Conclusion du chapitre

The electricity for supplying a mini-grid can come from a number of sources, ranging from the

conventional (diesel and gasoline engine or a distribution transformer supplied by the national or regional grid) to the non-conventional (wind, solar, or micro-hydropower). Before constructing a mini-grid, it is essential that whatever supply of electricity is proposed be available in the quantities and at the times it is needed. If not, this will not only reduce the end-uses to which electricity can be put but it may also complicate the generation of adequate revenues to cover the costs incurred in electrification. And the power supply should be located sufficiently near the load center to minimize costs in transmitting power to the village loads.

Several electricity supply options might be considered. In probable order of popularity, these are a distribution transformers fed by a national or regional grid, a diesel/gasoline generating set (genset), a micro-hydropower plant, a wind turbine, and a solar PV (photovoltaic) station. While the purpose of this guide is not to provide details about these various technological options, brief descriptions of issues that should be considered with each option are reviewed.

Grid extension

In cases where a MV line serving a number of larger load centers passes near a community, this is generally the cheapest approach to rural electrification. Electrification involves the local utility installing a distribution transformer of appropriate size near or in the village and making power available to the village. Presumably the utility is not interested in managing a small system within the community;

otherwise, there would be no need for the villagers to consider implementing their own system. In this case, the utility may only be willing to install an energy meter at the transformer location and provide a connection from which the villagers can extend the line into their community under appropriate

supervision and implement their own distribution system. The community would then be responsible for collecting the necessary tariff to pay the utility, based on the consumption that has been metered at the transformer.

Several advantages are associated with this option:

• Unless the electricity supply in a country is power-limited, much more power could be made available at the village level than would be the case with the other options. Electrification could therefore have a much broader impact on the community, far beyond lighting and entertainment.

With grid extension, employment generation and a much broader range of productive uses of electricity and social amenities are possible on a 24-hour basis.

• In implementing a project, the community has one less burden to address—the power supply. In this case, the national utility would usually ensure a functioning supply of electricity.

• Because of economies of scale in centralized generation, the cost of energy is relatively low.

(However, the community may also have to include the cost of bringing the power to the village in the overall project cost.)

The disadvantages associated with this options are the following:

• Some countries do not have a reliable supply. In this case, rural areas are usually the first to be cut off when the load on the entire system exceeds available generation capacity. An unreliable supply may then frustrate consumers who subsequently refuse to pay because of the poor service they receive. The system may then fall apart because of the lack of adequate revenues.

• If more conventional, urban-based, higher-cost design standards to which the utility subscribes must be adhered to, the distribution design adopted in this case may be more costly than would otherwise be the case.

In considering this option, several questions must be asked:

• Is the existing MV line sufficiently close to the community or must it be extended. Is the utility amenable to extending the line and what would be the cost for line extension and transformer placement?

• Are there provisions whereby the utility could enter into some agreement with communities willing to be responsible for their own distribution system?

• Based on experience to date, how reliable can the power supply be expected to be and is that adequate to meet the needs of the community?

In the Philippines, such an approach is routine. Within remote communities, utilities actually install the entire distribution system, with service connections and, through a memorandum of understanding, delegate the responsibility for the metering, billing, and collection to formally formed community groups.

The utility merely reads the meters at the transformers it installed in the various community and the community is responsible for collecting the fees and paying its bill.

Diesel/gasoline genset

Next to connecting to a grid-connected transformer, the use of gensets is the easiest approach to implement. Advantages of this technology are significant: gensets are readily available is all countries and they are low-cost and easy to transport and install (Fig. 5). But several disadvantages must be taken into consideration:

• Fuel must be delivered to the community on a year-around basis (unless it is stockpiled in the community). Can availability of fuel in the community be guaranteed in light of the reliability of transportation, accessibility by road during the rainy seasons, and political uncertainties?

• While the cost of fuel continues to be relatively low, are there any indications that cost will rise significantly or that supply will diminish sufficiently to discourage future use of this fuel?

• Gensets require expertise for regular engine

maintenance and, occasionally, major overhauls. A local source of expertise must be available in or to the community before this option is considered. Without this intervention, the life of the equipment may be short and may lead to frequent and costly replacement of equipment.

• Can environmental pollution commonly associated with internal combustion engines—noise, disposal of spent oil, and exhaust emissions—be adequately addressed?

While the availability of fuel and its cost may be of concern, it is interesting to note that diesel fuel or kerosene is already burned in wick lamps as a principal source of lighting in rural homes in many countries. Therefore, in these countries, fuel is already being purchased and imported into communities for lighting. Because burning fuel in a wick lamp for lighting is very inefficient, reliance on a diesel genset for electric lighting means that less fuel would need to be imported into a community to generate the same amount of lighting as the wick lamps currently use.

