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Powering Health

Electrification Options for Developing Country Health Facilities

Inverters

Power inverters

New power inverters at a health clinic in Rwanda. (Photo: Walt Ratterman)

Inverters convert DC to AC power. This capability is needed because batteries produce DC power while most common electrical applications and devices require AC power. DC-only systems are appropriate for health facilities with limited loads such as lighting, communication, and refrigeration.

Inverters fall into one of three classes, square wave, modified sine wave, and sine wave. Square wave inverters are the least expensive but their output, a square wave, is suitable only for resistive loads such as resistance heaters. Modified sine wave inverters produce a staircase square wave that more closely approximates a sine wave. This type of inverter is the most common. Most AC electronic devices and motors will run on modified sine wave AC. Some sensitive electronics, such as computers and stereo equipment, may not work with modified sine wave AC and require sine wave inverters. These inverters can produce power that is indistinguishable from what comes out of the outlet, but they cost more than the other types of inverters.

For off-grid applications the inverter must have stand-alone capability, i.e., the inverter does not need to be grid connected in order to regulate voltage and frequency. High conversion efficiency, especially at part load, is desirable. If the system includes a generator, paralleling capability allows the inverter to operate simultaneously with the generator.

Inverters are usually sized according to their maximum continuous power output. Most inverters however, are capable of handling significantly more power than their rated size for short periods of time. This surge capability is useful for meeting the occasional oversized load such as starting a motor.

Cost Considerations

Inverter costs are roughly $600-$1,000 per kW for good quality modified sine wave inverters. The technology for inverters larger than 5 kW is not as mature as for smaller inverters and costs may be somewhat higher.