In today’s popular solar publications the focus is unerringly on solar’s leading edge: technology, technique and policy, with occasional forays into finance and sales. What is often missing is cogent discussion on solar’s trailing edge: operations and maintenance.
The omission can be attributed to a pervasive misconception that solar is a static technology: build it and it will work error-free for 30 years. This philosophy works well in a dark vacuum. But in the unpredictable cycles of weather, solar facilities require a watchful eye, intuition, skilled hands and a budget. In a sense, operating a solar array of any size is similar to raising a child. They’re expensive and each one has its own personality requiring patience and considerable attention.
As director of Asset Management Services for Solar Power Partners, I oversee the health and wellbeing of over 40 solar electric facilities (SEFs) generating more than 21 GWh a year. Our principal business is in financing, owning and managing a constellation of commercial-scale SEFs including fixed rooftop arrays, fixed elevated arrays, tracking elevated arrays and single and dual axis ground mount arrays. System sizes range widely from 100 kW up to 2.4 MW. Our PPA customers include schools, universities, airports, retail stores and municipalities. All told, our portfolio of assets is a fine cross-section of distributed solar.
It is also a daunting exercise in O&M made manageable with accurate upfront performance modeling, a reliable and effective monitoring system, active monitoring by knowledgeable personnel, realistic preventative maintenance policies and budgets, solid warranty backing, efficient reactive repair policies and targeted analytics.
This is the second of three articles that will focus on O&M issues for solar electric facilities. We began in the September/October issue by examining some of the factors that contribute to an effective O&M strategy. In this article we will look at preventative maintenance strategies, warranties and budgets. Part III, which is scheduled for the January/February issue, looks at both critical and non-critical reactive repairs.
Preventative maintenance (PM) involves panel cleaning, inverter servicing, tracker maintenance (when present) and balance of system inspection. Frequency and scheduling of PM are dictated by environmental factors and warranty requirements; panel cleaning is usually performed as needed while the scheduling of inverter, tracker and balance of system maintenance tends to be at fixed intervals. The value of preventative maintenance is measured in system efficiency and system/component lifespan. System efficiency is immediately quantifiable at any given time while lifespan is a single long-term result. Yet both are affected by how well a system is maintained.
Balance of System
The lowest level of attention is often given to the balance of system components because of their inert stature within a system. Fixed mounting structures, conduit runs, junction boxes, combiners, communications equipment and disconnects are static components that require annual inspections to identify corrosion and loose connections. Rust and signs of water infiltration inside enclosures are immediate red flags while weak electrical connections in combiners and junction boxes are identified through thermal imaging. Loose mechanical connections are identified by touch.
Preventative maintenance of communications equipment involves thorough inspection of enclosures to ensure a clean and dry environment and tight wire connections. Periodic measurement of communication equipment power supplies and signal strength can identify weak or failed components. For systems without string level monitoring, measuring string current is an integral part of preventative maintenance.
Trackers: While balance of system components tend to be motionless, trackers are in constant, controlled motion. The continuous movement necessitates biannual inspection for wear and tear of friction components, motors, sensors, actuators, cables, hydraulic lines, gearboxes and anything else that moves or is exposed to the elements. Controllers and sensors must be checked periodically for calibration, alignment, weak signals and loose connections and the integrity of enclosures must be maintained. Gears and actuators must be greased, hydraulic fluids topped off and periodically replaced and corrosion must be kept in check.
Inverters: Inverters tend to receive the greatest preventative maintenance focus due to their susceptibility to failure, sizeable upfront cost and importance to system operability. Inverter maintenance varies according to the size of the inverter and the manufacturer. Normally it includes inspecting and tightening connections, ensuring water tightness, cleaning and replacing air filters, lubricating moving components like fans, handles and disconnects, running electronic diagnostics and performing thermal scans to identify hot spots. Twice-annual inspections are common with yearly maintenance mandatory to maintain the warranty with most manufacturers. The goal is to prevent equipment breakdowns and lengthen the inverter life. Many inverter manufacturers offer service plans and third-party servicing is available through numerous solar integrators and electrical contractors.
