Table of Contents
Factors Affecting PV Module Performance (Part 4)
1.2 Electrical Characteristics
The type of solar power produced by a photovoltaic solar cell is called direct current, or DC, the same as from a battery. Most photovoltaic solar cells produce a “no load” open circuit voltage of about 0.5 to 0.6 volts when there is no external circuit connected. This output voltage (VOUT) depends very much on the load current (I) demanded of the PV cell. For example, on a very cloudy day, the current demand would be low, and so the cell could provide the full output voltage, VOUT, but at a reduced output current. But as the current demand of the load increases, a brighter light (solar radiation) is needed at the junction to maintain a full output voltage, VOUT.
However, there is a physical limit to the maximum current that a single photovoltaic solar cell can provide, no matter how intense or bright the sun’s radiation is. This is called the maximum deliverable current and is symbolized as IMAX. The IMAX value of a single photovoltaic solar cell depends upon the size or surface area of the cell, the amount of direct sunlight hitting the cell, its efficiency of converting this solar power into a current and of course the type of semiconductor material that the cell is manufactured from either silicon, gallium arsenide, cadmium sulphide, cadmium telluride etc.
Most commercially available photovoltaic solar cells have solar power ratings which indicate the maximum deliverable solar power, PMAX, that the cell can provide in watts, and is equal to the product of the cell voltage V, multiplied by the maximum cell current I, and is given as:
PMAX = VOUT x IMAX
Where: P is in Watts, V is in Volts, and I is in Amperes
Various manufacturers refer to a PV cell’s output power at full sun as its “maximum output power”, “peak power”, “rated power”, “maximum power point” or other such terms, but they all mean the same.
1.2.1 Photovoltaic I-V Characteristics Curves
Manufacturers of photovoltaic solar cells produce current-voltage (I-V) curves, which give the current and voltage at which the photovoltaic cell generates the maximum power output, and are based on the cell being under standard conditions of sunlight and temperature with no shading.
Voltage (V) is plotted along the horizontal axis while Current (I) is plotted along the vertical axis. The available power (W) from the PV, at any point of the curve, is the product of current and voltage at that point.
1.2.2 Short Circuit Current (ISC)
A photovoltaic module will produce its maximum current when there is essentially no resistance in the circuit. This would be a short circuit between its positive and negative terminals. This maximum current is called the short-circuit current (Isc). This value is higher than Imax, which relates to the normal operating circuit current.
Under this condition, the resistance is zero and the voltage in the circuit is zero.
1.2.3 Open Circuit Voltage (VOC)
Open circuit voltage (Voc) means that the PV cell is not connected to any external load and is therefore not producing any current flow (an open circuit condition). This value depends upon the number of PV panels connected in series. Under this condition, the resistance is infinitely high, and there is no current
1.2.4 Maximum Power (PMAX or MPP)
This relates to the point where the power supplied by the array that is connected to the load (batteries, inverters) is at its maximum value, where Pmax = Imax x Vmax. The maximum power point of a photovoltaic array is measured in Watts (W) or peak Watts (Wp). Imax and Vmax value occurs at the “knee” of the I-V curve.
1.2.5 Fill Factor (FF)
The fill factor is the ratio of maximum power output (Pmax) to the product of the open-circuit voltage times the short-circuit current (Voc x Isc). The relationship is:
The fill factor gives an idea of the quality of the array. The closer the fill factor to 1 (unity), the more power the array can provide. Typical values are between 0.7 and 0.8.
1.2.6 PV Panel Energy Output
You have learnt previously that the power output of a photovoltaic solar cell is given in watts and is equal to the product of voltage times the current (V x I). The optimum operating voltage of a PV cell under load is about 0.46 volts at the normal operating temperatures, generating a current in full sunlight of about 3 amperes. Then the power output of a typical photovoltaic solar cell can be calculated as:
P = V x I = 0.46 x 3 = 1.38 watts
Now this may be okay to power a calculator, small solar charger, or garden light, but this 1.38 watts is not enough power to do any usable work. However, when the PV cells are connected in series (daisy-chained), the voltage is added, and when connected in parallel (side-by-side), the current is added. A suitable combination of PV modules in series and parallel gives you the desired voltage, current, and power output.
