Solar Photovoltaic Systems Explained

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Understanding Solar Thermal Systems

Solar Thermal Systems
Solar power benefit

A photovoltaic power system typically includes the following components:

  • An array of solar panels
  • A DC-to-AC power inverter
  • A battery pack to store energy (optional)
  • A charge controller (optional)
  • Solar racking, including interconnecting wiring, circuit breakers, circuit disconnect switches, fuses and voltage meters
  • Optionally, a solar tracking device (this equipment is expensive and unwarranted for most residential applications).

SOLAR PANELS

Monocrystalline panels and polycrystalline panels are the two most used residential solar panels. A different kind of panel, referred to as thin-film, is rarely used for residential and commercial building installations but is well-suited for utility-scale solar energy projects.

Monocrystalline panels
Monocrystalline panels

Monocrystalline panels are the most commonly used panel for residential applications, primarily due to their high efficiencies and aesthetic appeal (they are black and blend well with many roof finishes). Their name refers to their process of manufacture. Each solar cell contains a silicon wafer made from a crystal of silicon. The 'seed' crystal is dipped into a vat of pure molten silicon and removed – the molten silicon envelops the crystal seed, creating a larger silicon crystal known as an ingot. An ultra-thin diamond wire saw slices the ingot into thin wafers of some 180 microns thickness for use in solar cells. Typically, 60 or 72 solar cells comprise a monocrystalline solar panel, depending on panel size.

Monocrystalline panels exhibit efficiencies that range from 17-22 percent. The single wafer solar cell construction eases electron flow through the cell, thereby improving its electrical conversion efficiency. A higher efficiency equates to higher power production for a given panel area, reducing the space required for solar arrays to produce a specific power output.

Polycrystalline panels (also known as multicrystalline panels) are less expensive than their monocrystalline counterparts. However, they exhibit a lower efficiency and are considered somewhat less aesthetically pleasing due to their blue color.

Polycrystalline panels
Plycrystalline panels

In the manufacturing process, a seed crystal of silicon is placed into a vat of molten silicon, and the whole vat is cooled to form multiple crystals of a marbled blue color. Solar panels made of polycrystalline solar cells contain either 60 or 72 cells.

The polycrystalline structure of the silicon wafer found in each solar cell somewhat inhibits electron flow compared to monocrystalline wafers, reducing their efficiencies to the range of 15-17 percent. However, technological advancements have narrowed the efficiency gap between the two types of panels, and the nominal power output of 60-cell polycrystalline panels has increased from 260 watts to 300 watts.

Thin-film solar panels are the least attractive option for residential installations. Although they are a pleasing solid black color, flexible and lightweight, commercially available thin-film panels offer efficiencies rated only 10-13 percent. As a result, thin-film solar arrays require more panels and space to achieve a given power output – a problem for residential and commercial applications where space is limited. Thin-film panels make sense for large utility and industrial-scale applications, where their lower cost and reduced weight overcome concerns over lower efficiency ratings.

Thin-film solar panels
Thin-film solar panel

Solar cells within PV panels are connected in series electrically to produce the desired voltage and in parallel to generate the required direct current. The nominal power output of a manufacturer's PV panel under standard test conditions is the product of the panel's rated voltage and direct current.

Power (watts) = Voltage (volts) x Current (amps)

Commonly, solar panels produce nominal power outputs anywhere from 100 watts to 390 watts, and newer panels with higher power ratings over 500 W and up to 800 W are becoming commercially available. The realized power output from a PV panel under actual operating conditions will be less than the nominal power rating. External factors like ambient temperature, shading, less-than-optimal panel placement, and the accumulation of pollutants and dust on the panels reduce power output.

Solar panel efficiency directly affects the panel area required to produce the desired power. Systems using 10% efficient solar panels require double the number of panels as a system employing 20% efficient units to generate the same output.

INVERTERS

Solar inverters, also referred to as PV inverters, are power electronic devices. They convert variable direct current (DC) output power generated by a photovoltaic panel to alternating current (AC) power at a frequency matching the electrical utility grid (typically 60 Hertz or cycles per second in North America or 50 Hz in many other locations around the world). Inverters can feed surplus power generated by a solar array back into an interconnected utility power grid.

