by David Murphy and Cole Brawner, Fluor Corp.
Solar photovoltaic (PV) power plants continue to expand their impact as a dependable supply of electric power in the U.S. Growing implementations of PV onto the power grid will lead to greater emphasis on operating PV systems efficiently. Compared with traditional generating technologies, PV plants have low maintenance and subcontracting requirements. As PV generation output increases, it will become more important to improve operation and maintenance (O&M) performance to save cost, along with the reduction in overall PV capital cost.
PV plant design involves implementing a series of features aimed at achieving the lowest possible cost of electricity. Choosing the correct technology, such as modules and inverters, is important. Selecting a module requires evaluation of complex variables. This evaluation would include: capital cost; power output; technology benefits, restrictions or both; equipment quality; operational response; performance in low light; nominal power tolerance levels; degradation rate; and equipment warranty terms. The main components of a PV power plant are PV modules, mounting or tracking systems, inverters, transformers and the transmission interconnection.
Solar PV modules are made up of PV cells, most commonly from silicon. In general, silicon-based crystalline wafers provide high-efficiency solar cells. Good-quality PV modules are expected to have a life of 25-30 years, although their performance will degrade steadily over this period.
Remote monitoring capability of PV plants has advanced significantly during the past decade and can offer insights at an increasingly granular level. Given that most PV plants are unmanned and remotely supervised, remote monitoring improvements offer potentially huge performance and reliability benefits. Choosing the appropriate monitoring level is system- and equipment-specific. The value of increased system output and reduced downtime will need to exceed the associated costs of advanced monitoring and data analysis competence. Suitable monitoring needs will depend on system size, distance from O&M facilities, uptime priority, operating rationale and data analysis capability. Remote operation access can be set up as “read-only” without control, or with “write” access to allow for complete control remotely.
To achieve a high-performance PV plant, the incorporation of automatic data acquisition and monitoring technology is essential. This allows the yield of the plant to be monitored and compared with calculations made from solar irradiation data to raise warnings daily if there is a shortfall. Faults then can be detected and rectified before they have an appreciable effect on production.
In addition, power plants typically need to provide 24-hour forecasts (in hourly increments) to the network operator. Daily forecasts help network operators ensure continuity of supply.
Three types of maintenance strategies are used in any successful O&M program. Predictive, preventative and corrective maintenance each are performed at PV plants as they are at fossil-fueled plants.
Predictive maintenance. Predictive maintenance uses real-time and manually acquired data to prioritize and optimize maintenance and resources. Though largely incipient, an increasing number of third-party integrators are developing conditioned-based measures (CBMs) to offer greater O&M efficiency. The increased efficiency comes with a high upfront price tag, given the communication, monitoring software and hardware requirements. Manual predictive maintenance activities include thermography and cable diagnostics.
Preventative maintenance. Preventative maintenance (PM) requires routine equipment inspections to prevent breakdowns and unplanned production losses. Proper evaluation of the required original equipment manufacturer (OEM) inspections and maintenance activity should be analyzed and PM planned to maintain requirements as set forth in the warranty terms of the contract. A well-organized PM program can lower the probability of unplanned PV system downtime. A structurally deficient PM program can increase labor costs as a result of activities that are implemented incorrectly.
Corrective maintenance. Corrective maintenance (CM) addresses unplanned equipment breakdowns and is considered unplanned downtime. This “break-fix” utility practice allows for low upfront costs but creates a higher risk of major component failures and exorbitant costs. CM often renders a premium on negotiating warranty terms with suppliers and contractors. A certain amount of CM during a plant’s 20-year lifetime can be circumvented by deploying PM and predictive maintenance strategies.
Computerized Maintenance Management System
The most important feature in keeping O&M cost to a minimum is through a thorough review of the plant design, construction inspections and establishing adequate commissioning procedures. Commissioning data should establish the performance baseline and be recorded in the computerized maintenance management system (CMMS). This information can be compared to the ongoing plant performance data over the life of a system. Scheduling multiple O&M tasks can improve the cost benefit and frequency of performing routine tasks. Conduct PM work either early in the morning or late in the evening to avoid heat stress and production losses and minimize electrical hazards.
A CMMS software package maintains a computer database of information about an organization’s maintenance operations. CMMS is intended to help maintenance workers do their jobs more effectively by determining which machines require routine maintenance along with storeroom inventory and spare part allocation. This process allows management to make informed decisions by calculating the cost of machine breakdown repair vs. PM.
Impact to PV Plant Performance
Soiling. The accumulation of dust on solar panels decreases solar PV system efficiency and has a negative impact on the economics of PV systems. Rain can improve module efficiency by 3 to 5 percent and reduce the need for costly, labor-intensive, manual water washing. Analyze the yearly rain fall patterns in the location and do not commit to panel cleaning too soon. A detailed monthly soiling report can help in decision-making.
Inverters. These solid-state electronic devices convert DC electricity generated by the PV modules into AC electricity suitable for supply to the grid. Proper infrared scanning and monitoring can identify problems to be corrected before an inverter failure that could result in a loss in megawatt production.
Tracking systems. Tracking systems allow PV panels to move with the position of the sun and maximize the DC input to the inverter. Check and calibrate tracking systems a minimum of two times annually to adapt for summer and winter solstice.
Vegetation management. Manage weeds to avoid fire hazards and array shading. Also be aware of the unexpected consequences of contractors’ using weed control equipment and damaging solar equipment on-site by way of flying debris, etc.
Performance ratio. Performance ratio (PR) is a key metric in a PV power plant. The PR quantifies the overall effect of equipment failures and efficiency on the rated plant potential output. The PR can be used to compare PV systems independent of size and solar resource. A PR percentage varying from some 77 percent in summer to 82 percent in winter (with an annual average PR of 80 percent) would not be unusual for a well-designed solar PV installation or plant, depending on ambient conditions.
Contract warranty terms and claim procedures are vital to the success of an O&M program and must define clearly system performance, equipment repair responsibility, eligibility and response time requirements. Formulating strategic stipulations that cover the significant problem items and implementing a signed procedure that satisfies all affected parties is paramount in maintaining healthy relationships among plant owners, site managers, engineering, procurement and construction (EPC) contractors, system integrators and component suppliers. Performing a rigorous initial site commissioning ensures plant components are functioning correctly and helps guarantee performance adequacy and lessen life cycle O&M costs.
Utility-grade PV plant systems are becoming increasingly popular as the power industry migrates toward renewable business. Although the fuel is essentially free and staffing levels are kept extremely low, OEM required maintenance must be kept up-to-date. Adequate O&M practices ensure warranties are maintained, plant reliability remains optimum and overall plant performance is sustained.
David Murphy is senior director of operations at Fluor Corp.
Cole Brawner is an operations and maintenance specialist with Fluor Corp.
Fluor designs plants to meet customers’ specific needs and strategically selects inverters, trackers and distributed control systems (DCS) accordingly. Each plant has the option to be run locally or remotely and to be covered by a defined period warranty on all the equipment. Fluor-operated plants are monitored from the Fluor Monitoring Center in Dallas.