Panel Cleaning: Panel cleaning is where we can see immediate and dramatic results. It is also the least structured area of preventative maintenance with schedules being elusive and dependent upon the environment. However, because our aim is more about optimizing energy production and less about preventing component failure, we have a goal with measureable parameters (energy production) guiding us to a flexible cleaning schedule.
To statistically judge when a system should be cleaned, we take actual energy production as measured at the meter, adjust it using the ratio of actual solar insolation to expected solar insolation for the solar electric facility (SEF) and compare the result to projected production adjusted for degradation over time. The result is our system efficiency.
At SPP, we define a soiling boundary for each SEF below which any further loss in efficiency will cost us more than the cleaning cost of the SEF. Before we hit this boundary, a trigger tells us that we should start to consider cleaning the system.
Calculating system efficiency and knowing when an SEF should be cleaned is simple. Placing that information into the context of a budget is a different story.
For PPA (power purchase agreement) providers and large commercial and utility scale system owners, expenses associated with a system are modeled before a SEF is built. Doing this enables financial backers to calculate the risk and rewards of their investment. We’ll speak more on this subject later, but here the budget directly affects when we clean an array and has little connection to a capricious environment.
Ideally, we would like to clean an array the moment its efficiency drops to the calculated soiling boundary. In the real world we must work within the constraints of a budget that often provides for a set number of cleanings per year. We may cross the soiling boundary four or five times in any given year, but we are constrained to one or two cleanings. The question then is how to make the most efficient use of those cleanings. The answer depends upon countless variables that we whittle down to a few generalizations:
- When is peak production? We want to optimize production in peak season. Late spring, summer and early fall are peak production times in California so we want to optimize output during these seasons.
- What seasonal events create exceptional soiling? Pollen is a huge soiling problem in agricultural areas. Windblown dust collects in dry seasons. Birds tend to congregate in specific seasons (and pigeons all the time).
- Is it too hot to clean panels? Midsummer panel temperatures can reach precarious heights. Cleaning hot panels with water can result in thermal fractures.
- Is rain expected? Rain events can delay the need for cleaning.
- When is site access possible? Some businesses restrict access to their facilities. Examples include schools in session, airports requiring access pre-approval, and agricultural facilities in peak harvest.
You can see why scheduling panel cleaning is such a nuanced science requiring the insight and intuition of a seasoned asset manager with complete knowledge of each SEF and its operating environment. It is a balancing act between three disparate subjects: budget constraints, environment and production.
To add yet another layer of complexity, different installations require different cleaning methods. Ground-mount and flat-roof installations are relatively cheap and easy to clean provided a ready source of water exists. Tilted roof mounted arrays have personnel safety considerations and roof arrays built edge to edge require special equipment to reach long spans of panels without walking on them. Cleaning elevated arrays such as car shades often involves the use of a mechanical lift. Some arrays are designed in such a way that access with cleaning equipment is impossible and a water jet must be employed. These various methods all carry specific maintenance costs that must be budgeted before construction; otherwise, the systems will experience perennial budget shortfalls or improper cleaning.
Panel cleaning services exist in the marketplace today. Many are inexperienced entrepreneurs with inadequate PV knowledge, insufficient insurance coverage and an unrealistic pricing structure. At a minimum, each individual in a cleaning crew should know how to identify broken panels, deteriorated wiring and other dangerous SEF conditions; each worker should be covered under a general liability insurance policy; and pricing should reflect the nature of the job. The simplest of ground mounted and flat rooftop mounted systems should not cost any more than $.0025/watt to clean in 2009-10 dollars. More complex jobs will require a larger budget but should not exceed $.01/watt. As we tell every window cleaner who approaches us with a $.05/watt bid: “These aren’t windows.”