Solar inverters
View of the internals of a solar inverter.

Solar power installations employ one of three different categories of inverters.

  1. String and centralized inverters: String inverters (also referred to as grid-tie inverters) are standalone units mounted near the fuse or circuit breaker panel and convert the DC power from strings of PV panels to AC power. For residential applications, usually only one, maybe two, string inverters are required. They are relatively inexpensive, simpler to service, and are easy to replace in case of failure. They are best suited for dedicated local home or business use at low voltages. They are not ideal for solar panel arrays oriented in several different directions or subject to variable lighting conditions. String inverters do not easily accommodate additions to a PV array – changes can be expensive and complicated.

Centralized inverters are the standard choice for utility-scale solar installations as they offer higher power capacities. They also lessen DC-to-AC conversion losses, and given their higher power ratings, reduce the number of inverters required.

  1. Micro-inverters: Solar micro-inverters operate with a single PV panel and are installed on each panel's underside. They enable plug-and-play connections to create a larger array. They help optimize the power output of each panel regardless of orientation and shading variances. Significantly, they improve fire safety, reduce installation time and costs and minimize additional inventory. There is no need to size the micro-inverter to the system capacity, as is the case when using string inverters, and they allow monitoring of individual panel performance. They cost more than string inverters, adding $1,000 or more for a five-kilowatt residential solar power installation.
  2. Power optimizers: Like micro-inverters, power optimizers are installed on the backside of every solar panel in the array. They condition, clean, and voltage match the DC power output from each PV module, and the output from several such panels feeds a string inverter to convert from DC to AC power.

Some solar inverter designs maximize the power output available from PV panels at any given point in time. This feature is commonly known as maximum power point tracking or MPPT. Solar panels generate power that continually fluctuates in response to many different environmental and physical factors. An inverter with MPPT samples the variable output power along the PV panel's current-voltage curve and electronically adjusts conditions to maximize power output.

RACKING

Solar racking consists of the structural members designed to support photovoltaic panels. The racking is installed directly on the roof or other locations, and the solar panels mount into the racking.

Mounting systems are classified as ground mount, pole mount or roof mount. For buildings, a wide range of racking arrangements accommodate the size, slope or orientation of fixed roof-mounted systems. Pole mounting can accommodate fixed or tracking solar panels on the side or top of poles. Some locations take advantage of using a combination of pole-mounted panels to form a carport or canopy of shade for parking below the panels.

TRACKING

Solar tracking is the term used for mounting systems that move and tilt a solar panel array to follow the sun's course during the day or to focus on the brightest portion of a partly cloudy sky. These tracking systems vary from relatively simple single-axis design to more complex arrangements featuring two-axis movements for more accurate tracking.

Solar inverters
Solar Tracking

Solar trackers can do an excellent job of enhancing the performance of a solar array, especially in the early morning and late afternoon. Single-axis trackers routinely achieve gains of 20-25 percent, and 30% or more is possible when employing dual-axis trackers. These gains depend on the latitude of the location.

Solar trackers create the best performance gains in regions where there is plenty of direct sunlight. They make little difference to solar system performance on cloudy or foggy days where the ambient light diffuses.

Due to the initial cost outlay and routine maintenance requirements, solar tracking makes the most sense on large commercial and utility-scale solar installations of 1 MW output or more, where electrical production gains outweigh the tracker costs.

MONITORING AND METERING

A solar energy monitoring system, depending on its level of sophistication, can measure voltage, current, power and other output parameters that determine how well the solar electric system and its components are performing. They can measure the amount of electricity generated by the minute, hour, day, and year. The results recorded can establish trends and be benchmarked against design specifications to identify changes in performance and breakdowns.

Local and remote monitoring is possible. Sensors and systems are commonly classified into three tiers of accuracy: A, B or C or labeled as High Accuracy, Medium Accuracy or Basic Accuracy.

One crucial measurement in PV performance is on-site irradiance – the amount of solar power per unit area received from the sun. For fixed panel installations commonly found on most residential and commercial solar power systems, the irradiance sensor is placed in the same plane of orientation as the solar panels.