Another concern might be that diesel gensets generate carbon dioxide, a gas which is generally thought to contribute to global warming and its adverse impacts on the world environment. First, it should be recognized that the quantity of carbon dioxide generated by isolated grids for village electrification is insignificant in comparison to that generated by a country’s industrial or transportation sector or by its large powerplants supplying the urban areas. If the reduction of carbon dioxide emissions is truly of concern, it is in these areas that efforts can be cost-effectively focused, not in off-grid electrification.

However, at the same time, it should be noted that the introduction of diesel gensets for lighting in areas where wick lamps are being used can actually reduce carbon dioxide emissions.^*

Hydropower plant

All power systems harnessing renewable energy resources (wind, solar, and waterpower) have the advantage of low energy costs. However, the renewable resource with the lowest capital cost (cost per kilowatt installed) and possibly the only resource that can generate significant amounts of electricity on a

* For example, a typical wick lamp with glass mantel burns fuel at the rate of about 0.04 liters/hour and produces about 50 lumens. On the other hand, even relying on a very inefficient diesel genset (generating electricity at 1 kWh/liter rather than the 2 kWh/liter that is more typical for a small genset), a fluorescent unit (lamp and ballast) rated at about 10 W would consume only 0.01 liters/hour and produce about 400 lumens. So in this comparison, burning fuel in a genset produces 8 times the light, consumes fuel at the rate of one-quarter that consumed by a wick lamp (therefore emitting only one quarter the carbon dioxide), and keeps the emissions from combustion outside the home, reducing any respiratory problems that might be caused by one’s proximity to wick lanterns in the home.

Fig. 5. A low-speed 6 kVA diesel genset in southern Belize.

24-hour basis to feed into a mini-grid is waterpower. But having said this, the capital or up-front cost is still high. While the cost of a diesel genset might run several hundred dollars per kilowatt, the cost of a micro-hydropower plant (the equipment, powerhouse, and civil works) is usually five to ten times greater ($2,000 to $4,000 per kilowatt). Consequently, for such a plant to be viable, it is necessary to ensure that a significant portion of the available power is used for income-generating purposes (i.e., resulting in a high load factor). Otherwise, the plant will not generate the revenue required to cover this increased cost (Fig. 6).

One design option to reduce the cost of the micro-hydropower option is to share the cost of the civil works and the penstock (pressure pipe) with other uses for the water, such as irrigation or, occasionally, water supply. As noted in the case study of the project in the Dominican Republic, for example, the lengthy pipeline was initially purchased to bring water to the village for irrigation. It was this irrigation project that bore the cost of the pipe, resulting in an insignificant additional cost for the hydropower plant (p. 219). This was not possible in the plant in Youngsu, and in this case, the cost of the hydropower plant was a major contributor to total project costs (p. 209).

However, it should be noted that if several types of water projects are to be integrated to save costs, this must be known at the design stage. For example, the diameter of a pipeline designed only to supply a potable water system would normally be much smaller in diameter than one designed for a micro-hydropower plant. This results because a potable-water-supply pipeline usually handles a much lower flow and/or because excess water pressure is not needed to operate the system and can be dissipated in a small pipe. A micro-hydropower plant requires a large diameter to minimize energy loss through friction.

If a pipeline is to be used for both purposes, a large diameter pipeline would be required at the outset; it is

Fig. 6. This locally manufactured 14 kW micro-hydropower plant in Gotikhel generates power for about 110 households during the nighttime hours for a fee of $0.40 /month for a 25 W bulb.

During the day, it can run a range of electrical equipment, including a bandsaw and planer, as well as a mechanically-driven oil expeller. While electricity is an attractive product, it is the

mechanically-driven oil expeller which generates most of the plant's income.

not typically possible to incorporate a micro-hydropower plant in a pipeline for a project that originally was specifically designed to only supply domestic water.

Micro-hydropower also has a significant advantage in a village setting in that it first generates mechanical power that can easily and very efficiently be directly used to drive agro-processing, sawmilling,

refrigeration, and other productive-use equipment, in addition to driving a generator. In cases where the generation equipment encounters problems or the grid is not functional, it is still possible for the plant to serve the community and generate revenues by directly driving belt-driven equipment (Fig. 7).

In addition, with the little disposable income in many rural areas and the relatively high cost of electrification, the sale of electricity for household use usually does not generate adequate income to cover costs. Frequently, it is the other equipment that is directly driven by the turbine that generates the bulk of the revenues from the operation of a micro-hydropower plant.

In addition to the relatively high capital cost of micro-hydropower, several other factors must be considered:

• The mere availability of water or even a fall is no guarantee that sufficient resource exists. In addition to needing an adequate combination of flow and fall (head) to generate the required power, the terrain must be conducive to a cost-effective development of the hydropower scheme.

Are all these conditions met at the site?

• Actual projects costs are very site-specific, and someone with considerably experience

developing micro-hydropower sites should be involved in estimating cost. Furthermore, to ensure that the investment will yield expected returns, it is generally necessary to gather streamflow data for a period of at least one typical year prior to committing to the project, if a significant portion of the streamflow is to be used.

