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Everything You Need to Know About Operations & Maintenance (O&M) For Utility Scale PV Solar Plants

02.09.2017 — 0

From the key issues to contracts structure, all explained in brief.

Once the PV solar plant has been built, it needs to be efficiently operated and carefully maintained. Compared to other power generating technologies, solar PV power plants have low maintenance and servicing requirements.

However, as International Financial Corporation warns, “proper maintenance of a PV plant is essential to maximise both energy yield and the plant’s useful life. Optimal operations must strike a balance between maximising production and minimising cost”.

Indeed, while solar energy does require almost no maintenance at all as compared to the other generation sources, PV solar plants are investments that are likely to last for 20–25 years or more, and that’s why in order to arrive at an accurate ROI figure, one needs to address the operation and maintenance issues.

Thus, before turning to the actual process and stages of maintenance and operation, one needs to understand the issues involved in the functioning of a PV solar plant. Naturally, they can be divided into the groups according to the plant’s main components.

O&M Issues in PV Solar Energy

1. Natural Degradation

All solar cells naturally degrade over time, regardless of the environment they are in. This is called natural degradation, and is completely normal for all solar cells to experience once in operation. Depending on the material, the rate of degradation can vary. This is important to take into account in budgeting and investment planning.

The following table summarizes the degradation rates of solar panels made of different materials. As is clear from the table, Solar DAO PV plants are among the most robust ones, since the solar panels used are made of crystalline silicon which is characterized by the one of lowest annual degradation rates.

Natural degradation cannot be prevented, but must be taken into account in the planning process. It can also be covered by warranties. Usually, manufacturing companies that produce solar modules offer warranties if degradation rate exceeds certain amounts, for example, if it is more than 0.8–0.6% depending on the particular firm. The good news is that the higher quality panel, the less natural degradation.

The degradation rate must be weighed against the cost and the utility of particular materials from which the solar models are made. The following chart, provided by Scandia Labs, demonstrates the estimates for Average Utility-Scale Solar PV O&M Costs, by Technology ($/kWAC-yr), including different types of solar panels materials, as well as different types of trackers with which the panels are equipped. Here, again, crystalline silicon stands out, as do conventional solar panels as opposed to concentrating photovoltaics that uses lenses and curved mirrors to focus sunlight on the solar cells.

  • CdTe — cadmium telluride;
  • CIGS — copper indium gallium selenide;
  • c-Si — crystalline silicon;
  • SAT — single-axis tracking;
  • DAT — dual-axis tracking;
  • CPV — concentrating photovoltaics.

2. Grounding and Lightning Protection

PV solar plant is a structure of considerable size, which is why some lightning protection is in order. The first level of such protection is the ground mount system itself, whereby the grounding system redirects the energy from the lightning into the ground and away from the panels. Depending on the foundation, different forms of grounding can be used, as summarized in the following table provided by the Desert Research Institute:

Note that copper conductor may be tinned, and that aluminium is not allowed to be buried into the soil. It is also important to use the same type of metal in both the grounding system and in the protection equipment, so as to avoid corrosion.

Even with a proper grounding system, a PV installation can still be at risk from lightning. Even after the lightning energy has been discharged into the ground, it can still cause a power surge within the solar panels array, which is why a surge protection equipment is in order. In some cases it is not needed, if the grounding system is effective enough to reduce the lightning strike energy.

3. Component Failures (panels, inverters, trackers)

3.1. Panel cracking.

Different components of PV solar plant may fail during the operation. First, panels might crack, even in the new once, if they have been damaged in the manufacturing process. The micro-cracks are not always obvious, and that’s why the new panels must be inspected and a warranty must be secured. The cracks may lead to the failure of panels or losses of optimal efficiency.

3.2. Visual discoloration.

Visual discoloration is another common defect that reduces the amount of sunlight that penetrates into a solar cell. As a result, solar cells are less exposed to solar irradiation, and generate less energy. The reason it leads to loss of efficiency is because different color panels changes the wavelength of light that can be absorbed. As in the case with panel cracking, not much can be done once the panel became discolored, hence the solar panels must be carefully operated and maintained.

