Photovoltaic system performance is a function of the climatic conditions, the equipment used and the system configuration. PV performance can be measured as the ratio of actual solar PV system output vs expected values, the measurement being essential for proper solar PV facility's operation and maintenance. The primary energy input is the global light irradiance in the plane of the solar arrays, and this in turn is a combination of the direct and the diffuse radiation. [1]
The performance is measured by PV monitoring systems, which include a data logging device and often also a weather measurement device (on-site device or an independent weather data source). Photovoltaic performance monitoring systems serve several purposes - they are used to track trends in a single photovoltaic (PV) system, to identify faults in or damage to solar panels and inverters, to compare the performance of a system to design specifications or to compare PV systems at different locations. This range of applications requires various sensors and monitoring systems, adapted to the intended purpose. Specifically, there is a need for both electronic monitoring sensors and independent weather sensing (irradiance, temperature and more) in order to normalize PV facility output expectations. Irradiance sensing is very important for the PV industry and can be classified into two main categories - on-site pyranometers and satellite remote sensing; when onsite pyranometers are not available, regional weather stations are also sometimes utilized, but at lower quality of data; the Industrial IoT-powered sensorless measurement approach has recently evolved as the third option.
Sensors and photovoltaic monitoring systems are standardized in IEC 61724-1 [2] and classified into three levels of accuracy, denoted by the letters “A”, “B” or “C”, or by the labels “High accuracy”, “Medium accuracy” and “Basic accuracy”. A parameter called the 'performance ratio' [3] has been developed to evaluate the total value of PV system losses.
Photovoltaic system performance is generally dependent on incident irradiance in the plane of the solar panels, the temperature of the solar cells, and the spectrum of the incident light. Furthermore, it is dependent upon the inverter, which typically sets the operating voltage of the system. The voltage and current output of the system changes as lighting, temperature and load conditions change, so there is no specific voltage, current, or wattage at which the system always operates. Hence, system performance varies depending on its architecture (direction and tilt of modules), geographic location and the time of day, weather conditions (amount of solar insolation, cloud cover, temperature), and local disturbances such as shading, soiling, state of charge, and system component availability.
Solar parks of industrial and utility scale may reach high performance figures. In modern solar parks the performance ratio should typically be in excess of 80%. [4] [5] Many solar PV parks utilize advanced performance monitoring solutions, which are supplied by a variety of technology providers.
In rooftop solar systems it typically takes a longer time to identify a malfunction and send a technician, due to lower availability of sufficient photovoltaic system performance monitoring tools and higher costs of human labor. As a result, rooftop solar PV systems typically suffer from lower quality of operation & maintenance and essentially lower levels of system availability and energy output.
Most off-grid solar PV facilities lack any performance monitoring tools, due to a number of reasons - including monitoring equipment costs, cloud connection availability and O&M availability.
A number of technical solutions exist to provide performance measurement and monitoring for solar photovoltaic installations, differing according to data quality, compatibility with irradiance sensors as well as pricing.
Weather data acquisition is generally relying on physical weather sensors and remote sensing with satellites.
An essential part of PV system performance evaluation is the availability and the quality of energy generation data. Access to the Internet has allowed a further improvement in energy monitoring and communication.
Typically, PV plant data is transmitted via a data logger to a central monitoring portal. Data transmission is dependent on the local cloud connectivity, thus being highly available in OECD countries, but more limited in developed countries. According to Samuel Zhang, vice president of Huawei Smart PV, over 90% of global PV plants will be fully digitilized by 2025. [6]
In general, monitoring solutions can be classified to inverter manufacturer-provided logger and monitoring software solutions, independent data-logger solutions with custom software and finally agnostic monitoring software-only solutions compatible with different inverters and data-loggers.
Dedicated performance monitoring systems are available from a number of vendors. For solar PV systems that use microinverters (panel-level DC to AC conversion), module power data is automatically provided. Some systems allow setting performance alerts that trigger phone/email/text warnings when limits are reached. These solutions provide data for the system owner and/or the installer. Installers are able to remotely monitor multiple installations, and see at-a-glance the status of their entire installed base. All the major inverter manufacturers provide a data acquisition unit - whether a data logger or a direct means of communication with the portal.
These solutions have the advantage of providing of a maximum information from the inverter and of supplying it on a local display or transmitting it on the internet, in particular alerts from the inverter itself (temperature overload, loss of connection with a network, etc.).
Some of those monitoring solutions are:
Generic data logging solutions connected to inverters make it possible to overcome the major drawback of inverter-specific manufacturer solutions - being compatible with several different manufacturers. These data acquisition units connect to the serial links of the inverters, complying with each manufacturer’s protocol. Generic data logging solutions are generally more affordable than inverter manufacturer solutions and allow aggregation of solar PV system fleets of varying inverter manufacturers.
