Proper O&M practices are essential for acquiring accurate solar resource measurements. As addressed in this subsection, several elements in a chain form a quality system. Collectively, these elements produce accurate and reliable solar resource data: station location, measurement system design, equipment installation, data acquisition, and O&M practices. Proper O&M requires long-term consistency, attention to detail, and a thorough appreciation for the importance of preventative and corrective maintenance of sensitive equipment.
Calibrations are performed with clean instrument optics and a carefully aligned detector. To properly apply the calibration factor, the instrument should be kept in the same condition during field measurements. To maintain the calibration relationship between irradiance and radiometer output, proper cleaning and other routine maintenance is necessary. The maintenance process includes:
• Checking the alignment of the detector. Pyrheliometers must be accurately aligned with the solar disk for accurate DNI measurements. Pyranometer detectors must be horizontal for GHI and DHI measurements and accurately aligned with a flat-plate collector for POA measurements. The radiometer orientation should be checked periodically using the features described in Chapter 3. (In some cases, a carefully leveled pyranometer may produce GHI readings that are not symmetrical around solar noon under clear skies. If this cannot be attributed to any change in atmospheric composition (aerosols or water vapor), or optical asymmetries can be verified under strict laboratory conditions, the optical axis of the detector is probably not exactly vertical. This is a manufacturing defect.
• Cleaning the instrument optics. To properly measure the solar intensity, no contaminant should block or reduce the amount of sunshine falling on the detector. The outdoor environment provides many sources of such contamination, such as dust, precipitation, dew, plant matter, insects, and bird droppings. The sensors should be cleaned regularly to minimize the effect of contaminants on the measurements. Depending on the local conditions, this can require daily maintenance of unventilated or otherwise protected radiometers.
• Documenting the condition of the radiometer. For analysts to understand limitations of the data, conditions that affect the measurement must be documented. This includes substandard measurement conditions, but it is just as important to document proper operations to add credibility to the data set. Observations and notes provide a critical record of conditions that positively and negatively affect data quality.
• Documenting the environment. As a consistency check, note the sky and weather conditions at the time of maintenance when interpreting data from the radiometer, including measurements with unusual values.
• Documenting the infrastructure. The measurement station as a whole should be examined for general robustness. Any defects should be noted and corrected.
Maintenance frequency depends on prevailing conditions that soil the instruments. This includes dust, rain, snow, birds, and insects. It is also depends on instrument type. Radiometer designs based on optical diffusers (such as LI-COR LI-200) are less susceptible to dust contamination than are instruments with clear optics (Myers et al. 2002). This may be caused by, in part, the area subject to soiling (e.g., a larger dome versus a smaller diffuser). Also, fine dust on the surface of a diffuser can become an integral part of the diffuser, and it may lessen the impact of the dust on the diffuser transmittance compared to that on a precision-ground optical dome. Soiling of the windowed or domed radiometers can quickly affect the measurement and increase by many-fold the measurement uncertainty. This is especially relevant for pyrheliometers (Geuder 2006). As described earlier, a pyranometer in a ventilator can reduce this risk of contamination; thus, the frequency and cost of maintenance should be considerations in instrument specification.
If a remote site will be difficult to maintain for extended periods, a higher class windowed instrument might not be optimal, despite its potential for better measurements. The cost of maintenance for a remote site may dominate the estimated cost of setting up and operating a station. This aspect should be anticipated when planning a measurement campaign.
A conservative maintenance schedule will support the credibility of the measurement data set and provide the analyst a base of justification when assigning confidence intervals for the data. Daily inspection should be scheduled for instruments with clear optics, and twice monthly inspections should be scheduled for diffuser instruments. More frequent spot inspections should be conducted after significant weather events (e.g., dust storms, heavy rainfall, rainfall during periods with high optical depth, and storms). Radiometer optics may not necessarily soil within a 24-hour period, but the effects of soiling can best be mitigated with frequent inspection.
Radiometers should be carefully cleaned at each inspection, even if soiling appears minimal. Cleaning is generally a very short procedure, and it removes the possibility of differing
interpretations of the need to clean among different technicians. With such a procedure in place, the analyst can claim with confidence that the instruments were kept clean according to the documented schedule.
Maintenance at remote measurement sites away from institutional or corporate employment centers will require finding a qualified person nearby who can perform the necessary maintenance duties. The qualifications for maintenance are generally nontechnical, but they require someone with the interest and disposition to reliably complete the tasks. As a rule, compensating these people for time and vehicle mileage—rather than seeking volunteers— becomes a worthwhile investment in the long run, because it sets up a firm contractual
commitment to perform all necessary maintenance duties. Absent that formal relationship, it can become difficult to assert the need for reliable and regular attention.
All O&M should be carefully documented with log sheets or preferably with electronic databases that contain enough information to reveal problems and solutions or that assert that the
enables an analyst to identify potentially bad data, and it provides important documentation to determine and defend the overall quality of the measurements.