Solar Simulator Components and Design
Our prior chapter covered the history of the solar simulation field and the rise of solar simulators. Thanks to the standards created, manufacturers had a clear definition of what was required to make a solar simulator of varying quality. The first step was finding the right combination of components. Solar simulators comprise three key components: the light source, power supply, and optics. This chapter will be dedicated to exploring these three components in more detail.
Solar Simulator Light Sources
The main component of the solar simulator is the light source. The most widely used light source is a short-arc and long-arc Xenon (Xe) lamp, but other light sources are also used, such as metal halide (MH) arc lamps, quartz tungsten halogen (QTH) lamps, and, more recently, light-emitting diodes (LEDs).
Xenon Arc Lamps
Xenon arc lamps are the most widely used light source for solar simulators. The main reason for this is that it provides a stable spectral that matches reasonably well to sunlight even unfiltered.
Xenon arc lamps have a spectrum almost identical to that of a sun at 5800 K. It does generate strong emission lines in the infrared from 800 nm -1000 nm but these can be eliminated using optical filters.
An additional advantage that the xenon arc lamp has is that when there are variations in power, this does not result in any significant shift in the spectral balance. Also, high-pressure short arc lamps are able to produce high intensity light beams. However, there are several disadvantages with a xenon arc lamp when used in solar simulators. Below is a summary of pros and cons:
PROS
- Stable spectra
- Good match to sunlight spectrum
- Power variation has negligible effect in spectral balance
- Can produce high intensity beams
CONS
- Require complex and expensive power supplies
- High xenon gas pressure in the lamp during operation, is a safety concern
- Power supply instabilities significantly affect amplitude stability in the output
- Aging of the lamp alters the spectral irradiance enhancing the infrared contribution and reducing the ultraviolet
- Filters create a permanent mismatch that cannot be corrected and can burn from prolonged exposure to the Xenon bulb which results in a systematic drift in output that can be difficult to detect
- Bulbs have a short life and are relatively expensive
Metal Halide Arc Lamps
The metal halide arc lamp is an arc source utilizing a mercury vapor arc with metal halide additives and it produces illumination that matches the spectra typically for temperatures in the range of 5000 K to 6000 K.
It was introduced as the light source for solar simulators when the compact source iodide was developed. Below is a summary of pros and cons:
PROS
- High temporal stability
- Good spectral quality that closely matches the sun spectrum
- Lower pressure than xenon arc lamps
CONS
- Emits large amounts of IR energy and insufficient amounts of UV irradiation
- Generates a low collimation beams (meaning it disperses with distance) which limits its application in experiments that need high collimation requirements (ex. high concentrated solar simulators, standard solar simulators for PV testing).
- It experiences light flux reduction overtime
Quartz Tungsten Halogen Lamps
Commonly referred to as QTH, Quartz Tungsten Halogen is an incandescent lamp that consists of a tungsten filament in a sealed transparent container that has a mixture of an inert gas and a halogen.
They are more commonly used in multi-source solar simulators because the tungsten filament can only reach temperatures of less than 3400 K which is lower than the 5800 K of the surface of the sun. As a result, they radiate weaker in the shorter wavelengths (blue and UV portion) but stronger in the infrared portion.
Below is a summary of pros and cons:
PROS
- Ideal black body match in the infrared
- Minimal UV emission
- Relatively inexpensive
CONS
- Lower color temperature than the sun
- Poor spectral match in the visible range
- Usually need to be combined with another light source
LED Lamps
LEDs are solid-state devices that don’t require the maintenance or have the hazards related to a pressurized lamp.
When used in solar simulators, they are able to provide a more dynamic functionality solar simulator that meets the experimental needs for solar energy.
In the past, the main issue was the light intensity insufficiency when fully LED solar generators were designed but the issue was later solved when researchers were able to develop high power LED technology. In 2012 researchers from the University of Illinois at Urbana-Champaign (UIUC) presented a fully LED solar simulator design, which covers the AM1.5G solar spectrum and achieves Class C uniformity over an area of 100 mm × 50 mm.
