 1980s, NRECA recognized that consumption levels were so low in many areas that grid extension would not be a viable alternative for decades. NRECA began to investigate and use distributed power sources, including renewable-energy technologies in the early 1990s. So-called microgeneration systems, including solar home systems, have been employed in recent projects with some success. While satisfying an immediate demand for domestic lighting service and some very small appliance loads, these projects cannot be used effectively for economically productive activities because of limited capacity and high delivery costs.

Electrification Technologies
In 1993, partnering with the National Renewable Energy Laboratory (NREL), NRECA began to investigate the use of renewable hybrid-power systems for projects in South and Central America. Note that the interconnected grid remains the technology of choice for most rural electric utilities and users alike. In many countries, however, national grid systems are not complete for a variety of factors centered around their large capital requirements.

Distribution-system management has been challenging in many cases, especially where this job had been overseen by a government agency. Typically, technical and administrative losses are several times what they should be, collection rates are extremely low, consumer theft is problematic and system reliability suffers from maintenance and operational shortfalls. Moreover, grid-connected rural electric systems often suffer from frequent blackouts because of a lack of sufficient generation capacity.

In many instances, isolated generation systems have been used to service loads that are distant from regional or national electric grids. Most often, diesel generators power medium- and low-voltage distribution systems. Generally, isolated systems with diesel-fired generation provide service to communities of less than 1000 service drops for 4 to 8 hours per day. For larger communities (greater than 1000 consumers), service is provided for up to 18 hours per day.

In developing countries, however, isolated diesel-based grid systems have proven problematic. Poor maintenance practices, inadequate fuel-handling systems, and poorly trained operators and maintenance personnel are usually the sources of the problems, which cause less-than-satisfactory performance and higher-than-necessary power-delivery costs. These problems are particularly notable when systems have been owned and operated by state-owned electric utilities. For many of these reasons, state and private electric utilities are reluctant to invest in rural electric projects that depend upon diesel power stations at this time.

Renewable Technology Options

High costs, coupled with some of the problems that have arisen with management of grid-electric systems and even more so for diesel-powered minigrids, has resulted in increasing interest for renewable-energy alternatives to electrify households in isolated areas of sparse settlement. Under consideration are solar-photovoltaic (PV) systems for individual households, wind-energy systems, micro- and mini-hydroelectric systems, biomass generation plants and hybrid-energy systems that combine two or more generation technologies.

Perhaps most popular, thus far, is the solar home system (SHS), which can be scaled up or down in capacity to meet rather modest loads. A typical SHS can range in size from 12 to 100 peak watts or more (or Wp nominal capacity of the solar- PV panel). The electric energy delivered by these systems depends upon hours and intensity of sunlight, as well as the efficiency of the system components.

The greatest advantage of SHS is that it can cost less to install and operate than grid extension alternatives for areas with low population density and low productive demands. While generally not designed to provide 30 kWh to 40 kWh per month, by using high-efficiency lamps and appliances SHS can provide the same level of service that many rural electric systems provide to low-income households. Its principal limitation is a system cost of US$8 to US$15 per Wp of PV generation capacity. SHS becomes very expensive when sized to serve 24-hr base loads or most productive loads.

Architecture, Components and Service

Potential power requirements for remote applications vary tremendously-from small residential lighting and communications to water pumping and agricultural processing. Continuous 24-hr service may not be required in applications such as small-scale residential lighting. However, for applications such as remote telecom, continuous service is important. In each situation, responsive project-specific design is essential to optimize the admittedly large initial investment for renewable-based generation technology. For continuous service, hybrid systems can provide higher service quality than single-source (usually fossil-based) systems, at the same time potentially reducing the life-cycle cost of energy. This is accomplished by combining a renewable source, such as solar-PV and wind electric, with diesel or other traditional generation technology. This arrangement takes advantage of low operating costs of the renewable source on the one hand, and the highly dispatchable nature of the fossil-based source on the other hand.

The systems often use a dc and an ac bus integrated through an inverter that both charges batteries during periods of excess energy production and serves the loads. Figure 1 shows the architecture of a typical wind-electric hybrid-power system, which is comprised of the following main components:

* Wind turbine. On a tower from 39 ft to 98 ft (12 m to 30 m) high, a modern "permanent-magnet alternator" wind generator will typically produce "wild ac" power (variable voltage and frequency). Its nonsynchronous design allows it to operate closer to its optimum efficiency under variable wind conditions. The efficiency gain outweighs the corresponding losses in the necessary electronic-control componentry, and the design's inherent simplicity avoids the need for brushes and other relatively high-maintenance components.
The turbine-to-powerhouse conductor run typically will be three-phase "wild ac"-either directly from the turbine or at an elevated voltage by way of transformers on either end-for runs longer than approximately 1640 ft (500 m). One advantage of wind-electric technology is the ability to site the turbine tower where greater wind resource is present.

