Small wind energy systems can be used in connection with an electricity transmission and distribution system (called grid-connected systems), or in stand-alone applications that are not connected to the utility grid. A grid-connected wind turbine can reduce consumption of utility-supplied electricity for lighting, appliances, and electric heat. When the wind system produces more electricity than the household requires, the excess can be sold to the utility. With the inter-connections available today, switching takes place automatically.

Stand-alone wind energy systems can be appropriate for homes, farms, or even entire communities (a co-housing project, for example) that are far from the nearest utility lines. Either type of system can be practical if the following conditions exist.

Small wind generator sets for household electricity supply or water pumping represent the most interesting wind-energy applications in remote areas. Such generators can be very promising for the Third world countries as well where millions of rural households will be without grid connections for many years to come and will thus continue to depend on candles and kerosene lamps for lighting as well as batteries to operate radios or other appliances.
Wind turbines for domestic or rural applications range in size from a few watts to thousands of watts and can be applied economically for a variety of power demands.
In areas with adequate wind regimes (more than five meters per second annual average), simple wind generators with an output range of 100 to 500 W can be used to charge batteries and thus supply enough power to meet basic electricity needs. The families assign a very high priority to electricity and the range of services made possible by it (lighting, operation of radios and TVs). But relatively high investment costs of a complete wind-power system, which range from several hundred to a thousand US dollars or more, can be an obstacle for many households in developing countries.
In the past reliability of small wind turbines was a problem. Small turbines designed in the late 1970’s had a well deserved reputation for not being very reliable. Today’s products, however, are technically advanced over these earlier units and they are substantially more reliable. Small turbines are now available that can operate 5 years or more, even at harsh sites, without need for maintenance or inspections. The reliability and cost of operation of these units is equal to that of photovoltaic systems.

WIND vs. DIESEL OR GRID EXTENSION
Small wind mills are sometimes better than diesel generators or extension of grid because they offer a number of other socio-economic benefits. Wind systems are smaller, modular and have a shorter lead-time than grid extension. In many countries for grid extension distances as short as one kilometre a wind system can be a lower cost alternative for small loads. While they cost more initially than diesels they are much better from the users point of view. Some donor agencies, for example in developing countries, typically supply diesels at no cost, but leave operational costs (fuel, maintenance and replacement) to the local people. This requires scarce hard currency and usually results in limited utilization and a shortened life of the diesel because of inadequate maintenance. Many countries must also import their fossil fuels, further magnifying the burden imposed by diesels. In such case small wind mills seems to be the better alternative.
The economies of scale in small wind turbines makes them particularly competitive in cost for sizes above 250 watts. For daily loads as small as one kilowatt-hour per day a wind turbine will be less expensive than diesels, grid extension, or photovoltaics for virtually any wind resource above 4 m/s. This wind resource is available in most of the developing world. For larger daily load requirements the economics of wind power get progressively better. For a 10 kW wind turbine a wind resource of only 3-3.2 m/s will usually make wind the least cost option. There are not many areas of the world that have average wind speeds below 3 m/s .

In Asia, for example, 50 000 wind generators are currently in operation in Inner Mongolia. The success story in Mongolia was made possible by favourable climatic conditions, on the one hand, and a consistent development and marketing policy, on the other. A minimum monthly velocity above 5 m/s throughout the year in many parts of the vast grasslands provides for a continuous supply of electricity to the semi-nomads living in the region. Operating electric lights, a radio and a TV is one of the few modern technical conveniences available to the people living in these remote areas. On the other hand, several private companies competing with one another have developed cheap and affordable designs. The wind generators are sold locally. The local government subsidizes the price of the equipment with up to 50 % of the production costs.