Currently, LED based solar simulators are able to achieve any AM spectra to precision well in excess of that required by the standards.
LED as a light source has been shown to have several advantages when compared to traditional light sources such as lower cost, they are more compact, and typically consumes less power. Below is a summary of pros and cons:
PROS
- Output control of the light source with a 30 nm – 50 nm resolution
- Lower energy consumption
- Longer lifetime of 50 khours – 100 khours (compared to ~1 khour for arc lamps life expectancies) which reduces maintenance cost to a minimum
- LEDs can be controlled very fast within microseconds or operated stable at one light output intensity continuously for long time
CONS
- Only available in discrete wavelength
- It is expensive to have light below 360 nm and above 1100 nm.
- Intensities are too low for concentrating solar simulator
When used in solar simulators, they are able to provide a more dynamic functionality solar simulator that meets the experimental needs for solar energy.
Spectrum Comparison for Different Light Sources.
For the spectra generated by the xenon and the metal halide lamp, we get a very wide bandwidth ranging from 200 nm to well past 2500 nm. Additionally, the xenon and the metal halide lamp spectra show large spikes which are inconsistent with the target spectrum (these spikes usually have no significant impact on solar cell characterization).
On the other hand, for the LED system we see a narrow emission bands, tuned to match the 400 nm to 1100 nm range specified in the ASTM standard and no spikes are observed.
Note: For solar simulators, it is convenient to describe the irradiance of the simulator in terms of “suns.” One “sun” is equivalent to the irradiance of one solar constant, which is equal to 1000 W/m2 (1 sun=1000 W/m2).
Power Supply
These are typically dictated by the light source used. For example, arc lamp power supplies are typically highly complex devices that must manage a high-voltage ignition stage to establish the arc. For QTH lamps, a comparatively more straightforward DC source with a compatible power output is required. For LEDs, only a relatively simple power supply is required.
Optics of a Solar Simulator
Optics & filters are often used but aren’t necessary for all light sources. When using solar simulators, you are using an artificial light source to generate the spectra needed. However, there may be differences between artificial light sources and natural sunlight, both in intensity and spectral composition.
Filters and optics can be modified to cause the output light to resemble the desired spectra closely.
The optical layout of a solar simulator varies considerably depending on many variables, such as the light sources used, the area of illumination, and the spectral output.
Usually, significant concerns for the optical system of a solar simulation system, beyond the fact that it should be able to attain its required classification, is that it should be simple to use and maintain.
A collimator lens can be added to solar simulators to generate a collimated beam in which the electromagnetic radiation has parallel rays, and as a result, the light spreads minimally as it travels.
When a collimating lens is not used, the beam angle will be larger. Additionally, an integrator lens is also typically used in solar simulators to achieve beam uniformity.
Systems that do not use lenses to homogenize the light typically require a minimum of two mirrors to change the beam direction, homogenize the light beam, and collimate the light, which results in a loss of light.
A Lambertian back reflector can also be added to solar simulators to randomize the direction of the reflected light and provide a more uniform radiance.
The quality of a solar simulator is graded on its spatial uniformity across a defined illumination area, temporal stability throughout the experiment, and spectral match to the sun from a clearly defined reference spectrum. We will cover solar simulator classifications in a later chapter.
To recap, the three components of designing a solar simulator are the light source, power supply, and optics. When it comes to putting all three together, the light source sets the stage for the other two. Depending on what is chosen, the power supply complexity and type of optics will generally guide the design.
If you are looking for a solar simulator that uses LED technology, is low-maintenance, and offers incredible quality and performance, check out our LED solar simulators.
If you are still looking to continue your journey in solar simulation, our next chapter will explore the difference between pulsed and flashed vs continuous wave or steady-state solar simulators.