Some of those monitoring solutions are:
The last category is the most recent segment in the solar photovoltaic monitoring domain. Those are software based aggregation portals, able to aggregate information from both inverter-specific portals and data loggers as well as independent data loggers. Such solutions become more widespread as inverter-specific communication to the cloud is done more and more without data loggers, but rather as direct data connections.
On-site irradiance measurements are an important part of PV performance monitoring systems. Irradiance can be measured in the same orientation as the PV panels, so-called plane of array (POA) measurements, or horizontally, so-called global horizontal irradiance (GHI) measurements. Typical sensors used for such irradiance measurements include thermopile pyranometers, PV reference devices and photodiode sensors. To conform to a specific accuracy class, each sensor type must meet a certain set of specifications. These specifications are listed in the table below.
Sensor type | Class A High accuracy | Class B Medium accuracy | Class C Basic accuracy |
---|---|---|---|
Thermopile pyranometer | Secondary standard per ISO 9060 or High quality per WMO Guide (Uncertainty ≤ 3% for hourly totals) | First class per ISO 9060 or Good quality per WMO Guide (Uncertainty ≤ 8% for hourly totals) | Any |
PV reference device | Uncertainty ≤ 3% from 100 W/m2 to 1500 W/m2 | Uncertainty ≤ 8% from 100 W/m2 to 1500 W/m2 | Any |
Photodiode sensors | Not applicable | Not applicable | Any |
If an irradiance sensor is placed in POA, it must be placed at the same tilt angle as the PV module, either by attaching it to the module itself or with an extra platform or arm at the same tilt level. Checking if the sensor is properly aligned can be done with portable tilt sensors or with an integrated tilt sensor. [7]
The standard also specifies a required maintenance schedule per accuracy class. Class C sensors require maintenance per manufacturer's requirement. Class B sensors need to be re-calibrated every 2 years and require a heater to prevent precipitation or condensation. Class A sensors need to be re-calibrated once per year, require cleaning once per week, require a heater and require ventilation (for thermopile pyranometers).
PV performance can also be estimated by satellite remote sensing. These measurements are indirect because the satellites measure the solar radiance reflected off the earth surface. In addition, the radiance is filtered by the spectral absorption of Earth's atmosphere. This method is typically used in non-instrumented class B and class C monitoring systems to avoid costs and maintenance of on-site sensors. If the satellite-derived data is not corrected for local conditions, an error in radiance up to 10% is possible. [2]
Sensors and monitoring systems are standardized in IEC 61724-1 [2] and classified into three levels of accuracy, denoted by the letters “A”, “B” or “C”, or by the labels “High accuracy”, “Medium accuracy” and “Basic accuracy”.
In California, solar PV performance monitoring has been regulated by the State government. As of 2017, the governmental agency California Solar Initiative (CSI) provided a Performance Monitoring & Reporting Service certificate to eligible companies active in the solar segment and acting in line with CSI requirements. [8]
A parameter called the 'performance ratio' [3] has been developed to evaluate the total value of PV system losses. The performance ratio gives a measure of the output AC power delivered as a proportion of the total DC power which the solar modules should be able to deliver under the ambient climatic conditions.
Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially used for electricity generation and as photosensors.
A solar inverter or photovoltaic (PV) inverter is a type of power inverter which converts the variable direct current (DC) output of a photovoltaic solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical balance of system (BOS)–component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. Solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.
A pyranometer is a type of actinometer used for measuring solar irradiance on a planar surface and it is designed to measure the solar radiation flux density (W/m2) from the hemisphere above within a wavelength range 0.3 μm to 3 μm.
The National Renewable Energy Laboratory (NREL) in the US specializes in the research and development of renewable energy, energy efficiency, energy systems integration, and sustainable transportation. NREL is a federally funded research and development center sponsored by the Department of Energy and operated by the Alliance for Sustainable Energy, a joint venture between MRIGlobal and Battelle. Located in Golden, Colorado, NREL is home to the National Center for Photovoltaics, the National Bioenergy Center, and the National Wind Technology Center.
Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m2) in SI units.
An automatic weather station (AWS) is an automated version of the traditional weather station, either to save human labor or to enable measurements from remote areas. An AWS will typically consist of a weather-proof enclosure containing the data logger, rechargeable battery, telemetry (optional) and the meteorological sensors with an attached solar panel or wind turbine and mounted upon a mast. The specific configuration may vary due to the purpose of the system. The system may report in near real time via the Argos System, LoRa and the Global Telecommunications System, or save the data for later recovery.