3.3. Hotspots.

Contrary to the common misleading opinion, solar panels are most efficient when they gain maximum solar irradiance, not maximum temperature. Quite the contrary, high temperatures can actually damage solar panels, leading to the emergence of the hot spots. Hot spots occur when a panel is shaded, damaged, or electrically mismatched and decrease power output. Since solar cells are attached in strings, just one hot spot can lead to multiple cells functioning poorly. To solve this problem, all shading should be negated, and electrical connections should be optimized.

3.4. Inverters failure.

Generally, inverter faults are the most common cause of system downtime in PV power plants. Therefore, the scheduled maintenance of inverters should be treated as a centrally important part of the O&M strategy.

3.5. Trackers and Panel Orientation.

Panel orientation is an issue for static PV solar systems. It requires due diligence on the consumer’s part to make sure the installer is taking the proper steps necessary to determine an ideal panel orientation. Similarly, tracking systems also require maintenance checks. These checks will be outlined in the manufacturer’s documentation and defined within the warranty conditions. In general, the checks will include inspection for wear and tear on the moving parts, servicing of the motors or actuators, checks on the integrity of the control and power cables, servicing of the gearboxes and ensuring that the levels of lubricating fluids are appropriate. The alignment and positioning of the tracking system should also be checked to ensure that it is functioning optimally. Sensors and controllers should be checked periodically for calibration and alignment.

3.5. Structural Integrity.

The module mounting assembly, cable conduits and any other structures built for the solar PV power plant should be checked periodically for mechanical integrity and signs of corrosion. This will include an inspection of support structure foundations for evidence of erosion from water run-off.

4. Weather Conditions (snow, wind, soiling).

Finally, depending on the environmental conditions, the panels must be protected from wind, snow, and soiling (in dusty areas). Regular cleaning and maintenance will be enough in these cases. Solar DAO uses durable crystalline silicon panels that are built of lead-free, optically transparent, anti-reflective glass, which can withstand the tested shot of an ice ball with 35mm diameter at a speed of 30 m/s. Their serviceable life is up to 25 years, with 10 years of guaranteed performance.

5. Other issues

Other common unscheduled maintenance requirements include but are not limited to:

  • Tightening cable connections that have loosened.
  • Replacing blown fuses.
  • Repairing lightning damage.
  • Repairing equipment damaged by intruders or during module cleaning.
  • Rectifying SCADA faults.
  • Repairing mounting structure faults.
  • Rectifying tracking system faults.

O&M Approaches and Activities

Maintenance can be broken down in two parts:

  • Scheduled maintenance: Planned in advance and aimed at fault prevention, as well as ensuring that the plant is operated at its optimum level.
  • Unscheduled maintenance: Carried out in response to failures.

Another way to classify the PV O&M approaches is to break them down into three categories, each with different cost-benefit tradeoffs and risk profiles:

  • Preventative maintenance (PM) encompasses routine inspection and servicing of equipment — at frequencies determined by equipment type, environmental conditions, and warranty terms in an O&M services agreement — to prevent breakdowns and unnecessary production losses. Th is approach is becoming increasingly popular because of its perceived ability to lower the probability of unplanned PV system downtime. However, the upfront costs associated with PM programs are moderate and the underlying structure of PM can engender superfluous labor activity if not optimally designed.
  • Corrective or reactive maintenance addresses equipment repair needs and breakdowns after their occurrence and, as such, is instituted to mitigate unplanned downtime. The historical industry standard, this “break-fi x” method allows for low upfront costs, but also brings with it a higher risk of component failure and accompanying higher costs on the backend (perhaps placing a premium on negotiating extended warranty terms). Th ough a certain amount of reactive maintenance will likely be necessary over the course of a plant’s 20-year lifetime, it can be lessened through more proactive PM and condition-based maintenance (CBM) strategies.
  • **Condition-based maintenance (CBM) **uses real-time data to anticipate failures and prioritize maintenance activities and resources. A rising number of third party integrators and turnkey providers are instituting CBM regimes to offer greater O&M efficiency. The increased effi ciency, however, comes with a high upfront price tag given communication and monitoring software and hardware requirements. Moreover, the relative novelty of CBM can produce maintenance process challenges caused in part by monitoring equipment malfunction and/or erratic data collection.

