Design and operation factors impacting performance on PV installations.
The main goal of a PV system is to maximize the energy captured from the modules. We want to get the highest operational profits over the lifetime of the array, especially when the capital costs of solar systems are high.
However, there are several factors that may influence the power output of your system. Most of them can be considered in the design stage of the project and should be carefully evaluated in order to make an accurate sizing taking into account the restrictions and needs of the installation.
One of the fundamental factors that definitely affects the power output of your PV array is the available solar resource (sunlight) in the area selected for the installation.
Irradiance is generally referred to in units of W/ and represents the density of power from solar radiation on a surface.
On the other hand, insolation is the amount of solar energy received at a specific time interval. This parameter can be measured in days, months or years, but as a typical reference, it is usually measured in kW/m2/day.
These two parameters establish the amount of solar resource available in your area, but, the solar radiation levels will differ according to the region where the installation is to go.
These factors are the first constraints presented in the design:
- Is your solar resource good enough to consider a PV system? (If you’re in Australia the answer is yes. Many other countries suffer through months of poor to no sunlight.)
- Is the available space on your roof or in your backyard large enough to deliver the amount of energy needed with the solar resource?
These questions can only be answered by checking your available solar resource.
These resources are provided by the National Renewable Energy Laboratory (NREL) and they offer international solar data that you can easily access to take a look at your available solar resource.
NSRDB is more intended for design purposes as it allows you to download meteorological data (global, direct and horizontal irradiance values), but the PVWatts can be easily understood whether you are designing the solar system or are the homeowner.
PVWatts will provide you with the available average daily irradiance for your location.
For example, here are PVWatts results for a 4kW PV system in Melbourne-Australia [assuming electricity rate at 20 cents/kWh]:
As you can see, the tools show the solar radiation, estimated annual energy output and the estimated savings in electricity.
From this data, we will be particularly interested in the solar radiation. Then we will establish the available area for the installation (roof or ground) and also establish a derating factor that accounts for general losses in the system, that can be approximated to 0.82. That way we can obtain the energy output of the system for one day.
Typical efficiency values are within 14-18% so you can use an average of 0.16 as a reference. Once you obtain the total energy output of the system per day, you can compare it with the desired energy load for your household (just select the appliances that you want to cover with solar and sum up the power values in watts, then multiply them by the estimated number of hours each of them will work).
If the energy output of the system is higher than the energy demanded, then you have overcome the first performance issue.
One of the first loss factors in any PV design is the thermal loss.
Such parameter is referred to as the thermal behaviour of the field and is deeply related to the electrical performance of the array.
The operating temperature of the cell, ambient temperature of the location, irradiance, efficiency and mounting system will determine the thermal loss factor that will influence the amount of energy lost in the form of heat.
The best way to overcome this obstacle is to allow the panel to have air-flow on the underside.
Ground-mounted systems have the best performances in minimising thermal losses because the air freely circulates on both sides of the modules, making a better cooling system.
The roof-mounted system is not so good regarding thermal losses because the panels are placed close to the roof, leaving little room for the air-flow necessary to effectively dissipate the heat from the modules.
A minimum separation of 15 cm from the roof is crucial. This will allow enough air-flow behind the modules to reduce the thermal loss to an average figure.
Also called ohmic losses.
Wiring losses are associated with resistive losses in the cables that go from the PV modules to the inverter (DC side) and from the inverter to the load (AC side).
These ohmic losses are also associated with a voltage drop in the PV array. Therefore when there are long distances between modules and inverter, it is wise to consider detailed designs on wiring sizes to avoid undesirable voltage losses that will affect the performance of the array.
Unlike the wiring for typical electrical installations where voltage drop values are within 2-5%, for PV array designs, voltage drop values should be within 0.5-1.5% on DC side and 1% or less on AC side.
The optimal way to reduce ohmic losses is to increase the conductor size.
Module Quality Loss
This parameter is related to the reliability of the manufacturers’ power output. It is intrinsically related to the provided tolerance of the module.
A simple way to estimate this loss is to establish the tolerance provided as a reference and divide the difference (i.e. - 3% and +3%) by four.
Inclination and Orientation Angles
These are crucial factors that affect the performance of any PV array. In order to achieve the most accurate performance, the PV arrays should be oriented towards the north in Australia, as the highest solar radiation will be achieved in that position.
East-west selection with some modules facing east and others facing west is also a valuable option as the system will behave more in line with the average consumption patterns of a household.
