Solar cells produce less power at higher temperatures. Thermal heating from ambient conditions and local heating resulting from heat generated by the normal operation of the solar cells are contributing factors. Temperature coefficients are empirically determined and used to describe the negative effect of increased temperature on PV module power. A higher temperature coefficient has more impact upon energy production in warmer climates or where the PV modules are mounted in high-temperature locations, such as a dark roof. To obtain the coefficients, a sampling of PV modules from each product line must be tested by an independent lab at a specific irradiance over a prescribed range of temperatures. Products with a higher temperature coefficient will have lower LEP. 4. Nominal operating cell temperature (NOCT)
In addition to the influence of environmental temperature, operating PV modules generate heat that also reduces their power. Generally, as a PV module collects more solar energy at times when more sunlight is available, the temperature of the solar cells increases because of the increased current flow. At standard conditions this characteristic operating temperature is called NOCT. A higher NOCT amplifies the negative effect caused by the temperature coefficient.
5. Power at low irradiance / Power at high irradiance ratio
Most of the time PV modules are not operating under optimum insolation. Peak sunlight is available for a limited time each day, and much of the power is produced at off-peak conditions. Cloud cover will also create non-ideal, low irradiance operating conditions.
Some PV modules perform better than others in off-peak conditions. Recognizing the significance of this characteristic is important in selecting a superior PV module – one with higher Lifetime Energy Production. In order to calculate the impact on LEP, power must be measured at two different irradiance levels–typically 1,000 W/m² and 200 W/m². Thus, PSI has included the Power at low Irradiance / Power at High Irradiance Ratio characteristic in the PSI Rating: Plow/Pmax. Temperature and other test conditions are held constant during this test. Results from these measurements are used to determine a characteristic called the insolation response function of the PV module. The insolation response combined with the daily insolation is a key component of the LEP. 6. Annual power reduction
The power produced by PV modules may degrade over time for a variety of reasons, resulting in a characteristic PV module power reduction. Accelerated testing has been used to determine this degradation2, and it is of extreme significance to the manufacturers' warranty policies. Although the expected annual degradation data can be gathered in laboratory testing, the PSI Rating uses manufacturer’s warranty values to calculate an annual power reduction value. A lower coefficient of degradation increases a PV module’s PSI Rating.
7. Total area efficiency
PV modules are assembled from arrays of solar cells, and it is not always possible for the manufacturer to cover the entire surface of a module with solar cells. The amount of coverage combined with the solar cell’s efficiency results in the Total Area Efficiency, and is calculated by dividing the module power by its total surface area. Since more concentrated wattage in a PV module improves design flexibility and efficient use of space -- especially on rooftops -- a higher surface power density increases the PSI Rating. 8. Failure rate
Perhaps one of the most difficult PV module characteristics to ascertain, Failure Rate, can impact Lifetime Energy Production through of loss of productivity while a failed module is offline and being replaced. Accelerated testing methods can provide some guidance to long term failure rates, but long-term field failure rate information is rarely available or disclosed. Because of the general lack of data, this characteristic is not presently included in the PSI Rating number.
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