2. PV Module Output
For a specific load, PV module output depends on two major factors:
- Irradiance or light intensity
- Temperature
2.1 Solar Intensity
The amount of sunlight falling onto the face of the PV cell affects its output. The more sunlight entering the cell, the more current it produces. The voltage will remain the same. The figure below shows that under different test conditions, when daylight is 1000 W/m² v/s 600 W/m², the power out from the PV module varies in proportion
Dirt and dust can accumulate on the solar module surface, blocking some of the sunlight and reducing output. Although rigorous maintenance will clean off the dirt and dust regularly, it is more realistic to estimate system output considering the reduction due to dust buildup in the dry season. A typical annual dust reduction factor to use is 93% or 0.93. So, the “100-watt module” operating with some accumulated dust may operate on average at about 79 Watts (85 Watts x 0.93 = 79 Watts).
2.2 Temperature
PV cell performance declines at higher cell temperatures. The operating voltage drops with increasing cell temperature. So, in full sun, the output voltage reduces by about 5% for every 25°C increase in cell temperature. Then, photovoltaic panels with more solar cells are recommended for very hot climates than would be used in colder ones in order to offset power output losses due to high temperatures.
Most thin film technologies have a lower negative temperature coefficient compared to crystalline technologies. In other words, they tend to lose less of their rated capacity as the temperature rises. Refer graphical representation below:
As the temperature of a solar cell increases, the open circuit voltage Voc decreases, but the short circuit current Isc increases marginally.
3. PV Module Efficiency & De-rating Factors
PV module efficiency is the ratio of the electrical power output Pout, compared to the solar power input Pin, hitting the module. Pout can be taken to be PMAX, since the solar cell can be operated up to its maximum power output to get the maximum efficiency.
The efficiency of a typical solar array is normally low at around 10-12%.
Example:
On a clear sunny day, a 1kWp PV array received 6 Peak Sun Hours (PSH). The expected output can be determined as follows:
Peak Power Output x Peak Sun Hours = Expected Output
1kW x 6PSH = 6kWh
The calculation above shows the maximum theoretical energy output, which will never be produced in a real PV system. The actual output would be a lot lower than calculated because of inefficiencies and losses in the PV system. A summary of typical efficiency losses is provided in the table below.
Cause of loss | *Estimated Loss (%) | De-rating Factor |
---|---|---|
Temperature | 10% | 0.90 |
Dirt | 3% | 0.97 |
Manufacturer’s Tolerance | 3% | 0.97 |
Shading | 2% | 0.95 |
Orientation/Tilt Angle/Azimuth | 1% | 0.99 |
Losses due to the voltage drop in cables from the PV array to the battery. | 2% | 0.98 |
Losses in distribution cables from the PV battery to the loads | 2% | 0.98 |
Losses in a charge controller | 2% | 0.98 |
Battery losses | 10% | 0.9 |
Inverter | 10% | 0.9 |
Loss due to irradiance level | 3% | 0.97 |
Total de-rating factor (multiplying all de-rating factors) | 0.60 |
* Typical losses in PV systems. Actual loss will be as per site conditions
Energy Yield = Peak Sun Hour x Module Rated Power x Total Derating Factor
Example:
On a clear and sunny day, a 1kWp PV array received 6 Peak Sun Hours. The expected output can be determined as follows:
Expected Output = Peak Sun Hours x Peak Power Output x Total derating factor
= 1kWp x 6 x 60%
= 3.6 kWh
3.1 Performance degradation over the life cycle
The performance of a PV module will decrease over time. The degradation rate is typically higher in the first year upon initial exposure to light and then stabilizes. Factors affecting the degree of degradation include the quality of materials used in manufacture, the manufacturing process, the quality of assembly and packaging of the cells into the module, as well as maintenance levels employed at the site. Generally, degradation of a good quality module is about 20% during the module’s life of 25 years, @0.7% to 1% per year.
Example:
On a clear and sunny day, a 1kWp PV array received 6 Peak Sun Hours (hours). Total loss (derating factor) in the system is estimated at 0.70 (70%) expected output can be determined as follows:
Expected Output = Peak Sun Hours x Peak Power Output x Total derating factor
= 1kWp x 6 hour/day x 0.70
= 4.2kWh per day (1st year)
Now considering degradation of module as per the indicative profile above (example only, actual degradation of module will be based on module quality and climatic conditions.
Energy generation:
= 3.83kWh per day (on 10th year)
= 3.39kWh per day (on 25th year)
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References:
A. Bhatia, Course No: R08-002, https://www.cedengineering.com