SYSTEM MAINTENANCE

Owners of solar energy systems benefit from how easy they are to operate and maintain. However, a solar installation requires a proactive approach to inspection and cleaning to perform to specifications – typically once per year or as needed due to tree, storm or other damage.

Routine monitoring of the solar power system's output is key to detecting problems early. The energy produced under similar lighting conditions should remain relatively unchanged over time. Diminishing output and a flickering or extinguished monitoring light should trigger the call to inspect the system thoroughly. Even if the system appears clean and operates as expected, periodic inspections can reveal potential issues before they become expensive problems.

Professional solar panel cleaning and inspection companies will perform routine annual maintenance and charge anywhere from $150 to $500 per year. Factors impacting this price include the size of the solar system; the number of panels; the configuration (roof pitch and access versus ground-mounted or pole-mounted); special tools that may be required; obstructions that make access and operation more difficult; how long since the last cleaning; the home location; and whether service is performed under a maintenance agreement (such agreements with bundled services can lower the annual cost). Some companies bill on a price per panel basis, typically ranging from $5 to $15 per panel, where others may charge a flat fee of around $150 for annual inspections before adding for cleaning costs.

Solar arrays do not generally require snow removal. The operating temperature of the panel and the mounting angle help snow to melt and clear without assistance. Scraping ice and hardened snow from panels will likely cause damage – avoid the effort.

Using professional maintenance services lowers the risk of damage to sensitive PV panels. Qualified companies employ experts who locate and evaluate damage to components and issue a complete report on the system's status. Further, their training allows them to work safely in locations that present many perils to life and limb for the uninitiated.

solar panel cleaning
Worker Doing Maintenance

A professional undertaking solar panel and system inspections must be thorough. Their checklist should include:

  • Looking for signs of cracking, fractures, moisture penetration, corrosion and other panel damage
  • An inspection of the sturdiness and alignment of the panel mounting system, ensuring anchoring points are in good shape, making a note of any corrosion concerns and confirming ballasted (weighted) components remain solidly in place and clamps and fixtures are tight
  • Checking for damage in cabling, conduit and connections, and verifying that junction boxes remain sealed and watertight
  • Confirming that inverters, meters and displays function properly
  • An assessment of the system's overall performance and efficiency against specifications and on-site records.

The cost to repair system components can vary based on several factors – the following list indicates what one may expect to pay:

  • Repair damaged glass on solar panel – $20-$400
    • Options for repair include a silicone treatment or wrapping the pane with a transparent laminate film
    • Severe glass damage will likely require replacing the entire panel
  • Cracked panel frame – $150-$350
    • A common fix requires soldering the edges of the cracks or replacing a badly cracked section
  • Loose wiring – $100
    • A typical repair requires cutting through soft silicone to access the back of solar cells, re-solder loose connections and apply new silicone padding
  • Replace sections of panel frame corrosion – $150-$350
  • Replace an inverter – $1,000-$2,000
    • String inverters are less expensive to replace than micro-inverters
  • Removing and replacing damaged roof tiling – $2,000-$3,000
    • Required where panel installation has loosened or damaged roofing materials
  • Clearing obstructions like tree branches using a professional contractor – $50-$200.

Optional upgrades can assist an owner in maintaining a solar system passively, including:

  • Automated sprinkler-style cleaning systems – Consisting of strategically placed water nozzles, these systems use a programmable timer to adjust washing/rinsing frequency and duration. Drainage depends on gravity, requiring panels to be mounted at an angle to clear the spray from the panel faces. Costs can run to several thousand dollars per system.
  • Cleaning robots – A robotic cleaning system is a mechanical system employing microfiber brushes to wipe away dirt without the need for water. Such systems store solar energy in a dedicated local battery to power the robot for nightly panel cleaning. The costs for a typical residential/commercial array run about $3,000 and up.
  • Mesh system for bird deterrence – A solar array needs to remain free of birds choosing to nest within the system. Bird droppings, nesting materials, and damage to wiring and connections due to pecking can wreak havoc with a solar panel array. If birds pose a problem, professionals or do-it-yourselfers can install kits containing the mesh and connection clips. Costs range from less than $100 to hundreds of dollars.