COSTS
Small wind turbines can be an attractive alternative, or addition, to those people needing more than 100-200 watts of power for their home, business, or remote facility. Unlike PV’s, which stay at basically the same cost per watt independent of array size, wind turbines get less expensive with increasing system size. At the 50 watt size level, for example, a small wind turbine would cost about USD 8/W compared to approximately USD 5/ for a PV module. This is why, all things being equal, PV is less expensive for very small loads. As the system size gets larger, however, this “rule-of-thumb” reverses itself. At 300 watts the wind turbine costs are down to USD 2,5/W, while the PV costs are still at USD 5/W. For a 1500 W wind system the cost is down to USD 2/W and at 10 000 watts the cost of a wind generator (excluding electronics) is down to USD 1,50/W. The cost of regulators and controls is essentially the same for PV and wind. Somewhat surprisingly, the cost of towers for the wind turbines is about the same as the cost of equivalent PV racks and trackers. The cost of wiring is usually higher for PV systems.

SMALL WIND TURBINE COMPONENTS
The wind systems for remote or rural application is essentially the same as used with a PV system. Most wind turbines are designed for battery charging and they come with a regulator to prevent overcharge. The regulator is specifically designed to work with that particular turbine. PV regulators are generally not suitable for use with a small wind turbine because they are not designed to handle the voltage and current variations found with turbines.
Small wind turbines usually consists of : blades, alternator, regulation and control electronics.

Blades are usually made of carbon fiber reinforced composite that twists as the turbine reaches its rated output. This twisting effect changes the shape of the blade, causing it to go into stall mode. This limits the revolving of the alternator, preventing damage in high winds. Some small turbines do not have brakes and during period of strong winds they can change their orientation.Alternator is optimized to match as close as possible the energy available in the wind. It is constructed with permanent magnets and is usually brushless for best performance and maintenance-free operation.
Regulation and control electronics performs several functions to assure maximum output and safety for the user. The control electronics maintains a load on the alternator at all times to make sure that the turbine never over speeds, regardless of the condition of the battery. In case of battery charging, the sophisticated regulator periodically checks the line, correcting for voltage loss and monitoring charge rate. Once the battery has reached its optimum charge level the regulator shuts the current off, preventing the battery from being overcharged while maintaining a load on the alternator at all times to prevent over speeding.

APPLICATION OF SMALL WIND TURBINES

When considering renewable energy sources and their use in some remote areas wind energy is today once again a possible alternative to the diesel engine as an economical means of converting energy.The principal ways in which wind energy can be exploited in rural areas are as follows: for pumping water and producing compressed air; for generating electricity (and thus also for developing technologies which depend on electricity); for powering mechanical devices.

Water pumping
Wind energy has always been used extensively for pumping water, since there are no major problems involved in storing sufficient quantities of water without loss. Current estimates calculate that 100 000 wind pumps are installed around the world. Most of them are located in rural, non-electrified areas. They are used primarily by farmers for drinking water supply and livestock-watering. Wind pump technology is still of major interest for applications in the developing countries because of the importance of water supplies in rural areas, and the relative simplicity and transparency of the technology.
In view of the varying amount of wind energy available and the fact that, for economic reasons, the amount of storage capacity is limited, it can only be assumed in extremely rare cases that a single wind pump installation will be capable of ensuring a 100 percent reliable power supply. Hence, as a rule, these renewable energy sources can only be used as part of a combination of different systems appropriate to the case in question.
This means that for pumping water, be it for a drinking water supply, irrigation, or drainage, a suitable combination of different pumping systems with an optimized storage capacity should be installed. For small pump capacities up to approx. 10 m3/day, systems such as hand and foot pumps, capstans and, with certain limitations, solar pumps may be considered in addition to wind pumps where the water requirement is greater, motor pumps (diesel or electric) become competitive.
The question as to which combination of possible systems is the right one, i.e., the one which is most economical and best adapted to local conditions, depends on a variety of physical, socio-economic and sociocultural conditions which can differ considerably from one region to another. All of these conditions, which are not dealt with in more detail here for reasons of space, are of vital importance in the planning of rural water supply systems. Failures of projects for the introduction of wind pumps can, without exception, be traced back to the non-observance of one or more of these conditions or prerequisites.
Thus, for example, a combination of wind and hand pumps can be the right solution for providing a drinking water supply for a settlement, always provided that there is a sufficient amount of wind available. In the case of a small-scale irrigation system with wind pumps, a small, transportable diesel pump which can be used by several farmers is more suitable as a back-up system.
Other factors which have proved to be essential for dissemination on a larger scale are the existence and financial and technical capability of potential operators as well as the availability of marketing and service facilities in the area.
Today there are several water-pumping windmills on the market. They are designed to pump water in wind speeds as low as 2 m/s to 4 m/s from depths reaching 1000 meters. Typical water pumping windmill with a 3-m rotor can draw up to 2000 litres per hour from a depth of 10 meters at a wind speed of 3 m/s. Windmill with a 7-m rotor, can draw up to 8000 litres per hour under the same conditions. These systems can be used for irrigation, land reclamation or drinking water in remote areas. Windmills are designed for easy installation and require minimal maintenance.

