Designing off-grid energy systems can be complicated and should be done by a trained professional. First, it is important to emphasis the importance of doing a design! One-size fits all approaches to health facility energy system procurement are rarely effective. A proper design must be done by a professional, include input from several stakeholders, and should be reviewed after system installation to check if assumptions were correct.
Consider using the energy audit template to collect information for the design process. The information below reviews some lessons learned for PV or PV/diesel hybrid system design at health facilities:
One common design error lies in assuming that the watt hours arrived at during the energy load calculation study is equivalent to the energy design watt hours. A health facility’s need for energy grows. If a facility needs 10,000 watt hours/day, then it is standard to design around a load which is 20 percent larger. Once a system has been installed it is very common for additional loads to be added. A strong manager must be designated to ensure that loads are restricted to the original design size.
Watt-hour calculation charts typically include a calculation for daily and weekly average use of energy. The weekly average basically takes a facility that operates for 5 days/week, and averages out the energy requirement over the seven days in the week. Some people use this (lower) weekly average as a design watt-hour use. The weekly number is important for a designer to know (to compare how different the average is versus the daily), but should not be used as the basis for the design.
Solar resource assessments and resource availability data do not exist for many developing countries. There are, however, some guidelines that show annual average PSH as well as monthly averages. Some designers tend to use the annual average number for their design. In areas where there is almost no monthly deviation, this is fine. But where there is deviation, these designers are putting together systems that by virtue of their design alone, will not work half of the year.
When designing a solar system, one must consider the number of days that the system should operate without charging (e.g. cloudy weather) – called the “days automomy” Typically designers use two or three days.
More importantly, if the batteries are designed for 3 or 4 or 5 days of autonomy, and the panels are designed to produce one day’s worth of energy on a sunny day, then after the first bout of clouds, the batteries are never fully charged again unless all of the loads are shut off to let them charge … or unless the system has a generator.
This is probably the biggest problem with systems is that batteries never get charged. Batteries operating at the 50 percent level might last 6 months instead of batteries at the 80 percent level which can last 6 years. For this reason it is advisable to consider adding a generator to all but the smallest sized PV systems for health facilities.
The defaults on most inverters, for voltage cut-offs and cut-ins are generally designed for highly maintained systems in the U.S. Proper design of systems in developing countries often requires that low voltage cut-off levels be increased, in order to protect the batteries. Inverters which have an easily accessible low voltage bypass option are not a good choice for developing country health facilities.
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Last updated: September 28, 2011
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