Fig. 7. Electricity generation is only one of many end-uses for this 13 kW micro-hydropower plant at Phaplu, Nepal. Most of the uses are directly driven by belts coupled to the turbine.

• The location of the resource determines the placement of the powerplant and, with hydropower, the distance between this location and the load may be considerable. Additional costs would be incurred in transmitting power over this distance, adding to the cost of the project. The

powerhouse must also be easily accessible at all times to ensure proper operation.

• The availability of the water resource—the streamflow—is subject to the vagaries of the weather.

In countries with monsoons or pronounced rainy seasons, it is quite possible to have insufficient water for power generation for nearly half the year. The question that will then have to be asked is whether half a year of guaranteed power is sufficient to justify the project. If the plant can only be used for half the year, then the cost of energy to cover costs must be roughly twice as high.

Storing water originating during the rainy season for use is the dry season is only an option with large hydropower plants. However, small but sufficient streamflow might be available during the dry season for daily storage, such as for storing water during the late evening and daytime hours for use during several hours in the early evening. But this is only an option with higher-head sites with low energy demand. Furthermore, constructing storage capacity can increase cost

considerably and create additional operations and maintenance problems.

Wind turbine

Like hydropower plants, wind turbines must be located where the resource is found. In the case of wind, this may mean on ridges and hilltops, while communities are usually found lower down the slopes or in the valleys. At other times, it may be on the coast, even within a community. But before such an option is adopted, it is necessary to ensure that the wind regime is adequate, both in terms of wind speed and in terms of its availability over the day and over the year. The turbine, tower, battery bank, and electronics are costlier yet than the previous options, on the order of $6,000/kW for units in the 5 to 10 kW range.

Because of the variability of the energy typically associated with wind turbine, other costs are imposed on this option:

• Possibly the most significant problem with relying on the wind resource is that, since adequate wind speeds are not always present, energy generated when little use is made of the electricity has to be stored in a battery until it is needed. This battery bank needed to store energy adds

considerably cost to the initial as well as recurring cost of such a system. Also required are electronics for battery charging and an inverter to convert the stored dc power into ac power as needed, so that it can then be distributed over the mini-grid to the consumers.

• Because of the limited availability of energy, a special electrical meter is required in the home to limit the energy (kWh) which each household can consume daily. If this were not included, it would be possible for a few households to consume the entire day's allotment of stored energy before the others can access their share. These meters are not commonly available and introduce a further cost to this option. Current limiters, such as a simple fuse, cannot be used for this purpose, because these limit current or power (kW) to the consumers but do not adequately limit the energy (kWh) that they consume over the day.

And, as in the case with hydropower, a knowledgeable individual is needed, but this time to ensure proper measurement of the wind resource. This usually requires the collection of data for at least one year before making a decision. Data already gathered in the immediate vicinity should give some indication of this resource but care must be exercised in extrapolating the results because windpower is sensitive to the local topography.

Solar PV station

A solar-PV-based system supplying a mini-grid would generate electricity and store it in a battery bank in a central location and then automatically invert it to alternating current (ac) when it is needed by the grid to supply consumers.

Solar energy has an advantage over the other renewable options in that this resource is more evenly distributed throughout the world. Furthermore, the amount of solar energy reaching a specific point on the earth over the year—the insolation—is known with a greater certainty than are wind or hydropower resources. Therefore, a year of data collection is not required to assess the extent of the resource before committing to a solar system. However, in areas such as those where burning rice stubble in the field or slash-and-burn agriculture creates a heavy haze for a month or two each year or in the mountains where fog typically persists until late morning, one has to be cautious about predictions on insolation based on other areas in the country that may not encounter these conditions.

The principal drawback to solar power for mini-grid application is that this option relies on considerably costlier hardware to harness this energy and make it usable. A complete power supply, with batteries, electronic controls, inverters, etc., costs at least $10,000 per peak kilowatt. This is equivalent to roughly

$60,000 per "real" kilowatt, i.e., a kilowatt that generates 24 kWh daily.^*

Another significant drawback is the fact that solar-PV generated electricity is direct current (dc) and, like windpower, must be stored in this form in a costly battery bank until it is needed. In addition to the capital costs, these battery banks need to be replaced periodically. For example, a 3-kWp solar array that might generate 10 kWh daily would require a 30-40 kWh bank of deep-discharge batteries costing at least

$4,000 and having to be replaced every 5 to 10 years. As with windpower, an inverter is also required to convert dc power to usable ac power when needed by the grid, adding further to cost and complexity.

And as with a wind system, where a limited quantity of energy is generated daily, an electronic metering device would also be required with a solar-based system to ensure that this energy is equitably available

And as with a wind system, where a limited quantity of energy is generated daily, an electronic metering device would also be required with a solar-based system to ensure that this energy is equitably available