At SPP we clean panels with a weak vinegar solution (2 Tbsp/Gal. water). We do not use de-ionized water or special filters and we do not squeegee the panels dry. The vinegar breaks the water’s surface tension causing it to sheen from the glass without spotting. It is also safe for roofing surfaces and the environment and it is inexpensive. We rinse the heaviest material away then use wide, soft-bristle brushes to mechanically remove soil before following with a rinse. For larger unbroken spans of panels, we use extendable window washing poles and brushes. For those spans too large for such devices, we use a water pump that forces a large volume of water great distances. It is not a pressure washer and will not damage the panel seals. The key to all of these methods is to save money and time. We typically clean a 250kW flat-roof array mounted at 20 degrees in four hours with three laborers.
A Word on Budgeting
Perhaps the most hotly debated topic between finance and asset management is the level of O&M budgeting. The asset manager would enjoy a large budget to fulfill all necessary O&M obligations without stress. The finance department needs to keep expenses to a minimum. A compromise is needed.
Monitoring costs are known and calculated according to the type and quantity of equipment. Maintenance costs are more erratic ranging wildly according to:
- System size and location
- SEF construction (ground mount, roof mount, tracker and so on)
- Number of distinct arrays, meters and inverters in the entire SEF
- Availability of water
- Ease of access to the PV modules.
Balance of system, inverter, and tracker maintenance are relatively easy to budget; only skilled, experienced technicians can perform these tasks and the scope of work tends to be the same across different arrays. Discrepancies arise due to SEF geographic location and travel distances to them.
Panel cleaning is different because the level of work is difficult to judge until the SEF is constructed and the cleaning performed. After construction, costs are easier to estimate. This, however, does not help the financiers establish a budget in the pre-construction financial modeling. A buffer must be built into the initial O&M budget to account for the lack of complete cost knowledge. This balloons expenses that in turn weaken financial viability.
At SPP, to limit the need for a sizeable O&M budget buffer, we rely on our portfolio of projects to smooth the discrepancies between actual and budgeted expenses. We estimate O&M expenses for each SEF using assumptions based on historic pricing as well as assuming two maintenance visits/cleanings per year. With these estimates we negotiate maintenance contracts with outside service providers. Maintenance costs for some SEFs will end up over budget, others under, but they tend to sum very close to our overall portfolio O&M budget.
System warranties can be wonderful things by outlining who is responsible for system repairs. They often provide repair response requirements and system performance thresholds and they help maintain a healthy relationship between the system manager and the integrator. Most integrators provide some form of warranty coverage for the systems they construct while some localities mandate specific warranty coverage. In California most systems are built with financial assistance from the California Solar Initiative. In exchange for this assistance, the program mandates that for all SEFs a system integrator builds, they must provide a 10-year warranty on the SEF power generating equipment, which is virtually everything from the mounting hardware to the meter.
The trouble with warranties is that they are only as good as the company that backs them. If a poorly managed integrator goes out of business two months after completion of an SEF, then the warranty is worthless. This holds true for all system warranties from panel performance warranties to inverter up-time guarantees. To avoid being left with a worthless document, the system owner needs to perform a little due diligence before agreeing to a construction contract:
- In localities without mandated warranty coverage, what are the specifics of the integrator’s system warranty and what is the term of coverage?
- The owner should know the history and financial strength of the system integrator as well as those of the component manufacturers whose products are specified for the system.
- The owner must ensure that the integrator understands and has budgeted for warranty coverage over the length of the warranty period.
If these questions are met with solid answers, the SEF manager can most likely count on a friendly, long-term relationship with the integrator. Any weakness can result in confrontation and unforeseen financial burdens.
Our series concludes in the January/February issue with a look at critical and non-critical reactive repairs.
Bryan Banke is director if Asset Management Services for Solar Power Partners and has been with the company since November 2007. He was a private solar PV project analyst and feasibility consultant with Banke Energy. His career in energy began in the 1980s as an analyst with Southwest Gas Corp.