Over the years, many different types of photovoltaics (PV) have been created, and each possesses unique qualities, but one thing remains true, all applications aren’t necessarily satisfied by the same technology.
When determining which PV technology to adopt and which company to partner with, it’s important to think long and hard about your design and the most important requirements.
To make the best possible decision, you must look at many factors, including the operating environment, power budget and use case.
In this three-part series, we first tackle operating environment considerations.
Understanding the use environment is critical. Quoted efficiency of solar technology is based on a specific controlled light source and operating temperature.
The light source is the Air Mass 1.5 (AM1.5) solar spectrum with an intensity of 1000W/m^2. The AM1.5 spectrum contains a great deal of energy out into the infrared (wavelengths up to 2500nm).
This has become the standard measurement criteria for all solar since most solar is designed and installed outdoors.
Are you thinking about utilizing solar power indoors using ambient light?
There are additional factors to take into account.
The previously mentioned standard does not adequately represent how solar will perform when subjected to other light sources, light intensities, or temperatures found in indoor environments.
Over the past ten years, high-efficiency lighting has become increasingly common, especially with the phasing out of incandescent bulbs in many countries worldwide.
The reason that a 13W compact fluorescent (CFL) bulb, an 8W LED bulb and a 60W incandescent bulb all provide about the same level of illumination is that the light output of the CFL and the LED is contained primarily to the visual spectrum and there is less power converted to heat.
The visual spectrum of light from 400-750nm contains a little less than 50% of the usable light and 16% of the wavelengths included in the AM1.5 solar spectrum.
This means to a solar cell that the range of wavelengths available for absorption indoors is greatly reduced.
For very high-efficiency multijunction cells containing junctions focused on infrared absorption, one junction may produce little current, which will severely limit the output of the entire cell.
The second consideration when using solar indoors is the overall intensity available.
As previously stated, the measurement standard for solar intensity is 1000W/m^2 (roughly 100,000 lux).
Indoors, this value is significantly less between 20 and 1000 Lux. Low intensities will have a significant impact on efficiency.
Depending on the internal composition of the cell and its manufacturing process, varying levels of power loss to internal leakage paths exist.
Some technologies, such as CIGS, have a high intrinsic leakage making them perform very poorly in low-intensity light despite having efficiencies >10% at 1000W/m^2.
At the other end of the spectrum is amorphous silicon, which is about 6% efficient at 1000W/m^2, but will provide 1.5-2.5 more power per unit area than crystalline silicon (22% efficient), CIGS (10% efficient) or triple junction GaAs (28% efficient) due to its low internal leakage and sensitivity to the visual spectrum.
The last consideration is temperature.
Not all solar technologies are equal in temperature sensitivity, and operating temperature is vitally important.
Temperature increases will cause decreases in power output. The rate at which power decreases is dependent on solar technology.
The table below gives a general guide for temperature coefficients of common solar technologies.
As discussed earlier, the testing standard is set at 25°C. In most cases, this is an unrealistic operating point.
The outdoor operating temperature will vary depending on illumination, ambient temperature, and the system's installation.
Solar cells will heat up well above ambient temperature during operation due to the infrared heating caused by sunlight.
If the solar is installed so that airflow is limited or there is no wind, the temperature will be much higher than if air is flowing by.
Also, ambient temperature is important. Installations in cooler areas will produce more power than installations in hotter areas.
When deciding which type of solar material to use for your application and who to work with, it’s important to consider the operational environment.
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