Regarding the slope of the panels, the simplest and most practical estimation is to fix the inclination at the latitude degree. This is valid for fixed-mounted options, but for ground-mounted installations that allow some variation in the tilt angle across the year, the optimum inclination varies according to the season at hand (10°).
Losses due to mismatch can be defined as the difference between the sum of all maximum power points in each independent sub-module and the resulting maximum power point of the whole array’s I/V curve.
These losses occur due to several reasons:
- Variations in the module’s name-plate current and voltage ratings when connected in series and parallel
- Different ageing degradation rates in each module
- Manufacturing differences among modules with the same model number
- Non-uniform shading
- Different lengths of conductors from each string to the inverter
- Instantaneous irradiance variations within a string - e.g. partial shading of a panel
- Design of strings with different orientations towards a single MPPT input of the inverter.
Average mismatch range is generally established within 0.5% (good design systems) and 5% (systems with significant and non-uniform shadings).
The best way to minimize these losses is to install the same module models, keep the same number of modules on each string, try to establish similar distances among strings towards the inverter and establishing separate inverters for each string if there are different orientations due to space constraints.
Soiling losses depend on many complicated and site-specific factors.
Elements like the soil’s composition, wind speed and direction, relative humidity, industrial activities, snow, precipitation patterns and bird droppings can directly affect the performance of the PV system if accumulation is allowed.
Arid areas like the Middle East are most exposed to this type of loss, as besides considering the above factors, they must also take into account eventual sandstorms which are damaging for the PV array performance.
Typical annual losses from soiling can go from 1 to 10%
The only way to reduce these losses is with cleaning, and the most economical way to achieve low losses and low maintenance costs is to design the installation to clean itself when it rains.
In order to do so, the inclination angle of the PV array should never be below 15°.
You must take into account that bird droppings can create hot spots in the modules and since rainfall is not enough to clean them off, some maintenance will still be needed.
Here you can take a look at the dust intensity around the world in μg/m3. As you can see, Australia is located in the Zone 2 area, meaning that the dust intensity is not strong here.
This loss factor refers to the optical effects at the module surface due to the incidence and reflection of sunlight on the PV cells.
In other words, some of the incoming photons that should be transformed into electricity will be reflected off the glass or plastic surface of the module by reflection. Naturally, this causes loss of energy.
This factor is expressed through the Incident Angle Modifier (IAM) which is quantified as the ratio between the transmittance angle of incidence (AOI) of the direct normal irradiance (DNI) onto the array (in degrees) and the normal incidence angle (AOI=0°C).
The only way to reduce these losses is to buy modules with an anti-reflective coating. That way, the IAM factor will be lower and fewer sunbeams will be reflected.
Finally, but no less important, shading is the ultimate loss factor in PV systems.
Shading is the crucial element design in every PV system because when a module is shaded, not only does the shaded area stop producing energy, but a larger area of the module also stops producing.
The reason behind this is that the bypass diodes of the panels must be activated in order to avoid reverse currents that could damage the module.
Bypass diodes are spread for each sub-module (panels are generally divided into 3 sub-modules) which means that if just one part of the sub-module is shaded, then that whole section will not produce electricity.
Moreover, shading can also cause reduced current output in a string because the panel with the lowest current value (shaded module) is the one that establishes the value of the current in the whole string.
For these reasons, shading must be avoided at all cost.
A bad design of a PV system does not consider the variability of shading over time because the shade pattern is not fixed. It changes across the year according to the solar irradiance and the translational/rotational movement of the earth.
We have examined many performance considerations and loss factors that can be encountered in the design and operation of PV arrays.
Among the most crucial considerations, shading, mismatch losses, solar resource, inclination and orientation are some of the most important performance factors to consider in the design of a solar PV system.
Particularly for Australia, where the available solar resource is abundant, consider the installation of solar panels and investment over the long term including the available Feed in Tariff policy and the long-term lifespan of the solar modules.
In order to achieve the greatest benefits from your PV array just remember:
- Try to select the place with the least possible shading throughout the day, keeping the panels away from trees, fences, chimneys, etc.
- Try to keep strings of panels with the same number of modules in each string. This is not only more aesthetic but also reduces the possibility of mismatch losses.
- Buy the same modules for the whole array. One size, one brand.
- Consider only the necessary electrical appliances to add into the load panel.
- Study your available solar resource with the PVWatts tool, place your estimated system size, electricity rate and inclination according to your latitude and orientation towards the north (0° in PVWatts).
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Photo credit: Depositphotos