Hoping for a sunny day?

Jörg Schwartz

It seems like renewable sources of energy – and solar technology in particular – are rapidly gaining attention and momentum. Driven by rising energy prices, but equally by favourable tax breaks in a number of European countries, solar cells and panels are on everyone’s mind these days, with deployment growth rates on the order of 50 per cent being reported by the German Energy Agency.

As a result of this “market pull,” significant effort is being put into developing photovoltaics technology, with low-cost solar cell manufacturing high on the agenda and a number of vendors pushing this route. But making solar cells affordable for widespread use by developing and introducing mass-manufacturing processes is only one part of the equation. The other part is making them more efficient, so that the user gets maximum “bang for the buck,” or as much electricity as possible from the sunlight available.

Reproducible conditions are key

Although it may appear at first glance that only the efficiency of converting photons into electrons must be optimized, a second look reveals that the problem is much more complex and includes a variety of illumination conditions. This typically is done by measuring the current-voltage characteristics of cells under various illumination conditions and comparing them with different designs, or by repeating the measurements over time to investigate possible performance degradations over the life of the cell.

The practical problem is that these conditions rarely present themselves when needed, especially not at a particular place or time. This generates the need for devices that can emulate the sun, so that solar cells can be tested and optimized under reproducible standardized conditions whenever and wherever needed.

One main challenge in doing this lies in accomplishing an accurate emulation of the real sun’s spectrum, which depends on factors such as time of day and year, location on Earth, weather and pollution.

Simulator devices differ in how well they match the solar spectrum, in the temporal stability of the radiated power and in the spatial homogeneity of the illumination. Detailed classifications using these parameters are defined by standards such as ASTM E 927 or IEC 904-9.

Xenon lamps are close, but filters improve

The good news when looking for a light source emulating the sun is that the emission from xenon short-arc lamps already is close to the spectral characteristics of the sun. With their colour temperature of about 5500 K, they offer a very good starting point, and additional filters can be used to improve the match.

A specific type – an air mass filter – has been designed for solar simulators. Its purpose is not just to adjust the shape of the xenon spectrum but to do so in different ways, representing various conditions of sunlight exposure. The height of the sun above the horizon determines air mass – if the sun is low, its light obviously must travel a longer way through the atmosphere than when it is at zenith. Consequently, air mass values are higher when the sun is lower in the sky.

For example, air mass is 1 when the sun is directly overhead and the angle of sun to zenith direction is 0°. Air mass is 2 when the angle is 60°. The extraterrestrial spectrum is called air mass 0 because it passes through no air mass; e.g., an AM 0 filter leads to characteristics of sunlight outside the atmosphere, which is relevant for solar cells to be used for space and satellite applications. Near the equator, the sun spectrum is matched by what comes out of an AM 1 filter, and in North America or Central Europe, AM 1.5 G provides a good match.

Variations in time and space

Good temporal and spatial stability are other important features because any fluctuation will degrade solar cell efficiency measurements. The former can be addressed by choosing the right components, particularly on the power supply side, so that today’s solar simulators offer stabilities of better than 1 percent over time.

The spatial homogeneity, however, continues to be a challenge, particularly if large devices or areas of interest must be measured. The optical shape of solar simulators was designed to convert the output of the xenon lamp into a collimated parallel beam, with the light evenly distributed over the illumination area. The standards require a maximum intensity variation of ±2 per cent over the entire illuminated area to meet a class A classification. By comparison, only ±5 per cent is required for class B and ±10 per cent for class C, as per ASTM E 927.

It is worth noting that the xenon lamp’s electrode structure changes during its ~1500-h lifetime, which changes the emission characteristics. This affects the homogeneity and can require recalibration after lamp replacement, according to Michael Foos, sales and product manager at LOT-Oriel Laser Optik Technologie GmbH and Co. KG in Darmstadt, Germany.

Figure 1. The optical design of a state-of-the-art solar simulator, transforming the light from a short-arc xenon lamp into a homogeneous parallel beam, illuminates a solar cell.

His company represents a major solar simulator manufacturer, Abet Technologies Inc. of Milford, Conn., USA, which has revamped the optical design of solar simulators. The result is that a solar simulator with an illumination area of 100 × 100 mm2 and an intensity of up to four times that of the sun – about 5.2 kW/m2 – requires only a 550-W xenon lamp instead of the earlier 1000-W lamp.

The way forward for solar simulators is in increasing the illumination area. However, this means maintaining – or even improving – the homogeneity. Simulators with continuous emission are commercially available with illumination areas of 450 × 450 mm2 and above. Alternatively, there are flashing simulators that can cover areas of several square meters but that require special setups; e.g., when measurement must be performed while the flash lasts, such as within a few tens of milliseconds.

Figure 2. A detailed view is shown of the mirrors used inside the solar simulator to generate homogeneous illumination of the solar cell. The “optical engine” of a modern state-of-the-art solar simulator comprises a xenon lamp, an ellipsoidal reflector, a (dichroid) mirror and an optical integrator.

And, finally, there are a variety of other solar simulator applications, “some of which are what these simulators were originally intended for,” Foos said. These include the ability of materials, paints and finishes to withstand sunlight – either on Earth or in outer space – and sun-protecting glasses and cosmetics that enable relaxing with confidence while sunbathing by the pool.

Why It Matters

Devices emulating the sun initially were developed for testing of commodities such as sun creams and cosmetics. Now they also play an important role in developing and optimizing solar cells to achieve the best efficiency and operating conditions when converting sunlight into energy. Interestingly, one of the challenges is not to generate “ideal” sunlight but rather to duplicate the spectrum of the light reaching the Earth – which is mainly dependent on incident angle and weather conditions.