Preventative Maintenance (PM) includes the following activities:

  • Panel Cleaning
  • Water Drainage
  • Vegetation Management
  • Retro-Commissioning (identifies and solves problems that have developed during the course of the PV system’s life.)
  • Wildlife Prevention
  • Upkeep of Data Acquisition and Monitoring Systems (e.g., electronics, sensors)
  • Upkeep of Power Generation System (e.g., Inverter Servicing, BOS Inspection, Tracker Maintenance)
  • Site maintenance (e.g., security, road/fence repair, environmental compliance, snow removal, etc.).

Corrective/Reactive Maintenance typically includes:

  • On-Site Monitoring
  • Non-Critical Reactive Repair (addresses production degradation issues)
  • Critical Reactive Repair (high priority, addresses production losses issues)
  • Warranty Enforcement

Condition-Based Maintenance (CBM) usually consists in Active Monitoring — Remote and On-Site Options Equipment Replacement (Planned and Unplanned) and Warranty Enforcement (Planned and Unplanned).

Contracts & Obligations

1. Key Contractual Provisos (KCP)

KCPs in O&M contracts impact the O&M budgeting considerations and approaches, and typically include:

  • **Service-level agreements (SLA) **— specify compliance timeframes for responding to and resolving a range of plant conditions, based on equipment type and issue severity level.
  • Availability or “uptime” guarantees — define the percentage of time that a system must be fully able to produce electricity. Availability guarantees are typically set at 97–99% per year.
  • Performance ratio and yield guarantees — stipulate plant performance levels (e.g., a minimum amount of energy delivered) according to measured solar irradiation at a site, based on system design and modeled plant behavior — which can be variable, thus introducing risks. These guarantees account for Force Majeure events and warranty defects.
  • **Production guarantees **— state annual plant production levels, independent of weather conditions. Insurance coverage can be used to mitigate weather risk, though it can be an expensive policy to underwrite.
  • **Performance incentives **— reward/penalize for plant performance that misses, meets, or exceeds projected production levels.
  • Energy-based contracts — links plant production (kWh/yr) with O&M service provider revenues so that associated expenses are calibrated according to low (fall/winter) and high (spring/summer) revenue periods.

2. O&M Contract Contents

The purpose of an O&M contract is to optimise the performance of the plant within established cost parameters. To do this effectively, the O&M contract should clearly set out:

  • Services to be carried out by, and obligations of, the contractor.
  • Frequency of the services.
  • Obligations of the owner.
  • Standards, legislation and guidelines with which the contractor must comply.
  • Payment structure.
  • Performance guarantees and operational targets.
  • Methodologies for calculating plant availability and/or performance ratio.
  • Methodologies for calculating liquidated damages/ bonus payments in the event of plant under- or overperformance.
  • Terms and conditions.
  • Legal aspects.
  • Insurance requirements and responsibilities.

3. O&M Contractor Services and Obligations

The O&M contract should list the services to be performed by the contractor, including the following entries:

  • Plant monitoring requirements.
  • Scheduled maintenance requirements.
  • Unscheduled maintenance requirements.
  • Agreed targets and/or guarantees (for example, response time or system availability figure) Reporting requirements (including performance, environmental, health and safety, and labour relations reporting).
  • The contractor should also be contractually obliged to optimise plant performance. Additionally, it should be stipulated that all maintenance tasks should be performed in such a way that their impact on the productivity of the system is minimised.

The O&M contract will also typically define the terms by which the contractor is to:

  • Provide, at intervals, a visual check of the system components for visible damage and defects.
  • Provide, at intervals, a functional test of the system components.
  • Ensure that the required maintenance will be conducted on all components of the system. As a minimum, these activities should be in line with manufacturer recommendations and the conditions of the equipment warranties.
  • Provide appropriate cleaning of the modules and the removal of snow (site-specific).
  • Make sure that the natural environment of the system is maintained to avoid shading and aid maintenance activities.
  • Replace defective system components and system components whose failure is deemed imminent.
  • Provide daily (typically during business hours) remote monitoring of the performance of the PV plant to identify when performance drops below set trigger levels.

In an O&M contract, the obligations of the owner/ developer are generally limited to granting the O&M contractor access to the system and all the associated land and access points, obtaining all approvals, licences and permits necessary for the legal operation of the plant providing the O&M contractor with all relevant documents and information, such as those detailed above, that are necessary for the operational management of the plant.

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