IRRIGATION
The use of wind pumps for irrigation purposes seems to be problematic, since the water requirement and the availability of wind energy were generally subject to wide variations over the year. A good and above all constant wind regime is required to make them a viable option. Generally speaking, an annual average wind speed of four meters per second is a prerequisite for economic operation.
Typical project involving wind pump for irrigation was realised in Eastern Indonesia. This area has a short rainy season and traditional practice is for farmers to raise one rice crop per year. Two thirds of the time, during the dry season, the rice paddies are used only for grazing cattle. But many areas have substantial ground water resources which can be used for irrigation. In one project they dig wells, installed pumps, and trained the local farmers to use irrigation to raise higher value crops year-round. In most cases small 5 horsepower kerosene pumps are used for irrigation. These pumps are inexpensive and the fuel costs are partially subsidised by the government. But they also only last a few years and they operate at poor efficiency, so their life-cycle costs are quite high. Small wind systems cost more initially, but they have lower life-cycle costs. Project in Oesao, where the water table is only 2-5 meters below ground level, was based on use of the wind turbine which drives a surface mounted centrifugal pump. Pump is operated at variable voltage and frequency and its speed varies with the rotor speed of the wind turbine. The peak flow rate is ~3 litres/second. The system requires no fuel and no regular maintenance. A kerosene pump is, however, used for back-up. The Oesao system was installed in 1992 as a pilot project to show that wind power could be effective for water pumping in Eastern Indonesia. Since that time fifteen additional systems have been installed and more systems are planned.

TELECOMMUNICATION
Wind power is an excellent source of power for telecommunications sites because the height and exposure that make for a good antenna site also make for a good wind energy site. But wind turbines for this application must be particularly rugged because of the harsh conditions often encountered on mountains.

BATTERY CHARGING
Utilisation of small wind turbines for lighting, TV or refrigeration is quite simple through battery charging. Storing wind produced electricity in battery gives a homeowner a possibility to use this power whenever it is needed. Many small wind turbines directly produce 14 or 28 V . Some smaller wind turbines and other larger types produce higher voltages. 12 V o 24 V output from the battery can be used directly for DC appliances or inverted to 240 VAV current. For standard domestic appliances. It is usually best to directly charge the battery from the wind as this will not load the wind turbine at low speed causing stalling of the rotor.

HEAT STORAGE
If there is a need for hot water it is better to use direct wind generated electricity via an immersion heater to standard hot water tank and store the hot water. Battery storage is always more expensive than heat storage. The simplest system for water heating uses a thermostat to protect the water from boiling. The immersion heater should match the wind turbine rating. If a 1 kW turbine is used the immersion heater should also be rated at 1 kW (most domestic immersion heaters are 3 kW).

Wind - Solar Hybrid Systems
Solar and wind energy are complementing each other well under average seasonal conditions. In winter, when there is much wind, room heating is needed while in summer with much sun domestic hot water is needed. The combination of solar-wind is very interesting in the so-called off-grid electricity systems. These are self-supplying plants which are not coupled to the public electricity grid. A photovoltaic plant has a relatively high production in summer and a relatively small production in winter. This means that an off-grid system will either result in a heavy over-production in summer or should be equipped with a seasonal storage. Both solutions will be very expensive. A wind power supply can have serious problems in summer when periods with no wind may occur. The combination of solar-wind is therefore evident.
The important question, what the proportion between the solar and wind plant should be, have to be answered by the planner of the facility. It is obvious that the answer depends on energy needs during the year and a site conditions.

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