* Rectifier/charge controller. At the powerhouse, "wild ac" from the wind turbine is brought into a typical 48-V or 120-V rms voltage range via a dedicated three-phase transformer operating at variable frequency. It is then converted to dc through rectifiers on each phase. An electronic charge controller, which may be separate or within the rectifier enclosure itself, then conditions this dc power for battery charging.

* Photovoltaic array. Photovoltaics is another renewable source of electric power. As hybrid-system components, PV modules may be connected in a series to form panels, which may then be connected in parallel to form a larger array. Thus, PV generation is extremely modular yet is still relatively expensive at around US$4 per peak watt for relatively high-volume purchasers.

* dc source center. For installation streamlining, as well as for safety reasons (to facilitate high-current dc fusing and grounding), typically a dc source center encloses one dc bus bar that unifies all the dc components. For further convenience, battery state-of-charge (SOC) monitoring and other system readouts may be included here as well.

* Inverter/charger. Inverter technology, which has been a primary enabler of the viability of stand-alone hybrid-power systems, has made great strides during the last decade. Modern inverters efficiently convert low-voltage dc to standard utility-grade ac. A key feature of these inverters is that they can serve as battery chargers from an ac source. This arrangement allows efficient use of a backup generator because it avoids generator operation at low load conditions. For example, if batteries are discharged, the genset can run at its optimal efficiency by supplying ac service to the loads while simultaneously charging the batteries with any excess capacity. Such conditions may occur on a regular basis during periods of low wind resource or may not happen at all if the wind regime is adequate to maintain minimum battery SOC.
These technological advances notwithstanding, the reliance on power inverters is still one limitation of hybrids. The electronic components on which they depend are still developing, and their power throughput is still somewhat limited. Most rural electric hybrid systems are dimensioned for loads in the 5 kW to 50 kW range.

* Battery bank. Batteries are generally the most delicate components of any remote power system. Proper voltage must be maintained to prevent premature wear. Battery-bank replacement also is one of the principal ongoing costs associated with remote power-system maintenance. Hybrid-system architecture, with its flexible and dispatchable generation sources, can decrease the probability of battery mistreatment through excessive discharge.

* Back-up generator. A traditional fossil-based internal-combustion generator provides a flexible, dispatchable power source for high-peak loads and for battery charging during high demand and periods of low wind resource.

* ac loads. Powerhouse output is utility-grade ac power. Depending on the application and the site, residential loads might be served through a small, traditional ac distribution grid, or the power system might be dedicated to a single remote institutional load. In any case, given the relatively large capital investment required for renewable-energy generation, it is important to emphasize the use of efficient ac appliances, such as compact fluorescent lighting.

Renewable-hybrid systems are not yet fully commercialized for rural electrification applications. However, there has been sufficient field experience to draw some conclusions regarding their applicability and, more importantly, to focus on the issues that will affect their use in the future. The following examples illustrate some of the options applied in a developing country context.

Flor de Oro's Hybrid PV-Diesel
The Noel Kempf Mercado National Park is the second-largest protected area in Bolivia. The Friends of Nature Foundation (Fundacio'en Amigos de la Naturaleza [FAN]) administrates the park from its headquarters at Flor de Oro, situated on the Itenez River in northeast Bolivia, several hundred kilometers from the nearest power lines (Fig. 2). Population density is nowhere near high enough to make central grid power provision economically feasible.

At Flor de Oro, FAN not only orchestrates the park's daily operations, but also runs a scientific laboratory, visitors' center and ecotourism facilities on the site. FAN also provides support and lodging for park rangers. Flor de Oro is the cerebral cortex of Noel Kempf and as such requires 24-hr access to electric service. With efficient appliances installed wherever possible, the facility's daily electricity requirements average around 20 kWh (Fig. 3).

From the park's beginnings in 1989, electricity was supplied by a 12-kVA to 15-kVA diesel generator that was expensive to operate as well as noisy and messy. As the Flor de Oro facilities expanded, and as funding became available for more ecologically sound development, FAN wanted to explore new alternatives to fossil-based generation. Under these circumstances, a PV-hybrid system was the most attractive solution.

The PV-hybrid system was implemented in 1996 by NRECA, FAN and CRE, a large electricity cooperative active in the rural areas of subtropical Bolivia. In fact, CRE contributed a large part of the locally available equipment and installation contracting for the system's 0.6-mi (1-km) underground distribution grid. NRECA performed the system design and provided some funding, originally from USAID, for the procurement of most imported components. (Table 1.)

The system's 48-V bus connects to three 350-Ah batteries, totaling more than 50 kWh of energy storage. The batteries store energy from the sun (and the diesel genset if needed) and power a small ac grid with a 5-kW inverter. Most of the loads are supplied easily from the battery bank via the inverter.

For loads greater than a few kilowatts, however, these systems have two limitations. First, battery losses during high-current charge and discharge can reach 20%. Thus, for loads such as welding and clothes washing, it does not make sense to use expensive PV-generated electricity. Second, in this case, inverter capacity limits the power extracted from the battery bank to 5 kW (continuous). As a result, this system allows the 12-kVA diesel genset to operate not only as an automatic backup battery charger, but also as a stand-alone unit powering a separate circuit to which the larger loads are connected directly.