The Solar Radiation and Climate Experiment (SORCE) was a 2003–2020 NASA-sponsored satellite mission that measured incoming X-ray, ultraviolet, visible, near-infrared, and total solar radiation. These measurements specifically addressed long-term climate change, natural variability, atmospheric ozone, and UV-B radiation, enhancing climate prediction. These measurements are critical to studies of the Sun, its effect on the Earth's system, and its influence on humankind. SORCE was launched on 25 January 2003 on a Pegasus XL launch vehicle to provide NASA's Earth Science Enterprise (ESE) with precise measurements of solar radiation.
Maximum power point tracking (MPPT), or sometimes just power point tracking (PPT), is a technique used with variable power sources to maximize energy extraction as conditions vary. The technique is most commonly used with photovoltaic (PV) solar systems but can also be used with wind turbines, optical power transmission and thermophotovoltaics.
A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. The electrons flow through a circuit and produce direct current (DC) electricity, which can be used to power various devices or be stored in batteries. Solar panels are also known as solar cell panels, solar electric panels, or PV modules.
A solar tracker is a device that orients a payload toward the Sun. Payloads are usually solar panels, parabolic troughs, Fresnel reflectors, lenses, or the mirrors of a heliostat.
A stand-alone power system, also known as remote area power supply (RAPS), is an off-the-grid electricity system for locations that are not fitted with an electricity distribution system. Typical SAPS include one or more methods of electricity generation, energy storage, and regulation.
A photovoltaic system, also called a PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and convert sunlight into electricity, a solar inverter to convert the output from direct to alternating current, as well as mounting, cabling, and other electrical accessories to set up a working system. Many utility-scale PV systems use tracking systems that follow the sun's daily path across the sky to generate more electricity than fixed-mounted systems.
The Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) is a computer program designed to evaluate the surface solar irradiance components in the shortwave spectrum under cloudless conditions. The program, written in FORTRAN, relies on simplifications of the equation of radiative transfer to allow extremely fast calculations of the surface irradiance. The irradiance components can be incident on a horizontal, a fixed-tilt or a 2-axis tracking surface. SMARTS can be used for example to evaluate the energy production of solar panels under variable atmospheric conditions. Many other applications are possible.
A rooftop solar power system, or rooftop PV system, is a photovoltaic (PV) system that has its electricity-generating solar panels mounted on the rooftop of a residential or commercial building or structure. The various components of such a system include photovoltaic modules, mounting systems, cables, solar inverters battery storage systems, charge controllers, monitoring systems, racking and mounting systems, energy management systems, net metering systems, disconnect switches, grounding equipment, protective devices, combiner boxes, weatherproof enclosures and other electrical accessories.
Photovoltaic mounting systems are used to fix solar panels on surfaces like roofs, building facades, or the ground. These mounting systems generally enable retrofitting of solar panels on roofs or as part of the structure of the building. As the relative costs of solar photovoltaic (PV) modules has dropped, the costs of the racks have become more important and for small PV systems can be the most expensive material cost. This has caused an interest in small users deploying a DIY approach. Due to these trends, there has been an explosion of new racking trends. These include non-optimal orientations and tilt angles, new types of roof-mounts, ground mounts, canopies, building integrated, shading, vertical mounted and fencing systems.
Solar power forecasting is the process of gathering and analyzing data in order to predict solar power generation on various time horizons with the goal to mitigate the impact of solar intermittency. Solar power forecasts are used for efficient management of the electric grid and for power trading.
The Open Solar Outdoors Test Field (OSOTF) is a project organized under open-source principles, which is a fully grid-connected test system that continuously monitors the output of many solar photovoltaic modules and correlates their performance to a long list of highly accurate meteorological readings.
Morgan Solar, Inc. is a Canadian solar power and optical technology company based in Toronto, Ontario. Since 2017, the company has specialized in urban sunlight management, led by its SPOTlight platform. The company also produces in situ IV curve tracers and optical film technologies.
Multiple different photovoltaic module analysis techniques are available and necessary for the inspection of photovoltaic (PV) modules, the detection of occurring degradation and the analysis of cell properties.
Soiling is the accumulation of material on light-collecting surfaces in solar power systems. The accumulated material blocks or scatters incident light, which leads to a loss in power output. Typical soiling materials include mineral dust, bird droppings, fungi, lichen, pollen, engine exhaust, and agricultural emissions. Soiling affects conventional photovoltaic systems, concentrated photovoltaics, and concentrated solar (thermal) power. However, the consequences of soiling are higher for concentrating systems than for non-concentrating systems. Note that soiling refers to both the process of accumulation and the accumulated material itself.