Wind-Diesel Community Power
The goals of the Chilean Rural Electrification Program (PER) are to supply electric service to approximately 100,000 families. Although PER policy states that the ideal solution for all Chileans is interconnected grid-based service, renewable-energy options will be important in many areas where line-extension costs can run well above US$2500 per connection. Where renewables can provide "adequate" electric service for lower cost, they are equally open for subsidy under the PER.

Within the PER, three hybrid wind-electric systems have been installed in the coastal area of Region IX, La Araucania (Fig. 4). These projects, commissioned in January 1997, were implemented using a subsidy granted by the regional government to Frontel, the largest rural electric utility in the region. NRECA, under a contract funded by the NREL, provided program coordination and technical assistance to the government of Chile and to Frontel during the course of this program. The systems provide 5 Hz, 220 V ac for basic lighting and communications loads to remote villages. Frontel owns and maintains the systems and charges a tariff for service. A total of 38 users have been connected at the three sites, with costs ranging from US$2400 to US$4100 per connection.

These are the first and precedent-setting wind-electric systems to be funded through the traditional Chilean subsidy mechanism. An interesting aspect of the project will be the rigorous quantification of operation and maintenance costs, and their influence on the tariffs levels required for Frontel to profit from these systems. In these projects, a number of service outages have resulted from some combination of maintenance inadequacies and casual circumstance. One system in particular suffered several outages during its first two years of operations as a result of a deadly combination: nonautomated backup generation and low wind events.

Installed cost per user is somewhat too expensive for large-scale government subsidization. However, Chile's experience with hybrids has been positive enough that a large wind-based electrification project is now being planned for the southern archipelagic region of Chile. Lower-cost single-family wind systems also are under consideration for the expansion phase.

The question of what minimum level of service should be provided by renewable-energy systems has proven difficult from the regulatory perspective. In addition to wind-electric systems, solar-PV and microhydro projects recently have been subsidized with the PER. None of these alternatives is equal to the interconnected grid, but neither are they comparable with one another, because the exact service delivered depends upon the resource and technology used. Thus, standardization of "renewable" solutions and their financing becomes difficult.

Current Limitations
There are several limitations to the widespread use of hybrid systems in remote areas of developing countries:

* Capital cost. Although the sizes of the two (or more) generation sources can each be smaller than they would be in single-source configurations, together they imply a relatively high initial investment. Whereas grid extension projects usually cost less than US$1000 per new connection, hybrid systems with local minigrids can cost upwards of US$2000 per connection, even in a large program with economies of scale. The relatively high initial capital outlay required reduces the potential market for hybrids in poor areas.

* Technical expertise. These systems incorporate fairly sophisticated electronics to control the power flows, regulate the batteries and dispatch the fossil generator automatically. Technical expertise is often extremely difficult to access in remote areas, and a hybrid system may be out of service for an extended period while a repair is sought. In this sense, we might expect proper dealer and repair networks to appear only in those countries where the potential market for hybrid systems and their component technologies is relatively large.

* Preventive maintenance. A commitment to preventive maintenance is most important. Where implementation agencies lack a commitment to proper and systematic maintenance, a hybrid project is doomed to failure. Failures are not detectable from the dispatch center. As a result, reliable communications and service networks are required to monitor and repair these isolated systems. Where rural distribution utilities do not routinely maintain their grid system, it is difficult to imagine the successful adoption of hybrid-power systems on even a modest scale. Ongoing training of system-design engineers and field technicians is as integral a part of successful deployment of newer technologies, such as wind hybrids, as it is with grid-based systems.

Utility engineers of grid-based networks rely upon large information systems and databases to predict and control their distribution networks through time. For hybrids to achieve quality of service similar to that of the grid, and thus increase penetration rates to reach some economics of scale, a similar level of technical underpinning will likely be necessary.

Daniel B. Waddle is the director of Latin American programs for NRECA International Ltd., which he joined in 1980. Waddle has since worked in rural energy and rural electrification programs in Latin America, Africa and Asia in the areas of project and program management, technology analysis and selection, and electric-utility management consulting. He earned the BS and MS degrees from Virginia Tech and a PhD in engineering from Texas A&M University.

Joseph Andrew McAllister is a renewable-energy specialist with NRECA International Ltd. In addition, he is currently pursuing doctoral studies at the Energy & Resources Group (ERG) at the University of California-Berkeley. From 1992 to 1998, he served with NRECA providing technical and planning assistance to renewable-energy programs in several South American countries. He has worked as a researcher in the Energy Analysis Program at Lawrence Berkeley National Laboratory. He received the BA degree in engineering sciences and environmental studies from Dartmouth College and the MS degree from ERG. In addition, McAllister served as a member of the U.S. Peace Corps in Costa Rica.